Unit 4: Water Technology, Fuels & Coal PDF

Unit 4: Water Technology, Fuels & Coal 1. Hardness of water Water which does not produce lather with soap is termed as hard water. The hardness is usually expressed in terms of Ca & Mg salts like bicarbonate, carbonate, sulphate, chloride etc. Formation of Hard water: Hard water is for...

Unit 4: Water Technology, Fuels & Coal 1. Hardness of water Water which does not produce lather with soap is termed as hard water. The hardness is usually expressed in terms of Ca & Mg salts like bicarbonate, carbonate, sulphate, chloride etc. Formation of Hard water: Hard water is formed due to presence of minerals like Ca and Mg. they are not removed or separated by sedimentation or filtration. When hard water reacts with soap (sodium salt of stearic acid or pametic acid) gives curdy precipitate. In above reaction hard water react with sodium salt of stearic acid to form calcium stearate or magnesium stearate which being insoluble and separate out without producing lather. 2. Types of hardness a) Temporary hardness: Temporary hardness of water is caused by Ca and Mg bicarbonate. This can be removed by simply boiling of water. Due to boiling bicarbonate is converted into carbonate (insoluble precipitate) Temporary hardness can also be removed by adding hydrated lime to precipitate insoluble carbonate. b) Permanent hardness: Permanent hardness is caused by the presence of soluble salt of Ca and Mg other than bicarbonate such as chloride and sulphate. Permanent hardness cannot be removed by boiling of water or hydrated lime. It can be eliminate by water softening techniques like Lime-soda process, Zeolite, Ion-exchange resin, reverese osmosis etc. 3. Degree of hardness The unit in which hardness is usually expressed, known as degree of hardness. Degree of hardness is expressed in terms of calcium carbonate (CaCO3) equivalent because CaCO3 have molecular weight 100 and it is easily precipitate. Degree of hardness may be expressed as follows: Unit of hardness: i) Parts per million (ppm): it is the number of equivalent part CaCO3 present per million (106) part of water by weight. ii) Milligram per litre (mg/lit): it is the number of milligram of CaCO3 present in one litre of water. iii) Degree Clarke (oCl): it is the number of equivalent part of CaCO3 present per 70,000 part of water. iv) Degree French (oFr): it is the number of equivalent part of CaCO3 present per 105 part of water. Correlation between ppm, mg/lit, oCl and oFr: [1ppm= 1mg/lit=0.07 oCl= 0.1 oFr] 4. Boiler Trouble Boilers are used in industries and power station to generate steam. During conversion of water into steam in boiler, the dissolve and suspended solids are not removed. All the impurities are deposited in form of scale and sludge within the boiler and causes boiler troubles. a) Scales formation: Scales are hard deposits, which stick on the inner wall of the boiler. These are formed by CaCO3, CaSO4, Ca(HCO3)2, Mg(OH)2 etc. in hot portion of boiler. b) Sludge formation: Sludge form loose, slimy and soft precipitate in the colder area of the boiler. The sludge formed by the CaCl2, MgCl2, MgCO3, MgSO4 etc. Problems caused in boilers: i) Scales and sludge is poor conductor of heat & therefore prevent effective transfer of heat to water. ii) In this condition excessive heat is required which increase fuel consumption. iii) Scale often crack due to their uneven expansion allowing the water to come immediately in contact with overheated metal. This suddenly result in the formation of large quantity of steam which in turn leads to excess pressure and then to explosion. 5. Boiler trouble removal by internal treatment Internal treatment: i) Sludge formation can be removed by blow down operation. In this case impurities can be removed by an outlet present at the bottom of boiler. ii) Scale formation can be prevented by internal treatment that involve addition of chemical to the boiler water either to ppt the scale forming impurities in the form of sludge so that they can be removed by blow down method or to convert them into soluble compounds. a) Calgon treatment: Calgon is sodium meta hexa phoaphate, which can be used to covert CaSO4 into soluble complex. b) Phosphate treatment: Scale formation can be removed by adding sodium phosphate, which reacts with hardness of water and form soft sludge of Ca and Mg phosphates which can be removed by blow-down operation. Techniques for water softening (External treatment) a) Zeolite process Zeolite is a three-dimensional silicate. The chemical formula of zeolite is hydrated sodium aluminum silicate represented as Na2OAl2O3.xSiO2.yH2O (x = 2-10 & y = 2- 6). Zeolites are capable exchanging ions with sodium ions. So it is capable of exchanging hardness producing icons present in water. This process also called as permutit process. Zeolite can be written as Na2Ze The two Na+ icons is replaced by one Ca2+ or Mg2+ ions. Process:- The apparatus is made of cylindrical metallic vessel several beds are made inside it where zeolite salt is kept. Raw water is poured inside the apparatus through inlet that passes through beds and thus chemical ion exchange reactions are takes place. After the use of this process for a certain time, Zeolite is exhausted i.e all Na+ ions are replaced by Ca2+ or Mg2+ and therefore this will not be used for soften the water. Regeneration: Exhausted zeolite can be regenerated by treating it with brine solution (10% NaCl solution) Exhausted Zeolite on washing with cold water, CaCl2 & MgCl2 can be removed and regenerated zeolite is this ready to be reused. b) Ion-exchange resin: In this process cations and anions are completely removed by passing impure water into two different columns. First column contain sulphuric acid resin with acidic group –SO3H. This column is known as cation exchange resin because it exchange only cations like Ca2+, Mg2+, Na+ etc. Whereas, second column contain resin with basic group like –𝑁𝑅3+ 𝑂𝐻-. It is known as anion exchange resin because it exchanges anions like 𝐶𝑙-, 𝑆𝑂42- etc. The removal of H+ ions from the first column and 𝑂𝐻- ions from the second column react to form water. Regeneration: when both the column are exhausted, then first and second column are treated with dilute sulphuric acid or HCl (generate H+ ions) and aqueous NaOH (generate OH– ions) respectively. So, they have to be regenerated. c) Lime-soda process: In this process, hydrated lime & sada ash use to remove hardness from water. i) Hydrated lime: Hydrated lime is used to remove temporary hardness of water. It react with Ca(HCO3)2 & Mg(CO3)2 to form insoluble precipitate of calcium carbonate and magnesium hydroxide respectively. Hydrated lime is also used to remove permanent hardness (magnesium salt impurities only) from water. It reacts with MgSO4 & MgCl2 to form insoluble precipitate of magnesium hydroxide. In above reaction calcium based impurities like CaCl2 & CaSO4 also form, which are soluble in water. Therefore, calcium based impurities (CaCl2 & CaSO4) are not removed by lime treatment. ii) Soda ash: Sodium carbonate (Na2CO3) is used to remove permanent hardness of water which caused by MgSO4 & MgCl2 or CaCl2 & CaSO4. The amount of lime-sada required for the softening of hard water can be calculated by following formula d) Reverse osmosis: The minimum excess pressure that has to be applied on the solution to prevent the entry of the solvent molecule (pure water) into solution through semi permeable membrane is known as osmotic pressure (lower concentration to higher concentration). If a pressure higher than the osmotic pressure is applied on the solution, the solvent (pure water) will flow reverse, higher concentration solution to low concentration solution, the process is known as reverse osmosis. Reverse osmosis process can also be used in purification of sea water, for this purpose sea water is delivered under pressure through the semi-permeable membrane where water permeate the minute pores of the membrane & is delivered as purified water. Advantages: 1) Reverse osmosis system have low maintenance requirement. 2) It removes colloidal silica, which is not removed by demineralization. 3) RO system required less energy as compare to other technology. 4) The reverse osmosis is gaining ground at present for converting sea water into drinking water and for obtaining water for very high pressure boilers.  Fuels & Combustion Introduction A fuel is a substance that contains carbon and hydrogen undergoes combustion in presence of oxygen to gives large amount of energy. Classification of Fuel On the basis of occurrence fuel is classified into two categories; natural or primary fuels and artificial or secondary fuels. i) Natural/primary fuels: These fuels are naturally present. ii) Artificial/ secondary fuels: They are synthesized by primary fuels. Characteristics of Good Fuel i) Fuel should have high calorific value. ii) Must have moderate ignition temperature. iii) Fuel should have low moisture content. iv) Available in bulk at low cost. v) Should not burn spontaneously. vi) Fuel should burn efficiently, without releasing hazardous pollutants. vii) Handling, storage and transportation should be easy. Calorific value Calorific value of fuel can be define as the amount of heat evolved when one unit mass or volume of the fuel undergoes completely combustion in presence of oxygen. i) High or gross calorific value (HCV or GCV): it is defined as amount of heat evolve when one unit mass or volume of the fuel is completely burnt and combustible products are cooled to room temperature (25oC or 77oF). ii) Low or net calorific value (LCV or NCV): it is defined as amount of heat evolve when one unit mass or volume of the fuel is completely burnt and combustible products are permitted to escape. Therefore, net calorific value is lower than gross calorific value. One part by mass of hydrogen produced nine parts by mass of water molecule. Therefore, Determination of calorific value i) Bomb calorimeter Bomb calorimeter is used to determine calorific value of solid and liquid fuels experimentally. A bomb calorimeter contains a cylindrical bomb made by stainless steel. Combustion takes place in this cylinder. The lid contains two stainless steel electrodes. Oxygen is supplied through oxygen valve for combustion. The electrode is attached with a small ring which supports nickel or stainless steel made crucible. The bomb is taken in a copper calorimeter which is surrounded by air and water jacket in order to prevent heat loss by radiation. The copper calorimeter also contains electrically operated stirrer and Beckmann’s thermometer (take reading with temperature difference up to 0.01oC. Working: In a crucible, a known amount of the fuel is placed in the nickel or stainless steel crucible which is supported by a ring. A fine magnesium wire touches the fuel sample, which is already connected to the electrodes. The bomb lid is lightly screwed and filled with oxygen at about 25 atm pressure, is placed in copper calorimeter containing a known amount of water. The electrically operated stirrer is driven and notes the initial temperature of water (T1). After that both the electrodes are connected to a battery to complete the circuit. The fuel sample is burn and heat is liberated. To maintain the uniform temperature, water is continuously stirred and the final temperature (T2) of water is noted. Calculation: Mass of fuel (solid or liquid)= x g Mass of water taken = W g Water equivalent of calorimeter = w g Initial temperature of water in calorimeter = T1 K Final temperature of water in calorimeter = T2 K ii) Theoretical calculation by Dulong’s formula: The theoretical calculation of calorific value of a fuel can be approximately calculated by Dulong’s formula, based on the percentage of the constituents (C, H, O and S) present in the fuel. As per Dulong’s formula Where, C, H, O and S are percentage of carbon, hydrogen, oxygen and Sulphur present in fuel. In above formula the oxygen is assumed to be present in combined form with hydrogen or in form of water (H2O).  Coal Analysis of Coal The quality of coal can be analyzed by two analysis; proximate and ultimate analysis. Proximate analysis: In this analysis moisture, volatile matter, ash and fixed carbon can be determined. Moisture: A known amount of finely powdered air-dried coal sample is taken in crucible. The crucible is placed inside an electric hot air-oven, at 105o to 110oC for 1 hour. The crucible is then taken out, cooled in desiccators and weighed. Difference in the weight of sample gives the information about the weight loss due to removal of moisture. Lesser the amount of moisture content, better the quality of fuel. Volatile matter: The moisture free coal sample is taken in a crucible, covered with a lid and placed in muffle furnace (electric furnace) at 950oC for 7 minutes and then remove the crucible from the oven and cooled first in air, then cooled in a desiccator and weighed again. Loss in weight is due to presence of volatile matterin coal sample. Low quantity of volatile matter, better the quality of a coal. Ash: The residual coal sample taken in a crucible and then heated without lid in a muffle furnace at 700-750oC for an hour. The crucible is then taken out, cooled first in air, then in desiccators and weighed again. The process of heating, cooling and weighing are repeated until a constant weight is not obtained. The residue is reported as ash on percentage-basis. Ash is non-combustible substance which reduces the calorific value of a coal. Therefore, low quantity of ash contents, better the quality of a coal. Fixed carbon: The fixed carbon percentage is determined by following equation Ultimate analysis: Ultimate analysis is involving the measurement of C, H, N, S, and O. Carbon and hydrogen: In a combustion apparatus, about 1-2 gram of coal sample is burnt in a current of oxygen to convert C and H into CO2 and H2O respectively. The gaseous products CO2 and H2O are absorbed in KOH and CaCl2 tubes of known weights, respectively. The increase in weights of these (KOH and CaCl2) are then determined. Nitrogen: In Kjeldahl’s flask add accurately weighed powdered coal and heated with concentrated H2SO4 and K2SO4 as a catalyst. The solution becomes clear when all the nitrogen is converted into ammonium sulphate then it is treated with excess of NaOH which convert ammonium sulphate into ammonia; the liberated ammonia is distilled over and absorbed in a known volume of standard (N/10) H2SO4 solution. From the volume of H2SO4 used by liberated ammonia, the percentage of Nitrogen in coal, calculated as follows: The volume of unused H2SO4 is then determined by titrating against standard NaOH solution (N/10) Calculation: The amount of H2SO4 required to neutralize ammonia evolve from coal is calculated as follows: Sulphur: A known amount of coal sample is burnt in bomb calorimeter in presence of oxygen. After that, sulphur present in coal is converted into SO2 and SO3. The ash obtained from the bomb calorimeter, is extracted with dil. HCl. The washings (acid extracts) are treated with Barium chloride solution and the sulphates are precipitate as Barium sulphate. This precipitate is filtered, washed, dried and heated to obtain constant weight. Calculation: Ash: Percentage of ash calculated by method given in proximate analysis. Oxygen: It is calculated by subtracting the sum of total % of carbon, hydrogen, nitrogen, sulphur and ash from 100. Calculation: