Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 3, 1978
377
Electrolytic Reduction of Alumina with and without High Voltage Pulsing Sir: In his present communication, Diller is forced to selectively omit part of our published data (experiment no. 6, Acton et al., 1976) in order to computationally demonstrate “activation”. The back emf (intercept at zero current) of 1.6 V determined from all of our data (cf. our Figure 3, Acton et al., 1976) in fact agrees with the value quoted by Diller from the data of Schlain et al. (1963); i.e. our data show no perceptible “activation” regardless of whether or not other “baseline” data are invoked. Actually, by employing Diller’s method of data analysis, virtually any set of experimental current-voltage data could be incorrectly shown to demonstrate “activation” simply by exploiting the inevitable scatter in any suitably chosen pair of data points, and ignoring other valid data. I t is noteworthy that Diller now acknowledges that our measurements were carefully made, credible, and that our experimental parameters indeed fall within the ranges specified in his patent (Diller, 19741, as we, of course, intended. However, Diller’s recent communication contains no new experimental evidence, nor any valid criticism of our earlier,
documented experiments. Thus, there is no reason to alter our previous conclusion (Acton et ai., 1977) that high voltage pulsing causes no significant after-effect on the normal electrolysis current/voltage relation for parametric ranges specified in our paper.
Literature Cited Acton, C. F., Nordine, P. C., Rosner. D. E., Ind. Eng. Chem. Process Des. Dev., 15, 285 (1976).
Acton, C. F. Nordine, P. C., Rosner, D. E., Ind. Eng. Chem. Process Des. Dev., 16, 261 (1977).
Diller, I. M., U.S. Patent 3 244 604 (Apr 5, 1966). Schlain, D., Kenahan, C. B., Swift, J. H., U . S . Bur. Mines Rep?.Invest.,No. 6265 (1963).
High Temperature Chemical Reaction Engineering Laboratory Department of Engineering and Applied Science Yale University New Hauen, Connecticut 06520
Constance F. Acton*l Paul C . Nordine Daniel E. Rosner
Olin Metals Research Laboratories, New Haven, Conn. 06504.
CORRESPONDENCE
Survey of Propane Pyrolysis Literature Sir: The recent publication by Volkan and April (1977) includes features with which I agree but several features with which I disagree. This disagreement is, in part, caused by pyrolysis results reported in 1975 or later that were not included in the survey. Volkan and April have certainly made a major effort to collect and analyze 103 technical papers. Making sense out of so many publications that contain divergent conclusions is not easy. Several recent publications that were not surveyed, however, address themselves directly to some of the areas of controversy relative to propane pyrolysis. Papers in this category were presented in symposia at the 169th National Meeting of the American Chemical Society, Philadelphia, Pa., March 1975, and at the First Chemical Congress of the North American Continent in Mexico City (1975). Most papers in these two symposia were published in 1976 in the ACS Symposium Series. Features of the review by Volkan and April that I would like to discuss include the following. (A) By-products. Several products formed in smaller amounts have been discussed, but three by-products of importance have not been adequately discussed. Coke or carbon formation was never really discussed. Coke formation is, however, of major concern in all commercial pyrolysis units including those using propane as a feedstock. Several factors affecting coke formation include the material of construction of the reactor, past history or pretreatment of the reactor, and operating conditions (Brown and Albright, 1976; Crynes and Albright, 1976; Dunkleman and Albright, 1976a,b;Herriott and Eckert, 1972).Coke is formed by a t least two mechanisms. First the metal surface of a reactor catalyzes the growth of a filamentary type coke. The resulting coke contains metal granules and is magnetic in character; these 0019-7882/78/1117-0377$01.00/0
metal granules are extracted from the surface of reactors constructed of high alloy steels. Such coke occurs for example on the surfaces of nickel-chromium-iron alloys used in commercial reactors but not on glass or aluminized metal surfaces. Second, some coke is formed by condensation, polymerization, or agglomerating mechanisms in the gas phase. This coke is nonmagnetic in nature. Additional evidence on coke formation during pyrolysis was presented at the Anaheim ACS Meeting (Albright and McConnell, 1978; Albright and Yu, 1978; Albright et al., 1978). Acetylene that is a precursor for coke is always formed in small but nevertheless significant amounts whenever propane is pyrolyzed commercially. Coke formation from acetylene has recently been investigated in considerable detail (Baker, 1977; Baker and Waite, 1975; Baker et al., 1973; Bernard0 and Lobo, 1975; Lobo and Trimm, 1973). Many details of the coking mechanism have been determined. A filament-type coke is formed in some cases. Other probable coke precursors include ethylene, propylene, butadiene, and benzene, all of which are produced to at least some extent during most propane pyrolyses. Carbon oxides were mentioned in Table IV of the review; it is indicated, or a t least implied, that they are formed as acetylene and steam react in the gas phase. Carbon oxides are always formed in significant amounts during pyrolysis in high-alloy reactors (including commercial reactors) but are not formed in Vycor reactors. Most, if not all, carbon oxides are formed by reactions between coke or carburized metal surfaces and steam (Dunkleman and Albright, 1976a;Tsai and Albright, 1976). Metal surfaces act in part at least to catalyze the formation of carbon oxides. Heavy hydrocarbons include, in addition to those listed in the review, aromatics. (B) Surface Effects. In the review, it was stated that the walls of the reactor play a role in the initiation and termination
62 1978 American Chemical Society
378
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 3, 1978
Table I. Rate Constants for Reactions in Mechanistic Model Rate constantsa Forward reaction Reaction
No. 1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
13. 14. 15. 16. 17.
C3Hs * C2Hy + CHy C2Hy + C3H8 C2H6 + C3H7CH3. + C3H8 ~t CH4 C3H7Ha C3Hs Hz C3H7C3H7' CzH4 + CHy C3H7. F! C3Hs H. +
+
+
+
+
Reverse reaction A E
A
E
Ethane Reactions 1.0 x 1016 1.0 x 10'2 3.2 X 10l2 3.8 x 1013 1.3 x 1014
86.0 10.6 10.2 38.0 9.7
3.8 x 1013 1.0 x 101' 1.3 x 1014 4.0 x 1013 3.0 X 10"
11.0 12.6
85.0
4.2 x 1013
0.
Propane Reactions 1.2 x 1017 7.5 X 1O'O 8.1 X 10l1 3.2 x 1013 2.0 x 10'2 1.0x 1014
10.3 7.8 34.5 37.0
Termination Reactions 4.5 x 1013 1.5 X 10l2 1.7 x 1013 2.6 X 10l2 2.0 x 1013 6.3 x 1013
0. 0. 0. 0. 0. 0.
0. 1.6
10.8
10.0
2.5 X 6.3 X 3.3 x 2.5 x
10l1 10l2 10'1 1014
19.5 14.6 7.8 3.8
Secondary Reactions 1.9 x 1017 18. 86.0 7.9 x 10'2 19. 7.2 8.5 X 10l2 7.4 20.6 1.5 X 10l2 10.0 21. 3.2 X lo1' 13.0 22. 7.4 5.0 X 10" 6.3 X 10l2 11.0 23. 5.5 1.0 x 10'0 24. 4.0 x 1014 3.2 X 10l1 7.4 37.0 3.2 X 1O1O 25. 7.5 26. 46.0 3.2 x 1015 a A = frequency factor (s-l or cm3 g-mol-' s-l); E = activation energy (kcal g-mol-l. The literature sources for A and E values are given in the original paper by Dunkleman and Albright (1976a). A value disagrees with literature value by a factor of 10, but the reaction is of minor importance for modeling ethane pyrolysis data. of free radicals. This conclusion may have some validity for sub-atmospheric pyrolyses in glass reactors, but there is no known evidence to support such a conclusion for pyrolyses in metal reactors at higher pressures such as employed in commercial units. One might reason that hydrogen free radicals would be formed during coke formation on the surface. Destruction of ethyl and other alkyl radicals at the surface has also been suggested. The kinetics of pyrolysis, however, do not support the postulate that radicals are either destroyed or created at the surface. If they were either destroyed or created, presumably the overall kinetics of propane disappearance would be significantly changed. Experimentally it has been found that surface reactions leading to coke formation can have a major effect in metal reactors on the composition of the gaseous products but have little or no effect on the overall kinetics. Experimental evidence has, however, been obtained recently to prove all of the following surface reactions (Tsai and Albright, 1976): (1)coke formation (hydrogen is also formed); (2) decoking using steam (carbon oxides and hydrogen are produced) and using hydrogen (methane is also produced); (3) oxidation of alloy surfaces with steam (nickel, iron, and chromium oxides and hydrogen are formed); (4) reduction of metal surfaces with CO, hydrocarbons, and hydrogen (carbon oxides and steam are produced); (5) sulfiding of metal surfaces with hydrogen sulfide or other sulfur compounds present in the feedstream; (6) desulfiding of metal surfaces with steam. These results help to explain the differing conclusions
voiced by several previous investigators relative to the role of the surface during pyrolysis. I t is now clear that the composition of the inner surface of the reactor has a pronounced effect on both ethylene and coke yields (Dunkleman and Albright, 1976a,b; Herriot et al., 1972). The presence of trace amounts of oxygen and other additives as reported in the Volkan-April review is undoubtedly caused in part by changes in composition of the inner surfaces even when the reactors were constructed from glass or gold. Blakemore et al. (Blakemore et al., 1973), for example, showed that oxygen pretreatment of a gold reactor had a pronounced effect on the subsequent pyrolysis even though gold and oxygen do not "react" to form oxides. Hydrogen sulfide or sulfur-containing hydrocarbons are sometimes deliberately added in commercial units to propane or ethane feedstreams in order to reduce the formation of coke and carbon oxides and to increase the yields of ethylene. Sudden addition or elimination of hydrogen sulfide in the feed stream shows no immediate change in the composition of the product gases. Significant changes do occur, however, after a period of time. Clearly the surface is sulfided to some extent by the hydrogen sulfide or sulfur-containing hydrocarbons. The presence of small amounts of oxygen in the feedstream often has pronounced effects when the hydrocarbon feedstream is dry but generally not when it is wet. The role of the oxygen seems to be to affect the surface composition. (C) Kinetics a n d Modelling of Propane Pyrolysis. Although Volkan and April reviewed in quite some detail literature discussing the order of propane decomposition, they are
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 3, 1978
most correct in concluding that the orders of decomposition reported by numerous previous investigators have little meaning relative to any mechanistic understanding when considering pyrolyses involving substantial propane conversions. As they pointed out, the numerous consecutive and simultaneous reactions preclude use of a simple order of decomposition except a t very low propane decomposition levels. In fact, use of simple order models for light paraffins has been shown to lead t o large errors when conversion levels are considered over a wide range (Dunkleman and Albright, 1976a). Volkan and April present in Table V of their paper a model that is characterized as being comprehensive. A model such as this one can be rigorously integrated using digital computers, and the calculated results can be compared t o experimental data. There is no evidence that such a test has yet been made with the model of Table V. Another model by Dunkleman and Albright (1976a) as shown here in Table I has been tested and found to predict with excellent accuracy extensive experimental data for ethane pyrolysis a t 750 to 850 "C and from low to high conversions. The same model predicts with good accuracy comparable data for propane pyrolysis. Volkan and April, however, did not review this latter model. The model shown in Table I is based on a mechanistic approach; it incorporates terms for what are thought to be the most important gas-phase reactions. Rate parameters used for the various reaction steps are those reported in the literature (Kondratiev, 1972). The model shown here in Table I differs rather substantially from that of Table V of the Volkan-April paper. Both the model shown in Table I and the model presented by Volkan and April do not include reaction steps for production of several hydrocarbon products obtained in minor amounts. Formation of both coke and carbon oxides is also not taken in account; these materials are formed to a t least a significant degree by surface reactions. T o circumvent the latter problem, a correction technique that eliminates the effect of the major surface reactions has been proposed (Dunkleman and Albright, 1976a,b). In this technique, coke and carbon oxides are added back to increase ethylene yields and to decrease hydrogen yields. Such a procedure works exceptionally well in the case of ethane data and to a somewhat lesser extent for propane data. It was encouraging to learn that a t least one industrial firm employs a similar technique for correcting commercial data to essentially a "surfaceless" basis. In any case, models more complicated than either that shown in Table I or reported by Volkan and April will be required in order to account for certain hydrocarbons produced in small amounts. Allara and Edelson (1973,1975) proposed a model involving many more reaction steps and several additional products. Hopefully this model or its modification can be tested in the near future. Volkan and April have questioned the use of a quartz reactor a t temperatures above 800 "C. I t is not clear whether they are referring to static batch reactors or to flow reactors.
379
In any case reliable results can be obtained in Vycor or quartz glass reactors a t temperatures up to a t least 900 "C. Tubular flow reactors with several sample ports have resulted in data over a rather wide range of conversions (Crynes and Albright, 1976; Dunkleman and Albright, 1976a,b; Herriot et al., 1972). I t should be emphasized that careful attention to details is necessary in order to obtain the desired temperatures and residence times expecially at higher temperatures in any and all reactors used for pyrolysis. Sections of the book, "Industrial and Laboratory Pyrolysis" (Albright and Crynes, 1976) that discuss propane pyrolysis and that were not reviewed include Chapters 2,3,5,7,15,and 27. Chapters 6 and 13 discuss propylene pyrolysis that obviously is always of some importance during pyrolysis of propane. Chapters 7 and 15 report experimental results for pyrolyses of mixtures containing propane. Other important papers involving mixtures are by Froment and associates (Incl. Eng. Chem. Process Des. Deu., 15,495 (1976);AIChE J . , 23, 93 (1977)). Murata and Saito ( J . Chem. Eng. Jpn., 8, 39 (1975))and Illes (Acta. Chim. Acad. Sci. Hung., 67,44 (1971) both have interesting papers dealing with the kinetics and modeling of propane pyrolysis.
Literature Cited Albright, L. F., Crynes, B. L., "industrial and Laboratory Pyrolyses", ACS Symposium Series No. 32, American Chemical Society, Washington, D.C., 1976. Albright, L. F., McConnell, C. F.,presented at the 175th National Meeting of the American Chemical Society, Anaheim, Calif., March 1978. Albright, L. F., McConnell, C. F., Welther, K.. presented at the 175th National Meeting of the American Chemical Society, Anaheim, Calif., March 1978. Albright, L. F., Yu, H. C., presented at the 175th National Meeting of the American Chemical Society, Anaheim, Calif., March 1978. Allara, D. L., Edelson, D.. Int. J. Chem. Kinet., 7 (4), 479 (1975). Baker, R. T. K., Chem. Eng. Prog., 73, (4), 97 (1977). Baker, R. T. K., Harris, P. S.,Thomas, R. B., Waite, R. J., J. Catal., 30, 85 (1973). Baker, R. T.K., Waite. R. J., J. Catal., 37, 101 (1975). Bernardo, C. A., Lobo, L. S.,J. Catal., 37, 267 (1975). Blakemore, J. E., Barker, J. R., Corcoran, W. H., Ind. Eng. Chem. Fundam., 12, 147 (1973). Brown, S.M., Albright, L. F., "industrial and Laboratory Pyrolyses", ACS Symposium Series No. 32, Chapter 17, L. F. Albright and B. L. Crynes, Ed., American Chemical Society, Washington, D.C. 1976. Crynes, B. L., Albright, L. F., Ind. Eng. Chem. Process Des. Dev., 8, 25 (1969). Dunkleman, J. J., Aibright, L. F., "Industrial and Laboratory Pyrolyses", ACS Symposium Series No. 32, Chapter 14, L. F. Albright and B. L. Crynes, Ed., American Chemical Society, Washington, D.C., 1976a. Dunkleman, J. J., Aibright, L. F., "Industrial and Laboratory Pyrolyses", ACS Symposium Series No. 32, Chapter 15, L. F. Albright and B. L. Crynes, Ed., American Chemical Society, Washington, D.C., 1976b. Edelson. D.. Allera, D. L., AlChE J., 19, 638 (1973). Herriott, G. E., Eckert, R. E., Albright. L. F., AIChEJ., 18, 84 (1972). Kondratiev, V. N., "Rate Constants of Gas-Phase Reactions", NSRDS-COM10014, U.S. Dept. of Commerce, Washington, D.C., 1972. Lobo, L. S.,Trimm, D. L., J. Catal., 29, 15 (1973). Tsai, C. H., Albright, L. F., "Industrial and Laboratory Pyrolyses", ACS Symposium Series No. 32, Chapter 16, L. F. Albright and B. L. Crynes, Ed., American Chemical Society, Washington, D.C., 1976. Volkan, A. G., April, G. C., Ind. Eng. Chem. Process Des. Dev., 18, 429 (1977).
School of Chemical Engineering Purdue University West Lafayette, Indiana 47907
Lyle F. Albright
