Energy & Fuels 1995,9, 500-506
500
Kinetics of Hydrodesulfurization of Thiophenic and Sulfide Sulfur in Athabasca Bitumen Murray R. Gray* and Alan R. Ayasse Department of Chemical Engineering, University of Alberta, Edmonton, AB, T6G 2G6 Canada
Edward W. Chan and Maya Veljkovic Syncrude Canada Ltd., 10120, 17th St., Edmonton, AB, T6P l V 8 Canada Received September 14, 1994@
Fractions from Athabasca bitumen that had been hydrocracked at 430 "C and 13.9 MPa were analyzed for total sulfur content and organic sulfide (or thioether) content. The sulfides were oxidized to sulfoxides and then measured semiquantitatively by infrared spectroscopy. Thiophenetype sulfur was estimated by subtracting the sulfides from the total sulfur. After catalytic hydrocracking over Ni-Mo on y-alumina, the sulfur in the distillate fractions was predominantly thiophenic. The sulfur remaining in the residue fraction, however, was mainly in sulfides, and the proportion of sulfides increased with reaction severity. The fraction of sulfides was reduced when no catalyst was present and when the hydrogen pressure was reduced. This behavior was consistent with a simple kinetic model wherein sulfides were formed as intermediate species by hydrogenation of thiophenes. This type of reaction network, involving series and parallel reactions, can give apparent reaction orders for overall desulfurization ranging from negative values t o greater than 2, depending on the extent of conversion and the relative rate constants.
Introduction Removal of sulfur is an important step in the processing of petroleum residues and bitumens into more valuable products. This removal of sulfur is achieved by two major pathways: by thermal reactions when high-boiling feeds are cracked t o lighter products, and by catalytic hydrodesulfurization. When the cracking step is carried out in the presence of hydrogen and an active catalyst, such as promoted molybdenum on y-alumina, the extent of desulfurization can be significant.l The catalyst in residue hydrocracking processes serves to transfer hydrogen to the high-boiling components to help suppress c ~ k i n gand ~ , ~to remove sulfur compounds, particularly in the distillate fraction^.^ Sulfur occurs in two major forms in heavy fractions of petroleum and bitumens: thiophene and its benzologs (benzothiophene, dibenzothiophene, benzonaphthothiophene, etc.), and aliphatic sulfides, such as the cyclic sulfides (or thiolanes) identified by Payzant et al.5 in Athabasca bitumen. Representative structures are illustrated in Figure l . In catalytic hydrotreating of distillates, the sulfides are highly reactive and easily removed, while the thiophenic compounds are more resistant.6 Substituted dibenzothiophenes have frequently been identified as the major type of residual Abstract published in Advance ACS Abstracts, April 1, 1995. (1)Gray, M. R. Upgrading Petroleum Residues and Heavy Oils; Marcel Dekker: New York, 1994. (2) Miki, Y.; Yamadaya, S.; Oba, M.; Sugimoto, Y. J . Catal. 1983, 83, 371. (3) Sanford, E. C. Ind. Eng. Chem. Res. 1994, 33, 109. (4) Gray, M. R.; Khorasheh, F.; Wanke, S. E.; Achia, U.; Krzywicki, A.; Sanford, E. C.; Sy, 0. K. Y.; Ternan, M. Energy Fuels 1992, 6 , 478. (5) Payzant, J. D.; Montgomery, D. S.; Strausz, 0. P. AOSTRA J . Res. 1988, 4 , 117. (6)Weisser, 0.;Landa, S. Sulphide Catalysts, Their Properties and Applications; Pergamon: Oxford, U.K., 1973. @
0887-0624/95/2509-0500$09.00/0
ThiophenicSulfur Species Substituted Thiophene
Substituted Benmthiophene Dibenaothiophene
Q-@
Benzo(b]naphtho[2,34]thiophene
Benro[b]naphtho[l,24]thiophene
d
sulfide sulfur species Aliphatic Sulfide
Cyclic Sulfides
R14H2-S-CRZ-R2
O P
Figure 1. Representative structures of thiophenic and sulfide compounds. The side chains on thiophene and benzothiphene and the cyclic sulfides follow compounds identified in Athabasca b i t ~ m e n . ~ J ~
sulfur compound in hydrotreated products (e.g., Houalla et al.7 and Ma et a1.8 1. The disparity in reactivities between the various forms of sulfur has been used to explain why the kinetics of hydrodesulfurization can follow fractional-order kinetics.' The persistence of highly-resistant species in a multicomponent reacting mixture gives apparent (7) Houalla, M.; Broderick, D. H.; Sapre, A. V.; Nag, N. K.; de Beer, V. H. J.; Gates, B. C.; Kwart, H. J . Catal. 1980, 61, 523. (8)Ma, X.; Sakanishi, K.; Mochida, I. Ind. Eng. Chem. Res. 1994, 33, 218.
0 1995 American Chemical Society
Hydrodesulfurization of Thiophenic and Sulfide Sulfur Table 1. Composition of Athabasca Bitumen Feed
sulfur, wt % sulfide sulfur, wt % nitrogen, wt % hydrogen, wt % carbon, wt % microcarbon residue, wt % boiling fraction naphtha (IBP - 177 "C) middle distillate (177-343 "C) gas oil (343-525"C) residue (525"C+)
4.74 1.25 0.44 10.07 82.3 14.9 wt %
sulfur, wt %
0
7.0 38.0 55.0
2.6 3.3 6.3
reaction orders that approach 2,9 even though the rate of reaction of each component in the mixture follows first-order kinetics. The highest sulfur concentration in a given feed is usually found in the residue fraction, or the material boiling over 525 "C.Although the behavior of the sulfur species in the distillate fractions is well understood, the reactivity of the thiophenic and sulfide forms of sulfur in residues has not been investigated. The objective of this study was to determine whether the sulfide compounds in the residue fraction of bitumen during catalytic hydrocracking were easier to remove than the thiophenic compounds and to determine how the reactions of the two major sulfur types contributed to the overall kinetics of hydrodesulfurization.
Experimental Section Materials. Athabasca bitumen was supplied by Syncrude Canada Ltd., from the mining and extraction operation at Mildred Lake, Alberta, Canada. The properties of the bitumen feed are summarized in Table 1. The catalyst was obtained from a commercial supplier as 1 mm diameter x 4.5 mm long cylindrical extrudates of y-alumina, containing 12.5wt % MOO, and 3.5 wt % NiO. The surface area, by nitrogen BET determination, was in the range 150-200 m3/g, and the pore volume was 0.3-0.6 mug. Reactor Experiments. A simplified schematic of the reactor apparatus is shown in Figure 2. A hydraulic cylinder containing a floating piston, heated t o a temperature of 150 "C, was used to transfer the bitumen into the reactor. The cylinder was filled with bitumen from a heated feed drum using a Moyno pump. The bitumen was fed to the reactor at a set rate by displacing the piston with a light metering oil using a Milton Roy metering pump, driven by a Doerr electric motor. Hydrogen was supplied by Canadian Liquid Air Ltd. in either 24 MPa or 41 MPa Linde cylinders. A pressure regulator reduced this pressure to about 1400 kPa above the reactor operating pressure. The rate of hydrogen flow to the reactor was measured using a mass flow meter and adjusted using an automatic control valve. The reactor used in these experiments was a modified 2-L autoclave (Autoclave Engineers). The bottom half of the reactor was plugged t o give a 1-L working volume, with a height t o diameter ratio of 2:l. Catalyst (78g) was held in an annular 130 mL basket, made from 16 mesh stainless steel screen with a solid top and bottom. The basket sat in a baffle assembly approximately 1 cm from the bottom of the reactor. The stirrer was designed to force the liquid and bubbles of entrained gas and vapor radially outward through the catalyst basket and then draw it over and under the basket back into the centre. The stirrer was magnetically coupled outside the reactor and was equipped with a variable-speed drive. The reactor was also equipped with a heating element and was surrounded by permanent insulation except at the head, which (9)Ho, T. C.; Aris, R. AIChE J. 1987, 33, 1050.
Energy & Fuels, Vol. 9, No. 3, 1995 501 was covered by a removable insulatingjacket. A thermocouple well descended about three-quarters of the way t o the bottom of the reactor. The feed hydrogen, at a rate of 5 Umin (at 15 "C and 101.3 kPa), joined the bitumen through a valve, and then the twophase feed entered near the bottom of the reactor. This hydrogen supply rate was sufficient to maintain the partial pressure of hydrogen within the reactor at above 90% of the total operating pressure. The two-phase product left near the top of the reactor. The reactor pressure was regulated by a digital controller which adjusted the control valve on the outlet flow. This arrangement was able to keep the reactor pressure within 200 kPa of the set point, normally 13.7 MPa. At the end of each experiment, the liquid was removed from the reactor to determine the liquid holdup. The reactor liquid holdup was 376 i~10 mL of liquid, and approximately 500 mL of gas. The liquid holdup was not sensitive to feed rate, temperature, or pressure. After exiting the reactor through the control valve, the twophase product stream entered a knockout drum. This drum had a liquid outlet at the bottom which was directly connected t o a sight glass; it also had a vapor outlet a t the top which was also directly connected to the sight glass. The pressure in the drum was maintained at 2800 kPa by a digital controller and a valve on the outlet vapor line. The liquid level was maintained manually by inspecting the sight glass and turning a hand valve to drain the liquid from the drum when the level got too high, approximately hourly depending on the flow rates. The off gas from the knockout drum was sent through a water scrubber and then could either be directly vented or sent t o the gas chromatograph for analysis. Product Characterization. The liquid product was distilled into four cuts by Syncrude Canada Ltd.: naphtha (initial boiling point 177 "C), middle distillate (177-343 "C), gas oil (343-525 "C), and residue (525"C+). The naphtha and middle distillate were removed using an atmospheric spinning band distillation of the whole product, leaving a heavier fraction in the flask at the final temperature of 350 "C. This fraction was then distilled under vacuum to produce the gas oil and residue fractions following the ASTM D1160 procedure. The sulfur analysis of the whole product and each boiling fraction was done by combustion followed by fluorescence detection. The analysis was performed three times for each sample, with the final value given as an average of the three. Analysis of Sulfides. Aromatic and aliphatic sulfur species cannot be distinguished from hydrocarbons using IR spectrometry. Aliphatic sulfur species are much more easily oxidized than thiophenes, changing from sulfides to sulfoxides which then give a distinct band in the infra red spectrum. The sulfides in the bitumen and products were selectively oxidized by refluxing a 0.2g of sample of oil for 30 min with a solution of 0.2g of tetrabutylammonium periodate in 25 mL of toluene 5 mL of methanol, following the method of Green et a1.I0 This mild oxidation converted most of the aliphatic sulfur without affecting the aromatic sulfur. After extracting the periodate and removing the solvent, the remaining sample was weighed and dissolved in dichloromethane. Approximately 0.4 g of this solution was diluted with dichloromethane to make 1 mL of solution and the IR spectrum was measured using a Nicolet 730 Fourier-transform infrared spectrometer using a NaCl cell with 0.5 mm spacing. Pure solvent was used as a reference. The sulfoxide peak was at 1025 cm-', with an extinction coefficient of 6 x lo3 L mol-' cm-2.11 This semiquantitative analysis for sulfide sulfur was verified using 96% dioctyl sulfide (C16H&; 96% purity, Aldrich) in dichloromethane. Five samples were analyzed with an average error of 4% and a maximum error of 8%.
+
(10)Green, J.B.; Yu, S.K-T.;Pearson, C. D.; Reynolds, J. W. Energy Fuels 1993, 7, 119. (11)Bunger, J. W.; Thomas, K. P.; Dorrence, S. M. Fuel 1979, 58, 183.
502 Energy & Fuels, Vol. 9, No. 3, 1995
Gray et al.
Vent
I
vo It t ss 0
A
Feed Piston
Gas Chromatograph
Bitumen Kc.4
_ I
n
-
PM-11
Scrub ber
PR-2 0 1
I
y \
H-17
ZE
2 6-22
PR-10
PM-13
I4 Reactor
-
I
-1
Pressure Letdown
I
U Electronic Scale Figure 2. Schematic diagram of catalytic reactor apparatus.
Results and Discussion
sulfide sulfur. Three hypotheses are possible: (1)The thiophenic sulfur was converted to sulfides, which Sulfur and Sulfide Conversion. The total concenremained a t a steady concentration as a reaction tration of sulfur and the amount of sulfur in sulfides in intermediate. (2) The sulfides were much less reactive each boiling fraction are illustrated in Figures 3 and 4 than the thiophenes, and almost all of the sulfur as a function of mean residence time, which was in the converted was of the thiophenic type. (3) The analytical range of 0.47-1.88 h on a liquid volume basis or 2.2method was not effective in the heavy residue fraction. 8.8 (g of catalyst h)/mL on a mass-catalyst basis. The Prior studies by Payzant et al.13 and Green et al.1° sulfur content was reduced to low levels in the naphtha found that the method of oxidizing sulfides to sulfoxides and middle distillate fractions, consistent with the activity of Ni/Mo on y-alumina toward light f r a c t i o n ~ . ~ J ~ was effective even for residue or asphalt fractions, and similar results were obtained. Green et al.1° observed Sulfur was more persistent in the gas oil fraction that sulfides comprised 28-41% of the sulfur in four (Figure 31, although the sulfide content was reduced asphalts, compared to 28% in the residue fraction of consistently to a low level. Thiophenes can be estimated Athabasca prior to reaction. The determination of the from the data of Figure 3 as total sulfur minus sulfides. sulfoxides by infrared spectroscopy was semiquantitaIn model compound studies and hydrotreating of distiltive, but even allowing for errors due t o the use of a lates, thiophenic compounds such as dibenzothiophene mean extinction coefficientthis analysis would not show are much more resistant t o catalytic hydrodesulfurizaconstant concentration if the true concentration had tion than aliphatic sulfides.6 The data on the distillate decreased in proportion t o the decrease in the total fractions was consistent, therefore, with the prior work sulfur (Le., a factor of 3 or more). The third hypothesis, on model compounds. therefore, can be dismissed. The data for the residue fraction, however, were If the thiophenes were partly converted to sulfides, dramatically different as illustrated in Figure 4. The then the reaction network can be represented by the concentration of sulfide sulfur was almost constant at scheme in Figure 5. Thiophenes would react either by ca. 1.5 wt % of the residue fraction (525 " C f fraction), hydrogenolysis, giving hydrogen sulfide, or by hydrowhile the total sulfur was reduced by almost a factor of genation to form nonaromatic sulfide species. These 3 at the longest mean residence time of 1.8 h. The sulfides would be subject to thermal crachng and implication of these data was that the thiophenic sulfur catalytic hydrodesulfurization to give hydrogen sulfide. in the residue fraction was removed more rapidly than (12)Trytten, L. C.; Gray, M. R.; Sanford, E. C. Ind. Eng. Chem. Res. 1990,29, 725.
(13)Payzant, J. D.; Mojelsky, T. W.; Strausz, 0. P. Energy Fuels 1989, 3 , 449.
Hydrodesulfurization of Thiophenic and Sulfide Sulfur 1.0 0.8
0.6 0.4
1 I
I
I
I
Reaction 2 (kz) Hydrogenolysis
1
Naphtha Thiophenes
I
Middle Distillate
I.5
2*5 2.0
I\
catalyst inlet hydrogen pressure, MPa boiling fraction Gas Oil naphtha middle distillate gas oil \ I residue boiling fraction naphtha \rmiddle distillate \ gas oil m--+-----a I 1 I residue
I\ I
I.o [k \ 0.5 -
.
Table 2. Sulfide and Total Sulfur in Boiling Fractions from Catalytic and Thermal Hydroconversion of Athabasca Bitumen"
experiment
3'0
Reaction 1 (kr) Hydrogenation
H2S Reaction 3 (k3) Thermallcatalytic Removal Figure 5. Reaction scheme for hydrodesulfurizationof residue fraction.
:,m 1
.
H2S
Sulfides
0.2 0.o 3.0
Energy & Fuels, Vol. 9, No. 3, 1995 503
i
catalytic NVMo 13.7
0.1 0.4 0.3 1.6 0.61 1.9 0.27 0.56
thermal none 13.7
low pressure Ni/Mo 10.2
sulfide sulfur, wt % 0 0 0.3 0.2 0.5 0.2 1.0 0.7 sulfide sulfudtotal sulfur 0 0 0.13 0.36 0.14 0.16 0.20 0.18
a Reaction at 430 "C and mean residence time of 0.94 h or 4.4 (g of catalyst h)/mL.
removal of sulfur from the aromatic ring, would be strictly catalytic due to the resonance stabilization of the thiophenic ring. Hydrogenation (reaction 2) would be negligible except in the presence of a catalyst; therefore, any reduction in the catalytic function would reduce the conversion of thiophene to sulfides. Since sulfides could still react thermally t o form hydrogen 0 Sulfide sulfide, a reduction in catalyst activity would reduce the 0 Total S sulfide concentration relative to the data in Figure 4. This hypothesis was tested by reacting bitumen without Ni-Mo catalyst, so that thermal reactions would predominate, and at lower hydrogen pressure so that the hydrogenation reactions would occur at a lower rate. The sulfide concentrations and ratio of sulfide to total sulfur in the boiling fractions of the products are listed in Table 2. The sulfide concentrations in the residue fractions from thermal and low-pressure processing were lower than in residue from catalytic processing at any residence time, consistent with the 1 qualitative prediction based on the reaction scheme of Figure 5. Concentration differences between the distillate fractions from the three experiments were much 1 1 I 0 less significant. The ratio of sulfide to total sulfur in the residue fraction was also 2-fold lower, due to reduced desulfurization overall in the absence of catalyst and at reduced pressure. The same trend in the ratio of sulfide to total sulfur was also observed in the distillate fractions. The consistency between the behavior of the sulfides in the residue and the simple reaction model in Figure 5 suggested that the experimental results were representative of the actual distribution of sulfur The scheme in Figure 5 is consistent with the known species in the residue product. chemistry of catalytic hydrodesulfurization of comKinetics of Removal of Sulfur Types. The mass pounds such as benzothiopheneand diben~othi0phene.l~ balances for residue fraction and thiophenic sulfur in Hydrogenolysis of thiophenes (reaction 11, i.e., direct the residue in a continuous-flow catalytic reactor can be written as follows, assuming first-order kinetics: (14) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991,30,2021.
Gray et al.
504 Energy & Fuels, Vol. 9, No. 3, 1995
FiCi - FC = k,VLC
(1)
FiiCiWthio,i - F C W t h i o
= krVLCwthio,i+ (k1 + k2)V~Cwthi~ (2)
Table 3. Ratio of Sulfide Formation to Hydrogenolysis in Reactions of Thiophenic Model Compounds over Co-Mo on y-Alumina Catalyst
model compound
where Ci and C are the concentrations of residue (525 "C+ fraction) in the inlet and outlet, respectively, Fi is inlet flow, F is outlet flow, kr is the rate constant for cracking of residue, VL is the volume of liquid in the reactor at 15 "C, and Wthio is the weight fraction of thiophenic sulfur in the residue. The first term on the right-hand side of eq 2, krVLCWthio,i, represents removal of a portion of the inlet thiophene due to cracking of residue components and subsequent migration of sulfur into the distillate fractions. As illustrated in Figure 3, the thiophene that appears in the distillates would then be subject to catalytic reaction. The second term on the right-side of eq 2 is for catalytic reaction to form hydrogen sulfide or organic sulfides, following Figure 5. Rearranging eq 1 and 2 gives the following result for thiophenic sulfur:
w thio . =
thio,i
6
+ (k, + k2)z
(3)
t is the mean residence time of the liquid, calculated either as inlet liquid volumetric flow divided by the liquid holdup (F~IVL, both at 15 "C), or as inlet liquid volumetric flow divided by the mass of catalyst. The ratio of liquid holdup to catalyst was approximately constant a t 4.8 m u g for all experiments. The ratio of the outlet liquid volumetric flow rate to the inlet flow, 6 = FIFi, was in the range of 1.0-1.07. The corresponding material balance equation for the sulfide sulfur is given by FiCiwsulf,i - FCWsulf
=
krVLCwsulf,i
- k2vLcwthio
+ k3VLCWsulf
(4)
Solving for the outlet concentration of sulfide sulfur, terms of the inlet concentration, wsulii, gives
wsulf, in
Wsulf
=
+ kltWthio 6 + k3z
6wsulf,i
Since the concentration of sulfides changed very little with mean residence time, the trivial fit of eq 5 to the data would be given by k2 = 0 and k3 x 0 (i.e., hypothesis 2 above). These kinetic constants, however, were inconsistent with the data for the experiments without catalyst and at reduced hydrogen pressure (Table 2) and inconsistent with the kinetic behavior of sulfides versus thiophenes in model compound studies. The results of the thermal experiment gave a minimum estimate for 123 of 0.6 h-l (from eq 5 with kz = 0 since catalyst was absent), which would be increased by the addition of a catalyst. Model compound studies indicate that k 3 r k l , i.e., that sulfides are more easily reacted than thiophenes at equivalent conditions. When this constraint is applied, the curves in Figure 4 show the best fit of the kinetic model to the experimental data for total sulfur and sulfide sulfur, using values of the rate constants kI = 1.2 h-l, k g = 1.1h-l, and k 3 = 1.2 h-l. The concentration of thiophenic sulfur was approximated as wtotal - w,,Lf. Although the concentration of
thiophene benzothiophene dibenzothiophene benzo[blnaphtho[2,3-dlthiophene benzo[blnaphtho[1,2-dlthiophene
reaction reaction ratio of sulfide temperature, press., formation to "C MPa hydrogenolysis ref 300 300
300 300 250
3 3-10 10 7 3
0
16 15
0.73-2.2" 0.0015 0.32
24
0.51
25
7
a The ratio of sulfide formation to hydrogenolysis increased with hydrogen pressure.
sulfide sulfur in the residue did not change significantly as residence time was increased, this behavior was consistent with the kinetic model based on Figure 5 when the rate constants for thiophene and sulfide reactions were all of similar magnitude. These kinetic parameters, and the sustained concentration of sulfides as an intermediate, showed that the catalyst was less selective for hydrogenolysis of sulfur heterocycles in the residue than in the distillates. This loss of selectivity would be accompanied by more hydrogenation of carbon rings. A number of reaction networks for thiophenic compounds have been presented in the literature, based on model studies. The data of Table 3 summarize these results in terms of the ratio of the rate of hydrogenation of the thiophenic ring t o form the nonaromatic cyclic sulfide (reaction 2 in Figure 5) to the rate of hydrogenolysis, which is the direct catalytic removal of sulfur from the thiophene (reaction 1 in Figure 5). Clearly, the series of compounds from thiophene to benzonaphthothiophene did not exhibit consistent behavior, although differences in catalyst formulation and reactor conditions could contribute to the disparate results. The actual compounds in a residue fraction would be substituted with various groups, but data on the reaction networks for substituted thiophenes are not available. Benzothiophene had the highest initial rate of sulfide formation, mainly because Van Parijs et al.15 observed that the sulfide formation reaction was so fast that a hydrogenation equilibrium was established between benzothiophene and 1,2-dihydrobenzothiophene (the sulfide). No sulfide was detectable in the studies by Van Parijs and FromenP of hydrotreatment of thiophene, although the sulfide (tetrahydrothiophene in this case) has been suggested as a short-lived intermediate.l4The tendency of the thiophenes in the residue fraction to form sulfides as intermediates suggests that the thiophenic compounds behave more like benzothiophene and benzonaphthothiophene than dibenzothiophene. Implicationsof Kinetic Behavior of Sulfur Species. The importance of sulfide intermediates in removing sulfur from the residue fraction implies that the activity of the catalyst is less specific for sulfur heterocycles than in the distillate reactions. The residue fraction contains large and complex molecules, so that one would anticipate that steric hindrance would limit the ability of the sulfur to coordinate with the catalyst (15)Van Parijs, I. A,; Hosten, L. H.; Froment, G. F. Ind. Eng. Chem. Prod. Res. Deu. 1986,25, 437. (16)Van Parijs, I. A,; Froment, G. F. Ind. Eng. Chem. Prod. Res. Dev. 1986,25, 431.
Energy & Fuels, Vol. 9,No. 3, 1995 505
Hydrodesulfurization of Thiophenic and Sulfide Sulfur
surface. Once the adjacent side rings are hydrogenated, the sulfide is no longer aromatic and is much more reactive. Since sulfides are subject to thermal cracking due to the relative weakness of the carbon-sulfur bond, the removal of sulfur from the partly-hydrogenated residue components would be a combination of thermal and catalytic reactions. The kinetic analysis in eqs 2-5 lumped the sulfur species in the residue according to sulfur type. Within each lump, for example the thiophenes, we would expect t o find a variety of components with different reactivities. Following the reaction scheme of Figure 5 , the components in the lumps undergo series reactions, forming sulfide from thiophene, as well as parallel reactions to form hydrogen sulfide. All previous analysis of lumped kinetics has considered either parallel reactions, or simple series reactions of the type A B C. Analysis of the series-parallel case from Figure 5, given in the Appendix, shows that the apparent order of the hydrodesulfurization can range from negative values t o large positive values, depending on the extent of conversion and the rate constants. The observation of fractional orders of reaction for desulfurization of heavy hydrocarbon feeds, ranging as high as 2.5," may have two contributing factors: lumping of components with a wide range of reactivities as total sulfur, and lumping of species which are subject to a combination of series and parallel reactions. The latter factor is required to interpret apparent reactions orders of less than 1 or greater than 2.
-
-
eq 6 can give overall apparent reaction orders ( n ) ranging from first order to second order, depending on the range of reactivities within the lump and the level of conver~ion.~ If the intrinsic reaction order is less than 1,'then the possibility exists for observation of n 0.23 The experimental results presented in this paper suggest that when the sulfur species in residue are lumped together, both series (thiophene reacting to form sulfide) and parallel (hydrogenolysis) reactions will occur simultaneously. The apparent kinetics of lumped pseudocomponents with irreversible series reactions have not been considered previously. Consider the reaction scheme for desulfurization from Figure 5 , with lumping of the reactants to give the total concentration of organic sulfur: cT
(7)
= Cthio + Csulf
Taking the case of a batch reactor, to allow an analytical solution for the lumped kinetics, with firstorder kinetics for the reactions of Figure 5 dCthiJdt dCsulddt
-k
(8)
k2)cthio
(9)
= lzlCthio - k3Csulf
Solving for Cthio and Csulffor the simplest case of no initial sulfide, using initial concentrations Ctho,i and and CSulf,i= 0 gives (10)
Conclusions 1. Sulfide concentrations in the residue fractions of hydrocracked Athabasca bitumen were insensitive t o mean residence time when a catalyst was present. Lower concentrations of sulfides were detected in residue when catalyst activity was reduced, either by operating without catalyst or by reducing hydrogen pressure. 2. The behavior of the sulfides was consistent with the formation of sulfides as an intermediate step in the hydrogenation of thiophene compounds. 3. The presence of series reactions can account for unusual apparent kinetics in the overall disappearance of sulfur from heavy hydrocarbon fractions.
Appendix: Lumped Kinetics of Series and Parallel Desulfurization Kinetics Hydrodesulfurization of hydrocarbon fractions is usually analyzed using lumped kinetics, using the total sulfur content to represent the various sulfur species in the mixture. A common empirical approach is to fit the disappearance of the feed to power-law kinetics: rate of reaction = k,VLCn
(12) Nondimensionalizing eq 12 with XT= cT/cthio,i and z = (kl kz)t, and defining two characteristic ratios of rate constants, P = kd(k1 k2) and K = k3/(k1 k 2 ) gives
+
+
+
In this equation, /3 is the fraction of thiophenes that undergoes hydrogenolysis t o give H2S directly, while K is the ratio of sulfide reactivity to thiophene reactivity. The apparent reaction order, n, at any time z is given by
(6)
where C is the total concentration of sulfur, k, is the rate constant, and n is the apparent reaction order. In developing the theory for such lumped kinetic equations, most attention has been directed to lumping of irreversible parallel reactions1s-21 and networks of reversible reactions.22 Lumping of parallel first-order reactions by (17) Papayannakos, N.; Georgiou, G. J . Chem. Eng. Jpn. 1988,21, 244.
Substituting eqs 10 and 11 into eq 7 gives
Defining $J = (1- P)/(K - 11, eqs 13 and 14 give (18)A r i s , R. Arch. Rat. Mech. Anal. 1968, 27, 356. (19) Astarita, G.; Ocone, R. AIChE J . 1988,34, 1299. (20) Chou, M. Y.; Ho,T.C. AIChE J . 1989,35, 533. (21) A r i s , R. AIChE J. 1989, 35, 539. (22) Wei, J.; Kuo, C. W. Ind. Eng. Chem. Fundam. 1969,8, 124. (23) Astarita, G. AIChE J. 1989, 35,529. (24) Sapre, A. V.; Broderick, D. H.; Frankael, D.; Gates, B. C.; Nag, N. K. AIChE J. 1980,26,690. (25)Vrinat, M. L. Appl. Catal. 1983, 6, 137.
Gray et al.
506 Energy & Fuels, Vol. 9, No. 3, 1995
+ l/q)e-' - ~ ~ e - ~ ' ] / ( d x d d z(15) )~ dXddt = y[-(l + l/W)e-' + ~ e - ~ ' l (16)
5
n = X,y[(l
For the singular case of shown that
K
= 1 (II) infinite), it can be
4
B 3 0
s
2
\
3 1
The apparent reaction order from eq 15 is shown as a function of conversion in Figure 6, for the case @ = 0.3. For K < @, n 2 1at all levels of conversion whereas for K > @, n 5 1at all levels of conversion. The apparent reaction order, n, converged on a value of 1 at high conversion. This solution showed the importance of the conversion of part of the thiophene t o a sulfide intermediate; as @ 1the solution approaches the behavior of a simple first-order reaction. This analysis can be readily extended to continuous mixtures that follow the The r-distribution by extending the analysis of results in this case were qualitatively similar to Figure 6, except the values of the apparent reaction order deviated more from unity for low conversions (less than
-
PI.
This analysis shows that when the irreversible series and parallel reactions of hydrodesulfurization of residue are lumped together, the first-order reactions can give any apparent reaction order (-w < n < -1 if partial conversion occurs in the first step, i.e., 1 > ,8 > 0. Equations 8-17 cannot be applied directly to the experimental results, because they were derived for batch reactor, not a continuous-flow reactor, nor does the solution allow for a nonzero initial concentration of
B -
0
!i Ip -1 2 -2 ,&r=O. 75 -3
1
I
G1.0
4
0.0
0.2
0.4
0.6
0.8
1.o
Fractional conversion Figure 6. Apparent reaction order for lumped series reactions as a function of fractional conversion. Reaction order was calculated from eqs 15 and 17 with ,8 = 0.3.
sulfide. This analysis does illustrate, however, how lumping series of first-order reactions can give apparent power-law kinetics.
Acknowledgment. The authors are grateful t o Syncrude Canada Ltd. for financial and analytical support, and to Colin Winkelmeyer for assistance with sulfide determinations. EF940177C
