Some simple gas processing applications can use simple technologies - solid desiccant or absorber pellet beds or silica gel systems - each of which has limitations when applied to larger systems.  Where solid or liquid effluent or short bed life is likely to be unacceptable, the option of Molecular Sieve treatment prevails.

Molecular sieves are Alumino Silicates, which have a uniform, crystalline structure.  It is this structure, which is used to "filter" out molecules of contaminants.  The sieves are produced in pellets of different shapes and sizes to suit the application and bed weight.  They are also described by the size of the crystalline structure in Angstroms:

• 3A Used for drying unsaturated hydrocarbons (Ethylene, Propylene) and highly polar compounds (Methanol, Ethanol).
• 4A Used for general purpose dehydration service and some carbon dioxide applications.
• 5A Used for natural gas dehydration, Carbon Dioxide, Hydrogen Sulphide and Mercaptan removal.  Also for separating N-Paraffins from branched and cyclic hydrocarbons.
• 13X Used for simultaneous removal of the above contaminants as it has a larger (10A) pore size.

Applications include hydrocarbon gas and liquid dehydration and sweetening.  In each case the optimum system design will be where feedstock contamination is below 1%, and the desired product contamination level is in the PPM range.  Molecular sieve systems offer very high levels of product purity and are often the preferred choice for applications such as dehydration of natural gas into cryogenic plants or sweetening to pipeline Hydrogen Sulphide levels.

Typically, a system will comprise a multiple number of adsorber beds into which the feed gas flows, having initially been scrubbed in a filter-separator to remove free liquid and solid particles.  The beds remove essentially all the contaminants and gradually become saturated over a period of several hours.  Prior to reaching saturation, the gas flow is switched to an alternative bed, and the original bed is fed with hot, dry gas to drive off the contaminants and regenerate the sieve.  Purge gas is treated to recover the contaminants as appropriate.  Once regenerated, the bed is cooled and put back on stream.

KCC supplies full systems complete with inlet and outlet filters, adsorber vessels, regeneration heaters, automated controls and switching valve assemblies, and first fill of sieve material.