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Liquefaction in a LNG Facility operates at very cold temperaturs and requires extremely dry gas to avoid hydrate formation. Dehydration to a -260°F dew point is achieveable only with molecular sieve types desiccant. The operating cost of mole sieve dehydration gets expensive in large scale LNG projects due to the large ammount of heat required to produce regeneration gas at 550°F. This article discusses techniques to reduce both operating and capital cost of dehydration in large scale LNG facilities.
What is specific in sour gas dehydration ?
- Water saturation is increased by the presence of acid gases
- Extended use of stainless steel is required (extent depending on level of acid gas content)
- NACE application in presence of H2S
- HSE (presence of H2S) – isolation philosophy, waste & effluents management,…
- TEG off gases compresion and recycle may need to be considered in cases of significant presense of H2S
Dehydration of sour gases is not an issue by itself, but design and operation must be adapted.
Traditional design considerations to save energy in large mole dehydration :
- Waste heat recovery such as hot exhaust from gas turbines driving the boost compressors. Heat recovered by this design saves a considerables ammount of heat, but also involves safety and operationg problems.
- Longer adsorbtion and regeneration cycles to increase life of mole sieve media. Mole sieve are vulnerable to liquid water, corrosion inhibitors, and condensable heavy hydrocarbons. Special design and strict operating parameters can overcome these challangesto extend the life up to 4 years.
- High pressure regeneration (usually 50 psi less than adsorption pressure) in-lieu of low pressure regeneration. This method requires an additional compressor to recycle the regeneration gas.
- Twin bed designs replaced with four bed designs for smaller and shorter mole sieve beds along with lower regen gas flow rates. Lower regen gas flow reduces regen gas heater size, duty, size of switching valves, and size of associated piping. It is also easy to minimise pressure drop(lessthan 5 psi) in the adsorption cycle and maximise regeneration gas velocity (greater than 10ft/min).
- Feed gas chilling. Dehydration systems are usually preceded by an amine sweetening system for acid gas removal. Sweetening with amine saturates the sweet gas and increases the gas temperature by 5-15°F. Saturated water content in lb//million ft³ increases by around 33% for every 10 degrees rise in gas temperature above 100°F.
In addition to these traditional design improvements, there is one peculiar design aspect that is critical in evaluating large scale LNG dehydration system; ie. partial dehydration with Tri Ethylene Glycol (TEG). Partial dehydration with TEG upstream of mole sieve beds reduces total cost of dehydration by decreasing the capital and operating cost of mole sieve systems.
TEG vs Mol Sieves
- Mol sieves units are generally more efficient in terms of treated gas water dew point, but also much more expensive than TEG units
- Sour gas dehydration with mol sieves is not an issue in itself
- Care must be taken in Mol sieve selection to avoid COS formation from reactions between H2S and CO2
- The volume of regeneration gas to be compressed for recycling in MS, is far above the volume of TEG overhead gas, even considering impact of P on actual flowrates.
Mol Sieves should be preffered mainly when required water dew point cannot be achieved by a TEG procces.
Examples of Experience
Sour Gas Dehydration with TEG
Dehydration with TEG alone cannot achieve the required water dew point of -260°F. Any ammount of water removed with TEG upstream of mole sieve beds is called partial dehydration as TEG is not achieving the final dew point. Water dew points aroung 20°F are easily accomplised by TEG dehydration at moderate operating pressures of greater than 700 psig TEG units require less regeneration heat per pound of water removed, compared to mole sieves therefore lowering the operating cost.
High regen gas flow rates exponentially increase the operating cost of large mole sieve dehydration systems designed for water saturated feed. TEG can remove a significant amount of water with less heat requirement, reducing the overall utility costs. Combining dehydration designs with TEG and mole sieve will result in lower overall cost of dehydration.