Ion exchange - Principles of Chemical Separations with Environmental Applications - page 223

Table 8.2 Particle size in US
mesh values and in mm [4].

US mesh Diameter (mm)

16-20 1.2-0.85

20-50 0.85-0.30

50-100 0.30-0.15

100-200 0.15-0.08

200-400 0.08-0.04

CO 2 3 , and SiO 2 4 ). The preference series for some common anions is similar to that
for strong-base exchangers, but OH − will fall farther to the left depending on the strength
of the reactive group on the resin [8]. They can be regenerated with NaOH, NH 4 OH and
Na 2 CO 3 , and they have much higher regeneration efficiencies than strong-base exchang-
ers. Weak-base exchangers are sometimes used in conjunction with strong-base exchang-
ers to minimize cost and to attract organics that might otherwise foul the strong-base
resins [4].

8.5.1 Physical characteristics of the resins

Most synthetic resins are granular with a spherical diameter of 0.04-1.0 mm. In the United
States, the particle sizes are listed according to standard screen sizes or “mesh” values.
Table 8.2 shows a comparison of mesh sizes and metric sizes; the most common size
ranges used in large-scale applications are 20-50 and 50-100 mesh [4].
The particle size has a significant effect on the hydraulics of an ion-exchange column.
In about half of all ion-exchange applications, the design is based on hydraulic rather than
chemical limitations (the allowable pressure drop dictates the smallest particle size used)
[4]. The size of the resin particles also affects the kinetics of ion exchange; the rate of
exchange can be proportional to the inverse of the diameter or the inverse of the square of
the diameter (kinetics are discussed later). This tradeoff between hydraulic and transport
considerations with respect to particle size is again similar to the discussion in Chapter 7:
Adsorption.

8.6 Equilibria

Ion exchange is a chemical reaction, and in theory will continue until equilibrium is reached
between the bulk solution and the solution within the pores of the resin. However, in most
real processes equilibrium is not achieved and the effluent solution from an ion-exchange
column is not in equilibrium with the influent solution, so kinetics as well as equilibrium