Encyclopedia of Renewable Energy. James G. Speight

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      Alkazid Process

      The Alkazid process is a process for the removal of hydrogen sulfide and carbon dioxide from gas streams using concentrated aqueous alkaline (caustic) solutions of amino acids. 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 high concentration of up to 90% hydrogen sulfide. The process has been used to treat gas containing up to 10% v/v hydrogen sulfide and removes the hydrogen sulfide to 0.07 to 0.10% v/v. The process is also used for low pressure CO₂ removal where sulfur is not present or is present in small quantities.

      In the process, the purified gas stream leaves the unit from the top of the absorber or absorbers and the cold lean alkaline 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 alkaline solution is fed to the top of the stripper column. The stripper kettle temperature is maintained at approximately 105°C (220°F). The stripped gas (hydrogen sulfide and carbon dioxide) leaves the top of the stripper, and passes through a condenser and separator to remove condensate, thence out of the system to the Claus Unit or other disposal system.

      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 alkaline solution between 1.16 and 1.20. If the gravity of the solution gets too high or the solution gets too cold, solids will settle out. 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: (i) the DIK caustic version – the solution most commonly used – is a solution of potassium dimethyl or diethyl alpha amino-acetate and is used to remove hydrogen sulfide from gases containing carbon disulfide, and (ii) the M-caustic version which uses a solution of potassium methyl alpha amino-propionate which absorbs both hydrogen sulfide and carbon dioxide but is used only in the absence of carbon disulfide. The usual effective gas charge of the solutions ranges from 10 to 15 volumes per volume of caustic for best removal of hydrogen sulfide but can be increased to as much as 30 to 35 volumes per volume by use of mechanical contacting devices and longer contact time, or by permitting higher sulfur content in the outlet gas.

Absorption and stripping are at substantially atmospheric pressure, and the optimum absorption temperature is approximately 5°C (41°F), but a temperature up to 30°C (86 °F) can be used satisfactorily. The relative absorption of carbon dioxide by the DIK-caustic version increases with increased absorption temperature, and with increased time of contact over the normal 1 minute. Any oxygen that enters the system during the operation forms thiosulfates in the solution, destroying its effectiveness. Special precautions are taken to prevent air entering the system at pump packing, or in the solution storage vessels, etc. This difficulty limits the use of the process to oxygen-free gases.