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4. Synthesis of Isoparaffins

Isoparaffins were synthesized commercially in Germany by two (2) processes isobutylene polymerization followed by hydrogenation of the polymer and by alkylation of butylenes and isobutene. Of the two processes alkylation was much the more important from the stand point of volume produced. Both of the above processes have been more highly developed than present practice elsewhere. They are described below, however, together with the methods by which their raw materials are produced.

The production of isoparaffins other than those obtained from the two commercial processor was given extensive study. The synthesis of triptane was studied and a process was designed from this work. although triptane itself is not the end product. This development is described below.

The isomerization or normal butane was being carried out commercially to supply isobutylene to alkylation. The commercial process used is described in this section together with some new research on the isomerization of normal C 6 and C7 paraffins.

(a) Isobutylene Polymerization and Polymer Hydrogenation

This process for isooctane manufacture was employed at Leuna, Ludwigshafen-Oppau and Heydebrek.

Isobutyl alcohol was synthesized directly from CO and H₂ by the “Isobutyl Syntheses” (described by the U.S Naval Technical Mission in Europe Report titled Synthesis of Hydrocarbons and Chemical s from Mixtures of CO and H². The alcohol was dehydrated to isobutylene over precipitated alumina at 630 to680 degrees fahrenheit and normal pressure. In this temperature interval a 95 percent conversion of alcohol to olefin was obtained with a small accompanying yield of isobutyraldehyde. A pass operation was therefore employed. Isobutyraldehyde and water were separated from the isobutylene by simple distillation. The aldehyde was hydrogenated to alcohol and recycled back to dehydration feed.

Isobutylene from the alcohol dehydration was compressed to 20 atmospheres heated to 300 to 350 degrees Fahrenheit and polymerized over a catalyst of 25 percent phosphoric acid no activated carbon. Unpolymerized isobutyl was separated and recycled and combined dimmers and trimers were taken overhead in a second column, leaving only a small amount of high boiling polymers as bottoms. The dimmer-trimer mixture then hydrogenated under 200 atmospheres of hydrogen pressure at 660 degrees Fahrenheit, using a tungsten nickel-sulfide catalyst. A hydrogen recycle of four (4) to one (1) based on fresh hydrogen was employed.

The hydrogenated fraction, known as ET 110 or Di 1000, had the following properties:

Before the advent of the alkylation process, isobutylene was being produced by isobutene dehydrogenation at Leuna Pölitz, and Scholven. Polymerization and polymer hydrogenation systems were used to convert this isobutylene to T-52, a product nearly identical to ET 110. The processing of the isobytylene to T-52 differed from the ET 110 system only in that due to slightly different feed composition, the polymerization catalyst in the T-52 process was 50 percent phosphoric acid on asbestor instead of the 25 percent phosphoric acid on active carbon catalyst in the ET 110 System.

The following document, transmitted to the Bureau of Ships, relates to this process:

II.  Herstellung von Di. 1000. (Flow diagram of the Di 1000 or ET 110 process).

(b) Alkylation

Although research and development work on alkylation was started in Germany prior to 1940, the commercial production of alkylate did not begin until 1943. Prior to that time Leuna, Pölitz, and Scholven had been producing isobutylene by isobutene dehydrogenation, and those dehydrogenation plants were then shifted to normal butane feed.

In early 1944, these three plants were still the only operating aliplation units, but plants were being constructed in Wesseling, Brux, Bohlen, and Blechhammer. Had these plants all been completed and put into operation, Germany’s alkylate outturn would have risen about 50 percent above her actual attained production.

Normal butane dehydrogenation and isomerization processes were both in use in Germany. Appendix I to this report describes dehydrogenation, and the general subject of isomerization is discussed later.

Only butylenes alkylation was practiced in Germany. By the application of the processes of dehydrogenation, isomerization and alkylation C4 components from the large coal and coal tar hydrogenation plants could be totally converted to butylenes alkylate. (Some C4 fraction was still being need as liquefied gas, but nearly all of the large hydrogenation plant C4 outturn was to have gone ultimately into alkylates).

No propylene or alkylene alkylation was carried out commercially. While these operations had been completely explored in the laboratory, to supply an additional olefin to alkylation and thereby increase the volume of alkylate at a sacrifice) in quality. In calculating the optimum position on isoparaffin production, the most stress was placed on lean mixture performance rating. Rich mixture performance was at a lower premium apparently because of the relatively greater availability or aromatics and aromatizing capacity.

The alkylation plants varied in a few respects only from those in common use in America. (Complete plant descriptions are attached). Refrigeration of the reactor was accomplished by evaporating C4 from the surface, compressing and liquefying and returning the liquid to feed. The reactor itself use sometimes a stirred autoclave with no external recycle of reactor hydrocarbon phase being practiced. Only pure isobutene prepared from reactor product through a series of columns, was then used for recycle to build up the isobutene to olefin ratio.

In other plants, however reactor system was used which consisted of a mixing and cooling vessel, where vapor was withdrawn to the refrigerating cycle a circulating pump, and a time tank. Emulsion was recycled, and a portion of the emulsion was withdrawn to a settling vessel, from which acid was recycled back to the mixing vessel.

The important operating variables and yield figures for a butylenes plant employing the last described reactor system are summarized in Table III. Triisobytylene from ET 110 plants was used for alkylation feed when available, and the alkylate yield and quality were about equal to those obtained when using the equivalent amount of isobutylene.

Regeneration of spent sulfuric acid from alkylation was practiced in at least one location (Leuna). In that plant, alkylation acid was diluted to ca. 50 percent concentration, the liberated oil (tar) layer was separated off and the acid was reconcentrated in a “Pauling Kessel” to 93 or 94 percent acid. It was then fortified with SO3 to 98 percent concentration.

The following documents, transmitted to the Bureau of Ships, relate to alkylation:

III.  Herstellung hochklopffester isoparaffinischer Treibetoffe durch Alkylierung aliphatischer Kohlerwasserstoffe 

(I.G. Leuna - Dr. Pohl II report of 6 Jan. 1943)

IV. Alkylerung - Anlage-Leuna 

(I.G. Leuna - flow diagram of Alkylation Plant)

V. Alkylierung und Destillation 

(I.G. Leuna - report by Dr. Struts of about April 1944)

(c) The Peroptan Synthesis

The premium value of triptane as an aviation gasoline component was recognized in Germany and much effort was put forth to develop a method for its synthesis. The most extensive study was made by a research group from I.G. - Ludwigshafen-Oppau.

Some triptane was first made by a Grigrard reaction for testing to establish its anti-knock properties. In contemplating then what reaction could be used for its commercial production, the combination of isopropyl chloride (Chlorpropane-2) with isobutane was considered. Also, by the use of the same type of reaction, it was considered that tertiary butyl chloride and isobutene might yield 2,2,3,3 tetramethyl butane another octane with outstanding anti-knock properties.

In 1943 a program of study of the above type of reaction was undertaken. Propyl chloride was first made by direct reaction of propane and chlorine using ultraviolet light as a catalyst. An 8:1 mol. ratio of propane to chlorine was fed into a vertical iron tube, down the center of which was mercury in a tube. The feed inlet temperature was 70 degrees Fahrenheit and the best of reaction was adequate to raise the temperature of the system to 140 degrees Fahrenheit. A pressure of 20 atmospheres was maintained to keep the system totally liquid. Under these conditions complete reaction of the chlorine was obtained. The product was fractionated removing first hydrogen chloride then propane and then separating the two monochlor isomers. A very small yield of residue remained.

The isopropyl chloride was reacted with isobutane at 32 degrees Fahrenheit, using both the ultraviolet light and a slurry of aluminum Chloride as catalyst. One part of isopropyl chloride, five parts of isobutene, and one part of AlCl₃ were agitated under ultraviolet light until HCl liberation subsided. The HCl was removed, then isobutene was separated, and the higher boiling materials were examined. No triptane was ever found in the product, but essentially the entire yield was a mixture of isoparaffins boiling in the 190 to 370 degree Fahrenheit range. About 50 percent of the yield was 2,2,3 trimethyl pentane, and most of the product boiled between 210 and 230 degrees Fahrenheit. The octane number of the total mixture was 96 to 98 and the rich mixture rating exceeded that of 2,2,4 trimethyl pentane.

It was found that chlorpropyl-1 was equally as effective as isopropyl chloride for this reaction and the separation of the two isomers was discontinued. It was found also that all material boiling below 190 degrees Fahrenheit formed in the reaction could be recycled back into the system without build-up. Based on propane and isobutene feeds an 80 percent weight yield of product could be obtained.

The above operation was proposed as a process and the product was named “Peroptan”. A plant to produce about 100 barrels per day was being designed for construction at Ludwigshafen-Oppau, but by early 1945 it had not progressed beyond the design stage. The plant was to take propyl chloride available at Oppau from the synthetic glyceria plant. The reaction for propyl chloride isobutene was to be a 40 barrel autoclave. The 190 to 370 degrees Fahrenheit fraction was to be separated and given a purifying hydrogenation over Raney nickel catalyst to remove about one percent weight of chlorine that remained combined in that fraction.

Isobutyl chloride and tertiary butyl chloride were also reacted with isobutene, following the general procedure as given above for propyl chlorides. No 2,2,3,3 tetramethyl butane was over-detected in the products. It was found that the product from the two butyl chlorides were the same and surprisingly they were very similar to the products from propyl chloride. The composition and qualities were not significantly different.

Ethyl Chloride isobutene reaction was attempted but HCl liberation could not be obtained.

Other efforts synthesized highly branched paraffins were made in Germany but none had resulted in a practical process. For technical interest the following document transmitted to the Bureau of Ships relate to the chemistry of these studies.

VI. Die Herstellung von trimethylbutan (I.G. - Ludwigshafen - review by Dr. Bueren of 22 October 1943).

VII. 2.2.3 Trimethylbutan und andere verzweigte Kohlenwasserstoffe durch Hydrierung von Trialkylessigsäure.

VIII. (Die Wichtigsten Daten und Herstellungsweisen einiger Isoparaffin unter besonderer Berücksichtigung ihrer Verwendung als Motortreibstoffe. (I.G. - Ludwigshafen - tabulation of 16 March 1944)

(d) Isomerization of Normal Paraffins.

Normal butane isomerization plants producing isobutene for alkylation had been built in Blechhammer, Böhlen, Leuna, and Scholven.

The plants employed a vapor phase process over aluminum chloride as contact. The installations were not greatly different from the vapor phase plants in wide use in America.

The German reactors were operated at 200 to 210 degrees Fahrenheit under 16 atmospheres pressure. The normal butane feed to the reactor contained 10 percent weight HCl. The AlCl₃ catalyst (technical grade) was put into the reactors in crude lump form. At 200 degrees Fahrenheit and a liquid hourly speed velocity of 3.0 (volumes liquid normal butane per volume of catalyst per hour) a conversion of ca 30 percent was obtained and a 96 percent weight recovery of total C₄ was obtained. The aluminum chloride consumption was not above 1.2 percent weight, based on isobutene produced and the corresponding figure for anhydrous HCl was 0.6 percent weight.

The conversion of normal to isobutene could be increased to 40 percent by raising the operating temperature to 210 degrees Fahrenheit , but C₄ recovery dropped to 95 percent weight and catalyst consumption increased somewhat.

There are attached quite complete description and a flow diagram of the process. The reactor design described is interesting. The lump aluminum chloride catalyst was put in on top of a section of Raschie rings and both below the rings and above the catalyst layers there were large free space created in the vertical reactor. The feed butane-HCl mixture entered the bottom of the reactor and flowed upward . As the catalyst formed hydrocarbon complexes, it began to fluidize and run down over the surface of the Raschig Rings. By supplying and adequate height of ring layer, the fluid reaching the bottom and running off into the reactor free space was completely spent. The spent liquid collected in the bottom head of the reactor and was withdrawn. The free space above the layer of catalyst was to serve as a zone of “after reaction” in which sublined catalyst would react with the butane mixture, form a liquid and return to the catalyst bed rather than be carried out as sublined AlCl₃; (In practice this was not quite realized and AlCl₃ did carry over causing condenser tube plugging.)

Although chrome-nickel steels were preferred for use in the reactor, condenser, piping, etc., only low carbon steels were available. Some corrosion difficulties were originally experienced in the plants, but with good drying of the feed corrosion was not serious operating problem.

There was no commercial isomerization of pentane in Germany, but the process had been extensively studied in the laboratory.

Of technical interest was some research conduted by I.G. Leuna and the KaizerWilhelm Institute in Mulheim on the isomerization of C6 paraffins.

Hexane isomerization was carried out on a normal hexane fraction (from Fischer-Tropsch) at I.G. -Leuna. A 50 atmosphere pressure of hydrogen was applied and the temperature was 160 to 175 degrees Fahrenheit. The catalyst uses aluminum chloride with added HCl equal to ca. 30 percent weight of the AlCl3 in the system. (The AlCl₃ was mixed with SoCl₃ or chlorinated hydrocarbons or phosgene to obtain a liquid phase catalyst at the operating temperature). A particular experiment with a contact time of 5 hours gave a 70 percent conversion, and the approximate yield structure was 15 percent weight of 2,2 dimethyl butane 10 percent weight of 2,3 dimethyl butane, 10 percent weight of 3 methyl pentane, 15, percent weight of 2 methyl pentane, 30 percent unconverted normal hexane, and 20 percent weight of C₄ C₅ and other components. (Ethane and propane were usually absent in the products produced by cracking and isobutene was the main product of disproportionation reaction).

Less cracking is obtained in paraffin isomerization when hydrogen pressure is high and temperature is low. Of course, as temperature is lowered longer contact time is required in order to attain a given conversion.

In a K.W.I. experiment at 100 atmospheres of hydrogen, 160 to 175 degrees fahrenheit, 0.2 mols of AlCl₃ and 2 mols of HCI per mol of normal hexane, and a contact time of about 18 hours, a 90 percent conversion of normal hexane was obtained and very little cracking or disproportionation occurred.) Based on total hexanes, the yield was 57 percent of 2,2 dimethyl butane, 9 percent of 2,3 dimethyl butane, 31 percent of a mixture of the two methyl pantanes, and 3 percent of unconverted normal hexane.

I.G. consider that the practical application of normal hexane isomerization would be under conditions to obtain perhaps a 30 percent conversion, and the estimate that at such conversion, the dimethyl butane isomer would be more than half of the total isomer yield.

Isomerization of normal heptane was studied under hydrogen pressures up to several hundred atmospheres. It was found impossible even under these conditions to avoid substantial cracking of heptane in contact with AlCl₃.

The production of branched hexane by the isomerization of cyclohexane was studied. Cyclohexane was contacted with 15 percent weight of AlCl₃ and 7 percent weight of anhydrons HCl in the presence of 150 atmospheres of hydrogen and a temperature of 210 degrees Fahrenheit and a contact time of 6 hours, the product obtained was a mixture of one percent weight isobutene, 20 percent weight of 2,2 dimethyl butane, 6 percent of 2.3 dimethyl butane 18 percent of a mixture of 2 and 3 methyl pentanes cyclopentane, and the rest was unconverted cyclohexane. It was stated in a patent application that the AlCl3 can be recovered essentially unchanged from the operation.

The following documents transmitted to the Bureau of Ships, relate paraffin isomerization:

IX.  Die isomerisierung von n-Butan mit AlCl3.. (I.G. - Leuna - report by Dr. Pohl II, etc. of 22 February 1943)

X.  Schema der Isomerisierung. (I.G. - Leuna - flow diagram of butane isomerization plant).

XI.  Isomerisation. (I.G. - Leuna - report by Dr. Strätz of early 1944).

XII.  Über Isomerisierung von Paraffinen. (KWI - Mulheim copy of speech by Dr. Koch on 24 June 1943.)

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