Ethane/ethylene and heavier hydrocarbons recovery, from natural gas or refinery off gas feed streams, using a cryogenic turboexpander process. High ethane recovery (99%) is achievable with essentially complete C3+ recovery. Lower ethane recoveries are possible with the same process while maintaining C3+ recovery in the 99% range.
Raw feed gas that is to be processed cryogenically is treated to remove impurities such as water that would prevent cryogenic processing. Acid gas removal may be needed, if concentration is large enough to produce freezing in the plant’s cryogenic section.
Clean, dry and treated feed gas (1) is cooled against cold residue gas and against cold demethanizer streams. Feed is then sent to the cold separator (2) for
phase separation. Liquid (3) from the separator is sent to the demethanizer tower (4) as bottom feed. Vapor leaving the separator is split into two. The larger stream is sent to the expander (5) for isentropic expansion and sent as feed to the demethanizer.
The smaller stream (6) from the cold separator is cooled, partially condensed and sent to a reflux separator (7). Liquid (8) from the separator is sent to the demethanizer as feed. Vapor stream (9) leaving the vessel is condensed and sent as second feed to the demethanizer.
The demethanizer produces at the top, a methane and lighter stream, and at the bottom a C2+ stream containing the desired components to be recovered. Cold residue gas (10) is warmed up in exchangers and boosted in pressure by a compressor (11). The warm and intermediate pressure residue gas is boosted to pipeline pressure by residue compressors (12) and sent for further processing. A part of the high-pressure residue gas (13) is cooled, condensed and sent to the demethanizer as top feed. This top feed is very low in C2+ thereby enabling very high C2+ recovery levels.
Operating conditions: Due to the presence of two reflux streams, the scheme is able to process feed gases with a wide range of liquid content in the feed gas. The scheme requires the demethanizer tower to run at a somewhat higher pressure than a single tower system. This higher pressure will make the process more CO2 tolerant, and hence simplify or eliminate inlet gas treating.
Licensor: Randall Gas Technologies, ABB Lummus Global Inc., US Patent Pending.
