2790.     ---------------.  [REICHL, E. H.]  Synthesis of Hydrocarbons and Chemicals From Carbon Monoxide and Hydrogen.  Medium-Pressure Fischer-Tropsch Units.  3-Stage Operation.  Gas-Recycle Operation.  U. S. Naval Tech. Mission in Europe Rept. 248-45, 1945, pp. 23-27; PB 22, 841; TOM Reel 200.

        In connection with the introduction of medium-pressure synthesis, it was found desirable to revise operations to produce liquid products with a maximum of olefins.  The process used consisted in a combination of staging and gas recycle without substantial change in the operating temperature.  It was found that considerable improvement could be made in the operation and the products by lowering the H₂:CO ratio from the usual 2:1 to 1.94:1 then to 1.8:1 and 1.6:1.  This resulted in improved liquid yields, higher olefin content, lower CH₄ formation, and higher average space velocity.  The only apparent disadvantage was the decrease in the average catalyst life.  Because of the low H₂:CO ratio in the feed, a corresponding low ratio in the exit gas was obtained; therefore, since the ratio of consumption averages nearly 2.15:1, H₂ had to be added to convert the CO remaining from the first state in the second stage.  To correct the ratio, H₂ in the form of converted water gas with a ratio of 3.79:1 was added.  Even then the total conversion in the 2 stages was low.  It was decided, therefore, to add a third stage, preceded by another addition of H₂.  The favorable results of this operation with low H₂:CO ratio led to adsorption of the stage operation in the 3 medium-pressure units at Hoesch, Ruhrchemie, and Lurgi.  In spite of the general improvement in the operation, the olefin content of the products was still too low and finally was overcome by the use of gas recycle in the ratio of recycle gas to feed gas up to 3:1.  The olefin content of the gasoline cut (200° end point) is about 70%, the Kogasin cut (200°-325°) about 45%.  The temperature required to give adequate conversion is somewhat higher than in ordinary Co operation but does not exceed 225°, which can be reached with the existing steam-cooled reactors.  The advantages derived from the above operation are:  The shorter time required to bring the oven on stream; the increase in the yield per m.³ of synthesis gas.  Thus in the first stage above, the same yield may be reached as before in 2 stages:  higher space velocity with a corresponding decrease in the number of ovens required; lower catalyst cost; less disturbance to the operation from variations in the feed gas composition; higher olefin concentration in the products.  In general the application of recycle lowers the boiling point of the product toward lighter materials, but this may be corrected by raising the CO:H₂ ratio.