Raw natural gas must be dehydrated to remove water vapor. Water vapor causes the formation of hydrates, over-saturation of natural gas, and corrosion of equipment. Hydrates are solid, ice-like crystallized compounds formed of hydrocarbons and water. Hydrate formation occurs in high-pressure well streams with a low temperature. Hydrates can form, however, at temperatures above freezing point.
The high pressure increases saturation and creates more water vapor. Hydrates cause freezing and blocking of pipelines, valves and other equipment, bringing production to a halt. Over-saturated gas does not meet pipeline specifications of 7 pounds per mmcfd, and must be removed in order to sell.
The temperature and pressure determine whether water is in a gaseous state or has condensed into liquid. When in liquid form, water also causes corrosion to equipment. In order to prevent these harmful effects, gas is dehydrated by being condensed from vapor into liquid form, and then removed by either adsorption or absorption.
Dehydrating, the removal of water vapor from natural gas, can be done by either adsorption or absorption. Croft’s PDS uses adsorption, where water vapor is collected and condensed on the surface with the use of a solid desiccant. Solid desiccants have a high adsorptive capacity, a low resistance to gas flow to minimize pressure drop, is both non-flammable and non-corrosive, and is inexpensive. TEG units dehydrate by absorption, which uses liquid desiccants such as glycol or methanol, to remove the water vapor.