Water vapor is the most common undesirable impurity in gas streams. Usually, water vapor and hydrate formation, i.e. solid phase that may precipitate from the gas when it is compressed or cooled. Liquid water accelerates corrosion and ice (or solid hydrates) can plug valves, fittings, and even gas lines. To prevent such difficulties, essentially gas stream, which is to be transported in transmission lines, must be dehydrated as per pipeline specifications. The processing of natural gas to the pipeline specifications usually involves four main processes;
Most of the liquid free water associated with extracted natural gas is removed by simple separation methods at or near the wellhead. However, the removal of the water vapor requires more complex treatment, which usually involves one of the two process, either absorption or adsorption. In absorption, dehydrating agent (e.g. glycols) is employed to remove water vapors and in adsorption, solid desiccants like alumina, silica gel, and molecular sieves can be used. The absorption process has gain wide acceptance because of proven technology and simplicity in design and operation.
Glycols are extremely stable to thermal and chemical decomposition, readily available at moderate cost, useful for continuous operation and are easy to regenerate. These properties make glycols as obvious choice as dehydrating agents. In the liquid state, water molecules are highly associated because of hydrogen bonding. The hydroxyl and ether groups in glycols form similar associations with water molecules. This liquid –phase hydrogen bonding with glycols provides higher affinity for absorption of water in glycol. Four glycols have been successfully used to dry natural gas: ethylene glycol (EG), Diethylene glycol (DEG), Triethylene glycol (TEG) and Tetraethylene glycol (TREG). TEG has gained universal acceptance as the most cost effective choice because:
Diethylene glycol is preferred for applications below about 10oC because of the high viscosity of TEG in this temperature range.
As shown in Figure 1, wet natural gas is first flashed in an inlet separator to remove liquid and solid content. The gas stream from separator is dried in contactor using counter current glycol stream with temperature difference of around 5 oC with that of gas stream. The dry gas passes through a gas/ glycol heat exchangers to cool lean glycol. The rich glycol, from the bottom of the contactor, is flashed at reduced pressure in flash tank where dissolved hydrocarbon gases are recovered and further it is preheated with lean glycol in a glycol/ glycol heat exchanger. After exchanging heat, the rich glycol is further fed to the stripping column where TEG gets concentrated only upto 98.5%. Further purity in stripper is not recommended due to limitation of maximum reboiler temperature of about 204oC. Higher temperature in reboiler can lead to undesirable process of decomposition of glycol. The purity of 99.9% (wt) is achieved in a regeneration column where TEG is stripped by using some part of dried gas, processed in contactor. The regenerated TEG is recycled back to the contactor. Exceptional efficiency at low liquid rates improves separation performance and allows reduction in column height in new installations. High capacity for high liquid rates allows reduction in column diameter in new installations.
Contactor is most important mass transfer equipment in TEG dehydration since its performance has crucial impact on downstream processes. The typical contactor is provided with internals like Vane Inlet Device (VID) for gas distribution, Chimney tray for collection of liquid, trays/packings for mass transfer and demister to minimize TEG losses. Mass transfer in contactor can be achieved by using Bubble cap, Valve trays, Sieve trays or Structured packings. In earlier days, bubble cap trays were commonly used in contactor. But in recent decade, due to proven performance of the structured packing, TEG contactors are now designed or revamped with high capacity structured packing. Typical contactor for structured packing with all required internals is shown in Figure 2.