Base exchange process

When hard water is passed through a bed consisting of zeolites, which are synthetic material systems composed of complex sodium, aluminium and silicate salts (Na Z), the calcium and magnesium ions are exchanged. The displacing reactions for temporary hardness are  

The process is reversible and the bed (Na Z) can be regenerated by passing concentrated salt solution. This method of water softening yields a very soft water (0.5-1 hardness). 

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The base exchange process removes both the temporary and permanent hardness salts from the water by allowing the water to flow through resin beads containing sodium zeolite, Na2Z. 

In reverse osmosis, the osmotic pressure is increased manually to get the water to flow from a high-density area through a semipermeable membrane to the lower-density weaker solution. The water will pass through the membrane and leave the solids behind. A pressure of about 2.76 MPa will extract 90% or more of the dissolved absorbed solids further refinement may be achieved through a base exchange process. 

As an adjective applied to metals base represents the opposite of noble, i.e. a base metal would be attacked by mineral acids, base exchange An old term used to describe the capacity of soils, zeolites, clays, etc. to exchange their cations (Na, K, Ca ) for an equivalent of other cations without undergoing structural change. An example of the general process of ion exchange. ... 

One ion-exchange process, which was used for several years by Quebec Lithium Corp., is based on the reaction of P-spodumene with an aqueous sodium carbonate solution in an autoclave at 190—250°C (21). A slurry of lithium carbonate and ore residue results, and is cooled and treated with carbon dioxide to solubilize the lithium carbonate as the bicarbonate. The ore residue is separated by filtration. The filtrate is heated to drive off carbon dioxide resulting in the precipitation of the normal carbonate. 

Monovalent cations are good deflocculants for clay—water sHps and produce deflocculation by a cation exchange process, eg, Na" for Ca ". Low molecular weight polymer electrolytes and polyelectrolytes such as ammonium salts (see Ammonium compounds) are also good deflocculants for polar Hquids. Acids and bases can be used to control pH, surface charge, and the interparticle forces in most oxide ceramic—water suspensions. 

Base catalysis is most effective with alkali metals dispersed on solid supports or, in the homogeneous form, as aldoxides, amides, and so on. Small amounts of promoters form organoalkali comnpounds that really contribute the catalytic power. Basic ion exchange resins also are usebil. Base-catalyzed processes include isomerization and oligomerization of olefins, reactions of olefins with aromatics, and hydrogenation of polynuclear aromatics. 

Safety valves are often required on the shell side of exchangers and sometimes on the tube side. These valves may require sizing based upon process reaction, overpressure, etc., or on external fire. For details, see Chapter 7, Vol. 1 on safety-relieving devices. 

Where the feed contains a large proportion of treated water, softening is a minimum requirement and the raw water quality dictates whether a more sophisticated form of external treatment would be preferable. If the water has a high alkalinity it calls for de-alkalization and base exchange. De-ionization is the ideal water treatment, but is usually avoided if possible because of its cost and use of corrosive chemicals. Membrane processes giving partial de-ionization are not normally installed at present, but are certain to become important in the future. 

The liquid-liquid extraction process is based on the specific distribution of dissolved components between two immiscible fluids, for instance, between aqueous and organic liquids. The process refers to a mass exchange processes in which the mass transport of component (j) from phase (1) to phase (2) by means of convection or molecular diffusion acts to achieve the chemical potential (p) equilibrium (134) ... 

Replacement of positive ions of a soluble salt by passing sodium through a cation resin (softening or base exchange) or hydrogen through an acidic resin (as in one half of the demineralization process). 

This is usually prepared by either a base-exchange method using sodium zeolite, by a lime-soda ash process, or by the addition of sodium hexametaphosphate. In addition to the bacteria derived from fhe mains water, additional flora of Bacillus spp. and Staphylococcus aureus may be introduced into systems which use brine for regeneration and from the chemical filter beds which, unless treated, can act as a reservoir for bacteria. 

Liquid-solid distributions are involved in ion-exchange and other adsorption-based separation processes, separation processes based on crystallization or precipitation, flotation processes for ore dressing, and smelting processes. 

Fontes tt al. [224,225 addressed the acid—base effects of the zeolites on enzymes in nonaqueous media by looking at how these materials affected the catalytic activity of cross-linked subtilisin microcrystals in supercritical fluids (C02, ethane) and in polar and nonpolar organic solvents (acetonitrile, hexane) at controlled water activity (aw). They were interested in how immobilization of subtilisin on zeolite could affected its ionization state and hence their catalytic performances. Transesterification activity of substilisin supported on NaA zeolite is improved up to 10-fold and 100-fold when performed under low aw values in supercritical-C02 and supercritical-ethane respectively. The increase is also observed when increasing the amount of zeolite due not only to a dehydrating effect but also to a cation exchange process between the surface proton of the enzyme and the sodium ions of the zeolite. The resulting basic form of the enzyme enhances the catalytic activity. In organic solvent the activity was even more enhanced than in sc-hexane, 10-fold and 20-fold for acetonitrile and hexane, respectively, probably due to a difference in the solubility of the acid byproduct. 

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