Comprehensive Guide to Water Softening and Demineralization Processes for Engineering Students

Comprehensive Guide to Water Softening and Demineralization Processes for Engineering Students

• 1.
  1 Water Softening -3(Demineralization Process) 1st Year B Tech Students Dr. Vivek Pandey Rajasthan Technical University Kota Rawatbhata Road, Akelgargh, Kota-324010
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  Demineralization Softening Process Demineralization(DM)process is the ultimate technology for softening hard water. This is also called Deionization softening process. Water obtained from DM softener is mineral free, means water without any cationic and anionic impurities. Treated water is called Demineralized Water in which all the soluble salts of Na/K/Ca/Mg/Al/Fe etc. present in hard water has been totally removed. The efficiency of DM softening process is best among all the available softening processes and the soft water has hardness up to 0 to 2 ppm only. Demineralized water finds wide application in generating steam in boilers, electricity and in different process industries and also as coolants. Demineralization is a physical process which works on the principle of reversible ion exchange where all types of, cationic impurities (Na+/K+/Ca2+/Mg2+/Fe2+/ Al3+ etc.) and anionic impurities (Cl-/HCO3 -/SO4 2-etc.), are replaced by H+ and OH- ions, respectively, available in the softening material of DM plant. Softening material used in DM softeners are ion exchange polymeric resins and commonly used resin is styrene divinylbenzene coploymer which is water insoluble, stable, nonreactive, porous and are of high molecular weight. These water softening ion exchange resins have acidic H+ and basic OH- functional groups which reversibly replaces all the cationic and anionic impurities, respectively, present in hard water. Demineralizers gives soft water of exceptionally high purity to be used in industries. Structure of Styrene Divinylbenzene Copolymer
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  Softening Process DM softeneris a two column softener where one is cation exchange used to replace cationic impurities with its acidic H+ function group while other is anion exchange column used to replace anionic impurities with its OH- functional groups. The working of cation and anion exchange column's can be described as: CationExchange Column Styrene divinylbenzene copolymer after its carboxylation or sulphonation reactions, becomes cation exchange resin, which have H+ acidic functional groups used for the reversible replacement of cationic impurities present in hard water. The resin is represented as RH+ where R represents polymer matrix and H+ is freely available acidic functional group to be used for the removal cations from the water. Cation Exchange Resin Softening Reactions The softening reactions taking place when hard water is allowed to pass through RH+ resin of cation exchange column are as follows: 2RH + + MgCl2 → R2Mg + 2HCl 2RH + + CaSO4 → R2Ca + H2SO4 RH + + NaHCO3 → RNa + H2CO3 FreshResin ExhaustedResin In above reactions RH+ represents fresh Resin while R2Mg/R2Ca/RNa represents exhausted resin after removal of cations from the hard water while acids(HCl/H2SO4/H2CO3) are added into the treated water. The exhausted resin is further regenerated with H+ acidic functional groups for further replacement or removal of cationic impurities of hard water.
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  Regeneration of CationExchange Resin The exhausted resin R2Mg/R2Ca/RNa after replacing cations of hard water, it can be regenerated by spraying the HCl solution through the injector. When HCl solution passes through the exhausted resin, having Na+/K+/Ca2+/Mg2+/Fe2+/ Al3+ etc. cations, are reversibly replaced by the H+ ions of HCl. The exhausted/ saturated resin can be regenerated again and again and the available H+ acidic functional groups will further be used for removing cationic impurities from the hard water. The reactions taking place during regeneration of exhausted cation exchange resin are as follows: R2Mg + 2HCl → 2RH+ + MgCl2 R2Ca + 2HCl → 2RH+ + CaCl2 RNa + HCl → RH+ + NaCl Exhausted Resin Regenerated Resin In the above reactions CaCl2/MgCl2/NaCl released after regenerating the exhausted resin are removed through the sink valve of the cation exchange column of the softener during the regeneration process. Cation Exchange Column
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  Anion Exchange Column Styrenedivinylbenzene copolymer having quaternary ammonium (-NR3) group after its reaction with NaOH, becomes anion exchange resin, which have OH- basic functional groups used for the reversible replacement of cationic (Cl-/CO3 -/SO4 2-) impurities present as acids in the treated water obtained from cation exchange column. The resin is represented as ROH- where R represents Polymer matrix and OH- is freely available basic functional group to be used for the removal anions from the treated of cation exchange column. Softening Reactions The softening reactions taking place when treated water of cation exchange column is allowed to pass through ROH- resin of anion exchange column, are as follows: ROH - + HCl → RCl + H2O 2ROH - + H2SO4 → R2SO4 + H2O 2ROH - + H2CO3 → R2CO3 + H2O FreshResin ExhaustedResin In above reactions ROH- represents fresh Resin while RCl/R2SO4/R2CO3 represents exhausted resin after removal of anions from the treated water and the acids present in the treated water in cation exchange column is removed and we get only H2O from the anion exchange column of the softener. The exhausted resin is further regenerated with OH- basic functional groups for further replacement or removal of anionic impurities of treated water. Anion Exchange Resin
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  Regeneration of AnionExchange Resin The exhausted resin RCl/R2SO4/R2CO3 after replacing anions can be regenerated by spraying the NaOH solution through the injector. When NaOH solution passes through the exhausted resin, having Cl-/CO3 -/SO4 2- anions are reversibly replaced by the OH- ions of NaOH. The exhausted/ saturated resin can be regenerated again and again and the available OH- basic functional groups will further be used for removing anionic impurities of treated water. The reactions taking place during regeneration of exhausted anion exchange resin are as follows: RCl + NaOH → ROH- + NaCl R2SO4 + 2NaOH → 2ROH- + NaSO4 R2CO3 + 2NaOH → 2ROH- + Na2CO3 Exhausted Resin Regenerated Resin In the above reactions NaCl/Na2SO4/Na2CO3 released after regenerating the exhausted resin are removed through the sink valve of the anion exchange column the softener during the regeneration process. Anion Exchange Column
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  Demineralization Softener
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  Comparison of Lime-sodaProcess/Zeolite/Demineralization Processes