conversion of syngas to diesel - article ptq-english

7/29/2019 Conversion of Syngas to Diesel - Article Ptq-English

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Conversion of syngasto diesel

An overview of Fischer-Tropsch technologies for the production of diesel from syngas usinga variety of feedstocks. Process technology, reactor design and catalyst requirements to

achieve a successful industrial scale-up are discussed

Fischer-Tropsch (FT)-based XtLoptions (gas, coal, petroleum cokeor biomass to liquids) represent

viable routes for the production of non-crude oil-based ultra-high-performance

motor fuels and speciality products.Improvements in catalyst and reactortechnology, coupled with optimisedintegration and economies of scale, haveallowed gas-to-liquids (GtL) complexesto be competitive with LNG andpipeline projects. Most major oil andgas companies are closely following XtLdevelopments. Currently, several largeGtL industrial projects have been realised(Oryx in Qatar) or are under construction(Pearl in Qatar, Escravos in Nigeria).

Eni and IFP (French Institute ofpetroleum) have developed proprietary

FT and upgrading technologies in a closecollaboration between the two groups.These technologies are based onproprietary catalysts and reactor designsresulting from scale-up criteria developedover 12 years of common research anddevelopment. Large-scale hydrodynamicfacilities (slurry bubble columns),dedicated bench-scale pilot units and alarge-scale FT pilot plant have beendeveloped and operated to minimisereactor and ancillary unit scale-up risks.

A large-scale FT pilot plant has beenbuilt and operated since 2001. The

plant, located within the Eni refinery inSannazzaro, Italy, is fully integratedwith the refinery utilities and network.The 20 bpd FT synthesis section hasbeen used to optimise slurry handling(loading, make-up and withdrawal),reactor configuration and productseparation units. Large-size scale-up hasbeen accomplished through theimplementation of several specificallydesigned scale-up tools to assess thecatalysts stability under industrialconditions, and study fluid dynamicsand liquid-solid separation.

Fuel diversification:from GtL to XtLThe estimated amount of natural gasreserves has continuously increased over

the past ten years, moving from the1997 estimation of 152.2 Tcm to 181.5Tcm in 2007, with a reserve/productionratio of around 63 years, higher than oil

of 38 years.1

It is thus quite logical toenvisage using natural gas (or coal withits R/P ratio above 140 years) as a sourceof liquid transportation fuels, throughnatural gas conversion into liquidhydrocarbons, the so-called GtL, whichis characterised by an intermediate stepof natural gas conversion for producingsynthesis gas (hydrogen and carbonmonoxide). This production can bedivided into two main value chains: Production of methanol andmethanol-derived products (includingacetic acid, DME and so on) FT synthesis for production of high-quality middle distillates (ie, keroseneand diesel fuel), pure paraffinic naphtha,lube bases, waxes and, in somecases, olefins and speciality chemicals.

While the final market for LNG,pipeline and wire transportation is thetraditional natural gas one, GtLtechnologies open up new markets

(automotive fuel, chemicals) to naturalgas producers (Figure 1).Increasing global demand for ever-

cleaner fuels, particularly middledistillates, such as in Europe, shouldfavour the FT GtL route, which answersmarket needs.2

A relatively recent requirement, butfundamental for decades to come, isthat automotive fuels must not only becleaner, but the impact of their well-to-wheel lifecycle on the greenhouse effect(ie, overall CO

2emissions from

production through consumption of theliquid fuel, including biomass CO

2absorption [in the case of biomass])must be as low as possible.

This general requirement will betranslated into different regional targets.

Stphane Fedou, Eric Caprani, Damien Douziech and Sebastien Boucher

Axens

www.eptq.com PTQQ2 2008 1

Pipeline

Gasmarket

Electricmarket

Chemicalmarket

Fuelmarket

MeOHDME Fischer-Tropsch

LNG Gas-to-wire GtL

Figure 1 Gas to market options


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As an example, the European Councilstated on 89 March 2007: a 10%binding minimum target to be achievedby all Member States for the share ofbiofuels in overall EU transport petrol

and diesel consumption by 2020. As it iscommonly agreed that the first generationof biofuels (using vegetable oils andsugars) will not be sufficient, the so-calledsecond-generation biofuels will have tocome into force. And the most promisingtechnology to achieve this target is BtL-FT(biomass-to-liquid through Fischer-Tropsch: production of FT diesel from asyngas generated by the gasification ofbiomass), which allows for a decrease inthe equivalent CO

2emissions per km

down to 90% compared to a conventionaldiesel.3 Biomass may also be mixed with

petroleum residues or coke in order to

take advantage of refinery infrastructuresand operating experience.

All these XtL options (ie, gas, coal,petroleum coke or residue, biomass-to-liquids, or a mixture of some of them)

will allow diversifying fuel sources,producing cleaner fuels and limitingoverall GHG emissions. They all requirea reliable technology package totransform clean syngas from whateversource into ultra-clean liquid fuels, andthis is the scope of the Gasel technologysuite (Figure 2).

Fischer-Tropsch technologies:industrial applicationsFT technologies can be characterised/differentiated by four parameters: FT catalyst: two main types:

Iron-based catalysts

Cobalt-based catalysts. FT reactor: three main types ofreactor:

Fixed bed (the catalyst is locatedinside tubes)

Fluidised bed (the catalyst is main-tained in suspension by the syngas)

Slurry bubble column (three-phasereactor with synthesis gas, waxes, liquidproducts and solid catalyst).

Operating temperature: HT-FT: High-temperature Fischer-

Tropsch (around 350C and above) LT-FT: Low-temperature Fischer-

Tropsch (220240C). Final products obtained (after FTupgrading):

Middle distillates (diesel): paraffinicnaphtha (also in some cases waxes orlube base)

Gasoline: olefins and chemicalsspecialities.

However, it must be noted that thesefour parameters are not independent of

each other, and only three combinationsare feasible and have been developed sofar, as illustrated in Figure 3.

These three combinations orcategories include: HT-FT on iron catalyst in fluidised-bed reactor for gasoline, olefins andspeciality chemicals production LT-FT on cobalt catalyst in fixed-bedreactor for middle distillates (kero,diesel), naphtha and waxes production LT-FT on cobalt catalyst in slurrybubble column reactor (SBCR) formiddle distillates (kero, diesel), naphtha

and waxes production.

Syngasgeneration

F-Tsynthesis

Gaseltechnology scope

Productsupgrading

Utilities

H2 + CONat. gas

Coal

Blomas

Petcoke

Liquidfuels

Power

Figure 2Gasel technology scope

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Each of these three categories has itsown unique background and strengths:

Category 1: In the operation of afluidised bed reactor, where solid

catalyst is kept in suspension bysynthesis gas (and gaseous products),

liquid production must be limited oravoided. That is why this type of reactor

can only be accommodated with HT-FT

(above 350C) with iron catalysts (cobaltcatalyst selectivity towards methane istoo detrimental at these temperatures).

The corresponding catalyst selectivity(characterised by the Schultz-Flory [SF]

coefficient for paraffins distribution: a =C

n+1/C

n, with C

nbeing the number of

moles of paraffins formed with n atoms

of carbon) is in the order of 0.7, leadingto more than 50 wt% production of C

1

C4

and less than 20 wt% of diesel andabove (Figure 4). This technology

requires a heavy work-up of the raw FTproducts to obtain valuable products

like a relatively limited yield of gasoline,as well as olefins and speciality

chemicals.

Category 2: The LT-FT on cobalt catalystin fixed bed reactor technology for

middle distillates (kero, diesel), naphthaand waxes production was primarily

developed by Shell back in 1970. Thecatalyst is placed inside tubes. The

syngas passes through the tubes and thevapour-liquid products are recovered at

the bottom of the reactor. The a-paraffins SF coefficient of around 0.9

and above leads to a good final yield(after product upgrading) of ultra-clean-

FT diesel or middle distillates, with the

possibility of producing lube base andwaxes (Figure 4).

This fixed-bed technology has twoimportant advantages:

The scale-up to industrial reactors istheoretically simple (multiplying the

number of tubes) There is normally no problem with

liquid-solid separation, as the catalyst isfixed inside the tubes.

Certain mechanical constraints in the

reactor design should be noted with thistechnology (ie, temperature gradientalong the tubes, limitations in the

thickness of the main tubesheets), andtransfer limitations between the gas-

liquid phase and the solid catalyst canlead to a limited capacity per single

train: from 30006000 bpd per reactorof 78 m diameter. Also, catalyst

continuous make-up is not feasible. Forexample, in the case of catalyst

deactivation, the reactor must be shutdown, tubes emptied and refilled

(estimated time by Shell is about twoweeks).4 This issue is particularly critical

when the risk of having impurities/poisons in the syngas is higher (coal,

biomass). This is also why this type oftechnology is not combined with iron

catalysts, which are known to deactivate

much faster than cobalt-based catalysts.

Category 3: The LT-FT on cobalt catalyst

in slurry bubble column reactor (SBCR)technology for middle distillates (kero,

diesel), naphtha and waxes has beendeveloped over the past 20 years. It is the

most promising in terms of catalystproductivity, capacity per train and

operational flexibility. The reaction takesplace in a three-phase slurry bubble

column reactor, where the syngas iscontacted with solid catalyst mixed in

the produced waxes. The obtainedproducts are the same as in the fixed bed

case (mainly ultra-clean FT diesel).Compared to the fixed-bed technology,

the SBCR has the following advantages: Higher capacity per train: at least

15 000 bpd per FT train (with reactors ofup to 10 m diameter)

Easy isothermal operation in thereactor

Possibility of continuous catalyst

make-up/withdrawal, allowing for a

constant production to be maintainedin case of catalyst deactivation or partial

poisoning.But some important challenges must be

solved:

Catalyst mechanical stress in a largeSBCR Liquid-solid separation.

Table 1 summarises the mainindustrial applications. Today, only two

companies can claim to have broughttheir FT technology up to industrial

scale: Shell and Sasol.

XtL technology forconversion of syngas to dieselThe proprietary Gasel FT and upgradingtechnologies have been developed by

Eni and IFP in close collaboration,which began in early 1996. At an early

stage, Axens became associated with theproject to optimise the basic engineering

design, the operability of the chain ofprocesses, and to develop the


20

40

60

80

100

0

= chain growth probability

0.7750.75 0.80 0.825 0 .85 0.875 0.90 0.925 0 .95 0.990.7250.70

Product,wt%

C15C22C10C14C5C9

C3C4

C1 C2

C22+

Figure 4 Distribution of raw FT product as a function of the Schultz-Flory (SF)selectivity coefficient

HT-FT in fluidised bed with iron catalyst: Sasol CFB and FFB in South Africa

GTL

South Africa (Petro SA & Sasol) Main products

Capacity: 200 000 bpd Olefins specialities

Last start-up: 1993 Liquid fuels

LT-FT in fixed bed with cobalt catalyst: Shell SMDS in Malaysia

GTL

Malaysia (Shell) Main products

Capacity: 14 500 bpd Diesel

Last start-up: 1993 Naphtha/waxes

LT-FT in slurry bubble column with cobalt catalyst: Sasol SPD in Qatar

GTLQatar (QP/Sasol) Main products

Capacity: 33 000 bpd Diesel

Last start-up: 2007 Naphtha

Main FT industrial applications in the world

Table 1


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preparation and production of the

associated proprietary catalysts atindustrial scale. The Eni and IFP

technologies have been developedaccording to the following strategy:

Development of FT technologybased on a tailored cobalt-based catalyst

and a proprietary FT SBC reactor design Development of specific upgrading

catalysts (mild hydrocracking andisomerisation) to be adapted with

the industrially referenced Axenshydrocracking technology

Focusing on the reduction of thescale-up risk, developing in parallel a

complete FT pilot plant and necessarycomplementary tools to assess the

catalyst mechanical stability underreactive and hydrodynamic conditions

representative of a large SBCR

Engineering studies of fully

integrated GTL complexes, up to front-end engineering design, includingdetailed operating instructions.

Process descriptionGasel is a complete technology suite for

the conversion of a purified syngasoriginating from various sources (natural

gas, coal, biomass, petroleum residue orcoke) into ultra-clean liquid fuels. The

technology, as shown in Figure 5,consists of:

FT synthesis: LT-FT conversion of

syngas in a SBCR on a cobalt-basedcatalyst. The liquid products are

recovered after gas/liquid and liquid/solid separations and sent to the

upgrading section. The liquid/solidseparation takes place outside the

reactor in an external slurry loop for asafer operation and easier maintenance

(internal L/S separation is feasible butconsidered today too risky)

FT products upgrading: After apreparation step, which includes

the condensate stabilisation andhydrotreatment of light olefins, the

pretreated raw FT product ishydrocracked and isomerised. The fully

converted product is separated intoapproximately 70% of an ultra-clean FT-

diesel (no sulphur, no aromatics) havinga high cetane number (> 70) and

excellent cold flow properties (CFPP


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experience has been achieved andoperating instructions consolidated forthe industrial unit. The DCS platform ofthe pilot plant can be used as a dynamictraining simulator, to reproduce criticaloperations, offsets, emergencies, and soon. The large-scale FT pilot plant is stillin operation and is considered a valuabletool for troubleshooting of differentaspects of the technology

Special laboratory equipment for FTcatalyst scale-up has been designed inorder to assess the FT catalyst mechanicalstability under reactive and hydro-dynamics conditions representative of alarge SBC reactor. Indeed, this issue hasbeen identified early in the developmentas critical. This task has been achievedfrom knowledge acquired in thedevelopment of slurry bubble columnhydrodynamics. A proper stabilisation ofthe FT catalyst, as well as the ability toassess this stability through the righttools and tests assures FT catalyst

performance on an industrial scale A hydrocracking pilot plant unit atthe IFP R&D centre of Solaize, France,where the raw FT products from the FTpilot plant have been transformed intodiesel and naphtha with the dedicatedupgrading catalyst, allows for the testingof the final products characteristicsand quality

Axens has developed facilities at itscatalyst manufacturing plant of Salindresin the south of France for the productionof the FT catalyst (full line from oxideprecursor preparation up to catalystactivation) on a semi-industrial scalearound 100 kg/d, in particular toproduce the catalyst for the FT pilotplant. The catalyst manufacturing linesfor industrial production (15000 tons/

year) have already been designed, andwill be a simple scale-up of the semi-industrial facilities already existing, withthe same process and operation sequence,so the scale-up risk for catalystmanufacturing has also been reduced toa minimum. During operation of theindustrial GtL or XtL plant, Axens willmanage the entire lifecycle of the FTcatalyst, including manufacturing,transport, activation, recovery of usedcatalyst, recycle and cobalt recovery withspecialised companies, and use of therecovered cobalt for new catalyst

manufacturing.

Conclusion

The scale-up basis has been completedand the technology is ready for industrialdeployment. It has recently beenproposed for several GtL and BtLinitiatives and is currently looking for itsfirst commercial implementation. The


licensing package, available on anunrestricted basis, includes: Transfer of the FT and Upgr ad in gtechnologies Basic design package for the FT andUpgrading units Assistance duringdetailed engineering and unit start-up Manufacturing and supply of theassociated catalysts Guarantee of the process and catalyst

performances.

Axens licensing package, including theEni and IFP proprietary FT+Upgradingtechnologies (and associated catalysts), isunder the Gasel trademark.

References

1 Enis World Oil & Gas Review 2007.2 Source CPDP for EU+Switzerland+Norway+

Turkey+Island.3 JRC Concawe/Eucar\] 2006 study.4 Hoek A, Shell, presentation at NGCS VIII,

Rio, Brazil, 2007.


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