Softener Design for Co-current and Counter-current Operation

To design a co-current or counter-current plant, determine the resin operating capacity based on a set of operating conditions and then apply correction factors for the specific conditions of the design.

- First, you have to have feed water quality analysis. Determine Hardness Concentration and TDS concentration of feed water in ppm as CaCO3. To determine concentration of TDS in ppm as CaCO3, determine concentration of all ions present in water (cations and anions) usually in ppm, and convert to ppm as CaCO3. To convert ppm to ppm CaCO3:

ppm of ion x (molecular weight of CaCO3/atomic weight of element)x(valence of element/valence of CaCO3)

Say we need to convert 9.2 ppm (mg/L) of Na ion to ppm CaCO3 (mg/L CaCO3),

MW of CaCO3 = 100

MW of Sodium = 23

Valence of Sodium = 1

Valence of CaCO3 = 2

Na (ppm CaCO₃) = 9.2 ppm x (100/23)x(1/2) = 20 ppm

- Set desired regenerant level, refer to resin data sheet (60-320 g/L). Determine Base Operating Capacity, C_B (Kgr/ft³) based on Figure 1:

- Set Service Flowrate (5-10 GPM/ft²) and determine Correction Factor C₁ at the set Service Flowrate and TDS concentration of feed water based on Figure 2:

- Calculate Operating Capacity (Kgr/ft³): 0.9(C_B x C₁), applying the 90% design factor.

- LEAKAGE is calculated as follows:

Determine Base Leakage, K_b @ set regenerant level based on Figure 3:

And Correction factor, K₁, for the TDS value based on Figure 4

Hence permanent (kinetic) leakage (as ppm CaCO3)= K_b x K₁

- Calculate the capacity required to handle the total exchangeable cation content of the feed for the desired feed rate and cycle time. First, determine the total cation content in the feed water as ppm CaCO3. Subtract from this value the sodium content of the feed as ppm CaCO3. The resulting number is the total exchangeable cation content as ppm CaCO3, divide this by 17.1 to obtain grains/U.S. gallon (grpg).

- Resin Volume = (feed rate, in GPM x Cycle time in minutes x total exchangeable cation, in grpg)/(operating capacity in Kgr/ft³ x 1000)

Calculate the flow rate per unit volume. If this number is outside the range of 1 - 5 gpm/ft³, modify the cycle length and resin volume to bring it within this range.

- Size the bed to this volume, keeping bed depths ≥36 inches (0.91 m). Calculate Softener Tank diameter from set Service Flowrate in #3. Resin Volume must be 60% of tank volume.

Packed Bed DI System: A new but proven counter-current configuration in Ion Exchange Technology

In a Packed Bed DI System, feed water enters the packed bed upward in the service cycle and downwards in the regeneration cycles. In the upward service cycle, the resin bed is lifted up in a compacted state, which minimizes the need for regular backwashing. As the water progressively comes into contact with the more regenerated resin, which is the resin in the upper portion of the bed since the regeneration is downward, high quality water production is ensured with lower ionic leakage.

Effluent from the regeneration of Packed Bed DI System is generally neutral due to the equivalence of cations to anions, also, acid and caustic are both introduced into the packed bed at the same time during regeneration.

Fully packed resin tanks give higher throughput compared to conventional systems. Counter current regeneration provides better water quality compared to co-current design. It also facilitates efficient and shorter regeneration time.