Tuesday, December 6, 2011
Tuesday, June 28, 2011
Cation and Anion Leakage and the Factors Affecting Them
As feed water flows down the cation bed, cations are exchanged for H+, thus reducing the pH. The higher the total dissolved solids (TDS), the lower the pH and the higher the leakage of sodium. Higher percentages of influent sodium also will increase the sodium leakage because more sodium will be left on the cation exchanger after regeneration. This is why it is not recommended to soften feed water going to a DI system. Since the acid produced by the cation exchanger affects leakage, water that is high in chlorides and sulfates will cause a higher Na+ leakage. Conversely, waters that are high in bicarbonate will give more favorable cation leakage characteristics. One way to reduce overall cation leakage is to increase the acid dosage during regeneration. In fact, the regenerant level is the only controllable variable. However, there is a rapid drop off in the economics for increasing the acid dose compared to the increase in capacity and reduction of leakage. Typical acid levels are approximately 8 pounds per cubic feet of resin.
On the other hand, anion resins love strong acidic anions such as chlorides and sulfates but have a tougher time removing the weak ionized acidic anions such as bicarbonates and silica. This especially is true if there is a considerable Na+ leakage from the cation, which converts to NaOH in the anion exchanger. Efficient operation of the anion exchanger is not concerned with the overall TDS level because the removal of the acidic anions results in a neutral pH, as shown in the reaction:
HCl + R*OH → RCl– + HOH(H2O)
However, the anion is affected adversely by high levels of weak acids (which we already have deemed beneficial to the operation of the cation.) Again, increasing the regenerant level will improve the operation of an anion but the gains are limited by economics. Typical caustic levels are approximately 8 to 9 pounds per cubic feet of anion.
Typical two-bed effluent with 400 to 500 ppm feed water would be a conductivity of about < 5 microsiemens, a pH of 9.5 and silica leakage of 20 ppb.
On the other hand, anion resins love strong acidic anions such as chlorides and sulfates but have a tougher time removing the weak ionized acidic anions such as bicarbonates and silica. This especially is true if there is a considerable Na+ leakage from the cation, which converts to NaOH in the anion exchanger. Efficient operation of the anion exchanger is not concerned with the overall TDS level because the removal of the acidic anions results in a neutral pH, as shown in the reaction:
HCl + R*OH → RCl– + HOH(H2O)
However, the anion is affected adversely by high levels of weak acids (which we already have deemed beneficial to the operation of the cation.) Again, increasing the regenerant level will improve the operation of an anion but the gains are limited by economics. Typical caustic levels are approximately 8 to 9 pounds per cubic feet of anion.
Typical two-bed effluent with 400 to 500 ppm feed water would be a conductivity of about < 5 microsiemens, a pH of 9.5 and silica leakage of 20 ppb.
Sunday, January 2, 2011
GUIDELINES FOR SELECTING RESIN ION EXCHANGE OR REVERSE OSMOSIS FOR FEED WATER DEMINERALISATION
Prepared by: Purolite International
FACTORS TO BE CONSIDERED
a) Reliability
Both RO and IX are well established reliable technologies with a good track of performance world wide. Future technical developments are not expected to have a major influence on plant and process costs.
b) Feed Water Pre-treatment Required
Both processes require pre-treatment to remove suspended solids to a low level to avoid fouling. However IX is more tolerant of suspended solids and RO requires additional pre-treatment by micro-filtration. Membranes are also subject to scaling by hardness present in the feed water and require either a softening plant as part of the feed water pre-treatment or the addition of anti-scaling chemicals.
c) Quality of Treated Water
IX can produce demineralised water with a conductivity of less than 0.5uS/cm from a simple SAC-SBA combination and less than 0.1μS/cm with the addition of a mixed bed SAC/SBA unit.
Even the best performing RO plants cannot meet the treated water quality of a simple IX plant and a subsequent IX unit is required to achieve boiler feed water quality. (SAC is strong acid cation resin, SBA is strong base anion resin).
d) Flexibility
Ion exchange plants tend to be more flexible than RO, for example in terms of performance over a wider range of temperature variations and the ability to recover from high suspended solids in the feed.
e) Fouling by Organics
Both RO membranes and IX resins can be fouled by organics present in the feed water. IX resins are much more easily cleaned than RO membranes without long plant shut down and use cheap cleaning chemicals; salt and sodium hydroxide.
However RO has a place in producing demineralised water and when used in combination with IX can produce the highest quality boiler feed water. The role of RO is in treating high TDS waters and in order to establish guidelines for selection of RO versus IX it is necessary to carry out detailed cost comparisons.
FACTORS TO BE CONSIDERED
a) Reliability
Both RO and IX are well established reliable technologies with a good track of performance world wide. Future technical developments are not expected to have a major influence on plant and process costs.
b) Feed Water Pre-treatment Required
Both processes require pre-treatment to remove suspended solids to a low level to avoid fouling. However IX is more tolerant of suspended solids and RO requires additional pre-treatment by micro-filtration. Membranes are also subject to scaling by hardness present in the feed water and require either a softening plant as part of the feed water pre-treatment or the addition of anti-scaling chemicals.
c) Quality of Treated Water
IX can produce demineralised water with a conductivity of less than 0.5uS/cm from a simple SAC-SBA combination and less than 0.1μS/cm with the addition of a mixed bed SAC/SBA unit.
Even the best performing RO plants cannot meet the treated water quality of a simple IX plant and a subsequent IX unit is required to achieve boiler feed water quality. (SAC is strong acid cation resin, SBA is strong base anion resin).
d) Flexibility
Ion exchange plants tend to be more flexible than RO, for example in terms of performance over a wider range of temperature variations and the ability to recover from high suspended solids in the feed.
e) Fouling by Organics
Both RO membranes and IX resins can be fouled by organics present in the feed water. IX resins are much more easily cleaned than RO membranes without long plant shut down and use cheap cleaning chemicals; salt and sodium hydroxide.
However RO has a place in producing demineralised water and when used in combination with IX can produce the highest quality boiler feed water. The role of RO is in treating high TDS waters and in order to establish guidelines for selection of RO versus IX it is necessary to carry out detailed cost comparisons.
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