LNG Book

NGL recovery and Condensate processing

Dew point control

  • A major reason for dew point control is the fact that rich natural gas mixtures that contain heavier hydrocarbons exhibit retrograde condensation.
  • Dew point control (or “dew pointing”) is necessary when raw gas lines are constrained in liquid content as the liquid reduces gas throughput, causes slugging, and interferes with gas metering.
  • This situation can be a problem for offshore production, where several operators share a common gas line to shore. These lines may operate at 1,800 psig (125 barg) or higher and be at seafloor temperatures of 36 to 40 °F (2 to 4 °C).
  • Dew point control is also necessary if potential condensation is present in a process because of temperature or pressure drops.
  • The latter happens when the gas is in the retrograde condensation region.

Process components

  • External refrigeration

: used to cool the gas stream to recover a significant amount of C3+ and to lower gas temperatures as the gas goes into other stages of hydrocarbon recovery.

: It maybe the only source of refrigeration when inlet pressure is low.

  • Basic propane refrigeration process

1.Compression of saturated refrigerant vapor at point A to a pressure well above its vapor pressure at ambient temperature at point B

2.Condensation to point C by heat exchange with a cooling fluid, usually air

3.Expansion through a valve (JT expansion) to cool and condense the refrigerant to point D

4.Heat exchange with the fluid to be cooled by evaporation of the refrigerant back to point A

  • Compression – Condensation – Expansion – Refrigeration
  • Compression step

: Cycle analysis begins with propane vapor enteringthe compressor as a vapor at 14.5 psia (1 bar) and approximately −40 °F (−40°C), where it is compressed to 250 psia (17 bar) (point A to point B in Figure 7.2)

: The work of compression is simply

: Taking into account compressor nonideality, the actual enthalpy at the end of the expansion is

: Compressor power to the refrigeration system is the product of the mass flow rate and shaft work

  • Condensation step

: The warm gas goes to an air- or water-cooled con-denser, where the propane cools to 100 to 120 °F (38 to 50 °C), totally condenses, and collects in a receiver (point B′ to point C in Figure 7.2).

: This step is simply

  • Expansion step

: Propane liquid leaves the receiver and flashes through a JT valve, where the temperature and pressure drop to −40 °F (−40 °C) and 16 psia(1 bar) (point C to point D).

: No change occurs in the enthalpy, but the temperature drops to the saturation temperature of the liquid at the expansion-discharge pressure, and hC = hD if there are no heat leaks.

: The fraction, f, of propane condensed is

  • Refrigeration step

: The cold propane then goes to a heat exchanger, the chiller, where it cools the process stream by evaporation (point D to point A in Figure 7.2).

: Because the propane in the chiller is evaporating, and a minimal heat exchange occurs between cold propane vapor and the inlet gas, the inlet and outlet propane temperature remains constant. The propane returns to the compressor suction slightly above −40ºF (−40°C). The heat absorbed by the propane is simply hA − hD.

: The chiller typically has two zones of heat transfer. The first is exchange of boiling propane with gas above its dew point and will involve only sensible heat. The second zone has condensing vapors from the process stream and boiling propane.

: To complete the cycle, the propane vapors leave the chiller and go to the suction drum before being compressed again.

Effect of operating variables on refrigeration performance

  • Refrigeration-cycle performance is commonly stated in terms of coefficient of performance (COP), which is the ratio of the refrigeration obtained divided by the work required.
  • Condenser outlet temperature
  • Refrigerant purity
  • The contaminated propane with ethane can be purged into the plant-gas inlet stream for recovery and the system recharged

Turboexpansion

  • Until the 1960s, Joule-Thomson expansion was the only way used to cool gas-plant streams by pressure drop. Herrin describes the first turboexpander plant in 1966
  • Turboexpansion provides the maximum amount of heat removal from a system for a given pressure drop while generating useful work.
  • The major breakthrough for turboexpanders came when the design and materials made it possible for condensation to occur inside the expander.
  • The fraction condensed can be up to 50% by weight. However, the droplets must generally be 20 microns in diameter, or less, as larger droplets cause rapid erosion of internal components.
  • The expander is the unit on the right, and the compressor is the unit on the left.
  • Gas enters the expander through the pipe at the top right, and is guided onto the wheel by the aerodynamically shaped adjustable guide vanes, which completely surround the expander wheel. The swirling high-velocity inlet gas turns the wheel and transfers part of its kinetic energy to the wheel and shaft, and exits to the right through the tapered nozzle.
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