Exchange Design and Operating Practices

Ion exchange processes function by replacing undesirable ions of a liquid with ions such as H or OH- from a sohd material in which the ions are sufficiently mobile, usually some synthetic resin. Eventually the resin becomes exhausted and may be regenerated by contact with a small amount of solution with a high content of the desired ion. Resins can be tailored to have selective affinities for 

Material particles k /L weight % c pH range equivalenl/kg equivalenl/L 

Polystyrene phuphonate Polystyrene aminodiacetate Polystyrene amidosiine G.S 0.74 50-70 40 120 3-14 6.6 3.0 

Zirconium tungstate Anion eschannn strongly basic Pr ystyrene-ba Trimethyl benzyl ammonium (type Homogeneous, 8% CL G 1.15-125 - 5 0 150 2-10 12 10 

Triethyl hydroxypropyl ammonium Anion exchangers intermediately basic (pK It) Polystyrene-based 100 4-10 0.57  

Material particles kg/L weight % °C pH range equivalenl/kg equivalent/L 

Zirconium tungstate Anion exchangers strongly basic Pol yrene-based Trimethyl benzyl ammonium (type 1) Homogeneous, 6% CL G 115-1 25 -5 0 150 2-10 1.2 1.0 

Epray-polyamine Anion exchangers weakly basic (pK 9) S 0.72 -64 8-10 75 0-7 6.5 1.7 

Ion exchange processes function by replacing undesirable ions of a liquid with ions such as H+ or OH from a solid material in which the ions are sufficiently mobile, usually some synthetic resin. Eventually the resin becomes exhausted and may be regenerated by contact with a small amount of solution with a high content of the desired ion. Resins can be tailored to have selective affinities for particular kinds of ions, for instance, mercury, boron, ferrous iron, or copper in the presence of iron. Physical properties of some commercial ion exchange resins are listed in Table 15.4 together with their ion exchange capacities. The most commonly used sizes are -20 + 50 mesh (0.8-0.3 mm) and -40 -h 80 mesh (0.4-0.18 mm). 

Rates of ion exchange processes are affected by diffusional resistances of ions into and out of the solid particles as well as resistance to external surface diffusion. The particles are not really solid since their volume expands by 50% or more. For monovalent exchanges in strongly ionized resins, half times with intraparticle diffusion controlling are measured in seconds or minutes. For film diffusion, half times range from a few minutes with 0. N solutions up to several hours with 0.0017V solutions. Film diffusion rates also vary inversely with particle diameter. A rough rule is that film 

Ion exchange materials have equilibrium exchange capacities of about 5meq/g or 2.27 g eq/lb. The percentage of equilibrium exchange that can be achieved practically depends on contact time, the concentration of the solution, and the selectivity or equilibrium constant of the particular system. The latter factor is discussed in Section 15.2 with a numerical example. 

Operating cycles for liquid contacting processes such as ion exchange are somewhat more complex than those for gas adsorption. They consist of these steps  

A rinse for recovering possibly valuable occluded process solution. 

Material Shaj e particles Bulk wet density (drained), kg/L Moisture content (drained). % by weight Swelling due to exchange, % Maximum operating temperature. °C Operating pH range Exchange capacity  

Anion exchangers strongly basic Polystyrene-based  

I When two temperatures are shown, the first applies to H form for cation, or OH form for anion, exchanger the second, to salt ion NOTE To convert kilograms per liter to pounds per cubic fort, multiply by 6.238 X 10 °F % °C + 32. 

5 (c) Approximate Selectivity Scale for Anions on Strong-Base Resins 

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Considerable interest has been generated in turbulence promoters for both RO and UF. Equations 4 and 5 show considerable improvements in the mass-transfer coefficient when operating UF in turbulent flow. Of course the penalty in pressure drop incurred in a turbulent flow system is much higher than in laminar flow. Another way to increase the mass-transfer is by introducing turbulence promoters in laminar flow. This procedure is practiced extensively in enhanced heat-exchanger design and is now exploited in membrane hardware design. 

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