Energy & Fuels 1991,5, 459-463
459
Tritium as a Tracer in Coal Liquefaction. 4. Hydrogen-Exchange Reactions between Hydrogen in Coals and Tritiated Hydrogen Molecule Toshiaki Kabe,* Toshiyuki Horimatsu, Atsushi Ishihara, Hideo Kameyama, and Kyoko Yamamoto Department of Chemical Engineering, Tokyo University of Agriculture and Technology, Nakamachi, Koganei-shi, Tokyo 184, Japan Received August 28, 1990. Revised Manuscript Received January 22, 1991
The hydrogen-exchange reactions between Datong, Wandoan, or Morwell coal and tritiated gaseous hydrogen were investigated to estimate the hydrogen mobility of coals under coal liquefaction conditions. The hydrogen-exchange reaction of coal with gaseous hydrogen remarkably increased with a rise from 350 to 400 O C and the hydrogen-exchange ratio of Datong and Morwell coals approached nearly 50%. The hydrogen-exchange reaction between tetralin and gaseous hydrogen was very slow in this temperature range and the hydrogen-exchange ratio of tetralin was less than 1% at 400 "C. These results suggest that the radicals produced in the coal would strongly contribute to the hydrogen-exchange reaction with gaseous hydrogen. The hydrogen-exchange reactions between these three coals and gaseous hydrogen proceeded even at 300 OC and the hydrogen-exchange ratio increased in the order of Datog cv Wandoan < Morwell. It was suggested that hydrogens of functional groups such as -OH and -NH2 in coals would be exchangeable with gaseous hydrogen at 300 "C.
Introduction In order to develop practical processes for coal liquefaction, it is important to elucidate the mechanisms of coal liquefaction. Since the reactions in this process consist of hydrocracking by gaseous hydrogen and hydrogen transfer from donor solvents, the estimation of the mobility of hydrogen in coal can be regarded as essential for process design. Following the pioneering work of Fu and Blaustein,' in which the reaction of coal with deuterium oxide under microwave irradiation was conducted, deuterium has been used to trace the reaction pathways in coal liquef a c t i ~ n . ~Schweighardt -~~ et al. reported that deuterium nuclear magnetic resonance spectroscopy was a valuable method for tracing the incorporation of deuterium into a coal-derived liquid (synthoil).2 They showed that deuterium was introduced into a wide range of structural groups in produds, which included aromatic and aliphatic structures. Franz reported the incorporation of deuterium from deuterated tetralin into coal products.6 Cronauer et al. traced a similar reaction by deuterium NMR spec~
(1) Fu, Y. C.; Blaustein, B. D. Chem. Ind. 1967, 1257. (2) Schweighardt, F. K.; Bockrath,B. C.; Friedel, R. A.; Retkofsky, H. L.Anal. Chem. 1976,48, 1254. (3) Heredy, L. A.; Skowromki,R. P.;Ratto, J. J.; Goldberg, I. B. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1981,26,114. (4) Ratto, J. J.; Heredy, L. A.; Skowronski, R. P. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1979,24, 154. (5) Franz, J. A. Fuel 1979,58, 405. (6) Franz, J. A,; Camaioni, D. M. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel. C k m . 1981,26, 106. ( 7 ) Cronauer, D. C.; McNeil, R. I.; Young,D. C.; Ruberto, R. G.Fuel 1982, 61, 610. (8) Brower, K. P. J. Org. Chem. 1982,47, 1889. (9) W h n , M. A.; Collin, P. J.; Barron, P. F.; Vsesalo, A. M. Fuel Process. Technol. 1982,5, 281. (IO) Wileon, M. A,; Vaasalo, A. M.; Collin, P. J. Fuel Process. Technol. 1984,8, 213. (11) Skowromki,R. P.: Ratto, J. J.: Goldberg. - I. B.; Heredy, L. A. Fuel
Table I. Chemical Analysis of Coals ultimate, w t % daf coal carbon hydrogen nitrogen sulfur oxygen proximate, wt % ash volatile matter fixed carbon moisture
Datonn
Wandoan
Morwell
82.3 4.6
76.9 6.7
70.6
0.9
1.1
0.6
0.6 11.6
0.3 15.0
0.3 24.0
5.3 33.9 57.0 3.8
7.7
2.2 44.8
43.0
40.7
4.5
12.3
tros~opy.~ Wilson et al. studied the reaction of coal with molecular deuterium in the presence of Ni/Mo catalyst? They found that, a t 400 OC, extensive scrambling of hydrogen and deuterium occurred among aromatic and CY to aromatic aliphatic hydrogen and deuterium substituents and that deuterium entered aromatic and CY to aromatic groups in preference to alkyl groups remote from aromatic ring. Although a number of attempts have been made to elucidate the mechanisms of coal liquefaction using deuterium tracer and NMR, there were few examples that enable a quantitative analysis of the hydrogen-transfer process in coal liquefaction. Skowronski et ala1' reported that the use of deuterium tracer methods with IH NMR and GC-MS was applicable to the estimation of the hydrogen incorporation during liquefaction of a bituminous coal, and that the abstraction of hydrogen from tetralin by coal-derived radicals is involved in the rate-determining step in the formation of soluble products. We have already reported that the tritium and C-14 tracer techniques were effective in tracing the reaction pathways of hydrogen atoms in coal liquefaction and gave quantitative information related to the mobility of hydrogen in C O ~ S . ' ~ - ~ ~
1984,63,440. (12) Maekawa, Y.; Nakata, Y.; Ueda, 5.;Yoehida, T.; Yoshida, Y. In Coal Liquefoction Fundamentals; ACS Symp. Ser. 139; Whitehurst, D. D., Ed.; American Chemical Society: Waehington, DC, 1990; p 315. (13) King, H. H.; Stock, L. M. Fuel 1982,61, 129.
0 1991 American Chemical Society
Kabe et al.
460 Energy & Fuels, Vol. 5, No. 3, 1991 Table 11. Distribution of Products and Tritium' tritium mas8.g mass,% dpm 70 coal 1.56 3.68 13 843 residu e 3.45 60388 6.78 15.80 16.86 SRC 2.70 0.85 24012 light oil 0.80 4 984 0.56 0.43 0.46 naphtha 659927' 74.14 0.34 gas 0.32* 126929 14.26 72.92 77.81 solvent 890083 105.99c 93.72 96.03d total a Wandoan coal 25.0113 g, tetralin 74.9174 g, initial hydrogen pressure 62.9 kg/cm2, temperature 400 "C,reaction time 120 min, initial radioactivity in gas phase 839779 dpm, initial amount of hydro en in gas phase 1.29 g, final pressure in gas phase 64.1 kg/ cm2. %Carbondioxide, carbon monoxide, and hydrocarbons are involved. The sum of the partial pressure is 1.3 kg/cm2. Weight of molecular hydrogen is not included. eRegardedas radioactivity in gaseous hydrogen. Recovery (daf basis). e Recovery.
In the course of our studies, we have found that the hydrogen-exchange reaction between coal and gaseous hydrogen proceeds even a t 300 O C in Datong coal liquefaction, which led us to compare the hydrogen-exchange reactions of three kinds of coals having different coal ranks in the extended range of temperature. In the present paper, the hydrogen-exchange reactions of Datong coal as a bituminous coal, Wandoan coal as a subbituminous coal, and Morwell brown coal as a brown coal with tritiated gaseous hydrogen were investigated in the temperature range of 200-400 "C and the hydrogen mobility of coal under coal liquefaction conditions were estimated in detail.
-
0 300
350 Reaction Temp.
400
[TI Figure 1. Change in yields of residue and SRC with reaction temperature. Datong 0,A;Wandoan 0,A;Morwell 0, A;residue 0 , 0 , 0 ;SRC A, A, A. Reaction time 2 h. Residue T e t r a i i n Datong
0
H
MorweN
0
0
\ '0
P
Experimental Section Materials. The samples of Datong, Wandoan, and Morwell brown coals were ground to 100%
10
0 200
300
LOO
Reaction T e m p e r a t u r e ('C) Figure 3. Effect of reaction temperature on the hydrogen-exchange ratio of coal with gaseous hydrogen.
Results and Discussion Hydrogen-Exchange Reaction between Coals and Gaseous Hydrogen. Datong, Wandoan, and Morwell coals were liquefied at 3W400 OC for 120 min and results are shown in Figure 1. The yields of SRC (coal products) increased with an increase in temperature. The rate of liquefaction decreased in the order of Momell> Wandoan > Datong, which shows that coals with the higher carbon content are more difficult to liquefy. Figure 2 shows the change in the tritium concentrations of residue and tetralin with temperature. At 200-230 "C, the tritium concentrations were very low and the hydrogen exchange hardly occurred. At 300 "C, tritium was introduced to residue, which indicated that hydrogen-exchangereaction between coal (residue and SRC) and gaseous hydrogen occurred at this temperature (vide infra). The tritium concentration of residue increased with an increase in temperature. However, the tritium concentration of tetralin remained very low below 350 "C and it was much smaller than that of residue in the whole range of temperature. The hydrogen-exchange ratio is plotted against reaction temperature in Figure 3. Although the liquefaction scarcely proceeded at 300 OC, the hydrogen-exchange reaction of coal occurred. With a rise from 350 to 400 "C, the hydrogen-exchange ratio remarkably increased and nearly 50% of hydrogen in Datong coal or Morwell coal exchanged. For Wandoan coal, it was somewhat small. Since the hydrogen exchange between coal and gaseous hydrogen rapidly proceeded at temperatures required for significant coal liquefaction, it seems to be related to thermally produced radicals. The change of yields of residue and SRC with reaction time is plotted in Figure 4. Liquefactions of Datong, Wandoan, and Morwell coals were performed a t 400,400, and 350 "C, reapectively. When Morwell coal was liquefied a t 400 "C, yields became extremely large and it was difficult to obtain complete material and tritium balance. To make the yield of Morwell coal similar to those of Datong and Wandoan, liquefaction of Morwell coal was performed at 350 "C. A t 30 min, the yields of residues of Datong, Wandoan, and Morwell coals were 57,25,and 50%, and the yields of SRC were 42, 61,and 42%, respectively. Although the extent of liquefaction does not necessarily follow the rank or the carbon content of coals,2oDatong coal, which is the highest rank among three,was the most (20) Yarzab,
R. F. Fuel
1980,59,81.
1 2 3 6 Reaction Time[ hr I
5
Figure 4. Change in yields of residue and SRC with reaction temperature. Datong (400 "c) 0, A; Wandoan (400"c)0 , A; Morwell (350"c) 0, A; residue: 0 , 0 , 0; SRC A, A, A.
.t 0
50t "=/.I /
Lot
I
/
GI
Datong Wondoan Morwell
L 0
10
= I I
0
1
2
3
0 Q
0
LOO O C LOOT 35OOC 35OOCI
L
5
Reaction Time ( hr )
Figure 5. Effect of reaction time on the hydrogen-exchangeratio of coal with gaseous hydrogen. difficult to liquefy even at 400 "C. In contrast to these observations, the hydrogen-exchange reaction showed the different results. The hydrogen-exchange ratio is plotted against reaction time in Figure 5. The hydrogen-exchange ratio increased with the passage of time. After 300 min, the hydrogen-exchange ratio of Datong coal was over 50%. Even at 350 OC, the HER of Morwell coal approached nearly 50%. On the other hand, the HER of Wandoan coal was small, 30% even after 300 min. These results showed that, although Datong coal was the most difficult to liquefy, hydrogens in Datong coal were most mobile among the three coals. Since Datong coal has the highest rank, it can be presumed to have the most polycondensed structure. Since the radicals generated in liquefaction can be stabilized in aromatic molecules, they may promote the hydrogen-exchange reaction rather than the hydrocracking reaction. As coal was liquefied, tritium in the gas phase was introduced into the coal. However, the amount of hydrogen exchanged in coal seems to be different depending on the kind of coal structure. Figure 6 shows changes in the tritium distribution during coal liquefaction. Because in the noncatalytic system the amount of hydrogen added into coal was very small and the hydrogen distribution was nearly constant between initial and final stages, it was approximated by straight lines. In Figure 6,horizontal dotted and fulllinea represent the hydrogen distributions of coal (Morwell and Datong 13%; Wandoan 17%) and solvent (Morwell and Datong 74%; Wandoan 70%) among three phases, respectively.
Kabe et al.
462 Energy & Fuels, Vol. 5, No. 3, 1991
Reaction Time (h)
.,
Figure 6. Change in the tritium distribution during coal liquefaction. Datong (400 "C)0 , . , A; Wandoan (400 "C) 0 , 0, A; Morwell (300 "C)0, 0,A: coal 0 , 0 , 0 : gas phase 0, 0; solvent A,A,A. Upper full lines, hydrogen distribution of solvent among three phases; dotted lines, that of coal; arrow, that of gas phase. The arrow in Figure 6 representa the hydrogen distribution of gas phase (13%) among three phases. When the hydrogen-exchange reaction approaches equilibrium among three phases, the tritium distribution in each phase will approach the hydrogen distribution in the phase. When Datong coal was used, the hydrogen exchange between gas phase and coal has occurred at the initial stage of the reaction and then tritium has transferred from coal to solvent. In the cases of Morwell and Wandoan coals, the rate of tritium transfer from gas phase to coal and solvent was approximately equal. In Morwell coal, tritium introduced into coal transferred to solvent very slowly, while in Wandoan coal the tritium transfer from coal to solvent was very fast. The reason for this result has not been understood yet. The reactivity of hydrogen in coal decreased in the order of Datong > Morwell > Wandoan, which is consistent with the result from HER in Figure
2
4 6 8 R e a c t i o n Time[ hr ]
Figure 7. Change in yields of residue and SRC with reaction time at 300 "C. Datong 0,A;Wandoan 0 , A: MorwellO, A;residue 0 , 0 , 0; SRC A, A, A.
\
-
'0 U
c
.-0
t 0
L
c
t 0)
c 0 0
0
I
0
2
4
6
R e a c t i o n Time[ hr
8
1
Figure 8. Change in tritium concentrations with reaction time
at 300 "C.
5.
Since tetralin, which has aromatic and naphthene rings in its structure, can be regarded as a model of one type of structure in coal, the hydrogen-exchange reaction of tetralin with gaseous hydrogen was also investigated. The hydrogen-exchange ratio of tetralin was 0.2% at 350 O C and increased with an increase in temperature. However, the exchange ratio was below 1%even at 400 "C and the tritium concentration of tetralin was about one-tenth of that of coal. As shown in Figure 2, a substantial amount of tritium can be introduced into tetralin in the presence of coal. However, this result shows that tetralin could not be tritiated in the absence of coal. These results indicate that the exchange reaction of hydrogen in tetralin requires types of radicals produced from coal which are not produced from the thermolysis of neat tetralin. It seems that radicals produced in coal react easily with not only gaseous hydrogen but also hydrogen in tetralin to cause the hydrogen exchange. Hydrogen-ExchangeReaction at Low Temperature. Since it was clarified that the hydrogen-exchange reaction proceeded even at 300 "C,coal liquefaction was further investigated at 300 "C and the results are shown in Figure 7. Yields of residue and SRC were not changed with the elapse of time and Datong coal was hardly liquefied at 300 O C . Wandoan and Morwell coals were liquefied to give SRC in 20 and 25 wt % yields, respectively. However, these values did not change after 240-360 min, which indicates that hydrocracking reactions proceeded slowly at 300 OC. The change of the tritium concentration with the passage of time a t 300 "C is shown in Figure 8. The tritium concentration of each of the three coals approached
0
2
L
6
0
R e a c t i o n Time ( h r )
Figure 9. Effect of reaction time on the hydrogen-exchange ratio of coal with gaseous hydrogen at 300 O C .
low constant values below 3OOO dpm/g, and that of Datong coal was the highest among the three. Tritium transfers to coal through both hydrogen addition and exchange reaction. In order to estimate the hydrogen-exchange ratio, the amount of tritium transferred by hydrogen addition must be subtracted. The hydrogen-exchange ratio at 300 "C is plotted against reaction time in Figure 9. After the amount of hydrogen added was subtracted, the hydrogen-exchange ratio increased in the order of Datong H Wandoan < Morwell. The largest amount of tritium was transferred to Datong coal, however, since the amount of hydrogen added to Datong coal was larger than that to Wandoan and Morwell coals, the hydrogen-exchangeratio of Datong coal became small. The hydrogen-exchange ratio of Datong, Wandoan, and Morwell approached constant values, 4.5,5.0, and 7.8%, respectively. The HER of Morwell coal was largest and therefore the hydrogen exchange at 300 "C may be related to the exchange of
Energy & Fuek 1991,5,463-468 hydrogens in functional groups such as -OH and -NH2. Although detailed analysis of such active hydrogens has not been done, the comparable analysis of bituminous coals has been reported?lS Bituminous coals which have a chemical composition of C 75-85% and H 5.0-5.4 w t 90 contain 6-12 atom % of phenolic OH hydrogen for total hydrogen. Yokoyama et al.29has reported that high-rank coals which have a chemical composition of C 75-8470 and H 5.8-6.4% (daf) contain 3-9 atom % of phenolic OH hydrogen and carboxylic acid COOH hydrogen for total hydrogen, while low-rank coals which have a chemical composition of C 61-70% and H 5.3-6.0% (daf) contain 12-14 atom 7% of those. Kotanigawa et al.% have reported that the exchange reaction between deuterium gas and aromatic hydrogen in phenol took place rapidly at 350 "C with ZnO-Fe20q catalyst and that no such exchange reaction occurred in the absence of catalyst. They did not refer to the exchange reaction between deuterium gas and hydrogen of hydroxyl group in phenol. We attempted the reaction of phenol with tritiated gaseous hydrogen at 340 "C for 2 h in the absence of catalyst; 8.8% of the hydrogen in phenol underwent tritium exchange. Since it can be assumed that only the hydrogen of the hydroxyl group in phenol would exchange, this indicated that 53 % of the hydrogen of the hydroxyl (21) Pestryakov, B. V. Solid Fuel Chem. 1986,20(6), 3. (22) Mrekawa, Y. J. Jpn. Pet. Inst. 1976,18,746. (23) Yokoyama, S.; Itoh, M.; Takeya, G.Kogyo Kagaku Za~Shi1967, 70, 133. (24) Kotanigawa, T.;Shimokawa, K.; Yoshida, T.; Tamamoto,M. J. Phye. Chem. 1979,83,3020.
463
group in phenol exchanged with gaseous hydrogen at 340 "C for 2 h. Further, the reaction of aniline with tritiated gaseous hydrogen was also performed at 300 "C for 2 h in the absence of catalyst; 13.7% of the hydrogen in aniline underwent tritium exchange. Since it can be assumed that only the hydrogen of the amino group in aniline would exchange, this indicated that 48% of the hydrogen of the amino group in aniline exchanged with gaseous hydrogen at 300 "C for 2 h. These results support the suggestion that, in the present reaction of coal with gaseous hydrogen, OH and NH2 hydrogens of polycondensed aromatic compounds would have exchanged at lower temperatures.
Conclusions The hydrogen-exchange reactions of Datong coal (bituminous coal), Wandoan coal (subbituminous coal), and Morwell brown coal with tritium-labeled hydrogen molecules were investigated. The hydrogen-exchange reaction of coal with gaseous hydrogen largely increased with a rise from 350 to 400 "C. The hydrogen-exchangereaction of tetralin with gaseous hydrogen was more difficult to proceed than that of coal. These results suggest that the radicals produced on coal would be strongly related to the hydrogen-exchange reaction with gaseous hydrogen. The hydrogen-exchange reactions between coals and gaseous hydrogen proceeded even at 300 "C and the hydrogenexchange ratio increased in the order of Datong N Wandoan < Morwell. It is suggested that hydrogens of functional groups such as -OH and -NH2 in coals would be exchangeable with gaseous hydrogen at 300 "C. Registry No. Tetralin, 119-64-2; tritium, 10028-17-8.
Effect of Nonuniform Surface Reactivity on the Evolution of Pore Structure and Surface Area during Carbon Gasification R. H. Hurt,* A. F.Sarofim, and J. P. Longwell Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusett,j 02139 Received August 30, 1990. Revised Manuscript Received February 22, 1991 There is much experimental evidence that the gasification of carbon surfaces is often nonhomogeneous on a fine scale. Gasification of impure carbons, for example, often occurs in the immediate vicinity of catalytically active particles of inorganic matter, proceeding by the formation either of irregular pita or of channels of definits size and orientation. In the present paper, a model is presented which predicts the effect of nonuniform gasification on the evolution of surface area and pore size distribution for low-temperature porous carbons. Nonuniformity of surface reactivity can dramatically reduce the extent of surface area development during gasification or activation. Resulta of the model are used to explain several seta of previously published measurements of surface area evolution as a function of carbon conversion.
Introduction The evolution of pore structure and internal surface area during carbon gasification plays an important role in both the production of activated carbon and the kinetics of coal *Author to whom correspondenceshould be addressed. Current address: Combustion Research Facility, Sandia National Laboratories, Livermore, CA 945514989.
gasification. Several models treat the evolution of surface area and/or pore structure by considering the effect of widening of cylindrical pores due to uniform surface recession.'~ There is much experimental evidence, however, that the actual gasification process is more complex. The (1) Gavalae, G.R.AZChE J. 1980,36 (4), 577. (2) Simons, G. A. Combust. Sci. Tech. 1979, 19, 227.
0887-0624/91/2505-0463$02.50/0Q 1991 American Chemical Society
