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3. Synthesis - Operating Conditions. (See ref. II/1 to II/13 at end of this section).

(a) CO₂ Formation.

The CO and H₂ are consumed in the synol synthesis in the ratio of 1.1:1.0 (CO:H₂) but regular watergas (CO:H₂=1.0:1.1) may be used for the synthesis. The catalyst is susceptible to sulphur poisoning and requires a feed gas as equally pure as the FT plants. A maximum of 25% inerts in the gas was acceptable.

It was stated that the synthesis should be carried out at the lowest temperature possible.

It was found practical to use the formation of CO₂ as controlling variable for the maximum production of alcohols. The less the conversion proceeds in one stage, the less CO₂ formed and the more alcohols recovered. 10% is considered the maximum value for the volume % of CO₂ in the exit gas from the synthesis. The CO₂ may be scrubbed out before entering the next stage or returning the gas to the reactor (recycle).

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(b) Temperature

Over a given catalyst and at a definite pressure and CO content of the feed, the temperature in the reactor is the means for control of the conversion one wishes to obtain. This temperature is between 190°-195° C in the first stage and may exceed 210° C in the following stages depending on the inert content of the gas, particularly the CO₂ content. CO₂ in quantities of 5-6% has a poisoning effect in that it requires a higher synthesis temperature and thus lowers the alcohol yield.

Towards the end of the war the German government lost interest in the synol development and insisted on directing the research toward production of fuels. I.G. then planned to continue with the installation of the synol plant, but operate under conditions which would give little alcohol and mostly olefins (“Benzinfahrweise”). This could be done very simply by raising the temperature of the synthesis. The operation at 260-300° C over the same catalyst produced a gasoline of 40-60% olefins with 65-75 octane number (res.). This operation was admittedly a means to continue work on the project and is not considered an improvement over the synol operation.

In order to illustrate the difference in the two methods, the following data are listed below giving results from two types of operation:

Both columns refer to the identical fuzed iron catalyst:

The alcohol and olefine content of the various fractions from Synol are as follows:

Note: Between 200-400° the “Rest” varies from 3-8%.

In general the increase in temperature lowers the alcohol and increases the olefine yield. As the catalyst ages, it slowly loses its activity and requires increasingly higher temperature. Thus the alcohol yield decreases over the life of the catalyst.

The same general rules, applicable in FT operation apply equally in syno1: more alkali in the catalyst, lower temperatures, lower space velocity (within limits) give higher boiling products. There were however, indications that the desirable middle fraction could be increased individually by using precipitated instead of fuzed catalyst of identical composition. The reasons for this were not recognized.

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(c) Pressure.

The preferred pressure range for the synol synthesis is 18-30 atm. (25 at optimum). At higher pressure the Fe-carbonyl formation is substantial. At the same time it becomes more difficult to remove the heat of reaction and this in turn leads to carbon deposition on the catalyst. Thee alcohol/olefine ratio is little affected by pressure.

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(d) Recycle

The “Kreislauf” operation described in Section I(a) of this report may be used equally well in synol operation. It is simply necessary to remove substantially all water from the reaction product before returning the gas to the oven. This drying tends to suppress the CO₂ formation sufficiently to obtain 90-93% conversion (on a gas containing 6% inerts) without intermediate scrubbing of the CO₂. The removal of CO₂ particularly by way of the alkalized-type processes is not easy in this case due to the fatty acids formed in the synthesis.

It was also found that the life of the catalyst was dependent on the partial pressure of the water in the reactor. In a “Kreislauf” run when the water was removed from the reaction products before recycling, and using 25 V/H/V total gas load, the catalyst ran 7 months without losing its activity. This compares in the once through (3-stage) operation with a maximum of 3-4 months catalyst life.

Another beneficiary effect of the “Kreislauf” is of course the immediate removal of the synthesis products from the reactor. Thus any secondary dehydration or hydrogenation is suppressed. The higher alcohols in the C₁₄-C₁₇ range are particularly affected by this operation. Due to the increased gas volume, these products are swept out of the reactor as vapors. In the once through operation they condense on and are decomposed by long contact with the catalyst.

Using recycle space velocities of 2000-3000 V/H/V (based on total feed) allows 90-94% conversion in two stages only (while 4 are required in the once through operation.). Under these conditions 1 mol. CO₂ is then formed from 12 mol. feed gas.

Some operating figures may illustrate the Kreislauf operation:

The data are based on a fresh feed gas of the following composition:

Change of recycle gas composition with conversion (1 stage only).

These figures are all based on the same catalyst volume. Thus to obtain higher conversion at lower CO-H₂ content the synthesis temperature must be raised; this in turn increases the CO₂ production (the conversion to CH₄ was assumed as 8% in all cases).

Operation in liquid phase was not considered applicable for synol because long contact on the catalyst is characteristic for this technique. However, the use of “Kreislauf” may make liquid phase operation feasible. I.G. had proposed to study this process but the end of the war prevented further work).

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