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.

Dealkalization by Ion Exchange

Dealkalization is the reduction of Alkalinity in water. Alkalinity is caused by bicarbonates, carbonates or hydroxides in water. Two primary methods of Dealkalization by Ion Exchange are executed using the ff:

a. Weak Acid Cation (WAC)
b. Strong Base Anion (SBA) Chloride Form

Strong Base Dealkalization method utilizes SBA resins in the Chloride (Cl) form and is the most commonly used method for Dealkalization in the USA. The SBA method is most commonly used to dealkalize water for commercial applications such as boiler feed or RO pretreatment. It can also be used to dealkalize water for residential applications.

Weak Acid Dealkalization is preferred when the influent water is high in hardness and alkalinity and has hardness to alkalinity ratio of 1 or more. This process uses weak acid cation resin to exchange hydrogen for hardness that is associated with alkalinity. The treatment is most effective if followed by degasification to remove CO2.

Factors that Affect Resin Life

Ion exchange resins are manufactured to last for a long time. However, there are factors that can impact resin life. Some of the major factors are described below.

Temperature
Ion exchange resins have a recommended maximum operating temperature as indicated in their product data sheets. These temperature maxima are intended only as guides. Thus, a temperature limitation does not mean that the resin will be unstable above and stable below this temperature. It should also be recognized that thermal degradation is proportional to the product of time and temperature. When exposed to higher than the recommended temperature, however, the resin will often lose its functional groups, which will result in loss of capacity and reduced resin life.

Oxidation
Oxidants attack the polymer crosslinks, which weakens the bead structure, or by chemically attacking the functional groups. One of the most common oxidants encountered in water treatment is free chlorine (Cl2). Hydrogen peroxide (H2O2), nitric acid (HNO3), chromic acid (H2CrO4), and HCl can also cause resin deterioration. Dissolved oxygen by itself does not usually cause any significant decline in performance, unless heavy metals and/or elevated temperatures are also present to accelerate degradation, particularly with anion exchange resins.

Although weak base anion resins are more stable than strong base anion resins, they can oxidize and form weak acid groups. When this occurs, the resin tends to retain sodium and requires a greater than normal volume of rinse water following regeneration.

Chemical attack on a cation exchange resin usually results in the destruction of the polymer crosslinks, resulting in an increase in water retention capacity and a decrease in the total wet volume exchange capacity.

Fouling
It is a irreversible sorption or the precipitation of a foulant within resin particles can cause deterioration of resin performance. Common foulants for resins are Silica and Iron. It is better to prevent fouling by removing the foulant before the water flows through the resin beds, rather than try to clean the foulant from the resin. Where fouling conditions are prevalent, proper resin selection can minimize resin fouling.

Osmotic Shock
Exposure of resins to high and low concentrations of electrolytes can cause resin bead cracking and splitting due to the alternate contraction and expansion of the bead. Over time, there may be significant reduction in particle size and an increase in resin fines, causing increased pressure drop across the resin bed during system operation and subsequent resin losses during backwash and regeneration. Ion exchange resin particle size is an important factor related to osmotic shock. Smaller beads are more resistant to breakage than larger particles.

Physical Degradation
Bead breakage due to mechanical attrition can occur when the resin is subjected to unusual mechanical forces, such as a crushing valve, a pump impeller, or an abrasive action during the movement of resin particles from one vessel to another. The broken beads will maintain the same operating capacity as whole perfect beads, but they are more prone to fluidization during backwash, and may be lost. In addition, the small fragments will fill the void spaces between the whole resin beads, resulting in increased pressure drop across the bed. Large beads are more subject to mechanical attrition than smaller ones.

Radiation
Since ion exchange resins are organic polymers, they can be affected by radiation. Generally, cation exchange resins are adequately stable for almost all reasonable applications involving radioactivity. Anion exchange resins are less stable although generally adequate for use in radiation fields.

A new innovation in resin production is now in full blast..the Shallow Shell Technology. It creates high efficiency ion exchange resins. Under a microscope, “Shallow Shell” resins look very different from other resins because the resins have inert core. Only the outer shell is functionalized which shorten the ion exchange diffusion path. This leads to more efficient ion exchange and regeneration and better handling of iron and organic foulants.

With increasing demand for better performing resin coupled with lower operating costs, shallow shell resins is seen as a solution. These resins enable a more complete regeneration and provide a higher, more efficient utilization of the regenerant, lower leakage, and reduced rinse water requirements. When compared to conventional softening or demineralization resins, regenerant cost are seen to reduce by 20 to 50%, without sacrificing capacity.

Advantages of Shallow Shell Resin:

• Higher Recovered Capacity


• Lower Leakages at All Regenerant Levels


• Better Iron Removal


• Lower Rinse Requirements


• No equipment Modifications Needed


• Excellent For High TDS Waters


• Shorter Regeneration Cycles


• Superior Physical Strength


• More Resistant to Oxidation


• Lower Iron and Organic Fouling

To regenerate ion exchange without the use of commercial salt is highly desirable. For ion exchange water softeners treating brackish water feed to RO plants, such a solution already exists. By using shallow shell softening resin and some engineering, the reject from the RO can be used as “free regenerant” brine to efficiently regenerate this unique resin while adding no extra salt to the environment.

(Shallow Shell Technology)SST resin exhibit much higher regeneration efficiency than standard resin, permitting the use of more dilute brine concentrations and lower salt dosages than recommended for standard resins. Reason to this higher regeneration level is the unique outer shell and inner core structure of the resin bead. Ion exchange takes place only in the shell area with the core being totally inert. The diffusion path for cations is therefore shorter than that for standard resin, divalent cations (e.g. calcium, magnesium, barium, strontium) are not exchanged deep in the core of the beads unlike standard resin in which divalent cations migrate deep into the center of the resin beads. The efficiency of removal of these deeply trapped divalent cations essentially determines how well the resin performs during the next service cycle. With SST resin, the time for the brine to diffuse to the shell-core interface is lower, resulting in more highly regenerated beads.

Pilot studies showed that brine concentrations as low as 1% can be used to regenerate
shallow shell resin.

Resin Longevity: Expected Life Span of Ion Exchange Resins

The operating life of ion exchange resin depends on several factors. Degradation of resin can be attributed to mechanical, osmotic or thermal shock; temperature; dissolved oxygen; and chemical oxidation due to attack of chemical like chlorine.

DI resins usually last for many years. In general, cation resin for water softening and demineralization may last 5 to 10 years. Anion resins last anywhere from 3 to 5 years and are dependent on operational conditions. Some of the resin beads break during the swell cycle when regenerating. Moreover, resin life is partially dependent on the number of regenerations and partially on the quantity of oxidizers passed through the column.

You can prolong the life of your resin provided the following circumstances are met:

- Adequate pre-treatment is in place, i.e. organics and suspended solids are removed and kept to minimum.

- Chlorine content in feed water is zero to undetected (determined through water quality analysis). In the presence of chlorine or any oxidant, ion exchange resins will breakdown prematurely.


- Low levels of iron in feed water. Cation resin removes ferrous iron but removing the iron off of the resin is a difficult which will result in loss of capacity overtime due to iron being embedded into the cation bead.

- There is no sudden and significant increase in raw water quality that would affect the performance of pre-treatment system which will in turn affect the quality of feed water to the ion exchange resin. Ion exchange resin system is designed based on feed water quality, therefore include safety factor in the design to handle occasional “spikes” in feed water quality.


- Operators are aware of the proper operation and maintenance of ion exchange system.

Because many variables and factors are involved, it's difficult to predict the life span of a resin and we can only provide you with what is the expected life span given ideal conditions; in critical applications it’s best to start analyzing and benchmarking the resin at least once per year.