Hydrocarbon Compression

Archive for the ‘Design Considerations’ Category

Solid Bed Dehydrator Moisture Content of Inlet Gas

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An important variable that determines the size of a given desiccant bed is the relative saturation of the inlet gas. This variable is the driving force that affects the transfer of water to the adsorbent. If saturated gas (100% relative humidity) is being dried, higher useful capacities can be expected for most desiccants than when drying partially saturated gases. However, in most field gas dehydration installations the inlet gas is saturated with water vapor and this is not a variable that must be considered.

Written by Jack

September 21st, 2009 at 8:54 am

Solid Bed Dehydrator Pressure Drop

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Towers are sized for a design pressure drop of about 5 psi through the desiccant. The pressure drop can be estimated by:

 Solid Bed Dehydrator Pressure Drop

Pressure drops of greater than approximately 8 psi are not recommended.

Written by Jack

September 21st, 2009 at 8:52 am

Solid Bed Dehydrator Bed Height to Diameter Ratio

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In its simplest form, an adsorber is normally a cylindrical tower filled with a solid desiccant. The depth of the desiccant may vary from a few feet to 30 ft or more. The vessel diameter may be from a few inches to 10 or 15 ft. A bed height to diameter (L/D) ratio of higher than 2.5 is desirable. Ratios as low as 1:1 are sometimes used; however, poor gas dehydration, caused by non-uniform flow, channeling and an inadequate contact time between the wet gas and the desiccant sometimes result.

Written by Jack

September 21st, 2009 at 8:50 am

Solid Bed Dehydrator Gas Velocities

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Generally, as the gas velocity during the drying cycle decreases, the ability of the desiccant to dehydrate the gas increases. At lower actual velocities, drier effluent gases will be obtained. Consequently, it would seem desirable to operate at minimum velocities to fully use the desiccant.

However, low velocities require towers with large cross-sectional areas to handle a given gas flow, and allow the wet gas to channel through the desiccant bed and not be properly dehydrated. In selecting the design velocity therefore, a compromise must be made between the tower diameter and the maximum use of the desiccant. Figure 8-22 shows a maximum design velocity. Smaller velocities may be required due to pressure drop considerations.

The minimum vessel internal diameter for a specified superficial velocity is given by:

 Solid Bed Dehydrator Gas Velocities

 Solid Bed Dehydrator Gas Velocities

Written by Jack

September 21st, 2009 at 8:46 am

Solid Bed Dehydrator Cycle Time

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Most adsorbers operate on a fixed drying cycle time and, frequently, the cycle time is set for the worst conditions. However, the adsorbent capacity is not a fixed value; it declines with usage. For the first few months of operation, a new desiccant has a very high capacity for water removal. If a moisture analyzer is used on the effluent gas, a much longer initial drying cycle can be achieved. As the desiccant ages, the cycle time will be automatically shortened. This will save regeneration fuel costs and improve the desiccant life.

Written by Jack

September 21st, 2009 at 7:48 am

Solid Bed Dehydrator Pressure

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Generally, the adsorption capacity of a dry bed unit decreases as the pressure is lowered. If the dehydrators are operated well below the design pressure, the desiccant will have to work harder to remove the water and to maintain the desired effluent dew point. With the same volume of incoming gas, the increased gas velocity, occurring at the lower pressures, could also affect the effluent moisture content and damage the desiccant.

Written by Jack

September 21st, 2009 at 7:47 am

Solid Bed Dehydrator Temperature

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Adsorption plant operation is very sensitive to the temperature of the incoming gas. Generally, the adsorption efficiency decreases as the temperature increases.

The temperature of the regeneration gas that commingles with the incoming wet gas ahead of the dehydrators is also important. If the temperature of these two gas streams differs more than 15°F to 20°F, liquid water and hydrocarbons will condense as the hotter gas stream cools. The condensed liquids can shorten the solid desiccant life.

The temperature of the hot gas entering and leaving a desiccant tower during the heating cycle affects both the plant efficiency and the desiccant life. To assure good removal of the water and other contaminants from the bed, a high regeneration gas temperature is needed. The maximum hot gas temperature depends on the type of contaminants and the “holding power” or affinity of the dessicant for the contaminants, A temperature of 450°F to 60G°F is normally used.

The desiccant bed temperature attained during the cooling cycle is important. If wet gas is used to cool the desiccant, the cooling cycle should be terminated when the desiccant bed reaches a temperature of approximately 215°F. Additional cooling may cause water to be adsorbed from the wet gas stream and presaturate or preload the desiccant bed before the next adsorption cycle begins. If dry gas is used for cooling, the desiccant bed should be cooled within 10°F~20°F of the incoming gas temperature during the adsorption cycle, thereby maximizing the adsorption capacity of the bed.

The temperature of the regeneration gas in the regeneration gas scrubber should be low enough to condense and remove the water and hydrocarbons from the regeneration gas without causing hydrate problems.

Written by Jack

September 21st, 2009 at 7:32 am