PROCESS DESCRIPTION OF GAS DEHYDRATION
The principle of glycol dehydration is contacting a natural gas stream with a hygroscopic liquid which has a greater affinity for the water vapor than does the gas. Contactor pressure is subject to economic evaluation usually influenced by water removal duty, required water dewpoint, vessel diameter and wall thickness. After contacting the gas, the water-rich glycol is regenerated by heating at approximately atmospheric pressure to a temperature high enough to drive off virtually all the absorbed water. The regenerated glycol is then cooled and recirculated back to the contactor.
Triethylene glycol (TEG) is the most commonly used dehydration liquid and is the assumed glycol type in this process description. Diethylene glycol (DEG) is sometimes used for uniformity when hydrate inhibition is required upstream of dehydration or due to the greater solubility of salt in DEG. Tetraethylene glycol (TREG) is more viscous and more expensive than the other glycols. The only real advantage is its lower vapour pressure which reduces absorber vapor loss. It should only be considered for rare cases where glycol dehydration will be employed on a gas whose temperature exceeds about 50 °C, such as when extreme ambient conditions prevent cooling to a lower temperature.
TEG has been applied downstream of production facilities that use MEG or DEG as a hydrate inhibitor without apparently leading to contamination problems. Methanol used as a hydrate inhibitor in the feed gas to a glycol dehydration unit will be absorbed by the glycol, and according to the GPSA Engineering Data Book it can pose the following problems:
– methanol will add additional reboiler heat duty and still vapor load and therefore increase glycol losses;
– aqueous methanol causes corrosion of carbon steel. Corrosion can thus occur in the still and reboiler vapor space;
– high methanol injection rates and consequent slug carry-over can cause flooding.
Where there is upstream hydrate inhibition, credit should be taken for any favorable reduction in the water content of the vapor phase. This effect is less significant at lower
feed temperatures, i.e. equivalent to about 2 °C reduction in water dewpoint at 10 °C feed temperature at 9 MPa pressure and 60 percent by weight MEG in the aqueous phase.
Adherence to the recommendations in this DEP can minimize but not eliminate entrainment and vapor losses of glycol. Glycol entrainment may lead to the following downstream problems:
– coalescing and partial condensation in pipelines resulting in localised corrosion;
– in cryogenic plants, particularly at temperatures below -25 °C, freezing of TEG and plugging of equipment;
– reduced performance of downstream adsorption plant, e.g. molecular sieves or silica gel.
Any entrained glycol should be removed upstream of cryogenic plant in high efficiency gas/liquid separators to prevent possible plugging. A range of lean TEG concentrations can be achieved with the basic regeneration flow.
1. Gas Dehydration Field Manual, Maurice Stewart & Ken Arnold
2. Gas Dehydration by TEG and Hydrate Inhibition Systems, Arthur William
3. Fundamentals of Natural Gas, Arthur J. Kidnay & William R. Parrish