What is a desiccant air dryer?

Desiccant air dryers are typically designed to work efficiently in critical applications were dew points of -40°C to -73°C are needed. Desiccant air dryers; use desiccant beads such as activated alumina, silica gel, or molecular sieves 4A to adsorb the moisture found in the compressed air stream. Desiccant air dryers are engineered to remove water vapor in compressed air systems to dew points as low as -73°C. Desiccant air dryers are typically found in industries such as; food, pharmaceuticals, painting, paper and petrochemical’s that have very specific drying requirements.  

Molecular sieve is a material with pores (very small holes) of uniform size. These pore diameters are similar in size to the molecules, and thus large molecules cannot enter or be absorbed while the smaller molecules can go in. Due to the special performance of preferential adsorption, molecular sieves are widely used for gas dehydration, gas purification and gas production. In this article, we are going to mainly talk about the application of natural gas dehydration. 

By the molecular sieve dehydration units, the water vapor in natural gas stream will be removed well. The inlet gas in the natural gas processing is required to be free of water under the cryogenic temperature. High water vapor can freeze at low temperature forming hydrates. These hydrates can be a problem in the cryogenic process. The use of the molecular sieve will help avoid such kind of problem. 

For natural gas dehydration, our molecular sieve 4A is very workable. With an open crystal structure, the moisture will be removed well also along with the removal of some other molecules. The following is the technical specification of 4A.

Regeneration Molecular Sieve should be in typical cyclic systems constitutes removal of the adsorbate from the molecular sieve bed by heating and purging with a carrier gas.

Sufficient heat must be applied to raise the temperature of the adsorbate, the adsorbent and the vessel to vaporize the liquid and offset the heat of wetting the molecular-sieve surface. The bed temperature is critical in regeneration. Bed temperatures in the 175-260℃ range are usually employed for type 3A. This lower range minimizes polymerization of olefins on the molecular sieve surfaces when such materials are present in the gas. Slow heat up is recommended since most olefinic materials will be removed at minimum temperatures, 4A, 5A and 13X sieves require temperatures in the 200-315℃ range.

 After regeneration, a cooling period is necessary to reduce the molecular sieve temperature to within 15℃ of the temperature of the stream to be processed.

For optimum regeneration, gas flow should be counter current to adsorption during the heat up cycle and concurrent (relative to the process stream) during cooling.

Alternatively, small quantities of molecular sieves may be dried in the absence of a purge gas by oven heating followed by slow cooling in a closed system, such as a desiccator.

Molecular sieve is a alumino silicate with uniform pores. These pore diameters are similar in size to small molecules. Thus, molecules with diameter less than the pore size can enter molecular sieve and be adsorbed, while molecules with diameter bigger than the pore size cannot go into molecular sieve and be excluded (Tab. 1). Moreover, molecular sieve pore size can be precisely adjusted through introducing of different metal ions such as Na+(4A, 13X), K+(3A), Ca+(5A, CaX), Li+(LiLSX, LiLX). This property enables molecular sieve to separate molecules of different size. Another important property of molecular sieve is it is easier to adsorb polar molecules other than non-polar molecules, thus makes it possible to separate molecules by its polarity   (Fig. 1).