Introduction to Ion Exchange Part 1

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Courtesy of Kinetico Incorporated

IMPURITIES IN WATER

Water is often referred to as the 'universal solvent' as it will dissolve small amounts of everything it touches. Water collects or dissolves a variety of mineral salts, organic compounds and suspended solids as it flows over the earth's surface and through the ground. These impurities make water undesirable for use in many applications. It is, therefore, important to properly treat or condition the water before using it.

Ion exchange is most often employed to remove mineral salts in solution and in some cases is used for removal of low molecular weight organic contaminants such as tannins or lignin’s. The most common mineral salts found in solution are bicarbonates (HC03-), sulfates (SO4-), chlorides (Cl-), nitrates (N03-) or silicates (Si2O5-) of calcium (Ca++), magnesium (Mg++), sodium (Na+) or potassium (K+).

HARDNESS IN WATER

Hardness, one of the most common impurities in water, is defined as anything that will react with soap to form a scum or curd. Calcium, magnesium, iron are common minerals that will form a soap curd and therefore, are defined as hardness. Conversely, water devoid of these specific minerals is referred to as “soft” water. Calcium and magnesium may be present as part of several salts and when associated with bicarbonate (HC03-) or carbonate (C03-) is called carbonate hardness. Carbonate hardness can precipitate (become insoluble) and form scale simply by heating it (tea kettle, water heater, etc.). Because of this chemical characteristic, the carbonate hardness is often called temporary hardness.

When calcium and magnesium salts are present as sulfate (SO4-), chloride (Cl-) or nitrate (N03-), the hardness is called non-carbonate. Non-carbonate hardness will not precipitate by simply heating and thus is sometimes called permanent hardness. Both temporary and permanent hardness can exist together in a water supply.

Each different mineral ion has unique characteristics and reacts uniquely. An analysis of a solution containing several different ions cannot be used to predict chemical reactions unless the individual ions are expressed in similar terms. Equivalence serves as a common denominator so that several ions, all expressed the same way, can be added together and used collectively. Similarly, in mathematics, we use common denominators to calculate fractions. The use of calcium carbonate equivalence is the method of converting the value of each element to a common denominator.

Total hardness is the sum of calcium and magnesium. A typical water analysis will show calcium, magnesium, and total hardness in milligrams per liter (mg/l) or grains per US gallon (gpg). When the term hardness is used, the value is always expressed in terms of calcium carbonate equivalence (as CaC03). Most water analysis will report calcium and magnesium individually in mg/l as the elements. In addition they will report either total hardness in grains per gallon expressed as a calcium carbonate equivalent or as Total Hardness which is a total sum of hardness in mg/l as CaCO3. To convert Total Hardness to grains per gallon simply divide by 17.1 .

PRINCIPLES OF ION EXCHANGE

Water softening by ion exchange uses highly efficient sulfonated polystyrene base cation exchange resin. As water passes through a bed, or column, of resin the hardness ions are removed and replaced with sodium ions. Thus when hardness is removed the hard water is changed to soft water.

Cation exchange water softening resins are actually insoluble compounds of polystyrene¬divinyl benzene sulfonate. It may be visualized as a plastic sphere having tiny pores or microscopic channels through which water can pass. The bead itself is an anion with negatively charged exchange sites. The exchange sites hold positively charged cations, such as sodium, potassium, calcium, magnesium, iron, manganese, hydrogen, ammonium, and other metal ions.

Due to the charge characteristics inherent in ion exchange resins, water softening resin does have preferences for ions. This preference is called selectivity, or affinity. In general, cation (softening) resin has the greatest affinity for cations which have the largest number of positive charges. Secondly, within a group of ions having an identical number of positive charges, affinity will increase with increasing atomic numbers. For example cation resin will have a greater affinity for aluminum (three positive charges), than for calcium (two positive charges), than for sodium (one positive charge). The sequence below shows the order of affinity of standard cation resin. AI> Ba > Ca > Cu > Zn > Fe > Mg > Mn > NH4 > Na > H

In general, any ion in the sequence will displace any ion to its right. This characteristic of ion exchange resins can serve as a useful tool in processes aimed at separating one ion from another. But it also creates a problem commonly referred to as breakthrough or leakage. It is quite possible that one ion can be released from the resin bed while another is still being taken on. The release of any unwanted ion can limit the extent of the softening application. For example most common cation resin holds calcium more tenaciously than it holds magnesium, so magnesium hardness will break through first.

So far we have discussed the water softening process in relation to only one resin bead. In reality, a softener tank contains millions of resin beads in a bed, or column, which in industrial applications can be several feet thick. As water passes downward through the resin, the ion exchange reactions begin at the top of the resin column in a thin reaction zone. As the top layers become full of hardness - as the beads become exhausted - the reactions continue farther down the column where beads are still in the sodium form. As this reaction zone progresses downward it increases in thickness. The actual thickness of the zone is determined by water velocity (flow rate), water hardness, and resin bead size. The reaction zone will vary and may be only a few inches thick under the right operating conditions.

When the leading edge of the reaction zone is close to bottom of the resin column, hardness leakage will occur and will continue to increase. When leakage increases to a predetermined level, usually dictated by the application, the column is considered exhausted and the resin must be regenerated. Ideally the softening process would go off line to regenerate just moments before hardness leakage could be detected. Recalling the order of affinity discussed earlier, the first hardness leakage will be primarily magnesium with some calcium present. Calcium displaces magnesium in the upper exhausted beads forcing this magnesium to move further down into the resin column to find a new exchange site still holding sodium. This calcium-magnesium exchange continues until water flow is stopped. If a vertical sample of the resin bed were analyzed in the lab, the top layers would be rich in calcium, the lower layers would be rich in magnesium, and the resin in between would contain both. This selectivity phenomenon is sometimes called stratification and is similar to chromatographic separation. If water continues flowing through the softener, all magnesium in the bed can eventually be replaced with calcium.

The Exchange Process

The number of exchange sites occupied by a single cation depends upon the number of positive charges on the cation. For example, calcium and magnesium each have two positive charges so each occupies two ion exchange sites. Sodium, potassium, ammonium, and hydrogen• have only one positive charge so each occupies only one site.

Before the softening process begins, sodium ions occupy all the exchange sites. As hard water passes around the resin beads, the hardness ions displace the sodium ions and occupy the exchange sites previously occupied by sodium. This interchange of ions is called ion exchange.

The process works best when the influent water is less than 1000 mg/l total dissolved solids (TDS) because the resin bead has a greater affinity for hardness than it does for sodium. As the TDS concentration goes above 1000 mg/l, the water most often contains higher values of naturally occurring sodium. In raw waters with high sodium levels the chance for hardness leakage increases.

Each resin bead has a fixed number of exchange sites. As water is processed all the sites will become filled with calcium and magnesium ions. At this point the bead is exhausted and the calcium and magnesium ions have to be replaced with sodium so that the softening process can be repeated. The process of removing hardness ions and replacing them with sodium ions is called regeneration.

As explained cation resin has a natural preference for hardness ions. While ion exchange resin beads are porous and will allow water transport through the structure there is little actual water flow through the bead itself. Rather most of the hardness first attaches to the outside or shell area of the resin and as the bead surface becomes saturated, hardness migrates towards the core of the resin in a transfer phenomenon that goes from outside to inside.

Once the resin is fully saturated with hardness minerals we must recondition or “regenerate” the resin back to the sodium form. The resin has a preference for hardness minerals therefore to overcome this preference we must use a strong solution of sodium ions forcing the hardness off the resin beads. The most common and economical sodium salt is sodium chloride (NaCl) therefore it is used most often in the regeneration of water softeners.

Recall that hardness removal occurs best when total dissolved solids are less than 1000 mg/l (ppm). A 10% salt brine solution is 100,000 mg/l and this high concentration of brine is strong enough to overcome the resin's affinity for hardness and will place sodium ions back on the exchange sites.

The exact number of exchange sites that sodium can occupy is called the stoichiometric quantity. Regenerating with this exact number of sodium ions that the resin holds will not place sodium on all the exchange sites. It takes three times the stoichiometric quantity, or 300%, of sodium in a 10% salt brine solution to place sodium on all the sites. Obviously, 100% of the stoichiometric amount is consumed and 200% is wasted. This explains why the softener regeneration waste contains salt.

CAPACITY

Common cation resin has an ultimate (theoretical) capacity of about 43,000 grains per cubic foot. However, only about 75% of this capacity can be used efficiently through normal regeneration procedures when installed in treatment equipment. To use any of the remaining 25% of capacity requires excessive amounts of salt and increased regeneration time to obtain very small capacity increases making such efforts economically undesirable. The most common method used to regenerate water softener resin is called “co-current” or down-flow regeneration. Single tank softeners or co-current units use raw (hard) influent water mixed with the brine to regenerate resin in the down-flow direction. This requires the brine to displace all the hardness in the upper parts of the resin bed and “push” that hardness to drain down through the entire bed. Care must be taken to ensure hardness does not get retained in the lower portions of the bed therefore a generous amount of brine is used to minimize this possibility.

Conversely there are newer technologies that use an up-flow or “counter-current” brine process. This means that soft water containing brine solution is introduced at the bottom of the softener vessel during regeneration. This up-flow regeneration eliminates the potential for hardness to be trapped at the lower portion of the bed. The counter current regeneration using soft water improves regeneration efficiency resulting in salt savings as well as improved water quality from the softener system.

Water softening resin is most economical to regenerate when it is not completely exhausted. The most efficient setting for regeneration utilizes 66% of the resins total capacity and will regenerate with 60% less salt than if run to exhaustion. In co-current or down flow regeneration systems the resin has a maximum capacity of 30,000 grains of hardness removal (as CaC03) per cubic foot when regenerated with 15 pounds of sodium chloride (salt) per cubic foot. When operated at its most efficient level the resin will give 20,000 grains of hardness removal while regenerating with only 6.0 lbs of salt per cubic foot of resin. Running a system at its most economical settings yields definite savings to the owner of the system.

The use of counter current regeneration further lowers the salt used per cubic foot thus yielding savings for the operator of the water softener while simultaneously provides better water quality and co-current regenerating units.

The next article in this series will explore the different regeneration methods to optimize capacity and reduce brine discharge from ion exchange water softeners.

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