Ind. Eng. Chem. Process Des. Dev. 1888, 25, 221-229
221
Catalytic Hydrodesulfurization of Dibenzothiophene and a Coal-Derived Liquid Willlam S. O’Brlen,’ Juh Wah Chen, Ramkrlshna V. Nayak, and Gregory S. Carr Department of Mechanical Engineering and Energy Processes, Southern Illinois University at Carbondale, Carbondale, Illinois 6290 7
The catalytic hydrodesulfurization (HDS)reactions of both specific organosulfur compounds and a coalderived liquid
were studied in a continuous, back-mixed, internally circulated (gradlentless)laboratory reactor using a commercial cobalt-molybdenum on alumina catalyst. The HDS of dibenzothiophene yielded experimental reaction rates that were adequately correlated by a Langmuir-Hinshelwood relationship. The computed activation energy and heats of adsorption computed compare quite well with values reported in the literature. The HDS of other thiophenic compounds demonstrated the effects that different alkyl substituents located on the thiophene ring would have on the sulfur-removalreactivity of these compounds relative to the HDS reactivity of dibenzothiophene. Coalderived liquids were prepared by the batchwise solvent extraction and hydrogenation of a Herrin (IL) No. 6 coal with tetralin, toluene, and hydrogen. The coal oil was then catalytically hydrotreated by the same HDS procedure. The HDS rates measured were very adequately represented by a kinetic model that was first-order with respect to the total sulfur concentration and first-order with respect to hydrogen pressure. The value of the activation energy of the HDS pseudoreaction rate constant also compares quite well with those reported in literature, in spite of differences in catalysts, reactor designs, and feed coal liquid origins and compositions.
The depleting petroleum reserves of the United States and the economic-political undesirability of importing vast quantities of oil has increased the attractiveness of utilizing lower grades of petroleum crude or of converting coal, tar, sand, and oil shale into alternative fuels or petrochemical feedstocks. However these alternative fuels and lower grade crude oils contain high concentrations of undesirable heteratoms, such as sulfur, nitrogen, and oxygen. The coal oil refining industry must be particularly concerned with reducing the sulfur and nitrogen content of the coal-derived fuels since these compounds are precursers of several environmental pollutants. One can gain much insight into the HDS mechanisms by reacting model compounds such as some of the individual sulfur-containing organic species which have been identified as being in the heavy petroleum crudes and in the coal-derived fuel alternatives. There have been recent major research projects studying the reactivity of dibenzothiophene (DBT) at the Exxon Laboratories (Espino et al., 1978; Singhal and Espino, 1978; Singhal et al., 1979) and at the University of Delaware (Houalla et al., 1977, 1978, 1979; Broderick et al., 1978; Broderick and Gates, 1980,1981). These investigators have agreed that under lower hydrogen pressures, DBT reacts to form hydrogen sulfide and biphenyl (BPH). But at higher hydrogen pressures, cyclohexylbenzene (CHB) also forms. Evidence has been presented that these two compounds are formed in parallel reactions (Houalla et al., 1978), rather than succession. Broderick and Gates (1981) labeled the immediate removal of the sulfur from DBT to form the BPH as the “hydrogenolysisroute” and the parallel pathway of completely saturating one of the aromatic rings before removing the sulfur atom as the ”hydrogenation route”. The hydrogenation of BPH to CHB is very slow compared to either the BPH formation or the “hydrogenation route” reactions; therefore, the product solution will be composed mainly of a mixture of BPH and CHB, with some of the
* Person to whom correspondence concerning this paper should be addressed. 0196-4305/86/1125-0221$01.50/0
intermediate compounds in smaller concentrations. The initial programs on the HDS of coal-derived liquids usually utilized catalysts which had already been proven successful in hydrotreating petroleum liquids. Frye and Mosby (1967) studied the HDS reactions of a light catalytic-cycle oil in a liquid-phase trickle-flow reactor. Although the oil was a petroleum-derived liquid, the sulfur compounds present were identified to be in the range from benzothiophene (C,) to substituted dibenzothiophenes (C14).Sulfur removal was reported to be first-order with respect to both sulfur and hydrogen concentrations and “clearly” not diffusion-controlled. Gas chromatograph analyses revealed that the lower-boiling sulfur compounds in the oil mixture were significantly easier to desulfurize than were the higher-boiling species. Qader et al. (1968) at the University of Utah reported similar findings for the batchwise HDS of a low-temperature Utah-coal tar. Crynes (1977) described other HDS studies which have been performed by various researchers, and Stiegel(l981) presented a comprehensive summary of different catalysts and supports which have been used to date in the studies of the hydrotreatment of coal liquids. An extensive 3-year HDS study was performed by the Mobil Research and Development Corp., supported by the Electric Power Research Institute (Angevine et al., 1979; Stein et al., 1977, 1978). Nine proprietary catalysts were evaluated in a bench-scale, fixed-bed reactor as to their HDS activities with solvent-refined coal (SRC) liquids made from four different coals. It was reported that a good-quality, low-sulfur boiler fuel could be produced from all the coal-derived feed liquids, using commercial catalysts. However, a ”short-contact-time SRC”coal oil caused an increased catalyst deactivation because of its “crude properties” relative to a regularly formed SRC coal liquid. Frumkin et al. (1981), at the Chevron Research Co., hydroprocessed an SCR-I1 liquid formed from a West Virginia HVB coal in a fixed-bed pilot-scale reactor containing Chevron proprietary catalysts, correlating the total product quality (aromatic and heteratom content) with the severity of the hydrotreatment. As the treatment severity was decreased, both the aromaticity and the heteratom 0 1985 American Chemical Society
222
Ind. Eng. Chem. Process Des. Dev., Vol. 25,
No. 1, 1986
(
e
TO EXHAUST
RUPTURE DISK
TO EXHAUST
t
r
He
1
HIGH PRESSURE SEPARATORS
LIQUID
--
_-. -- --
I
*
I
w METERING PUMP
I
COOL I NG WATER
I
T
l
T
l
SAMPLE POINT
Figure 1. Gas-phase laboratory reactor system.
content of the product oil were significantly higher. Numerous other coal oil hydrotreatment studies have been reported in the literature; however, the data from these studies do not really allow direct comparisons between the results because of the wide variety of coal feed liquids studied, catalyst types tested, and reactor designs and hydroprocessing conditions used. This study was undertaken to systematically investigate the catalytic hydrodesulfurization processes of individual organosulfur compounds and of coal-derived oil mixtures in a continuous, back-mixed reactor. In this reactor system, the chemical behavior of the process can be isolated and identified by easily controlling or eliminating the interference of mass-transfer and catalyst particle structure hinderences. This is accomplished by the careful preselection of the appropriate experimental parameters during the investigation. Furthermore, the rate of reaction can be determined from the experimental data directly, making the chemical reaction rate information much easier to interpret.
Experimental Section Equipment. The experiments were performed in a high-pressure, continuous-flow, interpally circulated catalysis reactor (Autoclave Engineering, Erie, PA), originally designed by J. M. Berty (1974). The total reactor system is illustrated in Figure 1. A four-way solenoid valve in the inlet gas stream line allows the reactor system to heat up and stabilize under pure helium flow before switching to hydrogen or hydrogen/ helium mixtures at the start of the reaction. The organic liquid (i.e., dibenzothiophene or coal liquid dissolved in the toluene or benzene carrier) is pressurized by a calibrated, variable-flow metering pump and thoroughly vaporized before mixing with the gas stream and entering the reactor body.
The product stream leaving the reactor was cooled, and the condensed liquids were separated from the gas stream before the gas pressure was reduced to atmospheric. A large high-pressure liquid separator continuously separates the liquids. A smaller separator is bypass-connected so that liquid samples can be intermittently trapped for analysis. Materials. Catalyst. The commercial, hydrotreating catalyst used in this study was “Katalco 477” (formerly called “Nalcomo 477’7, a 14.0% MOO, and 3.3% COOon y-alumina (wt %) catalyst supplied by the Katalco Corp. The catalyst surface area was 208.6 m2/g and the total pore volume was reported by the manufacturer to be 0.55 X lo* m3/g. The catalyst was precalcined at 700 K for 1 h, as recommended by the manufacturer, and then crushed in a ball mill and sieved to the various desired particle size ranges. About 30 g of the sized, precalcined catalyst was placed in the draft-tube catalyst basket of the continuous backmix reactor described in the previous section. A stream of 10% hydrogen sulfide and 90% hydrogen (vol % ) was then passed through the reactor system at atmospheric pressure, while the reactor was heated to 673 K, held at that temperature for 2 h, and then cooled to room temperature. After purging with helium, the catalyst particles were then removed from the reactor chamber and stored in sealed glass containers until used in the hydroprocessing experiments. Chemical Reactants, Carrier Solvents, and CoalDerived Liquid. All the chemicals in this study were reagent grade and used as received. The coal used to make the coal liquid in this study was obtained from the Eagle Surface Mine of the Peabody Coal Co. near Shawneetown, IL. The proximate analysis of this Herrin (IL) No. 6 seam coal is given in Table I. This coal is ranked as high-volatile C bituminous. The -40+100-
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 1, 1986
Table I. Proximate Analysis and Other Properties of Reactant Coal (Herrin (IL) No. 6 Seam, Eagle Surface Mine, Peabody Coal Co., Shawneetown, IL) wt %, moisture-free basis ash 12.30 volatile matter 38.00 fixed carbon* 49.70 sulfur 3.085 28.028 MJ/kg (12053 Btu/lb) high heat of combustion a
% fixed carbon = 100 - ( % volatile matter
+ % ash).
Table 11. Range of Experimental Operating Parameters total pressure, MPa 3.4-12.2 temp, K 540-695 catalyst particle size (most expts) U.S. std mesh size -25-35 av diameter, wm 587 catalyst load range examined, g 2-45 2 used for most HDS expts, g hourly space velocity, gmol of S feed/(g of catalyst-h) model compd expts 1.24 x 10-3 to 10.5 x 10-3 to 266.9 X coal liquid expts 33.8 X
mesh cut fraction of this coal was oven-dried a t 383 K before being used to make the coal liquid. The coal liquid was prepared in a 300-mL, magnetically stirred, batch reactor (Autoclave Engineers, Inc., Erie, PA). A mix composed of 50 g of dried coal, 75 mL of toluene, and 25 mL of tetralin was charged into the batch reactor. After purging several times, the reactor and mix were then pressurized a t room temperature to about 5.5 MPa (800 psig) with hydrogen, and the reactor stirrer was set at 700 rpm. After 30 min of heating, the reactor and mixture reached about 645 K, and the internal hydrogen pressure was in the range of 20-21 MPa (2900-3100 psig). The reaction mix was held in the 640-665 K range for 1 h. Then, the reactor and contents were quickly cooled to room temperature, after which the reactor gas, usually in the range of 3.4-MPa (500 psig) pressure, was vented off. About 80-90 mL of dark liquid and approximately 25-30 g of solid coal residue were recovered from each reaction batch. The liquid product was filtered twice to assure complete removal of all suspended fines. The crude liquid was then vacuum-distilled (0.67-kPa absolute pressure of helium) into two fractions. The viscous black distillate liquid fraction boiling above 491 K (218 “C) represents the directly hydrogenated, coal-derived liquid product. After dissolving in the carrier solvent toluene, a centrifugation was necessary to separate out the micrometer-sized coal particles that had escaped earlier filtration. This coalderived liquid was stored under an inert gas (helium or argon) until it was used. Experimental Procedure. The desired quantity of the presulfided catalyst was placed in the reactor basket, and the reactor was purged with helium. The reactor was then heated under continuous helium or hydrogen bleed flow, taking about 3 h to reach the set-point temperature. The ranges of experimental operating parameters for this experimental study are summarized in Table 11. Near-steady-state conditions were found to be reached in a little over 1 h after the thermal stabilization of the reactor. When the HDS of the model sulfur compounds was studied, the reactor was usually kept operating for several days in a row, with about 2-3 h of operating time required for each set of operating parameters (pressure, temperature, specific feed composition, etc.). When it was necessary to cool the reactor, a flowing helium atmosphere was always maintained. A slight flow of helium was always
223
passing through the reactor whenever the reactor was on stand-by status. In the 38 experiments desulfurizing the dibenzothiophene, the original presulfided catalyst was not changed. The experiments were performed in random sequence, and no more hydrogen sulfide was injected at any time to resulfide the catalyst. No significant effect on the reaction rates was observed under similar experimental conditions. It was concluded, therefore, that enough H2S was being continually produced in the desulfurization reactions to maintain the catalyst in an active, sulfided state. In the case of hydrotreating of the coal liquid, a fresh aliquot of catalyst was used for each experimental reaction condition set. Therefore, after each experiment, the entire reactor system was cooled down, thoroughly cleaned, recharged with fresh presulfided catalyst, and then reheated to the next experimental operation conditions. In all the coal liquid experiments, the sulfur concentrations of the product samples taken every 15-min during the last hour of the 150-min experimental run were nearly constant, showing that steady-state reaction conditions existed. No indication of catalyst activity degradation was observed in any experimental run. The concentrations of these steady-state product samples were averaged to represent the experimental output-product data value. Analyses of Products. The composition of the liquid reactant and the product streams in the “model compounds” experiments were analyzed by gas chromatography (Nayak, 1979). The analyses of the feed and product sulfur contents of the coal-derived liquids were performed on a Fisher Model 475 sulfur analyzer (Fisher Scientific Co.). It was necessary to take special care in sample preparation and analysis procedure in order to obtain repeatable results for the below 0.1 % sulfur values of the more volatile coal liquid samples. By using the extra care and certain analytical procedure modifications (Carr, 1982), the accuracy of even the low-sulfur product oils was better than f3% of the available sulfur, with the repeatability computed to be a 2% standard deviation.
Results Computation Form of the Reaction Rate. The overall disappearance rate of the organosulfur species in a back-mixed reactor can be expressed in two forms: total reaction rate
r’ = L(Ci - C,)
(1)
where r’ = total sulfur disappearance rate (gmol of S consumed/h), L = liquid stream feed rate, L/h, Ci = concentration of sulfur compound in inlet flow stream, gmol/L, and C, = concentration of sulfur compound in outlet flow stream and specific reaction rate
r, = r’/m,
(2)
where r, = specific sulfur disappearance rate (gmol of S consumed/(g of catalyst-h) and m, = mass of catalyst in reactor, g. The methods of using and correlating the various reaction rates found during the experimental studies are discussed later in this paper. Evaluation of Reactor Operation Parameters. An initial set of experiments was designed to determine the reactor operating parameters which minimize the mass-
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Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 1, 1986
transfer resistances from the bulk fluid phase to the catalyst surface through the pores into the catalyst interior. Also, the operating parameters were chosen which would optimize the measurement and evaluation of the HDS reaction rate (Nayak, 1979). In these initial experiments, dibenzothiophene (DBT) was used as the model organosulfur chemical species to be examined. Benzene proved quite suitable as the carrier fluid for the reactants. The hydrogenation conversion of the benzene a t 601 K and 7.0-MPa total pressure was undetectable without the catalyst, and the conversion was 0.5% when the catalyst was present. At 660 K and without the catalyst, less than 3.4% of the incoming DBT reactant was desulfurized. The mass-transfer resistance to the movement of components in to and out of the pore structures of a catalyst particle is very much dependent on the internal pore length, which in turn is a function of the particle diameter. The as-received catalyst extrudates were crushed and sieved into three particle-size fractions ranging from 355 to 1400 pm. HDS experiments were performed with each particle-size fraction at 601 and 660 K. There was essentially no effect on DBT conversion rate when the catalyst particle sizes were less than about 700 pm. Thus, intrapore diffusion resistances did not affect the HDS reaction rates when the smaller-sized catalyst particles were used. The use of the 587-pm (-25+35 mesh) catalyst size in the rest of the experimental study was justified. The internal circulation ratio of the reacting gases passing through the bed of catalyst particles can be varied by increasing or decreasing the rotation rate of the impeller inside the reactor chamber. The changing gas flow recirculation could affect the reaction rate by increasing or decreasing the mass-transfer resistance, along with the degree of back-mixing. In three experimental runs, when the impeller rotation was slowed from 1500 rpm to values as slow as 500 rpm, the DBT conversion rates varied less than 1%. Therefore, the complete back-mixed condition was achieved at these recirculation rates. The use of the 1400 rpm rate during the rest of the experimental study was justified. The kinetic reaction rate of the DBT disappearance, rs, is most usually expressed in terms of the gram moles of DBT reacted per gram of catalyst present per hour. In designing a commercial reactor, it is desirable to provide an excess of catalyst sites to maximize the DBT conversion. However, in a study of the fundamental catalysis mechanisms, the experiments should be designed to maximize the reaction capability of each active catalyst site. This condition would be recognized by a linear relationship between the total reaction rate, r’, and the amount of catalyst present in the reactor. In HDS experiments at 562,576, and 618 K, the linearity of the reaction rate with catalyst amount was found to exist by using as much as 3 g of catalyst. If more than 3 g of catalyst was present, the availability of DBT apparently controls the reaction rates, while the available active catalyst sites are in excess and not used to their fullest capability. Therefore, for the remainder of the experimental study, 2 g was the amount of catalyst used in the reactor basket during each experiment. HDS of Model Compounds. Because it is almost always found in significant quantities in coal-derivedliquids, dibenzothiophene (DBT) was selected as the major organosulfur compound to be examined. The HDS kinetic rates of DBT can represent those found when hydrotreating coal-liquid solutions. Later in the experimental study, the HDS of five other thiophenic derivative species
were compared with that of DBT. Dibenzothiophene Hydrodesulfurization Experiments. Thirty-eight experiments were performed to develop a rate expression for the HDS of DBT. The following reactor operating parameters were maintained: catalyst, 2.0 g of “Katalco 477” (Katalco Corp.) 25/35 U S . mesh size (average 587 pm) particles; reactor internal impeller rotation rate, 1400 rpm; total pressure, 5.5-12.2 MPa (800-1770 psia); temperatures, 562, 576, 590, and 601 K (552, 577, 602, and 622 OF). At each temperature, 8-11 experiments were conducted to examine the different variables (hydrogen pressure, reactant flow rate, etc.) which could affect the correlation of the reaction rate relationships. The liquid products from each experiment were analyzed for unreacted DBT, biphenyl (BPH), and cyclohexylbenzene (CHB). Material balances computed for all the experiments indicated that the products could be accounted for with a deviation, defined by eq 3, of less than *3%. % unaccounted reactant = 100 100 (3) ([(Co)DBT + (Co)BPH + (C,)CHBI/(Ci)DBT) For each of the 38 experiments, the DBT disappearance rate, in the form of the “specific reaction raten of eq 2, was computed, as well as the partial pressures of all the major products. Correlation of DBT Disappearance Data. A total of 17 “single-site” and 17 “double-site” rate models of the Langmuir-Hinshelwood type were postulated, based on the following assumptions: (a) only one chemical step is rate controlling; (b) the heat of adsorption is independent of the amount or type of gas species adsorbed; (c) there is no attractive or repulsive interaction between the adsorbed molecules if two or more gases are present at the same activity site; and (d) the reverse reaction is negligible under the reaction conditions. The 34 rate equations were derived following the method outlined by Hougen and Watson (1947) and are listed in Nayak (1979). A least-squares fit regression computation method was used to compute the chemical reaction coefficient, k, and the adsorption equilibrium constants, Ki, for each of the postulated models. The resultant values of the constants and coefficients were then screened for physically unfeasible conditions such as (a) negative constants or coefficients, (b) non-zero adsorption coefficients when the model postulates that the component does not take a significant part in the reaction, and (c) a negative slope of an adsorption coefficient with increasing temperature. Among all the models considered, the only model meeting all three of the criteria listed above was that of a single-site model which assumes the reaction occurs on that site between adsorbed dibenzothiophene (DBT) and molecularly adsorbed hydrogen, with the surface reaction being the controlling mechanism. The rate expression for this model is rs =
~ ( K D B T ~ (KH~PHB) DBT) (1 +
(4)
(KDBTPDBT)
+ ( K H ~ ~+H(KH,S~H,S))~ J
The Arrhenius plot of the reaction rate constant, k vs. the reciprocal of the temperature is shown in Figure 2. The van’t Hoff plots of the adsorption equilibrium coefficients for the components DBT, hydrogen, and hydrogen sulfide are shown in Figure 3. Each point on Figures 2 and 3 represents the numerical constant/coefficient value a t that respective temperature as computed, using from 8 to 11experimental data points, in the regression solution of eq 4. The straight lines drawn in these plots represent the best least-squares linear fit of the points. The rela-
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 1, 1986 225 100.0 I-
t
ll.
0
-
0
z
z
t
5
I
I
I 75
I70
-
I80
- (KPX lo3 Figure 2. Arrhenius plot: dibenzothiophene HDS reaction rate coefficient. RECIPROCAL OF TEMPERATURE
Table 111. Experimental Deviation Parameters for Dibenzothiophene Hydrodesulfurization Reaction Rate Correlation
rs =
~KDBTKH~DBTPH~
01 I65
(1 + KDBTPDBT -!-K H p H 2 + K€I#H$d2
individual expt temp?
K 562.56 576.46 590.36 601.46 all exptsb
no. of exDts 8 10 11 10 39
106(d) 0.027 0.451 7.370 1.057 2.234
103(d’) 5.08 34.26 298.62 28.67 106.20
103(d’)0u 0.64 3.42 27.15 2.87 2.72
Correlation using temperature-associated coefficients shown on Figures 2 and 3. *Correlation using relationships given as eq 5, 6, 7, and 8.
tionship constants found from the Arrhenius plot and the van’t Hoff plots are listed below: It = 9.396 X 1O’O exp(-138.3/RT) (5)
KDBT= 1.800 X K H p= 1.134 X
exp(51.8/RT)
(6)
exp(138.5/RT)
(7)
KHZs= 8.955 X
exp(55.1/RT) (8) For comparison purposes, a term, (+), is defined as the s u m of the squares of the differences between the observed experimental reaction rate and the rate predicted by the correlating relationship being considered. n
(4) =
E i=l
[(TsIexptl-
(rs)calcd12
(9)
In order to eliminate the magnitude bias when comparing the observed vs. predicted rate values, another term, (V), is defined as the dimensionless sum of the square of the differences between the observed rate and the correlation-computed rate.
I
I70
I75
I 80
RECIPROCAL OF TEMPERATURE - (OK)-’ X IO3 Figure 3. van’t Hoff plots: dibenzothiophene HDS adsorption rate coefficients.
Table IV. Relative Hydrodesulfurization Rates of Different Sulfur Compounds specific HDS (HDS rate, gmol of S rate/HDS reacted/(g of rate of cata1yst.h) DBT) thiophene 21.1 x 10-3 6.9 2-methylthiophene 15.7 x 10-3 5.1 2-ethylthiophene 14.5 x 10-3 4.71 2,5-dimethylthiophene 10.8 x 10-3 3.51 dibenzothiophene (DBT) 3.07 X 1.0
This latter term can be averaged to help in evaluating and comparing different correlation relationships.
The values of these several terms relating the goodness-of-fit of the correlation relationship 4 to the dibenzothiophene HDS experimental data are listed in Table 111. The experimentally measured reaction rates very closely matched those predicted by eq 4. The averaged deviation of all 39 experimental rates was 5.2% of the rate values computed by using eq 4 and generalized coefficient relationships 5, 6, 7, and 8. HDS of Other Organosulfur Chemical Compounds. In an effort to compare the kinetic reaction rate of DBT disappearancewith the HDS of other chemical compounds, a set of five experiments was performed using 2 g of catalyst in the reactor, a reactor hydrogen pressure of 4.1 MPa, and a temperature of 575 K. Only the single organosulfur reactant species in the feed stream was varied. The feed rate of each reactant species feed stream was
226
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 1, 1986
proportioned so that the same molar sulfur input rate was maintained for all experiments in the set. The specific reaction rate of the sulfur compound disappearance was calculated according to eq 2 for each experiment. The kinetic rates of the HDS reactions of these five compounds are listed in Table IV, in decreasing order of reactivity. HDS of a Coal Liquid. Eighteen experiments were performed to study the hydroprocessing reactions of the coal-derived liquid. Coal Liquid HDS Experiments. In each experiment, 2 g of fresh, presulfided catalyst was used, and the internal impeller rotation rate was maintained at 1400 rpm: reaction temperatures, 543, 583, 643, and 693 K; reaction total pressure, 3.45, 6.90, and 10.35 MPa; coal-liquid concentration, 3%, 5%, and 8% in toluene (weight basis); total liquid flow rate, 0.7, 1.5, and 3.0 mL/min. For each of the 18 experiments, the desulfurization rate in the form of the specific reaction rate of eq 2 was computed. Correlation of the Coal-Liquid HDS Rate Data. The kinetic model for the total sulfur removal in an isothermal, completely mixed reactor was developed based upon the following assumptions: (a) The partial pressure of hydrogen was constant throughout the reaction. (b) The molar concentration of hydrogen was very large relative to that of the sulfur species or any other hydrogenconsuming secondary reactants. (c) The HDS reaction rate was first-order with respect to the sulfur-containing chemical species. (d) The HDS reaction rate was firstorder with respect to the hydrogen concentration. Therefore, the HDS reaction rate, r,, would be proportional to r, = k'CSPHZ
(12)
where k ' = pseudo-reaction rate constant, Cs = sulfur species concentration, P H 2 = partial pressure of hydrogen (MPa). The fractional conversion value can be defined as
X S = (Ci - CJ/Ci
(13)
V
E 3
/ /"
1 2 -
1
b BLL
0 6 -
LL
0 4 -
4c
0 2 -
0
643K
V 2 LL 3
1
I
01
0 2
1
03
04
I
0 5
I
06
07
I
08
I
09
LL
RECIPROCAL OF LlOUlD HOURLY SPACF VELOCITY-LHSV (m3/gm catalyst hr 1 -
Figure 4. First-order relationships for total sulfur removal from coal liquid. Y .
C
I--
5
-1.00
G
I
v)
z
8 z 2 I-
O
a W
LT 0
-2.00 '
n 3 W
v)
where Ci = sulfur concentration in the entering feed and C, = sulfur concentration in the exiting product. Since the reactor system is assumed to be completely mixed, with Cs = C,, the experimental data can be correlated in the form
xs/(l - x,)= k'PH,/LHSV
LL
0
I
4I c LL
\
a W
0
(14)
The liquid hourly space velocity (LHSV) is defined as the rate of the coal liquid volume (at room temperature and pressure) fed to the reactor per hour expressed per mass of catalyst. This expression of the superficial contact time is used with the realization that, in reality, the liquid feed is vaporized as it enters the reaction chamber. The k' value found for each coal liquid HDS experimental reaction is a pseudo-rate constant since it represents the combined effects of a series of simultaneous HDS reactions of a variety of organosulfur chemical species, each reacting according to its individual concentrations (Gates et al., 1979). The comparison of Xs/(1- Xs)with the reciprocal of the LHSV in Figure 4 for the constant-hydrogen-pressure experiments demonstrates that the HDS reaction kinetics can be represented in the form of eq 14. The Arrhenius plot for the pseudo-rate constant, k ', as shown in Figure 5, is linear, allowing the value of k ' to be represented by the relationship
k' = 71.5 exp(-31.7/Rn
a
(15)
-1 _I
$ 3
l-
a
z
-3.00
I
I 1.40
I
1.50
I
I
1.60
1.70
RECIPROCAL OF TEMPERATURE (K)"
1.80 X
I .90
IO3
Figure 5. Arrhenius plot: coal liquid HDS pseudoreaction rate coefficient.
The HDS rate relationship of eq 14, with the value of k' predicted by eq 12, very adequately correlates the entire set of coal liquid HDS experimental data, with varying mass ratios of coal-liquid to carrier solvent as shown in Figure 6 and with the different reaction pressures and temperatures as shown in Figure 7. The standard deviation of the experimentally found data from the total sulfur removal values predicted by the kinetic model is 6.6%.
Discussion of Results HDS of Model Compounds. The attempts to fit the experimental data from this study into the 34 models of the Langmuir-Hinshelwood type demonstrated that the best correlation involves a model which assumes that the
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 1, 1986 100
-
80 70 -
90
Table V. Comparison of Experimental Results, Activation Energy, and Heats of Adsorption this Exxon Delaware study Study Studyb reaction activation energy, 138.3 163.7 125.6 kJ/gmol heats of adsorption, kJ/gmol dibenzothiophene -51.8 -18.8 hydrogen -138.5 -105.5 35.2 hydrogen sulfide -55.1 -22.2
6 . 8 9 MPa
PRESSURE
TEMPERATURE = 643K
_J
9
2
60-
W
LL LL 3 LL
50
-
"Espino et al., 1978. bBroderick and Gates, 1980.
40-
with competitive adsorption of the reactants and products on a single site.
v)
30-
~G(KDBTPDBT)(KHJ'HJ
IO -
20
b
" = (1 + KDBTPDBT + KprodPprod) (l + KHzPH1)
1
1
.I
7 -
-3
4
5 -
6.
1
7
0
8
9
,
IO
W T % COAL LIQUID IN FEED
Figure 6. Relationship of % sulfur removed with sulfur content of coal liquid feed.
90 -
100
2 2
5 0
80
227
-
(16)
term in eu 16 refers to the properties The (K,,d'nmJ r.v.. w."". of all the reaction products: BPH, CHB, H2S,Ltc. Nayak (1979) Derformed a limited set of experiments with additionh-quantities of BPH and CHB mixed in the reactant DBT feed stream. He found that the inhibiting effects of the BPH and/or CHB on the disappearance of DBT Seem to be negligible, with H,S as the only product species significantly affecting the reaction race. At the University of Delaware, Broderick and Gates (1980) found that overall DBT desulfurization data were best correlated by the relationship
~ ~ K D B T ~(KH~CHJ DBT)
W
rs =
2-
Their liquid-phase reactions were performed in a highpressure flow microreactor containing a commercial Co/ Mo on y-alumina (American Cyanamid HDS-16A) crushed to 80-100 mesh. In a later work, Broderick and Gates (1981) defined the rate relationships for each of the two parallel DBT-HDS reaction paths. For the hydrogenolysis route, with BPH as the primary product, the rate equation was of the same form as eq 17. For the DBT hydrogenation pathway, with CHB as the principal end product, the rate equation was
m 60 c 0 50 LL a 1 40 > 70
;
A
0
543 K 583 K
[7 643 K
0
V
693 K
30-
LL
LL 3
20
-
s IO
1
I
I
1
I
I
I
1
1
20
30
40
50
60
70
80
90
100
(1 + KDBT~DBT + KH~&H,S)* (1 + KH~CHJ
r, =
~ ~ K D B T ~(KH~CHJ DBT) (1 + KDBT~DBT)
(17)
(18)
EXPERIMENTAL VALUES OF % SULFUR REMOVAL
Figure 7. Comparison of experimental coal liquid HDS results with % removal values predicted by eq 14.
reaction occurs on a single catalyst site between the adsorbed dibenzothiophene (DBT) and the molecularly adsorbed hydrogen. Similar conclusions have been made by Espino et al. (1978) at the Exxon Laboratories and Broderick and Gates (1980) at the University of Delaware. There are several significant differences between the experimental reactor systems of the three studies. Both the Exxon project and the Delaware project utilized a plug-flow, liquid-phase, fixed-bed microreactor, while this study was performed in a completely mixed reactor. While all three catalysts were Co/Mo on alumina, each was commercially manufactured by a different company. In spite of all these different reactor operating parameters, the developed reaction rate correlations were very similar. The numerators of all three correlations were identical, with only slight differences in the form of the denominators. Espino et al. (1978) found that their experimental DBT HDS data could be best correlated by a dual-site model
The coefficients found in the present gas-phase study are compared in Table V with those found by Espino et al. (1978) and by Broderick and Gates (1980). The reaction activation energy values were all quite close. The heat of adsorption values for hydrogen was -138 kJ/gmol in this study and -105 kJ/gmol in the Exxon study. The University of Delaware investigators found the value to be +35 kJ/gmol, with a positive value for the hydrogen heat of adsorption computed in all the reaction rate correlations considered. They acknowledge a large confidence limit about their data, and the data do list some negative values of this coefficient. Values between -84 and -251 kJ/gmol for the heat adsorption of hydrogen have been reported in the literature (Pannetier and Souchay, 1967; Sinfelt, 1975; and others). As reported previously by Gates et al. (1979) and Satterfield (1980), the HDS of other thiophenic compounds showed a decreasing reactivity with increased substitution on the thiophenic ring. Broderick and Gates (1981) reported that DBT was "one to two orders of magnitude less reactive than thiophene". In this study, the HDS reactivity of DBT was found to be about 15% as reactive as
228
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 1, 1986
Table VI. ComDarison of Desulfurization Activation Energies for Coal Liquids investigators this study
feedstock solvent-extracted, Herrin (IL) No. 6 coal, Eagle Surface Mine Angevine et a]., 1979 solvent-refined coal Herrin (IL) No. 6 coal, Monterey Mine Angevine et al., 1979 short-contact-time, solvent-refined coal, Western Kentucky carbonization coal tar, Utah coal Qader et al., 1968 Stein et al., 1977 SRC-I recycle solvent Stein et ai., 1977 H-Coal distillate
thiophene. Nag et al. (1979) reported that DBT was 4.4% as reactive to hydrotreating as was thiophene. HDS of Coal-DerivedLiquid. The experimental data obtained in this study indicate that the HDS reaction of the sulfur compounds in the coal liquid is first-order with respect to the overall sulfur concentration. Gates et al. (1979) states that, “the rate of hydrodesulfurization of any feed is generally consistent with the first-order reaction of each of a series of sulfur-containing compounds”. Other researchers (Crynes, 1977; Frye and Mosby, 1967; Garg et al., 1980; Greene, 1981; Qader et al., 1968; Stiegel et al., 1981; Thakkar et al., 1981) have developed kinetic models which involve first-order dependence with respect to both sulfur and nitrogen removal when processing a variety of coal-derived materials. The feed is considered as containing a mixture of organic sulfur compounds, each of which reacts at a rate proportional to its concentration (Gates et al., 1979). Since the liquid used in this study was derived from a high-volatile bituminous Herrin (IL) No. 6 coal, a higher-ranked starting material, the organic sulfur compounds present are more likely to be thiophenic in nature (Attar and Dupuis, 1979). Also, during the coal liquefaction step, the coal-derived material was exposed to a hydrogen/ hydrogen-donor system at 625 K. These liquefaction-formation reactions also rupture the highly reactive sulfide and disulfide linkages, as well as initiate the HDS of the less-reactive thiolic and thiophenic bonds. The reaction rate relationship developed in this study, as represented by eq 14, indicated that the HDS rate is linearly related to the hydrogen pressure throughout the 3.4-10.3-MPa range that was examined. This first-order dependence with respect to the hydrogen partial pressure has been suggested by several researchers (Frye and Mosby, 1967; Greene, 1981; Weller et al., 1951a,b; Gates et al., 1979) investigating the desulfurization and denitrogenation of coal-derived liquids. However, both Crynes (1977) and Qader et al. (1968),when reporting work in the hydrotreatment of carbonization coal tar, noted that the pressure has a strong influence on the HDS rate up to about 10.3 MPa, which is the upper limit of this study, but then the HDS rate becomes relatively insensitive with larger hydrogen pressures. A comparison of the HDS activation energy measured during this study is listed in Table VI with those found in other studies of the hydrotreating of coal-derived liquids. It is important to note that the feedstocks are all produced from different coals and by different liquefaction techniques. The coal liquid used for this study was produced batchwise by using a specific formula of toluene, tetralin, and hydrogen to obtain a reproducible starting material. Coal liquids derived by the SRC process are produced by solvent-refining the coal with a recycle solvent. The relatively higher desulfurization activation energy reported by Angevine et al. (1979) for the short-contact-time SRC may be expected, considering the feed coal was subjected
reactor type catalyst complete mixed CoMo “Katalco 477”
activation energy, kJ/gmol 31.7
fixed bed
NiMo “Harshaw 618X”
53.8
fixed bed
NiMo “Harshaw 618X”
100.2
batch fixed bed fixed bed
W S pMatheson, Coleman & Bell Co. American Cyanamid NiMo “HDS-SA” American Cyanamid CoMo “HDS-1441A”
46.0 65.7 85.3
to only a few minutes of reaction conditions compared to the conventional SRC procedure of 40-60 min of reaction time. Therefore, the degree of thermal destabilization and sulfur compound reduction by hydrogen may be less. Coal liquids from the H-Coal process are produced by subjecting coal, in a coal oil slurry, to hydrogen and a cobalt-molybdenum catalyst in an ebullating bed reactor. During the H-Coal’s original formation reaction, the readily reducible sulfur compounds may have already been catalytically reacted and removed, thus leaving the sulfur compounds remaining for hydroprocessing that are much less reactive, as is reflected in the relatively higher activation energy. Qader et al. (1968) report that for the 46 kJ/gmol activation energy determined, chemical reactions and not physical processes are rate-controlling. Frye and Mosby (1967),when discussing the HDS of light catalytic recycle oil, stated that the “high value” of 63-84 kJ/gmol for the activation energy indicates that the reaction was not limited by diffusion but was chemical-reaction-controlled.
Acknowledgment This research study was financially supported by the Coal Extraction and Utilization Research Center, Southern Illinois University at Carbondale. The catalyst used in the experiments was provided by the Katalco Corp., and the coal was donated by the Peabody Coal Co.
Nomenclature C, = concentration of sulfur, inlet flow stream, gmol of S/L C, = concentrationof sulfur, outlet flow stream, gmol of S/L C, = total sulfur concentration; gmole of S/L k = chemical reaction coefficient for individual model compound HDS, gmol of S consumed/(g of catalyst-h) k = pseudo-chemical reaction coefficient for coal liquid HDS, gmol of S consumed/ (g of catalyst-h) Ki = adsorption equilibrium coefficient, MPa-’ kI6, kl,, k I 8 = chemical reaction coefficientsfound in eq 16, 17, and 18, gmol of S consumed/(g of cata1yst.h) L = liquid stream flood rate, L/min LHSV = liquid hourly space velocity, m3 of coal liquid/ (g of catalyst-h) m, = mass of catalyst in reactor, g n = number of experiments in set Pi = partial pressure of individual chemical species i, MPa R = gas law constant, 0.00831434 kJ/(gmol.K) r = total sulfur disappearance rate, gmol of sulfur consumed/ h r8 = specific sulfur disappearance rate, gmol of sulfur consumed/(g of catalyst-h) T = temperature, K Xs = fractional sulfur conversion defined by eq 13 4 = correlation deviation parameter defined by eq 9 4’ = dimensionless correlation deviation parameter defined by eq 10
Ind. Eng. Chem. Process Des. Dev. 1988, 25,229-236
= average of the values of 4'for the experimental set, defined by eq 11
(r$)av
Abbreviation H2 = hydrogen H2S = hydrogen sulfide DBT = dibenzothiophene BPH = biphenyl CHB = cyclohexylbenzene HDS = hydrodesulfurization Registry No. DBT, 132-65-0;Co, 7440-48-4;Mo, 7439-98-7; thiophene, 110-02-1; 2-methylthiophene, 554-14-3; 2-ethylthiophene, 872-55-9;2,5-dimethylthiophene,638-0243, L i t e r a t u r e Cited Angevine, P. J., Becker, M.; Callen, R. B.; Babkowski, M. J.; Granchi, M. P.; Green, L. A.; Heck, R. H.; Simpson, C. A.; Shih, S. S.; Stein, T. R. Report No. EPRI AF-1255, Electric Power Research Institute, Palo Alto, CA, 1979. Altar, A.; Dupuis, F. frepr., Div. Fuel Chem., Am. Chem. SOC. 1979, 24, 1. Betty, J. M. Chem. Eng. f r o g . 1974, 70(5), 78. Broderick, D. H.; Gates, B. C. f r e p r . , Div. Fuel Chem., Am. Chem. SOC. 1980, 25 (I), 53. Broderick, D. H.; Gates, B. C. AIChE J. 1981, 2 7 (4), 663-673. Broderick, D. H.; Schuit. G. C. A,; Gates, 6. C. frepr.-Div. Fuel Chem ., Am. Chem. SOC. 1978, 23, (l), 92. Carr, G. S.. unpublished Master of Science thesis, Thermal and Environmental Engineering Department, Southern Illinois University at Carbondale, Carbondale, IL, 1982. Crynes, B. L. "Chemlcal Reactions As a Means of Separatlon: Sulfur Removal"; Marcel Dekker: New York, 1977; pp 73-148. Espino, R. L.; Sobel, J. E.; Singhal, G. H.; Huff, G. A,, Jr. Prepr., Div. offetr. Chem., Am. Chem. SOC. 1978, 23(1), 46. Frumkin, H. A.; Sulllvan, R. F.; Stangeland, B. E. I n "Upgrading Coal Liquids": Sullivan, R. F., Ed.; American Chemical Society: Washington, DC, 1981; ACS Symp. Ser. No. 156, pp 75-113. Frye, C. G.; Mosby, J. F. Chem. Eng. frog. 1967, 6 3 , 9. Garg, D.: Tarrer, A. R.; &In, J. A,; Clinton, J. H.; Curtis, C. W.; Paranjape, S. M. Fuel R o c . Technol. 1980, 3, 263-284. Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. "Chemlstry of Catalytic Processes"; McGraw-Hill: New York, 1979; pp 390-41 1. Greene, M. I.F u e l f r o c . Technol. 1981, 4, 117-144.
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Houalla, M.; Nag, N. K.; Sapre. A. V.; Broderick, D. H.; Gates, B. C. AIChfJ. 1978. 24 - (6). ,-,. 1015-1021. Houalla, M.; Nag, N. K.; Sapre, A. V.; Broderick, D. H.; Gates, B. C. Paper presented at the 6th North American Meeting, The Catalysls Soclety, Chicago, IL, March 1979. Houalla, M.; Broderick, D.; deBeer, V. H. J.; Gates, B. C.; Kwart. H. frepr., Div. o f f e t r . Chem., Am. Chem. SOC. 1977, 22(3), 941. Hougen, 0. A.; Watson, K. M. "Chemical Process Principles: Part 111"; Wiley: New York, 1947; pp 906-956. Nag, N. K.; Sapre, A. V.; Broderick, D. H.; Gates B. C. J. Catal. 1979, 57, 509-5 18. Nayak, R. V., unpublished Master of Science thesis, Thermal and Environmental Engineering Department, Southern Illinois University at Carbondale, Carbondab, IL, 1979. Pannetier, G.; Souchay, P. "Chemical Kinetics"; Elsvier: London, 1967; p 317. Qader, S. A.; Wiser, W. H.; Hill, G. R. Ind. Eng. Chem. Process Des. Dev. 1988, 7, 3. Sapre, A. V.; Gates, B. C. Prepr., Div. Fuel Chem ., Am. Chem. SOC. 1980, 25 (I), 66. Satterfield, C. N. "Heterogeneous Catalysis I n Practice"; McGraw-Hill: New York, 1980; pp 259-265. Sinfelt, J. f r o g . Solid-state Chem. 1975, 10 (2). 55. Singhal, G. H.; Espino, R. L. frepr., Div. f e t r . Chem., Am. Chem. SOC. 1978, 23 (l), 36. Singhal, G. H.; Espino, R. L.; Sobel, J. E. Paper presented at the 8th North American Meeting, The Catalysis Society, Chicago, IL, March, 1979. Stein, T. R.; Bendorwitis, J. G.; Cabal, A. V.; Dabkowski, M. J.; Heck, R. H.; Ireland, H. R.; Simpson, C. A. Report No. EPRI AF-444, Electric Power Research Institute, Palo Alto, CA, 1977. Stein, T. R.; Cabal, A. V.; Callen, R. 6.; Dabkowski, M. J.; Heck, R . H.; Simpson, C. A.; Shih, s. s. Report No. EPRI AF-873, Electric Power Research Institute, Paio Alto, CA, 1976. Stiegel, G. J.; Shah, Y. T.; Krishnamarty, S.; Panvelker, S. V. "Reaction Englneering in Direct Coal Liquefaction"; Addison-Welsey Publishing Co.: Boston, 1981; pp 285-381. Thakkar, V. P.; Baldwin, R. M.; Bain, R. L. Fuel R o c . Technol. 1981, 4 , 234-250. Weller. S.; Pelipetz, M. G.; Friedman, S. Ind. Eng. Chem. 1951a, 43 (7), 1572-1 575. Weller, S.;Pellpetz, N. G.; Friedman, S. Ind. f n g . Chem. 195lb, 43 (7), 1575-1579.
-.
Received for review October 3, 1983 Revised manuscript received March 25, 1985 Accepted July 3, 1985
Weighting Factors To Obtain Kinetic Parameters from Integral Reactors with Differential Reactor Methods Sorab R. Vatcha Massachusetts Institute of Technology, Cambridge, Massachusetts 02 139
Dady B. Dadyburjor" Depariment of Chemical Engineering, West Virglnia Unlversiw, Morgantown, West Virginia 26506-6 10 1
A new method of analyzing kinetic data is formulated. Kinetic data obtained from integral reactors are analyzed by a differential method, with weighting factors on either the overall reaction rates or on the exit conversions. This method combines the experimental advantages of the integral method with the simplicity and accuracy of the differential method. The weighting factors are evaluated for several common systems of single and multiple reactions. I n general, the weighting factors depend only weakly on the functional form of the kinetic model, and they are independent of the value of the rate constants for single reactions. Hence, this method does not require the kinetlc model to be reliably known a priori. A series reaction example illustrates the power, robustness, and ease of application of the technique.
The evaluation of parameters in a reaction rate model involves either a (pseudo) differential or an integral reactor. The latter requires a more complicated analysis of the
* To whom inquiries should be addressed. 0196-4305/86/1125-0229$01.50/0
experimental data, particularly for non-power law kinetics or multiple reactions, but entails a more tractable experiment. On the other hand, the former has the advantage of an easier analysis, but the disadvantage of an (virtually) impossible experimental constraint. The experimental constraint of the differential reactor refers, of course, to 0 1985 American Chemical Society
