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11. Adsorption of Hydrocarbons on Activated Carbon.

The art of adsorbing the heavier fractions from hydrocarbon gases on activated carbon with subsequent recovery of these fractions by steaming the carbon has been known and practiced for many years. The principal use made of this process by the Germans was the recovery of the so-called “Gasol” (C₃-C₄) fraction from the Fischer-Tropsch synthesis. Adsorption on carbon is peculiarly adapted to this purpose because a large proportion of the reaction product from the low pressure synthesis is not condensable at 20° C, but passes through the condenser with the residue gas. The process operates effectively at low pressures (atmospheric).

A diagram, (reference I(a)/15 at end of this section) showing the method of operating an activated carbon unit fro “Gasol” recovery is included in the appendix. In this scheme the feed gas is Fischer synthesis residue gas from which the gasoline and heavier components have been condensed by water cooling. In operation, at least four adsorbers are required. Each chamber is used successively for adsorption, drying and cooling.

The feed passes from bottom to top through adsorber one, which is in the adsorption stage. There the heavier components are adsorbed on the activated carbon. The lean stripped gas goes to the drying cycle, where it is picked up by flower 5m and heated from 100° to 150° C, in heater 6 along with some circulated dry gas. This gas then passes through adsorber 2, to dry the wet carbon bed which has just been steamed in the desorption stage. The hot wet gas from adsorber 2, is dewatered in cooler 7, and a large part of the dewatered gas is recycled through blower 5 as shown. The remainder, equivalent in volume to the gas from adsorber 1, goes to blower 8, and is further cooled in cooler 9. It then passes through the hot carbon bed of adsorber 3 to cool this bed preparatory to re-entering the adsorption stage. The gas leaving adsorber 3, is partially returned to the cooling cycle while the remainder leaves the system as lean residue gas to be sued as fuel or for another synthesis stage.

The circulation is so controlled that when adsorber 1 has reached the limit of its adsorbing capacity, adsorber 2 is dry and adsorber 3 is cool. At this point the valves are automatically changed as follows: Adsorber 1 to desorption, 2 to cooling, 3 to adsorption, and 4 (which has been steamed out) to drying. The series flow of the gas through the three stages serves the additional purpose of picking up any heavy hydrocarbons, that carried through the first adsorber near the end of the adsorption period, in cooling the carbon bed of the third adsorber. This, of course, permits a greater loading of the carbon with the attendant economy of steam.

The desorption is performed with steam, and the gases are driven out of the carbon in the order: CH₄, CO₂, C₂H₆, C₃H₈ and heavier. The three-way valve 12 in the outlet line is open to the inlet of the adsorbing carbon chamber until the CO₂ and CH₄ are driven off in order to recover the small amount of heavy ends that come off. As soon as the C₃ and heavier gases begin to appear, valve 12 is switched so that mixture passes to condenser 13. Here the gasoline and water vapor are condenser by indirect cooling. This condensate is sent to separator 14 where gasol, gasoline, and water fractions are removed. The gasol goes to holder 15. The gasoline goes through after cooler 16 and meter 17 to tank 18. The water condensate is removed from the system.

The gasol from the holder goes through a compression and liquifaction cycle from which the uncondensable portion is recycled to adsorption and the liquid is sent to an intermediate storage tank. The pressure on this tank is maintained by bleeding gas back to holder 15. The gasol and gasoline are both pumped over a cooler and fed together to stabilizer column 19, where stabilized gasoline and merchantable gasol are removed, the rich overhead condensed gas is fed into the stream to the adsorber of the desorption step. At the beginning of the desorption, this gas, since it is quite rich, displaces CO₂ from the carbon bad during the first minutes of desorption, and therefore does not appear in the recycle to the adsorption vessel but goes to condenser 13, thus again saving steam for the process by higher loading of the carbon.

The carbon adsorption is sometimes operated as a two stage process. The following operating data were given by the Lurgi Company.

In the above cases, the carbon used was known as “supersorbon”. It was made from peat and activated by a Zn Cl₂- steam treatment. The size of the carbon was 10%, 2.0-3.3 mm. A carbon charge will remove about 1000 kg. of gasoline and gasol per kilogram of carbon before reactivation became necessary. It can be reactivated with steam at 800° C in a rotary kiln.

The Lurgi Company has developed a new carbon known as “SK” which is activated with K₂S, and is said to have about twice the capacity for low boiling hydrocarbons as “Supersorbon”. The entire output of this carbon was used for gas masks during the war. Its absorption power for benzol from air as compared to that of the “Supersorbon” is presented below:

A reprint of an article by Drs. Herbert and Ruepping entitled “Benzin und Gasolgewinnung mit Aktivkohle aus den Resgasen der Benzin-synthese nach Fischer-Tropsch, Ruhrchemie”; is included in the appendix of this report. This German article gives data on the recovery of hydrocarbon from Fischer-Tropsch residue gas, including the composition of the desorbed gases from minute to minute.

(See also reference I(a)/14 at end of this section).

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