What are the methods for removing carbon dioxide from feed gas?

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The transformed raw gas contains a large amount of carbon dioxide. Carbon dioxide not only temporarily poisons the ammonia synthesis catalyst but also dilutes the feed gas, reducing the partial pressures of hydrogen and nitrogen ； Furthermore, when it comes into contact with copper-ammonia washing solution in the system, or with ammonia-containing circulating gas, ammonium bicarbonate crystals are formed, which can block the pipes. At the same time, carbon dioxide is also a raw material for producing urea, soda ash, dry ice, and so on. Therefore, carbon dioxide in the feed gas must be both removed and recovered. There are many methods for removing carbon dioxide, and the appropriate method should be chosen based on specific circumstances. Currently, various decarbonization methods at home and abroad mostly use solution absorbents to absorb carbon dioxide. Based on the absorption mechanism, it can be divided into two main categories: chemical absorption and physical absorption. In recent years, solid carbon dioxide removal methods such as pressure swing adsorption and membrane separation have emerged. (1) Chemical absorption method: Early methods in this category included the ethanolamine method (MEA) and the ammonia water method, but these are now rarely used. The commonly used chemical absorption method at present is the modified thermal potassium-alkali method, which involves adding a small amount of activator to the potassium carbonate solution in order to accelerate both the rate of CO2 absorption and the rate of desorption; the activator functions similarly to a catalyst. During the absorption stage, potassium carbonate reacts with CO2 to form potassium bicarbonate ； During the regeneration phase, potassium bicarbonate decomposes upon heating, releasing CO2; the solution is restored and can be reused. Depending on the type of activator, the modified thermal potassium-alkali method is further divided into the following types: I. Benfield method. II. Composite catalysis method III. Steric amine promotion method IV. Aminoacetic acid method (2) Physical absorption method Currently, the main physical absorption methods used domestically and internationally include the cold methanol method, the polyethylene glycol dimethyl ether method, and the propylene carbonate method. (3) Pressure swing adsorption (PSA) method: This method utilizes solid adsorbents to absorb CO2 under pressure, thereby purifying the gas; the adsorbents are then regenerated by reducing the pressure to release the absorbed CO2. It is typically carried out at room temperature, requires low energy consumption, is easy to operate, and causes no environmental pollution. The PSA method can also be used to separate and purify gases such as H2, N2, CH4, CO, and C2H4. China already has domestically produced PSA units, whose scale and technology have reached international advanced levels. (4) Product generation method: In addition to the methods mentioned above, it is also possible to consider combining decarbonization with the reuse of carbon dioxide, that is, the product generation decarbonization method. The main methods that have been widely applied in this regard are the carbon ammonium synthesis method and the soda ash synthesis method. Source: Chemical Process Engineering