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Among our experience there are units in sour service (more than 20 ppmv of H2S), the largest silica gel gas dehydration plants in the world and units located offshore.
Gas dehydration is a fundamental step in gas treatment and is included in nearly all gas processing units in order to prevent the formation of hydrates in high pressure natural gases during gas transmission or during cryogenic gas processing (such as LPG / NGL recovery or in LNGs). Dehydration is also applied to prevent corrosion from condensed water in sour gas streams.
Based on most common applications, the typical water dew points are the following:
For natural gas transportation, depending on geographic area, in the range 0 °C to -20 °C, but can be lower for long subsea pipelines
For Condensate / LPG recovery in the range -20 °C to -50 °C
For NGL recovery and for LNGs, a residual water content lower than 0.1 ppmv, corresponding to a water dew point lower than -80 °C, is required.
Different dehydration technologies can be utilized. When the dew point values to be achieved are not very stringent methanol injection and silica gel dehydration can be used, in particular, for gas transportation. If more stringent dew point values are required, which is the most common case, dehydration with glycols (MEG, DEG and mostly TEG) in the range -10 °C to -40 °C is applied. When a water dew point value lower than -80 °C is to be achieved for NGL or LNG, molecular sieves will be used.
The following is a list of available Gas Dehydration processes:
Glycol Dehydration & Regeneration: glycol dehydration is one of the most widespread gas processing technologies.
TriEthyleneGlycol (TEG) is the most used glycol due to the higher regeneration temperature resulting in lower residual water content in regenerated glycol and allowing lower treated gas dew points. DiEthyleneGlycol (DEG) and MonoEthyleneGlycol (MEG) are used as well but their typical application is for hydrate inhibition service.
Several different designs are available for glycol regeneration. By increasing the lean glycol purity both the treated gas dew point and the glycol circulation decrease but the complexity of regeneration increases:
Simple evaporation: only TEG reboiler is used for regeneration. The typical lean TEG conc. is 98.5% wt. allowing a water dew points depression in the range of 40°C
Conventional Stripping: conventional stripping column is added. Typical lean TEG conc. is 99.9% wt. allowing a water dew points depression in the range of about 70 – 80 °C
Improved Stripping: by increasing stripping gas flow and the complexity of the stripping column, a maximum typical lean TEG conc. of 99.95% wt. can be achieved with a dew point depression of about 80 - 90 °C
Siirtec Nigi patented glycol regeneration (DRIGAS™ - ECOTEG™): higher purities and lower dew points are available by means of Siirtec Nigi patented processes. DRIGAS™ is based on regeneration gas recycling allowing very high stripping rates with a lean TEG purity of 99.98% and a treated gas dew point depression in the range of 100 °C. The DRIGAS™ process does not require any additional solvent. Stripping gas is recycled and therefore consumption is very low. Operating costs are lower with lower pollution as well.
ECOTEG™ is based on a similar arrangement to DRIGAS™ and it is applied to dehydrate gases rich in aromatic compounds (BTEX) where the effluent control is critical and its emission of aromatics into the environment is nil. The main advantages of the ECOTEG™ process are the capability of meeting more stringent regulations for disposal without additional facilities, low operating costs and low gas dew point (about the same as DRIGAS™).
Molecular Sieves Dehydration: mol. sieves are crystalline, highly porous materials made by alumino-silicates and are characterized by a very high internal surface with high adsorption properties allowing very low residual water content in treated gas to be achieved. This is typically in the range of 0.1 ppmv to 1 ppmv. Mol. sieves can be also used for the dehydration of NGL or light condensate at ppm level.
Typical mol. sieve types used for gas dehydration are the 3A and the 4A depending on feedstock composition. More specifically, the 3A is suggested to minimize COS formation when the feedstock contains CO2 and H2S, while for NGL dehydration the 5A is normally used.
Operation is cyclical and after a mol. sieve bed is saturated with water it must be regenerated by heating and cooling to restore adsorption capacity. Regeneration is normally closed loop type and the regeneration gas is a slip stream of dry treated gas driven by means of regeneration compressor.
Silica Gel Dehydration: in principle similar to mol. sieve with a similar plant arrangement however silica gel only achieves a residual water content in treated gas in the range of 5 to 10 ppm allowing a dew point level of -40 °C to -50 °C which is intermediate between glycols and mol. sieves.
Methanol Injection: methanol injection is normally used for hydrate inhibition and dehydration for wellheads and multiphase transfer lines, for gas transportation in pipeline or in LPG or LNG recovery units when moderate dew points are required. Typical achievable water dew points are in the range of 0 °C to -40 °C depending on operating conditions, methanol purity and injection flowrate. Methanol is characterized by high evaporation losses. However sometimes the residual methanol liquid phase can be recovered by means of distillation.