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Kinetic studies on Victorian brown coal hydroliquefaction
Kinetic studies on Victorian hydroliquefaction”
brown
coal
Yukio Nakako, Shinich Katsushima, Shinich Oya, Toshiaki Okui, Tetsuo Matsumura, Toshio Osawa, Kaisaburo Saito, Akihiro Mawashima and Nobuo Tanaka Nippon
Brown
Coal Liquefaction
Co Ltd, 5-20
Yaesu, 1 -Chome,
Chuo-Ku,
Tokyo, Japan
Reaction kinetics of the liquefaction of Victorian brown coal in a process development unit (PDU) having three reactors in series have been studied at temperatures of 430_47O”C, and pressures of 15-25 MPa. It is shown that the rate of hydrogen consumption can be expressed as a function of the concentrations of coal and catalyst, hydrogen partial pressure, reaction temperature and residence time, and is controlled by the rates of hydrogenation of polynuclear aromatic components, and the rates of formation and stabilization of radicals. The relative contribution of these reactions, at any temperature, determine the influence of the hydrogen partial pressure on the rate of the hydrogen consumption. The kinetics of the decomposition reactions of brown coal to preasphaltene, asphaltene and to oil also have been studied. The apparent activation energies determined are 25 kJ mol-’ for the brown coal to preasphaltene, 50 kJ mol-’ for preasphaltene to asphaltene, 76 kJ mol-’ for asphaltene to oil, and 184 kJ mole-’ for oil to gases. (Keywords:
brown
coal; hydroliquefaction;
reaction
The hydroliquefaction of brown coal involves the thermal decomposition of the complex molecular structure of the coal in the presence of a hydrogen-donor solvent and hydrogen at high pressure. Relative to bituminous coals, brown coals are richer in oxygen and the variety of functional groups, and respond more rapidly to heating.’ Consequently, the hydroliquefaction reactions of brown coals, can be expected to differ also. In hydrohquefaction of brown coals, the rapid thermal decomposition will be followed by rapid condensation reactions of the primary products unless the availability and transferability of reactive hydrogen is sufficiently high. This Paper reports the results of a study of the hydroliquefaction brown coal with regard to the influence of catalyst concentration, reaction temperature, and hydrogen partial pressure, on the rates of hydrogen consumption and thermal decomposition of the coal, and of the intermediate products. EXPERIMENTAL A brown coal from Morwell, Victoria, was used; the analyses of the coal are given in Table 1. The solvent was the steady-state recycle solvent formed in the same continuous PDU facility as used in this study, during the hydrogenation of Morwell coal under conditions of temperature, pressure, etc. similar to those used in these experiments. Iron oxide and sulphur were used as catalyst. Test facility and procedure The brown coal liquefaction experiments were carried outina0.1 tday-’ PDU facility, installed at Kobe Steel, * Presented Liquefaction’,
at International Lorne, Victoria,
Workshop on the ‘Science Australia, 2428 May, 1982
w16-2361/82/1009S3-06$3.00 @ 1982 Butterworth & Co (Publishers)
Ltd.
of Coal
kinetics)
Ltd., which has three stirred, complete-mixing, tank reactors in series. Reaction temperatures were between 430 and 47O”C, total pressure between 15 and 25 MPa, residence time between 36 and 90 min and relative catalyst concentration between 0.3 and 1.0. The products from each experiment were fractionated by distillation into naphtha, solvent and bottoms. For each experiment, the coal, feed solvent, naphtha, product solvent and the bottoms were analysed and the mass balance for carbon, hydrogen, nitrogen, sulphur and oxygen, was checked. The hydrogen consumption was calculated from the material and element balances. The distillation bottoms fractions were separated into hexane-soluble materials (oil), hexane-insolubleebenzenesoluble materials (asphaltene), benzene-insolublee pyridine-soluble meterials (preasphaltene) and pyridineinsoluble materials (unconverted coal and char fractions). The kinetics of the decomposition reactions of the coal and the intermediate products was studied using the yield data for the solvent fractionations of the distillation bottoms. RESULTS The results obtained during the 14 experiments are summarized in Table 2 where the data is expressed relative to the catalyst concentration and the calculated hydrogen consumption. Four series of experiments were carried out. The first two series were to determine the effects of temperature and the hydrogen partial pressure, respectively, on the rate of hydrogen consumption, at a constant catalyst concentration and residence time. In the third series, the effect of reducing the catalyst concentration was investigated. In the fourth series, different residence times
FUEL, 1982, Vol 61, October
953
Victorian brown coal hydroliquefaction:
Y. Nakako et al.
were used, with two experiments (IV-l and IV-2) carried out under the mildest reaction conditions with regard to hydrogen partial pressure and catalyst concentration, and the third experiment (IV-3) carried out under severe reaction conditions with regard to all three variables. The average values of the hydrogen partial pressures at the inlet and the outlet of the three reactor system are given in Table 2. The effect of the residence time on the hydrogen consumption, obtained at the mildest and the most severe reaction conditions with regard to temperature and pressure in Table 2, are given in Figure I. At the mildest reaction condition (430°C and 15 MPa) the increase in hydrogen consumption is proportional to residence time. However, at the most severe reaction condition (470°C and 25 MPa), the increase in hydrogen consumption with the residence time is initially rapid but then decreases, following the thermal decomposition reactions of the coal and the intermediate products, which are rapid and approach completion in a short residence time. The effect of temperature on the rate of hydrogen consumption at total pressures of 20 MPa and 25 MPa is shown in Figure 2. The increase in the hydrogen consumption with increase in temperature from 430 to 460°C is small at both pressures, with the hydrogen consumption being higher for the higher pressure. at >46O”C, the hydrogen consumption However, increases markedly at the lower total pressure and when the temperature approaches 47O”C, exceeds that obtained at 25 MPa.
reaction temperature and the hydrogen partial pressure. The liquefaction of coal is believed to involve many reactions, such as the formation and stabilization of radicals and the hydrogenation of the hydrogen-donor components, which proceed simultaneously, and the kinetic equilibrium condition established for these which controls the rate of hydrogen reactions, consumption. 201 c
P
i
&5-
I
0.5
0
IO
15
20
Residence time (hf
Figure7 0,470’C
Effect of residence time on hydrogen and 25 MPa; a, 43O’C and 15 MPa
consumption.
DISCUSSION Model for coal liquefaction reactions and the rate of hydrogen consumption A simplified model of the liquefaction of coalzB6 is used to correlate the rate of hydrogen consumption with the
Tab/e
I Representative
analysis of Morwell Elemental
brown coal
compositionjwt
% dry basis)
Ash (wt %daf)
C
H
N
S
0
3.9
68.0
4.7
0.5
0.6
26.2 (diff.)
Tab/e2
Reaction
conditions
and hydrogen
0’
I
I
420
430
440 Reaction
450
460
temperature
470 F’C)
Figure2 Effect of reaction temperatures on hydrogen tion. 0,20 MPa;O, 25 MPa; residence time, 60 min
Relative catalyst concentration
Residence time (min)
470 460 450 44.0 430
147 158 161 165 166
1 .o 1 .o 1 .o 1 .o 1 .o
60 60 60 60 60
(3) (4)
470 460 450 430
186 197 197 199
1 .o 1 .o 1 .o 1 .o
60 60 60 60
1.25 1.08 1 .lO 1.04
III
(1) (2)
450 450
164 164
0.6 0.3
60 60
0.91 0.79
IV
11) (2) (31
430 430 470
126 122 176
0.3 0.3 1 .o
36 54 90
0.29 0.46 1 .41
I
(1)
(2) (3) (4) (51 II
(1)
(2)
954
FUEL, 1982,
Reaction 1°C)
consump-
consumption Average hydrogen partial pressure
SerieslExp. No.
480
temperature
Vol 61, October
Relative hydrogen consumption 1.28
1 .04 1 .oo 0.94
0.88
Victorian brown coal hydroliquefaction: Radicalformation reaction. The radicals are formed by the thermal decomposition of the coal (B), the intermediate products (RR, RH), and solvent (S):
Y. Nakako et al.
kDHDH2Ae(i + kRDHRY!e) - $bH,,Ae AH= 1 +~~+k,,,,R$6’
(*I
ku B+RR+RH+S-mR. Heat
(1)
Radical stabilization reaction. Some of the radicals produced by reaction (1) are stabilized by extracting hydrogen from coal and intermediate products, but most are stabilized by the reactions with donor molecules (DH) and with the radical (R.) itself:
R.+DH
k RDH -RH+D
(2)
where: K, =kDH/k_DH. k If FAtJ > kRDHRm + 1, equation equatidn
AH = DH(1 + kRDHRm) - DH,
(3)
Formation of hydrogen donors. Polynuclear aromatic derived from coal during thermal molecules, decomposition or present in the solvent, are hydrogenated and converted to donor molecules (DH) by reaction (4) and, in part, to naphthenic molecules (DHH) by reaction (5):
-DH
DT K, H,
kDH&AeB (4)
(10)
l+K,Hz+K,K,H,
where: DT, total of the concentrations of polynuclear aromatic donor, and naphthenic components (DH+DHH); K1 =kDH/k-DH; and K, =kDHH/k-DHH. Equation (9) and (10) predict that the rate of hydrogen consumption decreases with increase in hydrogen partial pressure when:
k DH D+H,+
(9)
The concentration of donor component (DH) approaches DH, at equilibrium and is expressed by equation (10): DHE =
RR RR
to
(9):
k R,+R.-
(8) is converted
However, converted
if k&k@8 to equation
kRDHRm + 1
<< kRDHRm (11):
+ 1, equation
(8) is
k-D, k DHH DH+H,:
-DHH k-
AH = ‘hi D H, Ad -
Hydrogen is consumed in the hydrogenation of the polynuclear aromatic molecules (D) by reaction (4) and donor molecules (DH) by reaction (5). If the rate of decomposition of naphthenic molecules is very small, reaction (5) attains equilibrium rapidly and hydrogen consumption by the reaction can be neglected. When the coal liquefaction reactions are carried out in the completemixing, tank reactor for residence time de, the hydrogen consumption (AH) is expressed by equation (6):
where: kDH, k_DH, respectively, the rate hydrogeneration of polynuclear aromatic and the reverse reaction (reaction (4)). The concentration of donor components by equation (7): DH=
km DH,Ae+DH, 1 +(kDH + kRDHR.)Ad
kRDHR,
(11)
(5)
DHH
AH = kDHD H,Ae - k_DH DHAB
k-DHD&,
(6) constants of components is expressed
Equation (11) predicts that the rate of hydrogen consumption increases with increase in hydrogen partial pressure. Thus, experimental observation of the dependence of the rate of hydrogen consumption on the hydrogen partial pressure, can be used to indicate which reaction step controls the rate of hydrogen transfer in the coal liquefaction reaction. Derivation of an experimental equation for the rate of hydrogen consumption The rate of hydrogen consumption as expressed by equation (8), which was derived on the basis of the proposed model for coal liquefaction, is complicated and difficult to apply in practice. Thus, the experimental expressions of equations (i2) and (13) are derived as a function of K, coal concentration (B), catalyst concentration (C), and hydrogen partial pressure (PH). K incorporates the rateconstants kDH,kRDH, kR and kRR, relating to the radical concentration and the chemical equilibrium constant (K,), and is a function of temperature:
(7)
where: kRDH,the rate constant in the radical stabilization reaction (2) of radical with donor molecule; DH, and R., the concentration of donor components in the feed and the radicals (reaction (l)), respectively. From the equation (6) and (7), the hydrogen consumption is expressed by equation (8):
AH ,,=KBCnP;; K, a constant as:
at constant
K
temperature,
=
A
(12) can be expressed
e-WRT)
FUEL, 1982, Vol 61, October
(13)
955
Victorian brown
coal hydroliquefaction:
Log relative
catalyst
Figure3 Effect of catalyst concentration 450” C, 20 MPa and for 60 min
Y. Nakako
et al.
concentration on rate of reaction
at
temperature from Figure 4 is a rectilinear function of the reciprocal of the reaction temperature in Figure 5. The value of m becomes unity at 430°C and zero at 470°C. These values of m indicate that at the lower temperatures at 2 43O”C, the rate determining step in hydrogen transfer reactions is the hydrogenation of polynuclear aromatic components (reaction (4)). At higher temperatures (~470”C), radical formation and stabilization appear to be rate controlling. Thus, on the basis of these results, it seems that the thermal decomposition of most of the coal and the intermediate products of low thermal stability is completed in the early stages of reaction at temperatures of ~470°C and, hence, that the rate of radical formation has decreased to a low level. From this discussion, it can be expected that even at 47O”C, the shorter residence time gives a larger value of m. A plot of log k calculated at each experimental temperature using equation (12), against the reciprocal of the temperature is shown in Figure 6. The inverse rectilinear dependence substantiates the validity of equation (13), and shows that the value of E/R is 104.7 x 103. Rates of decomposition of coal, preasphaltene, asphaltene and oil To assist in the design of an optimum reactor system, it is necessary to understand the factors that influence the rates of decomposition of coal, preasphaltene, asphaltene and oil, as well as the rate of hydrogen consumption. A Ayr (US) using Belle study’ on previous hydrogenated anthracene oil, and hydrogenated phenthanene, as solvents, has reported that different solvent species have different activation energies and frequency factors. This study has suggested that in kinetic studies of coal liquefaction reactions, the solvent should be in equilibrium with the coal. The equilibrated solvent was produced by repeated recycling, using the same coal
(OC) 460
31
470
460
450
440
430
420
I
’ IO
20
15 Log hydrogen
partial
pressure
(MPa)
Figure 4 Effect of hydrogen partial pressure on rate of reaction. ,430”C;0,460°C;0,4600C;~,4700C;residencetime,60 min
q
where: A and E, constants; R, the gas constant; and 7; the reaction temperature. Figure 3 shows the relation between KC” and C, obtained from the results of the experiments carried out with various catalyst concentrations at 45O”C, 60 min and 20 MPa total pressure (B and PH constant). The relation is rectilinear and from the slope a value of 0.19 for the exponent ‘n’ is obtained which reflects the fact that the effect of catalyst concentration on the rate of hydrogen consumption at 450°C is small (cf. Figure 2). Figure 4 shows the relation between KP; and the hydrogen partial pressure P, indicated by the experiments at four reaction temperatures for 60 min with C and B constant. The exponent m calculated at each
956
FUEL, 1982, Vol 61, October
I
I
I 35 Reciprocal
reactlon
I
I
I 40
I 45
temperature
1K-‘l x
10-3
figure 5 Relation between exponent m of hydrogen partial pressure and reaction temperature (equation (12)) for 60 min of residence time
Victorian brown coal hydroliquefaction:
Y. Nakako et al.
(OCI 470
480
460
450
I
430
420
410
400
I
I 35 Reciprocal
440
145 reacbon
I45
temperature
Relation between k and reaction temperature (13)) for 60 min of residence time
I
l-45
reactlon
temperature
Tab/e 3 Activation energies and frequency factors of reactions in the hydroliquefaction of Morwell brown coal
Reaction
Frequency factor (min-l)
I
Coal + Preasphaltene Coa I + Gas Preasphaltene -+ Asphaltene Asphaltene -+ Oil
13.1 755 221 10666
CP
Oil + Gas
5545 x
Brown
&cg
coal
b gases (CO, CO*,
H,O, C,
(K-')xl6"
Arrhenius plots for hydroliquefaction reactions of Figure 7 Morwell coal. A, kcp; 6, keg; C, kpa; D, k,,; E, keg
(equation
as that in the experiments (i.e. Morwell), at 430°C and 15 MPa total pressure, for a residence time of 60 min. To facilitate the analysis of the reaction kinetics, the following simplified reaction sequence occurring during the hydroliquefaction of the coal was assumed:
k
I
I.40
Reciprocal
( K-‘)x103
Figure 6
I
I.35
1
Activation energy (kJ mol-l) 24.95 54.22 66.07 76.03
109
184.43
I Preasphaltene
I kw t
Asphaltene-
k so-oil-
keg
-gases
(C,-C,)
An Arrhenius plot for each reaction in this sequence is given in Figure 7. The apparent first-order activation energy and frequency factor of each reaction, calculated from the Arrhenius plot is given in Table 3. CONCLUSIONS In this study of the hydroliquefaction of Morwell brown coal, it has been shown that the rate of hydrogen consumption can be expressed mathematically as a function of the concentration of the coal and the catalyst, the hydrogen partial pressure, and the reaction temperature. An analysis of a model for coal hydroliquefaction reactions, the experimental results for the rate of hydrogen consumption and the values determined for the exponent m on the hydrogen partial pressure P, in the expression relating the rate of hydrogen consumption to experimental variables, indicate that the rate-determining step in hydrogen transfer at 60 min of residence time is the hydrogenation of polynuclear
aromatic components in the lower temperature region at ~430°C and the radical formation and stabilization reactions in the higher temperature region at ~470°C. The thermal decomposition reactions of the brown coal to asphaltene proceed at a high rate even in the lowertemperature region (~430”C), because their activation energies are in the relatively low range of 255.50 kJ mol-i. ACKNOWLEDGEMENTS The authors thank the Kobe Steel Ltd. Company and SASOL and those researchers whost test results on liquefaction of Victorian brown coal in PDU facility allowed this work to be carried out. REFERENCES Schafer, H. N. S. Fuel 1979, 58, 667 Han, K. W. and Weh, C. Y. Fuel 1979, 58, 779 Neavel, R. C. Fuel 1976, 55, 237 Wiser, W. H. Fuel 1968, 47, 415 Cronauer, D. C., Shah, Y. T. and Ruberto, R. G. Ind. Eng. Chem. Proc. Des. Dev. 1978, 17, 281 Shalabi, M. A., Baldwin, R. M., Bain, R. L., Gray, J. H. and Golden, J. 0. Ind. Eng. Chem. Proc. Des. Dev. 1979, 18, 474 Cronauer, D. C. and Roberto, R. G. EPRI Report AF 913, Final Report under Project 713, 1979 March
FUEL, 1982, Vol 61, October
957
Victorian brown coal ~ydrolj~~e~actjon: Y. Nakako et al. Questions
and Answers
-
Mr
Y. Nakako
Dr Romey What is the activation coal to oil?
energy for direct conversion
from
Mr Nakako The activation energy of direct conversion from brown coal to oil is not estimated, because for brown coal liquefaction using distilled equilibrium solvent and iron catalyst, the major portion of oil is produced via preasphaltene and asphaltene, and the direct conversion of the brown coal to oil has a smaller role in liquefaction kinetics. Prof Trimm Many of your results would be expected if mass transfer was important. What tests for diffusion control were carried out? Mr Nakako The rate of hydrogen transfer will be an important factor in the earlier stage of the liquefaction reactions at higher reaction temperatures (~430°C) and the length of the
958
FUEL, 1982, Vol 61, October
period in which the rate of hydrogen mass transfer is significant will be longer at lower reaction temperature. We have studied the effect of hydrogen mass transfer on the liquefaction results using different types of reactors which give different hydrogen gas hold-up and interface area. Dr Larkins The liquefaction kinetics model you propose is a hydrogen-donor solvent model. It does not allow for a direct role of molecular hydrogen in radical stabilization. As iron catalysts do not readily rehydrogenate the solvent, do you not consider this latter molecular hydrogen route as important at pressure as high as 25 MPa? Mr Nakako Some of the radicals must be stabilized by direct reaction with molecular hydrogen, but as it has a higher activation energy than that of the hydrogens which are catalytically activated and/or reacted with donor solvent, the molecular hydrogen route is considered to be less significant in liquefaction reactions even at pressures as high as 25 MPa.
brown
coal
Yukio Nakako, Shinich Katsushima, Shinich Oya, Toshiaki Okui, Tetsuo Matsumura, Toshio Osawa, Kaisaburo Saito, Akihiro Mawashima and Nobuo Tanaka Nippon
Brown
Coal Liquefaction
Co Ltd, 5-20
Yaesu, 1 -Chome,
Chuo-Ku,
Tokyo, Japan
Reaction kinetics of the liquefaction of Victorian brown coal in a process development unit (PDU) having three reactors in series have been studied at temperatures of 430_47O”C, and pressures of 15-25 MPa. It is shown that the rate of hydrogen consumption can be expressed as a function of the concentrations of coal and catalyst, hydrogen partial pressure, reaction temperature and residence time, and is controlled by the rates of hydrogenation of polynuclear aromatic components, and the rates of formation and stabilization of radicals. The relative contribution of these reactions, at any temperature, determine the influence of the hydrogen partial pressure on the rate of the hydrogen consumption. The kinetics of the decomposition reactions of brown coal to preasphaltene, asphaltene and to oil also have been studied. The apparent activation energies determined are 25 kJ mol-’ for the brown coal to preasphaltene, 50 kJ mol-’ for preasphaltene to asphaltene, 76 kJ mol-’ for asphaltene to oil, and 184 kJ mole-’ for oil to gases. (Keywords:
brown
coal; hydroliquefaction;
reaction
The hydroliquefaction of brown coal involves the thermal decomposition of the complex molecular structure of the coal in the presence of a hydrogen-donor solvent and hydrogen at high pressure. Relative to bituminous coals, brown coals are richer in oxygen and the variety of functional groups, and respond more rapidly to heating.’ Consequently, the hydroliquefaction reactions of brown coals, can be expected to differ also. In hydrohquefaction of brown coals, the rapid thermal decomposition will be followed by rapid condensation reactions of the primary products unless the availability and transferability of reactive hydrogen is sufficiently high. This Paper reports the results of a study of the hydroliquefaction brown coal with regard to the influence of catalyst concentration, reaction temperature, and hydrogen partial pressure, on the rates of hydrogen consumption and thermal decomposition of the coal, and of the intermediate products. EXPERIMENTAL A brown coal from Morwell, Victoria, was used; the analyses of the coal are given in Table 1. The solvent was the steady-state recycle solvent formed in the same continuous PDU facility as used in this study, during the hydrogenation of Morwell coal under conditions of temperature, pressure, etc. similar to those used in these experiments. Iron oxide and sulphur were used as catalyst. Test facility and procedure The brown coal liquefaction experiments were carried outina0.1 tday-’ PDU facility, installed at Kobe Steel, * Presented Liquefaction’,
at International Lorne, Victoria,
Workshop on the ‘Science Australia, 2428 May, 1982
w16-2361/82/1009S3-06$3.00 @ 1982 Butterworth & Co (Publishers)
Ltd.
of Coal
kinetics)
Ltd., which has three stirred, complete-mixing, tank reactors in series. Reaction temperatures were between 430 and 47O”C, total pressure between 15 and 25 MPa, residence time between 36 and 90 min and relative catalyst concentration between 0.3 and 1.0. The products from each experiment were fractionated by distillation into naphtha, solvent and bottoms. For each experiment, the coal, feed solvent, naphtha, product solvent and the bottoms were analysed and the mass balance for carbon, hydrogen, nitrogen, sulphur and oxygen, was checked. The hydrogen consumption was calculated from the material and element balances. The distillation bottoms fractions were separated into hexane-soluble materials (oil), hexane-insolubleebenzenesoluble materials (asphaltene), benzene-insolublee pyridine-soluble meterials (preasphaltene) and pyridineinsoluble materials (unconverted coal and char fractions). The kinetics of the decomposition reactions of the coal and the intermediate products was studied using the yield data for the solvent fractionations of the distillation bottoms. RESULTS The results obtained during the 14 experiments are summarized in Table 2 where the data is expressed relative to the catalyst concentration and the calculated hydrogen consumption. Four series of experiments were carried out. The first two series were to determine the effects of temperature and the hydrogen partial pressure, respectively, on the rate of hydrogen consumption, at a constant catalyst concentration and residence time. In the third series, the effect of reducing the catalyst concentration was investigated. In the fourth series, different residence times
FUEL, 1982, Vol 61, October
953
Victorian brown coal hydroliquefaction:
Y. Nakako et al.
were used, with two experiments (IV-l and IV-2) carried out under the mildest reaction conditions with regard to hydrogen partial pressure and catalyst concentration, and the third experiment (IV-3) carried out under severe reaction conditions with regard to all three variables. The average values of the hydrogen partial pressures at the inlet and the outlet of the three reactor system are given in Table 2. The effect of the residence time on the hydrogen consumption, obtained at the mildest and the most severe reaction conditions with regard to temperature and pressure in Table 2, are given in Figure I. At the mildest reaction condition (430°C and 15 MPa) the increase in hydrogen consumption is proportional to residence time. However, at the most severe reaction condition (470°C and 25 MPa), the increase in hydrogen consumption with the residence time is initially rapid but then decreases, following the thermal decomposition reactions of the coal and the intermediate products, which are rapid and approach completion in a short residence time. The effect of temperature on the rate of hydrogen consumption at total pressures of 20 MPa and 25 MPa is shown in Figure 2. The increase in the hydrogen consumption with increase in temperature from 430 to 460°C is small at both pressures, with the hydrogen consumption being higher for the higher pressure. at >46O”C, the hydrogen consumption However, increases markedly at the lower total pressure and when the temperature approaches 47O”C, exceeds that obtained at 25 MPa.
reaction temperature and the hydrogen partial pressure. The liquefaction of coal is believed to involve many reactions, such as the formation and stabilization of radicals and the hydrogenation of the hydrogen-donor components, which proceed simultaneously, and the kinetic equilibrium condition established for these which controls the rate of hydrogen reactions, consumption. 201 c
P
i
&5-
I
0.5
0
IO
15
20
Residence time (hf
Figure7 0,470’C
Effect of residence time on hydrogen and 25 MPa; a, 43O’C and 15 MPa
consumption.
DISCUSSION Model for coal liquefaction reactions and the rate of hydrogen consumption A simplified model of the liquefaction of coalzB6 is used to correlate the rate of hydrogen consumption with the
Tab/e
I Representative
analysis of Morwell Elemental
brown coal
compositionjwt
% dry basis)
Ash (wt %daf)
C
H
N
S
0
3.9
68.0
4.7
0.5
0.6
26.2 (diff.)
Tab/e2
Reaction
conditions
and hydrogen
0’
I
I
420
430
440 Reaction
450
460
temperature
470 F’C)
Figure2 Effect of reaction temperatures on hydrogen tion. 0,20 MPa;O, 25 MPa; residence time, 60 min
Relative catalyst concentration
Residence time (min)
470 460 450 44.0 430
147 158 161 165 166
1 .o 1 .o 1 .o 1 .o 1 .o
60 60 60 60 60
(3) (4)
470 460 450 430
186 197 197 199
1 .o 1 .o 1 .o 1 .o
60 60 60 60
1.25 1.08 1 .lO 1.04
III
(1) (2)
450 450
164 164
0.6 0.3
60 60
0.91 0.79
IV
11) (2) (31
430 430 470
126 122 176
0.3 0.3 1 .o
36 54 90
0.29 0.46 1 .41
I
(1)
(2) (3) (4) (51 II
(1)
(2)
954
FUEL, 1982,
Reaction 1°C)
consump-
consumption Average hydrogen partial pressure
SerieslExp. No.
480
temperature
Vol 61, October
Relative hydrogen consumption 1.28
1 .04 1 .oo 0.94
0.88
Victorian brown coal hydroliquefaction: Radicalformation reaction. The radicals are formed by the thermal decomposition of the coal (B), the intermediate products (RR, RH), and solvent (S):
Y. Nakako et al.
kDHDH2Ae(i + kRDHRY!e) - $bH,,Ae AH= 1 +~~+k,,,,R$6’
(*I
ku B+RR+RH+S-mR. Heat
(1)
Radical stabilization reaction. Some of the radicals produced by reaction (1) are stabilized by extracting hydrogen from coal and intermediate products, but most are stabilized by the reactions with donor molecules (DH) and with the radical (R.) itself:
R.+DH
k RDH -RH+D
(2)
where: K, =kDH/k_DH. k If FAtJ > kRDHRm + 1, equation equatidn
AH = DH(1 + kRDHRm) - DH,
(3)
Formation of hydrogen donors. Polynuclear aromatic derived from coal during thermal molecules, decomposition or present in the solvent, are hydrogenated and converted to donor molecules (DH) by reaction (4) and, in part, to naphthenic molecules (DHH) by reaction (5):
-DH
DT K, H,
kDH&AeB (4)
(10)
l+K,Hz+K,K,H,
where: DT, total of the concentrations of polynuclear aromatic donor, and naphthenic components (DH+DHH); K1 =kDH/k-DH; and K, =kDHH/k-DHH. Equation (9) and (10) predict that the rate of hydrogen consumption decreases with increase in hydrogen partial pressure when:
k DH D+H,+
(9)
The concentration of donor component (DH) approaches DH, at equilibrium and is expressed by equation (10): DHE =
RR RR
to
(9):
k R,+R.-
(8) is converted
However, converted
if k&k@8 to equation
kRDHRm + 1
<< kRDHRm (11):
+ 1, equation
(8) is
k-D, k DHH DH+H,:
-DHH k-
AH = ‘hi D H, Ad -
Hydrogen is consumed in the hydrogenation of the polynuclear aromatic molecules (D) by reaction (4) and donor molecules (DH) by reaction (5). If the rate of decomposition of naphthenic molecules is very small, reaction (5) attains equilibrium rapidly and hydrogen consumption by the reaction can be neglected. When the coal liquefaction reactions are carried out in the completemixing, tank reactor for residence time de, the hydrogen consumption (AH) is expressed by equation (6):
where: kDH, k_DH, respectively, the rate hydrogeneration of polynuclear aromatic and the reverse reaction (reaction (4)). The concentration of donor components by equation (7): DH=
km DH,Ae+DH, 1 +(kDH + kRDHR.)Ad
kRDHR,
(11)
(5)
DHH
AH = kDHD H,Ae - k_DH DHAB
k-DHD&,
(6) constants of components is expressed
Equation (11) predicts that the rate of hydrogen consumption increases with increase in hydrogen partial pressure. Thus, experimental observation of the dependence of the rate of hydrogen consumption on the hydrogen partial pressure, can be used to indicate which reaction step controls the rate of hydrogen transfer in the coal liquefaction reaction. Derivation of an experimental equation for the rate of hydrogen consumption The rate of hydrogen consumption as expressed by equation (8), which was derived on the basis of the proposed model for coal liquefaction, is complicated and difficult to apply in practice. Thus, the experimental expressions of equations (i2) and (13) are derived as a function of K, coal concentration (B), catalyst concentration (C), and hydrogen partial pressure (PH). K incorporates the rateconstants kDH,kRDH, kR and kRR, relating to the radical concentration and the chemical equilibrium constant (K,), and is a function of temperature:
(7)
where: kRDH,the rate constant in the radical stabilization reaction (2) of radical with donor molecule; DH, and R., the concentration of donor components in the feed and the radicals (reaction (l)), respectively. From the equation (6) and (7), the hydrogen consumption is expressed by equation (8):
AH ,,=KBCnP;; K, a constant as:
at constant
K
temperature,
=
A
(12) can be expressed
e-WRT)
FUEL, 1982, Vol 61, October
(13)
955
Victorian brown
coal hydroliquefaction:
Log relative
catalyst
Figure3 Effect of catalyst concentration 450” C, 20 MPa and for 60 min
Y. Nakako
et al.
concentration on rate of reaction
at
temperature from Figure 4 is a rectilinear function of the reciprocal of the reaction temperature in Figure 5. The value of m becomes unity at 430°C and zero at 470°C. These values of m indicate that at the lower temperatures at 2 43O”C, the rate determining step in hydrogen transfer reactions is the hydrogenation of polynuclear aromatic components (reaction (4)). At higher temperatures (~470”C), radical formation and stabilization appear to be rate controlling. Thus, on the basis of these results, it seems that the thermal decomposition of most of the coal and the intermediate products of low thermal stability is completed in the early stages of reaction at temperatures of ~470°C and, hence, that the rate of radical formation has decreased to a low level. From this discussion, it can be expected that even at 47O”C, the shorter residence time gives a larger value of m. A plot of log k calculated at each experimental temperature using equation (12), against the reciprocal of the temperature is shown in Figure 6. The inverse rectilinear dependence substantiates the validity of equation (13), and shows that the value of E/R is 104.7 x 103. Rates of decomposition of coal, preasphaltene, asphaltene and oil To assist in the design of an optimum reactor system, it is necessary to understand the factors that influence the rates of decomposition of coal, preasphaltene, asphaltene and oil, as well as the rate of hydrogen consumption. A Ayr (US) using Belle study’ on previous hydrogenated anthracene oil, and hydrogenated phenthanene, as solvents, has reported that different solvent species have different activation energies and frequency factors. This study has suggested that in kinetic studies of coal liquefaction reactions, the solvent should be in equilibrium with the coal. The equilibrated solvent was produced by repeated recycling, using the same coal
(OC) 460
31
470
460
450
440
430
420
I
’ IO
20
15 Log hydrogen
partial
pressure
(MPa)
Figure 4 Effect of hydrogen partial pressure on rate of reaction. ,430”C;0,460°C;0,4600C;~,4700C;residencetime,60 min
q
where: A and E, constants; R, the gas constant; and 7; the reaction temperature. Figure 3 shows the relation between KC” and C, obtained from the results of the experiments carried out with various catalyst concentrations at 45O”C, 60 min and 20 MPa total pressure (B and PH constant). The relation is rectilinear and from the slope a value of 0.19 for the exponent ‘n’ is obtained which reflects the fact that the effect of catalyst concentration on the rate of hydrogen consumption at 450°C is small (cf. Figure 2). Figure 4 shows the relation between KP; and the hydrogen partial pressure P, indicated by the experiments at four reaction temperatures for 60 min with C and B constant. The exponent m calculated at each
956
FUEL, 1982, Vol 61, October
I
I
I 35 Reciprocal
reactlon
I
I
I 40
I 45
temperature
1K-‘l x
10-3
figure 5 Relation between exponent m of hydrogen partial pressure and reaction temperature (equation (12)) for 60 min of residence time
Victorian brown coal hydroliquefaction:
Y. Nakako et al.
(OCI 470
480
460
450
I
430
420
410
400
I
I 35 Reciprocal
440
145 reacbon
I45
temperature
Relation between k and reaction temperature (13)) for 60 min of residence time
I
l-45
reactlon
temperature
Tab/e 3 Activation energies and frequency factors of reactions in the hydroliquefaction of Morwell brown coal
Reaction
Frequency factor (min-l)
I
Coal + Preasphaltene Coa I + Gas Preasphaltene -+ Asphaltene Asphaltene -+ Oil
13.1 755 221 10666
CP
Oil + Gas
5545 x
Brown
&cg
coal
b gases (CO, CO*,
H,O, C,
(K-')xl6"
Arrhenius plots for hydroliquefaction reactions of Figure 7 Morwell coal. A, kcp; 6, keg; C, kpa; D, k,,; E, keg
(equation
as that in the experiments (i.e. Morwell), at 430°C and 15 MPa total pressure, for a residence time of 60 min. To facilitate the analysis of the reaction kinetics, the following simplified reaction sequence occurring during the hydroliquefaction of the coal was assumed:
k
I
I.40
Reciprocal
( K-‘)x103
Figure 6
I
I.35
1
Activation energy (kJ mol-l) 24.95 54.22 66.07 76.03
109
184.43
I Preasphaltene
I kw t
Asphaltene-
k so-oil-
keg
-gases
(C,-C,)
An Arrhenius plot for each reaction in this sequence is given in Figure 7. The apparent first-order activation energy and frequency factor of each reaction, calculated from the Arrhenius plot is given in Table 3. CONCLUSIONS In this study of the hydroliquefaction of Morwell brown coal, it has been shown that the rate of hydrogen consumption can be expressed mathematically as a function of the concentration of the coal and the catalyst, the hydrogen partial pressure, and the reaction temperature. An analysis of a model for coal hydroliquefaction reactions, the experimental results for the rate of hydrogen consumption and the values determined for the exponent m on the hydrogen partial pressure P, in the expression relating the rate of hydrogen consumption to experimental variables, indicate that the rate-determining step in hydrogen transfer at 60 min of residence time is the hydrogenation of polynuclear
aromatic components in the lower temperature region at ~430°C and the radical formation and stabilization reactions in the higher temperature region at ~470°C. The thermal decomposition reactions of the brown coal to asphaltene proceed at a high rate even in the lowertemperature region (~430”C), because their activation energies are in the relatively low range of 255.50 kJ mol-i. ACKNOWLEDGEMENTS The authors thank the Kobe Steel Ltd. Company and SASOL and those researchers whost test results on liquefaction of Victorian brown coal in PDU facility allowed this work to be carried out. REFERENCES Schafer, H. N. S. Fuel 1979, 58, 667 Han, K. W. and Weh, C. Y. Fuel 1979, 58, 779 Neavel, R. C. Fuel 1976, 55, 237 Wiser, W. H. Fuel 1968, 47, 415 Cronauer, D. C., Shah, Y. T. and Ruberto, R. G. Ind. Eng. Chem. Proc. Des. Dev. 1978, 17, 281 Shalabi, M. A., Baldwin, R. M., Bain, R. L., Gray, J. H. and Golden, J. 0. Ind. Eng. Chem. Proc. Des. Dev. 1979, 18, 474 Cronauer, D. C. and Roberto, R. G. EPRI Report AF 913, Final Report under Project 713, 1979 March
FUEL, 1982, Vol 61, October
957
Victorian brown coal ~ydrolj~~e~actjon: Y. Nakako et al. Questions
and Answers
-
Mr
Y. Nakako
Dr Romey What is the activation coal to oil?
energy for direct conversion
from
Mr Nakako The activation energy of direct conversion from brown coal to oil is not estimated, because for brown coal liquefaction using distilled equilibrium solvent and iron catalyst, the major portion of oil is produced via preasphaltene and asphaltene, and the direct conversion of the brown coal to oil has a smaller role in liquefaction kinetics. Prof Trimm Many of your results would be expected if mass transfer was important. What tests for diffusion control were carried out? Mr Nakako The rate of hydrogen transfer will be an important factor in the earlier stage of the liquefaction reactions at higher reaction temperatures (~430°C) and the length of the
958
FUEL, 1982, Vol 61, October
period in which the rate of hydrogen mass transfer is significant will be longer at lower reaction temperature. We have studied the effect of hydrogen mass transfer on the liquefaction results using different types of reactors which give different hydrogen gas hold-up and interface area. Dr Larkins The liquefaction kinetics model you propose is a hydrogen-donor solvent model. It does not allow for a direct role of molecular hydrogen in radical stabilization. As iron catalysts do not readily rehydrogenate the solvent, do you not consider this latter molecular hydrogen route as important at pressure as high as 25 MPa? Mr Nakako Some of the radicals must be stabilized by direct reaction with molecular hydrogen, but as it has a higher activation energy than that of the hydrogens which are catalytically activated and/or reacted with donor solvent, the molecular hydrogen route is considered to be less significant in liquefaction reactions even at pressures as high as 25 MPa.