LNG Book

Recovery process

  • Low temperature separator (LTS)

: LTS are used both onshore and offshore, and its process consists of cooling and partial condensation of the gas stream, followed by a low temperature separator.

: When inlet pressures are high enough to meet discharge-pressure requirements to make pressure drop acceptable, cooling is obtained by expansion through a J-T valve or turboexpander.

: Water usually is present, and to prevent hydrate formation the separator downstream of the expander is warmed above the hydrate-formation temperature to prevent plugging. An alternative to heating is injection of either ethylene glycol or methanol, which is then recovered and dried for reuse.

  • A LTS that uses ethylene glycol injection for hydrate prevention and uses J-T expansion for cooling.
  • The low temperature separator is set to maintain the proper dew point of the blended outlet gas.
  • Twister

: Simplicity – No moving parts and no utilities required, and small size and low weight.

: Limited turndown capacity. Flow variability is limited to ± 10% of designed flow.

Ethane recovery

  • the objective is to produce a lean gas and recover up to approximately 60% of the ethane in the feed gas.
  • Two process schemes are used to obtain this level of ethane recovery:

: Cooling by expansion or external refrigeration

: Lean-oil absorption

  • Direct-refrigeration process

: Inlet gas is initially cooled with cold residue gas and cold liquid from the cold separator before going to the propane chiller and to the cold separator.

Vapor from the separator is the sales gas, and the liquid goes to a fractionator to strip out light ends and recover liquid product.

  • A simplified conventional expander plant
  • It consists of a gas-gas heat exchanger with five gas streams that enter at different temperatures, cold separator, turboexpander, and demethanizer.
  • The inlet gas stream makes several passes through the gas-gas exchanger before going to the cold separator, where the vapor expands through a turboexpander.
  • Liquid from the cold separator is flashed through a J-T valve and fed to the middle of the demethanizer.
  • The incoming gas provides reboiler heat at the bottom, and then is cooled further in a second reboiler midway up the column.
  • The maximum ethane recovery with the conventional turboexpander configuration is about 80%.

Condensate processing

  • Figure 10.1 shows a schematic of one configuration for the condensate that leaves the inlet receivers of a plant.
  • Processing the condensate involves two steps: water washing and condensate stabilization.
  • After removal of free water, the condensate goes to the stabilizer, where remaining lighter hydrocarbons are stripped and recombined with the gas that leaves the inlet receiver.
  • The primary purpose of the stabilizer is to produce bottom product that has specifications to be sold as “natural gasoline”. Specifications on natural gasoline include:

: Volatility as measured by ASTM D323 (Standard Test Method for Vapor Pressure of Petroleum Products (Reid Method)) test. The values range from 10 to 34 psi (0.7 to 2.3 bar), but the common range is 9 to 12 psi(0.6 to 0.8 bar)

: Density set in the range of 65 to 85 °API (650 to 720 kg/m3), with a price penalty for higher gravities (lower densities).

  • The stripping column must operate at a temperature and pressure to ensure that the bottoms product meets the volatility and density specification. It usually operates in the 75 to 400 psig (5 to 27 barg) pressure range.
  • Sweetening

: Most of the H2S and CO2 will be removed along with mercaptans. To enhance acid gas removal, sweet natural gas can be used as a stripping gas. This process can reduce H2S levels to the 10 ppmv range

  • Dehydration

: Dehydration of natural gasoline usually is unnecessary. When in storage, free water drops out and the remaining low water content should meet customer requirements.

: However emulsions of water and natural gasoline occur, which require emulsion breakers to obtain a clean water-hydrocarbon separation.

NGL processing

  • Extensive fractionation of NGL
  • The fractionation train depends upon the current economics and customer requirements.
  • Frequently the first column takes an ethane-propane mixture (EP mix) overhead instead of pure ethane.
  • Regardless of whether it is ethane or EP mix, this stream is pipelined to customers. All of the other products are shipped by tank car, truck, or pipeline
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