MECHANISM OF THE HYDROGEN-TRANSFER PROCESS T O COAL AND COAL EXTRACT GEORGE P. CURRAN,
R O B E R T T. S T R U C K , A N D
EVERETT GORIN
Research Division, Consolidation Coal Co., Library, Pa.
The kinetics of coal extraction and of the transfer of hydrogen from Tetralin to bituminous coal and a shallow extract derived from coal were examined. Other active hydrogen-donor solvents gave about the same amount of hydrogen transfer a t equal time and temperature conditions. The depth of extraction was found to be primarily a function of the amount of hydrogen transferred and relatively independent of the solvent composition employed. The hydrogen-transfer results were interpreted in terms of a free radical mechanism wherein the rate-determining step is the rupture of covalent bonds. The experimental data could b e adequately represented by a model where bonds of two different strengths are involved. The dependence of hydrogen-transfer rate on Tetralin concentration is complex. A semi-empirical correlation adequately represents the data.
vast literature exists on the solvent extraction of coal (7, 4 ) , dealing for the most part with the effect of variation of chemical composition of the solvent and the nature of the coal on results achieved. Much less attention has been accorded to kinetic measurements of the dissolution process. Oele ( 5 ) and coworkers, however, studied in some detail the kinetics of coal dissolution in anthracene and 2naphthol solvents. No transfer of hydrogen is involved in the process and the term “extractive disintegration” was coined to describe it. A process has been under development for the past several years in the laboratories of the Consolidation Coal Co. for the production of synthetic gasoline from coal. The first step is the extraction of coal with a process-derived solvent which possesses hydrogen-donor properties. The process thus involves transfer of hydrogen from the solvent to coal, as studied by Pott and Broche (6) and others. Some data (3)were found relevant to the kinetics of this specific type of extraction process as applied to brown coal, but few rate data appear to be available for bituminous coals. The data reported were all obtained in a batch system with chemically pure solvents in order to simplify interpretation of the measurements. Kinetic data were also obtained in a continuous system, but this will be reported later. The reaction model presented in this paper gives a n excellent fit to the experimental data.
A
RATHER
temperature control was effected by adding a small amount of cold sand to the fluidized bed. At the end of the desired residence time, the sand bath furnace was lowered and a container of cold water, raised to surround the still shaking autoclave, cooled the autoclave to essentially room temperature in less than 30 seconds. The reaction times reported are taken as the total time from immersion to quenching less 1 minute for effective heatup time. I t is estimated from time-temperature histories that the times are correct within zt0.5 minute. The uncertainty thus has relatively little effect on the kinetic interpretation. All coal runs were made with a Pittsburgh Seam coal from the Ireland Mine in Northern West Virginia. The coal was crushed and screened to give two fractions-Le., 100 X 200 mesh and 28 X 48 mesh with the analyses shown in Table I, A. Initial runs showed identical results with the two fractions, so that all further work was carried out with the 28 X 48 mesh coal. The ranges of conditions studied were temperatures from 615’ to 825’ F. and residence times from 2.5 minutes to 2 hours. Each run was conducted with 2 grams of coal, while the total amount of solvent was varied between 4 and 8 ml. in individual runs. No attempt was made to remove air from the autoclave before sealing. The oxygen content of the air above the
Properties of Feed Materials Wt. % Dry Basis 700 X 200mesh 24 X 48mesh A. Analysis of Feed Coal
Table 1.
Experimental
T h e experiments were conducted in a microautoclave constructed of Type 316 stainless steel, 0.625-inch i.d. and 6 inches long with an internal volume of 30 ml. The autoclave was shaken vertically at 2300 cycles per minute by an electric motor whose rotary motion was converted to reciprocating motion by means of a crack and cross-head linkage. The shaker was supported above a large fluidized sand bed which could be moved up and down rapidly by means of pulleys to immerse or exposure the entire autoclave. Experiments with a Lucite model using coal-water mixtures at room temperature showed that the charge was well mixed and was distributed uniformly over the entire inner surface of the autoclave when the filling ratio was one third or less. First the sand bed was preheated to about 40’ F. above the desired run temperature, and then the autoclave was rapidly immersed in the sand bed. Blank experiments showed that the inner temperature of the autoclave rose to within 3’ F. of the desired temperature 2.5 minutes after immersion. Final 166
l&EC PROCESS DESIGN A N D DEVELOPMENT
H C N 0 (diff.) Organic S FeS2 Other ash Total ash
4.86 71.77 1.25 7.12 2.48 3.70 8.82 12.52 B. Analysis of Feed Extract
H C N 0 S
Heavy oil Asphaltenes Benzene-insolubles
4.66 70.21 1.48 6.51 2.25 6.25 8.64 14.89 6.52 83.39 1.23 6.28 2.58 22.6 47.0 30.4
charge varied between 0.3 and 0.7 weight % of the coal, depending on the filling ratio of the autoclave. T h e depth of extraction was measured by the amount of filter cake obtained which was insoluble in xylenol a t 220' F. The coal conversion was calculated from the weight of dry filter cake and the weight of coal charged as follows:
yocoal conversion (MAF)
Table 11.
Hydrogen Donor Extraction of Ireland Mine Coal at Severe Conditions Tetralin/coal, wt. ratio 1. 5 3.0 1 Time-temperature his-
torv. min. 715-825' F. Hold 825" F. 825-715" F.
=
100 (g. M F coal charged - g. dry cake) g. MAF coal charged Experiments were also conducted with a "shallow" coal extract (Table I, B). The extract was prepared by extraction of Ireland Mine coal in a 1-gallon stirred autoclave at 650' F. for 1 hour using Decalin as solvent. The depth of extraction was 23yo of the MAF coal. I n the bulk of the runs, Tetralin was the model for the hydrogen donor liquid, although other pure hydrogen donors were used. T h e amount of hydrogen transferred was determined by analysis of the recovered solvent. The principal product resulting from the transfer of hydrogen to the coal is naphthalene, with appreciable quantities of indane and C4 benzenes. Small quantities of cis- and trans-Decalin are frequently present. The weight per cent hydrogen in the recovered solvent was determined from the recovered solvent analysis, assuming that no materials boiling below indane or above naphthalene were formed from the Tetralin. The weight of hydrogen transferred was then determined from the difference in weight of hydrogen in the feed Tetralin and recovered solvent. The recovered solvent was analyzed by first vacuum-distilling the product filtrate to recover a cut with a nominal boiling range of 160' to 230' C. a t 1 atm. This fraction was extracted with 10% aqueous caustic to remove the tar acids and then analyzed with a Perkin-Elmer Model 154C gas chromatograph. Separate aliquot portions are analyzed on two different columns : COLUMN 1 (polar), 20y0 diisodecylphthalate on 60 X 80 M Chromosorb in '/,-inch 0.d. X 8-foot stainless steel column a t 175' C. COLUMN 2 (nonpolar), 20% DC-silicone oil on 60 X 80 M Chromosorb in same type column as above, maintained a t 160' C. T h e polar column is used to effect adequate resolution between the naphthalene and Tetralin peaks, which cannot be achieved with the nonpolar column. Similar gas chromatography analyses were made to define the amount of hydrogen transferred from other pure solvents. Some experiments were also conducted in either a 300-ml. Aminco rocking autoclave or a 1-liter Parr bomb, when the main purpose was not kinetic measurements but to obtain a more complete analysis of the reaction products. Results and Discussion
T h e main purpose of the kinetic equations developed here was to obtain a correlation which would be useful in predicting and interpreting results from continuous bench- and pilotscale units. T h e method developed here has proved useful for this purpose, as \vi11 be shown in future publications. T h e equations were derived on the basis of a theoretical model as developed below. I t would be presumptuous to assume in a system as complex as the one dealt with here that the model represents the true mechanism. Maximum Amount of Hydrogen Transfer. T h e emphasis in the discussion of the kinetic results is on measurement of the amount of hydrogen transferred, since this is identifiable with a precise chemical process. Experimental work indicated that there is a "practical" maximum quantity of hydrogen that can be transferred to either coal or extract in a purely thermal process. Some additional hydrogen transfer may be obtained by further treatment a t more severe conditions, but little in the way of new useful products is obtainedLe., very little asphalt is converted to liquid products. These
10 60 11
Yields, wt. o/o MAF coal HZ H& COS CO
+
0.3 1.6 5.0 4.5
13 60 4
715-750" F. Hold 750' F. 750-715" F.
4 60 3
0.3 1.0 3.3 3.3
0.1 0.9 3.5 1.3
5 ' 4 }18.4 16.2 26.4 29.0 18.4 35.0 16.6 4.8 8.0 .~ 7 . 4 1 0 2 . 4 102.5 2.4 2.5
t14.2
+
HzO C1-Ca hydrocarbons Ca X 400' F. dis-
tillate
400" F. X 750' F.
distillate Heavy oila Asphaltenesb Benzene-insolublesc Cresol-insolubles Total yo H transferred e
20.9 42.1 10.9 7.5 101.4 1.4
~
a Soluble in cyclohexane. Soluble in benzene, insoluble in cyclohexane. Soluble in cresol, insoluble in benzene.
maximum limits for the coal and extract used in this work are approximately 2.6 and 2.2 weight % of hydrogen transferred, respectively. The product distribution resulting from extraction of coal a t conditions approaching maximum hydrogen transfer is given in Table 11. Basic Nature of Reaction. T h e hydrogen-transfer reaction is first of all a purely thermal process-Le., attempts to accelerate the reaction with contact catalysts of the hydrofining type (cobalt molybdate on alumina) or with cracking catalysts (silica-alumina) were unsuccessful. The hydrogen-transfer reaction between Tetralin and extract may be treated as a homogeneous process. I t is our opinion that coal extracts prepared by thermal dissolution of coal in hydrogen-transferring solvents are true solutions. No evidence that they are colloidal mixtures is available. The extracts, for example: may be divided into three molecular weight fractions-Le., cyclohexane-soluble oil, benzene-soluble and cyclohexane-insoluble asphaltenes, and a benzene-insoluble, cresol-soluble fraction. The average molecular weights of these fractions can be precisely and reproducibly characterized by cryoscopic methods and are approximately 400, 750, and 1500 for the three respective fractions. These are much too low for colloidal particles. The coal extract solutions may be filtered without deposition of extract through a Millipore filter with pore openings 100 A. in diameter. Coal itself is, of course, not in solution during the initial stages of the extraction. One is dealing here with a heterogeneous process. It seems likely, however. that the solvent rapidly diffuses into the coal, dissolving in the coal "gel," and thus the initial reaction takes place within the coal particle. Mass transfer and diffusional effects, however, d o not seem to be important, since the rate is independent of particle size. I t may be natural therefore to regard the process as a simple second-order reaction involving coal or extract molecules and a hydrogen-donor molecule. There are two objections to this point of view. First, if this were so, the process should follow a simple second-order rate law : VOL. 6
NO. 2
APRIL
1967
167
Table IV.
Comparison of Hydrogen Donor Activity of Various Organic Substances
(Ireland Mine coal used in all cases)
Solvent A. Perhydrophenanthrene Decalin Indane o-Cyclohexylphenol Tetralin Dihydrophenanthrene B. Tetralin Cyclohexanol C. 50% Tetralin-50% Decalin 50% 2-propanol-50yo Decalin a Excludes heat-up time.
MINUTES
Figure 1. time
dt
- K(E)(HT - H)(Hl' - H ) 100
T h e terms used in Equation 1 are defined below. tegrated form of Equation 1 is:
(1) T h e in-
where H T is assigned the value of 2.6 in the case of Ireland Mine coal.
Hi'
=
(400/132) ( D I M )
The ratio 400/132 is the hydrogen transfer capacity in grams per 100 grams of pure Tetralin. The results of a test of the validity of the second-order mechanism are given in Table 111. Since the rate constant varies by a factor of 5 over the limited range of conditions tested, it is justifiable to discard the second-order mechanism. Another clue pointing in the same direction is the behavior of different solvents. A true second-order reaction would naturally involve very specific stereochemical effects. Thus, only solvents that are very similar in structure would be expected to function at all, and the rate of hydrogen transfer would depend markedly on the chemical structure of the donor. Data summarized in Table IV show that these properties of a second-order process are not found. A wide diversity of solvents function as hydrogen donors as long as they contain mobile hydrogen bonds. T h e rate of hydrogen transfer with the more active donors appears to be about the same and independent of their structure. It would seem, therefore, that the rate of thermal decomposition of the coal determines the extent of hydrogen transfer once a sufficiently reactive donor is used.
Table 111.
Tetral~n in Soloent
168
O
Hydrogen Transferred to M A F Coal,
% 0.21
716 734 716 716 716 716
60
620
60
0.22 0.42
20
1.36
0.28 0.33 0.98 1.12 1.21
620 800 800
1.14
First-order rate plots of cool conversion vs.
d_H_
Val. %
Temp., F.
Residencea Time, Min.
cT - c =
n
(3)
C,e-Ki' r=l
where C1:is the weight per cent of the coal which decomposes with the characteristic rate constant Ki.
Second-Order Rate Constants in Hydrogen Transfer to Coal at 730' F.
M, G. MAF Coal,
cc.
These observations point to the possibility of a free radical mechanism. The coal is dissociated by thermal decomposition into free radicals which are stabilized by capture of a hydrogen atom from a donor molecule. The hydrogensaturated radicals are now stable molecules, sufficiently low in molecular weight to be soluble in cresol. Thus, a direct relationship exists between solvent extraction depth and hydrogen transfer. Thus, if donor of sufficient activity or concentration is used, one in effect is studying the rate of thermal decomposition of coal or extract into free radicals. Coal Conversion Kinetics. I t is therefore logical to test the above proposition by treating the hydrogen-donor extraction process as a first-order reaction.\ The above treatment should apply where such a high ratio of donor to coal is used that change in concentration of donor during the course of the reaction may be neglected. Such a series of kinetic runs was made using a ratio of 4 cc. of Tetralin per gram of coal. Figure 1 shows a first-order rate plot for these runs for coal conversion where [ l / ( l - a)] is plotted against time. The points of longer residence times fall reasonably well on a straight line, but the short-time points fall well below the line. The treatment of coal conversion per se as a first-order process assumes that only a single thermal decomposition process occurs with a characteristic rate constant. As a matter of fact, it appears more reasonable to assume that two or more decomposition processes occur simultaneously with different rate constants. The rate of coal pyrolysis on this model is given by the expression:
t,
100
0.178
(DIM) 4.5
25
0.178
1.12
Hi '
HT
H
13.6
2.6
0.48 0.71 1.22 0.16 0.32 0.47
3.39
I & E C PROCESS D E S I G N A N D D E V E L O P M E N T
2.6
Min. 10
20 60 5
20 60
K , Min.-'
Cc. G. Atom-'
0.85 0.67 0.36
2.12 1.12 0.56
Assuming that only two first-order reactions occur in parallel, Equation 3 can be written as
CT
-C
= y
+ (1 - y) CTe-KzL
Table V.
First-Order Constants,
(34
An approximate solution of Equation 3a was made with a n analog computer, giving an initial set of values for rate constants K1 and Kz and parameter y. y represents the fraction of the coal which decomposes by the faster rate. Arrhenius plots of the reaction rate constants then gave an initial set of values for the five parameters which describe the system-Le., A I , El, 7, A z , and E B ,
Figure 2.
Rate plots of coal conversion
Min. -1
Temp., Process
OF.
616 670 730 670 700 730 800 668 700 727 800
H transfer to coal
B.
y
Ki
Kz
Summary of Rate Constants
A.
Coal conversion
H transfer to extract
T h e final values of the constants were obtained with a digital computer using a search rechnique. An initial set of values of A I , El, and y as indicated by the analog computer was employed and the values of K 2 at the three temperatures were calculated for each of the experimental points. The values of A2 and E2 were determined by a least squares analysis of a standard Arrhenius plot. Then, the coal conversion was calculated from Equation 3a for each of the experimental points and the deviations were determined. The procedure was iterated with an adjusted set of values for A1, E l , and y until a minimum value of the standard deviation was obtained. The above procedure did not lead to a clearly unique solution for the constarits, since several combinations gave an almost equally good fit. Where a choice had to be made between equally good solutions, the criterion that E l , the activation energy of the fast process, should be less than the corresponding value .E2 for the slow process was used. The final equations for rate constants for coal conversion as well as their numerical values are given in Table V, B and A, respectively. T h e quality of the fit to the experimental points is good, as illustrated in Figure 2.
Rate Constants
0.34 0.12
0.262 0.898 3.02 0.0474 0.109
0.239
0.13
1.30 0.0558 0.121 0.226 1.06
0.00514 0.0247 0.116 0,00132 0.00340 0.00833 0.0571 0.00077 0.00204 0.00442 0.0304
Equations for Rate Constants
COALCONVERSION =
2.5
x
30,000 -_ _
101oe
RT 38,250 -_ _ RT
Kz = 5 . 1 X 10"e H TRANSFER TO COAL K1
40,000 -_ _ RT
= 4.0 X 1012e
- 45,500 K D = 9.1 X 1012e R T H TRANSFER TO EXTRACT ~1
= 9.0
x
- 35,000 _1010e RT
43,600 -_ _ Kz = 1.25 X 1012e RT
Relationship between Coal Conversion a n d Hydrogen Transfer. T h e kinetic treatment of the coal conversion process is far from rigorous, since it is not possible to identify it with a precise chemical change. The kinetic treatment presupposes that in essence two types of coal co-exist, refractory and labile material. Hydrogen transfer can on the other hand be identified with a more definite chemical process. O n the basis of the free radical picture outlined above, each gram mole of hydrogen transferred can be identified with the dissociation of a specific chemical bond into two free radicals. T h e relationship between coal conversion and hydrogen transfer, therefore, might throw some light on the mechanism of the coal extraction process itself. T h e graphical relationship between the two quantities at 730' and 670' F. is shown in Figures 3 and 4, respectively. In this work the solvent consisted of pure 'Tetralin, or Tetralin diluted with various quantities of Decalin, paraffin hydrocarbons, or xylenol. Surprisingly enough, the coal conversion appears to be little affected by the composition of the solvent and is determined primarily by the amount of hydrogen transfer. T h e first 50% coal conversion is obtained with very little hydrogen transfer-Le., z
8
J
U
8 c z W
0
6
a
50% TETRALIN 50% MCALIN
W
a.
x 25% TETRALIN 75% DECALIN 75% TETRALIN 25% XYLENOL
+
tkw32Tb
2o0
1
.2
.3 A
.5
.6
.7
.8
.9
PERCENT HYDROGEN TRANSFERRED Figure 3. Coal conversion vs. per cent hydrogen transfer at 730" F.
IJ
z 0
v)
K W
>
"
z
8 a
-f .
8
1 A ~
A I
I-
I I
SOLVENT TETRALIN
! o 50% TETRALIN 50% DECANE
z W
u
K W
I
n
l
x 25% TETRALIN ?5% DECAUN ~
1 +
I
I
75% TETRALIN 2 5 % XYLENOL
/ I l l
I
1
PERCENT HYDROGEN TRANSFERRED Figure 4. Coal conversion vs. per cent hydrogen transfer at 670' F.
Kinetics of Hydrogen Transfer to Coal. T h e hydrogentransfer process was treated kinetically in the same fashion as coal conversion. There is less uncertainty in this case in identifying the process with a definite chemical change. The kinetic treatment can therefore be more rigorously applied. I t is necessary, in the kinetic treatment of the hydrogentransfer process, to assign a value to the maximum amount of hydrogen that can be transferred-i.e., HT. A value of HT = 2.6 was assigned, based on the fact that experimentally one approaches the same level of HT under widely different conditions with 100% Tetralin solvent as cited below: H
Transferred,
2.0 2.5 2.4 a
170
yo
Time, Min.
120 60 94a
Tp., F. 730 800
825
Unit
Kinetics Kinetics Batch autoclave
Total time above 700' F. l&EC PROCESS DESIGN A N D DEVELOPMENT
Again, a model was chosen based on only two types of bonds, one relatively strong and the other relatively weak. Solution for the rate constants was obtained by the method described previously. The derived rate equations and the corresponding rate constants are given in Table V. The quality of the fit to the experimental data is illustrated in Figure 5 . T h e fast hydrogen-transfer rate is the same order of magnitude as the slow conversion rate, though it is greater by a factor of 2. The above identification is reasonable, since the fast coalconversion rate is associated with little or no hydrogen transfer. The fast hydrogen-transfer rate apparently takes over in the coal conversion range of 50 to 90%, which corresponds to the slow coal conversion rate. The slow hydrogen-transfer rate corresponds in all likelihood to continued transfer of hydrogen to coal extract. Kinetics of Hydrogen Transfer to Extract. The extract used in this work was a shallow extract-i.e, obtained with Decalin solvent at 650' F. with a total coal conversion of only 23%. T h e kinetic treatment followed the pattern set forth for the coal work. I n this case a value of y = 0.13 gave the best fit. The derived rate equations and constants are given again in Table V. The quality of the fit to the experimental data is shown in Figure 6. I t might be expected that the slow rate in the case of Hz transfer to coal would correspond to the fast rate in the case of extract. This is not the case, since the fast extract rate is an order of magnitude greater than the slow coal rate. The lack of agreement is partly due to the fact that shallow rather than deep extract was used where the comparison would be more valid. Data obtained with deep extracts were insufficient for calculation of rate constants but showed that the rate is indeed slower. Entropy of Activation in Coal Conversion and Hydrogen Transfer to Coal and Extract. The activation energies and entropies of activation are summarized in Table VI. The entropy of activation was estimated by use of the theory of absolute reaction rates ( 2 ) . The exact quantitative value of
Table VI. Energies and Entropies of Activation in Coal Conversion and Hydrogen-Transfer Rate Processes
Process
Coal conversion Fast rate Slow rate
Hydrogen transfer to coal Fast rate Slow rate Hydrogen transfer to extract Fast rate Slow rate
Energy of Activation, Kcal. / Mole Mean Value
Entrojy of Activation, E.U., Mean Value
30.0
38.25
-20.7 -14.7
40.0 45.49
-10.6 - 9.0
35.0
-18.1 -13.0
43.60
TIME, Figure 5. H
Figure 6.
MINUTES
Rate plots of hydrogen transfer to coal
- HT
+
= y HTe-K1‘ (1 HT 2.62 y = 0.12
- y)e-K2c
Rate plots of hydrogen transfer to extract
H
- HT
a
+ - y)
yHTe-KI1 (1 H T = 2.20 y = 0.13
also treated in the same manner. T h e values of the energies and entropies of activation for the initial fast rates are probably of little significance, since the accuracy of determination of the characteristic rate constants is not high. T h e accuracy for determination of the values for the slow rates is much better, since this corresponds to 85 to 90% of the total observed reaction. The activation energy of the slow, rate is about the same for both coal and extract. The energy of activation in both cases is somewhat low, however, ca. 45 kcal. per mole for most normal covalent bonds. The heat of dissociation of carboncarbon bonds, for example, varies for most simple molecules between 6 5 and 85 kcal. per mole. The low value is indicative of a highly resonating structure in the free radicals produced by bond rupture which stabilizes them. This is analogous to the well-known stable free radicals of organic chemistry such as triphenyl methyl which are stabilized by the same mechanism. The entropies of activation are again negative. I n the case of simple known unimolecular reactions where dissociation to free radicals is involved, a positive entropy of activation is usually observed. T h e significance of the negative values is not clear. HzTransfer Rate as Affected by Tetralin Concentration and Tetralin-MAF Coal Ratio. The above kinetic treatment applies only to the case where high solvent-to-coal and/or extract ratios are employed and where high Tetralin concentrations are used. T h e hydrogen-transfer rate falls off when either of the above quantities is reduced, although it is much more sensitive to a reduction in the latter than to the former quantity. A wholly consistent kinetic model must predict the effect of variation of the above parameters. Experimentally only sufficient data are available in the case of coal for testing the applicability of kinetic models. Even here most of the data revolve about variation only of the Tetralin concentration. The testing of kinetic models in the case of coal is difficult, since the system is not truly a homogeneous one. A number of models were considered to represent the kinetics, but the one given below was chosen because of simplicity. This model considers that the extract or MAF coal has built-in hydrogen-donor structures which compete for the radicals (R) produced in thermal dissociation. Thus, the kinetic model consists of the following sequence of reactions:
e-K2‘
AS* is of little significance but its sign and order of magnitude are significant relative to the structure of the activated complex and hence the mechanism of the process. The low activation energy for the high coal conversion rate of 30 kcal. per mole is well below that of most “reasonable” chemical bond strengths. Thus, it appears doubtful that the reaction involves rupture of a conventional covalent bond. This is consistent with the fact that the fast rate (approximately ‘/a of the total conversion) occurs with no significant hydrogen transfer. The energy of activation may perhaps be attributed to the breaking of nonvalence bonds-Le., hydrogen bonds and the like. The high negative value of the entropy of activation indicates a much more ordered structure in the activated complex than in the original coal molecule. This may be rationalized as meaning that the surrounding coal molecules must be oriented in a very specific manner to permit escape of the small coal molecule into the surrounding solvent medium. The hydrogen-transfer rates for both coal and extract were
Rate Constant
Reaction
M+2R R D-. RH DR M-. R H MR -I- D-* R H (D R M--+ R H (M
++ +
-++
ko
kl
++ --H2) 132)
kl kt
kr
where I)- and M- correspond to radicals produced by abstraction of a hydrogen atom from Tetralin (tetralyl radical) and extract, respectively. (D-H2) and (M-H2) are, respectively, dihydronaphthalene and the corresponding dehydrogenated structure derived from the extract. Dihydronaphthalene is undoubtedly a much more reactive hydrogen donor than naphthalene itself, and rapidly undergoes similar reactions wherein it is converted to naphthalene. I t thus is present in a very small steady-state concentration such that its activity for hydrogen transfer is equivalent to that of Tetralin. From the kinetic point of view it is appropriate to disregard its presence. Experimentally, we have at present no definite evidence for or against the presence of dihydronaphthalene. Dihydronaphthalene boils between naphthaVOL. 6
NO. 2 A P R I L 1 9 6 7
171
lene and Tetralin and thus is likely difficult to separate even on a gas chromatograph. I t would, however, be of interest to prove definitely the presence or absence of this compound as an intermediate. T h e above mechanism can be used to derive an expression between the amount of hydrogen transferred from Tetralin, H i , and the total amount of hydrogen transferred by all mechanisms, H , as shown below : dHi dH (HI'
-
where k = ( k 2 / 2 k l ) . equation
(Hi' - H i ) Hi) k{HEo - ( H
+
T h e equations derived above must be regarded as semiempirical, but are useful for engineering purposes, particularly for estimating the effect of Tetralin concentration at constant total solvent-to-coal ratio. They cannot be safely used to estimate hydrogen-transfer rates at lower solvent-to-coal ratios. Equation 9 predicts that the amount of hydrogen transfer obtained should be the same at the same Tetralin-extract ratio irrespective of the Tetralin concentration in the initial solvent. The experimental facts are not in accord with this,
(7)
- Hi)}
Integration of Equation 9 leads to the H
- Hi
The left-hand side of Equation 8 may be expanded by the binomial theorem. As an approximation only the first term of the expansion is used, since H,/Hl' < 1. T h e result, after rearranging and collecting terms and noting that HL' = (400,' 132) ( D I M ) ,is Hi
H C = kHE'
where
(9)
(E)
Experimental data obtained at 670' and 730' F. using 4 cc. of solvent per gram of coal were used to test Equation 9. The test consists of plotting 1/H1 us. M / D . Each series of points at constant temperature and residence time should lie on a straight line. The slope ( C / H ) divided by the intercept ( 1 / H ) should be a constant-Le., C for each residence time series at constant temperature. The tests for validity of Equation 9 at 670' and 730' F. are shown in Figures 7 and 8. Constant values of C = 3.7 and 4.2 were used for all the 670' and 730' F. data, respectively. The fit to the experimental data is good, particularly for the 21.5- and 61.5-minute points.
Figure 7. Hydrogen-transfer rate at 670' fected by MAF coal-Tetralin ratio
F.
Cc. total solvent per gram MF coal = 4.0 1 1 = jj [ l .+- C(M/D)]
-
Hi
c
= 3.7
as of-
Cc. total solvent per gram MF coal = 4.0
1 K
1 = j-, [l = C ( M / D ) ]
C = 4.2
172
l&EC PROCESS DESIGN A N D DEVELOPMENT
Figure 9.
800" F.
Thermal decomposition of Tetralin at
since the amount of hydrogen transfer increases at constant ( D I M ) with increasing Tetralin concentration. A few data points, for example, are available a t 730’ F. and a lower solvent-to-coal ratio-Le., 2 cc. per gram. T h e experimentally determined amount of hydrogen transfer as compared with that estimated by means of Equation 9 is shown below:
Res. Time, Min.
Solvent/ Coal, Cc. /G.
Vol. yo Tetraltn
6.5 21.5
2 2
100
100
Hydrogen Transfer, H PreObdicted served 0.26 0.33
0.68
Rate Constant
M+2R
R - + ~ ~ R H + D R +D-+RH (D -H2)
+
I
0.52
Evolution of Free Hydrogen. Under severe operating conditions-Le., high temperatures and/or long residence times-a considerable fraction of the hydrogen released from the Tetralin appears as free hydrogen (cf. Table 11). Under mild operating conditions little free hydrogen is produced. These facts are also in accord with the general mechanism outlined above. The mechanism at high Tetralin concentrations where the recombination of radicals and hydrogendonor properties of the acceptor may be neglected may be written as follows: Reaction
sarily correspond to actual compounds produced, since these were not definitely identified. The peaks in the gas chromatograph labeled as Ca benzenes and indanes have, however, the same retention time as those of the corresponding pure compounds. Thus, unsaturated compounds such as a-ethylvinylbenzene and o-methylallyl are not excluded as possible major decomposition products. Increased yields of these decomposition products would be expected as a result of thermal decomposition of the tetralyl radical produced from the coal radicals as follows:
ko ki ka ks ks
Thus, molecular hydrogen may be formed by thermal decomposition of the tetralyl radical by way of Reactions 5 and 6. High operating temperatures will favor this reaction, since Reaction 5 will have a higher activation energy than Reactions 1 and 2. This is seen from the differential equation corresponding to the above reaction sequence.
Nomenclature
D = g. Tetralin/cc. reaction mixture M = g. extract or MAF coal/cc. reaction mixture Hi = g. atoms of H transferred from TetralinI100 g. extract or MAF coal
H
tract or MAF coal ~~
= g. moles free hydrogen produced/100 g. of extract or
MAF coal = g. atoms of H transferred from extract/100 g. extract =
= =
r
C
CT Q Q
where H 2 corresponds to moles of free hydrogen produced and H I corresponds to gram atoms of hydrogen actually transferred to the extract. Equation 10 likewise predicts that the relative proportion of free hydrogen will increase a t constant temperature with increasing residence time, because of decrease in rate of production of free radicals from the coal or extract. This is also in accord with other experimental data which will be presented in a future publication. Sensitized Thermal Decomposition of Tetralin. It has been observed for this work that decomposition of Tetralin is enhanced when it is used as a hydrogen donor. This is illustrated by Figure 9, where the decomposition products as a function of time a t 800’ F. in absence of extract are plotted. Comparison is made with yields of C 4 benzenes and indane produced when extract is present. T h e Tetralin decomposition products other than naphthalene are found largely in two peaks in the gas chromatograph which are labeled arbitrarily as C 4 benzenes and indane, respectively. These do not neces-
= g. atoms of H transferred from all sources/100 g. ex-
Q
E1 Ai E2 A2
= = = = = = = = = =
or MAF coal total g. atoms H transferred at infinite time/100 g. extract or MAF coal due to rupture of all bonds hydrogen-transfer capacity of Tetralin, g. atoms/100 g. extract or MAF coal = (400/132) ( D I M ) hydrogen-transfer capacity of extract, g. atoms/100 g. extract or MAF coal fraction of “weak” bonds in coal or extract coal conversion in per cent of MAF coal coal conversion in per cent of MAF coal achieved a t infinite time fraction of ultimate conversion C/CTfor coal conversion H / H T for hydrogen transfer activation energy for fast rate, cal. per mole pre-exponential factor for fast rates, sec-1 activation energy for slow rate, cal. per mole pre-exponential factor for slow rate, sec.-l
literature Cited
(1) Dryden, I. G. C., “Chemistry of Coal Utilization,” H. H.
Lowry, Ed., Supplementary Vol., p. 237, M’iley, New York, 1962. (2) Glasstone, Samuel, Laidler, A. J., Eyring, Henry, “Theory of Rate Processes,” McGraw-Hill, New York, 1941, (3) Jostes, F., Siebert, K., Oel Kohle, Erdoel, Teer 14,777-83 (1938). (4) Kiebler, M. W., “Chemistry of Coal Utilization,” H. H. Lowry, Ed., Vol. I, p. 715, Wiley, New York, 1945. (5) Oele, H. P., Waterman, H. I., Goedkoop, M. L., Van Krevelen,D. W., Fuel30, 169 (1951). (6) Pott, A., Broche, H., Zbid., 13, 91-5, 125-8, 154-7 (1934). RECEIVED for review February 28, 1966 ACCEPTED December 12, 1966 Division of Fuel Chemistry, Symposium on Pyrolysis of Fossil Fuels, 151st Meeting, ACS, Pittsburgh, Pa., March 1966.
VOL. 6
NO. 2
APRIL 1967
173
