Contactor column usually contain either tray random packing or structure packing. For large volume of gas, the contactor is usually a tray column consisting of 4 to 12 trays. The greater number of trays, more moisture can be removed.
Bubble cap column
Each tray has opening with bubble cap bolted over them. The up flowing gas is forced through these caps and bubbles evenly through the down flowing glycol. The gas gives up water and become dryer as passes upward through each subsiding tray. The glycol becomes more saturated with water as it flows downward over each tray. Weirs which dam like devices, maintain the level of glycol above the slot in the bubble cap. The downcomer carry the glycol to the tray below.
In smaller capacity unit gas contactor is having diameter 18 inches or less and random packing may be used instead of trays. The packing can be metal, plastic or ceramic. This structure provides large surface area for glycol solution to spread out and make better contact with the gas. Random packing is poured to the contactor on to a support grid. 4 feed of packing is usually standard and sufficient to achieve the dew point depression which is up to 45 – 65 F. If higher dew point depressions are needed, additional packing may be required.
Pack column utilize the same process as tray column where liquid glycol flows down over the packing and the gas flow up through the packing. Pack columns are less expensive however they tend to channel and have poor flow distribution. Channel is when liquid glycol flow in streams throughout the random packing. Channeling limits the surface area where the glycol and gas commit contact. To ensure that gas and glycol mixing will be continues throughout the packing and the glycol would not channel, a well designed glycol distribution header is installed above the packing.
Another gas dehydration method uses solid bed desiccant instead of liquid desiccant. The common example of this is the molecular sieves which are made of pellets that are electronically poured to water. When placed in line with the gas stream the polarity of the pellets attract the water out of the gas into molecule size pour on the surface of the pellets. The water is held there until the pellets are saturated. The pellets them self are then dehydrated by a small volume of heated gas so they can be used again.
Methanol injection gas dehydration method involves the injection of methanol in the gas stream to absorb water. The methanol and water mixture and then disposed off in the environmentally safe manner. Methanol injection is rarely used on dehydration because it is toxic, expensive and the disposal can be complicated.