UNIT-II Water Technology

Transcription

1 External Treatment of Water:- The process of removing hardness producing salts from water, is known as softening of water. In industry, main three methods employed for softening water are as well as oil, if present in water. Cold lime-soda process provides water, containing a residual hardness of 50 to 60 ppm. 1. Lime-Soda process 2. Zeaolite 3. Ion exchange process Lime-soda process:- In this method, the soluble calcium and magnesium salts in water are chemically converted into insoluble compounds, by adding calculated amounts of lime Ca(OH) 2 and Soda (Na 2 CO 3 ). Calcium carbonate (CaCO 3 ) and magnesium carbonate (Mg(OH) 2, so-precipitated, are filtered off. Lime removes temporary hardness, permanent magnesium, mineral acids and dissolved CO 2. Soda removes the calcium permanent hardness. Cold lime soda process:- In this method, calculated quantities of chemical (lime and soda) are mixed with water at room temperature. At room temperature the precipitates formed are finely divided, so they do not settle down easily and cannot be filtered easily. Consequently, it is essential to add small amounts of coagulants (like alum, aluminum sulphate, sodium aluminate, etc. ), which hydrolyse to flocculent, gelatinous precipitate of aluminum hydroxide, and entraps the fine precipitates. Use of sodium aluminate as coagulant also helps the removal of silica METHOD:-Raw water and calculated quantities of chemicals (lime+soda+coagulant) are fed from the top into the inner vertical circular chamber, fitted with a vertical rotating shaft carrying a number of paddles. As the raw water and chemicals flow down, there is a vigorous stirring and continuous mixing, whereby softening of water takes place. As the softened water comes into the outer co-axial chamber, it rises upwards. The heavy sludge or precipitated floc settles down in the outer chamber by the time the softened water reaches up. the softened water then passes through a filtering media which is usually made up of wood fibers to ensure complete removal of

2 sludge. Filtered soft water finally flows out continuously through the outlet at the top. Sludge settling at the bottom of the outer chamber is drawn off occasionally. ii) HOT LIME-SODA PROCESS:- This process involves treating water with softening chemicals at a temperature of 80 to 150 ᴼC. Hot lime-soda plant consists of essentially 3 parts:- (a) A reaction tank in which raw water, chemicals and steam are thoroughly mixed; (b) A conical sedimentation vessel in which sludge settles down; (c) A sand filter which ensures complete removal of sludge from softened water. Advantages: Since hot process is operated at a temperature close to the boiling point of the solution, so (a) The reaction proceeds faster; (b) The softening capacity of water increases to many fold; (c) The precipitate and sludge formed settle down rapidly and hence, no coagulants are needed; (d) Much of the dissolved gases like carbon dioxide are driven out of the water; (e) Viscosity of softened water is lower, so filtration of water becomes much easier. This in-turn increases the filtering capacity of filters; (f) Hot lime-soda process produces water of comparatively lower residual hardness of 15 to 30 ppm. (2) ZEOLITE OR PERMUTIT PROCESS (Na 2O.Al 2O 3.xSiO 2. yh 2O):- Zeolite is hydrated sodium alumino silicate, capable exchanging reversibly its sodium ions for hardness-producing ions in water. Zeolites are also known as permutits. Zeolites are of two types:- (i) (ii) Natural zeolites are nonporous. Ex- natrolite, (formula). Synthetic zeolites are porous and possess gel structure. They are prepared by heating together china clay,

3 feldspar and soda ash. Such zeolites posses higher exchange capacity per unit weight than natural zeolites. stopped and the exhausted zeolite is reclaimed by treating the bed with a concentrated (10%) brine (NaCl) solution. PROCESS:- For softening of water by zeolite process, hard water is percolated at a specified rate through a bed of zeolite, kept in a cylinder. The hardness-causing ions are retained by the zeolite as CaZe and MgZe; while the outgoing water contains sodium salts. Reactions taking place during the softening process are:- Na 2 Z + Ca 2+ CaZ + 2Na + Na 2 Z + Mg 2+ MgZ +2Na + (Ca 2+ /Mg 2+ )Z + NaCl Na 2 Z + CaCl 2 or MgCl 2 The washings containing calcium chloride and magnesium chloride are led to the drath and regenerated zeolite bed thusobtained is used again for softening purpose Advantages Regeneration:- After sometime, the zeolite is completely converted into calcium and magnesium zeolites and it ceases to soften water, i.e., it gets exhausted. At this stage, the supply of hard water is 1. Hardness of water is removed and it is about 10 ppm in the soft water obtained by this process. 2. It is easy to operate and occupies less space 3. Sludge or scales are not formed 4. This process is cheap because regenerated permutit is used again. 5. It can work under pressure.

4 Limitations: 1. Turbidity and suspended matter should not be directly fed to the zeolite softner because the pores of the zeolite bed will be clogged and the rate of flow will be unduly decreased. 2. Water containing excess of acidity or alkalinity may attack the zeolite. Preferably the ph of water must be around 7 3. Water containing large quantities of Fe 2+ and Mn 2+ when passes through the zeolite bed are converted into their respective zeolites which cannot be easily regenerated. 4. Hot water should not be used as the zeolite tends to dissolve in it. 5. The process does not remove the anions such as bicarbonates, chloride and sulphates. (3) ION EXCHANGE AND DE-MINERALISATION PROCESS: In this method all the cations and anions are completely removed from water and water of zero hardness is obtained. Ion-exchange resins are insoluble, cross-linked, long chain organic polymers with a micro porous structure, and the functional groups attached to the chains are responsible for ion-exchange properties. The ion-exchange resins may be classified as: contact. Cation exchange resins are mainly styrene-divinyl benzene copolymers, which on sulphonation or carboxylation, become capable to exchange their hydrogen ions with the cations in the water. E.g., sulphonated coal, tannin formaldehyde resin etc., Cation exchange resin Anion exchange resin Anion exchange resins (ROH - ):- These resins contain basi functional groups like NH 2, =NH etc., they exchange hydroxyl ions for other anions preset in water. Anion exchange resins are styrene divinyl benzene or amine-formaldehyde copolymers, which contain amino or quaternary ammonium or quaternary phosphonium or tertiary sulphonium groups as an integral part of the resin matrix. These, after treatment with dil. NaOH solution, become capable to exchange their OH- anions with anions in water. Cation exchange resins (RH + ):- Resins containing acidic fundamental groups exchanging H + ions with other cations, which are capable of comes in their

5 Thus, the water coming out from the exchanger is free from cations as well as anions. Ion-free water is known as de-ionized or demineralised water. REGENERATION: When capacities of cation and anion exchangers to exchange H + and OH - ions respectively are lost, they are then said be exhausted. The exhausted cation exchange column is regenerated by passing a solution of dil. HCl or Dil. Sulphuric acid solution. The regeneration can be represented as: PROCESS: The hard water is passed first through cation exchange column, which removes all the cations like Ca 2+ and Mg 2+ from it, and equivalent amount of H + ions are released from this column of water. Thus, Ca 2+ +RH 2 RCa + 2H + Mg 2+ +RH 2 RMg + 2H + After cation exchange column, the hard water is passed through anion exchange column, which removes all the anions like SO 4,Cl - etc., present in the water and equivalent amount of OH - ions are released from this column to water. Thus,H+ and OHions released from cation and anion exchange columns respectively get combine to produce water molecule. The column is washed with deionised water and washing is passed through sink or drain. The exhausted anion exchange column is regenerated by passing a solution of dil. NaOH. The regeneration can be represented as RCa + 2HCl RH 2 + CaCl 2 RMg + 2HCl RH 2 + MgCl 2 The column is washed with deionised water and the washings is passed through sink or drain. Advantage:- Highly acidic and alkaline water can be softened. All types of ions are removed and water of zero hardness is obtained. H OH H 2 O

6 Disadvantages:- Turbid water cannot be treated because the suspended matter affects the efficiency of the exchanger by blocking the pores. The chemicals used for regeneration are costly. SCALE AND SLUDGE FORMATION IN BOILERS: In boilers, water evaporates continuously and the concentration of the dissolved salts increases progressively. When their concentrations reach the saturation point, they are thrown out of water in the form of precipitates on the inner parts of the boiler. If the precipitation takes place in the form of loose and slimy precipitate, it is called sludge. On the other hand, if the precipitated matter forms a hard, adhering coating on the inner walls of the boiler, it is called scale. Sludge is a soft, loose and slimy precipitate formed within the boiler. Sludge can easily be scraped off with a wire brush. It is formed at comparatively colder portions of the boiler and collects in the areas of the system where the flow rate is slow at bends. Sludge s are formed by substances which have greater solubility s in hot water,eg.,,magnesium carbonate, magnesium chloride, calcium chloride, magnesium sulphate, etc. Disadvantages of sludge formation: (1)Sludge s are poor conductors of heat, so they tend to waste a portion of heat generated. (2)If sludges are formed along with scales, then the former gets entrapped in the latter and both get deposited as scales. (3)Excessive sludge formation disturbss the working of the boiler. It settles in the regions of poor water circulation such as pipe connection, plug opening, gauge-glass connection, thereby causing even choking of the pipes. Prevention of sludge formation: (1)By using well softened water.

7 (2)By frequently blow-down operation i.e., drawing off a portion of the concentrated water. Scales are hard deposits, which stick very firmly to the inner surfaces of the boiler. Scales are difficult to remove, even with the help of hammer and chisel. Scales are the main source of boiler troubles. Formation of scales may be due to: (1)Decomposition of calcium bicarbonate: (2)Deposition of calcium sulphate: The solubilty of calcium sulpahte decreases with rise of temperature. In other words, CaSO 4 is soluble in cold water, but almost insoluble in superheated water. Consequently, CaSO 4 gets precipitated as hard scale on the heated portions of the boiler. This is the main cause of scales in high-pressure boilers. (3)Hydrolysis of magnesium salts: - Dissolved magnesium salts undergo hydrolysis(at prevailing temperatures in the boiler)forming magnesium hyrdroxide precipitate which forms a soft type of scale,eg., (4)Presence of silica: (SiO 2 ), even present in small quantities, deposits as calcium silicate and/or magnesium silicate. These deposits stick very firmly with the inner sides of the boiler surface and are very difficult to remove. One important source of silica in water is the sand filter. Disadvantages of scale formation: (1)Wastage of fuel: Scales have low thermal conductivity, so the rate of heat transfer from the boiler to inside water is greatly decreased. In order to provide a steady supply of heat to water, excessive or over-heating is done and this causes increase in fuel consumption. The wastage of fuel depends upon the thickness and the nature of the scale. (2)Lowering the boiler safety: Due to the scale formation, overheating of the boiler is to be done to maintain a constant supply of steam. The over-heating of the boiler tube makes the boiler material softer and weaker and this causes distortion of the boiler tube and makes the boiler unsafe to bear the pressure of the steam, especially in high-pressure boilers. (3)Decrease in efficiency: Scales may sometimes deposit in the valves and condensers of the boiler and choke them partially. This results in the deficiency of the boiler. (4)Danger of explosion: When thin scales crack, due to uneven expansion, water comes suddenly in contact with over-heated iron plates. This causes formation of large amounts of steam suddenly, so sudden high-pressure is developed, which may even cause explosion of the boiler.

8 Removal of scales: (i) With the help of scraper or piece of wood or wire brush, if they are loosely adhering. (ii) By giving thermal shocks, if they are brittle. (iii) By dissolving them by adding chemicals, if they are adherent and hard. Thus calcium carbonate scales can be dissloved by using 5-10% HCl. Calcium sulphate scales can be dissolved by adding EDTA(ethylenediamine tetraacetic acid),with which they form soluble complexes. (iv) By frequent blow-down operation. If the scales are losing adhering. Prevention of scale formation: (1)External treatment includes efficient softening of water (soda lime, zeolite and Ion exchange process). (2)Internal treatment: An internal treatment is accomplished by adding a proper chemical to the boiler water either: (a)to precipitate the scale forming impurities in the form of sludges, which can be removed by blow-down operation, or (b) convert them to compounds, which will stay in dissolved form in water and thus do not cause any harm. Internal treatment methods:- are, generally, followed by blowdown operation, so that accumulated sludge is removed. Important internal treatment methods are: Colloidal conditioning: In low-pressure boilers, scale formation can be avoided by adding organic substances like kerosene,tannin,agar-agar,etc.,which get coated over the scale forming precipitates, thereby yielding non-sticky and loose deposits, which can easily be removed by pre-determined blow-down operations. (ii)phosphate conditioning: In high-pressure boilers, scale formation can be avoided by adding sodium phosphate, which reacts with hardness of water forming non-adherent and easily removable, soft sludge of calcium and magnesium phosphates, which can be removed by blow-down operation, eg. 3CaCl 2 + 2Na 3 PO 4 Ca 3 (PO 4 ) 2 + 6NaCl The main phosphates employed are(a) (NaH 2 PO 4 ),sodium dihydrogen phosphate (acidic),(b) (Na 2 HPO 4 ),disodium dihydrogen phosphate (weakly alkaline) (c) (Na 3 PO 4 ),trisodium phosphate (alkaline). (iii)carbonate conditioning: In the low-pressure boilers, scaleformation can be avoided by adding sodium bicarbonate to boiler water, when CaSO 4 is converted to calcium carbonate in equilibrium. CaSO 4 + Na 2 CHO 3 CaCO 3 + Na 2 SO 4

9 Consequently, deposition of CaSO 4 as scale does not take place and calcium is precipitated as loose sludge of CaCO 3 which can be removed by blow-down operation. (vi)electrical conditioning: Sealed glass bulbs containing mercury connected to a battery, are set rotating in a boiler, When water boils, mercury bulbs emit electrical discharges, which prevents scale forming particles to adhere/stick together to form scale. (vii)radioactive conditioning: Tablets containing radioactive salts are placed in boiler water at few points. This energy of radiations emitted by these salts prevent scale formation. (viii)complexometric method: It involves adding 1.5% alkaline solution(ph=8.5) of ETDA to feed water. The EDTA binds the scale-forming cations to form stable and soluble complex. As a result, the sludge and the scale formation in the boiler are prevented. Moreover, this treatment (i) prevents the deposition of iron oxides in the boiler. (ii) Reduces the carryover of oxides with steam (iii)protects the boiler units from corrosion by wet steam. BOILER CORROSION Boiler corrosion is decay of boiler material by a chemical or electro-chemical attack by its environment. Main reasons for boiler corrosion are: (1)Dissolved oxygen: Water usually contains about 8ml of dissolved oxygen per litre at room temperature. Dissolved oxygen in water, in presence of prevailing high temperature, attacks boiler material: 2Fe + 2H 2 O + O 2 2Fe(OH) 2 4Fe(OH) 2 + O 2 2[Fe 2 O 3. 2H 2 O] Removal of dissolved oxygen: a). By adding calculated quantity of sodium sulphite or hydrazine or sodium sulphide. Thus: 2Na 2 SO 3 + O 2 2Na 2 SO 4 N 2 H 4 N 2 + 2H 2 O Na 2 S + 2O 2 Na 2 SO 4 b). by mechanical de-aeration, i.e., water spraying in a perforated plate-filled tower, heated from sides and connected to vacuum pump. High temperature, low pressure and large exposed surface reduces the dissolved oxygen in water. (2). Dissolved carbon dioxide: Carbon dioxide is also called carbonic acid. CO 2 + H 2 O H 2 CO 3 which has a slow corrosive effect on the boiler material. Carbon dioxide is also released inside the boiler, if water used for steam generation contains bicarbonate, e.g., Mg(HCO 3 ) 2 MgCO 3 + H 2 O + CO 2 Removal of carbon dioxide: a) By adding calculated quantity of ammonia. Thus:

10 NH 4 OH + CO 2 (NH 4 ) 2 CO 3 + H 2 O b). by mechanical-de-aeration process along with oxygen. (3). Acids from dissolved salts: Water containing dissolved magnesium salts liberate acids on hydrolysis. e.g., MgCl 2 + 2H 2 O Mg(OH) 2 + 2HCl The liberated acid reacts with iron in chain-like reactions producing HCl again and again. Thus: Fe + 2HCl FeCl 2 + H 2 FeCl 2 + HCl Fe(OH) 2 + 2HCl PRIMING AND FOAMING When a boiler is steaming rapidly, some particles of the liquid water are carried along with the steam. This process of wet steam formation is called priming. Priming is caused by: a) The presence of large amount of dissolved solids; b) High steam velocities; c) Very high water level; d) Improper boiler design, and e) Sudden increase in steam production rate. Priming can be avoided by a) Maintaining the low water level b) Fitting mechanical steam purifiers; c) Avoiding rapid change in steaming rate and d) Using soften water. Foaming is the production of persistent foam or bubble in boilers, which do not break easily. Foaming is due to the presence of dissolved salts organic matter, clay, oil and grease in water. Foaming finally leads to excessive priming. Foaming can be avoided by: a) adding anti-foaming chemicals like castor oil, b) removing oil from water by adding compounds like sodium aluminate. Priming and foaming, usually, occur together. They are objectionable because: a) dissolved salts in boiler water are carried by the wet steam to super-heater and turbine blades, where they get deposited as water evaporates. This deposit reduces their efficiency ; b) dissolve salts may enter the parts of other machinery, where steam is being used, thereby decreasing the life of the machinery ; c) actual height of the water column cannot be judged properly, thereby making the maintenance of the boiler pressure becomes difficult. Caustic Embrittlement In high pressure boilers, Sodium carbonate is hydrolyzed to yield NaOH Na 2 CO 3 + H 2 O 2NaOH + CO 2 The NaOH thus formed concentrates after long use. It causes inter-granular cracks on the boiler walls, especially at the stress points. The concentrated alkali is dissolved iron as

11 sodium ferrote in cracks and causes brittlement of the boiler parts especially at the bends, joints and rivets, even causing failure of boiler. The formation of cracks in the boilers due to NaOH is called Caustic embrittlement. Caustic embrittlement can be avoided by 1. Using Na 2 SO 4, Na 3 PO 4 as softening reagent instead of Na 2 CO 3 for water softening 2. Adding lignin or tannin, which blocks the hair cracks. 3. Maintaining the ph of the water between 8 and Using crack resistance steel during the manufacturing (adding Al)