Ind. Eng. Chem. Process Des. Dev. 1901, 20, 349-358
949
Kinetics of Donor-Solvent Liquefaction of Bituminous Coals in Nonisothermal Experiments Govlndan Mohan and Harry Sllla’ Department of Chemistry and Chemical Englmrhg, Stevens Institute of Technobgy, Hoboken, New Jersey 07030
An Illinois No. 6 coal was liquefied in a batch reactor under nonisothermal conditions from 330 to 450 O C using tetralin as a hydrogendonor solvent at total pressures up to 70 atm and reaction times from 5 to 60 min. A separation scheme was developed using liquid-solid chromatography on alumina to separate the Ilquefled products into classes of compounds consisting of mixtures of aromatics, ethers, nltrogens, hydroxyls, and multifunctionals. Using these classes of compounds several kinetic models were tested by flttlng experimental nonlsothermal concentration-time profiies using a nonlinear parameter estimation technique. The multifunctional compounds were considered as reaction intermediates which then decomposed reversibly Into aromatics, ethers, nitrogens, and hydroxyls. It was found that this kinetic model predicts concentration-time profiles satisfactorily. This work has also established the use of preparative liquld-solkl chromatography as a reliable and useful method for separating coal liquids for kinetic studles.
Introduction In view of the need to develop commercially viable processes for deriving liquid fuels from coal a description of coal liquefaction kinetics is necessary for reactor design and scaleup. Most coal liquefaction processes involve dissolution of the coal by using a hydrogen-donor solvent with added gaseous hydrogen, but among the various solvents employed in laboratory studies tetralin has been found to be the most common hydrogen donor. Furthermore, synthetic solvents designed to simulate a typical process solvent invariably contain significant amounts of tetralin. The addition of gaseous hydrogen in donor-solvent liquefaction in several laboratory investigations has been shown to yield higher conversions (Han et al., 1978). Recently, Maddocks and Huffman (1978) proposed that no hydrogen gas be added in the first dissolution stage of a two-stage coal liquefaction process. Also, in recent laboratory studies Mitchell and Whitehurst (1978) and Longanbach et al. (1979) liquefied coal at short contact times in the absence of gaseous hydrogen. In our studies tetralin was used as the donor solvent without added gaseous hydrogen. The kinetics of coal liquefaction are very complex leading to the formation of numerous compounds. The approach in kinetic studies has been to attempt to separate these compounds into “kinetically similar” classes based on some separation technique. Most studies used methods of separation that depend on solubility in various solvents to separate coal liquids. These studies are summarized in Table I. Recently, Brunson (1979) used distillation to measure the kinetics of formation of gaseous and liquid products using a coal-derived hydrogen-donor solvent. Prather et al. (1977) indicated the feasibility of using analytical liquid-liquid chromatography (LLC) to characterize coal liquefaction solvents for studying kinetics, but they did not attempt to use this approach to obtain product compositions for estimating kinetic parameters. Whitehurst et al. (1977) separated coal liquid products by liquid-solid chromatography (LSC) on silica using various eluting solvents in sequence. They concluded that the coal liquefaction products contained a large number of compounds with very similar hydrocarbon skeletons differing only by the various functional groups, and they also did not propose any kinetic model based on the separated products. A summary of kinetic models and rate constants based on product compositions obtained by solubility in various 0196-4305/81/1 120-0349$01.25/0
solvents with the definition for the various solubility classea is given in Table I. These experiments were carried out with and without catalysts and donor solventa using mostly bituminous coals. The proposed kinetic models for liquefaction with or without donor solvents are in many cases similar. In many of the studies a hydrogen-donor solvent with an added catalyst was used. In the earlier proposed kinetic models the reaction products, oils and asphaltenes, were combined or treated separately to determine reaction rate constants. Pelipetz et al. (1955) and Hill et al. (1966) assumed that only a single thermal decomposition process occurs. Falkum and Glenn (1952) found that the hydrogenolysis of Spitsbergen coal occurred in two distinct stages and suggested that two constituents are present in coal, one of which degraded much faster than the other. Such a model, involving two reactive constituents, was investigated by Curran et al. (1967) and Struck et al. (1969) and was found to fit their experimental conversion data on coal extraction and coal-extract hydrocracking processes. Weller et al. (1951) proposed that the conversion of coal to oils involves two consecutive first-order reactions with asphaltenes as an intermediate product. They determined the rate constants for both steps as shown in Table I. On the other hand, Liebenberg and Potgieter (1973) proposed two simultaneous reactions for the formation of oils and asphaltenes in order to explain their experimental data. Yoshida et al. (1976) expanded this model to allow for oils formation by two paths. Recently, Chiba and Sanada (1978) assessed the previous models consisting of oils and asphaltenes and suggested using the formation of oils from two active coal components to explain the experiments in the literature. Recently, a third class of compounds, the preasphaltenes, was isolated from coal liquefaction products and was shown to be a key reaction intermediate by Sternberg et al. (1975). Thus a more complex model was proposed by Schweighardt and Sharkey (1976) using preasphaltenes in addition to oils and asphaltenes. Subsequently, simplified versions of this model have been satisfactorily tested in a batch reactor (Shalabi et al., 1979) as well as in a continuous reactor (Cronauer et al., 1978; Traeger, 1979). Squires (1978) proposed a more comprehensive working model for donor-solvent coal liquefaction, but the model has not been tested to date because the isolation procedure for the “product oil” and “semi-coke” has not been established. In this study LSC was employed to separate the coal 0 1981 American Chemical Society
350
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981
Table I. Summary of Coal Liquefaction Mechanisms Baeed on Solubility Separations investigators
kinetic schemea
coal
hydrogen donor solvent
added catalyst
rate constant at 400 OC, min-I
Pelipetz et al. (1955) Hill et al. (1966)
Wyoming Utah
none tetralin
none none
k , = 0.0030 k , = 0.0372
Falkum and Glenn (1952)
Spitsbergen
none
Ca-Cu-Cr
k , = 0.0068
Curran et al. (1967)
Pittsburgh
tetralin
none
k , = 3.0200 k , = 0.1160
Struck et al. (1969)
Pittsburgh extract
none
ZnO/ZnCI,
k , = 0.2400 k , = 0.0062
Weller et al. (1951)
Pittsburgh Anthraxylon
none
SnS, NH,CI
k , = 0.0270 k , = 0.0011
Liebenberg and Potgieter (1973)
bituminous
tetralin
none
k , t k , = 0.0039
Yoshida et al. (1976)
Japan
decrystallized anthracene oil
red-mud and sulfur
k , = 0.0037 k , = 0.0049 k , = 0.0170
Cronauer et al. (1978)
Belle Ayr
hydrogenated phenanthrene
none
k , = 0.1492 k , = 0.1979 k , = 0.0855 k , = 0.0240 k , = 0.1546 k , = 0.0735
Shalabi et al. (1979)
Kentucky
tetralin
none
k , = 0.00043 k , = 0.00007 k , = 0.00075 k , = 0.00005 k , = 0.00120 k , = 0.00005 G, gases; 0, oils (hexane solubles); A, asphaltenes (hexane insolubles, benzene solubles); P, preasphaltenes (benzene insolubles; tetrahydrofuran solubles).
liquids into classes of compounds for kinetic analysis. Chromatographic separation promises to be a more efficient and perhaps more fundamental method.
Experimental Section System Design. Since Storch et al. (1940) reported the use of batch reactors for kinetic studies, many investigators have employed these massive reactors of various capacities. These reactors require long heat-up and cool-down times, during which significant reactions can occur. The approaches used to minimize these nonisothermal heat-up times have been to use small reactors constructed from tubing (Neavel, 1976) or to first heat up the solvent to the reaction temperature and inject the coal slurry with a pump (Shalabi et al., 1979). If isothermal conditions cannot be obtained experimentally, then a calculation procedure is required to correct for the nonisothermal conditions, which is the approach adopted here. Earlier, Yergey et al. (1974) applied the nonisothermal analysis to estimate the kinetic rate parameters for the hydrodesulfurization of coal. Recently, Morita et al. (1979) used a nonisothermal analysis for estimating the reaction rate parameters in studying the effect of hydrogen pressure on the rate of direct liquefaction of some Japanese coals. The experiments discussed here were conducted in 1-L stirred reactor (Autoclave Engineers, Model AFP-1005, Magnedrive), shown in Figure 1, where all materials in contact with the coal slurry are 316 stainless steel. A coal slurry injection system was designed and added to the
reactor to minimize reactant heat-up times. This system included a double-pipe heat exchanger to prevent overheating of the Teflon seat of the ball valve caused by heat conduction from the reactor. Nevertheless, the Teflon seats still had to be cleaned or replaced after a few injections because of coal deposits or erosion. The holdup of the slurry in the injector, which was washed and mixed with the reactor wash at the end of a run, was found to be less than 0.5% of the slurry injected. This method of slurry injection caused a temperature drop of 30 "C of the reaction mixture upon injection, thus requiring a method to correct for the nonisothermal reaction conditions. The furnace temperature was controlled by a proportional action temperature controller using a chromel-alume1 thermocouple and solid-state switching (Barber-Colman, Series 530). The reaction temperature was measured manually (*l OC) with another chromel-alumel thermocouple inserted into the thermowell of the reactor using a cold-junction compensator and a potentiometer. The temperature was also recorded with a Leeds & Northup Speedomax X/L 680 recorder with a full scale response of 1 8. All the components of the Magnedrive impeller were disassembled and cleaned thoroughly after each run. Neither the graphite bearings of the Magnedrive impeller nor the stainless steel gasket needed replacement during the course of this work, because of the care and cleaning procedure adopted. The reaction mixture was stirred at 1500 rpm since the mass transfer resistance has been re-
Ind. Eng. Chem. hocess Des. Dev., Vol. 20, No. 2, 1981
351
NITROGEN REACTION
CHECK VALVE SOLVENT PREHEATING
WENCH
530 400 0
STATOR FOR TACHOMETER
BALL VALVE
ir
u'
28 300 200
DOUBLEPIPE HEAT EXCHANGER
IO0
0
I
I
I
I
I
2
3
4
TIME, h
Figure 2. Typical temperature-time profile. - W E N C H WATER INLET
COOLING COIL-
-FURNACE
THERMOWELL-
INJECTION TUBE IMPELLER
Figure 1. Coal liquefaction reactor system. Table 11. Ultimate and Proximate Analyses of Illinois No. 6 Coalo proximate ultimate mass % (moisture component free basis) hydrogen carbon nitrogen sulfur (total) oxygen ash
4.8 68.9 1.2
component
mass % (moisture free basis)
volatile matter fixed carbon ash
42.3 45.2 12.5
3.1
8.9 12.5
* Coal source, River King Mine, Ill. Donated by U.S. Department of Energy, Pittsburgh, PA. Dried prior to use. ported to be insignificant at this speed (Hill et al., 1966). Also, when the impeller speed was reduced to 1300 rpm in this investigation, there were no significant changes in the product concentrations. Materials. The coal used in all experiments was a high-sulfur and a high-ash Illinois No. 6 coal from the River King mine. The analyses of the coal are given in Table 11. The coal was dried, ball-milled and sieved through a 200 U.S.standard mesh screen and stored in a desiccator under nitrogen atmosphere prior to its use. Technical grade tetralin was used as received from City Chemicals Corp., New York, and was analyzed by LC and found to contain 25 mol % of decalin and less than 1% of naphthalene. The reagent grade solvents used for washing the reactor and for the chromatographic separations, and the alumina packing (Fisher No. A-540), used to separate the liquid reactions products, were obtained from Fisher Scientific Co.
Experimental Procedure. In a typical experiment, the reactor is assembled and leak tested with nitrogen. Then 264 g of tetralin is introduced into the reactor at ambient pressure, flushed with nitrogen, and heated to a steadystate temperature while stirring at 1500 rpm. A slurry consisting of 30 g of coal in 36 g of tetralin is injected from the feed reservoir using nitrogen at a pressure of 30 atm as shown in Figure 1. The coal will settle from more concentrated slurries in the feed reservoir whereas less concentrated slurries cool the reaction mixture appreciably. The final solvent/coal ratio in the reactor was 10 in all experiments to keep the tetralin concentration relatively constant during reaction (Hill et al., 1966; Shalabi et al., 1979). After injection of the cold slurry, the reaction mixture is cooled by approximately 30 O C after which the temperature rises steadily during the experiment as can be seen in a typical temperature-time profile shown in Figure 2. The pressure rise during reaction was about 25 atm. After the desired reaction time, the reaction is rapidly quenched by passing cold water through the cooling coils and by blowing cold air over the reactor after dropping the furance with the aid of laboratory jacks. The experiments were carried out at temperatures from 330 to 450 OC and pressures under 70 atm. Temperatures below 330 O C are ineffective for coal dissolution, resulting in low conversions (Guin et al., 1979) and temperatures greater than 450 "C lead to excessive coke formation (Storch et al., 1940). Reaction times of 5, 10, 15, 20, 30, 40, and 60 min were selected for all the experiments. Because injection of the coal slurry required 1 min and quenching 2 min, reaction times less than 5 min were not considered. After 60 min a limiting conversion was achieved. Product Separation Procedure. The separation procedure for the reaction products is shown in Figure 3. After the reactor is cooled down to room temperature, the gaseous products are bubbled through 0.4 M lead acetate solution to remove the H a and thus determine the amount of sulfur in the gas phase. The black precipitate of PbS formed is filtered, washed with 0.1 M HN03 and then with distilled water before drying at 110 "C to constant weight. Overall mass balances made for each experiment showed better than 96% recovery of the total charge to the reactor. The unwashed material sticking to the walls of the reactor, estimated to be 1%of the total charge, and the product gaws constituted the losses. In early experiments methane, ethane, propane, butane, and hydrogen were found from a GC-MS analysis of the product gases, but no attempt was
352
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981
THFWIB*OFREACTDR
4-4 REACTOR
GASES
t
0.25
r
L E A 0 ACETATE SCRUBBER
p-/-'T' SLURP"
I
' i '
A 800
1200
I € IO
VOLUME ELUTED, mi
TETRALIN
I
"*C""U
I
Figure 4. A typical preparative liquid-solid chromatogram for solvent refined coal. PACKING: BONDAPAK CI8/PORASIL
B
SOLVENT: METHANOLIWATER (77:23) FLOWRATE. 2.5 mllmin
PEAKS 1. DECALIN
2. TOLUENE (Internal Standard1
3. NAPHTHALENE 4. DIHYDRONAPHTHALENE 5. TETRALIN
NiTRDGlNS
HIOROXYLS *SH
UULTlFUNCTlONilLI
Figure 3. Separation scheme for coal liquefaction reaction products.
made to account for the gaseous products in subsequent runs. The slurry products were collected in two stainless steel flasks and centrifuged to separate the sediment (heavy liquids plus solids) from the liquids. After the reactor surfaces were washed with tetrahydrofuran (THF), the wash was combined with the sediment. The total wash was then extracted with THF in a Soxhlet extractor to recover THF solubles. Subsequently, the THF in the extract was evaporated in a Rotavap and the undissolved material dried to constant weight and its ash content determined. Any water produced during the liquefaction would be evaporated in this step. The Soxhlet extract was combined with the centrifuged liquids and the mixture was then distilled at 125 "C and 8 cmHg to remove most of the unreacted tetralin and its decomposition products, naphthalene and dihydronaphthalene. Volatile coal liquids could also be removed at this point but these losses are small because the total loss for the whole experiment is only 2%. The bottoms product, referred to as Solvent Refined Coal (SRC), was stratified. In order to obtain a uniform sample for liquid-solid chromatography (LSC) the SRC was dissolved in T H F to produce a homogeneous solution. Approximately 5 mL of this solution was evaporated in a Rotavap at 25 OC and 2 mmHg to eliminate THF in order to obtain an accurate sample (1.5 f 0.001 g) for LSC separations. Then the sample for the chromatographic column was redissolved in THF and mixed with 10 g of alumina in a gas washing bottle to produce a material resembling wet sand. The alumina was then dried at 25 "C and 2 mmHg in the wash bottle where the evaporating THF vapors smoothly fluidized the alumina with no visible solids entrainment. The dry alumina with the adsorbed sample was then loaded onto the top of a 25 mm i.d. X 600 mm preparative chromatographic column, which had been slurry-packed with 60 g of alumina in hexanes and equilibrated overnight. The adsorbed SRC was then eluted
DISTILLATE
SATURATES
Figure 5. Comparison of analytical liquid chromatograms of the distillate and the saturates fractions.
with the sequence of solvents used by Schiller and Mathiason (1977). Each eluted sample was concentrated in a Rotavap and the solutions transferred to tared vials where the solvents were evaporated at 25 "C and 2 mmHg and the residues weighed. The uneluted material, which was obtained by difference, is the irreversibly adsorbed, multifunctional and highly polar components of the SRC. Six fractions were produced, five eluted and one adsorbed, as shown in Figure 4. Reproducibility of the separations in duplicate determinations ranges from 2% for the small fractions and up to 5% for the large fractions. The coal products are divided into six classes of compounds, namely, aromatics, ethers, nitrogens, hydroxyls, multifundionals, and unreaded coal. The volatile portions of similar chromatographic fractions were analyzed by Schiller and Mathiason (1977) using high resolution mass spectrometry and it was found that the aromatics fraction contained polyaromatic hydrocarbons and some aromatic ethers; the ethers fraction, benzofurans and naphthofurans; the nitrogens fraction, carbazoles, quinolines, and azapyrenes; and the hydroxyls fraction, phenols, hydroxy aromatics, and hydroxy ethers. The distillate and the saturates fraction from the SRC were analyzed by analytical liquid-liquid chromatography (LLC) using a methanol-water mobile phase, and the chromatograms are compared in Figure 5. It is clear that both these fractions contain only decalin, naphthalene,
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981 353
s
1 0.16
i-
I / I I
3800
IO
20
30
40
50
60
TIME, mln
REACTION TIME, min
Figure 6. Chebyshev fit to the measured reaction temperature-time profile.
Figure 7. Desulfurization of Illinois No. 6 coal.
dihydronaphthalene, and tetralin. Furthermore, it was found that in the blank runs, using tetralin without coal under similar reaction conditions, the same product concentrations were produced within experimental error. Hence no significant amounts of naphthalene and dihydronaphthalene are derived from the coal under the reaction conditions used, and the saturates fraction contains the unrecovered tetralin and its reaction products from the distillation step. Therefore, the saturates fraction from the SRC was combined with the distillate and was not considered in the kinetic schemes. The aromatics fraction was partially soluble in the mobile phase and when analyzed showed the absence of decalin, naphthalene, dihydronaphthalene, and tetralin. The other fractions obtained from the preparative chromatographic separations are insoluble in the mobile phase. Kinetic Modelling and Parameter Estimation The kinetic models are obtained by performing a differential mass balance for each class of compounds present in reactor a t any instant. Thus, the general set of differential equations representing a proposed reaction scheme and the initial conditions are written as
The starting values for the rate parameters, Aj and Ej, were obtained by executing the isothermal version of the program using values for the rate constants that are consistent with those in Table I. The optimal parameter estimates of this program were the maximum likelihood estimates and they are neither unique nor global. Obtaining a satisfactory fit of a kinetic model to the experimental concentration-time profiles by this method does not verify the correctness of a kinetic scheme because good fits can always be obtained if the kinetic scheme is made sufficiently complex so as to introduce additional parameters. The method, however, does allow one to reduce the number of possibilities because some kinetic schemes may be eliminated because of poor fits. Once a satisfactory fit has been obtained additional experimental information is required before the kinetic scheme can be accepted. Also, parameter estimation is useful in planning experiments. Desulfurization. The coal used in this work is a very reactive Illinois coal with 3.7 mass % sulfur and 12.5% ash content. In order to estimate the extent of desulfurization, the mass fraction of sulfur removed as H2Swas calculated on the basis of total sulfur in the coal. With such high ash content any iron oxides in the coal might be converted to sulfides, and the pyrites into pyrrhotite and hydrogen sulfide. Some sulfur may be present as organic compounds in the THF solubles, but this was not determined. It was found that up to 18% of the total sulfur is removed as H a during the experiment with large desulfurization occurring at higher temperatures and extended times as shown in Figure 7. These results are consistent with those reported by Longanbach et al. (1979) for the liquefaction of a West Kentucky 9/14 coal using Wilsonville recycle solvent under similar conditions of temperatures and pressures except that the reaction times were less than 5 min and solvent/coal ratios less than 3. Coal Conversion Kinetics. Coal conversion is based on the fraction of liquefied coal that is insoluble in tetrahydrofuran. The coal converted on the dry and ash-free basis, defined as
Xi(0) = XIJi The rate, Rj, depends on the component concentrations, X i , and temperature. The kinetic models developed here are restricted to systems of fmt-order reactions, where the temperature dependence for the rate constant is given by the classical Arrhenius expression k j = Aj exp(-Ej/(RT)) (2) The objective then is to estimate the rate parameters, Aj and E,, for proposed kinetic models by using nonlinear regression techniques (Bard, 1974). The estimation procedure consists of fitting the solutions of eq 1to the concentration measurements made at different reaction times and temperatures since the experiments are nonisothermal. The temperature variation during an experiment (Figure 6) is fitted by a 10-term series expansion in Chebyshev polynomials (Esch, 1974) with the Chebyshev coefficients determined by unweighted least-squares fitting to the reaction temperature-time profile. Hamming’s modified predictor-corrector method is used to numerically integrate the set of simultaneous linear differential equations given by eq 1. The nonlinear estimation algorithm (Meeter, 1965) is based on Marquardt’s method (1963), which combines the best features of the Gauss-Newton and steepest descent methods.
x =
WC - WR WC
(3)
was reproducible within 4% absolute error. A first-order irreversible reaction scheme for coal dissolution is assumed in accordance with earlier investigations (Han et al., 1978) and the kinetic model is represented as dx - = -k(X, - X ) (4) dt x(0) = 0
354
Ind. Eng. Chem. Process
Des. Dev., Vol. 20, No. 2,
1981 hydroxyls ( H )
coal
(c)
nitrogens
y
ti."
(N)
2 multifunctionals ( M )
1.. aromatics
(5)
A ethers
(A )
(E 1
d -
dt
I/ REACTION TIME, min
Figure 8. Conversion of Illinois No. 6 coal during liquefaction. Table 111. Kinetic Parameters for Conversion of Illinois No. 6 Coal ~~
temp range,"C 330-390 390-450
limiting conversion, Xe
activation energy, kcal/g-mol
frequency factor, min-*
0.75 0.88
18.2 19.1
1.86 x 105 3.53 x 105
where the limiting conversion of a given coal, xe, obtained at long reaction times, is a function of temperature as well as the donor-solvent type (Cronauer et al., 1978). The numerical solution of the above model is compared with the experimental data in Figure 8. The best-fit rate parameters, determined by the estimation program, are given in Table 111. The value of activation energy for coal dissolution agrees very well with that of 18.7 kcal/g-mol for bituminous coals liquefied in a well-mixed batch reactor (Hanet al., 1978). Coal Liquefaction Kinetics. To obtain kinetic models for coal liquefaction, the liquefied coal products were separated into classes of compounds according to their chemical functionalities, namely, aromatics (A), ethers (E), nitrogen compounds (N), hydroxyl compounds (HI, large multifunctional compounds (M), and unreacted coal (C). Because significant qualitative differences in concentration-time profiles for the multifunctional compounds were observed at high and low temperatures, the entire experimental range was divided into a high temperature range (390-450 "C) and a low-temperature range (330-390 "C). Several models were tested in order to obtain the best-fit to the experimental concentration-time profiles. A. High-Temperature Liquefaction. The experimental concentration-time profile for the multifunctional compounds in the high-temperature range (390-450 " C ) suggests that they me reaction intermediates. Perhaps this is due to the initial thermal breakup of the "coal molecules'' to produce large free radicals, which are then stabilized by abstraction of hydrogen from tetralin. Subsequent bond breaking of these stabilized-multifunctional-large molecules, which are highly polar, produce several smaller and less polar compounds. Accordingly, kinetic models 1 and 2 were tested. Kinetic Model 1. Model 1 consists of a series-parallel reaction scheme of first-order irreversible reactions with the large multifunctional compounds as reaction intermediates (eq 5). The kinetic model corresponding to the kinetic scheme in eq 5 is given by the set of equations in matrix form in eq 6 and eq 7, 8, and 9.
e
'MAlI'AM
aromatics
(A)
ethers
(E)
all the reactions shown in eq 5 are made reversible with the exception of the formation of the multifunctional compounds from coal. The kinetic model corresponding to the kinetic scheme in eq 10 is given by the set of equations in matrix form in eq 11 and eq 12, 13, and 14. 0
0
0 0
kMN
ME MA
0 0 0
-kNM
0 0
0
0
EM 0 0
(12) [XC, XM, XH, XN, XE, XAlO = [I,0, 0, 0, 0, 01 P2 = ( ~ M H+ ~ M + N ME + MA) (13) x , = 0.88 (Table 111) (14) The concentration-time profiles for the proposed models, calculated according to the procedure outlined earlier, are compared with the experimental concentration-time profiles in the high temperature range in Figures 9 and 10. Undoubtedly, model 2 with all but one reaction reversible fits the data best, which is further confirmed by the lower value of the minimum sum of squares of the residuals shown in Table IV. Numerical values of the kinetic parameters for model 2 are given in Table V. Reversible type
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981 355 I.",
1
I Q
6
1
hMULTIFUNCTIONALS
NREACTEDCOAL
REACTION TIME, min
Figure 9. Comparison of kinetic model 1with experimental product concentrations in the high-temperature range.
REACTION TIME, min
Figure 10. Comparison of kinetic model 2 with experimental product concentrations in the high-temperature range.
Table IV. Kinetic Model Discrimination Using Regression Analysis sum of squares of residuals kinetic model
high temp
low temp
Based on Chromatography
f
H
1.
C -M
0.3378
4N
7;
0.61
\
0.1817
0.0346
a
REACTION TIME, min
Based on Solubility /O
4.
0.0696
c - 0
0.0256
1.01
\ P
5.
Figure 11. Comparison of kinetic model 1 with experimental product concentrations-in the low-temperature range.
c - A
\)
Model does not apply. ative rate constants.
0.0359
b
0.0336
b
r
i 0.61 \
Estimation converges to neg-
Table V. Rate Parameters for Kinetic Model 2 high temperature rate parameter CM k MH ~ H M ~ M N ~ N M
ME EM MA AM
A, min-'
8.21 x 2.21 X 2.57 X 6.41 X 1.54 X 1.52 X 4.32 X 1.18 X 4.74 x
105 lo8
lo5 10' lo8 lo8 lo5
lo8 105
E, kcal/ g-mol 19.1 29.1 19.3 29.2 28.8 28.1 17.2 29.3 20.0
low temperature
A, min-'
2.11 x 7.34 x 2.31 X 1.87 x 2.33 X 1.16 X 1.25 x 3.92 x 1.94 x
lo6 105 lo6 105 lo6 lo6 107
105
lo6
E, kcall gmol 19.5 19.4 19.5 19.1 19.2 19.6 20.1 19.6 19.0
reactions to describe coal liquefaction kinetics have been suggested by Squires (1978). Model 3 shown in Table IV
REACTION TIME, mm
Figure 12. Comparison of kinetic model 2 with experimental product concentrations in the low-temperature range.
will not fit the experimental data because it will not produce the peak shown in Figure 10, which is characteristic of consecutive reactions. The activation energies for all the reaction steps range from 17 to 29 kcal/g-mol. I t is found that all the reverse reactions are kinetically faster than the corresponding forward reactions, which agrees with the high concentration for the large multifunctional compounds obtained experimentally. It is also found that the hydroxyl compounds dominate the other functional compounds, which is consistent with the abundance of hydroxyl groups found in the bituminous coal structure
356
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981
Table VI. Combination of Coal-Derived Products Obtained from Chromatographic Separations t o Oils, Asphaltenes, and Preasphaltenes chromatographic fraction
elution solvent
saturates aromatics ( A ) ethers (E) nitrogens (N)
hexanes toluene chloroform chloroform
hydroxyls (H)
90% THF/10% ethanol
large multifunctionals (M)
uneluted
combination rule
+ 0.5E 0.5E + N + 0.5H 0.5H + M
A
(Whitehurst el al., 1977; Wiser, 1978). B. Low-Temperature Liquefaction. In the low-temperature range (330-390 "C)it is again found that not model 1 but model 2 (Table IV) fits the experimental concentration-time profiles as shown in Figures 11 and 12. It can also be seen in Figure 12 that the multifunctional compounds do not peak as was found in the high-temperature range because in the low-temperature range the net rate of formation of the multifunctional compounds is slower. Thus model 3, where the reaction products were assumed to form directly from the coal, was tested. Kinetic Model 3. In this kinetic scheme all reaction steps are assumed to be first order and irreversible. The multifunctional
(MI hvdroxvls
solubility description
classical definition
oils (0)
hexane solubles
asphaltenes (, A,)
hexane insolubles benzene solubles benzene insolubles THF solubles
preasphaltenes (P)
1.01
0.6-
E i
E 0.42 L
Y 8 0.2HYDROXYLS-,
( H)
10
20
30
50
40
I
60
70
REACTION TIME, mm
Figure 13. Comparison of kinetic model 3 with experimental product concentrations in the low-temperature range. aromatics
(A)
ethers
1.01
(E)
kinetic model corresponding to the above kinetic scheme is given by the set of equations in matrix form in eq 16 and
Cl.1
QUI
0 0 0 0 0 PREAWHALTENES
0
kCN kCE kCA
E].[ti.]
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
-kCN
(l-xe)
(16)
-kCA
eq 17, 18, and 19. (17) (18) 6 3 = ( k C M + k C H -t- k C N + ~ C + E kCA) x, = 0.75 (Table 111) (19) In Figure 13 model 3 is fitted to the experimental concentration-time data. A slightly better fit of the data, however, is obtained with model 2 as can be seen by the lower value of the s u m of squares of the residuals, possibly due to the greater number of parameters in model 2 as compared to model 3. The fact that model 2 fits both the experimental concentration-time data for both the high and low temperature range, whereas none of the other kinetic models tested do, may be significant. The activation energy for the various steps in model 2 has been found to be approximately 19 kcal/g-mol. Comparison of Kinetic Models Based on Solubility. As it has been discussed earlier, the classical definitions [XC, X M , X H , X N ,
XE, X A l O =
0, 0, 0, 0, 01
0
0
UNREACTEDCOAL
-k C E
XA
7
0
OA
Ib
io
3b
40
510
60
7
REACTION TIME, mm
Figure 14. Comparison of kinetic model 4 with experimental product concentration in the high-temperature range.
of SRC fractions based on their solubilities in various solvents are preasphaltenes, asphaltenes, and oils. Since LSC separations produced five fractions, some way of combining these fractions is required to obtain the fractions based on solubility. Whitehurst el al. (1977) suggested a formula for combining the nine chromatographic fractions obtained in their work into oils, asphaltenes, and asphaltols (pyridine solubles, benzene insolubles) on the basis of chemical functionality, C-H skeletal structures, and molecular weight rangea that might be common among the two different product classifications. A similar combination rule, as given in Table VI, is used in this investigation. Three simplified versions of the kinetic model recently proposed by Cronauer et al. (1978) shown in Table I were fitted to the concentration-time profdes obtained by these rules of combining the LSC fractions. The tested models are given in Table IV. Model 4 fits the data reasonably
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981 357
Table VII. Comparison of Kinetic Parameters for the Formation of Oils, Asphaltenes, and Preasphaltenes reactor technique coal solvent
tetralin
Shalabi et al. (1979).
PREMPHALTENES 7
UNREACTED COAL
0.2 L:ASQH~EST OILS
'0
IO
20
hydrogenated phenanthrene
lo3 10" lo3 10'
E 28.9 8.6 4.3 33.9
batch nonisothermal (330-390 "C) Illinois No. 6 (bituminous)
batchb isothermal (350-400"C) Kentucky No. 9 (bituminous)
tetralin
tetralin
A E A E A 2.10 X l o 8 19.1 1.58 X lo5 40.0 1.21 X 1 O ' O 9.63 X 10 19.4 3.61 X lo5 30.0 8.34 X lo6 4.94 19.2 1.20 X lo6 29.0 2.25 X lo6 2.48 X lo9
29.0 1.14 X lo7 16.0 1.42 X l o 3 25.6 1.53 X lo7
Cronauer et al. (1978).
0
hydrogenated anthracene oil
A E A 1.71 X lo5 14.1 3.11 X 1.44 X lo5 15.6 1.12 X 1.49 X l o 9 13.8 2.81 X 2.05 X 10' 12.8 9.66 X
E 19.8 18.9 29.3 29.2
reaction coal + oils coal ---f asphaltenes coal ---f preasphaltenes preasphaltenes + asphaltenes asphaltenes + oils a
CSTRa isothermal (400-470"C) Belle Ayr (subbituminous)
batch nonisothermal (390-450"C) Illinois No. 6 (bituminous)
30
0
0
7 40
REACTION TIME,
,
9
50
60
E , activation energy, kcal/g-mol;A, frequency factor, min-I.
1
UNREACTEDCOAL
70
mm
Figure 15. Comparison of kinetic model 4 with experimental product concentrations in the low-temperature range.
well in the low-temperature range but not as well in the high temperature range as shown in Figures 14 and 15. Models 5 and 6 were found to fit the data in the hightemperature range but yielded negative values for some of the rate constants. By analogy to model 2, model 7 with reversible steps is proposed and is found to fit the data very well in both high- and low-temperature ranges as shown in Figures 16 and 17. Unfortunately, no kinetic data exist in the literature for Illinois No. 6 coal based on solubility measurements to make a strictly valid comparison. It is of interest, however, to make the best comparison that can be made at this time. Thus, the kinetic parameters obtained by Cronauer et al. (1978) and Shalabi et al. (1979) are compared in Table VI1 using other bituminous coals. Model 6 (Table IV) is essentially identical with the kinetic model proposed by Shalabi et al. (1979) shown in Table I with the exception of the reaction where oils are formed from the preasphaltenes. Most of the kinetic parameters differ substantially from the values calculated here. This disagreement may be caused by the differences in the coalsolvent systems or the method of combining the LSC fractions to obtain the oils, asphaltenes and preasphaltenes. Conclusions It has been found that nonisothermal experiments with Illinois No. 6 coal liquefied in tetralin as the donor solvent will yield reliable kinetic data for evaluation of kinetic models. Liquid-solid chromatography has also been established as an adequate separation method to model donor-solvent coal liquefaction kinetics. The fact that the rate data, and in particular, that of the multifunctional
REACTION TIME, mm
Figure 16. Comparison of kinetic model 7 with experimental product concentrations in the high-temperature range.
R
cr1.-
el01
