Gas Processing – Cryogenic Plants

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Figure 9-3 shows a typical cryogenic plant where the gas is cooled to -100°F to ~150°F by expansion through a turbine or Joule-Thompson (J-T) valve. In this example liquids are separated from the inlet gas at 100°F and 1,000 psig. It is then dehydrated to less than 1 ppm water vapor to assure that hydrates will not form at the low temperatures encountered in the plant. Typically, a mole sieve dehydrator is used.

Simplified flow diagram of a cryogenic plant.

The gas is routed through heat exchangers where it is cooled by the residue gas, and condensed liquids are recovered in a cold separator at approximately -90°F. These liquids are injected into the de-methanizer at a level where the temperature is approximately -90°F. The gas is then expanded (its pressure is decreased from inlet pressure to 225 psig) through an expansion valve or a turboexpander. The turboexpander uses the energy removed from the gas due to the pressure drop to drive a compressor, which helps recompress the gas to sales pressure. The cold gas {-150°F) then enters the de-methanizer column at a pressure and temperature condition where most of the ethanes-plus are in the liquid state.

The de-methanizer is analogous to a cold feed condensate stabilizer. As the liquid falls and is heated, the methane is boiled off and the liquid becomes leaner and leaner in methane. Heat is added to the bottom of the tower using the hot discharge residue gas from the compressors to assure that the bottom liquids have an acceptable RVP or methane content.

The gas turbine driven compressor is required since there are energy losses in the system. The energy generated by expanding the gas from 600 psig to 225 psig in the turbo-expander cannot be 100% recovered and used to recompress the residue gas from 225 psig to 600 psig. In this particular plant it is only capable of recompressing the gas to 400 psig, Thus, even if the inlet gas and sales gas were at the same pressure, it would be necessary to provide some energy in the form of a compressor to recompress the gas.

Because of the lower temperatures that are possible, cryogenic plants have the highest liquid recovery levels of the plants discussed. Typical levels are:

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