Energy & Fuels 1993, 7, 473-478
473
Enhancing Chain Transfer during Coal Liquefaction: A Model System Analysis Concetta LaMarca,+ Cristian Libanati,l and Michael T. Klein' Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716
Donald C. Cronauer Research and Development Department, Amoco Oil Company, Naperville, Illinois 60566 Received December 15,1992. Revised Manuscript Received March 19,1993
The aggregation of coal liquefaction elementary steps into representative reaction families allowed further organization into the Rice-Herzfeld kinetic formalism. Solution of this model and subsequent imposition of the Evans-Polanyi structurereactivity correlation revealed the attributes of an optimal chain-transfer solvent. In particular, the optimal chain-transfer solvent will have an intermediate C-H bond strength for catalysis of coal degradation chemistry. The results suggest a homogeneous analogue of the classical principle of Sabatier in heterogeneous catalysis.
Introduction Coal liquefaction is a complex set of chemical reactions involving coal and solvent that is often summarized globally in terms of the reaction of solubility- or boiling pointdefined product classes. This modeling approach is warranted because both identification and kinetics solution of the large number of elementary reactions governing coal liquefaction is formidable. Nevertheless, the number of elementary reaction families is far less than the actual number of governing elementary reactions, which permits a mechanism-oriented lumpinganalysis of coal liquefaction kinetics. This also allows some of the rigor of molecular chemistry to be brought to bear on the synthesis of novel coal liquefaction process concepts. The purpose of the present paper is to use a mechanistic lumping approach in the analysis of coal liquefaction reaction families to suggest the possibility of optimal chain-transfer solvents for the initial stages of coal fragmentation. The liquefaction reaction families are organized in Figure 1. Bond homolysis, followed by radical capping, is a time-honoredviewof coal liquefaction that has recently been suggested' to constitute only a portion of the overall reaction set. Any coal- or solvent-derived radical can abstract hydrogen from a donor (within coal or from a solvent); these radicals can also induce cleavage of coal bonds through ipso substitution and radical-induced hydrogen transfer (RHT). Kinetically significant @cission steps are also available to coal- and solvent-derived radicals. Radical recombination and radical addition to olefins, the reverse of bond homolysis and fl scission, respectively, are also kinetically significant. As regards practical coal liquefaction, steps 1-4 of Figure 1 are desirable: they lead to molecular weight reduction.
* Corresponding author.
+ Current address:
DuPont Engineering, Newark, DE 19714.
t Current address: Washington Research Center, W. €2. Grace & Co.,
Columbia, MD 21044. (1)McMillen,D. F.; Malhotra, R.; Hum,G.P.; Chang, S.-J.HydrogenTransfer-PromotedBond Scission Initiated by Coal Fragments. Energy Fuels 1987,I , 193-198.
1. Bond Homolysis 2. Hydrogen Transfer (Radical Capping)
3. DScisrion 4. Radical-Induced Hydrogen Transfer (W)
1'.
* X + yH 5. Radical Recombination 6. Radical Addition
Figure1. Mechanism-orientedcategorizationof coal liquefaction reaction families.
Steps 5 and 6 are undesirable, since they lead to molecular weight growth. Increasing the rates of 1-4 relative to 5 and 6 during the initial stages of coal liquefaction would allow for better net fragmentation of the macrostructure. As developed below, this can amount to increasing the kinetic chain length during the initial stages of coal liquefaction, and can be accomplished by the action of chain-transfer solvents. The coal reaction families 1-6 of Figure 1 collectively possess aspects of a kinetic chain process. This mapping is accomplished in Figure 2. Reaction 1of Figure 1initiates and reactions 2-4 (and 6) of Figure 1propagate the chain. These are represented by steps 1-3 in Figure 2. Reaction 5 of Figure 1terminates a chain, as do steps 7-9 in Figure 2. The essential feature of a chain is its kinetic chain length, i.e., the rate of consumption of coal by the propagation cycle relative to that by initiation. At the condition of steady state,the initiation rate must be equal to the termination rate, which is a primary retrograde reaction. As illustrated in steps 4-6 of Figure 2,a chain-
0887-0624/93/2507-0473$04.00/0@ 1993 American Chemical Society
474 Energy & Fuels, Vol. 7, No. 4, 1993
LaMarca et al.
AI k' 2 81
81 + A i % 41H + pi
Together, the present analysis and experiments suggest that enhancement of chain transfer during coal liquefaction would be a useful process concept.
Analysis
2p,
Kinetics analysis of the steps of Figure 2 is phrased in terms of the Rice-Herzfeld transformation of reactant (A1 = plH) through free-radical intermediates 01 and p1 to products 01H and &I. Chain transfer occurs through the action of A2 (BzH), which can donate hydrogen to 01, or to p1 in the case of slow 0 scission. The thus-derived j32 radical can abstract hydrogen from A1 or terminate with another radical. Individually, A2 is stable and, in particular, j32 does not have a kinetically significant0-scission path. This results in no net consumption of the catalyst
2'
A2.
The rate laws follow from the elementary steps of Figure
2/4 2 +
( 7 - 12)
2 8 ~a PI
PI
Termination
' Products
+ 42 =
2, which also contains the associated rate constants. For the elementary steps of Figure 2, the long-chain rate of
consumption of 'A,
+& 2 ,
A1
is
= kllAl& + k21A1j32 - k11&2pl
(1)
This can be written in terms of observable species' concentrations by invoking the pseudo-steady-state approximation on radical concentrations. The 01 and pl balances provide the concentrations p1 and 0 2 in terms of 01. The equality of initiation and termination rates then provides the final information needed to express 01 (and, thus, p1 and 82) in terms of AI, Az, and the rate constants for elementary steps. The thus-derived rate expression for the pyrolysis of A1 in the presence of a chain-transfer solvent (A21 is
where
The influence of the chain-transfer solvent A2 is emphasized by considering the enhancement €1 = ? ' A ~ ( A ~ , & ) / ~ Aof~ (the A ~rate ) of consumption of A1 caused by the addition of A2. Recognizing rAl(Al), the rate in the
Energy & Fuels, Vol. 7, No. 4,1993 475
Chain Transfer during Coal Liquefaction 1.6 1.4
4
rA,(A19A2)
=
112
k12 k21
(T) + k, kllA1(
A2)/
[ + ($222+ (y) 1 +( ---
12 E
kinlAl
d (81-H)187kcaVmol
(1
1
k21 kll A 1
0.8
((2)lI2+(2)
0.6
---))I
Wk12kt12A,
(7)
k 2 l 1212 A 1
0.4
02
0 65
70
75
80
85 90 d(p2-H) I kcal mol-'
100
95
1G
110
Figure 3. DBE pyrolysis rate enhancement due to hydrogen-
Application of eq 7 to coal liquefaction reaction families suggests severalchemistry-drivensimplifications. In many cases /3 scission is fast relative to H abstraction ( k l l A I I k 1 0), which permits the simplification of eq 7 to
-
transfer solvent. d (pl -H) I 87 kcaVmol
€1 =
In this instance 6 1 and /32 are the most abundant reaction intermediates. For the reasonable limiting case of equal termination rate constants save the statistical factor of l/2 for self-collison, the denominator simplifies, allowing
2
Equal Termination 1
r-
.o
65
70
75
85
80
95
90
100
105
(9)
110
d@2-H) / kcal mol-'
Figure 4. DBE pyrolysis rate enhancement due to chain transfer solvent: effect of unequal termination rate constants. Table 11. DBE Pyrolysis with Chain-TransferSolvents solvent diphenylmethane triphenylmethane fluorene dihydroanthracene dihydrophenanthrene acenaphthene diphenylamine carbazole t-butylamine
c
d0m
21.7 336.0 19.8 70.4 59.2 0.81 20.6 6.00 2.7
80.5,84 f 2 75*4 80.5 75.3,77 2 83 82.3 -80 -80 102.6
absence of a chain-transfer additive, to be
€1
Further inspection of the controlling dimensionless groups k12/k21 and k2l/K11 shows them to be relative rate constants for a family of hydrogen-abstraction reactions. The Evans-Polanyi relation for activation energies E* = Eo + ~ A H R for exothermic reactions2 and E* = Eo + (1 - a ) W R for endothermic reactions should then provide a reasonable estimate of €1 in terms of reaction enthalpies and, ultimately, comparative bond strength^.^^ The dependenceof these dimensionless groups on the relevant bond strengths is illustrated in Table I, where kzl/kll is shown to be a function of d O&-H and d O f i r ~and k 1 d k 2 1 a function of d O f i l - ~ ,d O f i r ~ ,and d O P 1 - ~ . Equations 8 and 9 for €1, and the expressions in Table I for k21/kll and k12/k21, allow illustrationof the dependence of on d O f i r ~ ,d O f i l - ~ , and d O P l - ~ . For a model where coal species are lumped as pseudocomponent 1,and with the chain-transfer solvent as pseudocomponent 2, it is rea-
reduces as
When the geometric mean rule applies for termination rate constants (03 = 2(0102)1/2,05 = 2(0104)1/2, and 0 6 = 2 q ( o ~ 4 ) ~ the / ~ , denominator reduces to D,'/2
Thermochemical Constraint
= 1+
(2) (T)( 112
kllAl
1
+ ---) k12 k21 A 2 + 1221 k l l A 1
(2) Evans, M. G.; Polanyi, M. Inertia and driving force of chemical reactions. Trans. Faraday SOC.1938,34, 11. (3) Stein, S. E. A Fundamental Chemical Kinetics Approach to Coal Conversion. In New Approches in Coal Chemistry;Blaustein, Bockrath, and Friedman, Eds. ACS Symp. Ser. 169; American Chemical Society Washington, DC, 1981. (4) Benson, S. W. Bond energies. J. Chem. Educ. 1966,42 (9), 502. (5) Bockrath, B.; Bittner, E.; McGrew, J. Relative Rate Constants of Hydrogen Transfer to Benzyl Radical. J. Am. Chem. SOC.1984,106, 135-138. (6) Benson, S. W .Thermochemical Kinetics, 2nd ed.,John Wiley and Sons, Inc.: New York, 1969.
(7) McMillen, D. F.; Golden, D. M. Hydrocarbon bond dissociation energies. Annu. Rev. Phys. Chem. 1982,33,493-532. (8)Billmers, R.; Brown, R. L.; Stein, S. E. Hydrogen transfer from 9,lO-dihydrophenanthreneto anthracene. Int. J. Chem. Kinet. 1989,22, 375-386. (9) McMillen, D. F.; Malhotra, R.; Chang, S.-J.; Ogier, W.C.; Nigenda,
S. E.; Fleming, R. H. Mechanismsof hydrogen transfer and bond scission of strongly bonded coal structures in donor-solventsyatems. Fuel 1987, 66, 1611.
resulting in the reduced rate expression:
(10) Curtis, C. W.;Guin, J. A.; Kwon, K. C. Coal solvolyeis in a series of model compound systems. Fuef 1984,63,1404.
476 Energy & Fuels, Val. 7, No. 4, 1993 0.025
o.026
0.02
3 =
0.015
z
0.01
-25
- LaMarca et al.
s 0.005 0 0
10
20
30 40 timdmln
50
0
60
10
20
a
30 40 timmin
x)
0
60
llmdmln
b
DBE Neal
E
@
$- 0.015
z 8
f-
a 0
-e
t O.O1
0.02
0.015 0.01
8 0.006
:0
0.005
wlDH Anthrrrcone
-
0
0
0
0
60
30
90
f 0 . 00.02 15
a
40 llmdmin
50
9
[oan
DEE N w l
1
0.01
I1
0 0
h
15C
s 0.005
**-.'....'.-.*-
60
120
B f
0 30
90
f
W"k
0
60
Aconclphthone
tlmdmin
DBE N w l
20
30
e
d
10
f
0
llmdmln
0
l:NIl C
0.025
0.02
90
60
30
1 0 2 0 3 0 4 0 5 0 6 0 Umdmin
i
Figure 5. (a-i) Experimental kinetics of DBE pyrolysis at 400 "C neat and with (a) diphenylmethane; (b) triphenylmethane; (c)
fluorene; (d) dihydroanthracene; (e) dihydrophenanthrene; (0 acenaphthene; (g) diphenylamine; (h) carbazole; (i) t-butylamine.
sonable to consider B1H and plH as fixed by the coal type and PzH to be a design parameter. This renders e1 dependent on d O B ~ I . Iand the relative rate constants for 8 2 termination (odw1,o d w 1 ) for a given coal. This dependence is illustrated in Figure 3 for T = 400 "C and d 0 8 1 - ~ = 87 kcal/mol, the bond strength for fission of toluene, @1H,into H and 81. Inspection of Figure 3 reveals an extremum in the dependence of €1 on d For low d O,Y?H, the easily formed @2 radicals are also very stable. Here 82-H acta as an inhibitor and lowers the rate. At the high d O p r ~extreme, highly reactive 8 2 radicals are formed
--
only with great difficulty. In the limit d OBrH 00 the additive acta as an inert diluent, and €1 1. In an intermediate regime of d O B ~ H , the enhancement q > 1. Thus figure 3 summarizes the homogeneous equivalent of the classic Balandin Volcano curve illustrating the principle of Sabatier. The effect of unequal termination rate constants (eq 8) is illustrated in Figure 4. When, for example, 8 2 radicals terminate with one one-thousandth the rate constant of 81 radicals ( w I / ( J ~= 0.001),the enhancement is amplified by a factor of about 4.
Chain Transfer during Coal Liquefaction
Energy & Fuels, Vol. 7, No. 4, 1993 477 0.02 A
0.02
z.
,s 0.015 E
!i *
0.008
0'01 0.005
f 0.008
8 0.004
8 0.004
0.002 0
0.002 0 0
0 0
10
5
15 20 tlmshin
25
30
0
3
8
9 1 tlmohnln
a
2
-
60
30
90
llmdmln
b
C
0.025
0.025
0.02
z.
5 0.015 E
0.015
1
Three-parmeter modo1
E
Two-parameter model
0.W5
0.006
0
0
0
10
5
15 20 tlrolmin
25
0.01
30
0 30
0
bo
90
0
-I .
-z
cg
E3
0.02
0.01
5 0.005
0.02
0.01 0.005
L
I....l....l....
20
150
-5 0.015
0.015
15
120
0.025
I ~
10 IImJmln
90
f
0.025
5
60
tlmdmln
e
d
0
30
Umdmln
0
30
60
0
90
0
tlmohnln
g
5
h
10 llmdmln
15
20
i
Fwure 6. (a-i) Modeling DBE pyrolysie a t 400"Cwith (a) diphenylmethane; (b) triphenylmethane; (c) fluorene; (d) dihydroanthracene; (e) dihydrophenanthrene; (0 acenaphthene; (g) diphenylamine; (h) carbazole; (i) t-butylamine.
Table 111. Modeling DBE Pyrolysis at 400 OC with Chain-Transfer Solvents ~~
r1*
ha
solvent DPM TPM fluorene DHAnth DHPhen AceN DPA carbazole a
ki 1.23 5.565 1.46 2.88 0.485 6E-11 1.434 0.492
k2
X2
0.0404 0.0176 0.011 0.035 0.044 0.014 0.0580 0.0804
0.0145 0.542 0.0044 0.078 0.130 0.0156 0.00295 0.0608
*
kr 1.00 51.5 1.30 1.79 0.314 2.2E-11 0.878 0.210
k2 0.032 7.00 1.4E13c 5.63-13 6Ell 0.026 2.33-12 1.3E-11
ka 0.0014 0.019 4E4 6.23-4 0.00143 3.33-4 0.00196 0.00155
r1 = (0.062A19/2(1+ kl(AdAl))#/{l+ kz(AdAl)]. rl = (0.062A1~/~(1 + kl(AdAl)]/{(l + kZ(AdA1) + ks(Az/A1)2)1/2). E-n
X*
0.0179 0.475 0.004 0.048 0.121 0.0156 0.00264 0.055 X
lo-".
LaMarca et al.
478 Energy & Fuels, Vol. 7, No. 4, 1993
Comparison with Experiment The reaction of dibenzyl ether (DBE) with, separately, the chain-transfer solvents diphenylmethane, triphenylmethane, fluorene, dihydroanthracene, dihydrophenanthrene, acenaphthene, diphenylamine, carbazole, and t-butylamine provided experimental assessment of these ideas. The experiments were performed in independent batch reactions for each pyrolysistime of interest. Reactor vessels were preloaded with reactant(s1, purged with an inert gas (argon) and sealed, and then immersed in a constant-temperature fluidized sand bath for the desired reaction time. Products were collected in methylene chloride or acetone, identified by capillary GC/MSD, and routinely quantified by GC/MSD. Further details are available." The DBE pyrolysis data are shown in Figure 5a-i, and summarized in Table 11. The enhancement of dibenzyl ether (DBE, d owl-^ = 78.85 kcal/mol and d Ofll-H = 87 kcal/ mol) was observed upon addition of all chain-transfer solvents except acenaphthene,where a neutral to impeded effect was observed. The chain-transfer solvents span a range of both C-H bond strengths (75-102.6 kcal/mol) and termination rate constants, so a ranking of effects based solely on bond strengths is not possible. For example, a termination-controlled effect is observed in the presence of triphenylmethane. Triphenylmethyl radicals are known to be persistent due to steric hindrance to self-collision. In this case, W4/W1- 0 and wg/w1
