Dry dessicant dehydration

While dehydration with glycol is the most common process used to meet the water dew point specification for sales gas, under certain conditions solid adsorbents are also used for this purpose.

Solid compounds

Materials with a great affinity for water are:

• Silica gel
• Sorbead
• Alumina
• Molecular sieves

These solid compounds are prepared as round or slightly elliptical beads having a diameter of about 4 to 6 mm and, in the case of molecular sieves, are also manufactured in small cylindrical forms, about 2 to 3 mm in diameter and about 6 to 8 mm long. Each of these compounds has its own characteristic affinity and adsorptive capacity for water, and the designer has to choose the material best suited for the purpose.

Silica gel

Silica gel is silicon dioxide (SiO₂), manufactured as small round beads with a large pore surface area onto which the water, contained in the vapor phase in the gas, is adsorbed by the desiccant at relatively low temperatures. The affinity for water is temperature dependent, and the affinity for water is broken at high temperatures, such as 390°F. It is necessary to avoid liquid water droplets from contacting the silica gel, as liquid water damages the desiccant. Thus, it is important to have effective gas/liquid separation ahead of a dry desiccant unit.

Sorbed

Sorbead is also a silica gel product that contains about 3% aluminum oxide (Al₂O₃) that makes the gel more resistant to damage from water droplets. Certain manufacturers of Sorbead desiccants provide different grades of Sorbead, such as grade R for regular natural gas drying and grade WS for additional damage resistance from water droplets. A tower filled with Sorbead contains mainly Sorbead R and is topped off with a one-foot-thick layer of Sorbead WS to offer degradation protection from water droplets.

Alumina

Alumina is used mainly for drying air. Molecular sieves are artificial zeolites that are manufactured so that they result in solids with uniform small pore spaces. They have a great affinity for water.

Molecular sieves

Molecular sieves are usually installed in applications in which very low residual water content is required, such as ahead of a low temperature hydrocarbon extraction process. Molecular sieves are suitable for drying very sour natural gas that also contains aromatic compounds. The heavier hydrocarbons might be difficult to remove from the silica gel during the regeneration step.

Absorptivities

Adsorptivities and design water contents are best obtained from the vendors of the desiccant. In general, design water adsorption is on the order of 8 to 12 lbm of water per 100 lbm of silica and Sorbead desiccants. When new, the adsorption capacity is higher than these numbers. However, when desiccant ages with a number of regeneration cycles, a good design value is about 8 to 10 lbm of water per 100 lbm of desiccant.

Typical process equipment

Fig. 1 illustrates the typical equipment for a dry desiccant process for dehydrating natural gas. Two towers are required for gas dehydration. As the gas flows through one tower containing the solid desiccant, water is adsorbed onto the surface of the material. After adsorbing for several hours, the material becomes fully saturated with water. For this reason, the process incorporates two towers. While one tower is on adsorption, the other tower is being regenerated by the application of heat through a small slipstream of gas and subsequent cooling. The regeneration time is slightly less than the adsorption time of the first tower. Thus, when one tower is fully saturated, the second tower has been fully regenerated. The towers are then switched, and the process of drying the gas continues uninterrupted.

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  Fig. 1—Schematic drawing of typical dry desiccant dehydration process.

Design

To design a silica gel dehydration system, it is necessary to select the duration of the onstream time per tower before switching towers. This is usually 8 to 12 hours for fully water-saturated gas. After establishing the cycle time and the amount of water to be removed from the gas during one cycle, the bulk volume of desiccant can be estimated. The diameter of the tower is designed on the basis of the superficial gas velocity, and the height of the tower is determined by dividing the cross-sectional area of the tower by the determined bulk volume of the desiccant. The vendors of the desiccant usually provide the design criteria and perform the design.

Water dewpoint and hydrocarbon dewpoint control

Short-cycle dry dessicant process

Some processes can be used to control the water content as well as remove heavier hydrocarbons from natural gas. The short-cycle dry desiccant process can be used in this way.

This process is similar to the dry desiccant adsorption process covered in dehydration. It is usually selected for a gas stream that is fairly lean in propane and butane but contains sufficient amounts of C5⁺ so that the hydrocarbon dewpoint is not met. Silica gel is generally the preferred adsorbent.

There are basically two differences between using this process for water dewpoint control and for hydrocarbon dewpoint control, namely that three towers are required for hydrocarbon dewpoint control, and the cycle time for the towers is short, usually 20 to 30 minutes as compared to 8 hours or longer for dehydration. The desirable feature of the short cycle adsorption unit is that it recovers about 60 to 70% of the pentanes-plus in the gas stream and meets both the hydrocarbon and water dewpoints.

Fig. 2 shows the arrangement of a typical three-tower, short-cycle hydrocarbon recovery unit. While one tower is on adsorption, the other two towers are on their heating and cooling cycles. Usually, the towers switch every 20 to 30 minutes. The length of the cycle time depends on the design of the facility.

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  Fig. 2—Schematic drawing of typical dry desiccant dewpoint control process.

Performance

There are many factors that influence the performance of a short-cycle unit. First and foremost, the separator ahead of the unit must remove all liquids and solids in the gas. A slug of liquid into the bed would damage the adsorbent and perhaps necessitate a changeout. The adsorption temperature should be as low as possible without causing hydrates ahead of the unit, to adsorb more of the heavier hydrocarbons. Proper regeneration of the bed is very important. A portion of the process stream is used for this purpose. Regeneration gas temperatures are in the order of 480 to 530°F. The tower should be heated to an outlet gas temperature of 390°F.

Condensing

While adsorbing and desorbing are important, the adequate condensing of the desorbed hydrocarbons is also very important. If the condensing is inefficient, the heavier hydrocarbons have no means of exiting from the gas stream. In warm climates, this is a major problem, requiring that the condenser be conservatively sized. Upon condensing, the liquids must be separated from the lighter hydrocarbons in the regeneration stream. Conservatively-sized vertical separators are usually used in this service.

Seller

The design of the short-cycle dry desiccant units is usually performed by the seller of the desiccant. The seller develops the computer software and tests the simulation results in actual applications. Thus, the design results from the vendor are quite reliable.

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See also

PEH:Gas Treating and Processing

Gas Treating and Processing

Dehydration with deliquescing dessicants

Dehydration with glycol

Dehydration with refrigeration and hydrate suppression

Sour gas sweetening