The Journal of Engineering Research Vol. 3, No. 1 (2006) 38-42 Kinetics of Hydrodesulfurization of Dibenzothiophene on Sulfided Commercial Co-Mo/γ-Al2O3 Catalyst Y.S. Al-Zeghayer1 and B.Y. Jibril*2 1Chemical Engineering Department, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia 2Petroleum and Chemical Engineering Department, Sultan Qaboos University, P.O. Box 33, Al-Khoud, PC 123, Muscat, Oman Received 27 April 2005; accepted 17 September 2005 …QÉŒ »àjÈc …õØM πeÉY ≈∏Y dibenzothiophene IOÉŸ á«æ«Lh~«¡dG âjȵdG ádGPG äÓYÉØJ á«côM 2@πjÈL .….Ü h 1ÒgõdG .¢S.… á≤∏£e áLQO 683 ¤G 633 ÚH Ée ájQGôM äÉLQO ~æY …QÉŒ »àjÈc …õØM πeÉY ≈∏Y É«∏ª©e â°SQhO dibenzothiophene IOÉŸ á«æ«LhQ~«¡dG âjȵdG ádGPEG äÓYÉØJ áÑ«côJ :á°UÓÿG á«∏ª©ŸG èFÉàædG §Ñæà°ùj ¿G øµÁ áHPɵdG ¤h’G áLQ~dG øe »°VÉjQ êPƒ‰ ¿G âÑKG .âe~îà°SG á«æ«LhQ~«¡dG âjȵdG ádGPGC á«∏ªY øe É¡«∏Y π°UÉ◊G DBT IOÉŸ áØ«©°V äGõ«côJ .…ƒL §¨°V 10 ~æY h (CHB) ɢª˘g ɢ¡˘«˘∏˘Y ∫ƒ˘°üÙG ᢫˘˘°ù«˘˘Fô˘˘dG äɢ˘é˘˘à˘˘æŸG ¿G ~L,, 51.7 kcal/mol …hɢ°ùJ ɢ¡˘fG ~˘˘Lh DBT IOÉe π˘jƒ˘ë˘à˘d á˘£˘°ûæŸG á˘bɢ£˘dG .ᢵ˘∏˘¡˘à˘°ùŸG DBT IOɢ˘Ÿ »£©J á©HÉààŸG ájRGƒàŸG äÓYÉØàdG ¿G ~Lhh .âHôL á©HÉààŸG ájRGƒàŸGh …RGƒàŸG äÓYÉØàdG ɪgh äÓYÉØàdG áµÑ°T øe ÚæKG ¿Gh . Biphenyl (BP) and cyclohexylbenzene (EBP) BP IOÉe ¤G DBT IOÉe πjƒëàd ᣰûæŸG ábÉ£dG º«b ¿G .IQGô◊G áLQO ≈∏Y ~ªà©J CHBIOÉe ¤G BP IOÉe áÑ°ùf ¿Gh . BP and CHB ÚJOÉe ™jRƒàd π° aG ∞°Uh èFÉàædG ¿Gh . ECHB2 < EBP< ECHB1 ‹RÉæJ πµ°T ‘ É¡ª«b I~Lh CHB (ECHB2) ¤G BP IOÉe πjƒ–h (ECHB1) CHB ¤G IOÉŸG ¢ùØf πjƒ–h .iõØ◊G πª©dG ≈∏Y ÚJOÉe êÉàf’ äOÉb iõØ◊G πeÉ©dG ≈∏Y áØ∏àfl ™bGƒe OƒLh ìÎ≤J .…QÉŒ »àjÈc …õØM πeÉY, Cyclohexylbenzen, Biphenyl, CoMo/A12O3, ÚLhQ~«¡dG ᣰSGƒH âjȵdG ádGREG äÓYÉØJáÑ«côJ :á«MÉàØŸG äGOôØŸG Abstract: Kinetics of hydrodesulfurization of dibenzothiophene (DBT) has been studied on a commercial CoMo/γ-Al2O3 catalyst at 633 - 683 K and 10 atm. A low DBT concentration typically obtained in hydrodesulfurization operations was used. Pseudo-first-order model was found to fit the experimental data for the consumption of DBT. The activation energy for the conversion of DBT was found to be 51.7 kcal/mol. Biphenyl (BP) and cyclohexylbenzene (CHB) were obtained as dominant products. For the reaction network, both parallel and parallel-sequential routes were explored. The latter was found to give a better description of the BP and CHB distributions. The ratio of BP to CHB depended on the reaction temperature. The values of activation energies of DBT hydrogenolysis to BP (EBP), DBT hydrogenation to CHB (ECHB1) and hydrogena- tion of BP to CHB (ECHB2) were found to be in a decreasing order of ECHB2 > EBP > ECHB1. The result suggests the pres- ence of different catalytic sites leading to the two products on the catalysts. Keywords: Hydrodesulfurization, Dibenzothiophene, CoMo/Al2O3, Biphenyl, Cyclohexylbenzene, Kinetics 1. Introduction The Environmental Protection Agency (USEPA) guide- lines, for instance, restrict the amount of sulfur allowed -2 3 Research interest in catalytic hydrodesulfurization in diesel fuels to 1.3 x 10 g/dL by the year 2010 (HDS) of petroleum feedstock has been increasing recent- (USA-EPA, 2001). This requires reduction of about ly. This is partly due to the necessity of processing feed- 97% from the current level. Achieving this limit will stocks that have large amounts of sulfur-containing com- require a major improvement or redesign of the catalysts pounds and also as a result of more stringent environmen- employed for HDS. tal regulations with respect to emission of sulfur-contain- Therefore, it is necessary to gain further understanding ing gases to the atmosphere. Sulfur removal has econom- of the chemical reactions involved. This could be ic benefits of prolonging catalyst life and decreasing cor- achieved by studies of kinetics and networks of the reac- rosion of the process equipment downstream. Perhaps one tions. Dibenzothiophene (DBT), as a typical sulfur-con- of the most important factors that spurred interest in HDS taining compound in the petroleum feedstock, has been is a recent review of the allowable sulfur content of fuels, studied extensively. A recent study has shown that at cer- flue gases and other petroleum products, in the U.S. tain conditions, the DBT kinetic closely resembles that of ______________________________________ an overall behavior of DBTs in a light oil fraction (Steiner, et al. 2002). Aspects of the HDS of DBT kinetics have *Corresponding Author's e-mail: [email protected] 39 The Journal of Engineering Research Vol. 3, No. 1 (2006) 38-42 been reported. The reaction was shown to follow a pseu- solvent at 623 K until H2S breakthrough was observed. do-first-order kinetics and both parallel and consecutive reaction networks were identified (Girgis, et al. 1991; 2.2 Catalyst Testing Farag, et al. 1997; Wang, et al. 2004; Pille, et al. 1994; The catalysts evaluation was carried out by passing Broderick, et al. 1981). When the reaction of DBT was solutions containing dibenzothiophene at a concentration tested on CoMo/MCM-41, a pseudo-first-order kinetic of one or two weight percent in the hydrogen donor sol- model was found to represent the experimental data better vent (tetralin) through a fixed bed containing 1 g of the than a Langmiur-Hinshelwood model. This is a typical catalyst particles. The catalyst particle sizes and reaction observation for low DBT concentrations. It has also been conditions were chosen to minimize mass transfer limita- shown that the relative significance of hydrogenolysis and tions, based on separate preliminary experiments. The use hydrogenation routes depended on Co/Mo ratios (Wang, of a hydrogen donor solvent ensured an excess supply of et al. 2004). Furthermore, the two routes could also be hydrogen readily available for the reaction without a need substantially affected by the presence of Naphthalene or for adding gas phase hydrogen. The reaction was conduct- hydrogen sulfide for the HDS of DBT or 4,6- ed at temperature range of 633 - 683 K, pressure of 10 dimethyldibenzothiophene (Farag, et al. 1999). MPa, and flowrates required to give measurable conver- Sulfided promoted CoMo/Al2O3 and other transition sions were between 0.5 and 2 g/min. Details on catalyst metal catalysts have been widely researched and shown to testing and GC analysis were reported earlier (Al- be effective for the reaction (Topsoe, et al. 1984; Prins, et Zeghayer, 2005). al. 1989; Yang, et al. 2002; Venezia, et al. 2002; Damyanova, et al. 2003; Papadopoulou, et al. 2003; Al- 3. Results and Discussion Zeghayer, et al. 2005). Further understanding of the nature of interactions between the reactants and catalysts The major products identified were biphenyl (BP) and and effects of reaction conditions thereupon is important. cyclohexylbenzene (CHB). Negligible amounts of bicy- Therefore, the objective of this work was to describe the clohexyl were detected at high conversions. The conver- kinetics of DBT hydrodesulfurization on a sulfided com- sion and products distribution over the catalyst are shown mercial CoMo/γ-Al O catalyst. Effects of variation of 2 3 in Fig. 1. The figure shows variation in DBT conversion reaction temperature and feed flow on products were and BP and CHB selectivities with inverse of weight explored. hourly space velocity (WHSV) from 0 - 250 g catalyst h/g feed. The effect of hydrogen concentration on the reaction 2. Experimental was neglected as it was considered to be in excess. The DBT degree of conversion increased rapidly from 0 to 2.1 Materials 92% in the inverse WHSV range. At the initial conversion, The materials used for the catalyst testing were BP appeared to be an exclusive product indicating that it Dibenzothiophene (DBT), Tetralin, Decalin and is a primary product. At the highest conversion, its selec- Dimethyldisulfide (CH3)2S2 (DMDS). All chemicals were tivity decreased to 80% with a corresponding increase in of analytical grade quality supplied by Aldrich Chemical the selectivity to CHB from 0 to 20%. Company. A commercial catalyst was obtained from a At certain conditions, the kinetics of HDS of supplier. Its composition and physical properties as given by the manufacturer are shown in Table 1. The catalyst was sulfided to a catalytically active form using a solution of dimethyldisulfide (2 wt% sulfur equivalent) in decalin Table 1. Chemical and physical properties of the commercial catalyst Components wt% dry basis Molybdenum oxide (MoO 3) 15.40 Cobalt oxide (CoO 3.20 Sodium oxide (Na 2O) 0.03 Iron (Fe) 0.03 Sulphate (SO 4) 0.30 Conversion (%) and Selectivity Silicon dioxide (SiO 2) 0.10 Physical properties (1/WHSV)*1000 (g cat. h/g feed) Poured bulk density, lb/ft 3 33.0 Compacted bulk density, lb/ft 3 36.0 Figure 1. Variation of DBT conversion and products Crush strength, lb/m m 3.4 2 distribution with 1/WHSV Surface area, m /gm 310.0 Pore volume, cm 3/gm 0.8 40 The Journal of Engineering Research Vol. 3, No. 1 (2006) 38-42 Dibenzothiophene (DBT) was shown to represent that of an aggregate of DBTs in a petroleum feedstock (Steiner, 0.28 Effect of temperature Effect of flow rate 2002). In an earlier study of kinetics of HDS of DBT in liquid feed, the data were well fitted to a Langmuir- 0.24 Hinshelwood (L-H) model in a plug-flow reactor (Singhal, et al. 1981). However, at a lower DBT concen- tration, as expected, a first order relation between the rate of consumption of DBT and concentration was obtained.