1148
Ind. Eng. Chem. Process Des. Dev. 1985, 24, 1148-1154
is to facilitate understanding and does not necessarily imply its endorsement or favoring by the US Department of Energy. Nomenclature a = specific interfacial area, m-l CX(d,) = cumulative bubble size distribution CZ(A) = cumulative bubble length distribution D = column diameter, m d b = bubble diameter, m dbg = geometric mean bubble diameter, m D, = gas-phase dispersion coefficient, m2/s d,, = Sauter mean bubble diameter, m E , = energy dissipation in liquid motion, kg/(ms3) Epl = energy dissipation to wakes behind the bubbles, kg/(m s3)
Fr = Froude number for gas-phase flow regime transition
fbJkD)llzl
g = gYavitationa1 acceleration, m/sz
12 = ratio of molar volumes of low surface tension to high
surface tension components S2(X) = variance of bubble length distribution, m2 U , = bubble rise velocity, m/s U,, = intercell liquid recirculation velocity, m/s U,, = terminal bubble velocity, m/s 0 = superficial gas velocity, m/s = transition gas velocity to foaming flow regime, m/s = limiting gas velocity in bubble flow regime, m/s rsr", = superficial liquid velocity, m/s U, = bubble velocity due to buoyancy, m/s X = mole fraction of solute X ( d b ) = bubble size frequency distribution
6,
bLB
Greek Symbols cg = gas holdup y = surface tension, N/m, or retardation parameter yaIc = surface tension of ethanol-water mixture, N/m yiw= surface tension of water, N/m X = bubble length, m X = mean bubble length, m
pg, pL
u2 =
variance of bubble size distribution
Registry No.
Ethanol,
64-17-5.
Literature Cited Andrew, S. P. S. Int. Symp. Distill. Inst. Chem. Eng. 1980, 73-78. Davis, R. E.; Acrivos, A. Chem. Eng. Sci. 1988, 27, 681-689. Franz, K.; Borner, T.: Kantorek, H. J.; Buchhoiz. R. Chem.-1ng.-Tech. 1984, 5 6 , 154-155. Fuchs, W.; Smith, D. N. Proceedings of Symposium on Instrumentation and Control for Fossil Energy Processes, 1982, Houston, TX, ANL-82-62, 223-246. Gal-Or, 8.; Wasio, S . Chem. Eng. Sci. 1988, 2 3 , 1431-1446. Hess, M. U.S. DOE Final Report DE-AC22-82PC12320, 1982. Joseph, S.; Shah, Y. T.; Kelkar, B. G. Chem. Eng. Commun. 1984, 2 8 , 223. Joshi, J. B. Trans. Inst. Chem. Eng. 1980, 155-165. Joshi, J. B. Chem. Eng. J. 1982, 2 4 , 213-216. Keitei, G.; Onken, U. Chem. Eng. Sci. 1982, 3 7 , 1635-1639. Kelkar, B. G. Ph.D. Dissertation, University of Pittsburgh, Pittsburgh, PA, 1982. Kelkar, B. G.; Godbole, S.;Honnath, M.; Shah, Y. T.; Carr, N. L.; Deckwer, W . 4 . AIChE J. 1983, 2 9 , 361-369. Kulkarni, A.; Shah, Y. T.; Keikar, B. G. Chem. Eng. J. submitted. Kuo, J. C. W.; Gupte, K. M.; Leib, T. M.; Lovett, R. N.; Smith, J.; Van Kirk, J. F. US. DOE Quarterly Report DE-AC22-83PC60019, Nov 1983. Levan, M. D.; Newman, J. AIChE J. 1978, 22, 695-701. Levich, V. "Physicochemical Hydrodynamics"; Prentice Hall: Englewood Cliffs, NJ, 1962. Lindland. K. P.; Terjesen, S. G. Chem. Eng. Sci. 1985, 5 , 1-7. Marrucci, G. Ind. Eng. Chem. Fundam. 1985, 4 , 224-225. Meissner, H. P.; Michaels, A. S. Ind. Eng. Chem. 1949, 47, 2782-2787. Nicklin, D. J.; Wllkes, J. 0.; Davidson, J. F. Trans. Inst. Chem. f n g . 1982. 40, 62-68. Oels, U.; Lucke. J.; Buchholz, R.; Schiigerl, K. Ger. Chem, f n g . 1978, 7 , 115-129. Raymond, D. R.; Ziemenski, S. A. AIChf J. 1971, 17,57-65. Schiigerl, K.; Lucke, J.; Oels, U. Adv. Eiochem. Eng. 1977, 7 , 1-84. Shah, Y. T.; Kelkar, B. G.;Godbole, S. P.; Deckwer, W.-D. AIChf J. 1982, 28, 353-379. Smith, D. N.; Ruether, J. A.; Stlegel, G. J. Proceedings of the DOE Contractors' Conference on Indirect Liquefaction, Pittsburgh, PA, Oct 12-13, 1983. Smith, D. N.; Fuchs, W.; Lynn, R. J.; Smith, D. H.; Hess, M. ACS Symp. Ser. 1984, No. 237. Tsutsui, T.; Miyauchi, T. Int. Chem. Eng. 1980, 2 0 , 386-393. Zieminski, S . A.; Caron, M. M.; Blackmore, R. B. Ind. f n g . Chem. Fundam. 1987, 6 , 233-242.
Received for review May 2 2 , 1984 Revised manuscript receioed December 26, 1984 Accepted February 25, 1985
= gas and liquid density, respectively, kg/m3
Paratiel Thermal and Catalytic Kinetics in Direct Coal Liquefaction Sal V. Gollakota, James A. Guin,' and Christine W. Curtis Auburn Coal Conversion Labofafory, Depaflment of Chemical Englneerlng, Auburn University, Alabama 36849
The direct liquefaction of coal is studied in batch microreactors in the presence of a coal-derived solvent, hydrogen gas,and a Co-Mo/Al,O, supported catalyst. Kinetics experiments for reactions with powdered catalyst, extrudate pellets, and no catalyst (thermal reactions only) are performed. From these data, the relative influence of pore diffusion and the relative selectivity of thermal/catalytic reaction pathways is determined for the conversion of coal to liquids via a simple series reaction mechanism, viz. coal preasphattenes asphattenes oils. Effectiveness factors are evaluated for each reaction pathway.
-
The upgrading of heavy crudes, resids, and coal-derived liquids can be influenced to a large degree by catalyst pore diffusional factors. The effect of catalyst pore size on hydroliquefaction of West Virginia coal in the presence of Co-Mo-Alz03 was studied by Ho and Weller (1981). In these studies, the conversion of coal and yields of asphaltenes increased, as pore diameter increased from 100 8 to about 500 8. Kang (1983) also observed that coal conversion was controlled by restricted diffusion in catalyst 0196-4305/85/1124-1148$01.50/0
-
-
pores. Thus, an understanding of the role of catalyst as it affects different stages of liquefaction is essential to make the best use of catalyst and to provide the optimum reactor design. One method of classifying the coal liquefaction products is obtained by a solvent separation scheme in which the various products can be classified as oils, asphaltenes, preasphaltenes, and gases. In order of decreasing molecular weight, viscosity, and heteroatom content, these products proceed in the order preasphalt0 1985 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
enes, asphaltenes, and oils. The oil fraction is the most desirable as a product from a coal liquefaction facility. In order to form more oils from the preasphaltenes and asphaltenes fractions, a transition-metal catalyst, e.g., CoMo-A1203 or Ni-Mo-Aln03, is commonly used. Because of the large molecular sizes present in the preasphaltenes and asphaltenes fractions, as discussed in more detail in the following, diffusional resistances may be encountered in the catalytic hydrotreatment by using these supported catalysts. The extent and nature of these diffusional resistances for the various product classes is not well-known, and their determination is complicated by the fact that thermal reactions proceed in parallel with catalytic reactions during the liquefaction process. The so-called thermal reactions may themselves be catalyzed to a small but unknown extent by mineral matter indigenous to the coal; however, in the interest of simplicity, the term catalytic and catalyst when used in this paper will refer to the Co-Mo-A1203 catalyst. Recent investigations of the kinetics of thermal coal liquefaction have been summarized by Abichandani et al. (1982, 1984). To our knowledge, there has been little attempt to systematically separate thermal and catalytic reactions when occurring simultaneously in coal liquefaction. Thus, the objectives of the present investigation were to determine the nature and selectivity of the thermal vs. catalytic reactions occurring in coal liquefaction and to determine the magnitude of pore diffusion limitations for the various lumped product fractions occurring in coal liquefaction. Determination of the above factors is important for optimum design of catalyst support pore structure to maximize the oil yield. Literature Review and Kinetic Model Selection The kinetics of thermal liquefaction of coal have been determined by several investigators (Shalabi et al., 1979; Mohan and Silla, 1981; Cronauer et al., 1978; Abichandani et al., 1982). These workers considered liquefaction to be a multistep process wherein the reaction products were classified according to their solubilities in different organic solvents. The model used by Abichandani et al. (1982), given below, accounts for a possible net decrease in oil yield during short contact times. The model was based on the assumption that the coal in the feed reacts with species in the oil fraction to give heavier species. They assumed all reaction rates to be irreversible and pseudo-first-order with respect to the reacting species. c + o - c , gases
asphaltenes
-
C, - preasphaltenes
oils
Mohan and Silla (1981) summarized the literature on coal liquefaction mechanisms based on solubility separations. In the earlier proposed models, the reaction products, oils, and gases were combined to determine rate constants. Recent studies have depicted coal liquefaction in terms of sequential progression through a series of intermediate steps from preasphaltenes to asphaltenes to oils. Shal et al. (1978) studied the kinetics of catalytic liquefaction of Big Horn coal in a segmented bed reactor. The products were defined, as given below, according to boiling range rather than solubility. They proposed the
1149
following reaction mechanism. byproducts
light gases
- / \ coal
water
furnace oil
naphtha
204-343 "C
C4-204 "C
heavy fuel oil
343+ "C
Although there is a rough correspondence between the solubility fraction and distillate fraction, it is difficult to compare their catalytic liquefaction results directly with the thermal liquefaction results obtained by using solubility separations by other investigators (Abichandani et al., 1982). Shalabi et al. (1979) studied the rates of formation of oils, asphaltenes, and preasphaltenes during solvent extraction of a high-volatile bituminous coal. The following was one of the reaction models proposed. coal preasphaltenes asphaltenes oil + gas
-
-
-
Abichandani et al. (1982) have indicated the possibility of direct formation of asphaltenes and oils from an active intermediate of coal in the presence of a good hydrogen donor solvent. Although it is known that oil may be formed directly from coal apparently without going through the intermediate stages, this path to oil is relatively rare (Ruether, 1977). Abichandani et al. observed that the reaction rate constants for the formation of gases from preasphaltenes, asphaltenes, and oils were very small compared to those from coal. Thus, in the present modeling, gases were assumed to form directly from coal. During the initial stages of coal liquefaction, uncapped free radicals of coal adduct with the solvent resulting in a net decrease in oils (McNeil et al., 1983). Thus, a reaction mechanism where coal + oil together give combined products accounts for the net decrease in oil yield for short reaction times (Abichandani et al., 1982). The ratio in which coal and oil adduct to form products is not known. In the present work, it is assumed that 1g of coal reacts with a g of oil to form combined products. The factor a is then determined from a fit of the kinetic data. For simplicity of analysis and because of the systematic appearance of maxima in preasphaltenes and asphaltenes distributions indicating their occurrence as reaction intermediates in the present investigation, the sequential progression of coal liquefaction given below was used in the modeling of kinetic data. coal + aoil preasphaltenes asphaltenes oils (1)
-
coal
-
-
gases
-
(2)
Several possible pore diffusion limitations may occur in catalyzing the above reactions. Pelleted high surface area catalysts such as cobalt-molybdenum or nickel-molybdenum are employed in fixed or ebullated beds such as in the Synthoil and H-Coal processes. Because of the presence of large molecules, pore diffusion limitation of reaction rates can be serious in hydroprocessing of coal-derived liquids and heavy resids over pelleted catalysts. Spry and Sawyer (1975) discussed the problem of steric limitation of large molecules in pores. They suggested that the molecular sizes of petroleum-derivedasphaltene molecules would be in the range 25-150 A. Generally, the pore sizes in pellets are between 20 and 200 8, (Froment and Bischoff, 1979). Thus, although coal-derived asphaltenes are thought to generally be somewhat smaller than their pe-
1150
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
troleum-based counterparts, it is evident that many asphaltene molecules will encounter so-called configurational or restricted diffusion in the catalyst pores. The effect of pore diffusion limitations on the overall rate of a catalytic reaction is commonly viewed in terms of the effectiveness factor, defined as the ratio of reaction rate of a component in the presence of pore diffusion to that in the absence of pore diffusion resistances. A major objective of the present investigation was to evaluate the catalyst effectiveness factors in the various stages of liquefaction. The classical way of determining the pellet effectiveness factors is to analyze the reaction rates obtained in the presence (i.e., large particle size) and absence (i.e., small particle size) of pore diffusion resistances. Thus, in the present work, kinetic data were obtained by using 3/16-in.catalyst pellets and -200-mesh powder obtained by grinding the pellets. In analysis of the kinetic data, it is essential to delineate the thermal reaction from catalytic reaction to determine catalyst effectiveness factors; hence, experiments were also carried out in the absence of any external catalyst. Because of the multiplicity of reactions and also because of the occurrence of reactions both in the presence and absence of catalyst, the effectiveness factors will be complex functions of reactant concentrations. Thus, a detailed mathematical analysis of the reaction system has been developed.
Mathematical Modeling After some consideration and preliminary data analysis, the following reaction model was selected for the present investigation. kz
kl
thermal
k3
C +aO-P-A-O
(3)
subscript s refers to concentration in the bulk liquid (assumed to equal the pellet surface concentration). To solve these equations, the catalytic reaction fluxes must be determined in terms of known variables. In this computation, the thermal reaction is neglected in comparison to the catalytic reaction for the fluid in the catalyst pores. For the simple geometry of a slab of catalyst, when the z coordinate is oriented from the center line to the surface, the steady-state diffusion equations for species inside the pellet are d2P = k,’P dz2
(10)
Dp-
d2A = -k2’P dz
DAY
+ ks’A
with the boundary conditions B.C.1 at z = L P = P, A = A, 0 = 0, (pellet surface) d P dA dO B.C.2 at z = 0 - = - = - 0 (symmetry) dz dz dz The first steady-state diffusion equation leads to the standard solution (Froment and Bischoff, 1979) cosh yz P = P , - cosh y L where y = [k2’/Dp]’/2 (13) This solution is used to solve the other two equations to give the following concentration profiles inside the pellet
k4
C-G k,’
catalytic
A = 0 cash PZ - Cp cash
P-A-0
--
(14)
(4)
This model contains four thermal rate constants, two catalytic rate constants, and a stoichiometric parameter, cy. As a first approximation, the thermal and catalytic reactions are assumed to proceed in parallel in the presence of a catalyst. As shown later by analysis of the data, the reactions C + a 0 P and C G are mainly thermal in nature. The other two reactions, P A and A 0 are thermal as well as catalytic in nature as shown by eq 1and 2. Thus, pore diffusion limitations may arise only for these latter two reactions. The rate of disappearance of each component in a batch reactor can be written as the sum of thermal (homogeneous)and catalytic reaction rates for the products given in eq 4. dC, - = -k,C,O, - k,C, dt
-
YZ
k,’
-
Now, as concentrations of P, A, and 0 are known as functions of z , the mass flux at the catalyst pellet surface can be determined.
The above relation can be rewritten in terms of the effectiveness factor as NPIzeL= -mk2/Psvl/pw2 = - k 2 e P s / ~ 2
(20)
where p and m are the density and loading of the catalyst, respectively, and tanh y L 71 =
In the above equations, N p ,NA,and No are the mass fluxes at the surface of the catalyst pellet, and w2 is the catalyst external surface area per unit volume of liquid. The
~
(21)
YL For simplicity, mkiql/p is expressed as an effective rate constant, k2e. Similarly, mass fluxes of asphaltenes and
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
1151
oils can be determined. The final expressions are given below
Table 111. Compositions of the Materials Used i n Coal Liquefaction Experiments Harshaw 0402T: 3 / l e in. catalyst (as received) c o o = 3% (wt) Moos = 15% silica-promoted alumina pellet density = 1.4 g/cm3 surface area = 180 m2/g pore vol = 0.4 cm3/g crushing strength = 20 lbs cumul pore diam distribut, A 120 A, 0.29 cm3/g 350 A, 0.31 cm3/g 700 A, 0.31 cm3/g 1000 A, 0.32 cm3/g 10000 A, 0.38 cm3/g 150000 A, 0.40 cm3/g oils = 87.95 f 0.89 solvent, wt % asphaltenes = 10.40 f 0.78 preasphaltenes = 0.945 5 0.11 IMC" = 0.694 f 0.07 coal, w t % oils = 0.012 asphaltenes = 1.13 preasphaltenes = 2.47 IMC" = 96.38 (MAF basis)
= [k,,(l - E ) P ,- k 3 e A s ] / ~ 2 (22)
"Insolubles in methylene chloride and (10% V) methanol mixture.
Table I. Rate Equations Used i n Fitting t h e Kinetic Data thermal catalytic
Table 11. Typical Analysis of Elkhorn No. 3 Coal proximate ultimate sulfur, wt 70 anal., wt % anal., wt % tot s moisture 1.81 C 69.4 sulfate S H 4.88 pyrite S volat mat 37.6 N 1.00 org S fixed C 46.0 S 1.94 ash 14.60 0 (by 8.18 differ)
NAlr=L
= -DA
@I dz
1.94 0.04 1.19 0.75
z = ~
Table IV. I n S i t u Sulfiding and Aging Conditions for t h e Catalyst catalyst, g Co-Mo 3/16 in. x 3/16 in. pellets, 60 2000 solv, g cs29 g 20 autoc1av e 1 gal temp, "C 365 agitation, rpm 500 72 time, h Hz press, psig 1100-1250 ~~
where 12
=
tanh PL
pL
(24)
Table V. Experimental Conditions in Coal Liquefaction
Runs
The mass fluxes in eq 6-8 now can be rewritten in terms of the effective rate constants, k2, and k3,, to yield the following set of working equations.
-dOS - - -k1d2sOs + (k3 + k3,)As+ k2,tPs dt
(29)
The effective catalytic rate constants are functions of vl, q2, m, p, D A , etc. For a purely thermal reaction in the absence of a catalyst, one has k2, = k% = 0. For a catalytic reaction with powder (small L), eq 21 and 24 give v1 E v2 N 0. For the catalytic reaction with pellets, N 1 and preliminary analysis indicated that, possibly because of low diffusivity of large preasphaltene molecules or because of their exclusion from catalyst pores, the effectiveness factor for the reaction preasphaltenes asphaltenes was very low. Thus, when pellets are used, q1 N 0. By writing v1 and 712 in terms of y and P in eq 25, it can be shown that as v1 approaches zero, the value of t also approaches zero. The material balance equations used for the thermal and catalytic cases are summarized in Table I. Experimental Section Equipment. Batch experiments were carried out in tubing bomb microreactors constructed of seamless stainless steel. The reaction temperature was maintained by immersing the microreactor into a fluidized sand bath. Additional details and a schematic of the equipment used
-
coal, g solv, g catal, g temp, "C pH2 (init), psig agitation, cpm reactor vol, cm3
2.25 4.50 1.00 425 1250 850 46
are given by Brooks et al. (1983) and Curtis et al. (1983). Materials. Experiments were performed using bituminous Elkhorn No. 3 coal and an SRC I1 coal-derived solvent. The coal samples were sized to -200 mesh and dried under vacuum to remove moisture (2.6%). The ash content of this particular coal sample was 16.35%. A typical analysis of Elkhorn No. 3 coal is given in Table 11. The solvent composition is given in Table 111. Harshaw 0402T cobalt-molybdenum on alumina 3/16 X 3 / 1 6 in. was the catalyst used. To more closely simulate the conditions in an actual coal liquefaction reactor, the fresh catalyst was aged and presulfided in situ for 72 h in a batch autoclave by using a coal-derived solvent and carbon disulfide as a source of sulfur. The catalyst aging conditions are given in Table IV. While aging, the catalyst was held in baskets to avoid attrition of pellets. The aged and sulfided catalyst then was washed by using tetrahydrofuran under sonication until clear liquid was obtained. A part of the aged catalyst was ground to -200 mesh. Procedure. After charging, according to the conditions in Table V, the microautoclave reactor was attached to the agitation equipment and immersed in a preheated fluidized sand bath. Huang et al. (1981) reported that agitation had no effect on coal liquefaction product yields for speeds greater than 400 cpm. In the present investigation, ex-
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
1152
WASHING
WLSHIUG
SONICATE
SONICATE
CENTRIFUGE
CENTRIFUGE
FILTER
FILTER
ASPHALTEUES.02
1OlL.P E
q y q 60 JEG
Cl
4-, FREEZE D R Y
IOU
.
0 PREASPHALTENES
0 IOM
I
lnrolubl. OrganloMolter
OIL
ASPHALTENES
PREASPHALTENES
t
OM CAT MM
Figure 1. Diagram for product separation procedure.
R E A C T I O N TIME, MIN
Figure 3. Experimental and fitted product distributions for catalytic liquefaction (powder).
0.044
3
0 L? LL
0
0.03-
4 v)
0.02-
7 0.3 0.4
0.0 I 0.0
0.2
1
0.I
U
Figure 2. Sum of square of errors for different a values.
periments were carried out a t 850 cpm. The gaseous, liquid, and solid products were collected for analysis. The gas yield was determined from the total weight of gases collected, volume of gases, and the volume percentage of hydrogen. As illustrated in Figure 1, the liquid + solid products were separated into four fractions. oils asphaltenes preasphaltenes
pentane solubles pentane insolubles and benzene solubles benzene insolubles and solubles in methylene chloride-10% methanol mixture IOM (insoluble insolubles in methylene chloride and organic matter) 10% methanol mixture (IMC)
Further details on the product separation procedure are given by Curtis et al. (1983). In the calculation of the insoluble organic matter, as described below, only the reactive portion of the coal was considered. It has been noted previously that all the organic coal may not be converted to liquefaction products (Shalabi et al., 1979). The quantity of unreactive macerals depends on the temperature and solvent used (Cronauer et al., 1978). The insolubles in methylene chloride-10% methanol mixture obtained in the longest run (i.e., 90-min run with aged powder) were taken as the unreactive portion of coal. The unreactive fraction of coal was found to be approximately 11.43% of dry coal. The product losses during the product separation were generally within *3%. Results and Discussion The equations given in Table I were solved by using a program which performs nonlinear least squares to find
R E A C T I O N TIME, MIN.
Figure 4. Experimental and fitted product distributions for catalytic liquefaction (pellet).
best estimates of rate constants (Song, 1983). This program was developed by using the numerical and programming techniques given by Seinfeld and Gavalas (1970),Box (1960), and Ralston and Jennrich (1978). The empirical factor a , held constant during the estimation of rate constants, was determined by minimizing the sum of squares of errors of all three sets of data (Le., thermal, pellet, and powder). Figure 2 shows a plot of the sum of the square of errors a t different a values. An a value of 0.2 was used for subsequent modeling. The mass fractions of products obtained as functions of time are shown in Figures 3-5. These fractions include the total reactor product, derived from both coal and solvent. The maximum conversion of coal to oils ( about 75%) was obtained when the powdered catalyst was used. In general, the agreement between the model predicted values and the experimental data is sat-
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
1153
Table VI. First-Order Rate Constants (min-’) for Different Stages of Coal Liquefaction thermal Dellet nawder kl = 0.18 f 0.03 k, = 0.23 f 0.05 ki = 0.17 f 0.02 k2 = 0.068 f 0.008 122 + kze = 0.058 k2 + k2, = 0.21 f 0.05 f 0.01
kz k3
= 0.0094 f 0.01
0 OILS I-
I
n 0
k, =
ASPHALTENES PREASPHALTENES
0 IOU 0 GASES
-MOOEL
+ kze (I - 6) = 0.065 f 0.01
+
k,, = 0.025 f 0.006 0.0195 h 0.004 k4 = 0.024 f k3
k3
+ kse = 0.033 f 0.001
k, = 0.023 f 0. 004
0.009
T
thermal catalytic
coal % ! preasphaltenes h. asphaltenes b oils coal 4gases preasphaltenes % asphaltenes kL oils
here indicates that 70% of the pore volume was made up of pores smaller than 120 A in diameter. These small pores restrict the diffusion rates of large molecules with a typical size range of 25-150 A. Thus, the effectiveness factor for the reaction preasphaltenes asphaltenes is very low. As noted earlier, Spry and Sawyer (1975) also observed an intraparticle diffusion rate limitation, involving the transport of large petroleum-derived asphaltene molecules where the molecular diameter is at least of the pore diameter. They found that the relative catalyst activity rapidly decreased with increased molecular size. For the reaction, asphaltenes to oils, there was a successive increase in the rate constant groups in the order thermal < pellet < powder. The effectiveness factor for this reaction can be obtained as follows. For pellets, v2 can be determined from eq 26 repeated below. The catalytic surface rate constant, k3’,can be +
R E A C T I O N TIME, MIN.
Figure 5. Experimental and fitted product distributions for thermal liquefaction.
isfactory. It should be recalled, when verifying the curve fits, that the model is fitted as a whole, rather than each product, e.g., oils, asphaltenes, etc., being fit separately. The primary liquefaction products at short reaction times are preasphaltenes and asphaltenes. The asphaltene fraction degrades as further cleavage and hydrogenation reactions begin to predominate, yielding oils. A net decrease in oil yield was observed at short reaction times. This observation forms the basis for the use of C + a 0 P as one of the reaction steps. It is interesting to note that, in all cases, at longer reaction times, the experimental preasphaltene fractions were higher than those predicted by the model. This is possibly because of the formation of a stable fraction of preasphaltenes, which is resistant to further hydrogenation. The existence of stable preasphaltenes in the reactor at long reaction times was also observed by Shalabi et al. (1979). The values of rate constants and their standard deviations are given in Table VI. As observed from the table, the rate constants for the initial breakdown of the coal for the thermal as well as catalytic runs are essentially the same, indicating that the primary step in coal liquefaction, the rupture of coal to form lower molecular weight products, is predominantly thermal in nature, at least with the supported catalyst used herein. In this regard, it may be noted that the activation energies for the coal solvation reactions, obtained by Shah et al. (1978) indicated that coal solvation may be largely a thermal process. Similarly, the formation of gases from coal also appears to be mainly thermal in nature. There was a 3-fold increase in the rate constant groups between the thermal and powder runs for the intermediate reactions (i.e., preasphaltenes to asphaltenes to oils), indicating that these reactions were the most strongly influenced by the catalyst. However, for the reaction preasphaltenes to asphaltenes, pellets gave roughly the same rate constant groups when compared to the thermal reaction, indicating that kze = 0. One explanation of these data is that the relatively large molecules of preasphaltenes underwent an extremely restricted diffusion rate in the pores of the pellet. As shown in Table 111,the cumulative pore size distribution for the catalyst used (as received)
-
obtained from the powder data. When, the catalyst is used in powdered form, q1 N qz = 1, and eq 26 simplifies to (30)
Consideration of the above data reveals the following observations. The reactions resulting in asphaltene formation are susceptible to catalytic action; however, comparison of the pellet vs. powder rate constant groups for this reaction reveals that only the powdered catalyst is effective in enhancing the reaction. On the other hand, the oil-forming reactions, also susceptible to catalytic activity, are susceptible to catalysis both in pellet and powder form. These observations are consistent with the generally accepted decline in molecular size as the reaction progresses from preasphaltenes to asphaltenes to oils and the associated increased diffusion rate of the smaller molecules. Conclusions The following conclusions were obtained from the present investigation. Both thermal and catalytic reactions proceed in parallel as coal liquefaction takes place. The progressive reaction sequence given by the model C + a0 P -+A 0 C-G satisfactorily represented the kinetics of thermal as well as catalytic liquefaction of coal. This model also accounted +
-+
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Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 4, 1985
for the net decrease in oil yield obtained a t short reaction times. The formation of preasphaltenes from coal, the primary stage of coal liquefaction, and the formation of gaseous products were predominantly thermal in nature. The effectiveness factor for the reaction preasphaltenes asphaltenes was found to be very low when the aged 3/ls-in. pellets were used, whereas the effectiveness factor for the reaction asphdtenes oils was found to be about 0.66. Thus, the reaction P A is a critical limiting step in catalytic coal liquefaction. For effective conversion of large preasphaltene molecules, supported catalysts with large pores must be utilized. The optimum pore size distribution probably has yet to be developed. Work dong these lines has been done by Tischer (19811, Ternan (1982), and investigators a t Amoco (1982), among others. Acknowledgment We thank the US Department of Energy for support of this work under contracts No. DE-AC01-79ET13397 and DE-FG22-82 PC 50793. Nomenclature A = asphaltenes C = coal C1 = more reactive coal cpm = agitation rate, cycles/min Di= effective diffusivity of component i , cm3/(cmps) G = gas kl-k - first-order rate constants for thermal reactions, s-l ki,kl'-= first-order rate constants for catalytic reactions, cm3/(cm3, s) L = distance from center to surface of catalyst pellet, cm MAF = moisture and ash free m = catalyst loading, g/cm3 N = mass flux at catalyst surface, g/(cmp2s) 0 = oils P = preasphaltenes t = time, s w2 = catalyst external surface area per unit volume of liquid, cme2/cm3 z = distance inside a slab of catalyst, cm a = stoichiometric parameter y = (kz//Dp)1/2, cmP-l 6 = (k3'/DA)'I2,cmP-l
-
--
p
= catalyst pellet density, g/cm,3
0 = defined by eq 17 4 = defined by eq 18 t
= defined by eq 25
vl, q2 = effectiveness factors Subscripts e = effective s = concentration in the bulk liquid or pellet surface con-
centration pe = pellet PO = powder Registry No. Co, 7440-48-4; Mo, 7439-98-7. Literature Cited Abichandani, J. A.; Shah, Y. T.; Cronauer, D. C.; Ruberto, R. G. Fuel 1982, 61, 276. Abichandani, S.; Weibnd, J. H.; Shah, Y. T.;Cronauer, D. C. AIChE J. 1984, 30, 295. Amoco Oil Co. Report No. DOE/ET/14803-T6, Aug 1982,Napervilie, IL. Box, G. E. P. Ann. N . Y. Acad. Sci., 1080, 86, 792. Brooks, D. G.; Guin, J. A.; Curtis, C. W.; Piacek, T. D. Ifld. Eng. Chem. Process D e s . Dev. 1903, 2 2 , 343. Cronauer, D. C.; Shah, Y. T.; Ruberto. R. G. Ind. Eng. Chem. Process D e s . Dev. 1978, 17, 281. Curtis, C. W.; Guin, J. A.; Tarrer A. R.; Huang, W. J. Fuel Process Technol. 1003, 7 , 277. Forment, G. F.; Bischoff, K. B. "Chemical Reaction Analysis and Design"; Wiley: New York, 1979. Ho, P. N.; Weiier, S . W. Fuel Process Technol. 1981, 4 , 21. Huang, W. J.; Curtis, C. W.; Win, J. A.; Clinton, J. H.; Barwood, H. L.; Tarrer, A. R. Paper presented at the AIChE Meeting, Houston, March 1961. Kang, C. C. Paper presented at the Proceedings of the International Conference on Coal Science, Pittsburgh, 1983,p 75. McNell, R. I.; Young, D. C.; Cronauer, D. C. Fuel 1983, 62, 806. Mohan, G.; Siila, H. Ind. Eng. Chem. Process Des. Dev. 1081, 20, 349. Raiston, M. L.; Jennrich, R. I. Technometrics 1978, 20, 7. Ruether, J. A. Ind. Eng. Chem. Process Des. Dev. 1977, 16, 249. Seinfeld, J. H.; Gavalas, G. R. A I C M J. 1970. 16, 645. Shah, Y. T.; Cronauer, D.C.; Mclivried, H. G.; Paraskos, J. A. Ind. Eng. Chem. Process Des. Dev. 1078, 17, 288. Shabbl, M. A.; Baldwin, R. M.; Bain, R. L.; Gary, J. H.; Gelder, J. 0. Ind. Eng. Chem. Process Des. Dev. 1979. 18, 474. Song, S . K. Ph.D. Dlssertation, Auburn University, 1983. Spry, J. C.; Sawyer, W. H. Paper presented at the AIChE Meeting, Los Angeles, Nov 1975. Ternan, M., Packwood, R. H., Buchanan, R. M.; Parsons, R. I. Can. J. Chem. Eng. 1902, 6 0 , 33. Tischer, R. E. J. Catal. 1981, 72, 255.
Received for review May 24, 1984 Revised manuscript received February 28, 1985 Accepted March 25, 1985
