Energy & Fuels 1987, 1 , 198-202
198
When kl >> k 2 , and ASl" is assumed to be zero,
-
y,ArH2 - y,Ar)
[ArH'l,, = (K1[ArH~l[Arl)l/~
1
1
K11J2= exp[ -EAHfo(ArH' - f/zArH, - yzAr) d[N-XI --= dt
(K, [ArH,] [Ar])1/2kz[N-XI
Relative rates for step 2 can be expected to correspond roughly to the enthalpy changes for the RHT process (reaction 2), assuming entropy of activation effects (A factors) within a series of solvents are similar. For an endothermic RHT step, the activation energy will be given by the endothermicity of the transfer plus the intrinsic activation energy (the activation energy in the exothermic direction). E2 = AH," + Eint
The intrinsic activation energy in any given system will be equal to that observed for the thermoneutral case minus some fraction of the extent to which the transfer deviates from thermoneutrality. In the terms of the Evans-Polanyi relationship, this fraction is the proportionality constant, a.
Eint = ( E d 0 - a(mzo)
Collection of these terms provide EZ =
+ (1- a)(/w,")
then
exp[ -&((Eint)o
+ (1 - a)AHfo(NH-X + Ar - NX -
1 1
ArH'))
Multiplying the exponential terms and ignoring all enthalpy terms that are the same in each solvent system, we obtain the following expression for the relative rates of RHT: Q a Kll~zkz a
exp[(y2AH?(ArHZ- Ar)
+ aAHH,O(Ar-ArH'))/RT]
The terms are collected in this figure of merit, Q, such that the first term inside the brackets determines the expected relative rates if the intrinsic activation energy for the RHT step were independent of the extent of endothermicity (a = 0). In this limit, the stability of ArH' is completely canceled out. We also see that the term is larger when ArH, is less stable and when Ar is more stable (i.e., when the driving force for dehydrogenation is greater). The second term represents the extent to which higher endothermicity of the RHT step results in a lower ,Tint. This term is larger when Ar is less stable and ArH' is more stable (i.e., when the RHT step is more endothermic), consistent with the expectation commonly articulated as the Hammond postulate. The term Q has been generalized in Table I with the use of ArH, to apply to tetrahydro- as well as dihydroaromatics.
Effect of Water and Hydrogen Partial Pressures during Direct Liquefaction in Catalyzed Systems with a Low Solvent-to-Coal Ratio John A. Ruether,* Joseph A. Mima, Robert M. Kornosky, and Bao C. Ha Pittsburgh Energy Technology Center, U S . Department of Energy, Pittsburgh, Pennsylvania 15236 Received August 28, 1986. Reuised Manuscript Receiued November 17, 1986
Illinois No. 6 bituminous coal with small concentrations of coal-derived solvent was subjected to hydroliquefaction in batch reactors to which substantial amounts of water were sometimes added. The reactions were catalyzed by 0.1% molybdenum on mf coal, added as a water-soluble salt. It was shown previously that a substantial water partial pressure, at fixed hydrogen partial pressure, increases coal conversion in uncatalyzed systems. The present work investigated the effect of added water in catalyzed systems. Considerations of process economics led to the performance of series of reactions at constant total pressure while the partial pressures of water and hydrogen were varied. For catalyzed systems containing solvent, at fixed total pressure, highest conversions are obtained without added water. Conversion to THF solubles is independent of the solvent-to-coal (S/C) ratio to very low values, S/C 5 0.25, due to the catalyst. At 427 "C and 1500 psig hydrogen partial pressure, THF and benzene conversions exceeding 90% and 85%, respectively, were achieved. It is concluded that reaction systems with catalyst deposited from solution on the coal particles are highly reactive. Added water is preferentially excluded from the reactor, but water retains a role as a solvent for the catalyst and potentially as a transport medium for coal feed in continuous processing. Introduction With the present respite from the need of commercial quantities of synthetic fuels, it is an appropriate time to explore some unconventional processing routes to obtain distillate fuels from coal. If any unconventional ap-
proaches show promise based on small-scale tests, considerable time will be required to evaluate their potential for commercialization. In this spirit, a number of researchers are investigating the use of significant quantities of water in the direct liquefaction of coal. A variety of
This article not subject to U.S.Copyright. Published 1987 by the American Chemical Society
Direct Liquefaction in Catalyzed Systems
approaches are being pursued. Wender and Holder; Chao, Greenkorn, and Narayan; and Squires are all investigating the production of liquids using water above its critical pressure and temperature, with or without organic cosolvents, in the absence of a reducing Such approaches may be called aqueous pyrolysis. Stenberg and co-workers are investigating the use of water and hydrogen sulfide as hydrogen-transfer agents in the liquefaction of low-rank coals with synthesis gas.4 Ross and colleagues use water as a reactant, together with carbon monoxide, in base-catalyzed aqueous system^.^ Work at our laboratory has focused on use of water as an aid to hydroliquefaction of Illinois No. 6 bituminous coals. For systems containing approximately equal masses of coal and water, the dependence of THF conversions on the organic solvent-to-coal(S/C) ratio of the feed describes a plateau! Conversions are markedly higher with a small amount of solvent present than for similar systems containing only coal, water, and hydrogen. However, a conversion plateau is observed at remarkably low values of S/C, and the use of higher solvent concentrations causes no change in conversion. The use of small concentrations of catalyst consisting of a water-soluble salt of molybdenum has the effect of increasing the plateau value of conversion and decreasing the SIC ratio for the onset of the plateau compared to uncatalyzed reaction systems. For catalyzed systems, THF conversions in excess of 90% were reported for S/C as low as 0.2L6 Some effects of organic solvent compositions have also been reported.' Our earlier work showed that in uncatalyzed systems, increasing water concentration, expressed either as water/coal ratio of the feed or as water density in the vapor phase of the reactor, increased conversion.6 For potential process development, however, there are distinct disadvantages to the use of large quantities of water in a hydrogenation reactor. To name two, the water would significantly increase the heat load required of the preheater preceding the reactor. Moreover, the use of water increases the total pressure at which the liquefaction reactor must be operated, since the partial pressures of water and of a reducing gas, hydrogen in our case, are additive. Our early work and that of others using both water and a reducing gas have been conducted at pressures exceeding 4000 p~ig.~+ Economic considerations dictate use of the lowest pressure possible for liquefaction processing. Our earlier work showed that both hydrogen partial pressure and water partial pressure have a beneficial effect on coal conversion. Hydrogen is a reactant, and increased conversion with increasing hydrogen partial pressure is expected. Water is not a reactant? and the manner in which it aids conversion is unknown, although the effect is indisputable. (1) Deshpande, G. V.; Holder, G. D.; Bishop, A. A.; Gopal, J.; Wender, I. Fuel 1984, 63, 956-960. (2) Bienkowski, P. R.; Narayan, R.; Greenkom, R. A.; Chao, K. C. Ind. Eng. Chem. Res. in press. (3) Slomka, B.; Aida, T.; Squires, T. G. Prepr. Pap.-Am. Chem. SOC., Diu. Fuel Chem. 1985, 30(2), 368-374. (4) Stenberg, V. I.; Nowok, J. Proc.-Int. Conf. Coal Sci. 1985 1985, 103-105. (5) Ross, D. S.; McMillen, D. F.; Chang, Sou-Jen; Hum, G. P.; Green, T. K.; Malhotra, R. Exploratory Study of Coal Conversion Chemistry, Final Report; US.DOE Report Number DOE/PC/40785-12, Dec 1984. (6) Ruether, J. A.; Friedman, S.; Illig, E. G.; Bockrath, B. C. Proc.Int. Conf. Coal Sei. 1985 1985, 51-54. (7) Blaustein, B. D.; Bockrath, B. C.; Davis, H. M.; Friedman, S.; Illig, E. G.; Mikita, M. A. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1985, 30(2), 359-367. (8) Ruether, J. A.; Lett, R. G.; Mima, J. A. Prepr. Pap.-Am. Chem. Soc., Diu.Fuel Chem. 1986, 31(4), 140-151.
Energy & Fuels, Vol. 1, No. 2, 1987 199
At the outset of this study, the sensitivity of conversion reactions to incremental increases in either water or hydrogen partial pressure was unknown. The possibility was open that some gas phase composition containing both water vapor and hydrogen would represent the optimal reaction environment. It was the purpose of the present research to determine what composition of water vapor and hydrogen in the reactor vapor space gives the highest conversion for a bituminous coal at fixed total pressure. Also to be determined was the minimum total pressure required to achieve substantial conversions when the optimal vapor composition was employed. Experimental Section Because the earlier work had indicated the benefit of small concentrations of molybdenum salt catalysts, all reactions were carried out with 0.1% molybdenum expressed as a percentage of mf feed coal. Furthermore, most reactions were carried out a t a low value of solvent-to-coal ratio, S / C = 0.33, which nevertheless is on the conversion plateau for reaction systems with added water! Unless otherwise indicated, all reactions reported here used S I C = 0.33. Material. Illinois No. 6 (Burning Star mine) bituminous coal was used in the present study except for experiments in Figure 6, which used Illinois No. 6 (River King mine) coal. In experiments with no added water, the catalyst was loaded on the coal by aqueous impregnation. The feed coal was prepared by mixing it with water containing a dissolved transition-metal salt, ammonium heptamolybdate. Sufficient water was used to prepare a pumpable coal-water slurry. This was done to simulate conditions for potential continuous processing, as discussed further below. The slurry mixture was then dried in a vacuum oven a t 110 "C, and the catalyst was deposited on the coal particles. Aqueous impregnation was shown to be an effective method for introducing catalyst for direct coal liquefaction in early work a t J~ method for introducing the U.S.Bureau of M i n e ~ . ~Another low concentrations of dispersed ammonium molybdate catalyst is the Dow process, which employs a water-oil emulsion." In reaction experiments with added water, water containing soluble catalyst was charged to the reactor with the coal and solvent feeds. The liquefaction solvent for most reactions was hydrogenated deashed residuum prepared in run 242 a t the Wilsonville, AL, liquefaction test facility from Illinois No. 6 coal. The solvent used with coal from the River King mine was a heavy-fraction-containing residuum, produced from a bituminous coal in the Lummus two-stage liquefaction process. Compositions of the two coals and two solvents have been given previously.6is Procedure. Liquefaction experiments were carried out in two reactor systems. Experiments reported in Figure 6 were carried out in 40-mL shaking microreactors. All other experiments were conducted in a 1-L stirred batch reactor. The standard operating conditions for all experiments were a 427 "C reaction temperature and a 60-min reaction time a t temperature. The 40-mL microreactors were charged with coal, catalyst, solvent, and, optionally, water. The reactors were sealed, and 1200 psig cold hydrogen pressure was applied. During reaction no gas was added or removed, so pressure was not controlled. The reactor assembly was heated in a fluidized sand bath and shaken a t 1 Hz. The heat-up time from ambient to reaction temperature was about 1-8 min. Typically, the 1-L batch autoclave was charged with coal, catalyst, solvent, and, optionally, water and then pressurized to the desired pressure with hydrogen gas. The stirrer was operated a t 1000 to 1100 rpm. It took from 1to ll/z h to heat the reactor system electrically from ambient to reaction temperature. During heat-up time, the operating pressure was controlled by a backpressure regulator that allowed gas to escape the reactor. Water and other condensable vapors were retained in the reactor with (9) Weller, S.; Pelipetz, M. G. Ind. Eng. Chem. 1951, 43, 1243-1246. (10) Weller, S.; Pelipetz, M. G.; Friedman, S.; Storch, H. H. Ind. Eng. Chem. 1950,42, 330-334. (11) Quarderer, G. J.; Moll, N. G. US. Patent 4136013, 1979.
Ruether et al.
200 Energy & Fuels, Vol. 1 , No. 2, 1987 1001
80 L
2
-
I
I
I
THF
60-
z
0 In
40-
2
s 20 -
2ol
0 2000
2500 3000 3500 TOTAL PRESSURE, psig
4000 WATER PARTIAL PRESSURE, psi
Figure 1. Effect of total pressure on conversion for feed consisting of a pumpable aqueous coal slurry. 1001
I
I
i
I
Figure 3. Effect of water partial pressure on conversion for a total pressure of 2900 psig. 20
I
c
n
c
E4 60
m
9
z 8 8
Gonv.
a
a
0
0
0
TI
20
20
t
Ol
400
1 I I I 800 1200 1600 2000 2400 WATER PARTIAL PRESSURE, psi I
Figure 2. Effect of water partial pressure on conversion at a total pressure of 2400 psig. a water-cooled condenser. When the reaction temperature was reached, the pressure regulator was shut off. Then, the pressure normally dropped as hydrogen was consumed in the course of liquefaction reactions. The reported total pressures and partial pressures were those at the end of the reaction periods. Water partial pressure was determined as follows. The mass of water in the reactor at the end of the reaction period was measured. From the reaction temperature and water density in the reactor, partial pressure was read from a steam table. Analysis. The gaseous products were metered and analyzed by gas chromatography. The yield of C1 through C4hydrocarbons was determined as hydrocarbon gases. Water and light oils were then vacuum stripped from the reactor. The liquefaction residues were analyzed for THF insolubles by pressure filtration.12 The THF-insoluble portion was then ashed. The benzene insolubles were determined by a Soxhlet extraction method.12 The benzene soluble portion was further analyzed for asphaltene content by pentane precipitation. The heavy oils were calculated by difference.12 The conversions were determined by maf coal - organic insolubles % conversion = x 100% maf coal The hydrogenated residuum solvent contained 8.7 % benzene insolubles and no T H F insolubles. The benzene insolubles were subject to conversion during the test reactions. However, the extent of such conversion could not be determined by analysis of the reaction products, since insolubles deriving from coal or (12) Utz, B. R.; Appel, H. R.; Blaustein, B. D. Fuel, 1984, 63, 1671-1676. (13) Mima, M. J.; Schultz, H.; McKinstry, W. E. 'Method for the Determination of Benzene Insolubles, Asphaltenes, and Oils in CoalDerived Liquids"; PETC/RI-76/6; Pittsburgh Energy Technology Center: Pittsburgh, PA, Sept 1976.
o
:
01000
1500
2500
2000
3000
TOTAL PRESSURE, psig
Figure 4. Conversion and hydrocarbon gas yield in systems with no added water. lOOr
I
I
1
i
90 00 c
-
70W
n
z- 6 0 2
m
50> 'f 4 0 0
301 20
IO0
KEY
$
Symbol
,
:
A A
Water
,
;
Total Pressure, psig
i":
E
1376 2627
500 1000 1500 PARTIAL PRESSURE OF HYDROGEN, psi
I 2000
Figure 5. Hydrogen partial pressure correlation of conversion in systems with and without added water. solvent could not be distinguished. I t was arbitrarily assumed that the solvent was inert in calculating conversions. The assumption is not strictly true, and it is thought to give rise to some anomalous apparent conversions. In principle, contributions to increased benzene solubles in the product arising from coal and from solvent could be identified individually. If increased benzene solubles from conversion of the solvent were greater than the difference between T H F and benzene conversions of the coal, the calculated benzene conversion for the reaction would be greater than the calculated T H F conversion. This situation is apparently obtained in a reaction system with about 20% T H F and benzene conversions (Figures 1 and 2) and in a system with apparent THF and benzene conversions
Direct Liquefaction in Catalyzed Systems
Energy & Fuels, Vol. 1, No. 2, 1987 201
greater than 92% (Figures 3 and 5). Because the benzene insolubles content of the solvent was not large and because a relatively small fraction of the reaction charge was solvent (SIC = 0.33),the error in calculated conversions caused by chemical change of the solvent is not considered to be serious.
Results If coal were to be fed to a reactor in a continuous liquefaction process as an aqueous slurry, the maximum concentration at which the mixture would be pumpable is about 0.70 weight fraction of coal. Therefore, this concentration of coal in water was used to investigate the relation of conversion to total pressure in systems with a large quantity of water in the feed charge. The results are shown in Figure 1. The changing variable among the data points shown is the hydrogen partial pressures at which the reactions were conducted. The water partial pressure is constant at about 2300 psia. As expected, a total pressure of about 4000 psig was necessary before a THF conversion of 90% was obtained. By inference, Figure 1 shows that hydrogen partial pressure is very important in determining conversion in the reaction systems studied. The effects of water and hydrogen partial pressures on conversion are further shown in Figures 2 and 3. All the data in Figure 2 were taken at a single value of total pressure, 2400 psig. Similarly, for Figure 3, all experiments were performed at 2900 psig. For each figure, the independent variable is water partial pressure, computed by using the mole fraction of water present in the reaction system at the end of the reaction. Since water is produced in the reaction, the water partial pressure calculated in this way is higher than if it were computed on the basis of the reaction charge. In particular, in both figures, the data points shown for the lowest values of water partial pressure, about 480 psi, represent water-free feed. The hydrogen partial pressure is approximately equal to the total pressure less the water partial pressure. If some particular vapor phase composition containing both water and hydrogen had given optimal conversion at fixed total pressure, the curves in Figures 2 and 3 would exhibit maxima. No such behavior was found. It is concluded that at fixed total pressure, the most reactive environment contains no added water, so that hydrogen partial pressure is as high as possible. Given this finding, the effect of total pressure on conversion was investigated in systems where no water was charged to the reactor. The results are shown in Figure 4. I t is seen that THF conversions of 90% and benzene conversions greater than 85% are obtained at total pressures less than 2000 psig. Also shown is the hydrocarbon gas make for the reactions. As is true with reaction systems containing conventional S / C ratios, gas make is not a strong function of pressure. The foregoing data have shown that hydrogen partial pressure is the major determinant for conversion in the systems studied. This conclusion is underscored by the data shown in Figure 5 . In this figure, data are shown for reaction systems both with and without added water. The THF and benzene conversions for both types of reaction feeds are correlated by the hydrogen partial pressure at the end of the reaction period. The data contained in Figures 2 and 3 do not preclude the possibility that water is an aid to conversion in the systems studied. The results indicate simply that whatever the effect of water, reactivity is increased if the partial pressure is exerted by hydrogen instead of water. The results in Figure 5 are more definitive. Single curves correlate conversion data for systems with a wide range
I A
2
I
I
I
1
L
a 1
.
60
1
KEY
Symbol ~ _ Catalyst _ _ Water _
A
40
Yes
_
Yes
Ye5
NO
No
Yes
1
1 0.25 0.5 0.75 1.0 1.25 1.5 1.75
300
MASS VEHICLE /MASS COAL
Figure 6. Effect of solvent-to-coalratio on conversion in systems with catalyst and/or added water.
of water partial pressure. The conclusion to be drawn is that water has no measurable effect on either THF or benzene conversion in these catalyzed systems. All of the above results were obtained in reaction systems with a solvent-to-coal ratio of 0.33 in the charge. It has been reported previously that in reaction systems containing substantial water, the onset of plateau values of conversion occurs at values of S / C lower than 0.33.6 In Figure 6, it is now shown that catalyzed systems without added water also exhibit a similar ”plateau-type” dependence of conversion on S/C ratio of the reaction charge. The figure contains data both for catalyzed systems without added water and for catalyzed systems in which the water/coal ratio of the charge was 0.43. Also shown are previously reported data for uncatalyzed systems6 For catalyzed systems both with and without added water, the THF conversion plateau is maintained with S/C ratios at least as low as 0.25. In catalyzed systems with no added solvent (S/C = 0), conversion is much higher when water is present than when it is not. In Figure 6, the plateau conversions of catalyzed systems both with and without added water are indistinguishable. This finding agrees with the conclusion drawn from Figure 5 that the effect of water on conversion in catalyzed systems is not measurable.
Discussion At present the means by which water increases coal conversion in systems containing hydrogen but not carbon monoxide is unknown. Recently Mikita and Fish suggested that water acts to open new ionic reaction pathways through its solvation effect.’* The present findings do not necessarily conflict with this point of view. The absence of any positive effect of water in catalyzed reaction systems that we observed could be explained on the basis of a very strong catalytic effect of molybdenum in promoting hydrocracking and hydrogenation activities. If the effect of molybdenum is sufficiently pronounced, it could mask smaller effects due to water. The “plateau conversion” effect at low S/C ratios illustrated in Figure 6 has now been shown in catalyzed reaction systems both with and without added water and in uncatalyzed reaction systems containing added water. It is a remarkable phenomenon. In more conventional reaction systems, containing neither transition-metal salt (14) Mikita,
M.A.; Fish, H.T.P r e p . P a p - A m . Chem. Soc., Diu.
Fuel Chem. 1986, 3I(4), 56-63.
202 Energy & Fuels, Vol. 1, No. 2, 1987
catalyst nor added water, conversion becomes independent of S/C ratio at much higher values than those shown in Figure 6. Thomas conducted short-contact-time reactions (6.5 min) in the temperature range 350-400 “C with a bituminous coal in process-derived s01vent.l~ He reported that both THF and toluene conversions became independent of S/C ratio only for S/C I10. Likewise, Hariri measured the effect of the S/C ratio on the conversion of a Utah Spring Canyon high-volatile bituminous coal at 400 “C for reaction times of 6 h.16 The vehicle was tetralin. Again, benzene conversion was independent of S/C ratio only for S/C 1 10. The presence of either 0.1% molybdenum salt catalyst or water changes the dependence of conversion on S/C ratio dramatically (Figure 6). Catalyst is the more powerful aid to conversion. As has been shown, when both catalyst and water are present, conversions are indistinguishable from when catalyst alone is present above an S/C ratio of 0.25. Furthermore, the plateau conversions are higher and extend to lower values of S/C when catalyst is present compared to water only as a conversion aid. Nevertheless, the beneficial effect of water on conversion under two types of reaction conditions is indisputable. One such condition is catalyzed systems without solvent (S/C = 0). From Figure 6 it is seen that conversion without water is 36% and conversion with water is 80% when solvent is absent. The other condition is for uncatalyzed systems. Figure 6 shows that for uncatalyzed systems with a water/coal ratio of 0.43, a conversion plateau of about 86% is observed at S/C ratios as low as 0.5. Thus, water, solvent, and water-soluble catalyst are all aids to coal conversion. The importance of each depends on the particular reaction system. In uncatalyzed systems, water appears to exert a large effect in the absence of solvent. For systems without solvent, conversion with water but without catalyst was 62 % , while conversion with catalyst but with no added water was only 36% (Figure 6). On the other hand, as has been shown when both solvent and catalyst are present, the effect of water on conversion can (15) Thomas, M.G.In T h e Science and Technology of Coal and Coal Utilization; Cooper, B. R., Ellingson, W. A., Eds.; Plenum: New York, 1984; pp 231-261. (16) Hariri, H. Ph.D. Dissertation, University of Utah, 1965.
Ruether et al.
be diminished to the point of not being measurable. The reason for the strong effect of relatively small concentrations of water-soluble catalyst is poorly understood. Part of the explanation is thought to be the highly dispersed nature of the catalyst. Catalyst is deposited on the reaction mass from aqueous solution whether or not water is added to the reactor with the feed charge. In the latter case, deposition of the catalyst occurs at reaction conditions when the water is vaporized in the reactor. For reaction systems without added water, the catalyst is deposited from solution on the coal particles before they are charged to the reactor. In either case, the catalyst appears to be highly accessible to the reacting species. The present work has shown that for the catalyzed system studied, the highest conversions at fixed total pressure are obtained when no water is added with the feed. It has further shown that THF conversions over 90% and benzene conversions over 85% can be achieved at hydrogen partial pressures as low as 1500 psi (Figure 5). In the absence of added water, the required hydrogen partial pressure could approach the value of the total system pressure. Although the results indicate it is advantageous to minimize the amount of water fed t o a reactor for the catalyzed system studied, the role for water has not been eliminated. It serves as solvent for the catalyst, and as mentioned earlier, the dispersed form of the catalyst deposited on coal particles is thought to be important. Furthermore, water may yet serve as a transport medium for the feed coal in a continuous process even if it is largely excluded from the reactor. There is incentive in continuous processing to use as low a solvent-to-coal ratio as possible for the feed to maximize the throughput of coal per unit reactor volume. With the low S/C ratios found for the onset of the “plateau conversions” in catalyzed systems, it would not be possible to transport the coal into the reactor as a pumpable coal-oil slurry. The reaction chemistry discovered to date invites some innovative process development. Acknowledgment. We are grateful for helpful discussions with Bradley Bockrath and Eugene Illig. The reactions were conducted by William Staymates. Registry No. Mo, 7439-98-7; HzO, 7732-18-5.
