Pore size compounds from water

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 compound.s from entering the pores. However, carbon dioxide molecules are about the same size as H2S molecules and present problems. Even thougli die COi is non-polar and will not bond to the active sites, the CO2 will entci the pores. Small quantities of CO2 will become trapped in the pores In this way small portions of CO2 are removed. More importantly, CO ih obstruct the access of H2S and water to active sites and decrease the eflectiveness ot the pores. Beds must be sized to remove all water and to pi ovitte for interference from other molecules in order to remove all H i.S. 

The removal of PhCs by NF membranes occurs via a combination of three mechanisms adsorption, sieving and electrostatic repulsion. Removal efficiency can vary widely from compound to compound, as it is strictly correlated to (a) the physicochemical properties of the micro-pollutant in question, (b) the properties of the membrane itself (permeability, pore size, hydrophobicity and surface charge) and (c) the operating conditions, such as flux, transmembrane pressure, rejections/recovery and water feed quality. 

Molecular sieves are used in a variety of fuel processing applications. Uses include drying and water removal from fuel, product purification, hydrocarbon separation and catalysis. Molecular sieves are composed of sodium and calcium aluminosilicate crystals which have been produced from natural or synthetic zeolite compounds. The crystals are dehydrated through heating and are processed to ensure that pore sizes are tightly controlled. 

In water studies it is standard practice to filter the sample soon after collection, usually through a 0.45p,m membrane disc (made of cellulose acetate, cellulose nitrate or polycarbonate). This process arbitrarily divides the sample components into soluble and insoluble fractions, but as shown in Table 2.3, the average size of different chemical species varies widely, and some differentiation between species can be obtained through using filter media of different pore sizes. For example, fully dissolved compounds can be separated from finer colloidal forms by using gel filtration and dialysis, and sub-division of the total content into fractions based on particle or molecular size (see Section 2.3) has been used for speciation of elements in waters. 

These responses correspond to a void volume of 9.8% using toluene as the void-filling compound in the calculation and 13.3% when chloroform, a smaller compound, is used in the calculation. The voids apparently have not only maximum capacity but also pores with some size-discrimination ability. These amounts correspond to 86% of the saturation limit of toluene in water and only 65% for that of chloroform. Larger molecules with even greater partitions were found not capable of being adsorbed into Teflon AF . The assumption here is that the pore size in Teflon AF prevents large molecules, such as naphthalene and bis(2-ethylhexyl phthalate), a plasticizer, from getting in. 

Membranes having effective pore sizes between 0.001 and 0.01 pm are used in nanofiltration. NF is placed between reverse osmosis and ultrafiltration, and because of that it is sometimes considered as loose reverse osmosis. Typical operating pressures for NF are 0.3-1.4 MPa. The process allows to separate monovalent ions from multivalent ions, which are retained by NF membrane. The process can be used for separation of organic compounds of moderate molecular weight from the solution of monovalent salts. The very well-known application in nuclear industry is boric acid recovery from contaminated cooling water in nuclear reactor. There are some examples of nanofiltration applications and studies done with the aim of implementation in nuclear centers described in literature. Some of them are listed in the Table 30.4. 

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