321
Ind. Eng. Chem. Res. 1990,29, 321-324 Palladium(I1) Chloride. J. Am. Chem. SOC.1964,86, 3246-3250. Jhaveri, A. S.; Sharma, M. M. Kinetics of Absorption of Oxygen in Aqueous Solutions of Cuprous Chloride. Chem. Eng. Sci. 1967, 22, 1-6. Jira, R. Ethylene and its industrial deriuatiues; Benn: London, 1969; pp 639-659.
Smidt, J.; Hafner, W.; Jira, R.; Sedlmetier, J.; Sieber, R.; Ruttenger, R.; Kojer, H. The Oxidation of Olefins with Palladium Chloride Catalysts. Angew. Chem., Znt. Ed. Engl. 1962, I, 80-88.
Receioed for reuiew October 10, 1989 Accepted October 29, 1989
Desulfurization of Fuel Oil by Oxidation and Extraction. 1. Enhancement of Extraction Oil Yield Patrick S. Tam,tJ-§James R. Kittrell,+and John W. Eldridge**t Department of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts 01003, and KSE, Inc., P.O. Box 368, Amherst, Massachusetts 01004
An oxidative-desulfurization (ODS) process was used, instead of the conventional hydrodesulfurization (HDS) process, to remove sulfur from diesel oils. This ODS process comprises two stages a t ambient conditions: oxidation using nitrogen dioxide or nitric acid, followed by liquid extraction with solvents having the capability to dissolve aromatics and olefinic compounds. y-Butyrolactone is one of the best solvents. The extraction efficiency for atmospheric gas oil (AGO) was improved by preceeding the extraction with oxidation. For a given sulfur removal, the extraction results from oxidized oil showed higher extraction oil yield than the unoxidized oil. This oil yield enhancement was due to the oxidation changing the sulfur compound configurations such that they were more readily extracted by aromatic/ olefinic dissolving solvents. The distribution coefficients of extraction for several sulfur compounds identified in the AGO were increased by over 100% when the oil was oxidized.
A vast variety of sulfur compounds are present in crude oil and synfuel. These sulfur compounds can be classified into four main groups: mercaptans (thiols), sulfides, disulfides, and thiophenes. Fuels from petroleum-refining processes such as distillation (atmospheric or vacuum), cracking (thermal or catalytic), coking, and synfuel refining all contain sulfur compounds in various concentrations. In general, the proportion of sulfur compounds increases with the boiling point range of the oil. Sulfur compounds are undesirable in refining processes because they tend to deactivate some catalysts used in downstream processing and upgrading of hydrocarbons. In liquid products, they contribute to the formation of gummy deposits as reported by Thompson et al. (1949). These deposits could plug the filter of the fuel-handling system of automobiles and other engines or heating devices. Sulfur compounds in fuel oils also cause corrosion to parts of internal combustion engines and refineries, mainly through the formation of oxyacids of sulfur from the products of combustion. Sulfur oxides and oxyacids also contribute to environmental pollution problems, such as acid rain. In recent years, the regulation of sulfur oxide emissions has become much more stringent due to the Clean Coal Act. Thus, the sulfur compounds in petroleum and synfuel are generally undesirable from consideration of the refining processes, emission control, and the quality of the liquid products for direct combustion. Therefore, desulfurization of fuel (both liquid and gaseous) is extremely important in the petroleum-processing industry. A new desulfurization process is presented here in contrast to the conventional hydrodesulfurization (HDS) process presently employed in industry. This desulfurization process is composed of two stages: oxidation, followed by liquid extraction. Guth and Diaz (1974) and fKSE, Inc.
* University of Massachusetts.
$Present address: The Pritchard Corp., Overland Park, KS
66210.
0888-5885/90/2629-0321$02.50/0
Guth et al. (1975) disclosed the use of nitrogen dioxides followed by extraction with methanol to remove both sulfur and nitrogen compounds from petroleum stocks. Tam and Kittrell (1984) described a process for purifying hydrocarbonaceous oils containing both heteroatom sulfur and heteroatom nitrogen compound impurities, such as shale oils, by first reacting the oil with an oxidizing gas containing nitrogen oxides and then extracting the oxidized oil with solvents in two stages (amines and formic acid). The oxidation-extraction process discussed in this paper operates at ambient pressure and low temperature (typically 0-30 " C ) , using nitric acid or nitrogen oxides as oxidants, and one of several polar solvents for extraction. The oxidized products are composed of a liquid phase and a byproduct that is a semisolid-like residue with high sulfur content, hence the reduced sulfur level in the liquid phase. Liquid-liquid extraction is widely used to separate the constituents of a liquid solution by introducing another immiscible liquid. This process offers improved product quality and energy savings. In the petroleum industry, solvent extractions have been used to remove sulfur and/or nitrogen compounds from petroleum distillates and synfuels. The extract oil and solvent are then separated by distillation. In general, employing solvent extraction of petroleum products to remove sulfur creates an oil yield loss, and the sulfur removal is poor. In part 1 of this series, the effect of preoxidation of diesel fuel for sulfur reduction by solvent extraction is discussed, and the extraction oil yield enhancement due to preoxidation treatment is presented. The kinetic modeling of the oxidation reaction will be presented in part 2.
Experimental Section Oxidations were carried out in a semibatch reactor system with nitrogen dioxide bubbling through a porous disk (sparger) into the oil previously charged to the reactor. The extent of oxidation was controlled by the molar ratio, which is defined as the ratio of moles of oxidant charged within the given reaction time to the g-atoms of sulfur plus 0 1990 American Chemical Society
322 Ind. Eng. Chem. Res., Vol. 29, No. 3, 1990 Table 11. Nitrogen Dioxide Oxidation Extraction oil charge, g 300 NO2 flow rate, SIP cm3/min 73 1 air flow rate, SIP I,/min temp, " C 8-20 0.5 reaction time, h molar ratio 0.9 product yield, wt 70 93.0 S content, % 0.56
Table I. ODeration Conditions of Gas Chromatography" helium flow rate, cm3/min 1.0 in column 2.2 in purge 37.8 in vent 40: 1 split ratio temp, "C 300 injector 300 detector temp programming 80 (held for 1 min) initial temp, "C 270 (held for 2 min) final temp, "C 10 rate of temp rise, "C/min 22 total run time, min operating gas in detector, cm3/min 65 hydrogen 100 air
of AGO for Solvent 300 73 1
4-8 0.25 0.4 98.0 1.1
300 73 1 7-10
300 73 1
1.0 1.7
4-6 0.42 0.7
89.0 0.31
97.5 0.88
a Column: 10 m capillary column with bonded methyl silicone, 0.25 mm I.D., and 0.25 micron film thickness
k0
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zol
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6 N02-OYIDIZED AGO, 0 % % S 8 NOt-OXIDIZED AGO, 0 3 I % S SI0 RATIO FOR EXTRACTlaV OF OXIDIZED OIL ARE IN THE FOLLOWING OROER,
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SULFUR REMOVAL EY EXTRACTION
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Figure 2. Effect of oxidation severity on sulfur removal by extraction and on extraction oil yield.
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Enhancement of Extraction Oil Yields by Preoxidation Significant yield loss and poor sulfur removal have been observed in the solvent extraction of petroleum products. Improvement of sulfur reduction can be obtained by raising the solvent-to-oil ratio in extraction; however, this will cause a further decrease in yield. This phenomenon is illustrated in Figure 1. Arabian atmospheric gas oil (AGO) with a sulfur content of 1.19% and high content of paraffinic components was
oxidized in the semibatch reactor system with different oxidation severities, which were controlled by the molar ratio used, as indicated by the sulfur content of the oxidized oil (Table 11). These oxidized AGO batches represent products from mild through severe oxidation conditions, with sulfur contents of 1.1%, O M % , 0.56%, and 0.31%. Each of these oxidized oils was used for extraction by y-butyrolactone at four different S / O ratios: 0.1, 0.2, 0.5, and 1.0. The effect of oxidation of AGO on lactone extraction is shown in Figure 2. The results from extracting the unoxidized oil with lactone are also included for comparison. Figure 2 clearly shows that oxidation of AGO enhanced the raffinate oil yields in the extraction with y-butyrolactone. The lines relating oil yield to sulfur reduction from extracting the preoxidized oils are all located well above that of the unoxidized oil, indicating that extraction was much more effective when the oil was pretreated with oxidation. Note that all lines relating oil yield to sulfur reduction should converge to the 100% extraction oil yield when no sulfur is removed. For the same percent sulfur removal (in the extraction step only), the extraction oil yields were higher for oxidized oil than for unoxidized oil, even when a very mild oxidation treatment (1.1% S in product oil) was employed. This enhancement was further improved with increasing severity in oxidation. Associated with this enhancement there would also be a n extremely important saving in solvent recovery. A S/O ratio of 5.0 was required to remove 50% of the sulfur from unoxidized oil (1.19% S), whereas a S/O ratio of only 1.0 was sufficient to obtain the same percent sulfur reduction in the oxidized oil containing 1.1% sulfur, with a higher oil yield (85% vs 80%).
Effect of Oxidation on Other Feedstocks Two other petroleum feedstocks were used to evaluate the effect of preoxidation on extraction efficiency. These two stocks were an olefinic coker furnace oil (CFO) of 0.69% S and an aromatic midcontinent light cycle oil (LCO) of 1.62% S. The oxidized oils of these two stocks were extracted by lactone.
Ind. Eng. Chem. Res., Vol. 29, No. 3, 1990 323
AGO AS RECEIVED
UNOXlDlZED CFO,
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N02-OXIDIZED CFO, 0 5 3 % 5 N O z - O X I D I Z E D CFO, O 3 1 % S
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IO
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,
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SULFUR REMOVAL BY EXTRACTION (%)
Figure 3. Effect of oxidation of coker furnace oil on sulfur removal by extraction and on extraction oil yield.
SPIKED AGO
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1 BENZYL MERCAPTAN 2 I-OCTANETHIOL 3 EENZOTHIOPHENE 4 BUTYL DISULFIDE 5 HEXYL SULFIDE 6 PHENYL SULFIDE 7 DIBENZOTHIOPHENE 8 PHENYL DISULFIDE
RETENTION TIME [ m i d COLUMN TEMPERATURE ("C)
Figure 5. Comparison of sulfur chromatograms of oil as received with that spiked with model sulfur compounds.
A I
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,
40
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70
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SULFUR REMOVAL BY EXTRACTION (%)
Figure 4. Inertness of light cycle oil to effect of oxidation on sulfur removal by extraction and on extraction oil yield.
The effect of oxidation of CFO on extraction oil yield is shown in Figure 3. Oxidation of CFO did not produce any enhancement in the extraction of CFO. Samples of the oxidized CFO charges used had 0.53% and 0.31% sulfur. The oil yield vs sulfur reduction relations for the oxidized oils were all located below that for the unoxidized oil, indicating that the oxidation of CFO had an adverse effect on extraction. This loss in extraction oil yield was further magnified with increasing severity in oxidation. Lactone extraction of LCO (Figure 4) provided similar oil yields for given sulfur reductions in unoxidized and oxidized oil. The S/O ratios required, however, were less for oxidized oil than for unoxidized oil, indicating that both the oxidized oil and its oxidized sulfur compounds are more soluble in this solvent. Aromatic a n d Olefinic Dissolving Solvents Lactones are known to be aromatic dissolving solvents. Other similar solvents, such as furfural, sulfolane, and DMF, also showed similar results when they were used to extract oxidized AGO, CFO, and LCO. The AGO is basically a paraffinic stock with very low olefinic and aromatic content. The CFO has more olefins than the AGO, while the LCO has the most aromatic content. All the solvents cited here showing extraction oil yield enhancements are known to be solvents that can selectively dissolve olefins and aromatics as reported by Deal and Derr (1962) and by Gerster et al. (1960). Apparently, oxidation converted the sulfur-containing components into more polar compounds, which were then more readily dissolved by those solvents leading to higher sulfur removal. The CFO and LCO had more initial aromatics and olefins, and extraction oil yields were not improved
Table 111. S u l f u r Content of Model S u l f u r Compounds in Initial AGO (Weight Percent) retention S content in S comud bv, "C time. min AGO. % hexyl sulfide 230 7.8 0.021 0.023 phenyl sulfide 296 8.8 323 10.7 0.036 dibenzothiophene phenyl disulfide 310 11.5 0.029 Table IV. Distribution Coefficients of the Identified S u l f u r Compounds i n Lactone Extraction distribution coeff unoxidized oil oxidized oil S compd extraction extraction hexyl disulfide 4.1 undetermineda phenyl sulfide 2.3 2.3 dibenzothiophene 5.1 11.2 phenyl disulfide 1.8 4.5 a There
was no hexyl disulfide in the oxidized oil.
by oxidation due to the higher solubility of oil in solvent before oxidation treatment. Gas Chromatographic Analysis Eight model sulfur compounds including mercaptans (thiols), aliphatic sulfides, aromatic sulfides, and thiophenes were used to spike the AGO for identification of some sulfur compounds. Figure 5 shows the sulfur chromatgrams of the untreated and spiked oil. The use of the relative retention times and the comparison of the chromatograms between the model compounds and the spiked oil allowed one to identify precisely these sulfur compounds. Only four of the eight model sulfur compounds used were present in the initial AGO as received. Their boiling points, retention times in the GC column, and concentrations in the oil are listed in Table 111. The distribution coefficients of the four identified sulfur compounds from extracting unoxidized and oxidized AGO
324
Ind. Eng. C h e m Res 1990,29, 324-329
were calculated and are shown in Table IV. The distribution coefficient of dibenzothiophene in the unoxidized oil extraction was the highest (5.1). After oxidation, hexyl sulfide was not present in the oxidized oil. The distribution coefficients of dibenzothiophene and phenyl disulfide in the extraction of oxidized oil were increased: from 5.1 to 11.2 for dibenzothiophene and from 1.8 to 4.5 for phenyl disulfide. As for the phenyl sulfide, the distribution coefficient in both extractions was relatively low. These results are consistent with the results described previously. Conclusions Oxidation of the Arabian AGO by nitrogen dioxide has converted the sulfur compounds of the oil to forms that are more polar in nature. These sulfur compounds are more readily extracted by polar solvents, such as lactone, that have been demonstrated to be aromatic/olefinic dissolving, thus resulting in higher sulfur removal, lower solvent-to-oil requirement, and hence, higher extraction oil yield for the same sulfur removal. Oxidized oils from other petroleum stocks which have higher contents of olefins and aromatics do not show an oil yield increase in the extraction by lactone. The distribution coefficients
of dibenzothiophene and phenyl disulfide in lactone extraction of AGO are greatly increased for the oxidized oil as compared to the untreated oil. Registry No. NOz, 7697-37-2; nitrogen dioxide, 10102-44-0; ybutyrolactone, 96-48-0; hexyl disulfide, 10496-15-8; phenyl sulfide, 139-66-2; dibenzothiophene, 132-65-0;phenyl disulfide, 882-33-7.
Literature Cited Deal, C. H.; Derr, E. L., Selectivity and Solvency in Aromatic Recovery. Prep--Am. Chem. Soc., Diu.Pet. Chem. 1962, 7 (4), 214. Gerster, J. A.; Gorton, J. A,; Eklund, R. B. Selective Solvents for Separation of n-Pentane from 1-Penteneby extractive Distillation. J . Chem. Eng. Data 1960, 5, 423. Guth, E. D.; Diaz, A. F. Method of Removing Sulfur and Nitrogen in Petroleum Oils. US.Patent 3,847,800, 1974. Guth, E. D.; Helgeson, N. L.; Arledge, K. W.; Brienza, A. R. Petroleum Oil Desulfurization Process. US.Patent 3,919,402, 1975. Tam, P. S.; Kittrell, J. R. Process for Purifying Hydrocarbonaceous Oils. U.S. Patent 4,485,007, 1984. Thompson, R. B.; Druge, L. W.; Chenicek, J. A. Stability of Fuel Oils in Storage--Effect of Sulfur Compounds. Ind. Eng. Chem. 1949, ;I
I
121, 2713.
Received for review July 18, 1989 Accepted November 8, 1989
Desulfurization of Fuel Oil by Oxidation and Extraction. 2. Kinetic Modeling of Oxidation Reaction Patrick S. Tam,tJil J a m e s R. Kittrell,' and John W. Eldridge*J Department of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts 01003, and KSE, Inc., P.O. Box 368, Amherst, Massachusetts 01004
The oxidation of Arabian atmospheric gas oil (AGO) with nitric acid t o remove sulfur from the oil is similar to the gradual processes involved in the storage instability of petroleum distillates and synfuels, except that in this process the instability process is accelerated by adding a strong oxidizing agent, nitric acid. The elemental composition of these sediments is compared. A mathematical kinetic model is presented to describe the kinetics of sulfur removal in the oxidation of AGO using a CSTR. This model employs lumping of the sulfur compounds in the oil into four groups, S1, S2, and S3, according t o their retention times (hence, boiling points) in the gas chromatograph, and residue (R) containing some of the other three sulfur groups. The first group (Sl) had a very fast reaction rate. The second group (Sa) reacts with second-order kinetics with a rate constant of 17 g of oil/(g of S2.min) at 25 "C. The third group (S3)was not present in the unoxidized oil, and as it was formed, it equilibrated between the oil and the residue phases. The principle of desulfurizing Arabian atmospheric gas oil (AGO) by oxidative desulfurization (ODS) is to form a coproduct (residue) that has high sulfur content and can be separated from the oxidized oil. It has long been recognized that petroleum distillates and synfuels (both crude and refined) are unstable upon long-term storage in the presence of air at ambient conditions, with the formation of sediment (deposit, residue, sludge, and insoluble gum). The sediment formation from the oil is due to the gradual oxidation and polymerization of the reactive compounds present. The ODS process is, in many ways, very similar to the instability process of fuel oil, except that the oxidation rate in ODS is greatly accelerated by using a strong oxidizing agent, nitric acid or nitrogen oxides in this case. The gum formation from jet turbine and diesel fuel is clearly associated with oxidation and cooxidation of hyKSE, Inc. University of Massachusetts. §Present address: The Pritchard Corp.. Overland Park, KS 8
66210.
0888-5885/90/2629-0324$02.50/0
drocarbon mixtures (Mayo and Lan, 1983). Walters et al. (1949) studied the gum formation in cracked gasoline and found that the gum and peroxide formation is autocatalytic in nature. It was proportional to the amount of air available for reaction, indicating that oxidation played the major role in this process. Not all fuel oils show the same rate of sediment formation. For thermally cracked gasoline, heavier fractions are less stable than light fractions (Walters et al., 1949). In general, paraffins, aromatics, monoolefins, and diolefins are increasingly unstable toward oxidation in the order named. Offenhauer et al. (1957) also found the same trend. Straight run distillates which normally contain mostly paraffinic compounds are more stable, whereas catalytically cracked distillate fuels containing more unsaturated hydrocarbons are unstable (Brooks, 1926; Cassar, 1931; Flood et al., 1933). For shale oil and shale-derived fuels, the instability is even more pronounced (Inneen and Bickel, 1951; Cooney et al., 1984; Frankenfeld et al., 1982). Certain heterocyclic sulfur and nitrogen compounds in addition to those responsible for the gum in cracked petroleum distillates are involved in 0 1990 American Chemical Society
