JOHN G. ALLEN Ė "BUTANE DEHYDROGENATION"
J. G. Allen. The subject of "Butane Dehydrogenation" itself is interesting, but it is fairly well covered in the report on Leuna Ė that being the only plant in which I personally saw the dehydrogenation equipment. Since this subject is so closely tied in with the manufacture of aviation fuel through is-octane and alkylate, I feel that I might review a little of the background of butane dehydrogenation and leave the details of the process itself in the Leuna report.
The first process now for making iso-octane in Germany appears to have been the dehydration of isobutyl alcohol to make isobutylene, polymerization to di-isobutylene, and hydrogenation to iso-octane. The next process started with iso-butane instead of alcohol and prepared iso-butylene by catalytic dehydrogenation. Subsequent steps were the same as in the first process. The third process developed with the advent of alkylation, which in Germany was restricted to the sulfuric acid catalyst system commercially. At this time the iso-butane was needed as one of the components of the alkylation feed stock, so dehydrogenation equipment was adapted to n-butane instead of iso-butane. The effluent butylene containing a stream was combined directly with iso-butane in the alkylation unit, and excess n-butane was recycled back to the dehydrogenation reactors.
There was yet a fourth process under development by the I.G. Farbenindustrie which I have seen mentioned in documents but the details of which I have not studied. This process started with n-butane, and by chlorination and dehydrochlorination steps formed butylenes for alkylation.
A comparison of the two processes for making butylenes from n-butane is interesting. By direct catalytic dehydrogenation there was a yield of 20 to 25 percent butylenes per pas, and an ultimate of about 85 percent of the butane destroyed. This mean that four or five passes through the dehydrogenation unit were required, and then only about 85 percent of the butanes could be made into butylenes. In addition the total stream from dehydrogenation had to be circulated through the alkylation unit, including 75 to 80 percent of "headhead" n-butane, which was disadvantageous both in circulation quantity and also in affecting alkylate quality adversely.
Chlorination, on the other hand, would give a once-through yield of butylenes of almost 100 percent, but had the disadvantages of handling chlorine and HC1, and the necessity of recovery of facilities for chlorine by oxidation of the HC1 which was split off in the dehydrochlorination step.
A brief review of the dehydrogenation process as practiced at Leuna might now be in order. An almost continuous catalytic process was used. The published report shows pictures of the equipment, flow sheets, and the general process data. The reactors employed circular groups of eight vertical tubes all arranged in a large circle within a furnace shell. The center of the furnace was verturi-shaped and flue gas ducts were so arranged that combustion gases entering the reactor furnace circulated flue gases around the individual tubes by verturi action. The operation of this reactor involved concurrent flow of n-butane and catalyst through the multitude of individual tubes. The catalyst was made in spherical form with emphasis on the true spherical shape; otherwise the catalyst
Would plug the tubes, carbon would deposit on the plug even to the point of rupturing the tubes. Some of the early troubles were centered in the mechanic of insuring movement of catalyst in all the tubes. At one time a ring of tuning forks was centered below the individual tube outlets so that an operator with a stethoscope could listen outside the reactor for the "ping" that told him catalyst was moving in each tube. Later this method was abandoned.
At the top of the reactor was a hopper for regenerated catalyst which fed the individual tubes through a distributing system. At the bottom was a series of rotating pocket valves or "schlŁssel" which controlled the rate of flow of catalyst through the reaction tubes and dumped the effluent catalyst into a bottom hopper. This catalyst flow was continuous for about 3-1/2 hours, then the top hopper was empty and the bottom hopper full. The catalyst in the tubes was then closed in, but butane continued to flow through this fixed bed of catalyst for the next half hour while the tope hopper was refilled with regenerated catalyst and the bottom hopper was dumped to the regenerator. Then the operation would be repeated.
The regeneration of the catalyst was done in a chamber of similar size but with a different internal arrangement. The catalyst being regenerated was put in a central bed in the chamber and burned with a gas containing about 2 percent oxygen. Flue gas was circulated with an external blower. The original 3 to 4 percent carbon on the catalyst was reduced to about 1-1/2 to 2 percent in the regeneration. It was not considered economical to reduce the unburned carbon in the core of the catalyst spheres any further.
One regenerator would pace two dehydrogenation reactors; that is, regeneration time was about one-half the process time. The entire system was worked out automatically with a cycle timer. Catalyst was transported on an endless chain of buckets, which looked like a miniature of common German coal-handling systems.
I believe this overall description of the process is sufficient with details available in the formal Leuna report.
In the comparison with sulfuric acid alkylation in the United States where several types of reactor are used, the Germans seem to have used only one type commercially, in which cooling was accomplished by auto-refrigeration or evaporation of a part of the reaction mixture. The other parts of the process were very similar to ours: sulfuric acid of 96 percent concentration new and 90 percent spent, isobutene-olefin ratio about 6:1, and reaction temperature about 32oF. One limitation on alkylation operations was the presence of butadiene if the butane conversion in dehydrogenation was too high. This conversion of 20 to 25 percent butylenes per pass, was evidently satisfactory in holding said consumption down to a little over one pound per gallon of alkylate. There was evidently some research work going on concerning removal of butadiene from the alkylation feed, but it was not practiced commercially.
From the overall appearance of these facilities Ė seven butane dehydrogenation reactors and four regenerators about 12 feet in diameter by 16-1/2 feet in height, a dozen alkylation reactors, fractionating towers, and tons of steel in general Ė it is amazing that the Leuna plant actually produced less than a thousand barrels of alkylate per day, although the design figure was about twice that figure.
At the end of the war there had been only two alkylation plants operating in Germany, the other being at Scholven, I believe, about the same size as the Leuna plant. There were another ten or twelve alkylation units under construction in varying degrees of completion, including some complete, which had never run. There was a small amount of iso-octane produced through the di-isobutylene route, but in the whole picture there was a total of a couple of thousand barrels of alkylate per day, and in the choice blends of aviation fuel the Germans preferred 20 percent alkylate. Just compare these production figures without own, and itís almost unbelievable, but these production figures with out own, and itís almost believable, but that was the case. This ends my comments and for further details, I refer you to the Leuna report.
W. C. Schroeder. Are there any questions?
E. B. Peck. How did they make the catalysts?
J. G. Allen. That subject is covered in the same Leuna report, partially in the reports of these processes and partially in Dr. Horneís section on catalyst manufacture. The actual machine making spherical catalysts was a modification of a "Frankonia" candy-making machine, containing two parallel horizontal rotating discs. " believe Dr. Horne has that in some detail.
H. Schindler. I would like to mention that we are just finishing a report on Reel 18, which contains quite a bit of the report on earlier work on isobutene dehydrogenation, and also whatever experimental work they have done on selective hydrogenation of the butadiene to decrease the acid consumption. These data should be available in a week or so.
D. W. Gould. You spoke of the dehydrogenation through the chlorination step. Was there any mention at that point of the catalysis of this reaction?
J. G. Allen. That subject was not mentioned at all at Leuna but then in sorting documents, especially, I believe, from the Oppau or Ludwigshafen plants of I.G., there was some work on chlorination with special lamps.
D. W. Gould. We would like to see details of those lamps, if they were available.
J. G. Allen. There were some drawings on that subject. Iím not sure who studying that, but there is some information somewhere on the microfilm.
H. Schindler. There is some information on this Reel 18 too, on that.
W. A. Horne. Some on that in the supplement to the Ludwigshafen report, too.
V. Haensel. Were there any large size plants on dehydrogenation using the chlorine method?
J. G. Allen. In sorting these documents again, we ran into quite a few plans on that, as I remember, for Heyderbreck. I never was able to find out what the status of that plant was, but that was their method. They had about three processes using chlorination and dehydrogenation which they were considering for that plant, and one general oxidation plant for converting HC1 back to the original chlorine.
______________. I think it is right to say there was no large scale plant for dehydrogenation by chlorination. There were very small scale and semi-commercial plants, but there was certainly not a single large scale unit.
J. G. Allen. Well, I think there is one point that is important to establish, that is, they have apparently designed for Heydebreck (which was a more modern unit than at Leuna), dehydrogenation through chlorination, whereas the old chromia-alumina dehydrogenation system was apparently in that dehydrogenation through chlorination, followed by dehydrogenation, was a better method than direct dehydrogenation by means of catalytic agents.
H. Schindler. Well, the impression I got from reading the document was that it was merely a question of the cost of chlorine with anything else.
E. B. Peck. Well, that comes in on what youíre going to do with your chlorine. Take in some plants, the principal use of chlorine is hydrochloric acid, and they actually have to burn chlorine with hydrogen to make it. Well, if youíve got any very large consumption of hydrochloric acid, itís much better to take a total on your chlorine than to make your hydrochloric acid. Of course, if the chlorine is regenerated, they run it through a reversible reaction.
W. A. Horne. There was almost complete recovery of the chlorine; the chlorine recycle was recovered by electrolysis and reoxidizing and sent back to HC1. I think it was better than 98 percent chlorine recycle through the process.
H. V. Atwell. Did your run across any work on the concentration of butylenes by absorption prior to alkylation?
J. G. Allen. It seems that I have seen some documentary work on that. We didnít run into it at the plant, and I do not have any specific references.
H. Schindler. Iíd like to answer that. In this same Reel 18, there is a very extensive report on concentration by means of a 50 percent silver nitrate solution. They almost seemed to want to put that into practice. But it contains very complete data on solubilities, and all the equipment data except for this unfortunate choice of solvent.
H. V. Atwell. In some other reel, which I canít identify at the moment, I saw a reference to this same process which Dr. Schindler mentioned, of the stripping of the silver nitrate solution by iso-butane vapors from the alkylation.
H. Schindler. Yes, that is exactly right. They very nicely worked that out. They could use liquid iso-butane to strip the butylenes from the silver nitrate solution and thereby come out with a product which was just what they wanted for alkylation. They have some economics in that too, which compares with the heat requirements using the one and the other processes. As I said, we made a very detailed translation of that and gave all the figures in converted units, and so on. That report should come out next week or so, as I said.
W. C. Schroeder. Any further comments and questions?
P. K. Kuhne. Iíd like to ask the size of the dehydrogenation tubes, 16-12/ feet long?
J. G. Allen. I was speaking of the whole reactor. The tubes themselves in the dehydrogenation of isobutene were a little under 2 inch inside diameter, and about 16 feet long, but in the normal butane dehydrogenation, they had gone up to a little larger tube, about 2-1/2 inches inside diameter.
W. C. Schroeder. If there are no further comments, we will move on to the next paper, which is "Hydroforming and D H D," by Ernest Cotton.