The molecular sieve process uses synthetically manufactured solid crystalline zeolite in a dry bed to remove gas impurities. The crystalline structure of the solids provides a very porous solid material with all the pores exactly the same size. Within the pores the crystal structure creates a large number of localized polar charges called active sites. Polar gas molecules, such as H2S and water, that enter the pores form weak ionic bonds at the active sites. Nonpolar molecules such as paraffin hydrocarbons will not bond to the active sites. Thus, molecular sieve units will “dehydrate” the gas (remove water vapor) as well as sweeten it.
Molecular sieves are available with a variety of pore sizes. A molecular sieve should be selected with a pore size that will admit H2S and water while preventing heavy hydrocarbons and aromatic compounds from entering the pores. However, carbon dioxide molecules are about the same size as H2S molecules and present problems. Even though the CO2 is non-polar and will not bond to the active sites, the CO2 will enter the pores. Small quantities of CO2 will become trapped in the pores. In this way small portions of CO2 are removed. More importantly, CO2 will obstruct the access of H2S and water to active sites and decrease the effectiveness of the pores. Beds must be sized to remove all water and to provide for interference from other molecules in order to remove all H2S.
The absorption process usually occurs at moderate pressure. Ionic bonds tend to achieve an optimum performance near 450 psig, but the process can be used for a wide range of pressures. The molecular sieve bed is regenerated by flowing hot sweet gas through the bed. Typical regeneration temperatures are in the range of 300-40()°F.
Molecular sieve beds do not suffer any chemical degradation and can be regenerated indefinitely. Care should be taken to minimize mechanical damage to the solid crystals as this may decrease the bed’s effectiveness. The main causes of mechanical damage are sudden pressure and/or temperature changes when switching from absorption to regeneration cycles.
Molecular sieves for acid gas treatment are generally limited to small gas streams operating at moderate pressures. Due to these operating limitations, molecular sieve units have seen limited use for gas sweetening operations. They are generally used for polishing applications following one of the other processes and for dehydration of sweet gas streams where very low water vapor concentrations are required.