NGLs and LPGs, made up of C2, C3, C4 and C5+ hydrocarbons, are a valuable fraction of natural gas and associated gas streams and can be used both in Petrochemical Industry (mostly C2 and C3) or as fuel in civil and industrial applications (mostly LPGs).
Recovery of NGL/LPG is normally achieved by means of cryogenic processing, with minimum temperatures that can be as low as -80 °C (NGLs) or -100 °C (high efficiency C2 recovery). Cryogenic processing requires proper gas dehydration, which for lower temperatures is normally achieved by means of mol. sieves.
Several different processes are available, based on alternative cooling methods and with different plant arrangements:
- Joule Thomson (JT) Expansion: gas cooling is achieved by adiabatic gas expansion (in JT Valve) and by means of heat recovery. This method is normally applicable to high pressure gas streams. It allows a lower investment cost but it is characterized by lower recovery and by higher pressure losses.
- Direct Refrigeration by means of Chiller Unit (usually Propane): gas cooling is achieved by use of a propane Chiller Unit and by means of heat recovery. This method is applicable to medium or low pressure streams and it is normally used to treat medium pressure associated gas streams rich in LPGs
- Turbo-Expander: gas cooling is achieved by means of an isentropic gas expansion (inside the Turbo-Expander, which is normally an Expander/Compressor to recover the expansion energy) and by means of heat recovery. This method is applicable to a wider range of pressure and gas compositions. It is more efficient than JT Expansion and requires lower pressure losses but investment costs are higher
- High efficiency NGL recovery: normally based on the use of Turbo-Expander in conjunction with a refluxed De-Methanizer or De-Ethanizer Column to improve the recovery efficiency. The heat recovery arrangement is normally more complex.
Due to pressure losses associated to gas expansion, the treated gas stream must normally be re-compressed to be delivered to transport pipeline.
The recovered raw NGL/LPG fraction is normally fractionated to recover single components or mixtures with given composition (for examples a LPG with a given C3 to C4 ratio). This is normally achieved in a fractionation train composed of different distillation columns such the De-Ethanizer, De-Propanizer, De-Butanizer or the LPG Splitter. Recovered products are normally sent to dedicated storage while the heavy C5+ is normally sent to condensate stabilization.