Ind. Eng. Chem. Res. 1988,27, 1555-1556
The level-control unit has worked without any problems. Both stainless steel solenoid valves and a simpler type of pinch valves have been tested. The latter type works fairly well but needs to be adjusted a little too frequently to close and open properly. Although the solenoid valves are exposed to wet and corrosive gases, the valves work extremely well. They only have to be cleaned from corrosion products 2-3 times a year. The problem with gases that are readily soluble in water can be handled in various ways. Tests with biogas, containing almost 50% C02, have shown that the fraction of dissolved COz can be neglected even when fresh water is used. Eventually, the water will be saturated with respect to soluble gases. In order to prevent leakage of gases out via the external vessel, the following simple method was used. By connecting the lower tube of the jacket, through which water is pumped out and in, to the inflatable rubber
1555
insert of a football, a completely closed system was obtained. In this case, the external vessel and the jacket communicate via the rubber membrane.
Acknowledgment This work was supported by the Swedish Board for Technical Development. Mikael Carlsson is gratefully acknowledged for technical support. Britt K. Nilsson,* Ingemar Bjerle, Hans T. Karlsson Department of Chemical Engineering ZI Lund Institute of Technology P.O. Box 124 5’-221 00 Lund, Sweden Received for review June 29, 1987 Revised manuscript received March 11, 1988 Accepted March 29, 1988
CORRESPONDENCE Comments on “Effects of Hydrogen Treat Rate and Hydrogen Mass Transfer in SRC-I1 Liquefaction of Coal” and “Kinetics of Liquefaction of Coal Catalyzed by Coal Minerals” Sir: Recent articles by Singh (1987) and Singh and Carr (1987) have reported findings that have been interpreted by these researchers as evidence for inhibition of the rate of coal liquefaction by hydrogen sulfide. These findings are principally evidenced by an increase in coal conversion as a function of hydrogen treat rate in the liquefaction reactor. These authors hold that a decrease in H2S partial pressure alone is responsible for the increase in liquid yield found with an increase in hydrogen treat rate. While the correlations that they have developed seem to substantiate this finding, several alternate interpretations for this effect are possible. The literature is not in universal agreement on the inhibition of hydrogen sulfide in hydrogenation/ hydrogenolysis reactions as suggested by the authors. Hydrogen sulfide has been found to inhibit catalytic hydrosulfurization as might be expected (Morooka and Hamrin, 1979; Satterfield and Roberts, 1968), and inhibition of hydrogenation of aromatics such as biphenyl over sulfided catalysts has also been reported (Sapre and Gates, 1982). However, the presence of H2S has been also found to markedly increase hydrogenolysis reactions in the HDN of pyridine (Goudriaan et al., 1973) and quinoline (Shih et al., 1977; Yang and Satterfield, 1984) without significantly affecting the rate of hydrogenation of these species. A very recent study on the HDN of pyridine has shown the rate of hydrogenolysis of the saturated intermediate (piperidine) to increase with an increase in the ratio of the partial pressure of H2S to hydrogen (Hanlon, 1987). In the same study, the rate of hydrogenation of pyridine to piperidine was found to be independent of hydrogen sulfide concentration. These citations merely serve to illustrate the point that it is extremely difficult to extrapolate from the findings of experiments on model systems with typical industrial hydroprocessing catalysts to coal liquefaction conditions with a catalyst such as iron 0888-5885 18812627- 155X$O1.5010
pyrite. Unpublished work from Auburn University (Guin et al., 1988) on the catalytic hydrogenation of naphthalene to tetralin and decalin has recently shown that the yield of the partially saturated hydrogen donor intermediate (tetralin) is strongly increased by an increase in the partial pressure of H2S. These data have further shown that the rate of formation of decalin from tetralin can be reduced by maintaining the proper partial pressure of hydrogen sulfide (not zero). Hence, the formation of undesirable saturates such as decalin a t the expense of desirable hydroaromatics such as tetralin can be suppressed by control of hydrogen sulfide partial pressure at some intermediate level. Finally, it is difficult to reconcile the negative effect of hydrogen sulfide reported by Singh with the positive influences of H2S on the reactions of model coal mimics such as diphenylmethane hydrocracking (Ogawa et al., 1984; Hattori et al., 1987) and on coal liquefaction (Baldwin and Vinciguerra, 1983; Hirschon and Laine, 1985; Willson et al., 1985; Sofianos, 1987). An alternate interpretation on the positive effect of hydrogen treat rate can be found in the effect of this variable on the phase equilibria and resulting residence time distribution in the liquefaction reactor. A similar effect of hydrogen treat rate on the yield of liquids from brown coal liquefaction catalyzed by red mud was found by researchers at Bergbau-Forschung (Strobe1 and Friedrich, 1987). The effect of increased hydrogen treat rate was in this case, however, ascribed to a marked increase in the average residence time of the coal in the reactor caused by preferential removal of the more volatile species from the reaction vessel. By employing a tracer technique, these researchers were able to directly measure the effect of gas treat rate on the mean residence time in the liquefaction reactor, which was completely backmixed. These results showed that doubling the gas treat rate more than doubled the average residence time in the liquefaction 0 1988 American Chemical Society
1556
I n d . E n g . Chem. R e s . 1988, 27, 1556-1557
reactor, thus bringing about increased liquid yield by virtue of the increased reaction time for the heavier and more Seem much more to convert species‘ It likely that the very marked effect found for gas treat rate would be related to a direct effect on the system such as mean coal residence time, rather than an indirect effect such as hydrogen sulfide partial pressure. Literature Cited Baldwin, R. M.; Vinciguerra, S. Fuel 1983, 62, 498. Goudriaan, F.; Gierman, H.; Vlugter, J. C. J . Inst. Pet. 1973,59,40. Guin, J. A.; Rhee, Y. W.; Curtis, C. W-., submitted for publication in Fuel Proc. Technol. 1988. Hanlon, R. T. Energy Fuels 1987, 1, 424. Hattori, H.; Yamashita, K.; Kobayashi, K.; Tanabe, T.; Tanabe, K. Proceedings of the 1987 International Conference on Coal science, 285; Moulijn, J. A,, et al., Eds.; Elsevier Science: New York, 1987. Hirschon, A. S.; Laine, R. M. Fuel 1985, 64, 911. Morooka, S.; Hamrin, C. E., Jr. Chem. Eng. Sci. 1979,34, 521. Ogawa, T.; Stenberg, V. I.; Montano, P. A. Fuel 1984, 63, 1660.
Sapre, A. V.; Gates, B. C. Ind. Eng. Chem. Process Des. Dev. 1982, 21, 86. Satterfield, C. N.; Roberts, G. W. AZChE J. 1968, 24, 159. Shih, S. S.; Katzer, J. R.; Kwart, H.; Stiles, A. B. Prepr.-Am. Chem. SOC.Div. Pet. Chem. 1977,22, 919. Singh, C. P. P. Ind. Eng. Chem. Res. 1987, 26, 1565-1573. Singh, C. P. P.; Carr, N. L. Znd. Eng. Chem. Res. 1987,26,501-511. Sofianos, A. C. Proceedings of the 1987 International Conference on Coal Science. 247: Mouliin. “ , J. A.. et al... Eds.:, Elsevier Science: New York, 1987. Strobel, B. 0.; Friedrich, F. Proceedings of the 1987 International Conference on Coal Science, 395; Moulijn, J. A., et al., Eds.; Elsevier Science: New York, 1987. Willson, W. G.; Hei, R.; Riskedahl, D.; Stenberg, V. I. Fuel 1985,64, 1 OR I#”.
Yang, S. H.; Satterfield, C. N. Znd. Eng. Chem. Process Des. Dev. 1984, 23, 20.
Robert M. Baldwin Chemical Engineering and Petroleum Refining Department Colorado School of Mines Golden, Colorado 80401
Response to Comments on “Effects of Hydrogen Treat Rate and Hydrogen Mass Transfer in SRC-I1 Liquefaction of Coal” and “Kinetics of Liquefaction of Coal Catalyzed by Coal Minerals” Sir: The comments of the reader’s are based on the understanding that the reported inhibitive effects of hydrogen sulfide on the rate of coal liquefaction (Singh and Carr, 1987) were principally evidenced by an increase in coal conversion as a function of hydrogen treat rate (first paragraph). Such an understanding of the referred work is surprising since all the data used in the development of the kinetics (Singh and Carr, 1987) was generated at a constant hydrogen treat rate of 4 g of H2/100 g of slurry. Throughout this paper, there is no mention of any effect of hydrogen treat rate. Also, the kinetic analysis only showed H2Spartial pressure to be a key variable but not the only or the most important variable. The final reaction rate expression (eq 26 in Singh and Carr (1987)) shows partial pressure of hydrogen, mass fraction of iron in the reactor, and temperature to be more important than partial pressure of hydrogen sulfide. Therefore, significantly higher liquid yield (45.9 vs 31.7 wt % mf coal) could be obtained at higher hydrogen sulfide concentrations (2.8 vs 1.43 wt % mf coal) in the reactor (Table V in Singh and Carr (1987)). It is important to note that the work by Singh (1987) only used the kinetics developed in the preceding work (Singh and Carr, 1987). The hydrogen treat rate and mass-transfer study (Singh, 1987) had nothing to do with the development of kinetics of coal liquefaction (Singh and Carr, 1987), showing an inhibitive effect of hydrogen sulfide on the rate of reaction. On the basis of this available kinetics, the hydrogen treat rate and masstransfer study showed that an increase in hydrogen treat rate increased the rate of coal liquefaction mainly by decreasing the hydrogen sulfide partial pressure. In comparison to the latter, the effect of hydrogen partial pressure on the rate of coal liquefaction was found to be small. The second paragraph of the comments uses several references to indicate the differences in reported effects of hydrogen sulfide on hydrogenation/hydrogenolysisreactions. Also, “these citations merely serve to illustrate the point that it is extremely difficult to extrapolate from the findings of experiments on model systems with typical industrial hydroprocessing catalysts to coal liquefaction 0888-5885/8S/2627-1556$01.50/0
conditions with a catalyst such as iron pyrite.” It is a valid but an out-of-context statement. Singh and Carr (1987) considered and used the Langmuir-Hinshelwod rate equations which were used earlier by many workers. A few references to successful applications of these rate expressions in desulfurization and/or hydrogenation reactions or some references to the results of other works dealing with the effects of H2S on desulfurization/hydrogenation reactions do not make our work dependent on, much less an extrapolation of, any other work. The kinetics work would not have been influenced at all by the absence of one or all the references to other workers. However, the authors are obligated to provide references to relevant works to provide a reader with the overall context and significance of the work being reported. We would have been more appreciative of the preceding comment if its very basis of differences between model systems and industrial findings was not reversed in the subsequent lines which read “it is difficult to reconcile the negative effect of hydrogen sulfide reported by Singh with the positive influences of H2S on the reactions of model coal mimics such as diphenylmethane hydrocracking and coal liquefaction.” Singh and Carr (1987) also refer to these differences which existed before its publication. If we could, we would have, and in the future we would like to reconcile the differences between the observed effects of H2S on seemingly similar reactions. However, reconciliation of differences between the results of different workers was not within the scope of data, analysis, and results of the study of Singh and Carr (1987). It is obvious that the reader’s comments are based on a superficial study and, hence, an erroneous understanding of the two papers (Singh and Carr, 1987; Singh, 1987). However, we have looked into the basis (Strobel and Friedrich, 1987) of t h e suggested “alternate interpretations” of the effect of hydrogen treat rate on rate of coal liquefaction Strobel and Friedrich (1987) studied the effects of solvent boiling range and hydrogen treat rate on coal conversion. Their study showed that a lowering of the boiling range of the recycle solvent (used to make feed coal slurry to a coal liquefication reactor) and/or an 1988 American Chemical Society
