7. GAS PURIFICATION BY ACTIVATED CARBON (FEINSTREINIGUNG)
There are two activated carbon processes used in Germany for the purification of synthesis gas. One is the so called "Feinstreinigung" of Lurgi Company, which consists of the absorption of hydrocarbons on activated carbon. This process is used to remove the aromatics and resin forming hydrocarbons whenever gas is obtained from coal instead of from coke, not only to purify the gas but also to protect the catalyst used in the organic sulfur removal process (Feinreinigung). The other activated carbon purification process is that of the I.G. Forbenindustrie and is used to remove hydrogen sulfide. When this process is used it replaces both Alkazid and Grobreinigung wherever the inlet H2S concentration is less than 7 grams/m3. These processes are described below.
In the "Feinstreinigung process of Lurgi the raw gas from the Alkazid purification unit or other source enters the Lurgi carbon chambers saturated with water at about 30ºC. The water inhibits adsorption so that instead of a possible 80 hours on-stream absorption cycle time, a period of 20 hours is used. After the adsorption period the chamber is steamed for two hours, dried with heated gas for one hour, cooled for one hour and then put back into the adsorption cycle. An understanding of the process can be better obtained from the following concrete example of a Lurgi installation.
90,000 m3/hr of gas generated from coal is passed over 75,000 Kg of activated carbon located in five parallel chambers of 5m. diameter. A sixth chamber is in the regeneration cycle. After a chamber has been in service 20 hours, it is put into the regeneration cycle and the freshly regenerated chamber put back into adsorption. The regeneration requires a total of 4-1/2 tons of dry steam at 2-1/2 atm. pressure per chamber. The recovery of hydrocarbons amounts to about 0.4g/m3 of inlet gas and this material is added to the coke-oven gasoline. The drying cycle removes about 1500 kg of water.
One kilogram of carbon will process about 10,000m3 of gas before reactivation is necessary. The reactivation is carried out in a rotary kiln at 800ºC with steam.
The I.G. Farbinindustrie process for hydrogen sulfide removal over activated carbon is an old process that dates from 1928, but it is still considered very satisfactory for gases containing a relatively small amount of sulfur. Drawing S92 in the appendix shows a schematic diagram of this process. The drawing shows that the raw gas goes through a blower then to a water separator, next a water cooler, and then more separators. Next a small amount of ammonia air mixture is added to it. The mixed gases then pass over an activated carbon is added to it. The mixed gases then pass over an activated carbon bed at 800mm water pressure and 20-40ºC temperature where the reaction:
2H2S + O2 S + 2H2O
takes place with the ammonia acting as in intermediate or catalyst. The H2S removal is substantially complete. The ammonia leaves the absorber along with the gas and is subsequently washed out in the water absorption process for removal of the CO2 in the gas.
The sulfur found in the reaction is precipitated in the carbon bed, and when the bed is loaded the sulphur is washed out with a 20% ammonium sulfide solution in the form of ammonium polysulfide. Ammonium carbonate is also formed in the carbon and it is washed out at the same time. The ammonium carbonate must be removed from the wash solution and this is done by adding CaCl2 solution to give the reaction:
CaCl2 + (NH4)2CO3 CaCO3 + 2NH4Cl
This CaCO3 sludge is removed from the wash solution in a Dow type thickener. Sodium sulfide is added and the following reaction takes place in the solution:
Na2S + 2NH4Cl (NH4)2S + NaCl
The solution goes to a column operating at 2 atm. and 130º kettle temperature where it is distilled. The ammonium sulfide and part of the water go overhead; the sulfur in molten form and the sodium chloride solution are withdrawn from the two liquid phases of the kettle.
A concrete example of the use of this process is the treatment of 6000 m3/hr of gas containing 3.5g/m3 of H2S and 0.2g/m2 of organic sulphur. In this case about 12-13 liters of air plus 0.2g NH3 are added per cubic meter of gas and the mixture is passed over a 20m bed (7 tons) of activated carbon. This bed will take a "loading" of 6 tons of sulfur before sulfur removal becomes necessary. The regeneration is accomplished by filling the bed 3 times with a total of 200m3 of ammonium sulfide solution and allowing each of the washes to stand for 1 to 2 hrs. The solution remaining after the third draining is steamed out, the bed is then dried and cooled and put back into service.
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