It should be free from turbidity and oil. It should be free from hardness causing and scale forming constituents like Ca and Mg salts. FORMATION OF DEPOSITS IN STEAM BOILERS AND HEAT EXCHANGERS 1.5.1 Sludge and Scale formation in boilers In a boiler, water is continuously converted into steam. Due to this continuous evaporation of water, the concentration of soluble matters increases progressively. Then the salts separating out from the solution in the order of their solubility, the lease soluble ones separating out first. (i) Sludge If the precipitate is loose and slimy it is called sludges. Sludges are formed by substances like MgCl2, MgCO3, MgSO4 and CaCl2. They have greater solubilities in hot water than cold water. Scale 1) If the precipitate forms hard and adherent coating on the inner walls of the boiler, it is called scale. 2) Scales are formed by substances like Ca (HCO3)2, CaSO4 and Mg (OH) 2. 1.5.2 Disadvantages (i) Wastage of fuels Scales have a low thermal conductivity, so the rate of heat transfer from boiler to inside water is greatly decreased. In order to provide a supply of heat to water, excessive or over-heating is done. This causes increase in fuel consumption. The wastage of fuel depends upon the thickness and the nature of scale. (ii) Decrease in efficiency Scales sometimes deposit in the valves and condensers of the boiler and choke them partially. This results in decrease in efficiency of the boiler.
Boiler explosion When thick scales crack due to uneven expansion, the water comes suddenly in contact with over-heated iron plates. This causes in formation of a large amount of steam suddenly. So sudden high-pressure is developed, which may even cause explosion of the boiler. 1.6 PREVENTION OF SCALE FORMATION (OR) SOFTENING OF HARD WATER The process of removing hardness – producing salts from water is known as softening or conditioning of water. Since water is a source for industrial purpose. It is mandatory to soften water to make it free from hardness producing substances, suspended impurities and dissolved gases, etc. Softening of water can be done by two methods. External treatment Internal treatment. EXTERNAL TREATMENT It involves the removal of hardness producing salts from the water before feeding into the boiler. The external treatment can be done by the following methods. Zeolite (or) Permutit process Zeolite (or) Permutit process Zeolites are naturally occuring hydrated sodium aluminosilicate minerals. The chemical formula is Na2O.Al2O3.XSiO2.YH2O. The synthetic form of zeolite is called permutit and is represented by Na2Ze. In this process the hard water is allowed to perlocate through sodium zeolite. The sodium ions which are loosely held in this compound are replaced by Ca2+ and Mg2+ ions. When zeolite comes in contact with hard water, it exchanges its sodium ions with calcium and magnesium ions of hard water to form calcium and magnesium zeolites. As sodium ions do not give any hardness to water, the effluent will be soft. The exhausted zeolite is again regenerated by treated with 5 to 10 percent of sodium chloride solution. CaZe + 2 NaCl → Na2Ze + CaCl2 MgZe + 2 NaCl → Na2Ze + MgCl2
Advantages No sludge is formed during this process. Water of nearly zero hardness is obtained. This method is very cheap because the generated permutit can be used again. The equipment used is compact and occupies a small space. Its operation is also easy. The process can be made automatic and continuous. Disadvantages This process cannot be used for turbid and acidic water as they will destroy the zeolite bed. This treatment replaces only the cations, leaving all the anions like (HCO3)– and (CO3)2– in the soft water. When such water is boiled in boilers, CO2 is liberated. Free CO2 is weakly acidic in nature and extremely corrosive to boiler metal. Na2CO3 + H2O→2NaOH + CO2 Due to the formation of sodium hydroxide, the water becomes alkaline and can cause cause caustic embrittlement. Water containing Fe, Mn cannot be treated, because regeneration is very difficult. This process cannot be used for softening brackish water. Because brackish water also contains Na+ ions. So, the ions exchange reaction will not take place. DEMINERALIZATION 1.8.1 Ion exchange or Demineralisation process Ion exchange or demineralisation process removes almost all the ions (both anions and cations) present in the hard water. The soft water, produced by lime-soda and zeolite processes, does not contain hardness producing Ca2+ and Mg2+ ions, but it will contain other ions like Na+, K+, SO42–, Cl– etc., On the other hand demineralised (DM) water does not contain both anions and cations. Thus a soft water is not demineralised water whereas demineralised water is soft water. This process is carried out by using ion exchange resins, which are long chain, cross linked, insoluble organic polymers with a micro process structure. The functional groups attached to the chains are responsible for the ion exchanging properties.
(i) Cation exchanger Resins containing acidic functional groups (–COOH, – SO3H) are capable of exchanging their H+ ions with other cations of hard water. Cation exchange resin is represented as RH2. Examples: Sulphonated coals Sulphonated polystyrene R–SO3H; R–COOH ≡ RH2 (ii) Anion Exchanger Resins containing basic functional groups (–NH2, –OH) are capable of exchanging their anions with other anions of hard water. Anion exchange resin is represented as R (OH)2. Examples: Cross-linked quaternary ammonium salts. Urea-formaldehyde resin. R–NR3OH; R–OH; R–NH2 ≡ R (OH)2 Process The hard water first passed through a cation exchange which absorbs all the cations like Ca2+, Mg2+ Na+, K+, etc. present in the hard water. RH2 + CaCl2 → RCa + 2HCl RH2 + MgSO4 → RMg + H2SO4 RH + NaCl → RNa + HCl The cation free water is then passed through a anion exchange column, which absorbs all the anions like Cl–, SO42, HCO3–, etc., present in the water. R' (OH) 2 + 2HCl → R'Cl2 + 2H2O R'(OH) 2 + H2SO4 → R'SO4 + 2H2O