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Model test set up methodology for HDS to improve the understanding of reaction pathways in HDT catalysts
Paulo, D.1,2, Guichard, B.2, Delattre, V.2, Lett, N.2, Lemos, F.1
1 Instituto Superior Técnico, Chemical and Biological Engineering Department, Lisbon, Portugal. 2 Institut Français du Pétrole Énergies nouvelles, Catalysis and Separation Division, Catalysis by Sulfides Department, Solaize, France.
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.
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 on fuels so, a better performance from the hydrodesulfurization (HDS) catalysts is required.
The commercial catalysts commonly used for HDS reactions are molybdenum sulfides promoted by cobalt or nickel and supported over alumina. 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 and 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. In the HDS of middle distillates, as sulfur
conversion increases the remaining species are mostly dibenzothiophenes. As 4,6-DMDBT, these compounds are very difficult to decompose. Moreover, these compounds are decomposed through two main and 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. The objective of this work was 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.
2 Methodology 2.1 Catalysts Three types of catalysts were prepared – CoMo-A,
CoMo-B and CoMo-C. All used catalysts are 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).
CoMo-A was the only catalyst used directly on the catalytic tests after drying phase. 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. The catalysts samples (4 cm3) were loaded with silicon carbide in the reactor to ensure good thermal diffusion in the catalyst bed. They were sulfided in situ in the reactor using a sulfidation feedstock containing DMDS, xylene and cyclohexane. This mixture was injected at a starting temperature of 40ºC. After 3h, the temperature was raised to 350ºC at a 1,7ºC/min rate and was maintained at 350ºC for 1h, before decreasing to 290ºC at a 1,5ºC/min.
2.2 Reaction Conditions
For this study, the HDS of 4,6-DMDBT was carried out in a high-pressure fixed-bed microreactor (length: 18,2 cm; inner diameter: 1 cm) at 290-310 ºC, under 3 MPa (ratio H2/HC=240) of total pressure after in situ sulfidation. The feedstock established for standard conditions contained: 4,6-DMDBT (0,66 wt.%) dissolved in cyclohexane (57,14 wt.%) and xylene (40,00 wt.%), then dimethyl disulfide (1,20 wt.%) and quinoline (1,00 wt.%) were added to generate H2S and NH3, respectively .
In order to evaluate the effect of H2S the quantity of DMDS in the liquid feedstock was increased to 2,00 wt.%. Then, to study the influence of the NH3 partial pressure, one used the experimental results determined on previous works (feedstock containing 0,50 wt.% of quinoline) and the experimental tests done along the present work (feedstock containing 1,50 wt.% of quinoline). For all cases, the composition of the other compounds was maintained by adjusting the amount of cyclohexane.
Finally, the effect of the H2 partial pressure was studied by increasing the total operating pressure from 3 to 4 MPa. To maintain the composition of the
other compounds constant the ratio H2/HC was increased from 240 to 320. 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 effluents were analyzed by a gas chromatography (GC). The starting oven temperature was 50°C and then increased to 67ºC at a 15ºC/min rate, prior to increasing to 290ºC at a 30ºC/min rate, in order to separate the produced organic compounds. The analytic results obtained from the GC allowed determining the conversion of the liquid feedstock. Hence, to obtain the HDS conversion of 4,6-DMDBT (%𝑋!,!!!"!#$) the following equation was used:
𝐧𝐏𝐫𝐨𝐝𝐮𝐜𝐭𝐬 𝐨𝐟 𝟒,𝟔!𝐃𝐌𝐃𝐁𝐓 𝐜𝐨𝐧𝐯𝐞𝐫𝐬𝐢𝐨𝐧 𝐧𝟒,𝟔!𝐃𝐌𝐃𝐁𝐓,𝐭𝐨𝐭𝐚𝐥
×𝟏𝟎𝟎 (Eq. 1)
In addition, the conversion of 4,6-DMDBT through
both HYD (%𝑋!"#) and DDS (%𝑋!!") pathways were calculated by the following equations:
×𝟏𝟎𝟎 (Eq. 2)
%𝑿𝑫𝑫𝑺 = 𝒏𝑫𝑫𝑺
𝒏𝟒,𝟔!𝑫𝑴𝑫𝑩𝑻,𝒕𝒐𝒕𝒂𝒍 ×𝟏𝟎𝟎 (Eq. 3)
In Eq. 2 and Eq. 3, 𝑛!"# and 𝑛!!"
represent the number of moles of HYD products and DDS product produced during the catalytic test, respectively.
2.4 Kinetic Study
In this study, the kinetic model was created in the software ReactOp Cascade®, considering the decomposition of 4,6-DMDBT through three main pathways: HYD, DDS and HDA. Thus, the following reactions were inserted in the kinetic model.
With A – 4,6-DMDBT, B – HYD products and C – DDS product.
Then, the results obtained from the experimental tests performed were inserted into the software in order to establish the kinetic model and understand how the H2S, NH3 and H2 partial pressures influence the decomposition of 4,6-DMDBT. With the kinetic model established, the reaction rate constant (𝑘) and
A !!"# B (Eq. 4)
A !!!" C (Eq. 5)
C !!"# B (Eq. 6)
the activation energy (𝐸!) were calculated. Finally, to determine the kinetic partial order for each component, the logarithm of the reaction rate constant was plotted as function of the logarithm of the partial pressure of H2S, NH3 and H2.
3 Results and Discussion 3.1 Impact of H2S partial pressure
CoMo-based catalysts are known for being inhibited by H2S  . In this study, the results showed the same tendency as HYD pathway has proven to be strongly inhibited (Figure 1), at high H2S partial pressure. The experimental tests concerning CoMo-A and CoMo-B at standard conditions (1,20% DMDS) were performed in previous works.
Figure 1 - 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.
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 th