Hydrocarbon Compression

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Solid Bed Dehydration – Process Description #2

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For each component in the inlet gas stream, there will be a section of bed depth, from top to bottom, where the desiccant is saturated with that component and where the desiccant below is just starting to adsorb that component. The depth of bed from saturation to initial adsorption is known as the mass transfer zone. This is simply a zone or section of the bed where a component is transferring its mass from the gas stream to the surface of the desiccant.

As the flow of gas continues, the mass transfer zones move downward through the bed and water displaces the previously adsorbed gases until finally the entire bed is saturated with water vapor. If the entire bed becomes completely saturated with water vapor, the outlet gas is just as wet as the inlet gas. Obviously, the towers must be switched from the adsorption cycle to the regeneration cycle (heating and cooling) before the desiccant bed is completely saturated with water.

At any given time, at least one of the towers will be adsorbing while the other towers will be in the process of being heated or cooled to regenerate the desiccant. When a tower is switched to the regeneration cycle some wet gas (that is, the inlet gas downstream of the inlet gas separator) is heated to temperatures of 450°F to 600°F in the high-temperature heater and routed to the tower to remove the previously adsorbed water. As the temperature within the tower is increased, the water captured within the pores of the desiccant turns to steam and is absorbed by the natural gas. This gas leaves the top of the tower and is cooled by the regeneration gas cooler. When the gas is cooled the saturation level of water vapor is lowered significantly and water is condensed. The water is separated in the regeneration gas separator and the cool, saturated regeneration gas is recycled to be dehydrated. This can be done by operating the dehydration tower at a lower pressure than the tower being regenerated
or by recompressing the regeneration gas.

Once the bed has been “dried” in this manner, it is necessary to flow cool gas through the tower to return it to normal operating temperatures (about 100°F to 120°F) before placing it back in service to dehydrate gas. The cooling gas could either be wet gas or gas that has already been dehydrated. If wet gas is used, it must be dehydrated after being used as cooling gas. A, hot tower will not sufficiently dehydrate the gas.

The switching of the beds is controlled by a time controller that performs switching operations at specified times in the cycle. The length of the different phases can vary considerably. Longer cycle times will require larger beds, but will increase the bed life. A typical two-bed cycle might have an eight-hour adsorption period with six hours of heating and two hours of cooling for regeneration. Adsorption units with three beds typically have one bed being regenerated, one fresh bed adsorbing, and one bed in the middle of the drying cycle.

Internal or external insulation for the adsorbers may be used. The main purpose of internal insulation is to reduce the total regeneration gas requirements and costs. Internal insulation eliminates the need to heat and cool the steel walls of the adsorber vessel. Normally, a castable re factory lining is used for internal insulation. The refractory must be applied and properly cured to prevent liner cracks. Liner cracks will per mit some of the wet gas to bypass the desiccant bed. Only a small amount of wet, bypassed gas is needed to cause freezeups in cryogenic plants. Ledges installed every few feet along the vessel wall can help eliminate this problem.

Written by Jack

September 21st, 2009 at 7:07 am

Solid Bed Dehydration – Process Description #1

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Multiple desiccant beds are used in cyclic operation to dry the gas on a continuous basis. The number and arrangement of the desiccant beds may vary from two towers, adsorbing alternately, to many towers. Three separate functions or cycles must alternately be performed in each dehydrator. They are an adsorbing or gas drying cycle, a heating or regeneration cycle, and a cooling cycle.

Figure 8-21 is a flow diagram for a typical two-tower solid desiccant dehydration unit. The essential components of any solid desiccant dehydration system are:

1. Inlet gas separator.
2. Two or more adsorption towers (contactors) filled with a solid desiccant,
3. A high-temperature heater to provide hot regeneration gas to reactivate the desiccant in the towers.
4. A regeneration gas cooler to condense water from the hot regeneration gas.
5. A regeneration gas separator to remove the condensed water from the regeneration gas.
6. Piping, manifolds, switching valves and controls to direct and control the flow of gases according to the process requirements.

In the drying cycle, the wet inlet gas first passes through an inlet separator where free liquids, entrained mist, and solid particles are removed, This is a very important part of the system because free liquids can damage or destroy the desiccant bed and solids may plug it. If the adsorption unit is downstream from an amine unit, glycol unit or compressors, a filter separator is preferred.

In the adsorption cycle, the wet inlet gas flows downward through the tower. The adsorbable components are adsorbed at rates dependent on their chemical nature, the size of their molecules, and the size of the pores. The water molecules are adsorbed first in the top layers of the desiccant bed. Dry hydrocarbon gases are adsorbed throughout the bed. As the upper layers of desiccant become saturated with water, the water in the wet gas stream begins displacing the previously adsorbed hydrocarbons in the lower desiccant layers. Liquid hydrocarbons will also be absorbed and will fill pore spaces that would otherwise be available for water molecules.

 Solid Bed Dehydration   Process Description #1

Written by Jack

September 21st, 2009 at 7:04 am

Solid Bed Dehydration

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Solid bed dehydration systems work on the principle of adsorption. Adsorption involves a form of adhesion between the surface of the solid desiccant and the water vapor in the gas. The water forms an extremely thin film that is held to the desiccant surface by forces of attraction, but there is no chemical reaction. 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. A typical desiccant might have as much as 4 million square feet of surface area per pound.

The initial cost for a solid bed dehydration unit generally exceeds that of a glycol unit. However, the dry bed has the advantage of producing very low dew points, which are required for cryogenic gas plants, and is adaptable to very large changes in flow rates. A dry bed can handle high contact temperatures. Disadvantages are that it is a batch process, there is a relatively high pressure drop through the system, and the desiccants are sensitive to poisoning with liquids or other impurities in the gas.

Written by Jack

September 21st, 2009 at 6:57 am