Zeolites are used in adsorption processes widely in industry for drying of gases and purification of effluents to improve quality of raw materials and final products together with silica gei and aluminum oxide. In comparison with other adsorbents, zeolites have advantages in adsorption process.
Firstly, zeolites can adsorb water in much higher quantities than silica gel and aluminum oxide. Even at low temperatures, zeolites show significant adsorption capacities for adsorption of water vapor. Secondly, with an increase in temperature, adsorption capacities of zeolites do not decreases as sharply as occurring in other adsorbents. Thirdly, zeolites have lower hydraulic resistance compared to other adsorbents. Also they show molecular sieving property in which they are able adsorb the molecules of substances, the critical diameters of which are less then the size of input openings of zeolites, whereas larger particles are not adsorbed. Hence, when a complex system is to be treated, water molecules are adsorbed by zeolites and the other components of the system are not.
Although in some cases, synthetic zeolites show even oetter properties man natural zeolites, it is more expensive to synthesize than using natural zeolites. As a result of this dinoptilolite containing tuffs having almost similar sizes of the input opening of
pores to synthetic zeolites, are used successfully for drying natural and petroleum gases as well as dehydrogenation of gases .
Zeolites are also used for adsorptive drying of other raw materials. In the production of butyl rubber, a specific purity of monomers and solvent is required. Methyl chloride is used as solvent and water is a strongest polymerization poison. In most cases, synthetic zeolite KA - 3M has been used for drying methyl chloride in the butyl rubber industry. This zeolite, however, being a highly effective drier, is disadvantageous, since it catalyzes the hydrolysis of the methyl chloride with the subsequent formation of dimethyl ether, which in turn poisons the process. It is necessary to introduce an additional purification from dimethyl ether using aluminum oxide. Replacement of the synthetic zeolite by clinoptifolite-containing tuffs allowed successful drying of the methyl chloride.
Another area where zeolites used is the food sector. Often when drying foodstuffs, undesired product degradations will occur, such as changes in color, loss of functional characteristics, and loss of aromatic components. TNO "Environment, Energy and Process Innovation" has developed a recent technology, by which the product can be dried in a short period of time at low temperature ,
In this way the undesired product degradations do not occur and product quality is preserved. This new, patented method is based on the adsorption of water using zeolites. Solid products can be dried by mixing the product with grains of zeolite (2-5 mm). After a certain period the product is separated from the zeolite by sieving.
Drying at low temperature, possibility of precisely setting of the final liquid content of the product, retaining aromatic substances and no external heat is necessary in the drying stage are the advantages of drying with zeolite. The schematic diagram of the patented zeolite dryer is given on the next page.
TNO-MSP Zeolite dryer