Acid Gas Removal
Sulfur contaminant and CO2 removal typically occurs in the process referred to as the Acid Gas Removal (AGR) unit. In most syngas production systems, sulfur contaminants are in the form of Hydrogen Sulfide (H2S) and/or Carbonyl Sulfide (COS). The purpose of the AGR is to 1) remove the sulfur contaminants from the syngas to a level compatible with the requirements of downstream catalyst systems, to comply with plant environmental permits, and 2) selectively remove CO2 in order to provide a pure stream of another syngas constituent (such as H2) or when the CO2 is the desired product stream for use in other processes such as enhanced oil recovery, chemical production, and refrigeration applications.
The most common technologies for acid gas removal are cyclic, regenerable, solvent absorption processes. In these processes, a liquid solvent is counter-currently contacted with the syngas in an absorption tower to selectively remove the H2S and/or the CO2. It is then thermally regenerated in a stripping tower to liberate the acid gases, while rejuvenating the solvent to begin the absorption cycle again. There are four generic types of acid gas removal solvents:
- Chemical solvents are aqueous bases that undergo reversible acid-base reactions with the H2S and CO2 after they hydrolyze in water to weak acids. The absorption processes typically operate at room temperature.
- Examples: methanolamine (MEA), methyl-diethanolamine (MDEA)
- Physical solvents are polar molecules that have positive and negatively charged portions that “attract” the polar H2S and CO2 molecules (no chemical reaction; polar bonding). The absorption processes typically operate at low temperatures and require cooling or refrigeration.
- Example processes: Selexol™, Rectisol®, Purisol®, Morphysorb®
- Physical-chemical or mixed solvents are mixtures of chemical and physical solvents, which combine the features of the first two types. Cooling or refrigeration may be required for the absorption processes.
- Example processes: Sulfinol®, Amisol™
- Oxidative solvents react with the H2S and oxidize it to elemental sulfur in the solution. The sulfur is recovered as a solid.
- Example processes: Sulferox™, Lo-Cat®