LNG Book

Process flow diagram

  • The flow configuration shows the first bed in the adsorption cycle, the second bed cooling down after regeneration, and the third bed undergoing regeneration with hot gas.

Membranes

  • Membranes are used in natural gas processing for dehydration, fuel-gas conditioning, and bulk CO2 removal, but presently CO2 removal is by far the most important application.
  • Commercial membrane configurations are either hollow fiber elements or flat sheets wrapped into spirally wound elements. Presently, about 80% of gas-separation membranes are formed into hollow fiber modules (Baker, 2002)
  • The spiral configuration is inherently more resistant than the hollow fiber membranes to trace components that would alter the polymer permeability.
  • It alsoallows a wider range of membrane materials to be used. However, the hollow fiber membranes are cheaper to fabricate, and thus dominate the field.

Operating considerations

  • Flow pattern
  • To increase methanerecovery to 98.7%, a two-stage unit requires recompression of the first-stage permeate.
  • Greater levels of methane recovery are obviously possible by application of three or more stages.

Advantages and Disadvantages

  • Advantages
  • Low capital investment when compared with solvent systems
  • Ease of operation: process can run unattended
  • Ease of installation: Units are normally skid mounted
  • Simplicity: No moving parts for single-stage units
  • High turndown: The modular nature of the system means very high turndown ratios can be achieved
  • High reliability and on-stream time
  • No chemicals needed
  • Good weight and space efficiency
  • Disadvantages
  • Economy of scale: Because of their modular nature, they offer little economy of scale
  • Clean feed: Pretreatment of the feed to the membrane to remove particulates and liquids is generally required
  • Gas compression: Because pressure difference is the driving force for membrane separation, considerable recompression may be required for either or both the residue and permeate streams
  • Generally higher hydrocarbon losses than solvent systems
  • H2S removal: H2S and CO2 permeation rates are roughly the same, so H2S specifications may be difficult to meet
  • Bulk removal: Best for bulk removal of acid gases; membranes alone cannot be used to meet ppmv specifications
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