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Thermodynamics of the transformations of oxygen- and sulphur-containing functional groups during coal liquefaction in hydrogen and hydrogen donor
Thermodynamics of the transformations of oxygen- and sulphur-containing functional groups during coal liquefaction in hydrogen and hydrogen donor Lance Messenger and Amir Attar Department of Chemical Engineering, University of Houston - Central Campus, Houston, Texas 77004, USA (Received 11 May 1978; revised 12 March 1979)
The thermodynamic equilibrium constants for the reduction of oxygen and sulphur functional groups in hydrogen and in tetralin were calculated for the temperature range 327-527°C. All the reduction reactions of the oxygen groups are thermodynamically favourable both with hydrogen and with tetralin. The reduction reactions of all the sulphur functional groups in tetralin are favourable except for the reductions of thiophenes below 407°C. However, molecular hydrogen cannot reduce thiophenes or aryl sulphides below about 59O’C. Reactions in which oxygen from water is captured by the organic matrix are not favourable reactions; however, capturing of sulphur from hydrogen sulphide or alkyl thiols can occur in this range of temperatures. The most stable products are condensed thiophenic structures.
Very few direct measurements on coal can yield information on the structure of coal. However, it is plausible to assume that the organic groups in coal are very similar to those which are found in oils and in coal-derived materials. Moreover, if the well accepted principle that homologue compounds react in an analogous manner is applied to coal? one would assume that the chemistry and thermodynamics of the reaction of coal functional groups are the same as those of the homolog groups in homologous low-molecular-weight compounds. The kinetics of the reaction of coal functional groups cannot be calculated from knowledge on small molecules. Moreover, steric hindrance and mass transport can seriously reduce the apparent rate of reaction of coal-derived molecules. Different coals consist of essentially the same organic functional groups, but the concentration of each group is different in each coal. However, to a first-order approximation the chemistry and the thermodynamics of these groups are independent of the nature of the particular coal. The thermodynamic properties of a molecule can be calculated as the sum of the contributions of individual functional groups or atoms. This concept is well established and indeed it received very wide application in the petroleum industry. This concept implies that the thermodynamic feasibility of a reaction can be calculated taking into account only the functional groups that were transformed in the reaction. The kinetics of the reactions of the various functional groups could be assumed to be independent of the coal and a function only of the reagent, the reactive group and the temperature. The chemistry and the thermodynamics of reactions of functional groups in coal during liquefaction are examined. The results are consistent with experimental data on coal and on model compounds. In particular, data are available (e.g. Whitehurst et ~l.*~) that suggest that during liquefac-
0016-2361179/090655-06S2.00 0 1979 IPC Business Press
tion elimination of oxygen proceeds much more rapidly than elimination of sulphur. The results of the thermodynamic calculations show indeed that elimination of oxygen functional groups is much more favoured thermodynamically than elimination of sulphur groups. However, it should be recognized that the rate or the kinetics of the elimination reactions determines the results of the liquefaction. Two additional points should be made in this context: (A) The equilibrium conversion which is determined by thermodynamics is the upper limit of the obtainable conversion in a closed system. The value of this ‘upper limit’ is independent of the rate at which the reaction proceeds. (B) In many reactions, there is a ‘reasonable’ correspondence between thermodynamics and kinetics, e.g. reactions which are more favourable thermodynamically proceed at faster rates. The calculations in the paper are elementary but the results are interesting and useful since they permit explanation of some of the empirical facts observed in coal liquefaction.
FUNCTIONAL
GROUPS IN COAL: INVENTORY
Tingey and Morrey’ published an excellent review of the oxygen, sulphur and nitrogen functional groups in coal. Whitehurst et al.‘? published an excellent survey and data on the reactivities of coals and of functional groups during liquefaction. To keep the discussion short, work which has been referred to in the previously mentioned reviews will not be mentioned specifically. The types and relative concentrations of various groups are a function of the rank of the coal, the material considered, and the specific conditions to which the coal had been exposed. The most important groups believed to be
FUEL, 1979, Vol 58, September
665
Transformations of 0- and S-groups during coal liquefaction via hydrogen: L. Messenger and A. Attar Tab/e 7 Oxygen,
sulphur and nitrogen
Type of group (written
functional
for the oxygen analogue)
groups in coal Oxygen
analogue
ROH
Alcohol-aliphatic Alcohol-aromatic
@OH
Acid (aromatic)
@JC
40
Sulphur analogue
Nitrogen
RSHa @Ha
-. _
-
@-CN
analogueb
‘OH Ethers - Aliphatic Aromatic Mixed Furans (and thiophenes) Pyran (and pyridine) Quinones Condensed heteroaromatics 8Most recently bMost recently
R-S-Ra I#-S-f#la o-S-Ra
R-O-R O-0-3 o-0-R
discussed by Attar and Dupuis’ discussed by Schillers who studied coal-derived
liquids
present in coal are summarized in Table 1, which is an augumented version of Tingey and Morrey’s list’. Typical concentrations of organic oxygen in coal are in the range of 0.5-20 wt % and are usually around lo-14 wt % in bituminous coals. The concentration of organic sulphur varies in the range of 0.0-6 wt % and is often around l-3 wt % in bituminous coals. Less information is available on the concentration of nitrogen in coal; the available data suggest however that it varies in the range of 0.52.5 wt % and in bituminous coals it is often close to 1.5 wt %. The data of Blom6 suggest that the distribution of oxygen functional groups is approximately 35-50 wt % C=O, 30-50 wt % OH, O-25 wt % COOH, O-5 wt % OCH3, and O-l 5 wt % in rings. Etheric oxygen was detected by Blom only in low-rank coals, with carbon content below 65 wt % (daf). The extremely low concentrations of etheric oxygen are not in accord with more recent observations which suggest that the initial liquefaction step is associated with cleavage of etheric bonds (e.g. Whitehurst?. The distribution of the sulphur groups in several coals has recently been studied by Attar and Dupuis4, who estimate that about 22-28% of the organic sulphur in bituminous and low-rank coals is in the form of aliphatic and alicyclic sulphides. lo-30% of the sulphur is in the form of thiols, and the rest is in the form of thiophenes and condensed thiophenic structures. Larger ratios of condensed thiophenic structures were observed in higher-rank coals. The author is not aware of any systematic study of nitrogen functional groups in coal. Studies of the nitrogen compounds in coal tar and in liquefaction products suggest that most of the nitrogen is bound in pyridines, condensed pyridines and nitriles (e.g. Schiller’, Schultz et al. ‘, and Smith*). It has never been demonstrated that amines are not present in coal. In fact, the author believes that amines are the source of at least part of the ammonia that coal releases during pyrolysis. The H-C structure of coal has been a source of controversy for years; however, it seems that most researchers accept today the theory that coals consist of lamellae, of 2-7 condensed aromatic rings connected by short aliphatic bridges, oxygen and sulphur. Alicyclic rings condensed with aromatic rings are also believed to be present in coal, as well as short aliphatic bridges. Various qualitative and quantitative estimates of the H-C functionalities has been published recently. Tingey and Morrey’ summarized some of them; many others have appeared since then in the literature.
656
FUEL,
1979, Vol 58, September
TRANSFORMATIONS
OF FUNCTIONAL
GROUPS
Three kinds of liquefaction reactions should be differentiated: 1. Reactions which result in the formation of products with significantly smaller molecular weight than the original material, e.g. cleavage of etheric oxygen:
R-O-4
4[Hl_ RH + $JH + Hz0
(1)
cleavage of sulphidic sulphur:
R-S-R’
4’H_’ RH + H2S + R’H
scission of carbon-carbon
(2)
bonds:
R-P’ 2[H_! RH t R’H
(3)
2. Reactions which result in a small reduction in the molecular weight but in which the functionality is significantly reduced, e.g. reduction of a hydroxyl:
ROH 22 reduction
RH + H20
(4)
of a ketone:
0 R’C R T? elimination
RCH2R’ + Hz0 of a thiol:
RSH 2[H_! RH + H2S
(6)
3. Ring opening; e.g. reduction 0, desulphurization
a
RC,H,o+
H,O
(7)
of thiophenes: BIHI
Q
of furan:
C H ‘ m*
H*S
(8)
The first class of reactions is the most effective class of liquefaction reactions because it results in a product with a
Transformations of 0- and S-groups during coal liquefaction via hydrogen: L. Messenger and A. Attar
significantly lower molecular weight and functionality. The second and third classes of reactions reduce the molecular weight only to a limited extent. However, the functionality decreases significantly. Since the extractability of the products depends on the functionality and on the molecular weight, the rates at which reactions of the first kind occur will determine to a large extent the rate of liquefaction.
Step 4: Calculate A@!00, A&&,0
= -39.89 A%M
Thermodynamic calculations were made for the feasibility of the transformations of different functional groups during liquefaction in hydrogen and in hydrogen donors. The method that has been used was proposed by Ciola’. The data that were used were taken from Benson”. The method of calculation assumes that only the functional groups which change in a reaction affect the Gibbs freesenergy of the reaction and that this may be calculated via group contributions. Exact details and many examples are described in the comprehensive study of Benson”, and therefore only one example will be presented here. Example: Calculation of the free energy of reduction of an aliphatic ether by molecular hydrogen at 600 K. Step I: Write the reaction noting the functional groups that are transformed. + 2H2 + 2RCH3 + H20
(9)
Using Benson’s notation: 2(C-COH2)
+ (0-C2)
+ 2H2 + 2(C-CH3)
+ H20
(10)
Step 2: Obtain A@,, , A&g and cP values for the groups involved (e.g. tables pp 178-215 in Benson’s book’@). Group
AH$!,,
A%,
Aq(600
C-COH;! 0-Q C-CH3 H2 Hz0
-8.5 -23.0 -10.08 0 -57.8
10.3 8.68 30.41 31.2 45.1
9.43 3.8 10.79 7.0 8.7
Step 3: Calculate AL!$~~~, A,$,,,
K)
A$, (298 K) 4.99 3.4 6.19 6.9 8.0
_ 2(O)-2(-8.5) = -158.9
= -57.8
= 14.24 - 6.38 In ‘G
= Mj&,
- 6OOAS$,,,
=
--39.89 - 600 x 10-3(9.776) = -45.75
kcal/mol = -204.1
A&-
lnK=_-=-
-45.75
RT
kJ/mol
x 103
1.987 x 600
= -38.38
RESULTS AND ANALYSIS Calculations of the Gibbs free energy of various reactions were made for the temperature range of 600-800 K (327-527OC), which is the range important in coal liquefaction. Two sets of results are presented for the reduction of each functional group, the first with molecular hydrogen and the second with tetralin. It should be recognized that the difference between the Gibbs free energy of reduction of a functional group with hydrogen and thht of reduction of the same group with tetralin is fried. Its value is equal to the Gibbs free energy of a hypothetical reaction where tetralin is converted to naphthalene and two molecules of gaseous hydrogen. In addition the thermodynamics of several reactions in which skeletal changes occur or in which an interchange between sulphur and oxygen would occur are also presented. Deoxygenation Figures 1 and 2 show the logarithm
of the equilibrium
Reactlon
+ 2(-10.08) = -37.96
kJ/mol
+ Ac$joo In ‘g
and A&00
- (-23.0)
x 1O-3 c;boO = -37.96
kcal/mol = -167.0
= A%,,*
AG&,,,
A
ROH
+ H, -
6 C
O0H ROR
+ H,QH . H,O + 2H,-2RH * Hz0
D 9
r
E
401
c
30:
D
RH
+ 2 H,-
+ H,O
C,H,,+
H,O
$‘o@+2H2-20H+
F ?.2H,
-
H,O C,H,
+ H,O
kcal/mol
kJ/mol k
A$,,,
+ (600-298)
and K.
= 9.776 cal/mol K = 40.9 J/mol K
50
AH:298 = &_iAHi,,
A@,,,
+ 0.302 (-6.38)
Method of calculation
RCH2-0-CH2R
= Al&g
mj&r,,
= 45.1 t 2(30.41)
- 2(31.2) - 8.68 - 2(10.3)
al
$ J
20--F
E
toc
= 14.24 cal/mol K = 59.6 J/mol K
A
0
o-
Acsoo
= 8.7 + 2(10.79)
= -6.38 A&98
- 2(7.0) -- 3.8 -- 2(9.43)
cal/mol K = -26.7
J/mol K
= 8.0 + 2(6.19) - 2(6.9) -- 3.4 - 2(4.99)
= -6.8 cal/mol K = -28.5
J/mol K
-207 600K 327’C figure I hydrogen
650K
700K
750K
Temperature Reduction
of oxygen functional
FUEL,
800 K 527’C
groups by molecular
1979, Vol 58, September
657
Transformations of 0- and S-groups during coal liquefaction via hydrogen: L. Messenger and A. Attar
A
ROH
+‘I2
8
@OH
+
C
ROR
+@J)-
D p E
‘12 a
+
-
@‘Ii
2RH
m
l
@OO
-RH
0 co
-C,H,,+
+ m
‘I2
0 0 03
*Hz0
+‘I,@@
+ @@+H,O @@
-2’l’H
liquefaction since such reactions reduce significantly the molecular weight and the functionality of the product. Elimination of aliphatic and alicyclic ethers is thermodynamically a very favourable reaction in hydrogen and in tetralin. However, elimination of aromatic ethers is a substantially less favourable reaction.
+H,O
+ H,O
+ @$I
+H,O
Desulphurization
-10
1 L
-201 600K ’
’
8
8
’
650K
c
’
4
8
’
700K
’
L
’
’
’ n
750 K
’
’
327’C
’
’
600K 527°C
Temperature Figure
2
Reduction
of oxygen
functional
groups
by tetralin
constant for the reactions which involve elimination of oxygen functional groups in hydrogen and in tetralin respectively. The data show that elimination of oxygen as water is a thermodynamically favourable reaction in hydrogenation with either hydrogen or tetralin. All the oxygencontaining functional groups are removable except for aromatic ethers and possibly dibenzofuran*. The equilibrium constant is very sensitive to temperature variation when hydrogen is used and is not sensitive when tetralin is the reducing agent. At low temperatures, e.g. around 330°C, elimination of water from phenols by hydrogen is extremely favourable but it becomes much less favourable at 53O’C (although still favourable). The equilibrium constant drops from e5.05 to e1.68 in this range of temperature with Hz, while it is essentially constant throughout this range of temperature when tetralin is considered. The value of K in the latter case varies from e4.’ to e4.9. The value of the equilibrium constant for the reduction of phenols is the same for hydrogen and tetralin around 340345”C, but at higher temperatures tetralin is a better hydrogenation reagent. The implication of these calculations for the practice of hydroliquefaction is that a substantial fraction of the oxygen functional groups would be eliminated as water during the preheating of the coal slurry before it enters the liquefier. Elimination of water during the preheating period was observed experimentally in numerous studies. Furthermore, the organic matrix of coal contains alicyclic groups condensed to aromatic rings (Whitehurst”, Given” andbothers). Such alicyclic hydrogen can be donated to the oxygen functional groups during the preheating period. Therefore, formation of water in the preheater could be achieved in the presence and in the absence of hydrogen and/or hydrogen donor solvent, provided that mass transport in the organic matrix permits contact between the oxygen groups and the alicyclic hydrogens. Elimination of etheric bonds has a special importance in
A priori, one could assume that the behaviour of the sulphur functional groups would be very similar to that of the analogous oxygen functional groups. Indeed, similarity is observed in many cases; however, the thermodynamic feasibility of the elimination of sulphur is often significantly different from that of the analogous oxygen functional groups. Three factors determine the difference between the behaviour of the sulphur functional groups and that of the oxygen analogue: 1. The hydrogen-oxygen bond is much stronger than the hydrogen-sulphur bond. For example, the standard enthalpy of formation of water, H20, is -242.0 kJ/mol while that of H2S is 20.1 kJ/moi. 2. The carbon-oxygen bond is much stronger than the carbon-sulphur bond. For example, the standard enthalpy of formation of CO2 is -397.6 kJ/mol while that of CS2 is 117.2 kJ/mol. 3. The sulphur n-electrons can resonate with groups which contain n bonds, e.g. double bonds and aromatic rings. The tendency of oxygen electrons to participate in resonating bonds is substantially smaller. Figures 3 and 4 show the equilibrium constant for the hydrogenation reactions of sulphur functional groups in the temperature range of 600-800 K with hydrogen and with tetralin respectively. Hydrodesulphurization of the sulphur groups in which resonance with the hydrocarbon matrix is not possible could proceed readily with hydrogen or tetralin. However, where the sulphur can resonate with an aromatic ring or with double bonds, hydrogen does not reduce the sulphur. Tetralin is not very effective either; however, it becomes more effective at higher temperatures. For example, hydrodesulphurization of a thiophenic ring by tetralin is not feasible below about 407°C but it is thermodynamically favourable above these temperatures. React RSH+H,-
B
@SH
C
RSR
DQ t QSUI
LO
F
1
600 K 327’C
ion
A
-
+ 2H,+ 2H,-
700 K
H,S
cDH . H,S
ZRH . C,tQ
+ 2H,-
i9.2H2
650K
Rti*
+ H,
-
2@H C,H,
H,S
H,S + H,S + H,S
750K
Temperature
*The quality of the available data on dibenzofuran does not permit a conclusive statement about its amenability to hydrogenation
656
FUEL, 1979, Vol 58, September
Figure 3 hydrogen
Reduction
of rulphur
functional
groups by molecular
EOOK 527°C
Transformations of 0- and S-groups during coal liquefaction via hydrogen: L. Messenger and A. A ttar Reactton A 0
OSH
C
RSR
D Q E LO-
and the formation
+‘/z l
o co
C
ZO-
D A
ti
-
wlO$ -I -
0 E
O-
F
-
a
-2RH
+ a
-C&H,,+ -2@H
U’S@’ +GXI
FQ*W 30 -
-CC,H,+
l“200
U’H
+ @
6 1 b 1 1 3 j a 1 650~
m +
@9
@@j
+H,S
+ H,S + H,S + H,S
h ’
1 1 1 1 1 8 1 750K
700 K
800K 527°C
Temperature figure
4
Reduction
compound:
of sulphur functional
l
H,S
groups by tetralin
This interesting observation correlates extremely well with the experimentally determined optimal liquefactions temperature in hydrogen donors, 410450°C. The equilibrium constant for hydrodesulphurizations is less sensitive to the temperature than the equilibrium constant for hydrodeoxygenation. The implication of the results for the operation of liquefiers is that the aliphatic sulphur could be eliminated as hydrogen sulphide during the preheating period. However, hydrodesulphurization of thiophenic sulphur could be achieved only when hydrogen donor is used at an adequate temperature, e.g. when the donor is tetralin the temperature has to be above 407°C. The results imply also that coals that contain a substantial fraction of organic sulphur can be desulphurized in relatively mild conditions if most of the organic sulphur is in aliphatic structures. These conditions were deduced from experimental observations by Attar and Dupuis4, who suggested using the distribution of the organic sulphur groups to screen coals with a large fraction of organic sulphur into ‘easily desulphurizable’ coals and ‘stubborn coals’. Coals in which most of the organic sulphur is thiolic or is present as aliphatic sulphides can be easily desulphurized. Coals in which most of the organic sulphur is thiophenic cannot be desulphurized with hydrogen, and only in a Iimited range of temperatures with hydrogen donors like tetralin. Attar and Dupuis4 proposed also an analytical procedure for the determination of the distribution of the organic sulphur groups in coals.
During the liquefaction Hz0 and H2S are released into the reactor; therefore, one could conceive reactions between the organic matrix and H2S or Hz0 which could fuc these again in organic compounds. Two plausible reactions which could fix S or 0 are:
H
RHC = CHR
I
RHC=CHR
30
L
-201 600K 327°C
RCH,CHlOHlR
CH(OCHRCH,R) l-&D -e
l
CH,R + 2H,
8 1 1 c 1 - 0 a 0 1 I s I 650K
I I
700 K
a I I I I
750K
800K 527-C
Temperature Figure 5
Interchange
among oxygen
groups and the organic matrix
Reaction LO
H
RHC 2 CHR
I
RHC = CHR + RCH,CHI
l
H,S -
RCH,CHiSH)R SH )R -
RICH,I,CH(SCHRCH,RlCH,R L
-t 2Ot L
@--@I
+ H,S -
@--
+ 2H, a
r
k
m IO0” J
L
L OI
H
- 10
t
I-201 600K 327°C
(12)
_
RCH2CH10H)R
20
(11)
+
@I-@
+ H,O l
RICH,),
t
R(CH&CH(SCHRCH2R)CH2R
(13)
2H,
Reactton
LO
Trapping of oxygen and sulphur by the omanic matrix
RHC=CHR + RCH$H(SH)R
l
Figures 5 and 6 show the values of the equilibrium constant for these reactions in the temperature range 600-800 K for Hz0 and H2S respectively. Figure 5 shows that aliphatic alcohols could react with olefins and form stable compounds. Similarly, water could react with the aromatic structure and form a furanic ring. However, since in liquefaction hydrogen or hydrogen donors are also present in the reactor, and since hydrodeoxygenation of ethers and furans are feasible reactions in these ranges of temperature, fixation of water in the organic matrix is not likely to occur during the hydroliquefaction of coal. Figure 6 shows that trapping of sulphur by double bonds is not a feasible reaction in the temperature range of 600800 K. However, trapping of sulphur as a heteroaromatic thiophenic ring is feasible. Such structures are stable in the presence of hydrogen as indicated in Figure 3, and are stable in tetralin up to about 407’C. Therefore, fixation of sulphur into the organic matrix is a feasible reaction and could result in reduction of the efficiency of the hydroliquefaction and hydrodesulphurization. Experimental data exist which support the possibility of ftiing sulphur in the organic matrix. A detailed review of these reactions was published by Attar13’14.
30 E
RHC=CHR + H2S + RCH$H(SH)R
-
+ H,S
-lO-20 ’ 600K 327°C
of a heterocyclic
RSH+“@-RHr’l@$+H2S
figure
6
a ’
8 ’ 1 0 s 8 0 I 650K
700K
x I
I 1 * I I * I 750K
Temperature Interchange
800K 527°C
among sulphur groups and the organic matrix
FUEL, 1979, Vol 58, September
659
Transformations of 0- and S-groups during coal liquefaction via hydrogen: L. Messenger and A. A ttar
L”r
pant in the reaction is a gas. The pressure effect on AG, AG!, is given by: AG: = CaiRTh
ln(p%)
= R T(2hi)
(16)
+RTln(n(&)“)
K -10 -201 ’ 6Oi!K 327% Figure
7
’
’
’
’ 650K
0
E
8
8
’ ’ 700K
n
s
I
’ 750K
’
’
n
Temperature Interchange
between
’
’ 8OOK 527’C
For hydrogenation 136 atm: %
oxygen and sulphur functional
x 1.1 and #HsS = 0.78 RSR + 2H2 -+ 2RH + H2.S
QrOUPS
Substituting Interchangeability
of oxygen and sulphur
AGj(p
Aliphatic and aromatic alcohols can react with hydrogen sulphide and form thiols and water, as indicated in Figure 7. However, the thiols are unstable in the presence of hydrogen and/or hydrogen donors and decompose to form a hydrocarbon and hydrogen sulphide. Solvent and pressure effects
The effect of the solvent and of pressure on the thermodynamics of a liquid or solid reaction is in general small. The type of solvent is important when it can solvate the reactant or the products of the reaction. The energy associated with solvation of polar group like C=O or polar molecules like Hz0 is usually small, e.g. AGklve,,, < 1216 kJ/mol. Thus the effect of the solvent on the equilibrium is in general small. However, solvents can enhance the rate of reaction and prevent repolymerization of the primary products of the liquefaction. Only very high pressures can have a significant effect on the thermodynamics of reactions in liquid or solid phases. Somewhat larger effect can however be observed if one or more of the reactants or products is a gas. At high pressures, the solubility of hydrogen in liquefaction solvents increases. Therefore the rate of hydrogenation is enhanced by applying high pressures. Pressure effects on AG can be estimated from the change in the volume Av, due to the reaction: AVR = &iVi AG: s ApZ:cUiVi
(14) (1%
For typical liquefaction pressures, <14 MPa (<2000 psi), the pressure effect is negligible unless at least one partici-
660
FUEL, 1979, Vol 58, September
of an aliphatic sulphide at 4OO’C and
the numerical
values in equation
= 136 atm, pref = 1 atm) = -30.0
(17) (16) yields kJ/mol
(18)
At 4OO’C and 54.4 atm (800 psi) AGj(p = 54.4 atm, 1 atm) = 29.7 kJ/mol. The pressure effect could be important and may change significantly the values of the equilibrium constants when gases take part in the reactions. However, its main effect will be on the rates of the liquefaction reaction.
pref =
REFERENCES 1 2
3 4
Tingey, G. L. and Morrey, J. R., ‘Coal Structure and Reactivity’, Battelle Pacific Northwest Laboratories, 1973 Whitehurst, D. O., Farcasiu, M., Mitchell, T. 0. and Orchest, J. J., ‘The Nature and Origin of Asphaltenes in Processed Coal’, EPRI Rep. AF480, July 1977 Whitehurst, D. O., Farcasiu, M. and Mitchell, T. O., ‘The Nature and Origin of Asphaltenes in Processed Coals’, EPRI Rep. AF-252, Feb. 1976 Attar, A. and Dupuis, F., ‘On the Distribution of Sulphur Groups in Coal’, Prep. Div. Fuel Chem., ACS, 1978,23 (2), 44
5 6 I 8 9 10 11 12 13
14
Schiller, J. E. Anal. Chem. 1977,49, (14), 2292 Blom, L., Thesis, Univ. ofDelft, 1960. Quotedby Dryden, I. G. C., in Chemistry of Coal Utilization, Suppl. Vol. (Ed. H. H. Lowry), John Wiley, New York, 1963, Ch. 6, p 267 Shultz, J. L., Kessler, T., Friedel, R. A. and Sharkey, A. G. Jr Fuel 1972,51,242 Smith, J. W. Fuel 1966,45, 233 Ciola, R. Ind. Eng. Chem. 1957,49 (lo), 1789 Benson, S. W., 7’hermochemical Kinetics, John Wiley, New
York, 1968 Whitehurst 1976 Given 1976 Attar, A., ‘The Fundamental Interactions of Sulfur and Modelling of the Distribution of Sulfur in Products of Coal Pyrolysis’, presented at the AIChE Meeting, March 1977, Houston, Texas Attar, A., Fuel 1978,57,203
The thermodynamic equilibrium constants for the reduction of oxygen and sulphur functional groups in hydrogen and in tetralin were calculated for the temperature range 327-527°C. All the reduction reactions of the oxygen groups are thermodynamically favourable both with hydrogen and with tetralin. The reduction reactions of all the sulphur functional groups in tetralin are favourable except for the reductions of thiophenes below 407°C. However, molecular hydrogen cannot reduce thiophenes or aryl sulphides below about 59O’C. Reactions in which oxygen from water is captured by the organic matrix are not favourable reactions; however, capturing of sulphur from hydrogen sulphide or alkyl thiols can occur in this range of temperatures. The most stable products are condensed thiophenic structures.
Very few direct measurements on coal can yield information on the structure of coal. However, it is plausible to assume that the organic groups in coal are very similar to those which are found in oils and in coal-derived materials. Moreover, if the well accepted principle that homologue compounds react in an analogous manner is applied to coal? one would assume that the chemistry and thermodynamics of the reaction of coal functional groups are the same as those of the homolog groups in homologous low-molecular-weight compounds. The kinetics of the reaction of coal functional groups cannot be calculated from knowledge on small molecules. Moreover, steric hindrance and mass transport can seriously reduce the apparent rate of reaction of coal-derived molecules. Different coals consist of essentially the same organic functional groups, but the concentration of each group is different in each coal. However, to a first-order approximation the chemistry and the thermodynamics of these groups are independent of the nature of the particular coal. The thermodynamic properties of a molecule can be calculated as the sum of the contributions of individual functional groups or atoms. This concept is well established and indeed it received very wide application in the petroleum industry. This concept implies that the thermodynamic feasibility of a reaction can be calculated taking into account only the functional groups that were transformed in the reaction. The kinetics of the reactions of the various functional groups could be assumed to be independent of the coal and a function only of the reagent, the reactive group and the temperature. The chemistry and the thermodynamics of reactions of functional groups in coal during liquefaction are examined. The results are consistent with experimental data on coal and on model compounds. In particular, data are available (e.g. Whitehurst et ~l.*~) that suggest that during liquefac-
0016-2361179/090655-06S2.00 0 1979 IPC Business Press
tion elimination of oxygen proceeds much more rapidly than elimination of sulphur. The results of the thermodynamic calculations show indeed that elimination of oxygen functional groups is much more favoured thermodynamically than elimination of sulphur groups. However, it should be recognized that the rate or the kinetics of the elimination reactions determines the results of the liquefaction. Two additional points should be made in this context: (A) The equilibrium conversion which is determined by thermodynamics is the upper limit of the obtainable conversion in a closed system. The value of this ‘upper limit’ is independent of the rate at which the reaction proceeds. (B) In many reactions, there is a ‘reasonable’ correspondence between thermodynamics and kinetics, e.g. reactions which are more favourable thermodynamically proceed at faster rates. The calculations in the paper are elementary but the results are interesting and useful since they permit explanation of some of the empirical facts observed in coal liquefaction.
FUNCTIONAL
GROUPS IN COAL: INVENTORY
Tingey and Morrey’ published an excellent review of the oxygen, sulphur and nitrogen functional groups in coal. Whitehurst et al.‘? published an excellent survey and data on the reactivities of coals and of functional groups during liquefaction. To keep the discussion short, work which has been referred to in the previously mentioned reviews will not be mentioned specifically. The types and relative concentrations of various groups are a function of the rank of the coal, the material considered, and the specific conditions to which the coal had been exposed. The most important groups believed to be
FUEL, 1979, Vol 58, September
665
Transformations of 0- and S-groups during coal liquefaction via hydrogen: L. Messenger and A. Attar Tab/e 7 Oxygen,
sulphur and nitrogen
Type of group (written
functional
for the oxygen analogue)
groups in coal Oxygen
analogue
ROH
Alcohol-aliphatic Alcohol-aromatic
@OH
Acid (aromatic)
@JC
40
Sulphur analogue
Nitrogen
RSHa @Ha
-. _
-
@-CN
analogueb
‘OH Ethers - Aliphatic Aromatic Mixed Furans (and thiophenes) Pyran (and pyridine) Quinones Condensed heteroaromatics 8Most recently bMost recently
R-S-Ra I#-S-f#la o-S-Ra
R-O-R O-0-3 o-0-R
discussed by Attar and Dupuis’ discussed by Schillers who studied coal-derived
liquids
present in coal are summarized in Table 1, which is an augumented version of Tingey and Morrey’s list’. Typical concentrations of organic oxygen in coal are in the range of 0.5-20 wt % and are usually around lo-14 wt % in bituminous coals. The concentration of organic sulphur varies in the range of 0.0-6 wt % and is often around l-3 wt % in bituminous coals. Less information is available on the concentration of nitrogen in coal; the available data suggest however that it varies in the range of 0.52.5 wt % and in bituminous coals it is often close to 1.5 wt %. The data of Blom6 suggest that the distribution of oxygen functional groups is approximately 35-50 wt % C=O, 30-50 wt % OH, O-25 wt % COOH, O-5 wt % OCH3, and O-l 5 wt % in rings. Etheric oxygen was detected by Blom only in low-rank coals, with carbon content below 65 wt % (daf). The extremely low concentrations of etheric oxygen are not in accord with more recent observations which suggest that the initial liquefaction step is associated with cleavage of etheric bonds (e.g. Whitehurst?. The distribution of the sulphur groups in several coals has recently been studied by Attar and Dupuis4, who estimate that about 22-28% of the organic sulphur in bituminous and low-rank coals is in the form of aliphatic and alicyclic sulphides. lo-30% of the sulphur is in the form of thiols, and the rest is in the form of thiophenes and condensed thiophenic structures. Larger ratios of condensed thiophenic structures were observed in higher-rank coals. The author is not aware of any systematic study of nitrogen functional groups in coal. Studies of the nitrogen compounds in coal tar and in liquefaction products suggest that most of the nitrogen is bound in pyridines, condensed pyridines and nitriles (e.g. Schiller’, Schultz et al. ‘, and Smith*). It has never been demonstrated that amines are not present in coal. In fact, the author believes that amines are the source of at least part of the ammonia that coal releases during pyrolysis. The H-C structure of coal has been a source of controversy for years; however, it seems that most researchers accept today the theory that coals consist of lamellae, of 2-7 condensed aromatic rings connected by short aliphatic bridges, oxygen and sulphur. Alicyclic rings condensed with aromatic rings are also believed to be present in coal, as well as short aliphatic bridges. Various qualitative and quantitative estimates of the H-C functionalities has been published recently. Tingey and Morrey’ summarized some of them; many others have appeared since then in the literature.
656
FUEL,
1979, Vol 58, September
TRANSFORMATIONS
OF FUNCTIONAL
GROUPS
Three kinds of liquefaction reactions should be differentiated: 1. Reactions which result in the formation of products with significantly smaller molecular weight than the original material, e.g. cleavage of etheric oxygen:
R-O-4
4[Hl_ RH + $JH + Hz0
(1)
cleavage of sulphidic sulphur:
R-S-R’
4’H_’ RH + H2S + R’H
scission of carbon-carbon
(2)
bonds:
R-P’ 2[H_! RH t R’H
(3)
2. Reactions which result in a small reduction in the molecular weight but in which the functionality is significantly reduced, e.g. reduction of a hydroxyl:
ROH 22 reduction
RH + H20
(4)
of a ketone:
0 R’C R T? elimination
RCH2R’ + Hz0 of a thiol:
RSH 2[H_! RH + H2S
(6)
3. Ring opening; e.g. reduction 0, desulphurization
a
RC,H,o+
H,O
(7)
of thiophenes: BIHI
Q
of furan:
C H ‘ m*
H*S
(8)
The first class of reactions is the most effective class of liquefaction reactions because it results in a product with a
Transformations of 0- and S-groups during coal liquefaction via hydrogen: L. Messenger and A. Attar
significantly lower molecular weight and functionality. The second and third classes of reactions reduce the molecular weight only to a limited extent. However, the functionality decreases significantly. Since the extractability of the products depends on the functionality and on the molecular weight, the rates at which reactions of the first kind occur will determine to a large extent the rate of liquefaction.
Step 4: Calculate A@!00, A&&,0
= -39.89 A%M
Thermodynamic calculations were made for the feasibility of the transformations of different functional groups during liquefaction in hydrogen and in hydrogen donors. The method that has been used was proposed by Ciola’. The data that were used were taken from Benson”. The method of calculation assumes that only the functional groups which change in a reaction affect the Gibbs freesenergy of the reaction and that this may be calculated via group contributions. Exact details and many examples are described in the comprehensive study of Benson”, and therefore only one example will be presented here. Example: Calculation of the free energy of reduction of an aliphatic ether by molecular hydrogen at 600 K. Step I: Write the reaction noting the functional groups that are transformed. + 2H2 + 2RCH3 + H20
(9)
Using Benson’s notation: 2(C-COH2)
+ (0-C2)
+ 2H2 + 2(C-CH3)
+ H20
(10)
Step 2: Obtain A@,, , A&g and cP values for the groups involved (e.g. tables pp 178-215 in Benson’s book’@). Group
AH$!,,
A%,
Aq(600
C-COH;! 0-Q C-CH3 H2 Hz0
-8.5 -23.0 -10.08 0 -57.8
10.3 8.68 30.41 31.2 45.1
9.43 3.8 10.79 7.0 8.7
Step 3: Calculate AL!$~~~, A,$,,,
K)
A$, (298 K) 4.99 3.4 6.19 6.9 8.0
_ 2(O)-2(-8.5) = -158.9
= -57.8
= 14.24 - 6.38 In ‘G
= Mj&,
- 6OOAS$,,,
=
--39.89 - 600 x 10-3(9.776) = -45.75
kcal/mol = -204.1
A&-
lnK=_-=-
-45.75
RT
kJ/mol
x 103
1.987 x 600
= -38.38
RESULTS AND ANALYSIS Calculations of the Gibbs free energy of various reactions were made for the temperature range of 600-800 K (327-527OC), which is the range important in coal liquefaction. Two sets of results are presented for the reduction of each functional group, the first with molecular hydrogen and the second with tetralin. It should be recognized that the difference between the Gibbs free energy of reduction of a functional group with hydrogen and thht of reduction of the same group with tetralin is fried. Its value is equal to the Gibbs free energy of a hypothetical reaction where tetralin is converted to naphthalene and two molecules of gaseous hydrogen. In addition the thermodynamics of several reactions in which skeletal changes occur or in which an interchange between sulphur and oxygen would occur are also presented. Deoxygenation Figures 1 and 2 show the logarithm
of the equilibrium
Reactlon
+ 2(-10.08) = -37.96
kJ/mol
+ Ac$joo In ‘g
and A&00
- (-23.0)
x 1O-3 c;boO = -37.96
kcal/mol = -167.0
= A%,,*
AG&,,,
A
ROH
+ H, -
6 C
O0H ROR
+ H,QH . H,O + 2H,-2RH * Hz0
D 9
r
E
401
c
30:
D
RH
+ 2 H,-
+ H,O
C,H,,+
H,O
$‘o@+2H2-20H+
F ?.2H,
-
H,O C,H,
+ H,O
kcal/mol
kJ/mol k
A$,,,
+ (600-298)
and K.
= 9.776 cal/mol K = 40.9 J/mol K
50
AH:298 = &_iAHi,,
A@,,,
+ 0.302 (-6.38)
Method of calculation
RCH2-0-CH2R
= Al&g
mj&r,,
= 45.1 t 2(30.41)
- 2(31.2) - 8.68 - 2(10.3)
al
$ J
20--F
E
toc
= 14.24 cal/mol K = 59.6 J/mol K
A
0
o-
Acsoo
= 8.7 + 2(10.79)
= -6.38 A&98
- 2(7.0) -- 3.8 -- 2(9.43)
cal/mol K = -26.7
J/mol K
= 8.0 + 2(6.19) - 2(6.9) -- 3.4 - 2(4.99)
= -6.8 cal/mol K = -28.5
J/mol K
-207 600K 327’C figure I hydrogen
650K
700K
750K
Temperature Reduction
of oxygen functional
FUEL,
800 K 527’C
groups by molecular
1979, Vol 58, September
657
Transformations of 0- and S-groups during coal liquefaction via hydrogen: L. Messenger and A. Attar
A
ROH
+‘I2
8
@OH
+
C
ROR
+@J)-
D p E
‘12 a
+
-
@‘Ii
2RH
m
l
@OO
-RH
0 co
-C,H,,+
+ m
‘I2
0 0 03
*Hz0
+‘I,@@
+ @@+H,O @@
-2’l’H
liquefaction since such reactions reduce significantly the molecular weight and the functionality of the product. Elimination of aliphatic and alicyclic ethers is thermodynamically a very favourable reaction in hydrogen and in tetralin. However, elimination of aromatic ethers is a substantially less favourable reaction.
+H,O
+ H,O
+ @$I
+H,O
Desulphurization
-10
1 L
-201 600K ’
’
8
8
’
650K
c
’
4
8
’
700K
’
L
’
’
’ n
750 K
’
’
327’C
’
’
600K 527°C
Temperature Figure
2
Reduction
of oxygen
functional
groups
by tetralin
constant for the reactions which involve elimination of oxygen functional groups in hydrogen and in tetralin respectively. The data show that elimination of oxygen as water is a thermodynamically favourable reaction in hydrogenation with either hydrogen or tetralin. All the oxygencontaining functional groups are removable except for aromatic ethers and possibly dibenzofuran*. The equilibrium constant is very sensitive to temperature variation when hydrogen is used and is not sensitive when tetralin is the reducing agent. At low temperatures, e.g. around 330°C, elimination of water from phenols by hydrogen is extremely favourable but it becomes much less favourable at 53O’C (although still favourable). The equilibrium constant drops from e5.05 to e1.68 in this range of temperature with Hz, while it is essentially constant throughout this range of temperature when tetralin is considered. The value of K in the latter case varies from e4.’ to e4.9. The value of the equilibrium constant for the reduction of phenols is the same for hydrogen and tetralin around 340345”C, but at higher temperatures tetralin is a better hydrogenation reagent. The implication of these calculations for the practice of hydroliquefaction is that a substantial fraction of the oxygen functional groups would be eliminated as water during the preheating of the coal slurry before it enters the liquefier. Elimination of water during the preheating period was observed experimentally in numerous studies. Furthermore, the organic matrix of coal contains alicyclic groups condensed to aromatic rings (Whitehurst”, Given” andbothers). Such alicyclic hydrogen can be donated to the oxygen functional groups during the preheating period. Therefore, formation of water in the preheater could be achieved in the presence and in the absence of hydrogen and/or hydrogen donor solvent, provided that mass transport in the organic matrix permits contact between the oxygen groups and the alicyclic hydrogens. Elimination of etheric bonds has a special importance in
A priori, one could assume that the behaviour of the sulphur functional groups would be very similar to that of the analogous oxygen functional groups. Indeed, similarity is observed in many cases; however, the thermodynamic feasibility of the elimination of sulphur is often significantly different from that of the analogous oxygen functional groups. Three factors determine the difference between the behaviour of the sulphur functional groups and that of the oxygen analogue: 1. The hydrogen-oxygen bond is much stronger than the hydrogen-sulphur bond. For example, the standard enthalpy of formation of water, H20, is -242.0 kJ/mol while that of H2S is 20.1 kJ/moi. 2. The carbon-oxygen bond is much stronger than the carbon-sulphur bond. For example, the standard enthalpy of formation of CO2 is -397.6 kJ/mol while that of CS2 is 117.2 kJ/mol. 3. The sulphur n-electrons can resonate with groups which contain n bonds, e.g. double bonds and aromatic rings. The tendency of oxygen electrons to participate in resonating bonds is substantially smaller. Figures 3 and 4 show the equilibrium constant for the hydrogenation reactions of sulphur functional groups in the temperature range of 600-800 K with hydrogen and with tetralin respectively. Hydrodesulphurization of the sulphur groups in which resonance with the hydrocarbon matrix is not possible could proceed readily with hydrogen or tetralin. However, where the sulphur can resonate with an aromatic ring or with double bonds, hydrogen does not reduce the sulphur. Tetralin is not very effective either; however, it becomes more effective at higher temperatures. For example, hydrodesulphurization of a thiophenic ring by tetralin is not feasible below about 407°C but it is thermodynamically favourable above these temperatures. React RSH+H,-
B
@SH
C
RSR
DQ t QSUI
LO
F
1
600 K 327’C
ion
A
-
+ 2H,+ 2H,-
700 K
H,S
cDH . H,S
ZRH . C,tQ
+ 2H,-
i9.2H2
650K
Rti*
+ H,
-
2@H C,H,
H,S
H,S + H,S + H,S
750K
Temperature
*The quality of the available data on dibenzofuran does not permit a conclusive statement about its amenability to hydrogenation
656
FUEL, 1979, Vol 58, September
Figure 3 hydrogen
Reduction
of rulphur
functional
groups by molecular
EOOK 527°C
Transformations of 0- and S-groups during coal liquefaction via hydrogen: L. Messenger and A. A ttar Reactton A 0
OSH
C
RSR
D Q E LO-
and the formation
+‘/z l
o co
C
ZO-
D A
ti
-
wlO$ -I -
0 E
O-
F
-
a
-2RH
+ a
-C&H,,+ -2@H
U’S@’ +GXI
FQ*W 30 -
-CC,H,+
l“200
U’H
+ @
6 1 b 1 1 3 j a 1 650~
m +
@9
@@j
+H,S
+ H,S + H,S + H,S
h ’
1 1 1 1 1 8 1 750K
700 K
800K 527°C
Temperature figure
4
Reduction
compound:
of sulphur functional
l
H,S
groups by tetralin
This interesting observation correlates extremely well with the experimentally determined optimal liquefactions temperature in hydrogen donors, 410450°C. The equilibrium constant for hydrodesulphurizations is less sensitive to the temperature than the equilibrium constant for hydrodeoxygenation. The implication of the results for the operation of liquefiers is that the aliphatic sulphur could be eliminated as hydrogen sulphide during the preheating period. However, hydrodesulphurization of thiophenic sulphur could be achieved only when hydrogen donor is used at an adequate temperature, e.g. when the donor is tetralin the temperature has to be above 407°C. The results imply also that coals that contain a substantial fraction of organic sulphur can be desulphurized in relatively mild conditions if most of the organic sulphur is in aliphatic structures. These conditions were deduced from experimental observations by Attar and Dupuis4, who suggested using the distribution of the organic sulphur groups to screen coals with a large fraction of organic sulphur into ‘easily desulphurizable’ coals and ‘stubborn coals’. Coals in which most of the organic sulphur is thiolic or is present as aliphatic sulphides can be easily desulphurized. Coals in which most of the organic sulphur is thiophenic cannot be desulphurized with hydrogen, and only in a Iimited range of temperatures with hydrogen donors like tetralin. Attar and Dupuis4 proposed also an analytical procedure for the determination of the distribution of the organic sulphur groups in coals.
During the liquefaction Hz0 and H2S are released into the reactor; therefore, one could conceive reactions between the organic matrix and H2S or Hz0 which could fuc these again in organic compounds. Two plausible reactions which could fix S or 0 are:
H
RHC = CHR
I
RHC=CHR
30
L
-201 600K 327°C
RCH,CHlOHlR
CH(OCHRCH,R) l-&D -e
l
CH,R + 2H,
8 1 1 c 1 - 0 a 0 1 I s I 650K
I I
700 K
a I I I I
750K
800K 527-C
Temperature Figure 5
Interchange
among oxygen
groups and the organic matrix
Reaction LO
H
RHC 2 CHR
I
RHC = CHR + RCH,CHI
l
H,S -
RCH,CHiSH)R SH )R -
RICH,I,CH(SCHRCH,RlCH,R L
-t 2Ot L
@--@I
+ H,S -
@--
+ 2H, a
r
k
m IO0” J
L
L OI
H
- 10
t
I-201 600K 327°C
(12)
_
RCH2CH10H)R
20
(11)
+
@I-@
+ H,O l
RICH,),
t
R(CH&CH(SCHRCH2R)CH2R
(13)
2H,
Reactton
LO
Trapping of oxygen and sulphur by the omanic matrix
RHC=CHR + RCH$H(SH)R
l
Figures 5 and 6 show the values of the equilibrium constant for these reactions in the temperature range 600-800 K for Hz0 and H2S respectively. Figure 5 shows that aliphatic alcohols could react with olefins and form stable compounds. Similarly, water could react with the aromatic structure and form a furanic ring. However, since in liquefaction hydrogen or hydrogen donors are also present in the reactor, and since hydrodeoxygenation of ethers and furans are feasible reactions in these ranges of temperature, fixation of water in the organic matrix is not likely to occur during the hydroliquefaction of coal. Figure 6 shows that trapping of sulphur by double bonds is not a feasible reaction in the temperature range of 600800 K. However, trapping of sulphur as a heteroaromatic thiophenic ring is feasible. Such structures are stable in the presence of hydrogen as indicated in Figure 3, and are stable in tetralin up to about 407’C. Therefore, fixation of sulphur into the organic matrix is a feasible reaction and could result in reduction of the efficiency of the hydroliquefaction and hydrodesulphurization. Experimental data exist which support the possibility of ftiing sulphur in the organic matrix. A detailed review of these reactions was published by Attar13’14.
30 E
RHC=CHR + H2S + RCH$H(SH)R
-
+ H,S
-lO-20 ’ 600K 327°C
of a heterocyclic
RSH+“@-RHr’l@$+H2S
figure
6
a ’
8 ’ 1 0 s 8 0 I 650K
700K
x I
I 1 * I I * I 750K
Temperature Interchange
800K 527°C
among sulphur groups and the organic matrix
FUEL, 1979, Vol 58, September
659
Transformations of 0- and S-groups during coal liquefaction via hydrogen: L. Messenger and A. A ttar
L”r
pant in the reaction is a gas. The pressure effect on AG, AG!, is given by: AG: = CaiRTh
ln(p%)
= R T(2hi)
(16)
+RTln(n(&)“)
K -10 -201 ’ 6Oi!K 327% Figure
7
’
’
’
’ 650K
0
E
8
8
’ ’ 700K
n
s
I
’ 750K
’
’
n
Temperature Interchange
between
’
’ 8OOK 527’C
For hydrogenation 136 atm: %
oxygen and sulphur functional
x 1.1 and #HsS = 0.78 RSR + 2H2 -+ 2RH + H2.S
QrOUPS
Substituting Interchangeability
of oxygen and sulphur
AGj(p
Aliphatic and aromatic alcohols can react with hydrogen sulphide and form thiols and water, as indicated in Figure 7. However, the thiols are unstable in the presence of hydrogen and/or hydrogen donors and decompose to form a hydrocarbon and hydrogen sulphide. Solvent and pressure effects
The effect of the solvent and of pressure on the thermodynamics of a liquid or solid reaction is in general small. The type of solvent is important when it can solvate the reactant or the products of the reaction. The energy associated with solvation of polar group like C=O or polar molecules like Hz0 is usually small, e.g. AGklve,,, < 1216 kJ/mol. Thus the effect of the solvent on the equilibrium is in general small. However, solvents can enhance the rate of reaction and prevent repolymerization of the primary products of the liquefaction. Only very high pressures can have a significant effect on the thermodynamics of reactions in liquid or solid phases. Somewhat larger effect can however be observed if one or more of the reactants or products is a gas. At high pressures, the solubility of hydrogen in liquefaction solvents increases. Therefore the rate of hydrogenation is enhanced by applying high pressures. Pressure effects on AG can be estimated from the change in the volume Av, due to the reaction: AVR = &iVi AG: s ApZ:cUiVi
(14) (1%
For typical liquefaction pressures, <14 MPa (<2000 psi), the pressure effect is negligible unless at least one partici-
660
FUEL, 1979, Vol 58, September
of an aliphatic sulphide at 4OO’C and
the numerical
values in equation
= 136 atm, pref = 1 atm) = -30.0
(17) (16) yields kJ/mol
(18)
At 4OO’C and 54.4 atm (800 psi) AGj(p = 54.4 atm, 1 atm) = 29.7 kJ/mol. The pressure effect could be important and may change significantly the values of the equilibrium constants when gases take part in the reactions. However, its main effect will be on the rates of the liquefaction reaction.
pref =
REFERENCES 1 2
3 4
Tingey, G. L. and Morrey, J. R., ‘Coal Structure and Reactivity’, Battelle Pacific Northwest Laboratories, 1973 Whitehurst, D. O., Farcasiu, M., Mitchell, T. 0. and Orchest, J. J., ‘The Nature and Origin of Asphaltenes in Processed Coal’, EPRI Rep. AF480, July 1977 Whitehurst, D. O., Farcasiu, M. and Mitchell, T. O., ‘The Nature and Origin of Asphaltenes in Processed Coals’, EPRI Rep. AF-252, Feb. 1976 Attar, A. and Dupuis, F., ‘On the Distribution of Sulphur Groups in Coal’, Prep. Div. Fuel Chem., ACS, 1978,23 (2), 44
5 6 I 8 9 10 11 12 13
14
Schiller, J. E. Anal. Chem. 1977,49, (14), 2292 Blom, L., Thesis, Univ. ofDelft, 1960. Quotedby Dryden, I. G. C., in Chemistry of Coal Utilization, Suppl. Vol. (Ed. H. H. Lowry), John Wiley, New York, 1963, Ch. 6, p 267 Shultz, J. L., Kessler, T., Friedel, R. A. and Sharkey, A. G. Jr Fuel 1972,51,242 Smith, J. W. Fuel 1966,45, 233 Ciola, R. Ind. Eng. Chem. 1957,49 (lo), 1789 Benson, S. W., 7’hermochemical Kinetics, John Wiley, New
York, 1968 Whitehurst 1976 Given 1976 Attar, A., ‘The Fundamental Interactions of Sulfur and Modelling of the Distribution of Sulfur in Products of Coal Pyrolysis’, presented at the AIChE Meeting, March 1977, Houston, Texas Attar, A., Fuel 1978,57,203