3. I.G. ALKAZID PROCESS FOR GAS PURIFICATION
Chemical purification is employed where the contaminants are not removable by mechanical means. Gas impurities of a slightly acid nature such as hydrogen sulphide, carbon dioxide and hydrocyanic acid were sometimes removed from gases by basic substances such as alkali hydroxides and possibly sodium or potassium carbonate solutions that can be regenerated. However the process developed by the Girdler Corporation employing the alkylol aminos especially the mono, -di-, and triethanolamino and later diaminopropanol to absorb hydrogen sulphide and carbon dioxide was not found in use. Amines are strong bases and water soluble substances which readily absorb H2S and CO2 at normal temperatures and liberate them when heated to 100ºC, and absorb several times as much CO2 and H2S per unit of volume as do the potassium or sodium carbonate solutions.
The alkazid process was developed by I.G. Farbenindustrie to remove H2S or mixtures of H2S and CO2. Gas purification units of this type have been constructed in many countries all over the world. The process is used primarily to treat gas of high sulfur content before it goes to other stops for more complete purification. It also claimed the advantage of yielding hydrogen sulfide of quite high concentration, e.g., under favorable conditions up to 90% H2S. This unit is probably used more often in Germany to treat gases from hydrogenation of cracking operations than it is used to treat manufactured gas, but it is satisfactory for either service.
A description of the unit as applied to hydrogenation product gas treatment is given in U.S. Naval Technical Mission in Europe Report No. 87-45 "The Wesseling Synthetic Fuel Plant" and it is not repreated here.
A simplified general flow sheet is shown on drawing M 2028 (Item 3 in the appendix of this report). In this sketch the raw gas enters the bottom of one or both absorber columns (labelled "Abstreifer") which can be operated in parallel or on series. The purified gas ("Reingas") leaves the unit from the top of the absorber or absorbers. The cold lean caustic solution passes countercurrent to the gas down the columns, out the bottom through a screen and is pumped through a heat exchanger countercurrent to the hot lean caustic from the bottom of the stripper. From the heat exchanger the rich caustic is fed to the top of the stripper colmn. The stripped gas (H2S and CO2) leaves the top of the stripper, passes through a condenser and separator to remove condensate, thence out of the system to the Claus Unit or other disposal. The steam to heat the kettle of the stripper is distributed between direct and indirect heating in order to hold the specific gravity of the caustic between 1.16 and 1.20. The stripped caustic leaves the bottom of the absorber through a screen, passes through the countercurrent heat exchange mentioned above, through a water cooler and to the top of the absorber.
There are two types of solutions used in the Alkazid process. "DIK Caustic", the solution most commonly used, is a solution of potassium dimethyl or diethyl alpha aminoacetate and is used to remove H2S from gases containing CS2. "M-Caustic" is a solution of potassium methyl alpha aminoproprionate. It will absorb both H2S and CO2 but is used only in the absence of CS2. The normal effective gas charge of the solutions ranges from 10-15 volumes per volume of caustic for best removal of H2S but can be increased to as much as 30-35 volumes per volume by the use of mechanical contacting devices and longer contact time, or by permitting a higher sulfur content in the outlet gas.
The process has been used to treat gas containing up to 10% H2S, and will remove the H2S to 0.07 to 0.10%. Absorption and stripping are at substantially atmospheric pressure. The optimum absorption temperature is about 5ºC, but temperature up to 30ºC can be satisfactorily used. The relative absorption of CO2 by DIK-caustic increases with increased absorption temperature, and with increased time of contact over the normal one minute.
The stripper kettle temperature is maintained at about 105ºC.
A new development of this process, which has not yet been put into commercial operation, absorbs at 8-10 atmospheres and strips at 1 atm without the addition of heat to the kettle except for the direct steam necessary to control caustic gravity. The effective loading of the caustic under these conditions is only 4-5 volumes/volume, but it is claimed that the extra pumping and compression cost is more than compensated by the economy of steam.
The process is also used for low pressure CO2 removal where sulfur is not present or is present in small quantities.
A cost comparison made by the I.G. Farbenindustrie shows the Alkazid process to be cheaper than water wash, Girbotol, Koppers, Shell and several other processes; however, even if the figures are accepted, altered conditions in the U.S. may well change the results and no positive statement can be made of the superiority, of the process for all conditions. The main advantage of the process over over the ethanolamino process is the lower volatility of the salute.
Many operating difficulties have been experienced with the unit. The principle trouble has been with corrosion of the steel and aluminum used in the equipment. The Germans combat this corrosion by close control of the caustic gravity, control of the temperature in the aluminum equipment, careful exclusion of air from the system, and by "passifying" the aluminum through circulation of dilute sodium silicate solution through the equipment before operation. No mercury or mercury salts are used where they can possibly enter the system. The direct steam to the stripper must be maintained dry and free from impurities. Attached is a report of visit made to the Alkazid Plant in Lutzkendorf in regard to the corrosion.
Foaming of the solution occasionally causes trouble, but the I.G. have developed an antidote known as Schaumbekampfungsmittel "I" (Foaming attacking agent "I") in the Leuna plant.
Any oxygen that enters the system during operation forms thiosulfates in the solution, destroying its effectiveness. Special precautions are taken to prevent air entering the system at pump packings, or in the solution storage vessels, etc. This difficulty limits the use of the process to oxygen-free gases.
If the gravity of the solution get too high or the solution gets too cold, solids will settle out. Corrective measures are heating and/or dilution of the solution.
For more complete information on operating and supervision methods for the Alkazid process two reports are included in the appendix as follows:
A more complete flowsheet which includes a NaOH wash following the Alkazid absorption is shown on drawing No. M3200-1, included in the appendix. A comprehensive technical article on both this process and the Claus process was published in the June 1938 issue of the Refiner and Natural Gasoline Manufacturer pages 237-244 and many of the details given there are not repeated here.