These processes are based on the solubility of the H2S and/or CO2 within the solvent, instead of on chemical reactions between the acid gas and the solvent. Solubility depends first and foremost on partial pressure and secondarily on temperature. Higher acid-gas partial pressures and lower temperatures increase the solubility of H2S and CO2 in the solvent and thus decrease the acid-gas components.
Various organic solvents are used to absorb the acid gases. Regeneration of the solvent is accomplished by flashing to lower pressures and/or stripping with solvent vapor or inert gas. Some solvents can be regenerated by flashing only and require no heat. Other solvents require stripping and some heat, but typically the heat requirements are small compared to chemical solvents.
Physical solvent processes have a high affinity for heavy hydrocarbons, if the natural gas stream is rich in C3+ hydrocarbons, then the use of a physical solvent process may result in a significant loss of the heavier molecular weight hydrocarbons. These hydrocarbons are lost because they are released from the solvent with the acid gases and cannot be economically recovered.
Under the following circumstances physical solvent processes should be considered for gas sweetening:
1. The partial pressure of the acid gases in the feed is 50 psi or higher.
2. The concentration of heavy hydrocarbons in the feed is low. That is,
the gas stream is lean in propane-plus.
3. Only bulk removal of acid gases is required.
4. Selective H2S removal is required.
A physical solvent process is shown in Figure 7-6. The sour gas contacts the solvent using counter-current flow in the absorber. Rich solvent from the absorber bottom is flashed in stages to a pressure near atmospherie. This causes the acid-gas partial pressures to decrease; the acid gases evolve to the vapor phase and are removed. The regenerated solvent is then pumped back to the absorber.
The example in Figure 7-6 is a simple one in that flashing is sufficient to regenerate the solvent. Some solvents require a stripping column just prior to the circulation pump.
Most physical solvent processes are proprietary and are licensed by the company that developed the process.