Most glycol dehydration processes are continuous. That is, gas and glycol flow continuously through a vessel (the “contactor” or “absorber”) where they come in contact and the glycol absorbs the water. The glycol flows from the contactor to a “reboiler” (sometimes called “reconcentrator” or “regenerator”1) where the water is removed or “stripped” from the glycol and is then pumped back to the contactor to complete the cycle.
Figure 8-4 shows a typical trayed contactor in which the gas and liquid are in counter-current flow. The wet gas enters the bottom of the contactor and contacts the “richest” glycol (glycol containing water in solution) just before the glycol leaves the column. The gas encounters leaner and leaner glycol (that is, giycol containing less and less water in solution), as it rises through the contactor. At each successive tray the leaner glycol is able to absorb additional amounts of water vapor from the gas. The counter-current flow in the contactor makes it possible for the gas to transfer a significant amount of water to the glycol and still approach equilibrium with the leanest glycol concentration.
The contactor works in the same manner as a condensate stabilizer tower. As the glycol falls from tray to tray it becomes richer and richer in water. As the gas rises it becomes leaner and leaner in water vapor. Glycol contactors will typically have between 6 and 12 trays, depending upon the water dew point required. To obtain a 7 Ib/MMscf specification, 6 to 8 trays are common.
As with a condensate stabilizer, glycol contactors may have bubble cap trays as shown in Figure 8-4, or they may have valve trays, perforated trays, regular packing or structured packing. Contactors that are 12% in. and less in diameter usually use regular packing, while larger contactors usually use bubble cap trays to provide adequate contact at gas flow rates much lower than design. Structured packing is becoming more common for very large contactors.
It is possible to inject glycol in a gas line and have it absorb the water vapor in co-current flow. Such a process is not as efficient as countercurrent flow, since the best that can occur is that the gas reaches near equilibrium with the rich glycol as opposed to reaching near equilibrium with the lean glycol as occurs in counter-current flow. Partial co-current flow can be used to reduce the height of the glycol contactor by eliminating the need for some of the bottom trays.
The glycol will absorb heavy hydrocarbon liquids present in the gas stream. Thus, before the gas enters the contactor il should pass through a separate inlet gas scrubber to remove liquid and solid impurities that may carry over from upstream vessels or condense in lines leading from the vessels. The inlet scrubber should be located as close as possible to the contactor.