Scrubbers and reboilers

The separated gas may contain mist and other liquid droplets. Drops of water and hydrocarbons also form when the gas is cooled in the heat exchanger, and must be removed before it reaches the compressor. If liquid droplets enter the compressor, they will erode the fast rotating blades. A scrubber is designed to remove small fractions of liquid from the gas.

There are various types of gas-drying equipment available, but the most common suction (compressor) scrubber is based on dehydration by absorption in triethylene glycol (TEG). The scrubber consists of many levels of glycol layers.

A large number of gas traps (enlarged detail) force the gas to bubble up through each glycol layer as it flows from the bottom to the top of each section. Processed glycol is pumped in at the top from the holding tank. It flows from level to level against the gas flow as it spills over the edge of each trap.

During this process, it absorbs liquids from the gas and comes out as rich glycol at the bottom. The holding tank also functions as a heat exchanger for liquid, to and from the reboilers. The glycol is recycled by removing the absorbed liquid. This is done in the reboiler, which is filled with rich glycol and heated to boil out the liquids at temperature of about 130-180 °C (260-350 °F) for a number of hours. Usually there is a distillation column on the gas vent to further improve separation of glycol and other hydrocarbons. For higher capacity, there are often two reboilers which alternate between heating rich glycol and draining recycled processed glycol. On a standalone unit, the heat is supplied from a burner that uses the recovered vaporized hydrocarbons. In other designs, heating will be a combination of hot cooling substances from other parts of the process and electric heaters, and recycling the hydrocarbon liquids to the third stage separator.