Gas treatment units include following process packages:
- Gas dehydration with TEG
- Silica Gel HC/water dew point control Dryers
TEG Dehydration Unit
Water vapor is probably the most common undesirable impurity in gas streams which is required to be removed from them for a variety of reasons:
- To prevent hydrate formation in downstream process equipment and/or transmission pipelines
- To prevent corrosion in downstream process equipment and/or pipelines in acid gas streams
- To meet sales gas water dew point specifications
The factors that have led to the widespread use of glycols for gas dehydration are:
- Their unusual hygroscopic property
- Their excellent stability with regard to thermal and chemical decomposition
- Their low vapor pressures
Their ready availability at moderate cost
After flowing through a separator and through a knockout section to remove entrained liquid, the feed gas then flows up absorption section of the contactor. The concentrated glycol, normally containing 0.5 to 2% water, is fed to the top of the contactor and absorbs water from the gas while flowing downward through the column. The dried gas leaves the top of the contactor and is used to cool the glycol feed.
Rich glycol flowing out of the bottom of the contactor must be reconcentrated before it can be reused for water absorption. The rich glycol is often used to provide cooling and condense water vapor at the top of the reconcentrator. This raises the temperature of the rich glycol, which then may be further heated by heat exchange with hot lean glycol. It then enters a reduced pressure flash tank where dissolved hydrocarbon gases are released. The released gases are recovered and used for fuel or other purposes.
After flashing, the rich glycol passes through a filtration system and a second glycol/glycol heat exchanger where it is further heated, and finally enters the reconcentrator column above a short packed or tray section.
Water reflux must be provided at the top of the column to effect rectification of the vapors and minimize glycol losses in the overhead vapor stream. This is normally provided by condensing a portion of the overhead vapor.
Heat for the distillation is provided by a direct fired reboiler. Hot lean glycol leaves the reboiler and flows to a surge tank (which often contains cooling coils) and is pumped through the glycol/glycol heat exchangers and back to the contactor.
Solid Bed HC/water dew point control Dryers
Solid Bed dehydration systems work on the principle of adsorption.
The desiccant is a solid, granulated drying or dehydrating medium with an extremely large effective surface area per unit weight because of a multitude of microscopic pores and capillary openings.
Dry bed advantages:
- Producing very low dew points, which are required for cryogenic gas plants
- It is adaptable to very large changes in flow rates
- It can handle high contact temperatures.
Dry bed Disadvantages:
- The initial cost for a solid bed dehydration unit generally exceeds that of a glycol unit.
- It is a batch process.
- There is a relatively high pressure drop through the system
- The desiccants are sensitive to poisoning with liquids or other impurities in the gas.
This package will treat the gas for correcting HC and water dew point. The beds will be regenerated with feed gas being heated by use of heat medium and HC liquids and water which were adsorbed will be desorbed from the desiccant and will be condensed by use of air cooler and trim cooler and separated in the K.O. drum. The regeneration gas will separate and then be returned back to the desiccant package inlet.
The essential components of any solid desiccant dehydration system are:
- Inlet gas separator
- Two or more adsorption towers (contactors) filled with a solid desiccant,
- A high-temperature heater to provide hot regeneration gas to reactivate the desiccant in the towers.
- A regeneration gas cooler to condense water from the hot regeneration gas.
- A regeneration gas separator to remove the condensed water from the regeneration gas.
- Piping, manifolds, switching valves and controls to direct and control the flow of gases according to the process requirements.