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Comparative study of the isothermal pyrolysis kinetic behaviour of some oil shales and coals
Journal of Analytical and Applied Pyrolysis, 14 (1988) 49-71 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
COMPARATIVE STUDY OF THE ISOTHERMAL KINETIC BEHAVIOUR OF SOME OIL SHALES
H. BAR, R. IRAN
and Z. AIZENSHTAT
49
PYROLYSIS AND COALS
*
Casali Institute of Applied Chemistry, The School of Applied Science and Technology, and the Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem, 91904 (Israel) (Received
September
13th, 1987; accepted
May 9th, 1988)
ABSTRACT Data are presented on the kinetics of hydrocarbon evolution from isothermal pyrolysis of four oil shale and two coal samples. Similarities and differences in the pyrolysis kinetics of the six samples are discussed. A fluidized-bed reactor coupled with a flame ionization detector as a monitor was employed. The rate of generation of the volatile hydrocarbons at each isothermal temperature can be described as a linear combination of three parallel independent first-order reactions. This model was found to be suitable for the six samples studied despite the large differences in the mineral matrix and the type of the organic matter. The activation energies calculated from the measured half-lifetimes depend on the temperature range; thus the activation energies measured in the 400-450 ’ C range were about 10 kcal/mole lower than those measured in the 450-500 o C range. The kinetics of hydrocarbon evolution from samples previously exposed to 400 o C were slower than the reaction kinetics of the original matter measured at a higher temperature range. This indicates that the kinetics slow down with maturation of the sample (or, as in this case, with a simulated maturation). Coal; fuels; kerogens;
kinetics,
reaction;
oil shale; pyrolysis.
INTRODUCTION
Several above-ground oil shale retorting processes are characterized by rapid heating of the shale, followed by essentially isothermal retorting with subsequent rapid removal of the pyrolysis products. During the last six years several attempts have been made to determine the kinetics under such conditions: Wallman et al. [l] and Richardson et al. [2] employed a fluidized bed reactor with a flame ionization detector (FID) as a monitor, and used curves of ln(l - conversion) versus time for the data interpretation. Both Wallman and Richardson approximated these curves by two straight-line
50
segments, and described the generation rate of the hydrocarbons from some U.S. oil shales by two sequential first-order reactions; the reaction rate constants k, and k, were calculated from the slope of the two straight-line segments. Bar [3] employed a similar apparatus but different data analysis procedure in which k (the observed rate constant) was calculated directly from the FID versus time curves. This improved data analysis procedure indicated that the rate of organic pyrolyzate formation from an Israeli oil shale could be described by two parallel first-order reactions. A careful examination of a new set of data which was obtained from an improved experimental system [4] indicated that a set of two reactions is not sufficient to describe the pyrolytic reaction and that three or more independent first-order reactions are needed. Braun and Bumham [5] re-examined Richardson’s data and concluded that it could be represented either by a single reaction with an effective order of 1.57, or by two parallel first-order reactions. Carter [6] used the same technique and described the kinetics of Kentucky oil shale pyrolysis by two parallel first order reactions. Charlesworth [7] employed a technique in which a preheated fixed bed reactor is attached to a flame ionization detector, and suggested that the rates of hydrocarbon evolution from Australian oil shale could be described by various solid phase mechanisms. Ekstrom and Callaghan [S] followed Braun and Burnham’s approach and described the rates of the total hydrocarbon evolution from some Australian oil shales by two parallel first-order reactions, using an experimental technique similar to that used by Charlesworth. Ten years ago, Nsakala et al. [9] had already suggested that the rate of weight loss of four coal (lignite) samples could be described by two parallel first-order reactions (isothermal 808 o C entrained-flow furnace). Scaroni et al. [lo] described the weight loss of a Texas lignite with the same model using an apparatus similar to that of Nsakala. However, the experimental technique which was used by N&ala and Scaroni was very different from that described in refs. 1-8. One objective of the present study was to find out whether the two, or three parallel first-order reaction model, which is commonly used today for oil shales [3-6,8], is also suitable for coals. An understanding of the similarities and the differences in the kinetic behaviour of various samples is essential in order to facilitate the transformation of conversion technologies developed for one type of an oil shale or coal to another. A comparison of the data reported in refs. l-8 indicates large differences in the kinetic data and in the kinetic parameters obtained by the various authors. It is not clear whether these differences are due to differences in sample, experimental apparatus, method of data analysis, kinetic model employed, or some as yet unknown experimental errors. Therefore, the second objective of the present study was to obtain kinetic data on different fossil fuel samples under comparable conditions, i.e. the same experimental technique, the same temperatures, and the same data analysis procedure.
51 EXPERIMENTAL
Samples Sample designation Z - Zefa-Ef’e (Israel oil shale). G - Green River Red Point Mine (U.S. oil shale). C - Green River Colony Mine (U.S. oil shale). N - Kentucky New Albany (U.S. oil shale). P - Secretary Pretoria Waterberg (S.A. coal). I - Illinois No 6 (U.S. coal). 2400 - Z pre-exposed to 400 o C (see text). G400 - G pre-exposed to 400 ’ C (see text). The samples were received in bulk, crushed, sieved and dried (100 o C in vacua); the 0.2-0.3 mm fraction was used in this work. After crushing and sieving, the samples were stored in closed vessels. The elemental analyses of the isolated kerogens and the pyrite free coals are presented in Table 1. The hydrocarbon production potentials of the isolated kerogens and of the coals were estimated by the Rock-Eva1 techniques (using CDS-820-ZA [ll]). These values are given in Table 1 as hydrogen index (HI) (hydrogen index = [S2 (mg/g)/TOC (%,>Ix 100). The kerogens were isolated according to the procedure described previously [12]; coals were treated with LiAlH, (for pyrite removal). Experimental
technique
A small stainless steel fluidized-bed reactor (Fig. 1) was used to pyrolyze (under an inert introgen gas flow) small samples of oil shale or coal particles
TABLE 1 Analyses of the kerogens and pyrite-free coals
Carbon (W) Hydrogen ( W) Nitrogen ( W) Sulphur ( W) Ash (W) Oxygen by difference (W) H/C atomic ratio Hydrogen index (Rock-Eval) * Not determined.
N
Z
G
C
I
P
76.06 7.66
66.00 7.33
75.37 9.96
80.90 10.60
71.70 4.93
65.25 3.86
2.24 2.64 4.50
1.70 8.30 0.08
1.54 2.06 1.27
2.40 1.80 0.79
1.43 2.13 11.72
9.80
3.51
1.58
1.57
1058
1218
6.91 1.21 851
16.6 1.33 882
2400
G400
- * -
_
1.77 0.70 8.38
-
-
8.02
20.04
-
-
0.82
0.71
-
_
279
587
279
145
52
(0.2-0.3 mm). A weighed sample (not greater than 0.5% by weight of the bed) was dropped into the preheated reactor, producing a negligible drop in bed temperature. Heat transfer in the fluidized bed was very rapid; calculations [13] indicated that when the temperature of the preheated bed is 500°C it requires about 1 second to heat a 0.2 mm particle of an Israel oil shale to 495°C (which is 5 “C below the final temperature) and that the temperature is essentially uniform within the particle during the heat-up. The volatile products were rapidly swept by the fluidizing gas (nitrogen) to a flame ionization detector (FID). The FID produced a signal proportional to the concentration of the total organic carbon (TOC) in the product stream. Since the reaction is relatively fast at the beginning and much slower at the end, the speed of the recorder paper was decreased as the reaction progressed. The slow decay of the reaction rate makes it difficult to determine the termination point of the reaction, which leads to an appreciable under-
-----
Sample
inlet
filter
Preheater and metal sieve
device
Spent
Dust
shale
Oven
b-
rFiG-l
, meter
I
IL-l_
1 Manometer
(
( Temperature 4 I&
,/ Thermocouple
Fig. 1. The fluidized-bed reactor system.
controller
)
(Eurotherm
8101
1
53
estimation of the total reaction time. It was therefore imperative to continue the reaction until the FID output returned to the background value prior to the start of the run. A measured amount of n-butane was injected into the reactor (through the calibration gas inlet) before and after each experiment. This calibration permitted quantitation of the detector response, estimation of the time lag of the system and the degree of the peak broadening (convolution). It was found that a pulse of calibration gas which is injected into the reactor within 0.8 s was monitored by the FID immediately as a sharp peak which was 1.5 s wide at half height. This result, together with the temperature rise time calculations [13], indicates that fast reactions (with a half-life of a few seconds) can not be followed by this system. Therefore the temperature range studied was limited to 500” C (upper limit). At this temperature the half-life for all the samples studied was longer than 5 s. The experimental curves were digitized and the data processing was carried out by a computer. This experimental system is actually an improved version of a system which has been described previously [4]. The main improvements are: (1) better stabilization of the reactor temperature, (2) minimization of the temperature gradients in the fluidized bed, (3) minimization of the ‘dead volume’ of the system, and (4) addition of a water manometer for better control of the FID/vent ratio. These improvements were probably responsible for the slight differences between the present results and those presented previously [4]. Preparation
of samples pre-exposed
to elevated temperatures
A bed made of NaCl (crystalline) was introduced into the reactor through the removable top (Fig. 1); a sample of ca. 5 g was slowly introduced in portions into the preheated reactor, in order to keep the reactor temperature stable. The reaction time was adjusted to be twice the time required for the completion of the kinetic experiments (t + 00). The reactor was then cooled to 100 o C (under the nitrogen stream). The sample was separated from the NaCl bed by dissolution in water. Determination
of the total yield
The estimation of the total amount of organic carbon produced during a by difference as follows; a long exposure time (t + 00) was performed sample was exposed to elevated temperature as described above, then the contents of the reactor (i.e. the bed and the residue) were extracted with benzene-methanol (7 : 3) overnight. After removal of the solvents, the NaCl bed was dissolved in water, the solid residue was treated with HCl, and the organic carbon content of the carbonate-free residue was determined. The total yield of organic carbon was calculated by difference: (TOC of the original sample) - [(TOC in the solid residue ) + (TOC of the extractables)]. The amount of extractables was negligible.
54
Experiments with samples pre-exposed to 400 OC The samples were prepared as described above. In order to compare the original matter and the pretreated samples under the same experimental conditions the experiments were carried out in tandem (a pair of experiments at each temperature).
RESULTS AND DISCUSSION
The dependence of the total yield on temperature In our previous work [4] we reported that the total amount of products formed during a long period of heating (t + 00) increased with a rise in the temperature. This was obtained by calibration of the area under the FID curves. In the present work the total amount of carbon produced was estimated to be the difference between the organic carbon content of the original sample and that of a sample which was expoed to elevated temperature (see Experimental). The results of such experiments for one sample of coal and a sample of oil shale are summmarized in Table 2. These indicate that the ultimate yield of the Israeli oil shale increases with rising temperature until the point of complete decomposition of the kerogen is reached at about 425 o C. Weitkamp and Gutberlet [14] reported that the ultimate yield of the Green River oil shale increased exponentially with temperature until 400 o C. Allred [15] also reported that the total yield of the Green River oil shale increased with rising temperature. Table 2 indicates that the ultimate pyrolyzate yield from Pretoria coal increases with reactor temperature, in the temperature range studied. This result is in agreement with the results of Pitt [16], Anthony et al. [17] and others for various coal samples. Inspection of the NaCl in water solutions indicated that there is almost no char on the NaCl bed. This result supports the findings of Campbell et al. [18] who indicated that charring of the products is minimized when fast heat-up and high sweep rates are applied.
TABLE 2 The ultimate yield of Pretoria Coal (P) and Zefa Efe temperature range
oil shale (Z) at the 400-500”
Organic carbon volatilized (%) Z P
400°C
425OC
450°c
475OC
5oo”c
55.4 12.4
66.5 17.1
66.5 18.6
66.5 21.7
67.6 25.2
C
55
These experiments were conducted on an NaCl bed rather than on spent shale. We assumed that the dependence of total yield on temperature will manifest the same overall relations for different beds. However, the possible influence of the bed on the reaction needs careful examination, particularly the influence of the bed on coal samples which undergo softening at elevated temperatures. Kinetic results
Since the FID measures total organic carbon [19], the FID output is directly proportional to the generation rate of volatile organic carbon (dA/dt). The amount of organic carbon produced at any time t (A(t)), is proportional to the integral of dA/dt from t = 0 to t (i.e. the area under the curve from t = 0 to t). Conversion is defined as A( t)/A( t + co); according to this definition each isothermal experiment reaches 100% conversion. This definition is very useful, even though it has been pointed out that the total amount of product A( t -+ co) increases with increasing reactor temperature (see Table 2). Fig. 2 shows curves of reaction rate versus time (these curves
100
200
300
1300
J
,ll.,ll,l,,..I,IIII,II1,,,,,,
0
2300
#
loo
200
300
1300
3300 TIME
0
w 0
ID0
loo
200
300
I300
2300
4300
(s)
Fig. 2. Curves of reaction rate versus time at 450 o C. These are fundamentally the experimental FID curves, the arbitrary FID response scale was changed into a reaction-rate scale by adjusting the area under the original curves to 100%.
56
Ti ME (5)
Fig. ‘3. Curves of conversion versus time (log scale). ( -) (---) 45ooc, f-----) 475*c, (--.‘-.‘._) 5oo*c.
4OO”C, (---)
425’C,
were obtained by normalization of the area under the FID curves to 300%). It is obvious that different samples behave differently. No differences between the kinetic behaviour of the original and the extracted samples were observed. Fig. 3 shows the curves of conversion versus time (log scale). It is obvious that the temperature has a marked influence on the reaction rate, thus the half-lifetime (t,,,) of all the samples at 500” C is measured in seconds whereas the t,,, at 400 * C is measured in thousands of seconds. It is noteworthy that the Pretoria coal (P) exhibits a different thermal behaviour over the 400-425°C range (this surprising beha~our was reproduced over several sets of experiments). The activation energy of a chemical process if often estimated by plotting - ln( t,,,) against the reciprocal of the temperature. The pyrolytic reaction is fast at the be~nning and slow at the end (Fig. 2), therefore we have also used t,,, for E, estimation (t,,, is the time elapsed when the reaction reaches 75% conversion: Fig. 3). We should point out that the choice of t,,,, or t,,, is an arbitrary one; t,,, or t,,, could be used as well. In Fig. 4 the
57 a
A
1
-2-
-hJ \ d c, -4 C l-i i -e-
-6’,,,,
,l),/,,,‘/,,,,,
I()‘
1.25
1.30
1.43
1.35
l/T
1.50
( 103;;eoj
-2
B
1
-4-
-v ;;; ct -@C d
I
-8 J
-10
, 1.25
,
,
,
, 1.30
,
,
,
, 1.35
i/T
(
,
,
,
3 1.40 (IO /K )
,
,
,
,
1.45
,
,
,
1.50
Fig. 4. (A) -In fl,,* versus the inverse temperature ( f,,, = the time passed until the reaction (0) N, (X) 2, (Cl) C, (A) G, (a) reached 50% conversion). (B) -In t,,, versus l/temperature. k (0) p.
values of - ln( t,,z) and - ln( t,,4) obtained from the curves in Fig. 3 are plotted versus l/temperature. Fig. 4 indicates that the order of reaction speed is: fast reaction 6 N > Z > C = G > I > P -+ slow reaction (i.e. the t,,, or t,,, for P is longer than that for N). The activation energies (E,( t,,,) or ( f3Y4)) and the frequency factors (k,( t,,,) or ( t3,4)) calculated
58 TABLE
3
Activation energies and frequency factors received tsj4 values versus the inverse temperature The data are from the experiments log k, values are in log s--i. N
reported
Z
Temperature range: 400 - 500 o C 38.9 39.8 10.22 10.48
E, (h/s) 1% kl (t*,d Correlation coefficient
-% ($4) log k, (b/4) Correlation coefficient
0.999 38.4 9.62 0.999
0.992 40.3 10.08 0.987
Temperature range: 400 - 450 ’ C 38.3 43.0 E, (h/z) 10.04 8.65 1% x-0 (~,,2~ Correlation coefficient 0.999 0.980 E, ($4) log k, (t3,4) Correlation coefficient
36.5 9.03 0.999
32.6 7.64 0.986
Temperature range: 450 - 500” C 39.9 44.9 E, (h/z) 10.50 11.99 1% ko (t1,d Correlation coefficient 0.999 0.997 & (f3/4) log ko (t3,4) Correlation coefficient
41.4 10.49 0.998
49.6 12.78 0.999
from linear regression
of t,,,
and
in Fig. 3. The E, values are in kcal/mole,
the
G
C
I
39.6 9.99
40.8 10.38
41.8 10.58
0.987 39.5 9.593 0.989
28.2 7.38 0.994 29.4 6.36 0.995
51.0 13.31 0.999 50.0 12.66 0.999
0.998 41.6 10.26 0.997
36.4 9.00 0.999 35.9 8.45 0.998
46.3 11.97 0.999 47.2 11.89 0.999
analysis
P
0.996 41.0 9.82 0.997
39.7 9.91 0.986 36.7 8.51 0.995
46.6 11.96 0.999 47.7 11.81 0.999
42.6 10.69 0.999 37.4 8.65 0.999
are summarized in Table 3. These results were compared with the results obtained from three points (400, 425, 450 OC) and (450, 475, 500 o C): see Table 3. It is obvious that the 450-500 o C range yields E, values which are about 10 kcal/mole higher than those measured in the 400-450°C range. values calculated at the higher temperature range The higher EAti,,, t,J could be related to: (1) the formation of more products at the higher temperature range, which requires higher activation energies. (2) Light-end products are more abundant at high temperatures [2,20]; this may result from the cleavage of the chemical bonds which did not break at lower temperatures, the measured average activation energy therefore being higher.
59
-lot---e-l 0
20
40
60
/
/
80
loo %
-lO40
loo
CONVERSION
Fig. 5. Experimental (dashed) and theoretical (solid) curves of the natural logarithms of k (observed) (s-l) versus conversion. 1 = 400 o C, 2 = 425 o C, 3 = 450 o C, 4 = 475 o C, 5 = 500°C.
(3) This phenomenon could also stem from the loss of selectivity with temperature increase. For competing reactions, the higher the temperature, the reactions with higher activation energy become more dominant. (4) It will be shown later that the shape of the pyrolyzate evolution curve changes with temperature (in our terminology: the contribution of fast reactions increases with temperature). Global parameters, such as t,,, or f3,++,do not account for this change in the reaction profile. The activation energies calculated from the half-lives (Table 3) range from 38 to 56 kcal/mole; these values indicate that diffusion or volatilization are probably not the rate-determining steps. This conclusion is supported by the results obtained by other investigators: Ekstrom and Callaghan IS] prepared samples of shale oil absorbed on spent shale and compared the kinetics of the shale oil release with the kinetics of fresh oil shale pyrolysis. They found that, at 465”C, the time necessary for the shale oil release is negligible as compared with the reaction time. ~harlesworth [7] compared the kinetics of n-octatriacontane (C,,) release from spent shale and observed the same phenomenon (in the 350-450 ’ C temperature range). Gas chromatographic analyses of the pyrolyzate obtained under fluidized bed conditions from
60 TABLE 4 Results of fitting eq. 2 to the experimental curves These results are presented graphically in Fig. 5. Temp. PC)
-In k, (-ins-I)
-Ink, (-In s-l)
-In k, (-Ins-‘)
A, (%)
A2
A3
(%)
(%>
N
400 425 450 475 500
0.60 - 0.50 - 0.50 - 0.70 0.90
3.30 2.50 2.80 2.34 2.10
6.51 5.50 4.80 3.98 3.55
19.5 15.5 15.0 11.0 28.0
10.0 11.0 22.0 38.0 40.0
70.5 73.5 63.0 51.0 32.0
Z
400 425 450 475 500
2.40 2.10 1.10 0.70 0.15
4.60 4.10 3.20 2.70 2.20
6.94 6.38 5.39 4.69 3.90
16.0 22.0 20.5 25.0 34.0
25.0 25.0 20.0 30.0 35.0
59.0 53.0 53.5 45.0 31.0
G
400 425 450 475 500
2.50 2.00 2.00 1.20 0.80
4.80 3.90 3.50 3.41 2.85
7.13 6.40 5.63 4.76 4.40
7.0 6.8 6.3 6.5 11.5
9.0 6.0 9.5 40.0 65.0
84.0 87.2 84.2 53.5 23.5
C
400 425 450 475 500
1.00 1.50 1.20 1.50 2.30
3.90 4.60 4.60 3.80 2.80
7.19 6.47 5.68 5.10 4.45
6.0 6.8 4.6 3.0 7.0
8.9 21.0 50.0 74.5 75.0
85.1 72.2 45.4 22.5 18.0
I
400 425 450 475 500
3.10 1.50 1.20 0.00 -0.10
5.20 5.20 4.65 3.80 2.90
7.90 7.10 6.38 5.67 4.72
12.5 13.2 15.0 13.0 12.0
10.0 33.0 36.0 48.0 55.0
74.5 51.0 49.0 39.0 33.0
P
400 425 450 475 500
2.90 2.90 3.80 3.10 2.20
5.20 5.23 5.20 4.40 3.70
7.35 7.40 6.54 5.94 5.54
4.5 4.0 17.0 25.0 36.0
17.0 19.5 39.0 37.0 35.0
78.5 76.5 44.0 38.0 29.0
U.S. [2], Israeli [20], and Australian [21] oil shales indicate that the boiling points of the pyrolyzates are lower than that of C,,; these results indicate that there is probably no holdup of pyrolyzate in the liquid state. Gas chromatographic analyses [20] of pyrolyzates which were sampled at different time intervals during the reaction also indicated that the reaction is not diffusion controlled. The above discussion supports the idea that the products formed from the kerogen at the range of temperatures studied diffuse immediately from the particle into the fluidizing gas stream, and are
61
instantaneously purged. Thus there is no holdup due to interaction with the support bed particles. However, secondary cracking in the gas phase, on the fluidized bed support, or on the hot metal tubing leading to the FID probably does occur. Nevertheless, this secondary cracking would not affect the kinetics measured by the FID, since the FID measures total organic carbon. Fig. 5 shows experimental and theoretical curves of In k versus conversion, where k is the observed reaction rate constant assuming a first-order reaction. The theoretical curves were obtained by fitting the three parallel first-order reactions model to the experimental curves. The proposed model and the data analysis procedure have been described in detail [4], therefore only a brief description will be given here: the experimental reaction rate constant k was calculated according to the equation: dA/dt k=
[A(t+
co) -A(t)]”
Where dA/dt is the height of the curves in Fig. 2, [ A( t + co) - A(t)] is the area under the curve at the right side of the observed point (i.e. the area under the curve from t to the end of the experiment), and n is the reaction order (n = 1 was assumed). According to the proposed model [4], the evolution of pyrolyzates is described as a linear combination of three parallel first-order reactions: A(t)
= i A*[1 -exp(-kg)] i=l
(2)
-3-
‘\ -a-
),,, 1.25
‘w.l
,,,,,,,,,,,,,,,,,,,, 1.30
1.35
i/T
1.40
1.45
i.50
IlO3 /K 1
Fig. 6. Natural logarithms of the observed rate constant at the end of the reaction ( k3) versus the inverse temperature. (0) N, (X) Z, (a) C, (A) G, (0) I, (0) P.
62
where AIT = the contribution of the ith reaction to the combined reaction and ki = the rate constant of the i th reaction. The results of the curve fitting procedure are summarized in Table 4. The experimental curves in Fig. 6 indicate that at the early stages of the reaction, k (observed) is not constant (k decreases with conversion). That is, the reaction cannot be described as a first-order reaction in the early stages. However, at high conversion levels all samples exhibit first-order behaviour (k becomes constant). This discrepancy is more obvious at low temperatures (i.e. the deviation from a single first-order reaction behaviour increases with temperature). This behaviour is expressed in Table 4 by the decrease with temperature of the slow reaction contribution A, and the increase of the fast reaction contributions A, and A, with temperature. This phenomenon could be explained by: (1) the general concept: loss of selectivity with the elevation in temperature; and (2) the enhanced cracking at high temperature (which brings about the participation of more types of chemical bonds in the reaction). This result is in agreement with the results of Ekstrom and Callaghan [8] but different from the results obtained by Braun and Bumham [5], who have interpreted Richardson’s data by two parallel first-order reactions with constant contributions of 75 and 25%, respectively, over the 490-540 o C temperature range. The values of -In k, from Table 4 were plotted against the inverse temperature; the activation energy values ( Eno2,3j), the frequency factors
TABLE 5 Activation energy and frequency factors received from linear regression analysis of k,, k,, k, values (from Table 4) versus the reciprocal of the temperature The E, values are in kcal/mole, the k,
values are in s-l.
N
Z
G
C
I
P
E, (k3) log k; (k,) Correlation coefficient
30.8 7.25
32.0 7.53
29.3 6.46
28.3 6.08
32.1 6.99
26.5 5.16
E, (k2) log k, (kz) Correlation coefficient
* *
E, (k,)
*
0.996
25.6 6.32 0.878
0.995 24.3 6.83
l
Correlation coefficient
0.996
0.047
0.987
0.996 18.2 3.93
0.999 *
0.963
24.4 5.53
l
0.996 17.2 4.49
0.996
0.609 * 0.817
22.8 4.81
0.943 32.8 9.49
l
0.991
0.965
0.943 * * 0.525
* When the correlation coefficient is less than 0.9 the results are meaningless. For P, only four points 425-500 o C were selected.
63
(AY~~,~,~)) and the correlation coefficients obtained for the three parallel reactions are summarized in Table 5. These indicate that for k3 values which are expe~mentally obtained (k, is the value of k (observed) (in Fig. 4) at high conversion levels, see ref. 4), there is a good correlation for all of the samples (with the exception of P in the 400-425OC range). The rate constants k, and k,, which are probably chemically meaningless in the strict sense (since they were derived by regression analysis), exhibit a good match for samples Z, G and I. However, samples C, N and P exhibited unsatisfactory correlation, despite the fact that at each isothermal temperature there is a good match between the experimental and the theoretical curves. The E;, values obtained by this method (Table 5) are much lower in comparison to those derived from t,/, or t,,, (Table 3). This is probably due to the fact that (according to this model), the contribution of fast reactions increases with temperature. It is noteworthy that Braun and Burnham [5], who interpreted Richardson’s data 121 according to a two parallel first-order reactions model, obtained much higher activation energy values for k, and k, (50.3 and 46.1 kcal/mole, respectively). These values are also higher than the values obtained by Ekstrom and Callaghan [8]. The differences between the three studies probably stem from the fact that, according to the Braun and Bushy model, the contributions of the fast and slow reactions do not change with the temperature. Comparison of Fig. 6 with Fig. 4 reveals that the order of the reaction rate according to k, is the same as according to t,,, or t,,, i.e. N > Z > C = G > I > P. However, the activation energies obtained from t,,,, and t,,, are higher than the values found from k,, k,, k, and are closer to the values expected for chemical bond cleavage. Fig. 7 shows the experimental curves of conversion versus time from Fig. 2 together with curves calculated by the equation : A(t)
=
iA:{l-exp[-(I&,, exp(-E,,/RT))t]) i=l
The EGj and k,i values were taken from Table 5; Af values were taken from Table 4. Fig. 7 exhibits a good correlation between the experimental and the theoretical curves. It must be emphasized that, since the equation is empirical, it might be valuable for retort modelling calculations. However, the chemical significance of the derived kinetic parameters (i.e. E, and k,) is still questionable. Figs. 8-10 demonstrate the differences among the various samples. In Fig. 8 the same order of kinetics as stated above is shown (N > Z > C = G > I > P). Fig. 9 compares the curve of In k versus conversion of the various samples at 450” C. It indicates that the samples differ one from the other not only in the speed of the reaction (the position of the various curves along the y-axis) but also in the shape of the curve. Fig. 10 shows the deviation of the ‘kinetics from a single first-order reaction behaviour, e.g. in
100 80
60 40 20 0 100
5
tn
80
", 60 =
40
0" 20 s 0 I
TIME(s)
Fig. 7. Experimental (solid from Fig. 3) and theoretical (dashed) time.
curves
of conversion versus
coal I the initial rate constant is fourteen times faster than that of the end of the rection. Samples N and 2 also do not agree with the first-order model, but G, C and P follow the first-order model rather closely. The data presented by us so far indicate that much of the difference in the kinetic data and the kinetic parameters, which were obtained by various authors [l--S], can be attributed to differences in samples. However, in view of the close resemblance of the kinetic behaviour of samples G and C (both from the Green River Formation), it is interesting to compare the four studies which used the Green River oil shale as a feedstock, and fluidized bed-attached FID technique. Table 6 compares the values of k(fina1) and E,(final) obtained in the different studies, where k(final) is the observed rate constant (assuming first-order reaction) at the end of the reaction (i.e. k, in the present study). Both Wallman et al. [l] and Richardson et al. [2] obtained k(fina1) by est~ating the slope of the second strict-line segment on In{1 - conversion) versus time curves. This method is equivalent to the method used in the present study, i.e. the value of k, was obtained directly
65
o-
1
0
I
I
I
I
I
I 600
400
200
I
I
I iooo
800
TIME (s) Fig. 8. Curves of conversion versus time at 450 o C. (- . -. -) N, (- . . -. . -) Z, (- - -) C, (---) G, (-.-.---) I, () Pm
from the curves of In k versus converstion (Fig. 4). Table 6 shows that k(final) values obtained by Richardson are slightly higher than those observed in the present study. This small but consistent difference could stem from a constant difference of about 10” C between the two fluidized bed temperatures. The values of the activation energies are very similar. Wall-
-27
-7
1 0
I 20
I
I
I
~CWERS~N
I
I
I 80
I
I 100
%
Fig. 9. Curves of In k versus (---)G,(-------)I,(-)P.
conversion
at 450 o C. (-. -. -) N,
(-..-..-)
z,
(---)
c,
66 15-
3
lo-
2
-
t=
e Y
5-
0
20
SO %
100
4COON”ERs&N
Fig. 10. Curves of k/k(final) versus conversion. The k values from Fig. 9 were divided by the value of k at the end of the reaction. k/k(final) is an indicator of the deviation of the observed k from the first-order behaviour observed at the end of the reaction. Lines as coded for Figs. 8 and 9.
man obtained a somewhat lower E, value. However, Braun and Burnham [5], who reinterpreted Richardson’s data with a type of ‘blind regression analysis’ computer program, obtained a much higher E, value. This comparison indicates that the data analysis procedure itself might lead to different results. At the outset of this work we expected to find a relationship between the samples’ elemental analysis (i.e. H/C ratio, oxygen and sulphur content or hydrogen index (HI)) and the kinetic behaviour. A definite relations~p was not found. For example, coal P with the low H/C and HI (Table l), exhibits a slower kinetic behaviour than coal I. However, the kinetics of G and C oil shales, with the high H/C and HI, were slower than the kinetics of N and Z TABLE 6 Comparison of the rate constants and the E, values received by different authors for the end of the reaction (Green River oil shale) k (final) (s-r x 103)
This
work 460°C 500°C 550°c E, (final) kcal/mole l l
4.9 l 12.3 44.5 ** 29.3
By interpolation (Fig. 6). * By extrapolation (Fig. 6).
Richardson et al. [2] 6.33 18.2 58.6 29.7
Wallman et al. [l] 2.0 4.5 11.16 22.6
Braun and Burnham [ 51
46.1
67
oil shales with lower H/C and HI values. The kinetics of oil shale Z with the high content of oxygen and sulphur, are slower than those of oil shale N, having a lower content of oxygen and sulphur. This lack of correlation between the elemental analysis and the kinetic behaviour probably indicates that the kinetics depend on the structure of the organic matter as well as the distribution of chemical bonds manifested statistically by its elemental composition.
TIME
0
(s)
20
80 %
I
100
~ON”ERSkJ
Fig. 11. Kinetics of the original samples G (solid), compared with a sample pre-exposed to 400 o C G400 (dashed). W-500 o C every 20 o C increment.
-1
-2
~
-3
-4 C l-t -5
-0 Y
~
-7
,
0
I
I
I
1
20
0
I
4CDON”EFlS~N
I
I 80
4
I
100
%
Fig. 12. Kinetics of the original sample 2 (solid), compared 400 o C Z400 (dashed). 440-500 o C every 20 * C increment.
Experiments
with samples pre-exposed
with a sample
pre-exposed
to
to 400 “C
Figs. 11-14 compare original matter to samples preexposed to 400°C. The activation energies and frequency factors obtained from this set of experimental are summarized in Table 7. Figs. 11-13 and Table 7 indicate that kinetic behaviour diplayed by the pretreated samples is slower than that of the original samples. However, the calculated E0 values do not show a definite trend. Fig. 14 shows the curves of k/k(final) of the fresh and the
69 -l-
-2-
+I-6J \ r( u -4C l-l ’
-5-
-e-
-I -7:
II*I~
1.20
1
1.30
1.32
I
1.34
l/T
II0
$3, /K
’ 1.9, ’ Lb0 ’ 1
I!42
Fig. 13. Arrhenius plots of r1,2 values of G and Z compared with G400 and 2400. (0) G, Z, ( x ) G400, (0) 2400.
(A)
pretreated samples at 460 o C. The kinetic behaviour of the fresh Israeli oil shale (Z) deviates strongly from first-order behaviour; it seems that the pretreatment changed this behaviour quite dramatically, and the kinetic behaviour of the pre-exposed samples (2400) follows the first-order model
\ \ \ \
O II
0
I
1
20
I
1
1
4CmJ”ER&N
1
,
I
80
8
I
100
%
Fig. 14. Curves of k/k(final) (-.--.--) G400, (---) 2400.
versus conversion
at 460 o C. (-
) G, (---)
Z,
70 TABLE 7 Activation energy and frequency the original samples
factors
for samples
pre-exposed
to 400°C
compared
with
The E, and k, values were received by linear regression analysis of t,,, and t,,, values versus the reciprocal of the temperature. The data are from the experiments reported in Figs. 12 and 13 (440-500 o C, every 20 o C increment). 2400
Z
G400
G
E, (+) log kll (h/2) Correlation coefficient
41.0 10.18
45.5 12.21
55.3 14.26
50.8 13.22
E, ($4) log k, (t3,4) Correlation coefficient
37.5 7.34
0.997
0.999
0.992 50.5 13.13 0.998
0.999 55.0 15.03 0.998
rather closely. The Green River fresh sample (G) follows model quite closely, as does the pre-exposed sample (G400).
0.999 52.0 13.19 0.999
the first-order
ACKNOWLEDGEMENT
This work was partially supported by a grant from the Israel NCRD German BMFT (KFA-Jiilich & Bergbau Forschung).
and
REFERENCES 1 P.H. Wallman, P.W. Tamm and B.G. Spars, in H.L. Stauffer (Editor), Oil Shale Tar Sands and Related Materials, ACS Symposium Series 163, American Chemical Society, Washington, DC, 1981, p. 93. 2 J.H. Richardson, E.B. Huss, L.L. Ott, J.E. Clarkson, M.O. Bishop, J.R. Taylor, L.J. Gregory and J.C. Morris, UCID-19548, Lawrence Livermore National Laboratory, 1981. 3 H. Bar, MSc. Thesis, The Hebrew University of Jerusalem, April 1984. 4 H. Bar, R. Ikan and Z. Aizenshtat, J. Anal. Appl. Pyrolysis, 10 (1986) 153. 5 R.L. Braun and A.K. Burnham, Fuel, 65 (1986) 218. 6 S.D. Carter, Prepr. Am. Chem. Sot., Div. Pet. Chem., 32(l) (1987) 133. 7 J.M. Charlesworth, Ind. Eng. Chem. Process Des. Dev., 24 (1985) 1125. 8 A. Ekstrom and G. Callaghan, Fuel, 66 (1987) 331. 9 N.Y. Nsakala, R.H. Essenhigh and P.L. Walker, Combust. Sci. Technol., 16 (1977) 153. 10 A.W. Scaroni, P.L. Walker, Jr. and R.H. Essenhigb, Fuel, 60 (1981) 71. 11 Z. Aizenshtat, Israel Chem. Sot. Bull., 3 (1980) 266. 12 K.E. Gokler, T.J. Stoecker and R.F. Baddour, Ind. Eng. Chem. Prod. Res. Dev., 23 (1984) 308. 13 G. Esterson and Z. Aizenshtat, Fate of Organic Sulfur in U.S. and Israeli Oil Shales, Annual Report No. 8610, Prepared for Department of Energy USA and Ministry of Energy and Infrastructure Israel, 1986, p. 143.
71 14 15 16 17 18 29 20 21
A.W. Weitkamp and L.C Gutberlet, Ind. Eng. Chem. Process Des. Dev., 9 (1970) 386. V.D. Allred, Chem. Eng. Progr., 62 (1966) 55. G.J. Pitt, Fuel, 41 (1962) 267. D.B. Anthony, J.B. Howard, H.C. Hottel and H.P. Meissner, Fifteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1975, p. 1303. J.H. Campbell, G.H. Koskinas, T.T. Cobum and N.D. Stout, UCRL-52256, Lawrence Livermore National Laboratory, 1977. R.A. Jones, An Introduction to Gas-Liquid Chromatography, Academic Press, New York, 1970. H. Bar, R. Ikan and Z. Aizenshtat, J. Anal. Appl. Pyrolysis, 10 (1986) 167. J.M. Charlesworth, Ind. Eng. Chem. Process Des. Dev., 24 (1985) 1117.
COMPARATIVE STUDY OF THE ISOTHERMAL KINETIC BEHAVIOUR OF SOME OIL SHALES
H. BAR, R. IRAN
and Z. AIZENSHTAT
49
PYROLYSIS AND COALS
*
Casali Institute of Applied Chemistry, The School of Applied Science and Technology, and the Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem, 91904 (Israel) (Received
September
13th, 1987; accepted
May 9th, 1988)
ABSTRACT Data are presented on the kinetics of hydrocarbon evolution from isothermal pyrolysis of four oil shale and two coal samples. Similarities and differences in the pyrolysis kinetics of the six samples are discussed. A fluidized-bed reactor coupled with a flame ionization detector as a monitor was employed. The rate of generation of the volatile hydrocarbons at each isothermal temperature can be described as a linear combination of three parallel independent first-order reactions. This model was found to be suitable for the six samples studied despite the large differences in the mineral matrix and the type of the organic matter. The activation energies calculated from the measured half-lifetimes depend on the temperature range; thus the activation energies measured in the 400-450 ’ C range were about 10 kcal/mole lower than those measured in the 450-500 o C range. The kinetics of hydrocarbon evolution from samples previously exposed to 400 o C were slower than the reaction kinetics of the original matter measured at a higher temperature range. This indicates that the kinetics slow down with maturation of the sample (or, as in this case, with a simulated maturation). Coal; fuels; kerogens;
kinetics,
reaction;
oil shale; pyrolysis.
INTRODUCTION
Several above-ground oil shale retorting processes are characterized by rapid heating of the shale, followed by essentially isothermal retorting with subsequent rapid removal of the pyrolysis products. During the last six years several attempts have been made to determine the kinetics under such conditions: Wallman et al. [l] and Richardson et al. [2] employed a fluidized bed reactor with a flame ionization detector (FID) as a monitor, and used curves of ln(l - conversion) versus time for the data interpretation. Both Wallman and Richardson approximated these curves by two straight-line
50
segments, and described the generation rate of the hydrocarbons from some U.S. oil shales by two sequential first-order reactions; the reaction rate constants k, and k, were calculated from the slope of the two straight-line segments. Bar [3] employed a similar apparatus but different data analysis procedure in which k (the observed rate constant) was calculated directly from the FID versus time curves. This improved data analysis procedure indicated that the rate of organic pyrolyzate formation from an Israeli oil shale could be described by two parallel first-order reactions. A careful examination of a new set of data which was obtained from an improved experimental system [4] indicated that a set of two reactions is not sufficient to describe the pyrolytic reaction and that three or more independent first-order reactions are needed. Braun and Bumham [5] re-examined Richardson’s data and concluded that it could be represented either by a single reaction with an effective order of 1.57, or by two parallel first-order reactions. Carter [6] used the same technique and described the kinetics of Kentucky oil shale pyrolysis by two parallel first order reactions. Charlesworth [7] employed a technique in which a preheated fixed bed reactor is attached to a flame ionization detector, and suggested that the rates of hydrocarbon evolution from Australian oil shale could be described by various solid phase mechanisms. Ekstrom and Callaghan [S] followed Braun and Burnham’s approach and described the rates of the total hydrocarbon evolution from some Australian oil shales by two parallel first-order reactions, using an experimental technique similar to that used by Charlesworth. Ten years ago, Nsakala et al. [9] had already suggested that the rate of weight loss of four coal (lignite) samples could be described by two parallel first-order reactions (isothermal 808 o C entrained-flow furnace). Scaroni et al. [lo] described the weight loss of a Texas lignite with the same model using an apparatus similar to that of Nsakala. However, the experimental technique which was used by N&ala and Scaroni was very different from that described in refs. 1-8. One objective of the present study was to find out whether the two, or three parallel first-order reaction model, which is commonly used today for oil shales [3-6,8], is also suitable for coals. An understanding of the similarities and the differences in the kinetic behaviour of various samples is essential in order to facilitate the transformation of conversion technologies developed for one type of an oil shale or coal to another. A comparison of the data reported in refs. l-8 indicates large differences in the kinetic data and in the kinetic parameters obtained by the various authors. It is not clear whether these differences are due to differences in sample, experimental apparatus, method of data analysis, kinetic model employed, or some as yet unknown experimental errors. Therefore, the second objective of the present study was to obtain kinetic data on different fossil fuel samples under comparable conditions, i.e. the same experimental technique, the same temperatures, and the same data analysis procedure.
51 EXPERIMENTAL
Samples Sample designation Z - Zefa-Ef’e (Israel oil shale). G - Green River Red Point Mine (U.S. oil shale). C - Green River Colony Mine (U.S. oil shale). N - Kentucky New Albany (U.S. oil shale). P - Secretary Pretoria Waterberg (S.A. coal). I - Illinois No 6 (U.S. coal). 2400 - Z pre-exposed to 400 o C (see text). G400 - G pre-exposed to 400 ’ C (see text). The samples were received in bulk, crushed, sieved and dried (100 o C in vacua); the 0.2-0.3 mm fraction was used in this work. After crushing and sieving, the samples were stored in closed vessels. The elemental analyses of the isolated kerogens and the pyrite free coals are presented in Table 1. The hydrocarbon production potentials of the isolated kerogens and of the coals were estimated by the Rock-Eva1 techniques (using CDS-820-ZA [ll]). These values are given in Table 1 as hydrogen index (HI) (hydrogen index = [S2 (mg/g)/TOC (%,>Ix 100). The kerogens were isolated according to the procedure described previously [12]; coals were treated with LiAlH, (for pyrite removal). Experimental
technique
A small stainless steel fluidized-bed reactor (Fig. 1) was used to pyrolyze (under an inert introgen gas flow) small samples of oil shale or coal particles
TABLE 1 Analyses of the kerogens and pyrite-free coals
Carbon (W) Hydrogen ( W) Nitrogen ( W) Sulphur ( W) Ash (W) Oxygen by difference (W) H/C atomic ratio Hydrogen index (Rock-Eval) * Not determined.
N
Z
G
C
I
P
76.06 7.66
66.00 7.33
75.37 9.96
80.90 10.60
71.70 4.93
65.25 3.86
2.24 2.64 4.50
1.70 8.30 0.08
1.54 2.06 1.27
2.40 1.80 0.79
1.43 2.13 11.72
9.80
3.51
1.58
1.57
1058
1218
6.91 1.21 851
16.6 1.33 882
2400
G400
- * -
_
1.77 0.70 8.38
-
-
8.02
20.04
-
-
0.82
0.71
-
_
279
587
279
145
52
(0.2-0.3 mm). A weighed sample (not greater than 0.5% by weight of the bed) was dropped into the preheated reactor, producing a negligible drop in bed temperature. Heat transfer in the fluidized bed was very rapid; calculations [13] indicated that when the temperature of the preheated bed is 500°C it requires about 1 second to heat a 0.2 mm particle of an Israel oil shale to 495°C (which is 5 “C below the final temperature) and that the temperature is essentially uniform within the particle during the heat-up. The volatile products were rapidly swept by the fluidizing gas (nitrogen) to a flame ionization detector (FID). The FID produced a signal proportional to the concentration of the total organic carbon (TOC) in the product stream. Since the reaction is relatively fast at the beginning and much slower at the end, the speed of the recorder paper was decreased as the reaction progressed. The slow decay of the reaction rate makes it difficult to determine the termination point of the reaction, which leads to an appreciable under-
-----
Sample
inlet
filter
Preheater and metal sieve
device
Spent
Dust
shale
Oven
b-
rFiG-l
, meter
I
IL-l_
1 Manometer
(
( Temperature 4 I&
,/ Thermocouple
Fig. 1. The fluidized-bed reactor system.
controller
)
(Eurotherm
8101
1
53
estimation of the total reaction time. It was therefore imperative to continue the reaction until the FID output returned to the background value prior to the start of the run. A measured amount of n-butane was injected into the reactor (through the calibration gas inlet) before and after each experiment. This calibration permitted quantitation of the detector response, estimation of the time lag of the system and the degree of the peak broadening (convolution). It was found that a pulse of calibration gas which is injected into the reactor within 0.8 s was monitored by the FID immediately as a sharp peak which was 1.5 s wide at half height. This result, together with the temperature rise time calculations [13], indicates that fast reactions (with a half-life of a few seconds) can not be followed by this system. Therefore the temperature range studied was limited to 500” C (upper limit). At this temperature the half-life for all the samples studied was longer than 5 s. The experimental curves were digitized and the data processing was carried out by a computer. This experimental system is actually an improved version of a system which has been described previously [4]. The main improvements are: (1) better stabilization of the reactor temperature, (2) minimization of the temperature gradients in the fluidized bed, (3) minimization of the ‘dead volume’ of the system, and (4) addition of a water manometer for better control of the FID/vent ratio. These improvements were probably responsible for the slight differences between the present results and those presented previously [4]. Preparation
of samples pre-exposed
to elevated temperatures
A bed made of NaCl (crystalline) was introduced into the reactor through the removable top (Fig. 1); a sample of ca. 5 g was slowly introduced in portions into the preheated reactor, in order to keep the reactor temperature stable. The reaction time was adjusted to be twice the time required for the completion of the kinetic experiments (t + 00). The reactor was then cooled to 100 o C (under the nitrogen stream). The sample was separated from the NaCl bed by dissolution in water. Determination
of the total yield
The estimation of the total amount of organic carbon produced during a by difference as follows; a long exposure time (t + 00) was performed sample was exposed to elevated temperature as described above, then the contents of the reactor (i.e. the bed and the residue) were extracted with benzene-methanol (7 : 3) overnight. After removal of the solvents, the NaCl bed was dissolved in water, the solid residue was treated with HCl, and the organic carbon content of the carbonate-free residue was determined. The total yield of organic carbon was calculated by difference: (TOC of the original sample) - [(TOC in the solid residue ) + (TOC of the extractables)]. The amount of extractables was negligible.
54
Experiments with samples pre-exposed to 400 OC The samples were prepared as described above. In order to compare the original matter and the pretreated samples under the same experimental conditions the experiments were carried out in tandem (a pair of experiments at each temperature).
RESULTS AND DISCUSSION
The dependence of the total yield on temperature In our previous work [4] we reported that the total amount of products formed during a long period of heating (t + 00) increased with a rise in the temperature. This was obtained by calibration of the area under the FID curves. In the present work the total amount of carbon produced was estimated to be the difference between the organic carbon content of the original sample and that of a sample which was expoed to elevated temperature (see Experimental). The results of such experiments for one sample of coal and a sample of oil shale are summmarized in Table 2. These indicate that the ultimate yield of the Israeli oil shale increases with rising temperature until the point of complete decomposition of the kerogen is reached at about 425 o C. Weitkamp and Gutberlet [14] reported that the ultimate yield of the Green River oil shale increased exponentially with temperature until 400 o C. Allred [15] also reported that the total yield of the Green River oil shale increased with rising temperature. Table 2 indicates that the ultimate pyrolyzate yield from Pretoria coal increases with reactor temperature, in the temperature range studied. This result is in agreement with the results of Pitt [16], Anthony et al. [17] and others for various coal samples. Inspection of the NaCl in water solutions indicated that there is almost no char on the NaCl bed. This result supports the findings of Campbell et al. [18] who indicated that charring of the products is minimized when fast heat-up and high sweep rates are applied.
TABLE 2 The ultimate yield of Pretoria Coal (P) and Zefa Efe temperature range
oil shale (Z) at the 400-500”
Organic carbon volatilized (%) Z P
400°C
425OC
450°c
475OC
5oo”c
55.4 12.4
66.5 17.1
66.5 18.6
66.5 21.7
67.6 25.2
C
55
These experiments were conducted on an NaCl bed rather than on spent shale. We assumed that the dependence of total yield on temperature will manifest the same overall relations for different beds. However, the possible influence of the bed on the reaction needs careful examination, particularly the influence of the bed on coal samples which undergo softening at elevated temperatures. Kinetic results
Since the FID measures total organic carbon [19], the FID output is directly proportional to the generation rate of volatile organic carbon (dA/dt). The amount of organic carbon produced at any time t (A(t)), is proportional to the integral of dA/dt from t = 0 to t (i.e. the area under the curve from t = 0 to t). Conversion is defined as A( t)/A( t + co); according to this definition each isothermal experiment reaches 100% conversion. This definition is very useful, even though it has been pointed out that the total amount of product A( t -+ co) increases with increasing reactor temperature (see Table 2). Fig. 2 shows curves of reaction rate versus time (these curves
100
200
300
1300
J
,ll.,ll,l,,..I,IIII,II1,,,,,,
0
2300
#
loo
200
300
1300
3300 TIME
0
w 0
ID0
loo
200
300
I300
2300
4300
(s)
Fig. 2. Curves of reaction rate versus time at 450 o C. These are fundamentally the experimental FID curves, the arbitrary FID response scale was changed into a reaction-rate scale by adjusting the area under the original curves to 100%.
56
Ti ME (5)
Fig. ‘3. Curves of conversion versus time (log scale). ( -) (---) 45ooc, f-----) 475*c, (--.‘-.‘._) 5oo*c.
4OO”C, (---)
425’C,
were obtained by normalization of the area under the FID curves to 300%). It is obvious that different samples behave differently. No differences between the kinetic behaviour of the original and the extracted samples were observed. Fig. 3 shows the curves of conversion versus time (log scale). It is obvious that the temperature has a marked influence on the reaction rate, thus the half-lifetime (t,,,) of all the samples at 500” C is measured in seconds whereas the t,,, at 400 * C is measured in thousands of seconds. It is noteworthy that the Pretoria coal (P) exhibits a different thermal behaviour over the 400-425°C range (this surprising beha~our was reproduced over several sets of experiments). The activation energy of a chemical process if often estimated by plotting - ln( t,,,) against the reciprocal of the temperature. The pyrolytic reaction is fast at the be~nning and slow at the end (Fig. 2), therefore we have also used t,,, for E, estimation (t,,, is the time elapsed when the reaction reaches 75% conversion: Fig. 3). We should point out that the choice of t,,,, or t,,, is an arbitrary one; t,,, or t,,, could be used as well. In Fig. 4 the
57 a
A
1
-2-
-hJ \ d c, -4 C l-i i -e-
-6’,,,,
,l),/,,,‘/,,,,,
I()‘
1.25
1.30
1.43
1.35
l/T
1.50
( 103;;eoj
-2
B
1
-4-
-v ;;; ct -@C d
I
-8 J
-10
, 1.25
,
,
,
, 1.30
,
,
,
, 1.35
i/T
(
,
,
,
3 1.40 (IO /K )
,
,
,
,
1.45
,
,
,
1.50
Fig. 4. (A) -In fl,,* versus the inverse temperature ( f,,, = the time passed until the reaction (0) N, (X) 2, (Cl) C, (A) G, (a) reached 50% conversion). (B) -In t,,, versus l/temperature. k (0) p.
values of - ln( t,,z) and - ln( t,,4) obtained from the curves in Fig. 3 are plotted versus l/temperature. Fig. 4 indicates that the order of reaction speed is: fast reaction 6 N > Z > C = G > I > P -+ slow reaction (i.e. the t,,, or t,,, for P is longer than that for N). The activation energies (E,( t,,,) or ( f3Y4)) and the frequency factors (k,( t,,,) or ( t3,4)) calculated
58 TABLE
3
Activation energies and frequency factors received tsj4 values versus the inverse temperature The data are from the experiments log k, values are in log s--i. N
reported
Z
Temperature range: 400 - 500 o C 38.9 39.8 10.22 10.48
E, (h/s) 1% kl (t*,d Correlation coefficient
-% ($4) log k, (b/4) Correlation coefficient
0.999 38.4 9.62 0.999
0.992 40.3 10.08 0.987
Temperature range: 400 - 450 ’ C 38.3 43.0 E, (h/z) 10.04 8.65 1% x-0 (~,,2~ Correlation coefficient 0.999 0.980 E, ($4) log k, (t3,4) Correlation coefficient
36.5 9.03 0.999
32.6 7.64 0.986
Temperature range: 450 - 500” C 39.9 44.9 E, (h/z) 10.50 11.99 1% ko (t1,d Correlation coefficient 0.999 0.997 & (f3/4) log ko (t3,4) Correlation coefficient
41.4 10.49 0.998
49.6 12.78 0.999
from linear regression
of t,,,
and
in Fig. 3. The E, values are in kcal/mole,
the
G
C
I
39.6 9.99
40.8 10.38
41.8 10.58
0.987 39.5 9.593 0.989
28.2 7.38 0.994 29.4 6.36 0.995
51.0 13.31 0.999 50.0 12.66 0.999
0.998 41.6 10.26 0.997
36.4 9.00 0.999 35.9 8.45 0.998
46.3 11.97 0.999 47.2 11.89 0.999
analysis
P
0.996 41.0 9.82 0.997
39.7 9.91 0.986 36.7 8.51 0.995
46.6 11.96 0.999 47.7 11.81 0.999
42.6 10.69 0.999 37.4 8.65 0.999
are summarized in Table 3. These results were compared with the results obtained from three points (400, 425, 450 OC) and (450, 475, 500 o C): see Table 3. It is obvious that the 450-500 o C range yields E, values which are about 10 kcal/mole higher than those measured in the 400-450°C range. values calculated at the higher temperature range The higher EAti,,, t,J could be related to: (1) the formation of more products at the higher temperature range, which requires higher activation energies. (2) Light-end products are more abundant at high temperatures [2,20]; this may result from the cleavage of the chemical bonds which did not break at lower temperatures, the measured average activation energy therefore being higher.
59
-lot---e-l 0
20
40
60
/
/
80
loo %
-lO40
loo
CONVERSION
Fig. 5. Experimental (dashed) and theoretical (solid) curves of the natural logarithms of k (observed) (s-l) versus conversion. 1 = 400 o C, 2 = 425 o C, 3 = 450 o C, 4 = 475 o C, 5 = 500°C.
(3) This phenomenon could also stem from the loss of selectivity with temperature increase. For competing reactions, the higher the temperature, the reactions with higher activation energy become more dominant. (4) It will be shown later that the shape of the pyrolyzate evolution curve changes with temperature (in our terminology: the contribution of fast reactions increases with temperature). Global parameters, such as t,,, or f3,++,do not account for this change in the reaction profile. The activation energies calculated from the half-lives (Table 3) range from 38 to 56 kcal/mole; these values indicate that diffusion or volatilization are probably not the rate-determining steps. This conclusion is supported by the results obtained by other investigators: Ekstrom and Callaghan IS] prepared samples of shale oil absorbed on spent shale and compared the kinetics of the shale oil release with the kinetics of fresh oil shale pyrolysis. They found that, at 465”C, the time necessary for the shale oil release is negligible as compared with the reaction time. ~harlesworth [7] compared the kinetics of n-octatriacontane (C,,) release from spent shale and observed the same phenomenon (in the 350-450 ’ C temperature range). Gas chromatographic analyses of the pyrolyzate obtained under fluidized bed conditions from
60 TABLE 4 Results of fitting eq. 2 to the experimental curves These results are presented graphically in Fig. 5. Temp. PC)
-In k, (-ins-I)
-Ink, (-In s-l)
-In k, (-Ins-‘)
A, (%)
A2
A3
(%)
(%>
N
400 425 450 475 500
0.60 - 0.50 - 0.50 - 0.70 0.90
3.30 2.50 2.80 2.34 2.10
6.51 5.50 4.80 3.98 3.55
19.5 15.5 15.0 11.0 28.0
10.0 11.0 22.0 38.0 40.0
70.5 73.5 63.0 51.0 32.0
Z
400 425 450 475 500
2.40 2.10 1.10 0.70 0.15
4.60 4.10 3.20 2.70 2.20
6.94 6.38 5.39 4.69 3.90
16.0 22.0 20.5 25.0 34.0
25.0 25.0 20.0 30.0 35.0
59.0 53.0 53.5 45.0 31.0
G
400 425 450 475 500
2.50 2.00 2.00 1.20 0.80
4.80 3.90 3.50 3.41 2.85
7.13 6.40 5.63 4.76 4.40
7.0 6.8 6.3 6.5 11.5
9.0 6.0 9.5 40.0 65.0
84.0 87.2 84.2 53.5 23.5
C
400 425 450 475 500
1.00 1.50 1.20 1.50 2.30
3.90 4.60 4.60 3.80 2.80
7.19 6.47 5.68 5.10 4.45
6.0 6.8 4.6 3.0 7.0
8.9 21.0 50.0 74.5 75.0
85.1 72.2 45.4 22.5 18.0
I
400 425 450 475 500
3.10 1.50 1.20 0.00 -0.10
5.20 5.20 4.65 3.80 2.90
7.90 7.10 6.38 5.67 4.72
12.5 13.2 15.0 13.0 12.0
10.0 33.0 36.0 48.0 55.0
74.5 51.0 49.0 39.0 33.0
P
400 425 450 475 500
2.90 2.90 3.80 3.10 2.20
5.20 5.23 5.20 4.40 3.70
7.35 7.40 6.54 5.94 5.54
4.5 4.0 17.0 25.0 36.0
17.0 19.5 39.0 37.0 35.0
78.5 76.5 44.0 38.0 29.0
U.S. [2], Israeli [20], and Australian [21] oil shales indicate that the boiling points of the pyrolyzates are lower than that of C,,; these results indicate that there is probably no holdup of pyrolyzate in the liquid state. Gas chromatographic analyses [20] of pyrolyzates which were sampled at different time intervals during the reaction also indicated that the reaction is not diffusion controlled. The above discussion supports the idea that the products formed from the kerogen at the range of temperatures studied diffuse immediately from the particle into the fluidizing gas stream, and are
61
instantaneously purged. Thus there is no holdup due to interaction with the support bed particles. However, secondary cracking in the gas phase, on the fluidized bed support, or on the hot metal tubing leading to the FID probably does occur. Nevertheless, this secondary cracking would not affect the kinetics measured by the FID, since the FID measures total organic carbon. Fig. 5 shows experimental and theoretical curves of In k versus conversion, where k is the observed reaction rate constant assuming a first-order reaction. The theoretical curves were obtained by fitting the three parallel first-order reactions model to the experimental curves. The proposed model and the data analysis procedure have been described in detail [4], therefore only a brief description will be given here: the experimental reaction rate constant k was calculated according to the equation: dA/dt k=
[A(t+
co) -A(t)]”
Where dA/dt is the height of the curves in Fig. 2, [ A( t + co) - A(t)] is the area under the curve at the right side of the observed point (i.e. the area under the curve from t to the end of the experiment), and n is the reaction order (n = 1 was assumed). According to the proposed model [4], the evolution of pyrolyzates is described as a linear combination of three parallel first-order reactions: A(t)
= i A*[1 -exp(-kg)] i=l
(2)
-3-
‘\ -a-
),,, 1.25
‘w.l
,,,,,,,,,,,,,,,,,,,, 1.30
1.35
i/T
1.40
1.45
i.50
IlO3 /K 1
Fig. 6. Natural logarithms of the observed rate constant at the end of the reaction ( k3) versus the inverse temperature. (0) N, (X) Z, (a) C, (A) G, (0) I, (0) P.
62
where AIT = the contribution of the ith reaction to the combined reaction and ki = the rate constant of the i th reaction. The results of the curve fitting procedure are summarized in Table 4. The experimental curves in Fig. 6 indicate that at the early stages of the reaction, k (observed) is not constant (k decreases with conversion). That is, the reaction cannot be described as a first-order reaction in the early stages. However, at high conversion levels all samples exhibit first-order behaviour (k becomes constant). This discrepancy is more obvious at low temperatures (i.e. the deviation from a single first-order reaction behaviour increases with temperature). This behaviour is expressed in Table 4 by the decrease with temperature of the slow reaction contribution A, and the increase of the fast reaction contributions A, and A, with temperature. This phenomenon could be explained by: (1) the general concept: loss of selectivity with the elevation in temperature; and (2) the enhanced cracking at high temperature (which brings about the participation of more types of chemical bonds in the reaction). This result is in agreement with the results of Ekstrom and Callaghan [8] but different from the results obtained by Braun and Bumham [5], who have interpreted Richardson’s data by two parallel first-order reactions with constant contributions of 75 and 25%, respectively, over the 490-540 o C temperature range. The values of -In k, from Table 4 were plotted against the inverse temperature; the activation energy values ( Eno2,3j), the frequency factors
TABLE 5 Activation energy and frequency factors received from linear regression analysis of k,, k,, k, values (from Table 4) versus the reciprocal of the temperature The E, values are in kcal/mole, the k,
values are in s-l.
N
Z
G
C
I
P
E, (k3) log k; (k,) Correlation coefficient
30.8 7.25
32.0 7.53
29.3 6.46
28.3 6.08
32.1 6.99
26.5 5.16
E, (k2) log k, (kz) Correlation coefficient
* *
E, (k,)
*
0.996
25.6 6.32 0.878
0.995 24.3 6.83
l
Correlation coefficient
0.996
0.047
0.987
0.996 18.2 3.93
0.999 *
0.963
24.4 5.53
l
0.996 17.2 4.49
0.996
0.609 * 0.817
22.8 4.81
0.943 32.8 9.49
l
0.991
0.965
0.943 * * 0.525
* When the correlation coefficient is less than 0.9 the results are meaningless. For P, only four points 425-500 o C were selected.
63
(AY~~,~,~)) and the correlation coefficients obtained for the three parallel reactions are summarized in Table 5. These indicate that for k3 values which are expe~mentally obtained (k, is the value of k (observed) (in Fig. 4) at high conversion levels, see ref. 4), there is a good correlation for all of the samples (with the exception of P in the 400-425OC range). The rate constants k, and k,, which are probably chemically meaningless in the strict sense (since they were derived by regression analysis), exhibit a good match for samples Z, G and I. However, samples C, N and P exhibited unsatisfactory correlation, despite the fact that at each isothermal temperature there is a good match between the experimental and the theoretical curves. The E;, values obtained by this method (Table 5) are much lower in comparison to those derived from t,/, or t,,, (Table 3). This is probably due to the fact that (according to this model), the contribution of fast reactions increases with temperature. It is noteworthy that Braun and Burnham [5], who interpreted Richardson’s data 121 according to a two parallel first-order reactions model, obtained much higher activation energy values for k, and k, (50.3 and 46.1 kcal/mole, respectively). These values are also higher than the values obtained by Ekstrom and Callaghan [8]. The differences between the three studies probably stem from the fact that, according to the Braun and Bushy model, the contributions of the fast and slow reactions do not change with the temperature. Comparison of Fig. 6 with Fig. 4 reveals that the order of the reaction rate according to k, is the same as according to t,,, or t,,, i.e. N > Z > C = G > I > P. However, the activation energies obtained from t,,,, and t,,, are higher than the values found from k,, k,, k, and are closer to the values expected for chemical bond cleavage. Fig. 7 shows the experimental curves of conversion versus time from Fig. 2 together with curves calculated by the equation : A(t)
=
iA:{l-exp[-(I&,, exp(-E,,/RT))t]) i=l
The EGj and k,i values were taken from Table 5; Af values were taken from Table 4. Fig. 7 exhibits a good correlation between the experimental and the theoretical curves. It must be emphasized that, since the equation is empirical, it might be valuable for retort modelling calculations. However, the chemical significance of the derived kinetic parameters (i.e. E, and k,) is still questionable. Figs. 8-10 demonstrate the differences among the various samples. In Fig. 8 the same order of kinetics as stated above is shown (N > Z > C = G > I > P). Fig. 9 compares the curve of In k versus conversion of the various samples at 450” C. It indicates that the samples differ one from the other not only in the speed of the reaction (the position of the various curves along the y-axis) but also in the shape of the curve. Fig. 10 shows the deviation of the ‘kinetics from a single first-order reaction behaviour, e.g. in
100 80
60 40 20 0 100
5
tn
80
", 60 =
40
0" 20 s 0 I
TIME(s)
Fig. 7. Experimental (solid from Fig. 3) and theoretical (dashed) time.
curves
of conversion versus
coal I the initial rate constant is fourteen times faster than that of the end of the rection. Samples N and 2 also do not agree with the first-order model, but G, C and P follow the first-order model rather closely. The data presented by us so far indicate that much of the difference in the kinetic data and the kinetic parameters, which were obtained by various authors [l--S], can be attributed to differences in samples. However, in view of the close resemblance of the kinetic behaviour of samples G and C (both from the Green River Formation), it is interesting to compare the four studies which used the Green River oil shale as a feedstock, and fluidized bed-attached FID technique. Table 6 compares the values of k(fina1) and E,(final) obtained in the different studies, where k(final) is the observed rate constant (assuming first-order reaction) at the end of the reaction (i.e. k, in the present study). Both Wallman et al. [l] and Richardson et al. [2] obtained k(fina1) by est~ating the slope of the second strict-line segment on In{1 - conversion) versus time curves. This method is equivalent to the method used in the present study, i.e. the value of k, was obtained directly
65
o-
1
0
I
I
I
I
I
I 600
400
200
I
I
I iooo
800
TIME (s) Fig. 8. Curves of conversion versus time at 450 o C. (- . -. -) N, (- . . -. . -) Z, (- - -) C, (---) G, (-.-.---) I, () Pm
from the curves of In k versus converstion (Fig. 4). Table 6 shows that k(final) values obtained by Richardson are slightly higher than those observed in the present study. This small but consistent difference could stem from a constant difference of about 10” C between the two fluidized bed temperatures. The values of the activation energies are very similar. Wall-
-27
-7
1 0
I 20
I
I
I
~CWERS~N
I
I
I 80
I
I 100
%
Fig. 9. Curves of In k versus (---)G,(-------)I,(-)P.
conversion
at 450 o C. (-. -. -) N,
(-..-..-)
z,
(---)
c,
66 15-
3
lo-
2
-
t=
e Y
5-
0
20
SO %
100
4COON”ERs&N
Fig. 10. Curves of k/k(final) versus conversion. The k values from Fig. 9 were divided by the value of k at the end of the reaction. k/k(final) is an indicator of the deviation of the observed k from the first-order behaviour observed at the end of the reaction. Lines as coded for Figs. 8 and 9.
man obtained a somewhat lower E, value. However, Braun and Burnham [5], who reinterpreted Richardson’s data with a type of ‘blind regression analysis’ computer program, obtained a much higher E, value. This comparison indicates that the data analysis procedure itself might lead to different results. At the outset of this work we expected to find a relationship between the samples’ elemental analysis (i.e. H/C ratio, oxygen and sulphur content or hydrogen index (HI)) and the kinetic behaviour. A definite relations~p was not found. For example, coal P with the low H/C and HI (Table l), exhibits a slower kinetic behaviour than coal I. However, the kinetics of G and C oil shales, with the high H/C and HI, were slower than the kinetics of N and Z TABLE 6 Comparison of the rate constants and the E, values received by different authors for the end of the reaction (Green River oil shale) k (final) (s-r x 103)
This
work 460°C 500°C 550°c E, (final) kcal/mole l l
4.9 l 12.3 44.5 ** 29.3
By interpolation (Fig. 6). * By extrapolation (Fig. 6).
Richardson et al. [2] 6.33 18.2 58.6 29.7
Wallman et al. [l] 2.0 4.5 11.16 22.6
Braun and Burnham [ 51
46.1
67
oil shales with lower H/C and HI values. The kinetics of oil shale Z with the high content of oxygen and sulphur, are slower than those of oil shale N, having a lower content of oxygen and sulphur. This lack of correlation between the elemental analysis and the kinetic behaviour probably indicates that the kinetics depend on the structure of the organic matter as well as the distribution of chemical bonds manifested statistically by its elemental composition.
TIME
0
(s)
20
80 %
I
100
~ON”ERSkJ
Fig. 11. Kinetics of the original samples G (solid), compared with a sample pre-exposed to 400 o C G400 (dashed). W-500 o C every 20 o C increment.
-1
-2
~
-3
-4 C l-t -5
-0 Y
~
-7
,
0
I
I
I
1
20
0
I
4CDON”EFlS~N
I
I 80
4
I
100
%
Fig. 12. Kinetics of the original sample 2 (solid), compared 400 o C Z400 (dashed). 440-500 o C every 20 * C increment.
Experiments
with samples pre-exposed
with a sample
pre-exposed
to
to 400 “C
Figs. 11-14 compare original matter to samples preexposed to 400°C. The activation energies and frequency factors obtained from this set of experimental are summarized in Table 7. Figs. 11-13 and Table 7 indicate that kinetic behaviour diplayed by the pretreated samples is slower than that of the original samples. However, the calculated E0 values do not show a definite trend. Fig. 14 shows the curves of k/k(final) of the fresh and the
69 -l-
-2-
+I-6J \ r( u -4C l-l ’
-5-
-e-
-I -7:
II*I~
1.20
1
1.30
1.32
I
1.34
l/T
II0
$3, /K
’ 1.9, ’ Lb0 ’ 1
I!42
Fig. 13. Arrhenius plots of r1,2 values of G and Z compared with G400 and 2400. (0) G, Z, ( x ) G400, (0) 2400.
(A)
pretreated samples at 460 o C. The kinetic behaviour of the fresh Israeli oil shale (Z) deviates strongly from first-order behaviour; it seems that the pretreatment changed this behaviour quite dramatically, and the kinetic behaviour of the pre-exposed samples (2400) follows the first-order model
\ \ \ \
O II
0
I
1
20
I
1
1
4CmJ”ER&N
1
,
I
80
8
I
100
%
Fig. 14. Curves of k/k(final) (-.--.--) G400, (---) 2400.
versus conversion
at 460 o C. (-
) G, (---)
Z,
70 TABLE 7 Activation energy and frequency the original samples
factors
for samples
pre-exposed
to 400°C
compared
with
The E, and k, values were received by linear regression analysis of t,,, and t,,, values versus the reciprocal of the temperature. The data are from the experiments reported in Figs. 12 and 13 (440-500 o C, every 20 o C increment). 2400
Z
G400
G
E, (+) log kll (h/2) Correlation coefficient
41.0 10.18
45.5 12.21
55.3 14.26
50.8 13.22
E, ($4) log k, (t3,4) Correlation coefficient
37.5 7.34
0.997
0.999
0.992 50.5 13.13 0.998
0.999 55.0 15.03 0.998
rather closely. The Green River fresh sample (G) follows model quite closely, as does the pre-exposed sample (G400).
0.999 52.0 13.19 0.999
the first-order
ACKNOWLEDGEMENT
This work was partially supported by a grant from the Israel NCRD German BMFT (KFA-Jiilich & Bergbau Forschung).
and
REFERENCES 1 P.H. Wallman, P.W. Tamm and B.G. Spars, in H.L. Stauffer (Editor), Oil Shale Tar Sands and Related Materials, ACS Symposium Series 163, American Chemical Society, Washington, DC, 1981, p. 93. 2 J.H. Richardson, E.B. Huss, L.L. Ott, J.E. Clarkson, M.O. Bishop, J.R. Taylor, L.J. Gregory and J.C. Morris, UCID-19548, Lawrence Livermore National Laboratory, 1981. 3 H. Bar, MSc. Thesis, The Hebrew University of Jerusalem, April 1984. 4 H. Bar, R. Ikan and Z. Aizenshtat, J. Anal. Appl. Pyrolysis, 10 (1986) 153. 5 R.L. Braun and A.K. Burnham, Fuel, 65 (1986) 218. 6 S.D. Carter, Prepr. Am. Chem. Sot., Div. Pet. Chem., 32(l) (1987) 133. 7 J.M. Charlesworth, Ind. Eng. Chem. Process Des. Dev., 24 (1985) 1125. 8 A. Ekstrom and G. Callaghan, Fuel, 66 (1987) 331. 9 N.Y. Nsakala, R.H. Essenhigh and P.L. Walker, Combust. Sci. Technol., 16 (1977) 153. 10 A.W. Scaroni, P.L. Walker, Jr. and R.H. Essenhigb, Fuel, 60 (1981) 71. 11 Z. Aizenshtat, Israel Chem. Sot. Bull., 3 (1980) 266. 12 K.E. Gokler, T.J. Stoecker and R.F. Baddour, Ind. Eng. Chem. Prod. Res. Dev., 23 (1984) 308. 13 G. Esterson and Z. Aizenshtat, Fate of Organic Sulfur in U.S. and Israeli Oil Shales, Annual Report No. 8610, Prepared for Department of Energy USA and Ministry of Energy and Infrastructure Israel, 1986, p. 143.
71 14 15 16 17 18 29 20 21
A.W. Weitkamp and L.C Gutberlet, Ind. Eng. Chem. Process Des. Dev., 9 (1970) 386. V.D. Allred, Chem. Eng. Progr., 62 (1966) 55. G.J. Pitt, Fuel, 41 (1962) 267. D.B. Anthony, J.B. Howard, H.C. Hottel and H.P. Meissner, Fifteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1975, p. 1303. J.H. Campbell, G.H. Koskinas, T.T. Cobum and N.D. Stout, UCRL-52256, Lawrence Livermore National Laboratory, 1977. R.A. Jones, An Introduction to Gas-Liquid Chromatography, Academic Press, New York, 1970. H. Bar, R. Ikan and Z. Aizenshtat, J. Anal. Appl. Pyrolysis, 10 (1986) 167. J.M. Charlesworth, Ind. Eng. Chem. Process Des. Dev., 24 (1985) 1117.