Ethane, propane and heavier hydrocarbons recovery from natural gas feed streams using a cryogenic turboexpander process. High propane recovery (>99%) is achievable. Residue compression is minimized by using a high-pressure absorber (HPA).
Raw feed gas is treated to remove impurities such as water that would prevent cryogenic processing. Clean, dry and treated feed gas (1) is cooled against cold process streams and sent to the warm separator (2) for phase separation. Liquid from the separator is preheated against warmer streams and sent to the deethanizer (3) as bottom feed. Vapor leaving the warm separator is sent to a turboexpander (4) for isentropic expansion. The two-phase stream exiting the expander is sent as bottom feed to a high-pressure absorber (5). Liquid exiting the absorber bottom is preheated against warm process streams and sent to the deethanizer as top feed. The deethanizer produces C3+ liquid at the bottom and a C2 top stream. The tower is generally provided with an external heat source for bottom reboiling.
Vapor from the tower top is partially condensed and sent to a reflux accumulator (6). Liquid is pumped as reflux, while vapor leaving the accumulator is compressed (7), cooled, partially condensed and sent to the absorber as top feed. The HPA process can also be used to recover ethane and heavier components. Ninety percent ethane recovery is achievable.
Operating conditions: This is very efficient when feed is available at high pressure. The absorber column is run at a high-pressure to minimize residue compression, while the deethanizer is run at a lower pressure to keep the tower from critical conditions. Due to this decoupling, the scheme is easily able to process a range of feed gas pressures while minimizing residuecompression and maintaining stable deethanizer operation. Tower pressure decoupling in the ethane recovery mode, and running the absorber at a higher pressure than the demethanizer, has similar savings in
residue compression as with propane recovery.
Licensor: Randall Gas Technologies, ABB Lummus Global Inc.,