In this post, I want to share you about natural gas dehydration using desiccant using adsorption principle and how to size it.
In common commercial use, desiccant can be classified into three categories, which are gels (alumina or silica gels), alumina, and molecular sieves.
When used for natural gas dehydration, silica gel will give outlet dewpoints of approximately -70o to -80oF. As for alumina, outlet dewpoint is appoximately -100oF. Molecular sieves produced the lowest water dewpoints, as low as -150oF. For gas going into cryogenic processing, the only adsorbent that can obtain the required dehydration is a molecular sieve.
Table below shows characteristics of several type of desiccants .
Figure below shows a schematic of two-bed adsorber system.
Adsorber A dries natural gas, while the other bed, Adsorber B, goes through a regeneration cycle. The wet gas enters inlet gas pre-filter or separator that will catch entrained liquids before entering the active beds. Gas flow is top-down to avoid bed fluidization. The dried gas then goes through solid after filter that catch fines adsorbent before exiting the unit.
A small amount of gas is used for regeneration. Sales gas is sometimes used because it has advantage of being free of heavier hydrocarbon that can cause coking. This gas is usually 5%-10% of gas throughput.
Regeneration involves heating the bed, regeneration (removing the water), and cooling.
During heating and regeneration, gas is heated to about 600oF to heat the bed and removed adsorbed water from the adsorbent. Regeneration gas enters from the bottom of the bed (counter-current to flow during adsorption) to ensure that the lower part of the bed is the driest.
During adsorption cycle, the bed can be thought of operating with three zones.
- The top zone is called the saturation or equilibrium zone, where the adsorbate on the adsorbent is in equal with adsorbate in the inlet gas phase and no additional adsorption occurs.
- The middle zone is called mass transfer zone (MTZ), where mass transfer and adsorption take place.
- The bottom zone is unused desiccant and is often called the active zone, where no adsorption has yet taken place.
Table below lists guidelines for typical molecular sieves dehydration units.
Below is step-by-step process of sizing or design of natural gas dehydration unit using desiccants.
The first step is to define operating condition of the system, such as gas flow rate, pressure, temperature, gas density, gas viscosity, and gas molecular weight.
The second step is to determine desiccant type used.
After that, by using several equations and basis design, such as allowable pressure drop accross the bed, we can estimate geometry of adsorber. We can also estimate how much molecular sieve desiccant we need and how much gas we need during regeneration.
I prepared this free spreadsheet of natural gas dehydration using desiccant for you. This spreadsheet is prepared based on GPSA engineering data books. This is preview of the spreadsheet.
I hope this post is useful. Please spread it to the world!
 GPSA Engineering Data Books
 Solid Desiccant-based Dehydration Technology (http://www.frames-group.com/)
 Natural Gas Dehydration by Desiccant Materials (https://www.researchgate.net/publication/251659746_Natural_gas_dehydration_by_desiccant_materials)
 Fundamentals of Natural Gas Processing