Kinetics of Heterogeneously Catalyzed Coal Hydroliquefaction John A. Ruetherl Pittsburgh Energy Research Center, United States Energy Research and Development Administration, Pittsburgh, Pennsylvania 152 13
The heterogeneously catalyzed hydroliquefaction of coal in a vehicle oil under conditions similar to those employed in the SYNTHOIL process is investigated. The model of Weller et al. (1951a,b) that considers coal liquefaction to occur in two steps, coal asphaltene oil, is extended by considering asphaltene to be a hydrogen donor solvent of variable hydrogen composition. New data show that coal conversion occurs via catalyzed and uncatalyzed reactions, which appear to be coupled.
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Processes for the conversion of coal to liquid hydrocarbons will likely become increasingly important in coming years. In the near term the liquids produced will find use as fuel, but in the longer term they will surely also become a source for chemical feed stocks. One of the several processes currently under development for coal hydroliquefaction and hydrodesulfurization is the SYNTHOIL process (Akhtar et al., 1972). In this process pulverized coal suspended in a vehicle consisting of a recycled product liquid is passed in turbulent cocurrent upflow with molecular hydrogen through a fixed bed of supported cobalt-molybdenum catalyst. Process conditions are 425-450 "C and 2000-4000 psi (14-28 X lo6 N/m2). Because coals are complex, nonuniform substances, description of the thermochemistry and kinetics of hydroliquefaction is difficult. Such description is nonetheless necessary for reactor design and process scaleup. I t will probably never be possible to describe the reactions occurring during heterogeneously catalyzed hydroliquefaction with complete rigor, so it is important to identify simplifying assumptions that can be used to enable a process to be characterized with a tractable number of parameters. I t was to seek valid simplifying concepts and to investigate some aspects of the kinetics of coal hydroliquefaction under conditions that obtain in the SYNTHOIL process that the present study was undertaken. Previous Work The process of coal liquefaction and conversion to oil in hydrogen donor solvents is, in general, one of successive bond fissure. Weaker bonds break first, and these are frequently a t heteroatoms such as sulfur, nitrogen, and oxygen, which may be split out as gases such as H2S, ",I, and water. From the work of Guin et al. (1975) and Plett et al. (1975) it is known that a t temperatures in the range 350-450 "C, fragmentation of the coal particles to an extent sufficient to solubilize the coal occurs within a time on the order of 1 min. The fragments formed within this period are still relatively large, and the material may resolidify on cooling, due in part to recombination of the fragments. T o form products of smaller molecular weight requires longer reaction times, the time increasing with decrease in molecular size, due to the increasing stability of the bonds which must be broken. It is thought that fragments break from larger molecules by thermal cleavage and that the fragments are free radicals (Curran et al., 1966).A fragment may be stabilized by reacting with a hydrogen donor compound, in which case a molecule is formed of molecular weight essentially equal to that of the fragment. Alternatively, two fragments may react with each other, called repolymerization, resulting in larger molecules
' Permanent address: Department of Chemical Engineering, [Jni-
versity of Ottawa, Ottawa, Canada K1N 6N5.
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which are resistant to further fragmentation. When coal liquefaction is carried out in hydrogen donor solvent the hydrogen donor compound is usually thought of as the solvent. Recently, however, Neavel(l976) has shown that compounds within coal can also stabilize fragments, a process he calls autogeneous hydrogen transfer. If coal liquefaction is conducted continuously, hydrogen donor compounds spent in reaction with fragments must be regenerated by addition of hydrogen. This may either be done in the same place where reaction of the fragments with hydrogen donor species is occurring, as for instance in the SYNTHOIL process, or in a separate location where reactive coal fragments are absent, as for instance in the Exxon Donor Solvent Process (Furlong et al. 1976).The reaction by which hydrogen donor compounds are rejuvenated may be heterogeneously catalyzed, and cobalt-molybdenum catalysts are widely used. Tarrer et al. (1976) have recently shown that the ash in coal itself is also a catalyst for this reaction. Most coal liquefaction studies have been carried out in a hydrogen donor solvent, such as tetralin, where the hydrogen donor species is known. When coal liquefaction is carried out without solvent, or in a vehicle consisting of recycled coalderived liquids, the identity and nature of the hydrogen donor species are less clear. A classic early study of coal liquefaction in the absence of solvent or liquid vehicle is that of Weller et al. (1951a,b). A catalyst consisting of stannous sulfide and ammonium chloride was used. Extents of reaction were determined by measuring product solubilities in two solvents, benzene and hexane. Material soluble in benzene and insoluble in hexane (or sometimes pentane) is termed asphaltene, and material soluble in both benzene and hexane (or pentane) is called oil. Their experiments were conducted in the range 400-440 "C and usually with a hydrogen pressure a t reaction temperature of 6000 psi (41 X lo6 N/m2).These workers considered liquefaction to be a two-step process which, slightly oversimplified, can be exasphaltene oil. In a masterful piece of pressed as coal experimental work, they measured separately the rates of conversion for coal to asphaltene, and asphaltene to oil. Each reaction was found to be first order in the amount of unconsumed reactant, coal, and asphaltene, respectively. First-order rate constants calculated from data in which only a single reaction occurred were combined to predict quantitatively the product distribution when coal was hydrogenated for a range of reaction times. An indication of the increasing stability of the bonds that are broken during liquefaction as the size of the coal derived molecules becomes smaller is that the firstorder rate constant for the reaction coal asphaltene was 10-25 times greater than that a t the same temperature for oil, in the temperature range 400-440 "C. asphaltene The model of Weller et al. (1951a,b) is an obvious simplification of the reactions occurring during coal liquefaction,
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since it forces the entire spectrum of molecules and fragments present in the liquid phase into three categories: unconverted coal, asphaltenes, and oil. The use of only three categories of liquid species precludes a detailed explanation of the steps occurring during coal liquefaction. As well, the model does not provide insight into the reactions operating in the conversion of species from one category to another. I t does tell us that a useful generalization of the pathway followed in the formation of products during coal liquefaction is the stepwise one given. I t is now known that oil may be formed directly from unconverted coal, without apparently going through the intermediate asphaltene. This can be understood as being due to the splitting of a fragment, of molecular weight as required to give rise to a molecule of oil, from a species characterized as unconverted coal. However, because of the verification of their model that Weller e t al. performed with their data, we know that this path to oil is relatively rare. The majority of oil is formed via fragments of molecular weight such that they would be classified as asphaltenes. The success of the model of Weller et al. (1951a,b) to account quantitatively for the sequence of product formation during coal liquefaction in the absence of solvent suggests that it may also serve as the basis for gaining insight concerning hydrogen donor species under the same conditions. T o this end the data of Weller e t al. have been subjected to a new analysis, which is presented in the following section. Prediction of Hydrogen Consumption w i t h t h e Model of Weller et al. A more precise statement of the model of Weller et al. (1951b) than was given above is the following
+
[(I - cy) gas water ii.. maf coal -2I hr [ a asphaltenes -gas
+ water + oil
(1)
Unconverted maf (moisture and ash free) coal is measured as organic benzene insolubles. The coefficient cy is the weight fraction of converted maf coal that appears as asphaltene. Weller et al. reported values of h kp, and cy as functions of temperature. The coal used by Weller et al. (1951b) was obtained from the Bruceton mine (Pittsburgh seam), and their asphaltene feed stock was also derived from this coal. In their reactions using asphaltene feed, they determined that 91.2 wt % of the converted asphaltene appeared as oil for all temperatures studied. Sufficient data are given in the original work to compute the relation between hydrogen consumption and asphaltene conversion. This has been done, and the results are shown in Figure 1. I t is seen that the data describe a straight line, but that a t zero asphaltene conversion the line extrapolates to a hydrogen consumption of 0.010 g of hydrogen/g of asphaltene. We offer the following explanation for this behavior. I t is known that coal-derived liquids can assume varying chemical hydrogen contents depending on the severity of hydrotreatment. (Chemical hydrogen refers to hydrogen that would appear in the molecular composition of a substance, as distinct from molecular hydrogen dissolved in the liquid phase.) It appears that asphaltenes can have a range of chemical hydrogen values, the value of the abscissa of Figure 1a t zero asphaltene conversion being a measure of the possible variation. According to this view, the asphaltenes charged to the reactor in the experiments whose results are shown in Figure 1 had chemical hydrogen contents that were 0.010 g of hydrogen/g of asphaltene less than their maximum value. Under the experimental conditions employed, the rate of hydrogenation 250
Ind. Eng. Chem., Process Des. Dev., Vol. 16,No. 2, 1977
09,
I
I
I
2
3
I
Oat 6 g
06-
c
t
5
05-
a
+
5
04-
z 0
y Y 5
03-
L3
02-
a 01
0
-
-
.
1
1
tip CONSUMED,g/q asphollene c h a r g e d , X I 0 0
Figure 1. Hydrogen consumption in conversion of asphaltenes to oil, gas, and water, from Weller et al. (1951a).
of the asphaltenes was materially faster than the rate of conversion of asphaltenes to oil. Thus under reaction conditions the asphaltenes first underwent an increase in chemical hydrogen content of 0.010 g of hydrogedgram on average, then underwent conversion to oil. The amount of hydrogen required to convert asphaltenes to oil is given by the slope of Figure 1: 34 g asphaltene converted/g of hydrogen consumed. Equivalently, the reciprocal slope is 0.029 g of hydrogen/g of asphaltene converted. According to the proposed explanation asphaltene can exist with any value of chemical hydrogen content within the bounds described. We will find it useful later to consider asphaltene having a hydrogen content near its maximum possible value to be in a so-called hydrogen-rich form, and asphaltene having a hydrogen content near the lower end of the possible range to be in a hydrogen-poor form. An obvious simplifying notion is to consider asphaltene to exist in one of two states, hydrogen-rich or hydrogen-poor. Weller et al. (1951b) also presented data for reduction of benzene insolubles as a function of hydrogen consumption when coal was the feed. They found that for a given conversion of benzene insolubles, hydrogen consumption increased with temperature, thus making impossible a good correlation of all the data with a single line. There are two reasons for the effect of temperature on hydrogen consumption. A t the lowest temperature employed, 400 "C, the conversion of asphaltene to oil occurred to only a small extent, so that most of the hydrogen consumed was for the reaction of coal to asphaltene. At higher temperatures the conversion of asphaltenes to oil proceeded to a greater extent, however, causing an increased hydrogen consumption. By use of the correlation contained in Figure 1 it is possible to calculate that part of the total hydrogen consumption used for the second reaction, conversion of asphaltene to oil. Subtraction of this value from the total hydrogen consumption gives the consumption for the first reaction, conversion of coal to asphaltene, gas, and water. The data of Weller et al. (1951b) were thus treated to calculate the hydrogen consumption for the first reaction. Because hydrogen consumption is computed by difference, for the sake of accuracy data were used only if the consumption of hydrogen for the second reaction was 25% or less of the total. The values of hydrogen consumption for the first reaction calculated in this way still exhibited a temperature dependence, hydrogen consumption being greater a t higher temperature. This is caused by the dependence of
Table 1. Analyses of Coal and Vehicle Oil (SYNTHOIL) I. COAL: Bruceton Mine. hvab
.
t
D c W
> W
0 J
-8 0
Proximate analysis,
wt %
Moisture Volatile matter Fixed carbon Ash
38.9 56.0 3.9
1.2
Ioo.0
z
P
Ultimate analysis, Hydrogen Carbon Nitrogen Oxygen Sulfur Ash
5.4 79.6 1.6 8.3 1.2 3.9
100.0
V LL LT
wt%
11. SYNTHOIL
c 0
Ultimate analysis,
ti2 CONSUMED,g/p maf coal charged , x 100
Figure 2. Hydrogen consumption in conversion of coal to asphaltenes. gas, and water, from Wellrr e t al. (195la,b).
product distribution of the first reaction on temperature. Production of gas and water, which require relatively large amounts of hydrogen compared to production of oil, increases with temperature. I t was found that the data for all temperatures could be superposed if the extent of coal conversion times an empirical function of the coefficient cy was plotted vs. hydrogen consumption for the first reaction. This plot is shown in Figure 2, where the abscissa is (weight fraction of maf coal converted) divided by cy4. Figure 2 indicates that hydrogen consumption for the first reaction increases directly with extent of coal conversion. For a given value of a it is possible to calculate hydrogen consumption for the first reaction from the slope of the figure, which is 46 g of maf coal converted/g of hydrogen consumed. In the temperature range studied by Weller et al. the range of N was 0.881-0.939. A representative value of hydrogen consumption for the first reaction is then 0.031 g of hydrogen/g of maf coal converted. Thus the data indicate that about the same amount of hydrogen is required for the conversion of coal via the first reaction as is required for the conversion of asphaltene via the second reaction. By use of the information contained in Figures 1and 2 and the statement of the model contained in eq 1an expression can be written for total hydrogen consumption in the conversion of coal to any product distribution of asphaltene and oil. Let x = asphaltene in prodnct/g of maf coal feed, and y = g of oil in product/g of maf coal feed. Then if the asphaltenes are considered to be in the hydrogen-poor form
+
+
h = ( ~ - ; ' ( 0 . 0 2 2 ~0 . 0 2 4 ~ ) 0 . 0 4 3 ~
(2)
where h = g of hydrogen consumed/g of maf coal feed. If the asphaltenes are considered to be in the hydrogen-rich form
+
+
h = c ~ - ~ ( 0 . 0 2 2 0~. 0 2 4 ~ + ) 0.010~ 0 . 0 4 3 ~
(3)
When eq 2 is tested against the 20 data points a t four temperatures given in Table I of Weller et al. (1951b), it predicts the hydrogen consumption with an average absolute deviation of 10.0%. It is observed that eq 2 systematically predicts too large a hydrogen consumption, particularly a t large values of the variable, y . Empirically it is found that better agreement with the data is given by the equation
+
+
h = ( ~ - ~ ( 0 . 0 2 20~. 0 2 4 ~ ) 0 . 0 2 8 ~
(4)
which results in an average absolute deviation of 4.8%. The reason why eq 4 gives better agreement with the data than eq 2 or 3 is not immediately apparent. Weller e t al.
wt
Hydrogen Carbon Nitrogen Oxygen Sulfur
Oh
7.9 88.0 1.2 1.6 0.5 0.8 100.0
Ash
(1951a) reported that asphalt used in their studies was 93.8% pure, Le., that the liquid reactant contained 93.8 wt % asphaltene. In the calculation of eq 2 and 3 it was assumed in the present work that only the asphaltenes reacted with hydrogen. If in fact some of the other 6.2 wt % of material contained in the crude asphalt also reacted with hydrogen, this would cause the predictions of hydrogen consumption of eq 2 or 3 to be too high, as was observed. Another possible explanation is that the asphaltenes formed in situ in the experiments for which the feed was coal had a higher hydrogen content than the asphaltene feed in the other set of experiments. Experimental Section The apparatus used to conduct the reactions was essentially identical with that employed by Weller et al. (1951a,b). The reactors were cylindrical stainless steel pressure vessels of 1 L capacity that during reaction lay in a horizontal position within an electric heating mantle and rotated a t 25 rpm. Solid and liquid charge for a reaction was held in a glass liner that fit inside the pressure vessel. Temperature was measured with a thermocouple and continuously recorded, and pressure was measured with a calibrated Bourdon tube type pressure gauge. In all reactions coal and a vehicle oil were charged. The coal was from the Bruceton mine, dried at 110 "C and stored under nitrogen. I t was ground and the 100% -100 mesh fraction used. The vehicle oil was the liquid product from a SYNTHOIL reactor with a feed of Ireland mine, West Virginia (Pittsburgh seam) coal. Analysis of the coal and vehicle are given in Table I. Coal and vehicle were always charged in the ratio 1 part coal:2 parts vehicle, the proportions used in the continuous SYNTHOIL process (Akhtar et al. 1972). Two charge levels were employed: 33.3 g of coal 66.7 g of vehicle, and 50.0 g of coal 100 g of vehicle. The catalyst used was silica promoted cobalt-molybdenum on alumina support, designated 0402 by the Harshaw Co. Portions of the catalyst were crushed and sieved to prepare six fractions with varying average particle size. Mean particle size in the several fractions ranged from 100 p to 3.2 mm (?&in. cylinders). The catalyst was presulfided before use. All runs were conducted at 400 "C. Most runs were conducted in a hydrogen atmosphere with a cold hydrogen pressure of 1900 psi (13 X lo6 N/m2j which translated into a pressure at reaction temperature of 4100 psi (28 X lofiN/mL).The reactions ran a t nearly constant pressure, the maximum pressure reduction that occurred over the
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course of a run being 500 psi (3.4 X lo6 N/m2). Several tests were run in a nitrogen atmosphere with no gaseous hydrogen present. In these runs the pressure a t reaction temperature was 500 psi (3.4 X lo6 N/m2). Benzene insolubles in the liquid product of all reactions were determined by Soxhlet extraction, and analysis of the reaction gas a t the end of each reaction was made by mass spectrometry. Gas generation was minimal, the maximum content of all gases other than hydrogen for the hydrogenation experiments being 1 mol %. In addition, for some runs the amount of hydrogen gas consumption was determined by use of a wet test meter. Following the practice of Weller et al. (1951a,b),the effects of reaction during heat up and cool down time of the reactor are accounted for by adding 20 min to the time at reaction temperature to arrive a t a corrected reaction time. Reaction times quoted below have been thus corrected. Weller et al. (1951b) actually found that conversion of coal to asphaltene was first order in that fraction of organic benzene insolubles that was able to be converted under the experimental conditions at very long time. They found the active fraction to be 98.5 wt % of the total. In the present work a similar determination was made. It was found that 97.8 wt % of the total organic benzene insolubles charged, Le., those originating with both coal and vehicle feed, were subject to conversion a t long run times (4l/2 h) with a large amount of catalyst (20 8 ) .
Results In the experimental system studied, comprising solid, liquid and gas phases, several kinds of mass transfer resistance could influence the rate of reaction, thus preventing kinetics from being measured directly. These are gas absorption resistance, external particle-liquid mass transfer resistance, and intraparticle diffusion resistance. In three-phase systems with slurry catalyst it is often possible to determine the volumetric gas absorption coefficient by means of a series of experiments with different catalyst loadings. See, for example, Sherwood and Farkas (1966).This method was tried without success in the present work, the results being discussed below. An indication that gas absorption resistance was not important, however, can be obtained by comparing this work to that of Weller et al. (1951b). The highest rate of coal conversion measured in this work, using 100 g of catalyst, was smaller than the highest measured by Weller a t 400 "C. In turn at 430 "C Weller measured rates greater than 2l/2 times that a t 400 "C. Intraparticle diffusion resistance was negligible, since no effects of catalyst particle size on rate was found. This contrasts with the results of Yen et al. (1976), who found an effect of catalyst particle size both for asphaltene and oil production, for particle diameters of 270 and 720 M. However, the temperature was 455 "C. Since internal diffusion effects were absent in the present work, external particle diffusion resistance was negligible as well (Ruether and Puri, 1973). The importance of the catalyst for the reaction of molecular hydrogen with coal was established in a series of experiments run for 35 min with varying amounts of catalyst. In the presence of catalyst the reaction of hydrogen proceeded readily, the amount reacted being as large as 0.036 g of hydrogen/g of maf coal feed in the experiment with the largest amount of catalyst used, 100 g. In the absence of catalyst the reaction of hydrogen was so small as to be undetectable using the wet test meter. Thus a well known fact was confirmed, namely that Bruceton mine coal is essentially unreactive toward molecular hydrogen in the absence of catalyst a t the experimental conditions. The dependence of the rate of hydrogen reaction on charge of catalyst was smaller than expected, given that gas absorp252
Ind. Eng. Chem., Process Des. Dev., Vol. 16,No. 2, 1977
MASS CATALYST, g
Figure 3. Rate dependence on catalyst concentration: - - -, catalytic reaction only; - - - - -, catalytic and noncatalytic reactions in parallel. Charge 150 g; with no catalyst, rate = 0.83 X lo-' g of benzene insoluble/g of maf coal, s.
tion resistance was probably small. The results of the experiments were examined more closely using conversion of organic benzene insolubles as a measure of extent of reaction, since this variable could be determined more accurately than the consumption of molecular hydrogen. The data were used to compute initial reaction rates, under the assumption that the rate of conversion of benzene insolubles was first order in remaining active organic benzene insolubles, as found by Weller et al. (1951b).The results are shown as a log-log plot of initial rate vs. charge of catalyst in Figure 3. When conversion of benzene insolubles is used as the measure of reaction, the experiments with catalyst exhibit higher rates than the uncatalyzed experiment, but the rate in the latter case is a significant fraction of those with catalyst. The contrast of these findings with those for the consumption of molecular hydrogen in the same experiments indicates that different reactions are involved in the consumption of hydrogen and in the conversion of benzene insolubles. Assuming the reactions were kinetically controlled for all experiments, if the conversion of benzene insolubles proceeded solely via a catalytic reaction, the curve expected to describe the data would be a straight line with slope of unity as shown in Figure 3. On the other hand, if the process advanced through the action of parallel catalyzed and uncatalyzed reactions, one would expect the curve to have a slope of unity a t higher catalyst concentrations, where the catalytic reaction dominated, and a slope less than unity a t lower catalyst concentrations. As the catalyst concentration was reduced the rate would approach an asymptotic value equal to the rate with no catalyst, in the present work this being 0.83 X g of organic benzene insolubles/g of maf coal, second. This situation is also shown in the figure. The positions in the figure of the 2 lines describing these two situations are not important, rather it is their slopes and shapes that matter. I t is seen that neither of the two hypothesized situations describes the data. A possible explanation of these results is that two classes of reaction are operating. One involves reaction of molecular hydrogen, and is heterogeneously catalyzed. The other involves conversion of coal t o asphaltenes, and is uncatalyzed. The reactions are coupled, since hydrogen donor species are reactants in each: in the hydrogen-poor state in the catalyzed reaction, and in the hydrogen-rich state in the uncatalyzed reaction. To test this hypothesis a series of runs with different amounts of catalyst was made under a nitrogen atmosphere. The results are shown in Table 11, and it is obvious that catalyst does not aid in the conversion of coal. The average conversion for the runs shown in Table I1 was 15 wt %. An uncatalyzed experiment performed under conditions identical with those listed in Table I1 except for use of a hydrogen atmosphere resulted in a 19 wt % conversion of organic benzene insolubles. If in the latter experiment it was not possible to react molecular hydrogen due to an absence of catalyst, one would expect the same conversion as for the experiments
Table 11. Coal Conversion in a Nitrogen Atmosphere" Mass of catalysts,
a
Org. benz. insoluble converted, wt fraction
0.0 0.500
0.152 0.156 0.136
4.99
Conditions of reaction: 400 "C, 500 psi at reaction temperature, 35 min reaction time, charge 100 g. Table 111. Coal Conversion in a Hydrogen Atmosphere" Mass of catalyst, e
Reaction time, min
Mass of catalyst X time, e-min
Org. benz. insolubles converted, w t fraction
130 0.784 0.767 130 0.750 140 0.674 0.690 0.760 137 0.706 0.760 139 139 0.661 0.657 0.998 0.653 0.997 142 142 ( I Conditions of reaction: 400 "C, 4150 psi at reaction temperature, charge 150 g. 0.500
0.500
260 260 184 180
shown in Table 11. Because of the ability of coal ash to catalyze the absorption of hydrogen (Tarrer et al., 1976) probably some hydrogen reacted in the latter experiment despite the absence of Co-Mo catalyst. Thus the somewhat higher conversion achieved in the experiment with the hydrogen atmosphere is compatible with the hypothesis. The proposed sequence of coupled catalyzed and uncatalyzed reactions can account for the data of Figure 3. With a very small amount of catalyst, reaction of hydrogen proceeds to only a small extent. Most coal conversion is due to reaction of asphaltene precursors with hydrogen-rich solvent that was charged to the reactor. Since there is insufficient means for replenishing hydrogen-rich solvent the rate quickly falls to a low value. On the other hand, with a large amount of catalyst the scheme predicts that nearly all the donor species should exist in the hydrogen-rich form. Further increase in the amount of catalyst would not affect the composition of the reaction mass and would therefore not affect the rate of coal conversion. One would expect a smooth transition between the conditions that obtain in the limits of zero and large amounts of catalyst. In the past, attempts have been made to correlate coal conversion data in catalyzed systems using the assumption that the rate was directly proportional to the mass of catalyst in the system. The data in Figure 3 show that this assumption is not generally justified. This point was further illustrated by conducting a series of reactions in a hydrogen atmosphere using different amounts of catalyst and reaction time, but such that the multiplication product of mass of catalyst and run time was nearly constant for all reactions. For a catalyzed reaction in a batch reactor the extent of reaction should be invariant for a given value of the product, (mass of catalyst X reaction time), regardless of the sizes of the two constituent terms. This is true regardless of the form of the rate expression. The experimental results are shown in Table 111. I t is seen that in spite of a slight increase of the product, (mass of catalyst X reaction time), with increasing mass of catalyst, the extent of reaction decreases in the same order. The proposed reaction scheme is compatible with this finding, but of course other explanations are possible as well.
Discussion I t appears that the general picture of coal conversion in catalyzed systems under hydrogen pressure without vehicle, or vehicle consisting of coal derived liquid, is similar to that when pure donor solvents, such as tetralin, are used. In the former case asphaltenes, and perhaps other species, serve as hydrogen donors. The rate of coal conversion is controlled by two classes of reaction, one heterogeneously catalyzed, the other not, which are coupled. Hydrogen donor species enter into each class of reaction. Recently Tarrer et al. (1976) have come to similar conclusions for the liquefaction of coal in creosote oil. The importance of the ratio, (mass catalyst/mass coal vehicle), in applied kinetics studies has also been shown. Until the process chemistry is better understood, kinetics studies aimed a t determining rates in industrial or pilot plant reactors should give thought to assuring that the above named ratio in the laboratory studies approximates that in the larger reactors. In the absence of special precaution the ratio may be quite different in the two cases. In the present work for most experiments the ratio was on the order of 0.005 g of catalyst/g of coal vehicle. For representative gas and liquid flow rates used in the continuous SYNTHOIL pilot plant reactors the ratio is much larger, being on the order of 20.
+
+
Nomenclature h = g of gaseous hydrogen consumed/g of maf coal feed k l = first-order rate constant for conversion of coal to asphaltene hn = first-order rate constant for conversion of asphaltene to oil x = g of asphaltene in product/g of maf coal feed y = g of oil in product of maf coal feed 01 = fraction of maf coal converted that goes to asphaltene (see eq 1) L i t e r a t u r e Cited Akhtar, S., Friedman, S., Yavorsky, P. M., AlChE Symp. Ser., 70 (137), 106 (1974). Akhtar, S., Lacey, J. J.. Weintraub. M., Reznik, A. A,. Yavorsky, P. M.. Paper No. 35b, presented at 67th Annual AlChE Meeting, Washington, D.C., Dec 1-5, 1974. Curran, G. P.. Struck, R . T.,Gorin, E., Proceedings, Symposium on Pyrolysis Reactions of Fossil Fuels, Division of Petroleum Chemistry, American Chemical Society, p C-130, Pittsburgh, Pa., Mar 23-26, 1966. Furlong, L. E., Effron, E., Vernon, L. W., Wilson, E. L.. Chem. Eng. Prog., 72 (8), 69 (1976). Guin, J. A., Tarrer, A. R., Taylor, 2. L., Green, S. C., Proceedings, Symposium on Low Sulfur Liquid Fuels from Coal, Division of Fuel Chemistry, American Chemical Society, p 66, Philadelphia, Pa., April 1975. Guin, J. A., Tarrer, A. R., Pitts, W. S., Prather. J. W., Proceedings Symposium on Coal Liquefaction,DivisionofFuel Chemistry, American Chemical Society. p 170, San Francisco, Calif., Aug 1976. Neavel, R. C., Fuel, 55, 237 (1976). Plett, E. G., Alkidas, A. C., Roger, F. E., Mackiewicz, A. 2 . . Summerfield. M., paper presented at Universitv-ERDA Contractors Conference, Salt Lake City. Utah, Oct 22-23, 1975. . Ruether, J. A., Puri, P. S., Can. J. Chem. Eng., 51, 345 (1973). Sherwood. T. K., Farkas, E. J., Chem. Eng. Sci., 21, 573 (1966). Tarrer, A. R., Guin, J. A., Pitts, W. S., Heniey, J. P., Prather, J. W., Styler, G. A,. Proceedings, Symposium on Coal Liquefaction, Division of Fuel Chemistry, American Chemical Society p 59, San Francisco, Calif., Aug 1976. Weller, S., Pelipetz, M. G.. Friedman, S., lnd. Eng. Chem., 43 (7), 1572 ( 195 1a). Weller, S., Pelipetz, M. G., Friedman, S., Ind. Eng. Chem., 43 (7) 1575 (195 1b). Yen, Y. K., Furlani, D. E., Weller, S. W., Ind. Eng. Chem.. Prod. Res. Dev., 15, 24 (1976).
Received for revieu' September 3, 1976 Accepted December 21, 1976
Facilities provided by Professor J. C. Charpentier and the Laboratoire des Sciences du GBnie Chimique C.N.R.S.,E.N.S.I.C., Nancy, France. during the preparation of this manuscript are gratefully acknowledged.
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