Gas Dehydration:
Natural gas dehydration is the process of removing water vapor from the gas stream to lower the dew point of that gas. Water is the most common contaminant of hydrocarbons. It is always present in the gas-oil mixtures produced from wells. The dew point is defined as the temperature at which water vapor condenses from the gas stream. The sale contracts of natural gas specify either its dew point or the maximum amount of water vapor present. There are three basic problems for the dehydration of natural gas streams:
1. To prevent hydrate formation. Hydrates are solids formed by the physical combinations of water and other small molecules of hydrocarbon compounds of about 10 % hydrocarbons and 90% water. Hydrates grow up as crystals and can build up in orifice plates, valves and other areas not subjected to full flow. Thus, hydrates can plug lines and retard the flow of gaseous hydrocarbon streams.
2. To avoid corrosion problem
3. Downstream processing requirements
Dehydration methods:
The most common dehydration methods used for natural gas processing are as follows:
1. Absorption, using the liquid desiccants in our case the liquid is the TEG ( triethylene glycol)
2. Adsorption, using solid desiccants (alumina and silica gel)
3. Cooling/ condensation below the dew point , by expansion and refrigeration
Absorption system:
The system is as following, the wet natural gas enters the absorption column (glycol contactor) near its bottom and flows upward through the bottom tray to the top tray and out at the top of the column. The column is 8 trays. Lean glycol is fed at the top of the column and it flow down from tray to tray absorbing water vapor from natural gas. The rich glycol leaves from the bottom of the column to the glycol regeneration unit. The dry natural gas passes through mist mesh to sales line.
The glycol regeneration unit id composed of a reboliler where steam is generated from the water in the glycol. The steam is circulated through the packed sections to strip the water from glycol. Stripped water and any lost hydrocarbons are vented at the top of the stripping column. Rich glycol is preheated in heat exchangers using the hot lean glycol, before it enters the still column of glycol rebolier. This cools down the lean glycol to the desired temperature and saves the energy required for heating the rich glycol in rebolier.