Page 348 - Water and wastewater engineering

ION EXCHANGE 8-17

The column should be operated long enough to achieve complete saturation of the bed
through several cycles of service and regeneration. To determine the optimum SFR, the flow rate
must be varied during the saturation loading tests. The main goal in determining the optimum
SFR is to reduce the capital cost of the column.
In the simplest expression, the resin volume required to treat a given flow rate of water is

Q
V R (8-22)
SFR
One method for estimating the resin mass is based on the principle of mass balance. It is
illustrated in the following example.

Example 8-2. As part of the preliminary design for a softening plant, a sodium-based ion ex-
change column is to be evaluated. For the evaluation of alternatives, estimate the mass of moist
resin required to soften the Hard Times water (Example 7-6) to a hardness of 80 mg/L as CaCO 3 .
3
The design flow rate is 275 m /d. Assume that there is no leakage from the column, that is,
C treated 0.0 mg/L as CaCO 3 , that the moisture content of the resin is 44%, and the operat-
ing temperature is 10 C. Also assume that iron and turbidity concentrations are negligible. The
manufacturer’s resin operating capacity to breakthrough is 67% of the exchange capacity.
The laboratory scale column was 7.5 cm in diameter and the height of the resin in the column
3
was 150.0 cm. The resin density on a moist basis is 0.85 g/cm . The moisture content is the same
3
as the full scale column. The flow rate through the column was 0.18 m /h.

Solution:
a. Begin by computing the meq of hardness removed per g of resin on a dry weight basis.
The mass of dry resin in the column is computed from the column dimensions, the unit
weight of the resin, and the moisture content of the resin.

⎡ (7 5 cm ) ⎤ 3
2
.
⎢ ⎥ (150cm )(0 85 g/cm )(10 44) 3 154 35, . g
.

.
)
⎣ 4 ⎦
b. From the breakthrough curve of the laboratory column ( Figure 8-5 ) the meq of hardness
removed at breakthrough ( V ) was 15,444 meq.
b
c. The meq/g of dry resin is
15 444 meq
,
489 meq/g
.
,
.
3 154 35 g
2
2
d. The total hardness of the Hard Times water is equal to the sum of the Ca and Mg or
238 mg/L as CaCO 3 90.6 mg/L as CaCO 3 328.6 mg/L as CaCO 3 .
e. A material balance on the flow downstream of the ion exchange column is used to deter-
mine the bypass flowrate.