In a refrigeration plant the inlet gas is cooled to a low enough temperature to condense the desired fraction of LPG and NGL. Either freon or propane is used as the refrigerant. Figure 9-2 shows a typical refrigeration plant.
The free water must be separated and the dew point of the gas lowered before cooling the feed to keep hydrates from forming. It is possible to dehydrate the gas with TEG or mole sieves to the required dew point. It is more common to lower the hydrate temperature by injecting glycol in the gas after separation of free water. The glycol and water separate in the cold separator where they are routed to a regenerator, the water is boiled off and the glycol is circulated back to be injected into the inlet stream. Some glycol will be lost with time and will have to be made up, The most common glycol used for this service is ethylene glycol because of its low cost and the fact that at the low temperatures it is not lost to the gas phase.
The chiller is usually a kettle type exchanger. Freon (which is cooled in a refrigeration cycle to -20°F) is able to cool the gas to approximately -15°F. Propane, which can be cooled to ~40°F, is sometimes used if lower gas temperatures and greater recovery efficiences are desired.
The gas and liquid are separated in the cold separator, which is a three phase separator. Water and glycol come off the bottom, hydrocarbon liquids are routed to the distillation tower and gas flows out the top. If it is desirable to recover ethane, this still is called a de-methanizer. If only propane and heavier components are to be recovered it is called a de-ethanizer. The gas is called “plant residue” and is the outlet gas from the plant.
The tower operates in the same manner as a condensate stabilizer with reflux. The inlet liquid stream is heated by exchange with the gas to approximately 30°F and is injected in the tower at about the point in the tower where the temperature is 30°F. By adjusting the pressure, number of trays, and the amount of reboiler duty, the composition of the bottoms liquid can be determined.
By decreasing the pressure and increasing the bottoms temperature more methane and ethane can be boiled off the bottoms liquid and the RVP of the liquid stream decreased to meet requirements for sales or further processing. Typical liquid recovery levels are:
These are higher than for a lean oil plant. It is possible to recover a small percentage of ethane in a refrigeration plant. This is limited by the ability to cool the inlet stream to no lower than -40°F with normal refrigerants.
Most refrigeration plants use freon as the refrigerant and limit the lowest temperature to ~20°F. This is because the ANSI piping codes require special metallurgy considerations below -20°F to assure ductility.