Energy Fuels 2010, 24, 518–524 Published on Web 12/21/2009
: DOI:10.1021/ef900876j
Comparative Studies on the Bio-desulfurization of Crude Oil with Other Desulfurization Techniques and Deep Desulfurization through Integrated Processes Prachi Agarwal and D. K. Sharma* Centre for Energy Studies, Indian Institute of Technology (IIT) Delhi, Hauz Khas, New Delhi 110016, India Received August 13, 2009. Revised Manuscript Received December 4, 2009
With an increase in environmental degradation, the environmental norms are becoming more and more stringent day by day. Research work is currently being undertaken around the world to bring about deeper desulfurization of crude oils. Presently, two crude oil samples, i.e., heavy crude oil (HCO, 1.88% S) and light crude oil (LCO, 0.378% S) were procured from a local petroleum refinery. Bio-desulfurization (BDS) of LCO using Pantoea agglomerans D23W3 resulted in 61.40% removal of sulfur, whereas HCO showed 63.29% S removal under similar conditions. The use of P. agglomerans D23W3 under anaerobic conditions showed marginally better results than those under aerobic conditions. However, the use of thermophile Klebsiella sp. 13T resulted in 62.43% S removal from LCO and 68.08% S removal from HCO. Studies have also been extended on the use of reactive adsorption techniques for the removal of sulfur from crude oils. Among all of the different adsorbents studied, residual coal obtained after the solvent extraction of Samla coal showed the maximum removal of sulfur, i.e., 78.90% from LCO and 74.46% from HCO. Hydrodesulfurization is an established technology for the deep desulfurization of petroleum and its products. Widening the choice and exploring further developments in deeper desulfurization of petroleum alternative technologies may also be developed. Comparative studies were extended on the use of BDS, reactive adsorption, oxy-desulfurization, photo-desulfurization, and solvent extraction for the removal of sulfur from HCO and LCO. Among all of these techniques, oxy-desulfurization was found to be the best. To study the deep desulfurization of LCO and HCO, two- and three-step integrated processes were developed for the removal of sulfur from HCO and LCO. BDS under anaerobic conditions followed by oxydesulfurization followed by reactive adsorption integration resulted in maximum removal, i.e., 95.21% removal of sulfur from HCO and 94.30% removal of sulfur from LCO.
way, this process has an advantage over the other processes of desulfurization of oils. The average amount of total sulfur in crude oil may vary from 0.03 to 7.89 wt %. The type of sulfur present in crude oil can be divided into two forms: inorganic and organic sulfur. Inorganic sulfur is present as elemental sulfur, sulfides, pyrites, H2S, etc. Organic sulfur is present as an aromatic or a saturated form of thiols, thiophenes, heterocyclic sulfides, etc. Crude oil with higher viscosity and density contains more sulfur compounds. Distillation fractions with a higher boiling point contain higher concentrations of sulfur compounds.2,3 Aromatic compounds, such as DBT or its derivatives, are of significant importance because they have higher boiling points (more than 200 °C) and it is difficult to remove them from an atmospheric tower outlet stream (e.g., middle distillates).4 Various processes have been reported in the literature, such as solvent extraction (SE), oxidative desulfurization, adsorption, bio-desulfurization (BDS), etc., which are efficient and cost-compatible. In the future, it may be desirable to bring down S to an almost 0 level,1 forcing intensive research into the development of different processes. Because of the increasing trend in desulfurization of fossil fuel by SE, there is a need to economize desulfurization energy requirements and decrease CO2 production that is
1. Introduction Fossil fuels, such as crude petroleum oil, are contaminated with sulfur compounds along with other N-heterocyclic and metallic compounds. Direct combustion of these fuels results in environmental pollution because of SOx and NOx emissions and the greenhouse effect because of CO2 emissions. Environmental norms are becoming more stringent and demand deeper desulfurization of liquid fuels. The new requirements for sulfur content in liquid fuels demand the use of novel methods of desulfurization. The current technology of hydrodesulfurization (HDS) is quite adequate for the present sulfur standards;1 however, the nature of the active sites of these catalysts remains somewhat unclear. Besides, these HDS processes generally require extreme conditions of temperature and pressure to obtain significant desulfurization. These also involve the use of costly hydrogen. HDS processes are also plagued by the problems of catalyst poisoning, which are costlier. Even most of the S-heterocyclic compounds, such as dibenzothiophene (DBT) and its derivatives, are quite refractory to HDS and usually require sophisticated catalysts. However, the HDS process is commercially successful in bringing about the deeper desulfurization of liquid fuels using sophisticated catalysts and higher hydrogen pressure. In this
(2) Kropp, K. G.; Fedorak, P. M. Can. J. Microbiol. 1998, 44, 605– 622. (3) Schulz, H.; Bohringer, W.; Ousmanov, F.; Waller, P. Fuel Process. Technol. 1999, 61, 5–41. (4) Shennan, J. L. J. Chem. Technol. Biotechnol. 1996, 67, 109–123.
*To whom correspondance should be addressed: Centre for Energy Studies, Indian Institute of Technology (IIT) Delhi, Hauz Khas, New Delhi 110016, India. Telephone: 91-11-26591256. E-mail: sharmadk@ ces.iitd.ernet.in. (1) Song, C. Catal. Today 2003, 86, 211–263. r 2009 American Chemical Society
518
pubs.acs.org/EF
Energy Fuels 2010, 24, 518–524
: DOI:10.1021/ef900876j
Agarwal and Sharma
Table 1. Percentage Desulfurization of Crude Oil through BDS (Aerobic and Anaerobic Conditions) sample
S in untreated sample (%)
S in treated sample (%)
desulfurization (%)
S in HCO S in LCO BDS of HCO with P. agglomerans D23W3 under aerobic conditions OD of HCO OD of LCO BDS of HCO with P. agglomerans D23W3 under anaerobic conditions BDS of HCO with Klebsiella sp. 13T BDS of LCO with P. agglomerans D23W3 under aerobic conditions BDS of LCO with P. agglomerans D23W3 under anaerobic conditions BDS of LCO with Klebsiella sp. 13T
1.88 0.378 1.88 1.88 0.378 1.88 1.88 0.378 0.378 0.378
1.88 0.378 0.690 0.200 0.090 0.580 0.600 0.150 0.138 0.142
63.29 89.36 76.19 69.14 68.08 60.32 63.40 62.43
associated with the other desulfurization processes, such as HDS. However, the recovery and recycling of these solvents during the desulfurization process would be required.4 Thus, only that organic solvent extraction process can be used, which may afford recycle or recovery of these liquids. Of the various non-HDS methods, adsorption is an emerging area as one of the desired unit operations because of its simplicity, being energy-efficient, and also meeting the stringent environmental legislation.5,6 A few examples include softening of water using zeolitebased material, adsorption of CO2 and H2S by suitable adsorbents, demetalation of waste streams from industries, and adsorbents for the pretreatment of fuels for fuel cell applications.7-9 Research on adsorption desulfurization of liquid fuels has gained much importance over the past few years. Adsorption, which is a surface phenomenon, depends upon the higher specific surface area, the narrow particle size distribution, and the porosity of an adsorbent.10 Developments in this area are mainly because of the strict environmental regulations that are being implemented by the United States Environmental Protection Agency (EPA) and other environmental agencies around the world for SOx emission (which are the precursors for acid rain) and also the limitations of the current refinery process, i.e., HDS of fuels for sulfur removal. HDS uses H2 under high pressure and high temperature, which uses hydrogen to remove the sulfur compounds present in the fuel. To combat the issues involved with HDS, researchers have been working on adsorption desulfurization of S compounds with various adsorbents.5,6,11,12 However, the sulfur specificity is a big problem with this process. There have been many adsorbents that have been developed for adsorption, but none has not been commercialized on an industrial level.12,13 In the present study, high ash coal has been used because it is polyaromatic and is known to possess good microporosity and, thus, a large surface area, although the use of the pore in the reaction/interaction of the surface is poor. Attempts to improve either the surface
or porosity of coal by different coal conversion processes, such as oxidation, pyrolysis, and SE, separately or in combination seem to be a rewarding field of research on adsorption. SE of coal may result in increased porosity (because of selective dissolution of specific organic components, i.e., leaching of coal by solvent) and a reduction of particle size. These, in turn, may improve the adsorption capacity of coal. Fly ash is inexpensive because of the availability and its good adsorption characteristics; therefore, this makes it an alternative media for the removal of organic compounds from aqueous solutions. In the present study, fly ash was used as a low-cost adsorbent. Another process of desulfurization that has been used for the desulfurization of liquid fuels these days is oxydesulfurization (OD). The general steps involved in OD are, first, oxidation of organosulfur compounds in fuels, followed by the removal of oxidized sulfur-containing compounds from the treated fuels. The greatest advantage of OD as compared to conventional HDS is that it can be carried out in the liquid phase under very mild conditions, about near room temperature and, above all, under atmospheric pressure. The chemistry involved in the OD process is that the divalent sulfur can be oxidized by the electrophilic addition reaction of oxygen atoms to form the hexavalent sulfur of sulfones, because of the chemical and physical differences of sulfones as compared to hydrocarbons in fuel oil. Therefore, they can be easily removed using distillation, SE, adsorption, and decomposition. Researchers have studied several oxidants.14,15,6 Oxidants, such as nitrogen oxides, nitric acid, hydrogen peroxide, ozone, t-BuOOH, oxygen, air, and per acids, may be used. Another approach to produce ultra-low sulfur fuels is BDS, which can selectively remove sulfur from DBTs. BDS has the potential benefits of lower operation cost and production of valuable byproducts. Sulfur compounds can be converted into hydroxylbiphenyl and its derivatives.16 BDS can be considered either an alternative or a complementary method to the conventional oil-refining technology. Some of the isolated microorganisms capable of sulfur removal are not effective in commercial uses. Therefore, there is still a need to increase the rate of sulfur removal that may efficiently bio-desulfurize the liquid fuels. One of the major objectives of the present work was to develop processes of desulfurization of crude oil under milder conditions of atmospheric pressure without using hydrogen under pressure and to integrate different desulfurization techniques for bringing about the deep desulfurization of petroleum crude oil.
(5) Earle, M. J.; Esperanc-a, J. M. S. S.; Gilea, M. A.; Lopes, J. N. C.; Rebelo, L. P. N.; Magee, J. W. Nature 2006, 439, 831–834. (6) Song, C. S.; Ma, X. Int. J. Green Energy 2004, 1 (2), 167–191. (7) Hernandez-Maldonado, A. J.; Yang, R. T. J. Am. Chem. Soc. 2004, 126 (4), 992–993. (8) Choi, J. H.; Kim, S. D.; Kwon, Y. J.; Kim, W. J. Microporous Mesoporous Mater. 2006, 96 (1-3), 157–167. (9) Namasivayam, C.; Sumithra, S. J. Environ. Manage. 2005, 74, 207–215. (10) Kao, P. C.; Tzeng, J. H.; Huang, T. L. J. Hazard. Mater. 2000, 76, 237–249. (11) Li, Y. W.; Li, Y.; Yang, R. T. J. Phys. Chem. B 2006, 110 (34), 17175–17181. (12) Velu, S.; Song, C. S.; Engelhard, M. H.; Chin, Y.-H. Ind. Eng. Chem. Res. 2005, 44, 5740–5749. (13) Wang, Y. H.; Yang, F. H.; Yang, R. T. Ind. Eng. Chem. Res. 2006, 45 (22), 7649–7655.
(14) Aida, T; Yamamoto, D.; Iwata, M.; Sakata, K. Heteroatom Chem. 2000, 22, 241–249. (15) Zannikos, F.; Lois, E.; Stournas, S. Fuel Process. Technol. 1995, 42, 35–45. (16) Luo, M. F.; Xing, J. M.; Liu, H. Z. Biochem. Eng. J. 2003, 13, 1–6.
519
Energy Fuels 2010, 24, 518–524
: DOI:10.1021/ef900876j
Agarwal and Sharma
2. Materials and Methods
Table 2. Percentage of Desulfurization Using RA in HCO (Sulfur = 1.8%)
2.1. Petroleum Oils. The light crude oil (LCO) and heavy crude oil (HCO) containing 0.378 and 1.88% of sulfur, respectively, were procured from a local petroleum refinery in India. 2.2. Microorganism. The DBT-desulfurizing isolates enriched and cultivated from the soil samples earlier in the laboratory of the authors viz. Pantoea agglomerans D23W3 and Klebsiella sp. 13T were used in the present work.17 The surfactant producing bacterial strains Bacillus cereus SG I and Staphylococcus sp. LFA has been used for the production of a surfactant that has been used for the enhancement of the desulfurization activity. The surfactant produced by B. cereus SG I has been assigned a notation as SG I, and the surfactant produced by Staphylococcus sp. LFA has been assigned a notation as LFA. 2.3. Medium. For the growth of organic-sulfur-degrading microorganisms, 5.0 g of sulfur-free medium (SFM) glucose, 2.0 g of NH4CI, 6.3 g of KH2PO4, 8.0 g of K2HPO4, 0.2 g of MgCl2 3 6H2O, 2 mL of metal solution (2.0 g of CaCl2, 1.0 g of NaCl, 0.5 g of FeCl2 3 4H2O, 0.5 g of ZnCl2, 0.5 g of MnCl2 3 4H2O, 0.1 g of Na2MoO4 3 2H2O, 0.05 g of CuCl2, 0.05 g of Na2WO4 3 2H2O, 10 mL of 10 M HCl, and 1000 mL of distilled water), 1 mL of vitamin mixture (400 mg of calcium pantothenate, 200 mg of inositol,400 mg of niacin,400 mg of pyridoxine-HCl, 200 mg of p-aminobenzoic acid, 0.5 mg of cyanocobalamin, and 1000 mL of distilled water), and 1000 mL of distilled water were used. Storage media for preserving microorganisms at -70 °C Luria-Bertani (LB) broth with 10% glycerol (v/v) were used. 2.4. Chemicals. The chemicals were purchased from E. Merck Ltd., India, through Lab Sales Corp., New Delhi, India. All other chemicals and reagents were of analytical grade and were obtained from Qualigens Fine Chemicals, Loba Chemie Co. Ltd., Hi Media, and Rankem (Ranbaxy Fine Chemicals). 2.5. Preparation of Adsorbents. Samla coal (10 g) was taken in a 500 mL round-bottomed flask containing ethylenediamine (EDA) (10 mL) and N-methyl-2-pyrolidione (NMP) (170 mL). A coal/EDA ratio of 1:1 (wt/vol %) and a coal/NMP ratio of 1:17 (wt/vol %) were used. The mixture was refluxed for 2 h and filtered. The residual coal (RC) obtained after filtration was dried overnight in an oven at 105 °C. The RC was washed with 2% aqueous HCl to remove EDA and then with distilled water to remove excess acid. Final washing was performed in a Soxhlet apparatus with a 1:1 methanol/water mixture. Activated carbon and activated charcoal were procured through Lab Sales Corp., New Delhi, India. Fly ash was obtained from a local thermal power station. A fluidized catalytic cracking (FCC) catalyst and Z catalyst were procured from Indian OiI Corporation Ltd. (IOCL) Research and Development, Faridabad, India. 2.6. Desulfurization Using Isolate P. agglomerans D23W3 and Klebsiella sp. 13T. Fresh cultures of P. agglomerans D23W3 and Klebsiella sp. 13T were inoculated separately in SFM containing petroleum oils individually, keeping an initial concentration of 2%. Inoculated flasks were incubated at 30 °C for 72 h (for desulfurization experiments) under shaking conditions (200 rpm). For the BDS experiment, after 72 h, the culture broth was harvested. The oil was separated by centrifugation at 10000g for 10 min. The total sulfur content was then measured in the treated oil. The experiments were performed in triplet, and the average of the values was taken as the final result. 2.7. Desulfurization Using Oxidative Desulfurization, i.e., OD. Oxidation of DBT or crude oil was performed by adding the catalyst sodium hydrogen sulfate (0.207 g) and sodium tungstate (0.495 g) to the octane solution of DBT while stirring and heating the mixture to 40 °C in a water bath. A solution of 30% aqueous hydrogen peroxide was then added dropwise. After 3 h, the reactant was taken to a separating funnel and
adsorbent S in HCO RC activated charcoal fly ash super clean coal activated carbon Z catalyst FCC catalyst
S in HCO before treatment (%)
S in HCO after treatment (%)
desulfurization in HCO (%)
1.80 1.80 1.80
1.88 0.48 0.58
73.33 68.77
1.80 1.80
0.62 0.60
68.77 67.02
1.80
0.60
68.05
1.80 1.80
0.605 0.59
68.05 68.61
Table 3. Percentage of Desulfurization Using RA in LCO (Sulfur = 0.378%) adsorbent S in LCO RC activated charcoal fly ash super clean coal activated carbon Z catalyst FCC catalyst
S in LCO before treatment (%)
S in LCO after treatment (%)
desulfurization in LCO (%)
0.378 0.378 0.378
0.378 0.08 0.12
78.90 68.20
0.378 0.378
0.14 0.10
63.00 73.50
0.378
0.15
60.30
0.378 0.378
0.20 0.09
47.08 76.30
shaken mechanically for 10 min. The top layer was separated, washed successively with H2O2, and finally, dried over anhydrous calcium chloride. 2.8. Desulfurization Using SE. The general procedure for SE was to mix the appropriate amount of solvent and crude oil in a separatory funnel and to shake them mechanically for 30 min. The phases were then allowed to separate, and their volumes and weights were measured. Finally, the solvent was removed by washing repeatedly with distilled water, and the hydrocarbon phases were analyzed for their sulfur content. 2.9. Desulfurization Using Reactive Adsorption (RA). The adsorption studies were performed by taking an oil/adsorbent ratio of 100 mL g-1 in a conical flask. The adsorption studies were carried out at different time intervals to attain the maximum desulfurization. 2.10. Desulfurization Using Photo-desulfurization (PD). The studies were performed using a crude oil/solvent ratio of 1:10. The solvent used was sulfur-free toluene. To this mixture, titanium oxide as a catalyst was added. After this, the reaction mixture was subjected to UV radiation for 24 h and the mixture was filtered and dried.
3. Results and Discussion Earlier research work in the laboratory of the authors was concentrated on the development of integrated processes for the deep desulfurization of coals and lignites.17 Research work was then extended on the development of suitable and convenient processes of BDS of DBT and its derivatives. Different microbes were isolated that followed a nondestructive 4S pathway in removing S from DBT. One such bacteria, i.e., P. agglomerans D23W3, was used to degrade DBT and its derivatives. This was further used to study the BDS of diesel, HDS of diesel, air turbine fuel (ATF), LCO, and HCO.
(17) Bhatia, S. Studies on biorefining of fossil fuels (lignites, coals and petroleum oils). Ph.D. Thesis, Indian Institute of Technology (IIT) Delhi, New Delhi, India, 2007.
520
Energy Fuels 2010, 24, 518–524
: DOI:10.1021/ef900876j
Agarwal and Sharma
Table 4. Deep Desulfurization of Petroleum Crude Oils Achieved through the One-Step Process sample
treatment
S before treatment (%)
S after treatment (%)
desulfurization (%)
HCO LCO HCO LCO LCO HCO HCO treated with biosurfactant of B. cereus SGI LCO treated with biosurfactant of B. cereus SGI HCO treated with biosurfactant of Saphylococcus sp. LFA LCO treated with biosurfactant of Saphylococcus sp. LFA
SE with NMP SE with NMP PD PD OD OD BDS BDS BDS BDS
1.88 0.378 1.88 0.378 0.378 1.88 1.88 0.78 1.88 0.378
0.52 0.18 0.69 0.19 0.09 0.20 0.48 0.08 0.58 0.11
72.34 65.00 63.00 66.00 77.00 89.36 74.46 78.84 69.14 70.89
Table 5. Desulfurization of Petroleum Crude Oils Achieved through Two-Step Integrated Processes sample
S present (%)
HCO LCO HCO LCO HCO LCO HCO
1.88 0.378 1.88 0.378 1.88 0.378 1.88
LCO
0.378
HCO LCO HCO LCO HCO LCO
1.88 0.378 1.88 0.37 1.88 0.378
first-step treatment RA RA OD OD OD OD BDS (AN) BDS (AN) OD OD RA RA OD OD
S present after the first step (%)
desulfurization (%)
0.48 0.08 0.20 0.09 0.20 0.09 0.58
74.46 78.90 89.36 77.00 89.36 77.00 69.14
0.138 0.20 0.09 0.48 0.08 0.20 0.09
second-step treatment
S present after the second step (%)
desulfurization (%)
BDS (AO) BDS (AO) BDS (AO) BDS (AO) BDS (AN) BDS (AN) RA
0.39 0.07 0.18 0.04 0.17 0.069 0.34
79.25 81.50 90.42 83.60 90.95 81.70 81.90
63.40
RA
0.041
89.07
89.36 77.00 74.46 78.90 89.36 77
RA RA OD OD SE SE
0.24 0.03 0.34 0.056 0.19 0.031
87.23 92.00 81.90 85.00 89.89 91.8
conserving the fuel value of oils.17 The use of Klebsiella sp. 13T in the BDS of HCO was found to remove 68.08% S. The use of Klebsiella sp. 13T was found to show more desulfurization of LCO than that of HCO. The use of P. agglomerans D23W3 was found to be better in BDS than Klebsiella sp. 13T (Table 1). 3.2. Studies on the RA of S from Crude Oils. Earlier research work from the laboratory of the authors had shown that adsorption was a good technique for the removal of hazardous wastes, such as phenols, from the wastewater.18 Because RA is also known to remove sulfur from crude oils, therefore, the use of this technique was extended for the removal of S from the HCO and LCO. Efforts were made to find inexpensive and more effective adsorbents. The use of the following adsorbents, such as activated carbon, Z catalyst, FCC catalyst, fly ash, N-methyl-2-pyrolidione (NMP), extracted RC obtained after extraction with NMP containing a small amount of EDA, i.e., e,N solvent system, fly ash, and activated charcoal, was made. The use of the Z catalyst and FCC catalysts was found to show about 68% removal of S from the HCO (Table 2). These are the traditional materials used for the RA of S compounds from crude oil. In the present study, it was decided to use the RC obtained after the solvent extraction of coal by using NMP as a solvent and EDA as a co-solvent. In fact, in the earlier studies from the laboratory of the authors, the RC was found to show very good adsorption potential for phenol.18 Interestingly, the RC was found to show almost 75% (Table 2) removal of sulfur from HCO. The use of activated carbon had shown the removal of 68% S from the HCO. The use of super clean coal (SCC)
Presently, research work was further extended on the BDS of LCO and HCO using P. agglomerans D23W3. About 61.4% desulfurization was obtained from the LCO. In fact, P. agglomerans D23W3 is a facultative anaerobe; therefore, attempts were also made to study the BDS of LCO under anaerobic conditions, which showed 2% improvement in the desulfurization of LCO (Table 1). Because the BDS of crude oil has mass-transfer limitations, therefore, it was decided to use the biosurfactants (SGI and LFA) isolated by the authors from the oil field bacteria. The use of those produced by the B. cereus SGI and Staphylococcus sp. LFA resulted in enhancing the desulfurization of LCO through BDS using P. agglomerans D23W3 up to 78.84%, whereas that of LFA resulted in enhancing the same up to 70.8%. The action of B. cereus SGI was found to be better than that of Staphylococcus sp. LFA (Table 1). The BDS of HCO using P. agglomerans D23W3 under aerobic conditions resulted in the removal of 63.3% S. Interestingly, the use of P. agglomerans D23W3 under anaerobic conditions showed the removal of 69.14% S. The BDS of LCO resulted in the removal of more S than that of HCO (Table 1). 3.1. BDS of Crude Oil Using Thermophilic Bacteria. There are several advantages of using thermophiles for the BDS of crude oils. In fact, the temperature inside the crude oil wells is high. The use of thermophiles would help in restricting the contamination by keeping the conditions sterile. Plenty of water is already available in the oil wells, and therefore, the removal of water may not be required. The use of higher temperatures for BDS may afford faster kinetics. Research work in the laboratory of the authors had resulted in the isolation of a thermophile Klebsiella sp. 13T from the contaminated oil field soils. This bacteria was found to degrade DBT by following the 4S pathway, thus
(18) Ahmaruzzaman, M.; Sharma, D. K. J. Colloid Interface Sci. 2005, 287, 14–24.
521
: DOI:10.1021/ef900876j
Agarwal and Sharma
0.58 0.138 0.69 0.15
69.14 63.40 63.29 61.4
OD OD OD OD
0.11 0.07 0.23 0.08
94.15 81.48 87.76 78.84
RA RA RA RA
0.09 0.053 0.10 0.022
95.21 85.9 94.6 94.3
obtained after extraction of Samla coal with an e,N solvent system also showed only 68% removal of S from the HCO. Table 3 shows the removal of S from the LCO through RA. Here again, the RC showed almost 79% removal of S from the LCO (Table 3). The use of the FCC catalyst showed 76.3% removal of S from the LCO. Activated carbon showed only 60%, while activated charcoal showed 63.2% removal of sulfur from the LCO. The SCC also showed good results of the removal of 73.5% sulfur from the LCO. It was found that the RC was a good adsorbent for the removal of S from the HCO as well as LCO. The reason for this could be as follows: The RC is obtained after the removal or leaching of organic compounds (including thiophenic S compounds) from the bituminous coal. This would result in increasing the porosity, surface area, and active sites on coal. Because coal has predominantly aromatic character, therefore, thiophenic sulfur compounds from the LCO and HCO would be attracted toward the active sites on the coal surface by following the like dissolves like principle. The role of mineral matter present in the RC cannot be under-emphasized. Removal of thiophenic and other polar compounds from the coal by SE using NMP containing the small amount of EDA may also leave deficient sites for the sulfur compounds on coal. This was supported by the earlier studies on the organic desulfurization of coals and lignites from the laboratory of the authors.19 In fact, the studies of the authors also confirmed that the Brunauer-Emmett-Teller (BET) surface area of the Samla coal in the RC increases after the extraction of coal by e,N solvent systems. This was also supported by the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies.20 Surprisingly, the Z catalyst was found to show poor (47%) desulfurization of LCO. This could be due to the reason that LCO may be containing certain other N- or O-heterocyclic compounds along with substituted aromatics, which had a larger tendency for physical bonding and coordination at the active sites of the Z catalyst. Thus, these would have become preferentially adsorbed on the Z catalyst. This would reduce the efficiency of the Z catalyst for adsorbing S compounds from the LCO. It seems the HCO did not have such compounds to deactivate the Z catalyst. However, further research on this would be required. 3.3. Comparison of BDS with Other Techniques of Desulfurization of Crude Oil. Studies were further extended to compare the results of BDS of LCO and HCO with other desulfurization techniques, such as RA, SE, PD, and OD. Table 4 shows the results. It was found that about 72.34% S was removed from the HCO by SE using NMP as a solvent. About 65% S was removed from the LCO, showing that it was possible to remove more S compounds from the HCO, which was obviously because of the fact that the HCO contains more aromatic S compounds and NMP is an aromatic compound extracting solvent. PD of LCO showed the removal of 66% S from the LCO and 63% S from the HCO, showing that desulfurization of the former was a bit better. Interestingly, OD of HCO showed the removal of 77% S (Table 4). These studies showed that OD was a good technique for the removal of S from the crude oils. HCO showed better results in comparison to those of the LCO, which may be attributed to the fact that HCO offered more polycyclic thiophenic-type compounds for OD reaction (Table 4). The OD was also found to be a better technique of desulfurization of HCO in comparison to the BDS (even in
1.88 0.378 1.88 0.378 HCO LCO HCO LCO
BDS (AN) BDS (AN) BDS (AO) BDS (AO)
third-step treatment desulfurization (%) S present after the second step (%) second-step treatment desulfurization (%) S present after the first step (%) first-step treatment S present (%) sample
Table 6. Percentage Desulfurization of Crude Oils Achieved through Three-Step Integrated Processes
S present after the third step (%)
desulfurization (%)
Energy Fuels 2010, 24, 518–524
522
Energy Fuels 2010, 24, 518–524
: DOI:10.1021/ef900876j
Agarwal and Sharma
the presence of a biosurfactant, i.e., biosurfactant produced by B. cereus SGI). The OD was also found to be better than RA for the removal of sulfur from HCO (Tables 2 and 4). The OD of LCO was found to show comparable results to that of BDS using P. agglomerans D23W3 under aerobic conditions and RA (Tables 1, 3, and 4). About 78% S was removed from the LCO using the desulfurization techniques individually, such as OD, BDS, and RA. The reason for this could be that, if the amounts of S compounds in crude oil are low, then either of these techniques could oxidize adsorb or biochemically oxidize these compounds. When the amount of organic S compounds is higher, the chemical oxidation would be more effective. 3.4. Deep Desulfurization of Crude Oils by Two- and ThreeStep Integrated Processes. Because one of the aims of the present work was to develop the proceses of deep desulfurization of petroleum crude oils, therefore, research work was further extended to develop two- or three-step integrated processes. First, attempts were made to develop two-step integrated processes by integrating BDS with RA or OD. Table 5 shows the results of desulfurization of HCO through the two-step integrated processes. It was found that the twostep integrated process of OD followed by BDS using P. agglomerans D23W3 (under anaerobic conditions), i.e., OD-BDS (AN), showed the maximum desulfurization (91%) (Table 5). This was followed by the two-step integrated process of OD followed by BDS using P. agglomerans D23W3 (under aerobic conditions), i.e., OD-BDS (AO), showing 89.36% S removal (Table 5). Overall, the two-step integrated processes of the desulfurization of HCO showed the following trend: OD-BDS (AN) g OD-SE g OD-BDS (AO) g OD-RA > RA-OD g BDS (AN)-RA > RA-BDS (AO) > BDS (AO)-RA (Table 5). The LCO was also subjected to desulfurization by different two-step integrated processes, and the results have been shown in Table 5. Table 5 shows the results of desulfurization of LCO by two-step integrated processes. The two-step integrated process of OD followed by RA, i.e., OD-RA, showed the maximum desulfurization of 92% (Table 5). This was followed by the two-step integrated process of OD followed by SE, i.e., OD-SE, showing almost the same (92%) desulfurization of LCO (Table 5). Overall, the following trend was observed: OD-RA g OD-SE > OD-BDS (AO) > BDS (AO)-RA > RA-BDS (AO). These studies showed that it was possible to remove an enhanced amount of S from both LCO and HCO by integrating the processes of desulfurization. OD followed by SE was found to be a suitable process for the desulfurization of both LCO and HCO. Among other integrations, it was found that RA should be selected as a last treatment step because this involves adsorption of S compounds. Although OD may be performed as a first step, it may still be preferable to use BDS as the first step because the BDS is performed under milder conditions and, thus, may not alter the chemical composition of crude oils. On the basis of these observations, studies were further extended on developing and
designing three-step integrated processes for obtaining the deeper desulfurization of crude oils. The following three-step integrated processes were selected as follows: BDS using P. agglomerans D23W3 under aerobic conditions followed by OD and then further followed by RA, i.e., BDS (AO)-OD-RA, was the first process. The second process of three-step integration was BDS using P. agglomerans D23W3 under anaerobic conditions followed by OD, which was then followed by RA, i.e., BDS (AN)-OD-RA. The first three-step integrated process, i.e., BDS (AO)-OD-RA, showed 94.3% desulfurization of LCO. The second three-step integrated process, i.e., BDS (AN)-OD-RA, showed 85.9% desulfurization of the LCO. The results are shown in Table 6. Similarly, the first three-step integrated process, i.e., BDS (AN)-OD-RA, showed 94.6% removal of sulfur from the HCO (Table 6). The second three-step integrated process, i.e., BDS (AN)-OD-RA, showed 95.21% desulfurization of the HCO. Among these processes, BDS (AN)-OD-RA was found to be a potential three-step integrated process because it is known that P. agglomerans D23W3 follows the “4S pathway”, i.e., nondestructive pathway for the BDS of crude oil. Although P. agglomerans D23W3 is a facultative anaerobe that brings about BDS under anaerobic conditions, it was still found that the mechanism of BDS of crude oil under anaerobic conditions was not very clear. In fact, HDS is a well-established technique of deeper desulfurization of crude oil. The techniques studied presently involved the use of different chemicals, and these do not bring about more than 95% desulfurization of crude oils. Further, research work may be required before these three-step integrated processes could show better results than HDS. However, because the integrated processes developed presenltly involved milder ambient pressure conditions, therefore, these may require simple reactors and milder reaction conditions and, thus, may be extended to develop these integrated processes further. 4. Conclusions On the basis of the present studies, the following conclusions may be drawn: It is also possible to bring about BDS of crude oils under anaerobic conditions using P. agglomerans D23W3, facultative anaerobic bacteria. The use of thermophillic bacteria Klebsiella sp. 13T, which follows the 4S pathway in BDS of crude oil, results in the removal of 68.08 and 63.43% S from HCO and LCO, respectively. It is possible to enhance the removal of S by BDS using biosurfactant possibly by overcoming the mass-transfer limitations. RC obtained after SE was found to be an effective adsorbent for the removal of S by RA of crude oil. This is an important finding. Among different techniques of desulfurization of crude oils, OD was found to be a potential technique. Some techniques gave better results of desulfurization of HCO in comparison to that of LCO. Among two-step integrated processes of desulfurization of crude oils, OD-SE was found to be good for both LCO as well as HCO. However, among the two-step integrated processes, the following order of desulfurization was obtained: OD-RA g OD-SE > OD-BDS (AN) g OD-SE g OD-BDS (AO). It was also observed that RA may be integrated as the last step. Among the three-step integrated processes, BDS (AO)-OD-RA was found to be good for both LCO and HCO. Almost 95% desulfurization
(19) Das, A.; Sharma, D. K. Energy Sources, Part A 2001, 23 (8), 687– 697. (20) Agarwal, P. Studies towards development of integrated processes for the desulfurisation of fossil fuels and production of biosurfactants for microbial enhanced oil recovery and other uses. Ph.D. Thesis, Submitted to Indian Institute of Technology (IIT) Delhi, New Delhi, India, 2009.
523
Energy Fuels 2010, 24, 518–524
: DOI:10.1021/ef900876j
Agarwal and Sharma
was observed using this three-step integrated process. The integrated processes may also be used for the desulfurization of not only crude oils but also the different petroleum fractions, such as diesel, gasoline, kerosene, ATF, vacuum residue petroleum coke, etc. These integrated processes involve milder atmospheric conditions and are therefore convenient to be adapted in petroleum refineries. HDS of crude oil is plagued by the problems of the use of conditions of high hydrogen pressure at elevated temperature, catalyst poisoning, and hydrogenation of thiophenes, olefins
and other aromatic compounds, besides the cost of hydrogen. However, the two- and three-step integrated processes developed presently may also be further integrated with the HDS process for the ultra-deep desulfurization of crude oils and its different premium fuel fractions as may be desired by environmentalists in the future. Acknowledgment. The authors are thankful to Mr. Shimbu Singh, Panipat Refinery, India Oil Corporation Ltd., Panipat, Haryana, India, for helping us with the analysis of sulfur in crude oil.
524
