Water Purification Methods & Water Softening

Module 7 Water purification methods - zeolites, ion- exchange resins and reverse osmosis Fuels and combustion -LCV, HCV, Bomb calorimeter (numericals), anti-knocking agents) Protective coatings for corrosion control: cathodic and anodic protection - PVD technique Chemic...

Module 7 Water purification methods - zeolites, ion- exchange resins and reverse osmosis Fuels and combustion -LCV, HCV, Bomb calorimeter (numericals), anti-knocking agents) Protective coatings for corrosion control: cathodic and anodic protection - PVD technique Chemical sensors for environmental monitoring - gas sensors  Hardness of water o Hardness of water is the characteristic of preventing lather formation of water with soap. Generally salts like chlorides, bicarbonates and sulfates of Ca2+, Mg2+ and Fe2+ make water hard. o This hard water on treatment with soap which is stearic or palmitic acid salts of sodium or potassium causes white precipitate formation of calcium or magnesium stearate or palmitate. o o Thus the cause of hardness is the precipitation of the soap and hence prevents lathering at first. When the hardness causing ions are removed as insoluble soaps, water becomes soft and forms lather.  Types of Hardness Permanent Hardness (or) noncarbonate Temporary Hardness (or) carbonate hardness hardness - Dissolved bicarbonate salts of Ca and Mg - presence of chlorides and sulphates of and other heavy metals such as iron Ca, Mg and other heavy metals) - Can be removed easily by boiling - Can’t be removed by simple boiling - Can be removed through zeolite, Lime- Ca(HCO3)2 CaCO3 + H2O + CO2 soda, ion-exchange processes Mg(HCO3)2 Mg(OH)2 + 2CO2  Water Softening methods The process of removing the hardness producing substance from the water is called softening of water In Industry three main methods are employed for softening of water – Lime soda process – Zeolite (permutit) process – Ion-exchange and Mixed bed ion-exchange process Lime-Soda process Soluble calcium and magnesium salts in water are chemically converted into insoluble compounds by adding calculated amount of lime [Ca(OH)2] and Soda [Na2CO3]. Calcium carbonate [CaCO3] and Magnesium hydroxide [Mg(OH)2] so precipitated, are filtered off. 1. Lime soda a) Batch process b) continuous process - Cold lime-soda 4 - Hot lime-soda  Zeolite or Permutit Process o Zeolite is hydrated sodium aluminium silicate having a general formula, Na2OAl2O3.xSiO2.yH2O. o It exchanges Na+ ions for Ca2+ and Mg2+ ions. o Common Zeolite is Na2OAl2O3.3SiO2.2H2O known as natrolith. o Other gluconites, green sand (iron potassium phyllosilicate with characteristic green colour, a mineral containing Glauconite)etc. are used for water softening. o Artificial zeolite used for water softening is Permutit. o These are porous, glassy particles having higher softening capacity compared to green sand. o They are prepared by heating china clay (hydrated aluminium silicate), feldspar (KAlSi3O8-NaAlSi3O8 – 5 CaAl2Si2O8) are a group of rock-forming tectosilicate  Natural Zeolite Natrolite Artificial Zeolite China clay Al2Si2O5(OH)4 Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8)  Zeolite process o Method of softening: Na2Ze + Ca(HCO3)2 2 NaHCO3 + CaZe Na2Ze + Mg(HCO3)2 2 NaHCO3 + MgZe Na2Ze + CaSO4 Na2SO4 + CaZe Na2Ze + CaCl2 2 NaCl + CaZe o Regeneration of Zeolite: CaZe (or) MgZe + 2 NaCl Na2Ze + CaCl2 or MgCl2 Brine solution 7 Zeolite process equipment diagram  Zeolite Process Advantages: o Residual hardness of water is about 10 ppm only o Equipment is small and easy to handle o Time required for softening of water is small o No sludge formation and the process is clean o Zeolite can be regenerated easily using brine solution o Any type of hardness can be removed without any modifications to the process Disadvantages: o Coloured water or water containing suspended impurities cannot be used without filtration o Water containing acidic pH cannot be used for softening since acid will destroy zeolite. 9  Ion-Exchange Process  Ion-exchange resins are insoluble, cross-linked, long chain organic polymers with a microporous structure and the functional groups attached to the groups are responsible for the ion-exchanging properties. o Cation exchange resins will exchange cations with H+. o Anion exchange resins will exchange anions with OH-. o Functional groups present are responsible for ion- exchange properties. o Acidic functional groups (-COOH, -SO3H etc.) exchange H+ for cations & 10 o Basic functional groups (-NH , =NH etc.) exchange  Ion-Exchange Process A. Cation-exchange Resins(RH+): - Styrene divinyl benzene copolymers - which on sulphonation or carboxylation, become capable to exchange their hydrogen ions with the cations in the water 11  Ion Exchange Process Styrene-divinyl benzene or amine-formaldehyde copolymers, which contain amino or quaternary ammonium or quaternary phophonium or tertiary sulphonium groups as an integral part of the resin matrix. These after treatment with dil. NaOH solution capable to exchange their OH¯ ions with the anions in the water 12  Ion Exchange Process The Process of Ion-exchange is: 2 RH+ + Ca2+/Mg2+ R2Ca2+/R2Mg2+ + 2 H+ (Cation exchange) R’OH- + Cl- R’+ Cl- + OH- (anion exchange) 2 R’OH- + SO42- R’2 SO42- + 2 OH- (anion exchange) 2 R’OH- + CO32- R’2 CO32- + 2 OH- (anion exchange) Finally, H+ + OH- H2O Regeneration of exhausted resins: Saturated resins are regenerated by treating with strong mineral acid or alkali respectively 13  Ion-exchange process e: Hard water should be first passed through the cation exchanger and on exchanger to avoid hydroxides of Ca2+ and Mg2+ getting formed 14 The outgoing water from the mixed-bed contains even less than 1 ppm of dissolved salts 15  The mixed bed deionizer consist of cation and anion exchange resins mixed together in a single pressure vessel. When water is passed through mixed bed it comes in contact, a number of times, with the two kinds of exchanges alternatively. As a result the net effect of mixed bed exchanger is equivalent to passing water through a series of several cation and anion exchangers. The quality of water obtained from mixed bed is appreciably higher than the water produced from two bed plants. Mixed bed exchange produce water with hardness less than 1 ppm Regeneration: The mixed bed is back washed by forcing water in the upward direction. This separate the cation and anion exchanges from the mixed bed. Being lighter the cation resin occupes upper part and the denser on at the bottom. Now they layers will be washed with NaOH and H2SO4 respectively to regenerate anion and cation exchange resins. After regeneration again they are mixed by forcing compressed air. Generally soften water (eg RO, etc) will be further purified by this method 16 Advantages & Disadvantages of ion- exchange process o Advantages: - Can be used for highly acid and highly alkaline water - Residual hardness of water is as low as 2 ppm. - Very good for treating water for high pressure boilers o Disadvantages: - Expensive equipment and chemicals - Turbidity of water should be < 10 ppm. Otherwise output will reduce; turbidity needs to be coagulated before treatment. - Needs skilled labour 17  Principle of osmosis and reverse osmosis When two solutions of unequal concentrations are separated by a semipermeable membrane, solvent will flow from lower to higher solute concentration This phenomenon can be reversed (solvent flow reversed) by applying hydrostatic pressure on the concentrated side  Osmosis Reverse Osmosis Hydrostatic pressure in excess Cellulose acetate of osmotic pressure is applied, Polysulfone the solvent flow reverses Polysulfone amide Polyamide Poly-acrylonitrile Advantages: Reverse OSMOSIS 1. Removes ionic, non-ionic, colloidal and high molecular weight organic matter 2. Removes colloidal silica 3. Low Maintenance cost - Life time (~2 years) 4. Easy membrane replacement 5. Low operating cost, high reliability  Reverse osmosis Reverse osmosis filters have a pore size around 0.0001 micron After water passes through a reverse osmosis filter, it is essentially pure water In addition to removing all organic molecules and viruses, reverse osmosis also removes most minerals that are present in the water Reverse osmosis removes monovalent ions, which means that it desalinates the water  Brackish Water The water containing dissolved salts with a peculiar salty taste – Salt Water Desalination of Brackish Water The process of removing common salt from water Electrodialysis Reverse osmosis  CORROSION and CORROSION CONTROL The secret of effective engineering lies in controlling rather than preventing corrosion, because its is impossible to eliminate corrosion. -Michael Henthorne  Corrosion Control Material selection:  The chosen metal should be as pure as possible because the presence of impurities enhance the rate of corrosion.  The choice of noble metals are preferable because they are highly resistant to corrosion.  Avoid the contact of dissimilar metals in the presence of a corroding environment.  If two dissimilar metals in contact have to be used, they should be as close as possible to each other in the electrochemical series.  Corrosion Control Proper designing  When anodic and cathodic materials are used together, then the area of anodic material should be large.  The anodic part should not be painted or coated because any damage in coating would cause rapid localized corrosion.  Whenever the direct joining of dissimilar metals, is unavoidable, an insulting fitting may be applied in-between them to avoid direct metal-metal electrical contact.  Angles, corners, edges etc. should be avoided in construction. For this reason L, T and U shaped structures should be avoided as far as possible some better shapes of L, T and U structure are given below:  Corrosion Control The material should not have sharp corners and recesses because they help in accumulation of impurities. It should be avoided by proper designing as show below. Corrosion Inhibitors Anodic Inhibitors Cathodic inhibitors