model test set up methodology for hds to improve the understanding of reaction pathways … ·...

Model test set up methodology for HDS to improve the

understanding of reaction pathways in HDT catalysts

David Manuel Paulo Negreiro

Thesis to obtain the Master of Science Degree in

Chemical Engineering

Supervisors

Dr. Bertrand Guichard (IFPEN)

Prof. Francisco Manuel da Silva Lemos (IST)

Examination Committee

President: Prof. José Manuel Félix Madeira Lopes (IST)

Supervisor: Prof. Francisco Manuel da Silva Lemos (IST)

Members of the Committee: Prof. Maria Filipa Gomes Ribeiro (IST)

October 2015

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iii

Acknowledgements

Firstly, I would like to start by thanking to Prof. Filipa Ribeiro the great opportunity provided for

doing my master thesis at IFPEN. I would also like to thank Joana Fernandes for her support and help

during the beginning of this internship.

To my IFPEN supervisor Dr. Bertrand Guichard, for your availability, patience and helpful advises

for my professional career. I am also very thankful to Véronique Delattre and Nathalie Lett, for all the

formation they gave me and, above all, I am grateful for their kindness, joy, teaching ability and good

humour which made this experience much enriching. To the people from the Catalysis by Sulfides

Department (R066S), for all support they gave me and for the very good working environment.

I want to express my gratitude to Prof. Francisco Lemos, for his support and for believing in my

capabilities.

I would like to specially thank to Fabien, Leonor, Rubén, Sónia, Mafalda, Svetan, Mathieu, Ana

Rita, Max, Leonel, Marisa and Alberto for their support. I also thank to Larissa and Alexis for the

amazing moments we shared together.

A big “thank you!” to my portuguese friends, Ana, Loios, Joana, Casinhas, Catarina, Solange and

Diogo. Thank you for your support, your friendship and, above all, for the great moments we shared

together. You have become undoubtedly my second family. I have also to thank Pedro, for your words

of wisdom about IFPEN and helping me along my internship.

Finally, I would like to thank my family, especially my mother and brother. Your encouragements

and cheering words over these six months made easier the fact of being far away from home. To

Susana, thank you for everything, because even far away you made everything much easier. It would

not be the same without you by my side…

v

Abstract

In this present work, the hydrodesulfurization (HDS) of 4,6-dimethyldibenzothiophene (4,6-DMDBT)

was studied over three CoMo/Al2O3 catalysts (dried, calcined and additive impregnated) in a fixed-bed

reactor under standard conditions close to those usually used in diesel fuel hydrotreating following

particularly the HYD and DDS pathways behaviors.

The main focus was to identify some strong differences in behavior between the various catalysts

and evaluate the effect of H2S, NH3 and H2 partial pressures on their relative catalytic performances.

It was found by experimental and modelling results that, at standard conditions, the additive

impregnated catalyst performs better and was less impacted by H2S adsorption than dried and

calcined. Though, in the presence of high amounts of H2S, the additive impregnated showed to be the

one differing mostly from H2S partial pressure.

In addition, the study on the impact of nitrogen-based compounds (quinoline) revealed that all three

catalysts are similar inhibited.

In the same way, modifying the partial pressure of H2 was found to enhance the activity of all

catalysts, especially the HYD pathway.

A more detailed study and additional experimental tests should be performed in order to improve

the understanding on the relation between quinoline and H2S within the deep HDS of 4,6-DMDBT and

to further develop the kinetic model created.

Keywords: Diesel, middle distillates, 4,6-DMDBT, CoMo/Al2O3, inhibition effect

vii

Resumo

Neste trabalho, estudou-se a hidrodessulfurização do 4,6-dimetildibenzotiofeno em três tipos de

catalisadores CoMo/Al2O3 (seco, calcinado e aditivado), num reactor de leito fixo em condições

standard próximas das usadas no hidrotratamento do gasóleo de forma a estudar particularmente as

duas reacções principais do processo – hidrogenação (HYD) e dessulfurização directa (DSD).

O principal objectivo foi identificar as maiores diferenças entre os três catalisadores e avaliar o

efeito da pressão parcial de H2S, NH3 e H2 nas performances catalíticas.

A partir dos resultados experimentais e de modelação cinética obtidos, a condições standard, o

catalisador aditivado mostrou ser o catalisador com maior actividade e menor impacto pela adsorção

de H2S comparativamente com os catalisadores seco e calcinado. No entanto, para grandes

quantidades de H2S, o catalisador aditivado foi o mais afectado.

Adicionalmente, do estudo do impacto de compostos azotados (quinolina), determinou-se que os

três catalisadores são inibidos de forma idêntica.

Da mesma forma, da alteração da pressão parcial de H2 determinou-se que a actividade de todos

os catalisadores é aumentada, em particular na via de hidrogenação.

Um estudo mais detalhado e mais testes experimentais devem ser efectuados de forma a melhor

compreender a relação intrínseca entre a quinolina e o H2S na reacção de hidrodessulfurização do

4,6-DMDBT e progredir no desenvolvimento do modelo cinético estabelecido.

Palavras-chave: Gasóleo, destilados médios, 4,6-DMDBT, CoMo/Al2O3, efeito inibitório

ix

List of Contents

List of Figures ..................................................................................................................................... xi

Abbreviation List ............................................................................................................................... xiv

1 Introduction .............................................................................................................................. 1

2 Bibliographic Study .................................................................................................................. 3

2.1 Context ................................................................................................................................ 3

2.2 Overview on hydrotreatment process .................................................................................. 5

2.3 Diesel: specifications and characteristics ............................................................................ 8

2.4 HDS Catalysts ..................................................................................................................... 9

2.4.1 Sulfidation process .................................................................................................... 10

2.4.2 Non-promoted catalysts: MoS2/Al2O3 ........................................................................ 11

2.4.3 Promoted catalysts: CoMo/Al2O3 ............................................................................... 12

2.5 Main compounds in HDT ................................................................................................... 15

2.5.1 Reactivity of sulfur compounds .................................................................................. 16

2.5.2 4,6-DMDBT HDS pathways ....................................................................................... 17

2.6 Inhibition effect................................................................................................................... 20

2.6.1 Ammonia .................................................................................................................... 20

2.6.2 Hydrogen sulfide ........................................................................................................ 24

3 Methodology .......................................................................................................................... 29

3.1 Experimental Part .............................................................................................................. 29

3.1.1 Catalysts preparation ................................................................................................. 29

3.1.2 Unit T033 ................................................................................................................... 30

3.1.3 Unit loading ................................................................................................................ 31

3.1.4 Sulfidation .................................................................................................................. 32

3.1.5 Model feedstock......................................................................................................... 34

3.1.6 Operating conditions .................................................................................................. 35

3.1.7 Data analysis ............................................................................................................. 36

3.2 Kinetic Study ...................................................................................................................... 38

4 Results and Discussion ......................................................................................................... 43

4.1 Comparison of Catalysts in Standard Conditions .............................................................. 43

4.2 Impact of H2S partial pressure ........................................................................................... 50

x

4.2.1 Apparent comparison ................................................................................................ 50

4.2.2 Kinetic comparison .................................................................................................... 52

4.3 Impact of NH3 partial pressure .......................................................................................... 57



4.4 Impact of H2 partial pressure ............................................................................................. 66



4.5 Summary of Results .......................................................................................................... 73

5 Conclusion and Future Perspectives ..................................................................................... 77

6 Bibliography ........................................................................................................................... 79

Appendix 1 – GC Chromatogram ..................................................................................................... 82

Appendix 2 – Kinetic model .............................................................................................................. 83

xi

List of Figures

Figure 1 – Yearly evolution of world consumption of primary energy [1] ........................................... 3

Figure 2 – World`s percentage shares of oil demand by sector in 2011 and 2040, [2] ..................... 4

Figure 3 – Representation of the maximum sulfur limit for diesel all over the world (2014) [2] ......... 4

Figure 4 – Schematic of a typical oil refinery [5] ................................................................................ 5

Figure 5 – Once-through hydroprocessing unit: two separators and recycle gas scrubber, [7] ........ 7

Figure 6 – Scheme of the top of a hydrotreatment reactor, [7] .......................................................... 7

Figure 7 – Typical shapes of catalysts – (A and B) trilobe and cylindrical pellets, (C) spheres, (D)

rings, [9] ................................................................................................................................................... 9

Figure 8 – Different steps of hydrotreating catalyst synthesis and life [12] ........................................ 9

Figure 9 – Schematic representation of the sulfidation process of a CoMo/γ-Al2O3 catalyst, [13] .. 10

Figure 10 – Evolution of the activity versus time on stream during the HDS of DBT on NiMo/Al2O3

[14] ......................................................................................................................................................... 11

Figure 11 – Top and side views of a MoS2 cluster [17] .................................................................... 11

Figure 12 – STM images of triangular (A) and hexagonal (B) MoS2 nanocluster [19] ..................... 12

Figure 13 – Structural illustration of different structures present in a sulfided CoMo/Al2O3 catalyst

[12] ......................................................................................................................................................... 13

Figure 14 – Co distribution on the sulfide CoMo/Al2O3 catalyst [21] ................................................ 13

Figure 15 – Schematic of the (a) MoS2 and (b) CoMoS active phases (adapted). The yellow

spheres represent sulfur atoms, purple spheres represent molybdenum, and finally, green spheres

represent cobalt [23] .............................................................................................................................. 14

Figure 16 –HDS rate of DBT as function of the computed EM-S [24] ................................................ 14

Figure 17 – Organosulfur compounds converted in HDT reactions [28] .......................................... 15

Figure 18 – Main organic-nitrogen compounds found in pre-treated crude oil. Underlined

compounds represent the non-basic nitrogen compounds and the others represent basic-nitrogen

compounds ............................................................................................................................................ 16

Figure 19 – Organosulfur reactivity in HDS process as function of aromatic ring sizes and positions

of alkyl substitutions [30] ....................................................................................................................... 16

Figure 20 – Scheme of the reaction pathways of the hydrodesulfurization of 4,6-DMDBT – HYD on

the left and DDS on the right, [31] ......................................................................................................... 17

Figure 21 – Scheme reaction for HYD pathway adapted from [20] ................................................. 18

Figure 22 – Scheme reaction for DDS pathway adapted from [20] ................................................. 18

Figure 23 – Schematic examples of a hydrogenation (A) and C-S bond cleavage (B) sites [20] .... 19

Figure 24 – HDS of 4,6-DMDBT (green) DDS and HYD concentration product (blue and red,

respectively), [33] Adapted .................................................................................................................... 20

Figure 25 – Molecular modeling results for various sulfur- and nitrogen-containing organic

compounds. The bond order value in bold next to a green symbol indicates the bond with highest bond

order, while the underlined blue number indicates the net electronic charge on the heteroatom in a

given molecule.[36] ................................................................................................................................ 21

xii

Figure 26 – Global activity as function of H2S partial pressure, for 4,6-DMDBT transformation on

CoMo and NiMo catalysts [31] .............................................................................................................. 24

Figure 27 – Overview of the T033 unit ............................................................................................. 30

Figure 28 – Schematic of a VALCO® valve with six ways ................................................................ 31

Figure 29 – Representation of the micro-reactor used in the T033 unit........................................... 32

Figure 30 – Temperature program for the sulfidation process, before each catalytic test ............... 33

Figure 31 – Temperature program for the catalytic test ................................................................... 36

Figure 32 – Schematic of the two reaction pathways for the HDS of 4,6-DMDBT, adapted [32] .... 36

Figure 33 – Kinetic fitting obtained for CoMo-C at standard conditions ........................................... 43

Figure 34 – Selectivity DDS/HYD as function of the total HDS conversion, at 290°C ..................... 46



Figure 37 – Ratio DMDCH/MCHT as function of the conversion of DMBPh (HDA), at 310oC ........ 48

Figure 38 – Selectivity DDS/HYD as function of the temperature .................................................... 49

Figure 39 – HYD conversion of 4,6-DMDBT as function of temperature for the three catalysts

prepared. In the graph, full lines represent the 1,2 wt.% DMDS feed and gapped lines represent the 2

wt.% DMDS. .......................................................................................................................................... 50

Figure 40 – DDS conversion of 4,6-DMDBT as function of temperature for the three catalysts

prepared. In the graph, full lines represent the 1,2 wt.% DMDS feed and gapped lines represent the 2

wt.% DMDS. .......................................................................................................................................... 51

Figure 41 – Kinetic fitting obtained for CoMo-B, for Feedstock-2 conditions (2 wt.% DMDS) ......... 52

Figure 42 – ln(ko) as function of ln(ppH2S) for HYD ......................................................................... 55

Figure 43 – ln(ko) as function of ln (ppH2S) for HDA ........................................................................ 55

Figure 44 – ln(ko) as function of ln(ppH2S) for DDS ......................................................................... 55

Figure 45 – H2S inhibition factor for CoMo-A, CoMo-B and CoMo-C. These results took into

account the results obtained at standard conditions and Feedstock-2 ................................................. 56

Figure 46 – HYD conversion of 4,6-DMDBT as function of temperature for the three catalysts

prepared. In the graph, full lines represent the 0,5 wt.% Quinoline model approximation and gapped

lines represent the 1,0 wt.% Quinoline (standard conditions). .............................................................. 58

Figure 47 – DDS conversion of 4,6-DMDBT as function of temperature for the three catalysts

prepared. In the graph, full lines represent the 0,5% Quinoline model approximation and gapped lines

represent the 1,0% Quinoline (standard conditions). ............................................................................ 59

Figure 48 – Kinetic fitting obtained for the HDS of 4,6-DMDBT using CoMo-B - 0,5% quinoline .... 60

Figure 49 – Kinetic fitting obtained for the HDS of 4,6-DMDBT using CoMo-C - 1,5% quinoline .... 60

Figure 50 – ln(ko) as function of ln(ppNH3) for HYD ......................................................................... 63

Figure 51 – ln(ko) as function of ln(ppNH3) for HDA ......................................................................... 63

Figure 52 – ln(ko) as function of ln(ppNH3) for DDS ......................................................................... 63

Figure 53 – Overall NH3 inhibition factor comparing standard conditions (1 wt.% Quinoline) with

both 0,5 wt.% and 1,5 wt.% Quinoline ................................................................................................... 65

xiii

Figure 54 – HYD pathway conversion as a function of the temperature for different catalysts and

total pressures ....................................................................................................................................... 66

Figure 55 – DDS pathway conversion as a function of the temperature for different catalysts and

total pressures ....................................................................................................................................... 67

Figure 56 – Kinetic fitting obtained for the HDS of 4,6-DMDBT using CoMo-C, at 40 bar .............. 68

Figure 57 – Effect of H2 partial pressure on the selectivity of HDS over the prepared catalysts, for

310°C. .................................................................................................................................................... 70

Figure 58 – ln(ko) as function of ln(ppH2) for HYD ........................................................................... 70

Figure 59 – ln(ko) as function of ln(ppH2) for HDA ........................................................................... 71

Figure 60 – ln(ko) as function of ln(ppH2) for DDS ........................................................................... 71

Figure 61 – H2 activation factor on the three catalysts tested (global activity, see Eq. 27) ............. 72

Figure 62 – Relative differences in activity for HYD reaction between the three catalysts .............. 74

Figure 63 – Relative differences in activity for the DDS reaction between the three catalysts ........ 75

Figure 64 – Example of a GC chromatogram .................................................................................. 82

Figure 65 - Kinetic fitting obtained for the HDS of CoMo-B at standard conditions ......................... 83

Figure 66 - Kinetic fitting obtained for the HDS of CoMo-A at standard conditions ......................... 83

Figure 67 – Kinetic fitting obtained for the HDS of CoMo-C at 2 wt.% DMDS ................................. 84

Figure 68 – Kinetic fitting obtained for the HDS of CoMo-A at 2 wt.% DMDS ................................. 84

Figure 69 – Kinetic fitting obtained for the HDS of CoMo-A at 0,5 wt.% Quinoline ......................... 85

Figure 70 - Kinetic fitting obtained for the HDS of CoMo-C at 0,5 wt.% Quinoline .......................... 85

Figure 71 – Kinetic fitting obtained for the HDS of CoMo-A at 1,5 wt.% Quinoline ......................... 85

Figure 72 – Kinetic fitting obtained for the HDS of CoMo-B at 1,5 wt.% Quinoline ......................... 86

Figure 73 - Kinetic fitting obtained for the HDS of CoMo-B at 40 bar .............................................. 86

Figure 74 - Kinetic fitting obtained for the HDS of CoMo-A at 40 bar .............................................. 86

xv

Abbreviation List

CoMo/Al2O3 – CoMo catalyst supported over alumina

DBT – Dibenzothiophene

DDS – Direct desulfurization

DFT – Density Functional Theory

DMBPh – Dimethylbiphenyl

DMDCH - Dimethyldicyclohexyl

DMDS – Dimethyl disulfide

E2 – Elimination reaction

GC – Gas Chromatography

HDA – Hydrodearomatization

HDN – Hydrodenitrogenation

HDS – Hydrodesulfurization

HYD – Hydrogenation

H2S – Hydrogen sulfide

LHSV – Liquid Hourly Space Velocity

MCHT – Methylcyclohexyltoluene

ppH2S – Hydrogen sulfide partial pressure

ppH2 – Hydrogen partial pressure

ppNH3 – Ammonia partial pressure

ppQuinoline – Quinoline partial pressure

SiC – Silicon carbide

SCR – Selective Catalytic Reduction

STM – Scanning Tunneling Microscopy

4,6-DMDBT – 4,6-Dimethyldibenzothiophene

wt.% – Percentage weight fraction

1

1 Introduction

In order to decrease pollution caused by automobile vehicles, the sulfur content in diesel fuel has

been drastically reduced over the years as witnesses the restrictive regulations. However, refining

industries are processing heavier feedstocks and facing an increasing demand of diesel/gasoline, so

the need for more efficient hydrodesulfurization (HDS) catalysts is required, i.e. low sulfur product at

even highest LHSV (the HDS process taking place in fixed bed reactors).

The commercial catalysts commonly used for HDS reactions are molybdenum sulfides promoted

by cobalt or nickel and supported over alumina. Those catalysts are well known and the way to

prepare them is well monitored. Nevertheless, if one aims to improve their performances, it is now

necessary to identify precisely the limitations. It can be achieved by characterizing deeply the catalyst,

using powerful tools or by carrying a kinetic study to identify the main limitation/inhibiting/activating

parameters. To do so, it is necessary to evaluate the catalyst in representative conditions witnessing

the way it will have to work in real conditions. The feedstocks being too much complicated, the matrix

needs to be simplified and adapted to the goals.

The main compounds contained into the feedstock are generally aromatics, olefins, nitrogen and

sulfides and the catalysts will simultaneously have to perform hydrogenation, hydrodesulfurization and

hydrodenitrogenation reactions. All those reactions improve the feed quality but could also be

competitive ones. That is why if one aims to study the reactivity, all those compounds would need to

be introduced in the model feed. Indeed studying separately the HDS could lead to a wrong

interpretation.

In the HDS of middle distillates, as sulfur conversion increases the remaining species are mostly

dibenzothiophenes. These compounds, such as 4,6-dimethyldibenzothiophene (4,6-DMDBT), are the

most refractory compounds, since they are very difficult to decompose. These compounds are

converted through two distinct pathways namely, Hydrogenation (HYD) and Direct Desulfurization

(DDS).Furthermore, the presence of the methyl groups on 4,6-DMDBT highly limits the reactivity and

leads the HDS to selectively process through the hydrogenating route compared to the DDS one.

Moreover, there is a high interest to obtain the best catalysts and operating conditions to achieve

HDS trough the DDS pathway since it consumes less hydrogen than HYD.

The objective of this work is focused on the comprehension of the HDS mechanism on various

representatives CoMo catalyst types in order to study the deep HDS of middle distillates in the range

of operating conditions dedicated to the low pressure HDS (i.e. around 30-50 bar with CoMo

catalysts). The inhibiting and activating effects should be taken into account by modifying some of the

working conditions (temperature, LHSV, pressure). The partial pressure in H2S and NH3 will be

monitored trough DMDS and quinoline incorporation into the prepared feedstocks, respectively.

For every conditions, there was a mixture of model molecules, in the presence of 4,6-DMDBT, with

the purpose to discriminate the catalytic performance of each CoMo-based catalyst and to compare it

to the well-established ranking provided by the real feed evaluation. The aim is to point out some

strong differences in behavior between the various catalysts and to go deeper in the comparison than

2

the direct ranking provided by diesel HDS, i.e. supplying activation energy, inhibition effects and

changes according to the HDS conversion or partial pressure. Thus, this work should lead to propose

the best conditions for each type of catalyst and also to improve their way of working induced by the

preparation methodology.

3

2 Bibliographic Study

2.1 Context

Today, energy is a mix of multiple resources, as we are able to convert them in many ways in order

to power high-consuming societies.

In the past few decades, the world energy market has entered in a period of dynamic changes due

to the economic growth in developed and developing countries. It led to a rapid growth in primary

energy. As it can be seen in Figure 1, the resource that plays a vital role in order to successfully

satisfy this demand is oil, as it was representing 33% of the world’s energy consumption in 2013 [1].

Figure 1 – Yearly evolution of world consumption of primary energy [1]

Therefore, as crude oil exploitation is one of the most pollutant activities worldwide, environmental

protection, cleaner fuels have been required.

Practically, the sulfur released to the atmosphere has to be controlled and lowered as much as

possible because this compound is responsible for many environmental problems such as production

of acid rains which cause the acidification of soils, lakes and streams, and accelerates corrosion of

buildings and monuments.

This environmental phenomenon is produced by the reaction of water molecules, present in the

atmosphere, with SOx produced within the diesel engine. The reaction is described as follows:

(Eq. 1)

(Eq. 2)

4

Furthermore, as one can see in Figure 2, the percentage shares of oil demand is mainly constituted

by the transportation and industrial sectors. In the next decades the percentage share taken by the

industrial sector will suffer a minor decrease of 2%, contrasting with the 4% increase within the

transportation sector.

Figure 2 – World`s percentage shares of oil demand by sector in 2011 and 2040, [2]

Moreover, post-combustion catalyst used to reduce NOx emission and massively introduced in the

automobile engines (SCR systems) are very sensitive to poisoning by sulfur so it strengthened the

need to reduce the sulfur amount in commercial diesels.

Taking into account that crude oil quality decreases over the years, the sulfur content tends to

increase thus regulatory specifications will be harder to satisfy. Furthermore, in Figure 3, it is possible

to see that in developed countries the maximum sulfur limit allowed is from 10 to 15 ppm (m/m%) but

the same level is expected in the coming years or decades in developing countries [2].

Figure 3 – Representation of the maximum sulfur limit for diesel all over the world (2014) [2]

5

2.2 Overview on hydrotreatment process

As crude oil quality decreases the need for new technologies capable of producing cleaner and

better fuels is a continuous challenge for process engineering companies. Indeed, in modern refineries

HDT units are the most common process units. The typical composition of unprocessed crude oil is

shown in Table 1.

Table 1 – Typical composition of crude oil, [3]

Element Percentage (%)

Carbon 84 - 87

Hydrogen 11 - 14

Nitrogen 0,1 – 1,0

Oxygen 0,1 - 0,5

Sulfur 0,5 - 6

Metals < 0,1

As one can see in Table 1, sulfur is the main contaminant within crude oil. On one hand,

sulphurous compounds are “poisonous” but, on the other hand, as reported by Rana et al.[4],

nitrogenous compounds lead to catalyst inhibition even at minute concentrations. Therefore,

hydrotreatment to remove sulfur and nitrogen is usually applied in many sections in the refinery as

shown in Figure 4.

Figure 4 – Schematic of a typical oil refinery [5]

6

Usually, the HDS process takes place in a catalytic fixed-bed reactor in presence of hydrogen gas

and the liquid feedstock to be “processed”. The typical operating conditions for the hydrotreatment

process depend on the feedstock reactivity, composition and with the product`s specifications. In

Table 2 are shown the operating conditions used in past as well as the conditions used nowadays.

As one can see, the conditions used in current days are much more “aggressive” (especially for

hydrogen pressure and LHSV) due to the increasing amount of sulfur and other contaminants present

in crude oil, as already mentioned, and to the lower limits that are imposed by legislation.

Table 2 – General hydroprocessing conditions used in industry, [6]

A typical flow diagram of a two reactors HDT process in which the feedstock and hydrogen gas are

supplied from the top of the reactor is shown in Figure 5.

There are two types of hydrotreatment processes, the single and the two or multiple-staged

processes [7].

With the increasingly stringent regulations on diesel oil, a lot of attention has been paid to reduce

sulfur content of distillate fuels. The two-stage process is an upgrade of the conventional

hydrotreatment process, since it removes the hydrogen sulfide and ammonia produced in the first

reactor, enhancing the reactivity within the second reactor. Therefore, with staged processes, high

decomposition of sulfur and nitrogen-based compounds are easier to achieve.

7

Figure 5 – Once-through hydroprocessing unit: two separators and recycle gas scrubber, [7]

As there are many types of compounds to be decomposed or removed, the choice of the catalyst is

therefore crucial to the process. So, to meet the required specifications, HDT catalysts have to be

efficient in order to accomplished hydrodesulfurization (HDS), hydrodenitrogenation (HDN) and

hydrodearomatization (HDA) reactions.

Typically, the reactor (Figure 6) consists in more than one catalytic bed depending on the impurities

found in the feedstock and operating conditions such as the liquid hourly space velocity (LHSV). In

these reactors several reactions are found in order to remove the sulfur and nitrogen-based

compounds. In order to avoid cracking reactions and to maximize the quality of the liquid fuels

produced, quenching hydrogen gas is commonly injected at various points along the reactor to cool

down the reaction temperature because HDT reactions are highly exothermic. Liquid-phased products

are additionally fractionated according to their boiling points into the required products in a column

according to their boiling points.

Figure 6 – Scheme of the top of a hydrotreatment reactor, [7]

8

2.3 Diesel: specifications and characteristics

To better understand why HDT is important to produce a high quality diesel, it is essential to know

the main characteristics and specifications for this fuel.

Diesel is a fuel produced from crude oil and consists mainly of aliphatic and some aromatics

hydrocarbons comprising, normally, 13-25 carbon atoms with boiling points in the range of 230-380°C

[8]. Moreover, diesel is an oil fraction heavier than gasoline, with lower H/C mass ratio.

The properties and reactivity of diesel feeds, composed mainly of paraffins and aromatics, are

deeply dependent on their source.

In Europe, the latest specifications imposed in 2009 were mainly pointed to decrease the sulfur

content on diesel fuels. In Table 3 are shown the main specifications for diesel composition. The other

one is directly linked to its use in the diesel engine.

Table 3 – Specifications for diesel fuel [9]

Generally, diesel quality is essentially related to its cetane number. Hence, the higher n-paraffinic

and naphthenic content, the greater will be the quality of the diesel produced.

Moreover, additives are generally added to reach better properties depending on the purpose and

country, such as to lower the freezing point, which is essential in some countries where the

temperatures are very low during winter.

9

2.4 HDS Catalysts

The catalysts used in the hydrotreatment process contain a metal sulfide usually molybdenum

promoted with either nickel or cobalt, supported over a refractory oxide carrier (e.g. alumina). To

minimize diffusion limitations and the process pressure drop, these catalysts must have a certain

shape. The most commonly used are generally trilobe shaped pellets, spheres and rings (Figure 7).

Figure 7 – Typical shapes of catalysts – (A and B) trilobe and cylindrical pellets, (C) spheres, (D) rings, [10]

For industrial purposes, the most commonly used support is γ-alumina since:

- it provides a greater surface area (230-350 m

2/g) than other supports

[11];

- it allows to maximize the dispersion of the active phase, due to its acid-basicity properties;

- and exhibits a high mechanical strength [12].

The preparation of a HDT catalyst (Figure 8) involves several steps, including:

Impregnation – the impregnation solution is added to the support;

Maturation – guarantees that the solution is well dispersed into the support pores;

Drying – remove the excess of solvent from the support.

Figure 8 – Different steps of hydrotreating catalyst synthesis and life [8]

(A) (B) (C) (D)

10

These steps can be complemented by an optional calcination which was frequently performed

before but is less common nowadays. An additivation step is also usual to be realized in order to

promote the catalytic activity of the HDT catalyst. This step can be accomplished, for instance, with a

glycol molecule [8]. Then, the catalysts are sulfided, leading to the active state.

2.4.1 Sulfidation process

HDT catalysts might be subjected to a sulfidation process in order to form the active phase. This

step of transformation from oxide to sulfide (and with molybdenum reduction) plays a crucial role in

what concerns to the catalytic activity and the catalysts stability during hydrotreatment reactions.

As this transformation is exothermic, temperature has to be carefully controlled in order to avoid

“poisonous” side reactions, i.e. metallic oxide reduction by hydrogen and coke formation, which would

reduce the catalytic activity (Figure 9).

Figure 9 – Schematic representation of the sulfidation process of a CoMo/γ-Al2O3 catalyst, [13]

Due to handling and loading problems associated with the active sulfided form, hydrotreating

catalysts are typically produced and shipped in their inactive form. Then, in order to be used these

catalysts must be first activated by a sulfidation agent, promoting the O–S exchange. This process is

performed using either a gas mixture of H2S/H2, an organo-sulfidation agent like dimethyl disulfide

(DMDS) or even directly the diesel feedstock to be desulfurized [14].

When DMDS is used, it decomposes into CH4 and H2S, which the latter acts as the actual

sulfidation agent. As one can see in Figure 10, Texier et al. [14] observed that using organo-sulfide

compounds like DMDS slightly increases the catalytic specific activity.

Hence, it has been stated that a correct activation of hydrotreating catalysts depends greatly on

temperature and H2S proportion.

11

Figure 10 – Evolution of the activity versus time on stream during the HDS of DBT on NiMo/Al2O3 [14]

Moreover, Hallie [15] reported that the use of organo-sulfide agents (such as DMDS) increases the

HDS activity of VGO (Vacuum Gas Oil) by as much as 60% when compared to a gas-phase H2/H2S

sulfidation procedure particularly for CoMo/Al2O3 catalysts.

All these results underline the key role of the sulfidation step in the catalytic performances.

2.4.2 Non-promoted catalysts: MoS2/Al2O3

In HDT processes, most part of the catalysts used are based in molybdenum sulfide. This is why

one first describes the non-promoted catalysts.

These catalysts are constituted by a well dispersed active phase of MoS2 on an alumina surface

whose the primary unit cell consists in a single hexagonal slab. Every single slab exhibits the same

structure, where molybdenum ions are coordinated with six sulfur ions in a trigonal-prismatic

configuration. Also, the slabs interact with each other by Van der Waals forces, creating a layered

structure with interposed molybdenum between two layers of sulfur atoms.

In addition, depending on the crystallographic plan terminating the obtained structure exhibits two

types of edges, either Mo-edges or S-edges as evidenced in Figure 11.

Figure 11 – Top and side views of a MoS2 cluster [16]

12

Both edges can lose a sulfur atom by reaction with hydrogen. The resulting vacancies lead to the

exposure of Mo-cations, and are known as coordinately unsaturated sites (CUS). CUS are “deficient”

in electrons and thus interact with electron donor compounds (Eq. 3).

(Eq. 3)

These sites are capable of adsorbing organosulfur compounds, which will bond to the unsaturated

Mo ions creating a metal-sulfur bond, becoming more active in HDS reactions [8]. Nevertheless, some

thermodynamic calculations show that this is not the only way to react for sulfur compounds.

Raybaud et al. [17] studied the morphology of MoS2 catalysts and observed that it depends on the

sulfidation conditions, such as temperature and partial pressure of H2 and H2S. It is the relative

thermodynamic stability of the two types of edges (under specific conditions) that determines the

morphology of the MoS2 nanoclusters. Under strongly sulfidation conditions (high H2S partial

pressures) triangular-shaped MoS2 particles should be obtained, whereas under more reducing

conditions the MoS2 particles might exhibit a hexagonal shape. Lauritsen et al. [18] observed these

two possible shapes for MoS2 nanoclusters by STM imaging of MoS2/Au (Figure 12).

Figure 12 – STM images of triangular (A) and hexagonal (B) MoS2 nanocluster [18]

2.4.3 Promoted catalysts: CoMo/Al2O3

Since the activity of CoMo catalysts will be the main subject of this work, they will be described in

more detail.

As observed by Bataille et al. [19], for the decomposition of DBT and 4,6-DMDBT (the most difficult

compounds to decompose) the overall catalytic activity increases from 0,4 to 7,2 and 0,65 to 2,3 mol.h-

1.kg

-1, respectively, when Co is added to Mo/Al2O3. Moreover, this increase is not homogenous for

both compounds since their main reaction pathway, as further discussed, is different.

However, cobalt by itself does not present any activity, which is why it is considered a promoter of

MoS2 activity [20]. It is also believed that the substitution of Mo by Co atoms at S-edges enhances the

formation of sulfur vacancies, CUS [21].

Although there are many attempts to describe the structure of these Co promoted catalysts,

Topsøe et al. [6]

have proposed the mixed phase "CoMoS" model, which is currently the most accepted

model.

13

As one can see in Figure 13, cobalt and molybdenum are dispersed into different structures at the

catalyst surface [8]. During the sulfidation process, there are cobalt atoms, which react with sulfur

atoms resulting on Co9S8 crystallites (Co-sulfide phase). Other cobalt atoms, influenced by the

calcination stage of the catalyst preparation, occupy tetrahedral sites inside the catalyst support

(Co/Al2O3). Finally, there is also the molybdate species, which remain from the precursor, will be

dispersed along the catalyst surface.

Figure 13 – Structural illustration of different structures present in a sulfided CoMo/Al2O3 catalyst [8]

The mentioned Co9S8 crystallites do not present any catalytic activity, therefore their formation has

to be minimized in order to produce selectively the active catalytically structure CoMoS. The formation

of all these structures must be controlled all along the catalysts preparation steps, including

impregnation and maturation. In Figure 14, one can observe how the quantity of cobalt impregnated in

the support influences the formation of each individual phase of cobalt.

Figure 14 – Co distribution on the sulfide CoMo/Al2O3 catalyst [20]

As can be seen, adding Co to a given support leads to an increasing amount of the CoMoS phase

up to a certain Co/Mo ratio. As Co increases, the edge positions will be occupied until certain point

when all these positions are completely filled and, then Co atoms will start to form Co9S8 crystallites.

This is a complex set of transformations, which is difficult to monitor, as there are many geometrical

14

and structural constraints, which will limit the CoMoS phase amount that is formed. Therefore, for high

Co/Mo ratios the HDS activity decreases. Raybaud et al. [22] have proposed, based on DFT

simulations, that the final morphology of MoS2 structures is influenced by cobalt atoms since their

presence changes the shape of the MoS2 slabs. Actually, it would be explained by the fact that cobalt

would be incorporated into the S-edges of MoS2 particles, which would enhance the stabilization of the

S-edges relatively to the situation of pure MoS2.

In Figure 15, are shown the structural modifications on the active phase from a MoS2 to a CoMoS

nanocluster.

Figure 15 – Schematic of the (a) MoS2 and (b) CoMoS active phases (adapted). The yellow spheres represent sulfur atoms, purple spheres represent molybdenum, and finally, green spheres represent cobalt [22]

Concerning the added promoter, it is believed that the substitution of Mo by Co atoms at S-edges

increases the formation of sulfur vacancies and creates new and more active sites. Indeed, it is

assumed that Co-S bond is weaker than the Mo-S bond, thus vacancy formation is expected to be

much easier. Also, it is known that Co increases the electronic density on the sulfur atoms, enhancing

the basicity of specific S2-

centers important to HDS reactions.

Indeed, it is possible to relate the S-Metal bonding energy (EM-S) with catalytic activity, using the

volcano curve (Figure 16) proposed by Raybaud et al. [23].

Figure 16 –HDS rate of DBT as function of the computed EM-S [23]

15

In Figure 16, it is possible to verify that CoMoS phase present an optimal EM-S (which corresponds

to the maximum activities), contrary of what happens to Co9S8.

Moreover, Besenbacher et al. [21] reported that the support interacts with the active phase,

influencing the catalyst activity. Further studies suggested a relation between the catalyst structure

and activity [24]. Thus, it was proposed that CoMoS has two distinct types - Type I (with low catalytic

activity) and Type II (with high catalytic activity).

On one hand, Type I structures are known to be incompletely sulfided, presenting bonds with the

support. These bonds correspond to the interaction between Mo and the surface of the alumina

support which produces monolayer-type structures thus influencing the catalytic properties of CoMoS.

On the other hand, Type II structures have the same interactions with the support, as Type I,

however they are much weaker, therefore the sulfidation of Mo and Co is easier. This CoMoS-type

presents a multilayered slab structure.

Furthermore, it has been reported the existence of S-H groups as an active site, created due the

adsorption of hydrogen on the S-edge. It is believed that these sites play an important role in

hydrogenation and hydrogenolysis reactions since the evidence of adsorption of sulfur containing

molecules, as well as the dissociation reaction of H2 seems to occur [25] [26].

2.5 Main compounds in HDT

In the HDT process, all reactions take place in the liquid phase. The compounds to be converted

diffuse through the liquid feed, filing the catalyst pores and adsorb on the catalyst surface where

reactions take place.

Generally, crude oil contains a huge amount of organosulfur compounds as they can be divided

into two main families: the non-heterocyclic and the heterocyclic compounds. On one hand, non-

heterocyclic compounds include mercaptans, sulfides and disulfides. On the other hand, heterocyclic

compounds have sulfur atoms within the cyclic structure (e.g. thiophene) and others with adjacent

aromatic rings and alkyl groups. In Figure 17 are shown examples of non-heterocyclic and

heterocyclic structures, which are converted within HDT reactions.

Figure 17 – Organosulfur compounds converted in HDT reactions [27]

Besides sulfur compounds, organic-nitrogenous compounds are also found in crude oil. Depending

on its origin, crude may contain amounts of these compounds between 0,1% and 1,0% (wt.%) [28].

This nitrogen content appears in crude oil especially in the form of nitrogen-containing polycyclic

aromatic rings, such as quinoline, indole, acridine and carbazole (Figure 18). These nitrogen-based

16

compounds are divided in basic and non-basic which thus influences their reactivity (basic compounds

adsorb easily and will compete severely with the sulfur compound to be desulfurized).

Figure 18 – Main organic-nitrogen compounds found in pre-treated crude oil. Underlined compounds represent the non-

basic nitrogen compounds and the others represent basic-nitrogen compounds

2.5.1 Reactivity of sulfur compounds

As illustrated in Figure 19, the reactivity of the sulfur compounds depends highly on the molecule

structure. Thus, thiols, sulfides and disulfides are easier to be converted compared to heterocyclic

compounds. Moreover, the overall reactivity decreases with the increasing number of aromatic rings.

Figure 19 – Organosulfur reactivity in HDS process as function of aromatic ring sizes and positions of alkyl substitutions

[29]

17

Song et al. [29] reported that when sulfur level in diesel is reduced to 30 ppm, the residual

compounds are essentially alkyl-dibenzothiophenes, such as 4-MDBT and 4,6-DMDBT. These

compounds exhibit a low HDS reactivity.

In fact, the low reactivity of 4,6-DMDBT could be explained by the steric hindrance caused by the

methyl groups and also by the electronic factors around the sulfur atom even if the demonstration has

never been provided.

Ultra-deep HDS of diesel fuel is thus a huge challenge since the lower is the sulfur composition of

the crude oil, the more difficult is the HDS and because the heavier feedstocks that have to be used

nowadays contain the highest alkyl-dibenzothiophenes proportions. Therefore it becomes important to

improve the catalysts activity towards refractory compounds such as 4,6-DMDBT.

2.5.2 4,6-DMDBT HDS pathways

As established by many authors [6]

[19], it is known that the HDS of DBT-type compounds occurs

by two parallel pathways – hydrogenation (HYD) and direct desulfurization (DDS) (Figure 19).

HYD pathway implies that the molecule undergoes numerous hydrogenation reactions before the

intended sulfur removal, while DDS pathway goes through the direct elimination of sulfur, producing

biphenyl-type compounds.

Figure 20 – Scheme of the reaction pathways of the hydrodesulfurization of 4,6-DMDBT – HYD on the left and DDS on the

right, [30]

18

The final products obtained for the hydrogenation pathway are methylcyclohexyltoluene (MCHT)

and dimethyldicyclohexyl (DMDCH). For the DDS pathway the main final product is dimethylbiphenyl

(DMBPh) [30].

There are several intermediate products involved in the 4,6-DMDBT conversion but they are not

always observed due to their high reactivity.

Bataille et al. [19] have suggested that 4,6-DMDBT conversion starts with its partial hydrogenation

to form a dihydrointermediate. This first step is considered to be the most difficult step as it partially

saturates the benzene ring. In general, nine isomers of dihydrointermediates may be formed by 1,2 or

1,4-addition of two hydrogen atoms. However, compounds formed by 1,4-addition are not favored as

the double bonds present in their aromatic ring are not conjugated which means that electrons

cannot be delocalized over the electronic system. In Bataille et al. [19] proposed mechanism, there is

a same first intermediary compound.

For the HYD pathway, the different steps which 4,6-DMDBT undergoes are shown in Figure 21.

Figure 21 – Scheme reaction for HYD pathway adapted from [19]

1. Firstly, dihydroisomers is hydrogenated, producing tetrahydroisomers;

2. Secondly, an elimination reaction leads to the first C-S bond cleavage;

3. The second aromatic ring is partially hydrogenated (1,2-addition of two hydrogen atoms);

4. Then, a second elimination reaction breaks the second C-S bond, thus forming MCHT;

5. Finally, hydrogenation of MCHT can occur, producing DMDCH.

In the DDS route, the vicinity of the sulfur atom must not contain a double bond in order to directly

break the C-S bond by an elimination step. To respect this configuration, only two of the

dihydrointermediates over the nine possible can be converted through this route. The next steps can

be considered for the DDS pathway, as illustrated on Figure 22.

Figure 22 – Scheme reaction for DDS pathway adapted from [19]

1. An elimination reaction leads to the first C-S bond cleavage;

2. The second aromatic ring is partially hydrogenated (1,2 addition of two hydrogen atoms);

3. A final elimination reaction performs the second C-S bond cleavage, consequently forming

DMBPh.

Finally, Bataille et al. [19] suggested that the catalytic sites, responsible for the hydrogenation and

the C-S bond cleavage, are basically the same, namely that they are made of sulfur vacancies

19

associated to neighboring sulfur anions (Figure 23). In this hypothesis, HYD and DDS sites would only

differ in the availability of adsorbed hydrogen and in the basicity of the associated sulfur anions.

Figure 23 – Schematic examples of a hydrogenation (A) and C-S bond cleavage (B) sites [19]

On one hand, the hydrogenation site would be composed by:

- a vacancy (CUS);

- associated with a SH group

- and with a hydrogen atom (adsorbed on a Mo atom).

So, the vacancy should adsorb the substrate and the neighbouring SH- group associated with the H

atom should undergo an immediate hydrogenation reaction.

On the other hand, the C-S bond cleavage site, which may be involved in the elimination step of

the proposed DDS route, would be then composed by:

- two vacancies (CUS)

- associated with a S2-

anion.

Subsequently, one vacancy should adsorb the substrate (e.g. 4,6-DMDBT), as the second retains

the sulfur atom. Subsequently, the sulfur anion should act as a basic site to favour the elimination

reaction and directly breaks the C-S bond.

(A)

Hydrogenation site C-S bond cleavage site

(B)

20

2.6 Inhibition effect

According to Prins et al.[31], HDS reactions are commonly inhibited by compounds such as H2S

and NH3. On one hand, the presence of H2S during diesel hydrotreatment is inevitable, since it is

produced from sulfur compounds decomposition and is necessary to remain in a sulfided form. On the

other hand, NH3 is a product of the hydrodenitrogenation (HDN) of compounds such as quinoline and

carbazole, which occurs within the HDT reactors and so is often inevitable.

2.6.1 Ammonia

As reported by Kwak et al. [32], HDS is markedly suppressed by the presence of nitrogen-type

compounds such as quinoline and carbazole, even at low concentrations.

In Figure 24 is reported the concentration of products formed in the HDS of 4,6-DMDBT in

presence of different concentrations of basic and non-basic nitrogenous compounds, carbazole and

quinoline, respectively.

Figure 24 – HDS of 4,6-DMDBT (green) DDS and HYD concentration product (blue and red, respectively), [32] Adapted

As one can see, the 4,6-DMDBT conversion decreases when small amounts of both nitrogen

compounds are present and drops to near 40% and 38% when using 650 ppm of carbazole or 500

ppm of quinoline in the feedstock, respectively. The inhibiting effect of quinoline is higher than the one

imposed by carbazole, as smaller quantities of the basic-nitrogen compound is added have nearly the

same effect on 4,6-DMDBT conversion when using higher concentrations of the non-basic one.

Hence, as reported by literature [30] [33]

[34], basic-nitrogen compounds inhibit much more HDS

reactions than basic-nitrogen compounds.

To further explain how nitrogen-based compounds influence each HDS pathway (HYD and DDS),

Ma et al. [34] have correlated by molecular modeling calculations the bond order and net electronic

21

charge on the heteroatoms for nitrogen- and sulfur-containing organic compounds typically found in

diesel and jet fuel feedstocks.

The bond order and net electronic charge on the heteroatom have been associated to the activity

for hydrogenation and hydrogenolysis, respectively. Among a set of molecules, it is expected that:

- the molecule having a bond with the highest order is expected to have the greatest reactivity for

hydrogenation.

- the molecule having the highest electronic charge is expected to have the greatest reactivity for

hydrogenolysis. Figure 25 depicts the bond order for each molecules considered.

Figure 25 – Molecular modeling results for various sulfur- and nitrogen-containing organic compounds. The bond order value in bold next to a green symbol indicates the bond with highest bond order, while the underlined blue number indicates the

net electronic charge on the heteroatom in a given molecule.[35]

The decreasing order of this bond is reflected in the following classification of the molecules in

Figure 25:

Acridine > Quinoline > Carbazole > 4,6-DMDBT

Quinoline, by virtue of having a bond with the highest bond order as compared to those in

carbazole and 4,6-DMDBT, for instance, could be expected to have the highest reactivity for

hydrogenation. In other words, quinoline would be the first to undergo HYD reaction. Furthermore,

quinoline could also be expected to adsorb first and more strongly inhibit a hydrogenation site in a

catalyst.

Moreover, Satterfield et al. [36] also reported that secondary amines (reaction intermediates)

produced by the decomposition of quinoline have a strong adsorption into the catalyst hydrogenation

sites.

22

The net charge on the heteroatom in the molecules listed in Figure 25 decreased in the following

order:

4,6-DMDBT > Carbazole > Acridine > Quinoline

The sulfur atom in 4,6-DMDBT possesses a higher electronic charge than the nitrogen atoms in

carbazole and quinoline. Consequently, 4,6-DMDBT would have a much higher tendency to undergo

hydrogenolysis as compared to carbazole or quinoline. So DDS would be poorly inhibited by quinoline.

Thus, due to the net charge, DDS would be less inhibited by NH3 than HYD.

To summarize the available data, in Table 4 are reported the various observations and conclusions

from the literature on the inhibition effect caused by NH3 on HDT catalysts. It is clear from this Table

that nitrogen compounds are very strong inhibitors for HDS of DBT or 4,6-DMDBT.

There are still some disagreements in the literature concerning the inhibition of HYD or DDS

pathways. It could probably be due to many experimental differences (feedstock composition,

operating conditions and catalysts).

23

Table 4 – List of the results obtained in literature for NH3 inhibition

Publication Reactant studied

Catalysts

Operating Conditions Observations

Type Composition

9

4,6-DMDBT

NiMo/Al2O3

CoMo/Al2O3

3,0%/4,0% NiO/CoO

16,0%/19,0% MoO3

2,6% P2O5

P = 25, 40, 55 bar

Temperature = 340°C

DDS centers are less sensitive to nitrogen-basic compounds than HYD sites.

[37] CoMo/Al2O3

4,0% CoO

16,0% MoO3

2,6% P2O5

P = 30 bar

Temperature = 330ºC

In presence of quinoline, HDS is greatly inhibited. Quinoline undergoes HYD easily thus inhibits

hydrogenation sites faster.

[35] CoMo/Al2O3 5,8% CoO

27,0% MoO3

P = 45 bar

Temperature = 350°C Quinoline inhibits HYD sites primarily than DDS sites.

[38] DBT

4,6-DMDBT

CoMo/Al2O3

NiMo/Al2O3

3,0% CoO/NiO

16,0% MoO3

P = 50 bar

Temperature = 300 to 340°C

Amines strongly decrease the 4,6-DMDBT global HDS rate and especially the HYD pathway.

[32]

DBT

4-MDBT

4,6-DMDBT

CoMo/Al2O3 4,0% CoO

17,0% MoO3

P = 40 bar


Basic-nitrogen compounds inhibit 4,6-DMDBT global HDS even at low concentrations. Inhibition of

quinoline is higher on DDS than HYD.

24

2.6.2 Hydrogen sulfide

Concerning H2S, Rabarihoela-Rakotovao et al.[30] have clearly established the unavoidable impact

of this molecule as it is a by-product of HDT reactions. Moreover, H2S is essential to maintain the

sulfided state of HDT catalysts.

Generally, H2S is admitted to have an inhibition effect on hydrotreating reactions however,

discrepancies remain on its influence on the two consider routes (HYD and DDS) of HDS of DBT-type

compounds.

Which types of active sites are subjected to H2S poisoning and in which step of the reactions is not

clear either. Some point out electronic changes or active sites variations with H2S partial pressure,

related to nature or/and number, as well as mechanism and kinetics reasons have been proposed in

order to explain the influence of H2S, and, thereby theoretical investigations were also carried out

[19][39][40].

As shown by Rabarihoela-Rakotovao et al. [30] in Figure 26, the activity for NiMo or CoMo catalysts

differ depending on the H2S partial pressure used on the catalytic test.

Figure 26 – Global activity as function of H2S partial pressure, for 4,6-DMDBT transformation on CoMo and NiMo catalysts [31]

However, there may be two ways for H2S to inhibit the HDS reactions. As reported by Besenbacher

et al. [21], H2S is possible adsorbed on the sulfur vacancies, and, as they seem to be the sites for the

C-S bond break, DDS pathway is then more impacted than HYD. The second effect is related to the

available S2-

atoms in the S-edge, which will be protonated by H2S, which has Brönsted acid

properties, lowering their basicity and consequently their reactivity [19].

25

Finally, in relation with the objectives of deep HDS, the effect of H2S on the activity of hydrotreating

catalysts is a very important issue in industrial practice. As already seen, depending on the sulfur

content in the feed and on the operating conditions, the choice of the catalyst may be crucial. And the

choice could also depend on the position into the reactor.

To further describe the understanding on the influence of H2S partial pressure on the 4,6-DMDBT

HDS, other observations are reported Table 5 and Table 6.

From those results, DDS is always more inhibited by H2S compared to HYD and no promoting

effect has ever been observed.

26

Table 5 – List of the results obtained in literature for H2S inhibition


Catalysts


Type Composition

[41]

DBT

4,6-DMDBT

NiMo/Al2O3

2,9% NiO

15,3% MoO3

P = 50 bar

ppH2S = 0 to 0,88 bar


H2S may be adsorbed on to the hydrogenolysis sites (DDS)

of 4,6-DMDBT more strongly than into hydrogenation sites

(HYD).

[42] CoMo/Al2O3

NiMo/Al2O3

3,0% CoO/NiO

16,0% MoO3

P = 50 bar



HDS reactivity, and the selectivity between DDS and HYD

pathways depend on the competitive adsorption between

the reactant (DBT or 4,6-DMDBT) and H2S. Some

adsorption conformation data are provided to explain that

DDS should be more inhibited than HYD.

[30] CoMo/Al2O3

NiMo/Al2O3

3,0%/4,0% NiO/CoO

16,0%/19,0% MoO3

2,6% P2O5

P = 25, 40, 55 bar

ppH2S = 0,058 to 1,00 bar


H2S would adsorb preferentially on the DDS centers of

DBT-type compounds. The centers could be identical but

owing to the fact that both reactions have not necessarily

the same rate-limiting steps, the reactions would be altered

differently by H2S partial pressure.

27

Table 6 – List of the results obtained in literature for H2S inhibition (continuation)


Catalysts


Type Composition

[19] DBT

4,6-DMDBT

NiMo/Al2O3

CoMo/Al2O3

3,1% CoO/NiO

14,0% MoO3

P = 30 to 50 bar



Steric effects upon adsorption on the catalyst active sites

could not be responsible for differences in reactivity of

DBTs.

29

3 Methodology

3.1 Experimental Part

3.1.1 Catalysts preparation

For this work, three types of catalysts were prepared – CoMo-A, CoMo-B and CoMo-C. All the

prepared catalysts were CoMo trilobe extrudates supported on γ-alumina. In order to process to their

catalytic evaluation, their length has been calibrated between 2 and 4 mm for hydrodynamic

considerations.

These catalysts were prepared by three common stages: active phase impregnation (dry

impregnation), maturation (in air atmosphere for 1,5 hours) and drying (at 90°C for 24 hours).

On one hand, CoMo-A was the only catalyst used directly on the catalytic tests after drying phase.

On the other hand, CoMo-B was calcined under air at 450°C, for 120 minutes, and to produce CoMo-

C catalyst, an organic solution was added as an additive, at pore volume, which was then dried under

nitrogen flow at 140°C and two hours to preserve the organic compound but eliminating the solvent.

In every test performed, it was used the exactly same volume of catalyst (Vcat = 4 cm3). Hence, the

mass needed for each test was obtained by the density of the catalytic bed (Densité Rempli Tassé -

DRT). Along with this characteristic, some other features were also analyzed. In Table 7 are shown

these characteristics.

Table 7 – Characteristics obtained for each catalyst

Catalyst DRT (g/cm3)

Oxide Density

(g/cm3)

Support

surface area (m2/g)

MoO3 (wt.%) Co/Mo

CoMo-A 0,89 0,83

182 X Y CoMo-B 0,85 0,84

CoMo-C 1,17 0,84

30

3.1.2 Unit T033

To simulate as real as possible the industrial conditions of HDS, every catalytic test was carried out

in the T033 unit (Figure 27), at IFP Energies nouvelles.

Figure 27 – Overview of the T033 unit

The T033 unit consists in a fixed-bed reactor, under hydrogen pressure. The reactor was fed up

with a liquid (feedstock) and gas supply (hydrogen). On one hand, the liquid stream was constantly fed

to the reactor by a HPLC pump and controlled by a QuantimTM

. For this flow meter, the liquid passes

through a U-shaped tube which vibrates in an angular harmonic oscillation. Oscillating forces will then

deform the tube and a further vibration component gets added to the already oscillating tube. This

added vibration results in a phase shift or twist in few parts of the tubes. This phase shift which is

directly proportional to the liquid mass flow rate is measured with the help of sensors. The measured

information is further transferred to the electronics unit where it gets transformed to a voltage

proportional to mass flow rate. This high performance instrument allows the simultaneous

measurement of mass flow, volumetric flow, density and temperature of the fluid. Nonetheless, this

device only operates in the presence of a differential pressure. This differential pressure is engaged by

a manual valve and at the same time allows pumping the liquid stream at high pressure. On the other

hand, the gas streams, nitrogen and hydrogen, were provided by the local networks at high and low

pressure.

Before introducing both gas and liquid into the reactor, the streams are combined and mixed. Then,

the mixture enters into the reactor in a down-flow mode. Furthermore, to heat up the reactor an oven

is used and a multipoint cane monitors the increasing temperature with four thermocouples attached,

two in the middle and one at each end. The operative pressure is measured by a KellerTM

indicator

and regulated by a KammerTM

valve. To avoid the effluent condensation the reactor outline is

thermally insulated since if effluent temperature reaches values below the dew point and, as the

solvent is not liquid, it could lead to precipitation and could have to face top plugging of pipes.

31

To fully control the unit during the reaction process, every line has flow indicators, manual control

valves and even particles filters.

Finally, in order to analyse the reactor effluent, a nitrogen stream is added at the outlet line and

then fed to the gas chromatographer. All effluent samples are automatically injected in the

chromatographer by a VALCO® valve with six ways. Moreover, as one can see in Figure 28, this valve

alternately rotates acquiring two distinct positions.

Figure 28 – Schematic of a VALCO® valve with six ways

First, the valve assumes the “balayage” position when the effluent analysis is not required. Then,

the “injection” position is taken when an effluent sample is injected within the GC. The initial

temperature of the GC column is 50°C, which then rises to 67°C throw a 15°C/min heating rate,

followed by another increase until 290°C (30°C/min heating rate).

3.1.3 Unit loading

As mentioned before, all catalytic tests were made in the same reactor which has 10 mm of

diameter and 18,2 cm of height. To load the reactor three main steps have to be taken:

1. Fill-in 4 cm3 of inert silicon carbide (SiC);

2. Load 4 cm3 of SiC mixed with 4 cm

3 of the chosen catalyst;

3. Load again with SiC until the top of the reactor.

Nevertheless, between each loading step the reactor has to be shaken to minimize the void spaces

along the reactor (intra-particular void). Indeed, the feed flow must be homogeneously distributed

along the reactor to minimize the risk of preferred path and ensure the wetting of all the grain of

catalysts. To prevent any leak, the reactor is topped with a porous joint and sealed with a torque tool

(80 N.m-1

). In Figure 29 is shown a schematic of the micro-reactor used to perform the catalytic tests.

32

Figure 29 – Representation of the micro-reactor used in the T033 unit

3.1.4 Sulfidation

For industrial purposes, the sulfidation of HDT catalysts is accomplished by using organo-sulfide

compounds as an activating agent [14]. For practical reasons, organo-sulfide compounds (e.g. DMDS)

are much easier to handle than H2S and might deliver sulfur gradually to the catalyst through a control

of their kinetics of decomposition.

In this work, it was used DMDS. Its decomposition is carried out in accordance with the following

reaction:

To begin the test, the catalyst was activated in order to produce the CoMoS phase. Hence, the

sulfidation was made in situ with a liquid feedstock composed by DMDS, xylene and cyclohexane. The

composition of this feedstock is presented in Table 8. Then, in Table 9 are presented the operating

conditions which the catalysts went through in the sulfidation step. These operating conditions were

used for almost every test. In fact, the sulfidation pressure used to study the impact of H2 was the

pressure at which the actual catalytic test was performed (40 bar).

Table 8 – Mass composition of the liquid feed used for the sulfidation process

Compound wt.%

DMDS 5,88

Xylene 20,00

Cyclohexane 74,12

(Eq. 4)

33

Table 9 – Operating conditions for the sulfidation stage

Parameter Value

Pressure (bar) 30,0

LHSV (h-1

) 4,0

As can be seen in Figure 30, the sulfidation process is formed by 4 steps:

- It began with a temperature ramp from 40°C to 350°C (1).

- Then this temperature was maintained in order to produce the sulfide form of the catalyst (2).

- Then the temperature was decreased until testing value and all the sulfidation operative

conditions are changed for the conditions of the catalytic test, respectively (3 and 4).

Figure 30 – Temperature program for the sulfidation process, before each catalytic test

Thus, one used always the same activating conditions in order to be sure that the catalysts

performances were only depending on the test conditions and not the activation procedure.

1,7°C/min

1,5

°C/min

1

2

3

4

(1)

(2)

(3) (4)

34

3.1.5 Model feedstock

In the first part of the study, the aim was to simulate, as real as possible, the last part of a HDT

reactor diesel feed with high sulfur content to study the HDS activity of each CoMo catalyst prepared.

As already mentioned, the most refractory sulfur compound to decompose is 4,6-DMDBT. Hence, this

was the model molecule chosen to study in this work. The composition of the “standard” model

feedstock (Feedstock-1) prepared is shown below (Table 10):

Table 10 – Model liquid feed characteristics, Feedstock-1

Compound Weight Fraction (wt.%) ppm S or N

Cyclohexane 57,14 -

DMDS 1,20 8170

Quinoline 1,00 1085

4,6-DMDBT 0,66 996

Xylene 40,00 -

On one hand, quinoline was chosen as a model basic-nitrogen compound to evaluate how these

nitrogen-based species influence the HDS. In this study, it was found that quinoline is not fully

converted into NH3, whatever the temperature.

On the other hand, xylene was added to the liquid feedstock to increase the dissolution of 4,6-

DMDBT, while cyclohexane was added to decrease the boiling point of the mixture. The mixture of

both an aromatic solvent and a non-aromatic one is also more representative of a diesel.

Two other feedstocks were prepared in order to determine the conversion of 4,6-DMDBT with

higher H2S and NH3 partial pressures, changing the mass composition of DMDS from 1,2 wt.% to 2

wt.% (Feedstock-2) and quinoline from 1,0 wt.% to 1,5 wt.% (Feedstock-3), respectively. However, to

go even further in the study of the influence of NH3, experimental results from previous works were

taken into account. These previous tests were performed with a feedstock with a lower concentration

of quinoline than the one considered in this present study (i.e. 0,5 wt.%).

All feedstocks were prepared in a vessel by adding 4,6-DMDBT to xylene as it is commercialized in

a powder form. Then, cyclohexane, quinoline and DMDS (all liquids) were added to these

components.

35

3.1.6 Operating conditions

Regarding the operating conditions of the catalytic test, the goal was to use similar conditions as

industrial HDS. The applied conditions are summarized in Table 11.

Table 11 – Operating standard conditions used in the catalytic tests

Parameter Value

Pressure (bar) 30,0

Temperature (°C) 290 to 310

Catalyst volume (cm3) 4

H2/feed ratio (NL/L) 240

The LHSV conditions were changed in order to to evaluate the model feedstock in the same range

of HDS conversion, i.e. not too high (to avoid some saturation and reduce the crossing between the

HDS pathways). In this way, it was possible to evaluate the DDS/HYD selectivity without being

influenced by supposed thermodynamic effects. In Table 12 is shown the value of LHSV input for each

catalyst. The values were chosen taking into account the activity obtained in some previous tests

(before this present work).

Table 12 – LHSV used for each catalyst

Catalyst LHSV (h-1

)

CoMo-A 4,0

CoMo-B 3,0

CoMo-C 6,5

During the catalytic test, for each tested temperature, 11 samples of the reactor effluent were

analysed within a regular time interval of 45 minutes (1). The temperature program of the catalytic test

is shown in Figure 31:

36

Figure 31 – Temperature program for the catalytic test

Along the process, a nitrogen stream (15 NL/h) is injected at the reactor outlet in order to dilute the

effluent before enter the GC. Afterwards, the reactor is washed with xylene and then dried (at the final

test temperature) (2) and cooled (3) with a nitrogen and hydrogen stream at descending temperature

until 40°C, thus avoiding the catalyst being stuck to the walls of the reactor and to improve the

downloading.

3.1.7 Data analysis

As mentioned in the bibliographic study, the HDS of 4,6-DMDBT produces many intermediary

compounds and products. However, in this study, to simplify the analysis, one considers that 4,6-

DMDBT is converted into two main HYD products (MCHT and DMDCH) and one DDS product

(DMBPh).

Figure 32 – Schematic of the two reaction pathways for the HDS of 4,6-DMDBT, adapted [32]

1 2

3

0,8°C/min

DMBPh

DMDCH

MCHT

(1) (2)

(3)

37

The analytic results obtained from the GC Galaxie® software allowed to determine the conversion

of the liquid feed.

Hence, the catalytic performance of each catalyst was evaluated according to the HDS conversion

of 4,6-DMDBT ( ):

(Eq. 5)

In addition, the conversion of 4,6-DMDBT through both HYD and DDS pathways were calculated

by the following equations:

(Eq. 6)

(Eq. 7)

Where,

(Eq. 8)

(Eq. 9)

In Eq. 8 and Eq. 9, and

represent the number of moles of HYD products and DDS

product produced during the catalytic test, respectively.

38

3.2 Kinetic Study

In order to obtain the kinetic model and the parameters from the studied reactions in this work it

was used the software ReactOp® Cascade. Hence, the software allows the user to create its own set

of reactions to better evaluate and estimate the kinetic parameters of a complex mechanism, based on

available sets of experimental data.

Although it was considered, for the first data analysis, just two main reactions (HYD and DDS), for

the kinetic modeling it was added an equilibrium reaction concerning the hydrogenation of DDS

products leading to enhance the apparent proportion of HYD products. This reaction was taken into

consideration because, at the mentioned operating conditions, DMBPh may be hydrogenated into

DMDCH or MCHT and vice-versa (see experimental results chapter 4.1, Figure 37).

Additionally, this kinetic study was made in order to better understand the different catalytic

performances and how they are influenced by the operating conditions, i.e. H2S, NH3 and H2 partial

pressures.

For the decomposition of 4,6-DMDBT, the global kinetic model is:

As already mentioned, for this part of the study, three reactions were taken into account – HYD,

DDS, HDA/HDAe. Thus, the kinetic equation for both main pathways (HYD and DDS) is the following:

Where,

(Eq. 10)

(Eq. 11)

(Eq. 12)

(Eq. 13)

39

In addition, the reaction constant rates for both HYD and DDS are influenced by H2, H2S and NH3

partial pressures. So, the reaction constant rates can also be written as:

With – reaction constant rate (h-1

), – pre-exponential constant rate (h-1-m-s-p

), – activation

energy (J/mol), – gas constant (J/mol.K), – temperature (K), – kinetic partial order for H2,

H2S and NH3, respectively, and – pressure (bar). For all the reactions the order relatively to reactant

is supposed to be 1.

Considering this, the new kinetic model has been created by selecting the ReactOp® Cascade tool

Model Wizard and then, the following reactions were introduced into the software:

With A – 4,6-DMDBT, B – HYD products and C – DDS product.

Then, in order to input the experimental results, other software tool had to be selected, namely

Experiment Wizard. The experimental data was introduced in the software (Table 13), for a given

temperature, as follow:

Table 13 – Example on how the experimental results, fixing a given temperature, were introduced in Experimental Wizard

(ReactOp software)

Time (hour) A (mol/100g feed) B (mol/100g feed) C (mol/100g feed) Temperature (K)

0,00 0,300 0,000 0,000 563

0,25 0,250 0,042 0,002 563

0,33 0,200 0,080 0,005 563

(Eq. 14)

(Eq. 15)

(Eq. 16)

(Eq. 17)

(Eq. 18)

(Eq. 19)

40

Additionally, the value input for time is directly linked with the LHSV at what the test was

performed, as established on Eq. 20 (Plug-Flow Reactor).

Thus, the result obtained will be equal to the reaction contact time during the catalytic test.

Finally, loading the experimental results already introduced in the software for all test temperatures

on the Estimation Wizard, it was possible to determine the kinetic parameters selected for the created

model ( and ) for each reaction giving a total of eight parameters.

Moreover, the activation energy does not change for any reaction when increasing or lowering the

partial pressure of H2S and NH3. For the equilibrium reaction (HDAe) the and were fixed since

they represent a thermodynamic characteristic equal for all the catalysts. In the following table (Table

14) is shown the assumptions made to fit the eight parameters.

Table 14 – Assumptions made to establish each kinetic parameter’s model

CoMo-A

(Dried)

CoMo-B

(Calcined)

CoMo-C

(Additive impregnated)

HYD

ln(ko) (hour-1

) May change with H2S, NH3 and H2 partial pressure

Ea (kJ/mol) For each catalyst, the value should be constant independently the

operating conditions

DDS

ln(ko) (hour-1




HDA

(equilibrated)

ln(ko) (hour-1




ln(ko,eq) (hour-1

)

For all catalysts, these values were considered to be the same and would only change if the H2 partial pressure is modified

Eaeq (kJ/mol)

(Eq. 20)

41

Then, for each reaction considered, the rate constant was determined by the following equation:

Moreover, in order to determine the selectivity between the two main reaction pathways (DDS and

HYD) and the global activity of each catalyst the following equations were used, respectively:

Furthermore, to evaluate the HYD and DDS relative activity between the additive impregnated

catalyst and the other two catalysts (dried and calcined) the next equation were used, respectively:

In this way, it was possible to determine in what conditions the catalytic performance of the additive

impregnated catalyst would be enhanced or inhibited.

Finally, an inhibition factor has been also calculated in order to understand the influence of H2S,

NH3 on the HDS pathways. Nevertheless, to evaluate the influence of H2, an activation factor has been

determined since in literature it is stated that H2 does not have a negative impact in the 4,6-DMDBT

HDS. Thus, the inhibition and activation factors were calculated by a direct ratio between global

activities, as following:

With, (h-1

) – global activity determined for a given study condition (Eq. 23) and

(h-1

) – global activity determined for standard conditions.

Finally, it has to be mentioned that it was not possible to add adsorption constants or inhibiting

effects directly into the software model. So, for instance, the nitrogen compounds and/or NH3 inhibition

were not included in the kinetic model.

(Eq. 21)

(Eq. 22)

(Eq. 23)

(Eq. 24)

(Eq. 25)

(Eq. 26)

(Eq. 27)

43

4 Results and Discussion

4.1 Comparison of Catalysts in Standard Conditions

To evaluate the base kinetic parameters for each catalyst, one used the experimental results

obtained from previous works and the experimental data accomplished during the internship. These

experimental results were performed at standard conditions thus using Feedstock-1.

In Table 15 are shown the experimental tests performed with CoMo-A, CoMo-B and CoMo-C and

the respective LHSV used at standard conditions.

Table 15 – Operating conditions used in previous and present works to evaluate standard conditions

Catalyst

LHSV (h-1

)

Previous work Present work

CoMo-A 3 and 4 5

CoMo-B 3 and 4 2,5

CoMo-C 3 5 and 6,5

The kinetic parameters were then optimized with ReactOp® Cascade software. For example, the fit

obtained for CoMo-C, the additive impregnated catalyst, is reported on Figure 33. The other fits are

present in Appendix 2.

Figure 33 – Kinetic fitting obtained for CoMo-C at standard conditions

44

However, the fit did not simulate perfectly the experimental results. Indeed, one can see that, for

high temperatures, the experimental curve obtained did not follow exactly the same tendency as for

other temperatures. It can be due to various reasons:

- First, the variability of the test and GC analysis;

- Secondly, the average kinetic parameters could change with the conversion rate of 4,6-

DMDBT owed to many changes in the mechanism such as inhibitions and kinetically limiting

steps;

- Also, the adsorption of 4,6-DMDBT might be stronger than expected thus changing the

reaction kinetic order (lower than 1), hence denying the assumption made.

Nevertheless, it was not possible to improve the model due to the absence of adsorption constants

in the kinetic model. This point could be studied modifying the programmed model.

To decide between the various hypotheses, it would be interesting to evaluate the kinetic and

simulate it without any quinoline. Indeed, if the inhibition is responsible for the fitting error, it should be

drastically improved suppressing the nitrogen inhibitor.

The work aiming to study the catalysts in closed conditions compared to real HDT units (LHSV and

NH3 partial pressure). The choice was done to continue with quinoline even if the fit did not represent

exactly the experimental values. In the following discussion, the parameters shown will be the average

parameters obtained, keeping in mind that these are probably not the good ones nevertheless, the

tendency will be discussed.

The various kinetic parameters determined by the experimental results and depending on the

catalyst are summarized in Table 16.

Table 16 – Kinetic parameters obtained for each catalyst used, at standard conditions

CoMo-A

(Dried)

CoMo-B

(Calcined)

CoMo-C

(Additive impregnated)

HYD

ln(ko) (hour-1

) 34,6 32,2 30,0

Ea (kJ/mol) 164,0 152,0 140,4

DDS

ln(ko) (hour-1

) 33,7 31,7 29,4

Ea (kJ/mol) 166,5 156,5 143,1

HDA (equilibrated)

ln(ko) (hour-1

) 19,5 19,8 20,2

Ea (kJ/mol) 102,4 101,3 96,1

ln(ko,eq) (hour-1

) 6,8

Eaeq (kJ/mol) 134,3

45

First, looking at the three catalysts, the rate constants obtained corresponding to HYD pathway

were slightly different and present the following hierarchy:

CoMo-A (dried) > CoMo-B (calcined) > CoMo-C (additive impregnated)

The activation energy was also ranked likewise. Moreover, the activation energies found for the

three catalysts correspond to the same order of magnitude as reported by literature, which is around

130 kJ/mol [41].

A possible explanation to why the dried catalyst has the highest activation energy may be due to a

higher adsorption of inhibitory compounds such as H2S, NH3 or aromatic compounds. Indeed,

considering the inhibition effect as a Langmuir-Hinshelwood-type, its “strength” should decrease with

temperature. Practically, it could be due to the fact that the additive impregnated catalyst (CoMo-C)

exhibits more active sites so, it would be less impacted by inhibitors adsorption (at same partial

pressure) and as a consequence, the rate constant will be less impacted by the temperature changes.

Whatever the catalyst, the values obtained for the activation energies of HYD and DDS appear to

be very similar to each other (at least in the same order of magnitude), which might confirm the fact,

reported by Bataille et al.[19] that, at least, HYD and DDS sites share a similar nature. Nevertheless,

one can also see that for the three CoMo-based catalysts, the HYD activation energy was slightly

lower than for the DDS pathway, which might additionally indicate that DDS is more sensitive to

reaction temperature and could also be indicative that DDS is more impacted by the inhibitors

adsorption.

The second hydrogenation reaction considered (HDA reaction after to DDS) did not change much

from catalyst to catalyst. However, it is indeed a secondary reaction and the differences found for the

kinetic parameters are not significant enough between the catalysts to be discussed.

To better visualize the differences between the catalysts, one compared the rate constants,

calculated from the average kinetic parameters for the temperatures of interest (Table 17).

Table 17 – Kinetic rate constants obtained for both HYD and DDS reactions, at standard conditions

k, rate constant (h-1

)

Temperature (°C)

HYD DDS

CoMo-C CoMo-B CoMo-A CoMo-C CoMo-B CoMo-A

290 1,00 (100) 0,76 (76) 0,65 (65) 0,31 (100) 0,18 (58) 0,15 (48)

300 1,69 (100) 1,34 (79) 1,19 (70) 0,53 (100) 0,32 (60) 0,29 (55)

310 2,80 (100) 2,32 (83) 2,15 (77) 0,88 (100) 0,56 (64) 0,52 (59)

330* 6,36 (100) 5,64 (89) 5,61 (88) 2,04 (100) 1,39 (68) 1,38 (68)

46

The values presented in parenthesis represent the relative difference between in activity between

the three catalysts considering the additive impregnated rate constant as the base value. Moreover,

the rate constant values obtained for 330°C were extrapolated from the experimental results and it

represents the typical temperature used on HDS process.

From Table 17, one can see that the difference between the additive impregnated catalyst rate

constant and the other catalysts rate constant becomes lower with increasing temperature, especially

for hydrogenation. For direct desulfurization, the rate constants obtained for CoMo-B are higher than

the ones found for CoMo-A, at low temperatures. However, with increasing temperature, the relative

difference between them disappear which could might be related with the increasing desorption of

inhibitory compounds thus leading to a higher activity on the dried catalyst.

Concerning selectivity, it was first calculated as the direct ratio of DDS and HYD rate constants and

plotted as function of the global HDS conversion. The results at various temperatures are presented in

Figure 34, Figure 35 and Figure 36 (290°C, 300°C and 310°C, respectively).

Figure 34 – Selectivity DDS/HYD as function of the total HDS conversion, at 290°C

Increasing contact

time

47



Increasing contact

time

Increasing contact

time

48

No unique tendency was found regarding the conversion. Nevertheless it can be said that:

- at 290°C, the apparent DDS selectivity increases with the global conversion, for all catalysts;

- at 300°C, it seems that the apparent DDS selectivity decreases (probably due to the

“activation” of hydrogenation pathway);

- finally, at 310°C, the apparent DDS selectivity decreases except at very high conversion,

which could be due to the HDA equilibrated reaction.

To confirm the hypothesis relatively to the increase of DDS selectivity at high conversion and

temperatures (which could be due to HDA decrease), the importance of HDA in the mechanism was

evaluated.

A catalytic test was carried out with Feedstock-1 but replacing 4,6-DMDBT by DMBPh. This test

was performed with the most active catalyst (CoMo-C) at 310°C (only). The results obtained are

reported in Figure 37.

Figure 37 – Ratio DMDCH/MCHT as function of the conversion of DMBPh (HDA), at 310oC

As one can see in Figure 37, the hydrogenation of DMBPh to form MCHT and DMDCH is not

negligible (about 20%). Therefore, MCHT and DMDCH may be produced from both HYD and DDS

reaction pathways, as also proposed by [33].

To go further in the calculations of selectivity, the ratio of DDS and HYD rate constants (i.e. other

way to express selectivity independently from the contact time or LHSV used) was plotted as function

of the temperature (Figure 38).

49

Figure 38 – Selectivity DDS/HYD as function of the temperature

It can be seen that the dried and calcined catalysts showed the lowest DDS selectivity whereas the

additive impregnated exhibited the highest. This fact could be due to the lower inhibition effect on the

additive impregnated catalyst. Moreover, the selectivity did not change significantly with temperature

but, the considered range was probably not large enough.

50

4.2 Impact of H2S partial pressure

The influence of the H2S partial pressure was studied by increasing the content of DMDS on the

model feed, while maintaining constant the composition of the other compounds. The partial pressure

was estimated by flash calculation using pro-II software. In Table 18 are shown the two model

feedstocks used to evaluate the effect of H2S on the HDS of 4,6-DMDBT.

Table 18 – Liquid feeds used to study the effect of ppH2S

Model Feedstock DMDS (wt.%) Sulfur composition (ppm) ppH2S (bar)

Feedstock-1 1,20 8710 0,35

Feedstock-2 2,00 13617 0,55

In addition, the partial pressure of H2S was calculated taking only in account the H2S produced by

the total decomposition of DMDS. The H2S provided by 4,6-DMDBT HDS was considered to be

negligible.

4.2.1 Apparent comparison

As reported by literature, the hydrogenation reaction in CoMo-based catalysts is inhibited by H2S

[14] [30]. In this study, the results showed the same tendency as HYD pathway has proven to be

strongly inhibited (Figure 39), whatever the catalyst, at high H2S partial pressure. Moreover, the tests

with CoMo-A, CoMo-B and CoMo-C were performed at different LHSV (4 h-1

, 3 h-1

, 6,5 h-1

,

respectively). The experimental tests concerning CoMo-A and CoMo-B at standard conditions (1,2%

DMDS) were performed in previous works.

Figure 39 – HYD conversion of 4,6-DMDBT as function of temperature for the three catalysts prepared. In the graph, full

lines represent the 1,2 wt.% DMDS feed and gapped lines represent the 2 wt.% DMDS.

51

Moreover, as CoMo-C is the catalyst with the lowest activation energy, the almost negligible effect

of H2S may confirm that, in additive impregnated-type catalysts, the adsorption of H2S is much weaker

than on the other catalysts tested. However, the results found for the additive impregnated catalyst are

difficult to discuss as it shows a low activity compared to the others. Thus, this test should be redone

with a lower LHSV.

Concerning the results obtained for the DDS pathway, for all catalysts the selectivity on

hydrogenolysis products was enhanced (Figure 40), with higher H2S partial pressure.

Again, CoMo-A (dried catalyst) seems to be slightly less activated which could be owed to the

highest competitive adsorption of inhibitors (such as NH3, H2S or aromatics) as the activation energy is

witnessing and as already shown for standard conditions.

Figure 40 – DDS conversion of 4,6-DMDBT as function of temperature for the three catalysts prepared. In the graph, full

lines represent the 1,2 wt.% DMDS feed and gapped lines represent the 2 wt.% DMDS.

A possible explanation for why all catalysts exhibit higher DDS conversion, with higher H2S partial

pressure, could be the existence of two distinct and specific active sites each of them being selective

for one of the two reactions considered. Thus, H2S would adsorb favourably on the hydrogenolysis

centers or closed to them (Figure 23) and, at some point, would change the structure of these active

sites.

In another hypothesis, it could be considered that both active sites are similar in nature and that the

catalytic centers are dual sites made of a sulfur vacancy associated to a sulfur anion (Figure 23).

Hence, the different sensitivity for DDS and HYD pathways to H2S partial pressure could be attributed

to the fact that, due to its acidic properties, H2S preferentially adsorbs on the most basic sulfur-anions

which are expected to be particularly active in C–S bond cleavage, hence in DDS.

52

However, no experimental data were found in literature about the promotion of DDS under H2S

partial pressure conditions. So, another explanation, which can be pointed out, is the fact that the

catalysts used in this study have much more molybdenum (up to 20 wt.% MoO3) than the catalysts

mentioned in literature (Table 5). Therefore, the catalysts prepared probably exhibit much more

dispersed active phase due to the recent way of preparation. However, one cannot confirm this

assumption because it is not possible to characterize the catalysts from the literature. So, it is possible

that the catalysts prepared, in this study, contain more active sites than the catalysts stated in

literature. Therefore, it is possible that to reach full saturation of the catalytic surface, by H2S

adsorption, and consequent inhibition of the S-edges, where supposedly hydrogenolysis reaction

takes place [43], the H2S partial pressure should have to be higher. To go further into this assumption,

some calculation would need to be done because, usually it is considered that the H2S adsorbed

quantity does not depend on the amount of molybdenum.

Eventually, the operating conditions presented in the literature were not the same as the conditions

used in this study. As one can see in Table 5, the H2S partial pressure, temperature and working

pressure used to perform the catalytic tests were always different.

In fact, regarding the feedstock composition, most part of the previous studies in this subject did

not use quinoline into their model feedstock in order to evaluate the effect of H2S on HDS. Hence, one

could also suggest that there is an indirect effect within the DDS pathway if H2S is introduced in the

presence of quinoline.

4.2.2 Kinetic comparison

The kinetic parameters were determined and optimized with ReactOp® Cascade software. For

example, the fit obtained for CoMo-B, the calcined catalyst, is reported on Figure 41. The remaining

fits are shown in Appendix 2.

Figure 41 – Kinetic fitting obtained for CoMo-B, for Feedstock-2 conditions (2 wt.% DMDS)

53

Nevertheless, taking into account that for each catalyst only one experimental test was performed,

the parameters determined, in the software, may not show the perfect approximation to what happens

throughout the decomposition of 4,6-DMDBT for different H2S partial pressures. Indeed, the three

parameters ( , and ) were fitted with only six experimental values. Thus, more

experimental tests should be performed to confirm the model.



Table 19 – Kinetic parameters obtained for each catalyst used, for Feedstock-2 conditions (2 wt.% DMDS)

CoMo-A CoMo-B CoMo-C

HYD

ln(ko) (hour-1

) 34,2 31,8 29,5

Ea (kJ/mol)* 164,0 152,0 140,4

DDS

ln(ko) (hour-1

) 34,2 32,1 29,7

Ea (kJ/mol)* 166,5 156,5 143,1

HDA (equilibrated)

ln(ko) (hour-1

) 19,0 19,3 19,5

Ea (kJ/mol)* 102,4 101,3 96,1

ln(ko,eq) (hour-1

) 6,8 6,8 6,8

Eaeq (kJ/mol) 134,0 134,0 134,0

*same value as in standard conditions

Looking at the three catalysts, the rate constants obtained for both HYD and DDS pathways exhibit

the following hierarchy:


The ranking obtained for the rate constants is conserved compared to the standard conditions.

There is no inversion of tendency.

The values presented in Table 20 represent the rate constants calculated of each study

temperature. The relative difference in activity between the three catalysts considering the CoMo-C

rate constant as the base value is shown in parenthesis. Moreover, the rate constant values obtained

for 330°C were extrapolated from the experimental results and represent the typical temperature used

on HDS process.

54

Table 20 – Kinetic rate constants obtained for both HYD and DDS reactions, at 2 wt.% DMDS.


)

Temperature (°C)

HYD DDS


290 0,61 (100) 0,52 (85) 0,43 (71) 0,42 (100) 0,25 (59) 0,25 (59)

300 1,03 (100) 0,91 (88) 0,80 (77) 0,71 (100) 0,45 (63) 0,47 (66)

310 1,71 (100) 1,57 (92) 1,44 (84) 1,19 (100) 0,79 (66) 0,86 (72)

330* 3,88 (100) 3,83 (99) 3,75 (97) 2,76 (100) 1,97 (71) 2,28 (82)

From Table 20, one can see that the most active catalyst for the hydrogenation reaction is always

the additive impregnated. But, it can also be pointed out that the rate constant values determined for

CoMo-A and CoMo-B have increased with temperature. This could actually be due to the fact that the

desorption rate of inhibitory compounds increases faster with temperature for these two catalysts than

for the additive impregnated.

Concerning the direct desulfurization, the rate constants obtained for CoMo-B were similar with the

ones found for CoMo-A. However, increasing temperature shows a quicker growing tendency for

CoMo-A DDS constant rate than to CoMo-B. In addition, CoMo-A (dried) and CoMo-B (calcined)

hydrogenolysis sites may have a higher inhibition effect comparing with CoMo-C (additive

impregnated) caused by the competitive adsorption of H2S and other inhibitory compounds, which

could be then reduced by increasing temperature.

Then, to determine the kinetic partial orders for each reaction, and, to study the influence of H2S

the logarithm of the pre-exponential rate constants obtained were then represented as function of the

logarithm of the partial pressure of H2S.

In Figure 42, Figure 43 and Figure 44 are represented, for the three catalysts, the ln(ko) for HYD,

HDA and DDS, respectively, as function of ln(ppH2S). Nonetheless, as the parameters for the

equilibrium reaction (HDAe) depend only on thermodynamic, H2S does not have any activation or

inhibition effect.

55

Figure 42 – ln(ko) as function of ln(ppH2S) for HYD

Figure 43 – ln(ko) as function of ln (ppH2S) for HDA

Figure 44 – ln(ko) as function of ln(ppH2S) for DDS

56

The obtained slopes are the kinetic partial orders concerning HYD, DDS and HDA pathways. The

results are presented in Table 21.

Table 21 - Kinetic partial orders with respect to H2S

As previewed from the majority of the previous works in literature, H2S has a negative impact on

HYD pathway for all three catalysts [30] [44].

Moreover, looking particularly to the results obtained for the hydrogenation routes (HYD and HDA),

one can see that the additive impregnated catalyst was the most inhibited, comparing with the other

two catalysts. Surprisingly, this conflicts with the fact that the additive impregnated was the one with

the lowest activation energy attributed to a lower H2S partial pressure effect. Additionally, this may be

linked to the fact that, at low H2S partial pressure, the catalytic surface coverage of the dried and

calcined catalysts by H2S was already high. Thus, for a higher H2S partial pressure, they would be

much less affected than the additive impregnated catalyst.

In Figure 45 is presented the inhibition factor imposed by higher H2S partial pressure compared to

standard conditions.

Figure 45 – H2S inhibition factor for CoMo-A, CoMo-B and CoMo-C. These results took into account the results obtained at standard conditions and Feedstock-2

From Figure 45 one can see that additive impregnated catalyst has the highest inhibition effect

confirming the results obtained in Table 20.

.

Catalyst HYD DDS HDA

CoMo-A -0,9 1,1 -1,1

CoMo-B -0,9 0,8 -1,1

CoMo-C -1,1 0,7 -1,6

57

4.3 Impact of NH3 partial pressure

In order to study the influence of NH3 partial pressure, some changing in the feed composition

(quinoline) were done.

First, a lower quantity was used (0,5 wt.%). The corresponding results are taken from a previous

work. Then some complementary experimental tests were carried out with 1,5 wt.% of quinoline. In

addition, the NH3 partial pressure introduced in the model was just an equivalent approximation since,

in real operating conditions, quinoline is not completely decomposed. Using pro-II software one could

calculate the partial pressures of NH3 and quinoline, assuming an average HDN conversion of 85%

(corresponding to the experimental HDN conversion). In Table 22 are shown the various partial

pressures obtained.

Table 22 – Composition of quinoline used in order to study the effect of NH3 partial pressure

Quinoline (wt.%)

Nitrogen composition (ppm)

ppNH3 (bar)

ppQuinoline (bar)

Theoretical Real

0,50 543 0,05 0,048 0,002

1,00 1085 0,10 0,084 0,016

1,50 1628 0,15 0,126 0,024

In Table 23 are presented which results were obtained from experimental tests and which were

actually simulated from the other ones with the purpose of direct comparison at same LHSV hereafter.

Table 23 – Tests performed on previous (*) and present experimental works to study the influence of NH3 partial pressure

Catalyst LHSV (h-1

) Quinoline

(wt.%) ppNH3 (bar) Kinetic Model Experimental Test

CoMo-A

4

0,5 0,05 X*

1,0 0,1 X*

3 1,5 0,15 X

CoMo-B 3

0,5 0,05 X*

1,0 0,1 X*

1,5 0,15 X

CoMo-C

6,5

0,5 0,05 X

1,0 0,1 X

3 1,5 0,15 X

58


The following results (Figure 46) show the difference between the chosen standard conditions and

the condition with less concentration of quinoline. It was chosen to discuss firstly these results in order

to understand how basic nitrogen-based compounds influence the HDS of 4,6-DMDBT within the

concentration range commonly found in real feedstocks. Again, the tests with CoMo-A, CoMo-B and

CoMo-C were performed at different LHSV (4 h-1

, 3 h-1

, 6,5 h-1

, respectively).

Figure 46 – HYD conversion of 4,6-DMDBT as function of temperature for the three catalysts prepared. In the graph, full lines represent the 0,5 wt.% Quinoline model approximation and gapped lines represent the 1,0 wt.% Quinoline (standard

conditions).

As one can see in Figure 46, the HYD conversion increased for all three catalysts with a lower NH3

partial pressure. In other words, with a higher amount of quinoline, the inhibition effect within the

hydrogenation reaction increased among all catalysts which is in agreement with literature [32].

In addition, it is also clear that the most impacted catalyst was the additive impregnated.

59

In Figure 47 are presented the results concerning the DDS conversion. For CoMo-A and CoMo-B,

the DDS conversion slightly decreased, for high temperatures, when using a feedstock containing a

low quinoline concentration. So, it seems that there was a slight inhibition of the DDS pathway.

Therefore, this fact is in agreement to what was found in literature [32]. It is also noticeable that, for

both dried and calcined catalysts there was a similar impact of NH3 partial pressure. This may again

point out the similar adsorption of inhibitory compounds by these two catalysts.

Figure 47 – DDS conversion of 4,6-DMDBT as function of temperature for the three catalysts prepared. In the graph, full lines represent the 0,5% Quinoline model approximation and gapped lines represent the 1,0% Quinoline (standard conditions).

Additionally, literature reports that DDS is more inhibited by quinoline than HYD [32]. However, at

this level, this fact was not seen in the results as there was a higher apparent inhibition on the

hydrogenation reaction, for all three catalysts (Figure 46). The only experimental work showing a

stronger effect of nitrogen compounds on HYD was actually obtained with piperidine [38]. Ma et al.

[34] and Turaga et al. [35] have also found from molecular modeling experiments, that the inhibition

effect caused by nitrogen-based compounds could be stronger for HYD route.

60


Once again, the kinetic parameters were determined and optimized with ReactOp® Cascade

software. The fits obtained for CoMo-B and CoMo-C catalysts for 0,5 wt.% and 1,5 wt.% quinoline

concentrations are reported in Figure 48 and Figure 49, respectively. The remaining fits are present in

Appendix 2.

Figure 48 – Kinetic fitting obtained for the HDS of 4,6-DMDBT using CoMo-B - 0,5% quinoline

Figure 49 – Kinetic fitting obtained for the HDS of 4,6-DMDBT using CoMo-C - 1,5% quinoline

Nonetheless, taking into account that for each catalyst, again, only one experimental test was

performed with feedstock-3, the parameters determined by the kinetic model may not show a perfect

approximation to what happens in the decomposition of 4,6-DMDBT at this NH3 partial pressure.

The kinetic parameters determined for 0,5 wt.% and 1,5 wt.% quinoline concentration are

summarized in Table 24 and Table 25, respectively.

61

Table 24 - Kinetic parameters obtained for each catalyst for 0,5 wt.% quinoline


HYD

ln(ko) (hour-1

) 35,5 33,0 31,3

Ea (kJ/mol)* 164,1 152,0 140,4

DDS

ln(ko) (hour-1

) 34,6 32,5 29,8

Ea (kJ/mol)* 166,5 156,5 143,1

HDA (equilibrated)

ln(ko) (hour-1

) 20,6 20,9 21,1

Ea (kJ/mol)* 102,4 101,3 96,1

ln(ko,eq) (hour-1

) 6,8

Eaeq (kJ/mol) 134,3


Table 25 – Kinetic parameters obtained for each catalyst for 1,5 wt.% quinoline, Feedstock-3


HYD

ln(ko) (hour-1

) 33,7 31,2 29,6

Ea (kJ/mol)* 164,0 152,0 140,4

DDS

ln(ko) (hour-1

) 33,9 31,7 29,4

Ea (kJ/mol)* 166,5 156,5 143,1

HDA (equilibrated)

ln(ko) (hour-1

) 18,3 18,7 18,9

Ea (kJ/mol)* 102,4 101,3 96,1

ln(ko,eq) (hour-1

) 6,8

Eaeq (kJ/mol) 134,3


Again, the three catalysts present the same HYD and DDS rate constants hierarchy:


The ranking obtained for the rate constants was conserved compared to the standard conditions.

To observe easily the differences between the catalysts, one compared the rate constants, calculated

from the average kinetic parameters for the temperatures of interest (Table 26 and Table 27).

62

Table 26 – Kinetic rate constants obtained for both HYD and DDS reactions, for 0,5 wt.% quinoline.


)

Temperature (°C)

HYD DDS


290 3,54 (100) 1,61 (45) 1,55 (44) 0,48 (100) 0,39 (81) 0,38 (79)

300 5,98 (100) 2,84 (47) 2,86 (48) 0,83 (100) 0,70 (84) 0,70 (84)

310 9,91 (100) 4,91 (50) 5,17 (52) 1,38 (100) 1,24 (89) 1,28 (93)

330* 22,52 (100) 11,93 (53) 13,50 (60) 3,19 (100) 3,09 (96) 3,40 (106)

Table 27 – Kinetic rate constants obtained for both HYD and DDS reactions, for 1,5 wt.% quinoline


)

Temperature (°C)

HYD DDS


290 0,67 (100) 0,28 (42) 0,25 (37) 0,31 (100) 0,17 (55) 0,19 (62)

300 1,14 (100) 0,49 (43) 0,47 (41) 0,53 (100) 0,30 (57) 0,35 (66)

310 1,89 (100) 0,85 (45) 0,84 (44) 0,88 (100) 0,53 (60) 0,64 (73)

330* 4,28 (100) 2,07 (48) 2,20 (51) 2,04 (100) 1,32 (65) 1,69 (83)

Again, the values presented in parenthesis in Table 26 and Table 27 represent the relative

difference in activity between the three catalysts considering the CoMo-C rate constant as the base

value. Again, the rate constant values obtained for 330°C were extrapolated from the experimental

results and represent the typical temperature used on HDS process.

Once again due to the activation energy, one can note that, by increasing the temperature, the

difference between the additive impregnated catalyst (CoMo-C) rate constants and the other catalysts

becomes lower, especially for the hydrogenolysis reaction (DDS), which increases relatively faster

than HYD route.

It can also be pointed out that, for the two conditions studied, the additive impregnated catalyst was

undoubtedly the most active catalyst, especially for hydrogenation reaction.

In Figure 50, Figure 51, Figure 52 are represented, for the three catalysts, the ln(ko) for HYD, HDA

and DDS, respectively, as function of ln(ppNH3).

63

Figure 50 – ln(ko) as function of ln(ppNH3) for HYD

Figure 51 – ln(ko) as function of ln(ppNH3) for HAD

Figure 52 – ln(ko) as function of ln(ppNH3) for DDS

64


results are presented in Table 28.

Table 28 – Kinetic partial orders with respect to NH3

As can be seen in Table 28, the HYD pathway was greatly inhibited for all three catalysts by NH3

partial pressure. Additionally, one may confirm the fact, already explained by Table 26 and Table 27,

that the HYD reaction is less inhibited for the additive impregnated catalyst than for the other. Again,

HDA follows the same tendency as HYD since it is a hydrogenation reaction.

Regarding the results obtained for DDS, one can notice that the kinetic orders are negative, for the

three catalysts, although the experimental results show an increase on DDS selectivity for the higher

NH3 partial pressure. Actually, this could probably be an artefact linked to the HDA inhibition that

provides higher DDS selectivity, but in appearance.

Nevertheless, DDS is less inhibited than HYD. Regarding literature, this difference in the inhibition

factor between DDS and HYD is possible, as shown by simulation [34] [35]. Actually, some

experimental work carried out with quinoline in presence of H2S show that the HDN rate is highly

impacted by H2S [36]. So the remaining quinoline could be hold responsible for the high inhibition of

HYD, being known that quinoline undergoes HDN mainly by hydrogenation.

Additionally, in Figure 53 are shown the inhibition factor for NH3 comparing standard conditions

with both 0,5 wt.% and 1,5 wt.% quinoline, respectively. To discuss the results from Figure 53, it has

to be noted that the standard value for the NH3 inhibition factor should be logically 1.


CoMo-A -1,6 -0,7 -2,0

CoMo-B -1,6 -0,8 -2,0

CoMo-C -1,5 -0,4 -1,9

65

Figure 53 – Overall NH3 inhibition factor comparing standard conditions (1 wt.% Quinoline) with both 0,5 wt.% and 1,5 wt.% Quinoline

As one can see, on one hand, decreasing the concentration of quinoline (compared with standard

conditions) resulted in a decrease of the inhibition factor for all catalysts. On the other hand, a higher

concentration produced a higher inhibition factor in all catalysts.

Moreover, one can note that the additive impregnated catalyst exhibits the highest decrease of

activity at high concentrations of quinoline but the lowest increase for low concentrations of quinoline.

This fact may confirm again the hypothesis in which the additive impregnated catalyst suffers

differently from inhibition compared to the two other catalysts, probably due to its highest active phase

dispersion.

66

4.4 Impact of H2 partial pressure

In order to study the influence of the H2 partial pressure within the reaction the H2/feed ratio was

increased from 240 to 320, thus increasing the total pressure from 30 to 40 bar. To study the influence

of H2 partial pressure the feed used was only Feedstock-1.


In Figure 54 are shown the performances concerning the HYD pathway from the catalytic test

performed with a total working pressure of 40 bar. The tests with CoMo-A, CoMo-B and CoMo-C were

performed at different LHSV (4 h-1

, 3 h-1

, 6,5 h-1

, respectively). The experimental tests concerning

CoMo-A and CoMo-B at standard conditions (30 bar) were performed in previous works.

Figure 54 – HYD pathway conversion as a function of the temperature for different catalysts and total pressures

Concerning the results obtained, one can see that the decomposition of 4,6-DMDBT through HYD

route increases with the increasing of H2 partial pressure. This fact is supported by most part of the

results found in literature [33] [38] [45].

However, throughout the experimental test of the calcined catalyst (CoMo-B) there were some

problems, at 300°C, associated with the data exploitation in the GC. So, one may consider that at this

temperature the conversion through HYD pathway would be higher attending to the tendency of the

experimental results. Thus, this test should be redone in order to better evaluate the influence of H2

partial pressure on the calcined catalyst.

67

In Figure 55 are shown the performances concerning the DDS pathway from the catalytic test

performed with a total working pressure of 40 bar.

Figure 55 – DDS pathway conversion as a function of the temperature for different catalysts and total pressures

Looking at the results, as mentioned previously, the DDS conversion for CoMo-B catalyst may not

be correct taking into account that at 300°C there were some problems with the GC. Nevertheless,

there is a clear decrease on the DDS conversion. The only way to explain this tendency should be to

assume that HDA pathway consecutive to DDS is enhanced. The same results were found for CoMo-

A.

For the additive impregnated catalyst, the decrease on DDS conversion is not as high as the one

found for the other catalysts. In fact, for high temperatures, one can see a slight increase. However,

these results might be bias considering the high LHSV used (low conversion rates). Indeed, this test

should be redone, with a lower LHSV (higher contact time) in order to evaluate how the activity of

additive impregnated catalyst is influenced by these working conditions than for the standard ones.

68


The kinetic parameters were then optimized with ReactOp® Cascade software. The fit achieved for

CoMo-C, the additive impregnated catalyst, is reported on Figure 56. The remaining fits are present in

Appendix 2.

Figure 56 – Kinetic fitting obtained for the HDS of 4,6-DMDBT using CoMo-C, at 40 bar



Table 29 - Kinetic parameters obtained for each catalyst at 40 bar


HYD

ln(ko) (hour-1

) 34,9 32,5 30,2

Ea (kJ/mol)* 164,0 152,0 140,4

DDS

ln(ko) (hour-1

) 33,5 31,6 28,9

Ea (kJ/mol)* 166,5 156,5 143,1

HDA (equilibrated)

ln(ko) (hour-1

) 19,8 20,1 20,4

Ea (kJ/mol)* 102,4 101,3 96,1

ln(ko,eq) (hour-1

) 6,6

Eaeq (kJ/mol) 145,6


69

Looking at the three catalysts, the rate constants obtained corresponding to HYD pathway follow

the previous hierarchy:


This ranking was the same as previously. The change in pressure did not inverse the tendency.

However, one can see that the activation energies found for the HDA route are slightly higher, for all

catalysts, than at standard conditions (see Table 16). This could be due to the changing on the

reaction thermodynamic by increasing the H2 partial pressure.

To better visualize the differences between the catalysts, one compared the rate constants,

calculated from the average kinetic parameters for the temperatures of interest, for 40 bar (Table 30).

Table 30 - Kinetic rate constants obtained for both HYD and DDS reactions, for 40 bar.


)

Temperature (°C)

HYD DDS


290 1,23 (100) 1,03 (83) 0,87 (71) 0,19 (100) 0,16 (83) 0,12 (64)

300 2,08 (100) 1,81 (87) 1,60 (77) 0,32 (100) 0,28 (88) 0,22 (70)

310 3,44 (100) 3,13 (91) 2,90 (84) 0,54 (100) 0,49 (92) 0,41 (76)

330* 7,83 (100) 7,61 (97) 7,55 (96) 1,24 (100) 1,23 (99) 1,08 (87)

Once again, the values presented in parenthesis in Table 30 represent the relative difference in

activity between the three catalysts considering the CoMo-C rate constant as the base value. The rate

constant values presented for 330°C were deduced from the experimental results and represent the

typical temperature used on HDS process.

From Table 30, it is clear that the rate constants determined for the hydrogenation reaction are

even much higher than for DDS pathway compared to standard conditions (Table 17). This confirms,

for all three catalysts, that the HYD route was more promoted by H2 partial pressure than DDS.

70

In Figure 57, the ratio / , at 310°C, is reported.

Figure 57 – Effect of H2 partial pressure on the selectivity of HDS over the prepared catalysts, for 310°C.

As one can see in Figure 57, all three catalysts have a higher selectivity for HYD by increasing the

H2 partial pressure supporting the facts already discussed.

However, it is important to note that the ratio HYD/DDS for the calcined catalyst does not increase

(+34%) as much as for the dried (+42%) and additive impregnated (+51%) catalysts.

Then, to determine the kinetic partial orders, for each reaction, the logarithm of the pre-exponential

rate constants obtained were then represented as function of the logarithm of the partial pressure of

H2.

In Figure 58, Figure 59, Figure 60 are represented, for the three catalysts, the ln(ko) for HYD, HDA

and DDS, respectively, as function of ln(ppH2).

Figure 58 – ln(ko) as function of ln(ppH2) for HYD

71

Figure 59 – ln(ko) as function of ln(ppH2) for HDA

Figure 60 – ln(ko) as function of ln(ppH2) for DDS


results are presented in Table 31

Table 31 – Kinetic partial orders with respect to H2

As previewed, increasing H2 partial pressure has a positive impact on HYD pathway for all three

catalysts.


CoMo-A 1,0 -0,5 1,0

CoMo-B 1,0 -0,4 1,0

CoMo-C 0,7 -0,3 0,4

72

Moreover, it appears that the hydrogenation reaction for both dried and calcined catalysts is more

promoted by the H2 partial pressure than for the additive impregnated.

Concerning the DDS pathway, one can point out that the changes in values found were almost

negligible. Moreover, the values obtained could be result of two made assumptions:

- First, the acid S-H groups (i.e. especially S-edge), present in the catalyst active sites (Figure

23), may be inhibited by H2 thus leading to a lower DDS rate;

- Secondly, one may also consider that these results could be due to the lack of experimental

points and the error in the model fitting done.

As a consequence, at this point, one considered that DDS pathway was not modified by the H2

partial pressure.

Finally, in Figure 61 the results obtained for the H2 activation factor are shown.

Figure 61 – H2 activation factor on the three catalysts tested (global activity, see Eq. 27)

Again, this factor was determined by the ratio between global activities at 30 (standard conditions)

and 40 bar.

As one can see, all catalysts were “activated” by the increase of H2 partial pressure. Moreover,

these results also show that dried and calcined catalysts were more “activated” by H2 than the additive

impregnated, which confirms the results found for the catalysts kinetic partial orders.

73

4.5 Summary of Results

Based on the kinetic study that has been performed, it is possible to propose three equations per

catalysts to describe the variation in reactivity according to the different pathways considered (HYD,

DDS and HDA). Those equations are summarized hereafter:

For CoMo-A,

(Eq. 28)

(Eq. 29)

(Eq. 30)

For CoMo-B,

(Eq. 31)

(Eq. 32)

(Eq. 33)

For CoMo-C,

(Eq. 34)

(Eq. 35)

(Eq. 36)

Even if the fitting of these equations did not describe exactly the experimental set of results, it

allows to compare apparently the various catalysts according to their reactivity and to the operating

conditions in a representative range of partial pressures, temperatures and feedstocks.

From the previous equations, it was possible to suggest the best working conditions for the additive

impregnated catalyst and to point out the most relevant differences in behaviour for the three

catalysts. To do so, one plotted the relative differences between the additive impregnated and the

other two catalysts (dried and calcined). As already explained, these relative differences were

determined by using the rate constant of the additive impregnated catalyst as base value. Thus, in

Figure 62 and Figure 63, were plotted the relative differences between the calcined and dried

catalysts to the additive impregnated for HYD and DDS pathways, respectively.

74

Figure 62 – Relative differences in activity for HYD reaction between the three catalysts

As one can see in Figure 62, concerning the HYD pathway, it is clear that increasing the

concentration of quinoline results in a decrease on the relative activity of the other two catalysts. Thus,

one can confirm that the additive impregnated catalyst would perform better than the dried and

calcined in the presence of high concentration of quinoline.

In addition, it is also noticeable that for high partial pressures of H2 and/or H2S the difference of

activity between the additive-impregnated catalyst and the dried or calcined decreases. So, the

additive impregnated catalysts would perform better by the HYD route when in the presence of

relatively low H2 and H2S partial pressures.

55

60

65

70

75

80

85

285 290 295 300 305 310 315

Re

lati

ve d

iffe

ren

ce w

ith

Co

Mo

-C f

or

HY

D (

%)

Temperature (oC)

CoMo-A - Std. Conditions

CoMo-B - Std. Conditions

CoMo-A - 40 bar

CoMo-B - 40 bar

CoMo-A - 0,5%Quino

CoMo-B - 0,5% Quino

CoMo-A - 1,5%Quino

CoMo-B - 1,5%Quino

CoMo-A - 2%DMDS

CoMo-B - 2%DMDS

75

In Figure 63 are shown the results obtained for the relative differences in activity between the three

catalysts for the DDS route.

Figure 63 – Relative differences in activity for the DDS reaction between the three catalysts

Looking at the results in Figure 63, one can note that the relative DDS activity of the additive

impregnated catalyst was especially enhanced, comparatively to the other catalysts, with high

concentrations of quinoline (1,5 wt.%).

As for HYD, low ppH2S is also enhancing the relative activity of additive impregnated catalyst (via

DDS), but the relative ranking is not sensitive to ppH2.

Finally, to summarize, one can say that low H2S partial pressure, high NH3 partial pressure, and

low H2 partial pressure are supposed to be the conditions in which the additive impregnated performed

the best, comparatively with the other two catalysts.

50

55

60

65

70

75

80

85

90

285 290 295 300 305 310 315

Re

lati

ve d

iffe

ren

ce w

ith

Co

Mo

-C f

or

DD

S (%

)

Temperature (oC)

CoMo-A - Std. Conditions

CoMo-B - Std. Conditions

CoMo-A - 40 bar

CoMo-B - 40 bar

CoMo-A - 0,5%Quino

CoMo-B - 0,5%Quino

CoMo-A - 1,5%Quino

CoMo-B - 1,5%Quino

CoMo-A - 2%DMDS

CoMo-B - 2%DMDS

77

5 Conclusion and Future Perspectives

In this study, the main focus was to evaluate the deep HDS of middle distillates in the range of

operating conditions dedicated to the low pressure HDS i.e. with sulfided CoMo-based catalysts.

After a description of the HDT catalysts, their operating conditions and known mechanisms for the

refractory sulfur compounds desulfurization, the experimental work has been dedicated to improve our

knowledge using high activity catalysts, especially the additive-impregnated one.

Three CoMo-based catalysts were prepared and catalytic tests were performed in order to

compare their performances within the same range of temperature and HDS global conversion.

Moreover, the inhibiting and promoting effects of H2S, NH3 and H2 on the three catalysts were

compared and discussed for each HDS pathways, namely HYD and DDS. NH3 has been provided by

quinoline decomposition and H2S by DMDS.

Regarding the effect of the partial pressure of H2S, the three catalysts were inhibited by H2S more

or less in the same range. Nevertheless, only the hydrogenation seems to be inhibited while DDS

appears to be promoted. These results strongly disagree with the literature and could be due to many

origins, like the working conditions, the molybdenum amount or the partial pressure of nitrogen.

Moreover, it has been shown that the additive impregnated catalyst is less affected by H2S adsorption

onto the catalytic surface. It is proposed that it is due to its higher number of available active sites,

namely dispersion.

However, increasing the amount of H2S, the additive impregnated catalyst ended up being inhibited

in a higher extent. This behavior could be related to the dispersion or also to some cross effect due to

quinoline.

Concerning the impact of nitrogen-based compounds (quinoline), it was proved that the three

catalysts undergo a strong inhibition over both pathways and especially the hydrogenation, in

agreement with some literature data. The additive impregnated seem to suffer less from this inhibition

comparatively to other catalysts which allow us to propose that it will be relatively more active than the

other catalysts at high NH3 partial pressure.

Finally, concerning the effect of H2, the catalysts showed a quite similar promoting impact although

the additive impregnated was slightly less enhanced.

All in all, the conditions in which the additive impregnated catalyst would relatively perform the best

compared to non-additivated catalysts were with high NH3 partial pressure, low H2 partial pressure and

relatively low H2S partial pressure.

As perspectives for this study, an improvement regarding the kinetic model created should be done

in order to fully understand the HDS of 4,6-DMDBT within the model feedstock considered. Indeed the

fit does match the experimental values. The following propositions should be taken into account:

- The reaction concerning the decomposition of quinoline should be introduced into the kinetic

model to conclude if quinoline inhibits more than NH3 or if there is some cross effect between

H2S and quinoline;

78

- A lower amount of 4,6-DMDBT should be introduced into the model feedstock in order to study

the adsorption/desorption relation between the 4,6-DMDBT and the catalytic surface. Then, it

would be possible to determine if the apparent reaction order considered for the kinetic model

implementation is lower or equal to 1;

- The model molecule in study could be changed to DBT with the aim to strengthen the DDS

selectivity and compare its behavior to the one found with 4,6-DMDBT;

Some other perspectives, regarding the hypothesis made to explain the promoting effect over DDS

with high H2S partial pressure, should also be considered. On one hand, the same tests should be

redone for all three catalysts decreasing the amount of MoO3 within the catalysts and/or increasing the

H2S partial pressure, in order to directly compare the results with literature. On the other hand, the

amount of H2S could be increased to a higher extent to observe if some inhibition occurs, providing a

more accurate H2S partial order.

Finally, all the experimental tests performed should be redone replacing the CoMo/Al2O3 catalysts

by NiMo/Al2O3 or even without promoter (Mo/Al2O3). This would allow understanding if the promoter

used has an impact on the kinetic mechanism of the 4,6-DMDBT HDS.

79

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Appendix 1 – GC Chromatogram

Figure 64 – Example of a GC chromatogram

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Appendix 2 – Kinetic model

Standard Conditions

Figure 65 - Kinetic fitting obtained for the HDS of CoMo-B at standard conditions

Figure 66 - Kinetic fitting obtained for the HDS of CoMo-A at standard conditions

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Impact of H2S

Figure 67 – Kinetic fitting obtained for the HDS of CoMo-C at 2 wt.% DMDS

Figure 68 – Kinetic fitting obtained for the HDS of CoMo-A at 2 wt.% DMDS

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Impact of NH3

Figure 69 – Kinetic fitting obtained for the HDS of CoMo-A at 0,5 wt.% Quinoline

Figure 70 - Kinetic fitting obtained for the HDS of CoMo-C at 0,5 wt.% Quinoline

Figure 71 – Kinetic fitting obtained for the HDS of CoMo-A at 1,5 wt.% Quinoline

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Figure 72 – Kinetic fitting obtained for the HDS of CoMo-B at 1,5 wt.% Quinoline

Impact of H2

Figure 73 - Kinetic fitting obtained for the HDS of CoMo-B at 40 bar

Figure 74 - Kinetic fitting obtained for the HDS of CoMo-A at 40 bar

model test set up methodology for hds to improve the understanding of reaction pathways … ·...

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