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Figure 3 records the results of experiments in each of which hydrogen, and methane, would be as valuable as or more 1 cubic foot (28.3 liters) of water gas was passed over 70 valuable than the original water gas. g r a w (gross volume about 75 cc.) of catalyst No. 6 a t 275 O C. From the fractionation analysis given above it has been The catalyst in these experiments, after a preliminary calculated that the total yield of all hydrocarbons higher than reduction, was moistened with water and molded into pellets methane would be about 2.3 times the yield of liquid hydroout of contact with air. I n this case the pellets did not carbons actually obtained. This would bring the yield powder up as before. The figure shows how the yields of of total higher hydrocarbons up to about 124 grams per cubic the various p r o d u c t s meter of water gas. Or, if all hydrocarbons from butane up 0.7 ,50 vary as the rate of flow were retained with the liquid product, we would have about of gas is changed at con- 85 grams of liquid hydrocarbons per cubic meter of water stant temperature. gas. The cost of the water gas to produce 1 gallon of liquid From Fimre 3 we find fuel would then be 22 cents instead of 28 cents. If the t h a t , t o Form 0.68 cc. value of the hydrocarbons lower than butane and higher (0.46 gram) of oil, there than methane were taken into account, this figure could has been 0.37 cubic foot probably be further reduced to about 15 cents. (10.7 liters) of water gas A rough estimate shows that about 2 to 3 per cent of the a c t u a l l y used. This carbon monoxide in the original gas appears as carbon figure, c o r r e c t e d f o r deposited on the catalyst. The process would be much methane caught in the improved if this could be avoided. Doubtless much of this receiver and for carbon deposition of carbon occurs a t the higher temperatures. monoxide and hydrogen Theoretically the only way to improve the yields of liquid occluded in the con- hydrocarbons would be to eliminate the formation of any lower hydrocarbons. From the commercial standpoint, densed p r o d u c t s, be comes about 0.3 cubic of course, it would be desirable to obtain more product per foot (8.5 liters). After volume of catalyst in a given time. extraction of liquid hyI n conclusion, the difficulties inherent in these experiments drocarbons, carbon di- must be emphasized. First, as mentioned before, a strictly oxide and water, there quantitative separation of all the various products formed remains a gas containing, in these small-scale experiments is very difficult. Further, FATE OF FLOW OF GAS, C. C. PER %llNUTE the relative results a t different temperatures may be inbesides carbon Figure 3-Results w i t h 70 Grams and hydrogen, fluenced to some extent by changes in the activity of the (Gross Volume 75 cc.) Catalyst No. 6, atn275* C. Der cent of methane in ad- catalyst during use. The apparently large effect of temperadition to small amounts ture on these reactions makes non-uniformity of the catalyst of low-boiling saturated and unsaturated hydrocarbons. Us- temperature a disturbing factor. This work is being continued and recent experiments on a ing a more efficient fractionating condenser, which would recover all hydrocarbons from pentane up, the yield of liquid larger scale with a much more active catalyst and more hydrocarbons could be brought up to about 0.56 gram per accurately controlled conditions give promise of more favor0.3 cubic foot (8.5 liters) of water gas. This corresponds to a able results. Acknowledgment yield of about 66 grams per cubic meter. On this basis i t would take 1400 cubic feet of water gas to make 1 gallon of The authors wish to take this opportunity to express their oil. At 20 cents per 1000 cubic feet the water gas actually appreciation to A. C. Fieldner, whose interest and encourageused to make 1 gallon of oil would cost 28 cents. In actual ment as chief chemist of the Bureau of Mines made this work practice the unused gas, consisting of carbon monoxide, possible.
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Design of Fractionating Columns’ D. B. Keyes, Roy Soukup, and W. A. Nichols, Jr. UNIVBRSITY OF ILLINOIS, URBANA, ILL.
HE estimation of the theoretical number of plates required to separate a binary liquid mixture into its com-
T
ponents has received considerable attention in recent years. An excellent review of the various methods has been given by Shirk and Montonna.2 They show quite conclusively that the most practical method is the graphical scheme devised by McCabe and Thiele.s This method requires as data the composition in mol per cent of the feed, the product, and the residue, also the reflux ratio. Equilibrium is necessary between vapor and liquid on each plate. This is, of course, never attained in 1 Received 1
January 30, 1928.
IND. ENO. CHBM., 19, 907 (1027).
* Ibid., 17, 605 (1925). See also Walker, Lewis, and McAdams, “Principles of Chemical Engineering,” 2nd ed.
practice, for many reasons. The most evident reason is faulty plate design. A safety factor, known as the plate efficiency, is applied to the theoretical number of plates in order to determine the actual number necessary. This safety factor varies greatly with changing conditions, and cannot be determined in advance. It is quite probable, therefore, that a still simpler graphical method can be devised which will give practical results of equal accuracy. I n many cases it is merely a question of choosing one of four columns-a lo-, a 20-, a 30-, or a 50-plate column. Obviously, the column chosen will be the one with the fewest number of plates that will operate under the specified conditions and with a reasonable reflux ratio (5 to 1 or less). The practlical application of the McCabe and Thiele method using the proper safety factor (found by experience) would
IXDUSTRIAL AND ENGINEERING CHEMISTRY
May, 1928
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/o
r
I lure 2
Figure 1
ever, that the deterretical minimumnumber of plates (infinite
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Figure 3
t o produce a product containing 94.5 per cent by weight of alcohol and a residue containing 0.1 per cent b y weight of alcohol is found to be 18.’ (Figure 3) In this case the equilibrium curve r u n s close a n d nearly parallel to the straight-line curve for an a p p r e c i a b l e distance a t the upper ! 4 L p p end. Furthermore, Figure 4a the composition of the residue is very close to zero. It is quite evidentthat a higher safety factor must be applied if a reasonable reflux ratio is to be used. A safety factor of 3 has been found satisfactory. This means 54 plates, or the selection of the 50-plate column. McCabe and Thiele have found 36 plates to be the theoretical number required for a 5 to 1 reflux ratio, using the same data. Their result has been checked by the authors. Dividing 36 by the plate efficiencies 0.6 and 0.8 mentioned above, the practical number of plates appears to lie between 45 and 60. Therefore the two methods seem to be in agreement again. Water-Acetic Acid System
Repeating the process once more, the theoretical minimum number of plates required for a feed containing 10 per cent by weight of acetic acid to produce a “product” (condensate) containing 0.1 per cent by weight of acetic acid (99.9 per cent by weight of water) and a ((residue”containing 80.0 per cent by weight of acid is found to be 22.6 (See Figures 4 and 4a) I n this case the equilibrium curve approaches the 45-degree line at the upper end a t an angle that is not nearly so wide as in the benzene-toluene case. Furthermore, the composition of the condensate is very nearly 100 per cent water. It is therefore evident that the higher safety factor (3) should be used. This gives 66 plates, but a 70plate column (two columns, one of 50 plates and one of 20) would probably be chosen in practice. The theoretical number of plates required using a 5 to 1 reflux ratio (McCabe and Thiele method) is 49. Dividing this figure by 0.6 and 0.8 (plate efficiencies), the practical number of plates appears to lie between 61 and 82-fair agreement with the other method. 5
The data used were the same as those used by McCabe and Thiele,
loc. cit. 6
Data taken from Hausbrand‘s “Distillation.”
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Relative Position of Feed It has also been noticed in these three examples that the ratio between the theoretical number of plates above the feed and those below as determined by graphical methods changes very little when the reflux ratio is changed from a practical value to infinity. This indicates that the relative position of the feed can be determined satisfactorily by noting the theoretical number of plates above and below the feed composition on the chart which has no enrichment lines and the reflux ratio of which is infinite. This simpler method of determining the relative position of the feed for practical operation is probably as accurate as the more complicated method because of the invariability of the
Vol. 20, No. 5
ratio mentioned above and because of the neutralizing effect of a sometimes large error that is common to both methods. This error is caused by the possible wide variations of plate efficiencies within the same column. Conclusion The results seem to indicate that the practical number of plates required in a fractionating column and the relative position of the feed can be determined satisfactorily by an extremely simple graphical method together with the use of a simple safety factor indicated by the character of the equilibrium curve.
The Building of Containers for Severe Service' T. McLean Jasper A . 0.SMITH CORPORATION, MILWAUKEE, Wrs.
ONTAINERS of today for use in chemical engineering, engineer shall not be curtailed by lack of knowledge of the oil-cracking engineering] and steam engineering are working conditions confronting the materials and designing required to carry increasingly higher pressures, with- engineer. stand greater variations of temperature, and resist the corMaterials and Working Conditions rosive effects of a greater variety of conditions as compared to the requirements of only a few years ago. Within the past The fabricator of containers is frequently asked to make few years the demands on the builder of containers have been quotations on a vessel of a certain diameter and wall thickness, for larger size, increased variation in working conditions, and without being given any information as to the service congreater resistance to continuous application of very high ditions under which the vessel is to operate. Sometimes and very low temperatures. that information is not readily forthcoming] and in conThe most economical and intelligent use of materials sequence if the fabricator is not extremely careful a vessel constitutes good engineering. I n order to accomplish this may be built which he knows beforehand will not adequately a considerable amount of fundamental knowledge must be take care of the working conditions. This company now inacquired and logically applied. This becomes increasingly sists that it be informed of the working conditions to which important because the hazard of building for the safe appli- vessels are likely to be subjected before proceeding with cation of the new processes discovered by the chemical fabrication. By this means the user as well as fabricator of engineer demands a protection for which the old cut-and-try vessels is protected. methods are inadequate. To meet these demands, laboraThe strength of various steels a t elevated temperatures tories have been established for studying the effects of new is another important consideration. This is a matter in working conditions. which short-time tests give very erroneous results, conI n general the problem of containers divides itself into four sidering the long-time service to which the metal in a vessel major parts, as follows: (1) an adequate knowledge of work- is subjected. This company has been furnishing vessels for a ing conditions and of materials necessary to successfully service which calls for continuous operation a t 900' F The meet the working conditions, (2) the safe design of apparatus strength of metal a t this temperature is of vita) importance, and has necessitated the into insure sufficient strength stallation of a considerable in all parts of the container; battery of testing machines (3) the protection of vessels which run c o n t i n u o u s l y during construction so as to night and day to furnish test eliminate unknown fabricatdata on various materials. ing stress conditions; (4) For ordinary low, 0.08 to the testing of such vessels 0.12 per cent carbon steel the to insure their ability to ultimate strength a t 900" meet the working conditions F. for continuous operation to be imposed. is 15,000 pounds per square It behooves the materials inch. Yet when this metal and designing engineer to is e v a l u a t e d by using a work closely with the chemishort-time test, a value of cal engineer, because it is 35,300 is obtained for this just as important to know temperature. Obviously, if the facts about the material the latter value is used and used as it is to anticipate a working stress assumed VPI the service conditions. This based on a certain factor of is necessary so that the adsafety, the value of the facvancement which has been Figure 1-Diagram of Testing'Apparatus Showing System of Levers tor actually available in the andsSystem!oflHeating pioneered by the chemical container for continuous ser1 Received March 5 , 1928.
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