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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
I n d . Eng. Chem. Res. 1988, 27, 1557-1558
increase in hydrogen treat rate increased the coal residence time in the reactor due to vaporization of the solvent and liquid products from coal. The increased coal residence time in the reactor resulted in higher liquid yield from coal. Qualitative relationships between the increase in extent of coal conversion with an increase in coal residence time resulting from vaporization of liquid in the reactor are correct. However, it is very important to understand that Strobel and Friedrich (1987) used high volatile solvents (ASTM D86 50% below 222 and 340 “C) which along with the light and middle distillates from coal were vaporized in the reactor. The vehicle (solvent) for slurrying of coal in the studies of Singh and Carr (1987) and Singh (1987) consisted of the heavy ends (Figure 1 in Singh and Carr (1987)). The composition of heavy ends was almost identical with that of the slurry in the reactor. A study of the effect of variations in vaporization equilibrium constant for a large range of extent of coal conversion in the SRC-I1 process showed that the effect of the latter on the slurry residence time in the reactor was small (Singh and Carr, 1983). A large effect of hydrogen treat rate on coal (slurry) residence time observed by Strobel and Friedrich (1987) resulted from the use of high volatile materials as vehicle (solvent). It appears that all the comments and supportive references have been used to conclude (last paragraph, last line) that an indirect effect such as hydrogen sulfide partial pressure is unimportant, if not irrelevant, and our studies should conform with the direct (measured) effect of hydrogen treat rate on the mean coal residence time. Use
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of inappropriate terms (direct vs indirect effects in place of physical and reaction or chemical effects) or citations for differences in reported results cannot support the above erroneous conclusions. The latter merely reflect the difficulties one may have in dealing with complex systems such as coal liquefaction. A scientific approach to the development of an understanding of such complex processes requires that each chemical and physical aspect of the process be given due consideration. In all likelihood an overzealous attempt to generalize the results of one study to all related studies can only limit the scope of the possible developments. It is obvious that all the comments on studies of Singh and Carr (1987) and Singh (1987) are a result of such an effort to generalize the observed relationship between hydrogen treat rate and slurry residence time (Strobel and Friedrich, 1987). Literature Cited Singh, C. P. P. Znd. Eng. Chem. Res. 1987, 26, 1565-1573. Singh, C. P. P.; Carr, N. L. Znd. Eng. Chem. Process Des. Dev. 1983, 22, 104-118. Singh, C. P. P.; Carr, N. L. Znd. Eng. Chem. Res. 1987,26,501-511. Strobel, B. 0.;Friedrich, F. Proceedings of the 1987 International Conference on Coal Science; Moulin, J. A., et al., Eds.; Elsevier Science: New York, 1987; pp 395-398.
Chandra P. P. Singh Aristech Chemical Corporation Research Laboratory 1000 Tech Center Drive Monroeville, Pennsylvania 15146
Comments on “The Axial Laminar Flow of Yield-Pseudoplastic Fluids in a Concentric Annulus” Sir: Hanks (1979) published an article on laminar flow of a Herschel-Bulkley fluid in a concentric annulus in which 900 computed values of the parameter X were reported. X is the basic parameter for the solution of the problem, namely, the value of the dimensionless radial coordinate for which the ICZ component of momentum flux tensor 7, = 0. About 10% of these values was chosen to check a computer program developed for the evaluation of a more general case of laminar flow: Herschel-Bulkley fluid with Newtonian slip layers in a concentric annulus (Buchtelovl, 1984). No differences in Hanks’ published values and my values of X were found for any cases where the power law index of the Herschel-Bulkley model m L 0.2; however, for m = 0.1, the values did not agree in several cases. The whole set of Hanks’ value of X for m = 0.1 was then checked, and 24 differences were found (Table I). The reasons for publishing the corrected values of X are that the evaluation of velocity profiles is very sensitive to the correctness of the X value, especially for highly plastic and pseudoplastic materials (Buchtelovl, 1986), and that the Herschel-Bulkley model or the power law model with a low value of m is often used for approximation of material behavior (for instance, in drilling technology). We should keep in mind, however, that in some cases nonregistered or incorrectly measured behavior could be hidden behind the low power law index. Justification of my values of X is presented below. Table I of Hanks (1979) lists values of X for various values of annulus aspect ratio, u; the rheological parameter in yield power law, m; and the parameter To= ( 2 r 0 ) / ( P R ) (ro being the yield stress, P the total head loss, and R the
Table I 0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
2mJPR 0.75 0.80 0.85 0.65 0.70 0.75 0.55 0.60 0.65 0.45 0.50 0.55 0.35 0.40 0.45 0.25 0.30 0.35 0.15 0.20 0.25 0.05 0.10 0.15
values of A, according to Hanks (1979) Buchtelova (1984) 0.3211 0.3215 0.3195 0.3259 0.3179 0.3349 0.4515 0.4525 0.4501 0.4589 0.4487 0.4700 0.5512 0.5520 0.5501 0.5616 0.5489 0.5765 0.6353 0.6359 0.6344 0.6488 0.6334 0.6624 0.7095 0.7093 0.7247 0.7086 0.7314 0.7079 0.7767 0.7762 0.7757 0.7865 0.7752 0.7882 0.8378 0.8380 0.8412 0.8374 0.8428 0.8371 0.8951 0.8954 0.8949 0.8954 0.8960 0.8947
radius of the outer pipe of the annulus). In all columns of that table, the values of X for each u start with the highest value at the top-for To = 0.0-and change in relatively even steps to the lowest value at the bottom for the highest value of To.This holds for all values of m, with the exception of m = 0.1; for it (for all u from 0.1 to 0.8),
0888-5S8~/88/2627-1557$01.50/0 0 1988 American Chemical Society
