Water in The Chemical Industry

University:
Washington State University
Course:
CHEM 101 | Introduction To Chemistry
Academic year:

2021
Views:

124

Pages:

52
Author:

BitProud

Chapter 2 Water in the Chemical Industry • Water is used as an input for several industries for washing the reactors, as a reactant, for steam generation and as coolant in heat transfer equipments. 1 Sources of water: ✓ Rain water - purest form obtained as a result of evaporation, Contains : dissolved gases ( CO2, SO2,NO ETC.) & suspended solid particles ✓ River water-contains dissolved minerals of inorganic matter and uprooted and decomposed plants as a result of its flowing in the surface of the land. ✓ Lake water- stagnant water which allows growth of algae and aquatic animals Which also contains large amount of organic mater and less amount of dissolved salts. ✓ Sea water: most impure form due to continuous evaporation of water from the sea. ✓ Underground water: the water which percolates down the earth till meets the hard rock. ✓ Spring water : after the percolated water level built up in the hard rocks it starts to move upwards and comes out in the form of spring water. ✓ Well water 2 parameters • Dissolved gases- very small/negligible amount • Dissolved liquids are almost absent • Dissolved solids are the most reason for the impurities of natural water which are present either in suspended or in dissolved form. Determination of total solids • by evaporation of a known amount of water sample by steam bath in porcelain dish residue obtained (in mg) Total solids ( TS) = volume of water sample taken Determination of total suspended solids (TSS) • by filtration and drying technique. TSS=weight of suspended solids / total volume of water sample ( mg/l) Total dissolved solids ( TDS) = total solids – total suspended solids. 3 Hardness of water Hardness - the presence of Ca and Mg salts which impart a property to the quality of water. For example; • It prevent water from giving foam or lather when mixed with soap. 2C17H35COONa + CaCl2 → (C17H35COO)2Ca + 2NaCl sodium stearate (soap) calcium stearate (Insoluble) • Soap reduces the surface tension of water, • if it forms a ppt, it cannot reach the surface of water and affects the cleansing action of soaps. Types of hardness a) Temporary hardness – water containing certain hardness causing salts undergo a chemical change in the form of insoluble salts when merely boiled, which can be thrown out as ppt settle down at the bottom. Ca(HCO3)2 heat CaCO3 (ppt) + H2O + CO2 Mg(HCO3)2 heat Mg(OH)2 (ppt) + 2CO2 4 b) Permanent hardness: it cannot remove by merely boiling and it is mostly caused by the salts of chlorides and sulphates of Ca & Mg and to some extent by Fe. Eg. MgCl2, CaSO4 Equivalents of Calcium Carbonates • To get the total hardness of water ,weight of hardness causing salts cannot be merely added. • Hardness of water is expressed as ppm CaCO3 eq. which is chosen as a standard salt. • This is due to CaCO3 is most insoluble salt in water and its M.Wt. is 100. CaCO3 Eq. mass of hardness causing salt∗MWt of CaCO3 MWt of the hardness causing salt = 5 Units of hardness Include two quantities: ▪ Quantity in mg of hardness causing salt. ▪ Quantity (volume, L) of water sample. 1mg/L = 1mg of CaCO3 Eq. of hardness causing salt per 1 liter of water sample. 1ppm = 1 part of CaCO3 Eq. of hardness causing salt per 1 million parts of water sample. Activity : Show the equality relation between mg/L & ppm that 1ppm = 1 mg/L Example; Calculate temporary hardness and total hardness of a sample of water containing: Mg(HCO3)2= 7.5 mg/L; Ca(HCO3)2=16 mg/L; MgCl2= 9.0 mg/L; CaSO4= 13.6 mg/L Answer: Temporary hardness= 19.474 mg/L Permanent hardness = 15.014 mg/L 6 Determination of hardness by EDTA method Involves the determination of Ca & Mg salts presents in the water using volumetric analysis by titration. EDTA as a titrating agent is capable of forming complex with divalent metal ions. pH is maintained at 10 by NH4OH/NH4Cl buffer. EBT indicator form unstable wine red complex with Ca2+ and Mg2+ in water sample. After it is titrated against EDTA ,it forms stable steel blue coloured complex.` Example: Standard hard water contains 19 g of CaCO3 per liter. 25 mL of this consumed 31 mL of EDTA solution. 100 mL of hard water sample consumed 23 mL of EDTA solution. The same sample after boiling consumed 14 mL of EDTA solution. Calculate the temporary hardness of water sample in ppm or mg/L? 7 Determination of alkalinity of water sample ✓water is alkalized due to the presence of OH-, CO32 ‾ & HCO3 − ✓Determination of the alkalinity of water sample is by titration with standard acids in the presence of phenolphthalein and methyl orange indicators. 8 Let P = phenolphthalein end pt & M = methyl orange end pt. P = OH- + ½ CO3-2 , M = OH- + CO3-2 + HCO39 Let P = phenolphthalein end pt & M = methyl orange end pt. P = OH- + ½ CO3-2 , M = OH- + CO3-2 + HCO3From the P & M end pts obtained in the titration, which alkalinities are present & what is the volume of the acid consumed corresponding to alkalinity present can be arrived from the following table. Conditions Alkalinity present OH- CO32- HCO3- P=0; M=0 0 0 0 P=0; M>0 0 0 M P=M P or M 0 0 2P=M 0 M 0 2PM 2P-M 10 Example • A 20 mL of water sample upon titration with 0.05 M H2SO4 had been consumed 25 mL of the acid at the phenolphthalein end point and 32 mL at methyl orange end point of titration. Identify and quantify the alkalinities present in the water sample? • Solution: Exercise: Upon titration of water sample with 0.01 M HCl solution, the phenolphthalein and methyl orange end points had been found to be 8 mL and 27 mL respectively for a 100 mL of water sample. Identify and determine the amount of total alkalinity present in the given water sample? 11 Requisites of Water in Industries Water must be free from suspended and coloured solids to be used in chemical industries. In textile industry- used for washing the fabrics before dyeing In dyeing industry- used as a suspension medium . Sugar industry-used to extract sugar from sugar cane by solvent extraction. Paper industry- used to extract the cellulosic material form as a wood pulp and paper. Concrete making- hardening of concrete mixture leading to the strength of the building is due to hydrolysis and hydration 12 Sludge and scale formation in boilers ⩥ When hard water is used for boilers, on continuous evaporation, the salts present in the hard water are get deposited in the areas where the flow is slow. ⩥ Sludge- when these precipitates are loose and slimy in nature. it is formed due to salts of MgCl2, MgCO3, CaCl2 & MgSO4 which have greater solubility in hot water and is removed by blow down operation by filling the boiler with fresh water. ⩥ Scale- when these precipitates are hard, and they adhere strongly to the inner surface of the boilers. its formation is due to decomposition of Ca(HCO3)2, deposition of CaSO4 and hydrolysis of Mg2+ salts. ⩥ Mg and Ca- silicates also can form very hard scales which cannot be removed easily by blow down operation as in sludge. 13 Cont’d… Deposition of CaSO4 • its solubility in water decreases with increase in temperature; CaSO4 is soluble in cold water, but almost completely insoluble in superheated water. It may be due to increased ionisation at high temperature so Ksp < Kionic prod. and less availability of molecules of solvation at high temperature. Consequently, CaSO4 gets precipitated as hard scale on the hotter part, of the boiler . ▪ Calcium sulphate scale is quite adherent and difficult to remove, even with the help of hammer and chisel. Hydrolysis of Magnesium Salts dissolved magnesium salts gets hydrolysed (at prevailing high temperature in side the boiler) forming magnesium hydroxide precipitate, which forms a soft type of scale. MgCl2 + H2O → Mg(OH)2 + 2HCl Presence of Silica: it may deposit as CaSiO3 and MgSiO3 . These deposits adhere very firmly on the inner side of the boiler surface and are very difficult to remove. One important source of silica in water is the sand filter. 14 Removal of Scales Scales can be removed by mechanical (i-iii) and chemical methods (iv). i. If the scales are loosely adhering, it can be removed with the help of scraper or piece of wood or wire brush. ii. If the scales are brittle , it can be removed by giving thermal shocks (i.e. heating the boiler and then suddenly cooling with cold water). iii. If the scales are loosely adhering, it can be removed by frequent blow-down operation . Blow –down operation is partial removal of hard water through a tap at the bottom of the boiler, when extent of hardness in the boiler becomes alarmingly high. Make up water is addition of fresh softened water to boiler after blow-down operation. iv. If the scales are adherent and hard, they can be removed by dissolving them by adding chemicals. Example CaCO3 scales can be dissolved by using 5-10% HCl CaSO4 scales can be removed by adding EDTA since the Ca-EDTA complex is highly soluble in water. 15 Cont’d…. Disadvantages of scale formation Wastage of fuel Decrease in boiler efficiency Lowering of boiler safety Explosion of boiler Caustic embrttilement: Eroding of the wall of boiler operation by the action of caustic alkali (high concentration of NaOH) which can result breaking up of boiler. ✓ During softening process, Na2CO3 is added and it gives rise to NaOH at elevated temperature according to the following reaction. Na2CO3 + H2O → 2NaOH + CO2 • • Reacts with surface of boiler and create cracks (NaFeO2) Sodium phosphate is better than using sodium carbonate in softening process to prevent this problem. 16 Water Treatment Objective of water treatment ✓ To improve the quality of water which is safe for drinking, release and reuse or recycle. ✓ To improve the aesthetic property of water ✓ To provide water at a reasonable cost Types of impurities ✓Particulate matter ✓ suspended impurities ✓ colloidal ✓ dissolved 17 Cont’d…. Numerous factors must be taken into considerations in designing and operating an industrial water treatment facility. For example: ✓ water requirement ✓ quantity and quality of available water sources ✓ sequential use of water (successive uses for applications requiring progressively lower water quality). ✓ water recycle ✓ discharge standards 18 Cont’d… The treatment of water may be divided into 3 major categories: a) purification for domestic use b) treatment for a specialized industrial applications c) treatment of waste water to make it acceptable for release or reuse. —The type and degree of treatment strongly depend on the source and intended use of the water. —Water for domestic use- must be thoroughly disinfected, but may contain dissolved Ca2+ and/or Mg2+ salts —water to be used in boilers – may contain bacteria but must be quite soft to prevent scale formation. Therefore, the physical and chemical processes used to treat water involve similar phenomena, regardless of their specific application or end use. 19 Municipal water treatment If the water source is taken from ground/well water, aeration is very important to remove volatile solutes like H2S, CO2, CH4 and volatile odorous substances such as methanethiol and bacterial metabolites. ✓ Aeration helps to oxidize Fe2+ /Mn2+ into Fe3+ /Mn4+ insoluble (ppt) 1 2 2Fe2+ + 4HCO3 − + O2 + H2 O ⟶ 2Fe(OH)3 (ppt) + 4CO2 ✓Fe2+/Mn2+ exist in acidic and reducing environments (e.g. wetlands and in aquifer below wetlands) ✓ Fe2+/Mn2+ are soluble and remain in water following conventional treatment but precipitates at the point of use, causing stains on plumbing fixtures and in laundry, support growth of bacteria (iron slime) in well screens and distribution systems 20 Fe and Mn can be addressed in a variety of ways 1. In-situ treatment: injection wells around water supply wells to precipitate Fe and Mn in the ground 2. Sequestration: phosphate chemicals added to water to bind with and sequester Fe and Mn, preventing later precipitation. 3. Ion-exchange treatment with greensand (glauconite) —Is a natural ion exchanger capable of exchanging with Fe and Mn Removal reaction: Z-MnO2 (Mn-coated glauconite) + Fe2+ /Mn2+ Z-Mn2O3 + Fe3+, Mn3+ & Mn4+ ✓Regeneration is with KMnO4 Z-Mn2O3+KMnO4 ⟶ Z-MnO2 21 Municipal water treatment (Cont’d) Addition of lime as CaO or Ca(OH)2 after aeration raises pH and inturn aids for the formation of ppts containing Ca2+/Mg2+ hardness ✓ much of the solid material remains in suspension and needs addition of coagulants like Fe3+ salts and Al3+ sulphate which form gelatinous metal hydroxides to settle the colloidal particles. Definition: ✓ Coagulation – destabilization and initial coalescing of colloidal particles. ✓ Flocculation – formation or agglomeration of larger particles from smaller particles. 22 Cont’d…. Colloids persist as a small particles because they carry negative surface charge and therefore repel each other. Colloids by definition, do not settle and colloid removal requires that they must be agglomerated into larger particles. This requires surface charge to be destabilized. Coagulants are added to precipitate Ca2+/Mg2+ salts and they will settle in the primary basin Coagulant-aids (i.e. activated silica or synthetic electrolytes) are added to stimulate coagulation/flocculation settle in secondary basin after addition of CO2 to lower pH. The sludge from the 10 and 20 basins is pumped into a sludge lagoon and the water is finally chlorinated, filtered to be used for drinking. 23 Lime – Soda softening Used to remove certain chemicals from water and wastewater. from natural water- Fe, Mn, As, hardness, nitrate, radionuclides, organic chemicals etc. from industrial waste water – virtually any chemical , but especially metals and organic chemicals. For water treatment Carbonate hardness is removed by adding lime (Ca(OH)2) Single stage softening 1st lime reacts with any CO2 Then the lime reacts to remove carbonate hardness And finally to remove non carbonate Mg-hardness 24 Cont’d… Water recarbonation + ✓Recarbonation is needed because the addition of Ca(OH)2 raises pH of water to 10.2-10.6. ✓ recarbonation consist of bubbling CO2 through lime treated water, lowering pH to 8.7 - 9.0 For waters with non-carbonate hardness, single stage softening is insufficient. Therefore, it must be removed by addition of soda ash. 25 Schematic of a municipal water treatment plant 26 Drinking Water Disinfection There three basic strategies to keep microbiological contaminants out of drinking water: 1. Keeping microbiota out of water source 2. Treating water to remove contaminants. 3. Maintaining safe water distribution system Disinfection methods 1. Free Chlorine – most common 2. combined chlorine residual (chloramines) 3. Ozone- strong oxidant 4. Chlorine dioxide 5. UV-light 27 Chlorine Disinfection Disinfection has two components 1. Primary disinfection – inactivation of microorganisms in the water 2. Secondary disinfection – maintaining disinfecting residual in distribution system Chlorine disinfection is; • • • • Most widely used method. Effective at low concentration, inexpensive and forms residual • Drawback- forms trihalomethanes (THMs) • Applied as chlorine gas or hypochlorite • Acts by oxidizing enzymes of cells, preventing essential metabolic processes Chlorine gas: Cl2 + H2O → HOCl (Hypochlorous acid) + HCl 𝐻𝑂𝐶𝑙 ⇌ 𝐻 + + − 𝑂𝐶𝑙 (favoured below pH 7.5) (favoured above pH 7.5) 28 Cont’d Hypochlorite Ca(OCl)2 (s) + H2O → Ca2+ + 2OClNaOCl (l) + H2O → Na+ + OCl- (equilibrates with HOCl) — HOCl is more effective disinfectant than OCl- but both are excellent. Combined Chlorine ✓ HOCl reacts with ammonia NH3 + HOCl → NH2Cl (monochloramine) + H2O (favoured at pH ≥ 6) NH2Cl + 2HOCl → NHCl2 (dichloramine) + 2H2O (favoured at pH ≈5) NHCl2 + 3HOCl → NCl3 (trichloramine) + 3H2O ✓ Chloramines are effective against bacteria (e.g. pipe growth) and much less effective against viruses ✓ Chloramine contributes to chlorine residuals along with residual free chlorine (HOCl + OCl-) ✓ Chloramines are longer lasting Drawbacks: chlorine reacts with organics and produce disinfection-byproducts (DBP). These byproducts are suspected human carcinogens. • With phenol to form chlorophenols which gives a strong taste and odour • With natural organic matter (NOM), example, humic acids form THMs (CHCl3, CHCl2Br, CHClBr2 and CHBr3) 29 Cont’d… ▪ 𝑁𝐶𝑙3 𝑖𝑠 𝑢𝑛𝑠𝑡𝑎𝑏𝑙𝑒 𝑎𝑛𝑑 𝑤𝑖𝑙𝑙 𝑖𝑚𝑖𝑑𝑖𝑎𝑡𝑒𝑙𝑦 𝑑𝑒𝑐𝑜𝑚𝑝𝑜𝑠𝑒 𝑡𝑜 𝑁2 & 𝑓𝑟𝑒𝑒 𝑎𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒 𝐶𝑙2. 30 Chlorine removal in wastewaters 31 3. Ozonation • Ozone is produced when O2 molecules are dissociated by an energy source into oxygen atoms and subsequently collide with an O2 molecule to form an unstable gas O3, • generated by imposing a high voltage alternating current across a dielectric discharge gap that contains an oxygen-bearing gas. • Ozone is generated onsite because it is unstable and decomposes to elemental oxygen • Ozone is a very strong oxidant and effective against viruses. The mechanisms of disinfection using ozone include: → Direct oxidation of the cell wall with leakage of cellular constituents outside of the cell → Reactions with radical by-products of ozone decomposition → Damage to the constituents of the nucleic acids (purines and pyrimidines) 32 Cont’d… Advantages: • • • • effective against bacteria’s, cryptosporidium and viruses powerful oxidizing agent than HOCl only need short contact time (seconds to few minutes) have not disinfection by-products Disadvantage: • reactors are prone to short-circuiting • Do not form disinfection residuals • Ozone by itself is an explosive chemical 33 UV-Disinfection ❖disinfects by: Damaging nucleic acids – DNA, RNA —Forms hydroxyl radicals- strong oxidant —200-300 nm wavelength is absorbed by DNA – disinfecting range for UV (also range likely to cause skin cancer) —Very effective against cryptosporidium Radiation produced by lamps: • Low-pressure UV-lamp- 254 nm • Medium pressure UV-lamp 210-300 nm range • Medium pressure UV lamp disinfects more but takes more power Interferences – absorption by dissolved substances in water and shading of microorganisms by particulates ✓UV contactors have very short residence time (seconds-to minutes) ✓Short circuiting is a potential problem ✓Expensive than chlorination 34 Cont’d… 1. Sedimentation (gravity settling and coagulation) process ❖ Any settleable suspended solids present in the water are removed by the action of gravitational force in a big reservoir or tank. ❖ To accelerate the sedimentation process coagulants like Al3(SO4)2, FeCl3 & FeSO4 are added to the impounded water in order to form gelatinous and flocculants ppt which absorb the suspended particles in the sedimentation tank. Al3(SO4)2.14H2O + Ca(HCO3)2 → 2Al(OH)3↓(flocculent particle) + 3CaSO4 + 14H2O + 6CO2 Al(OH)3 helps in agglomerating the colloidal particles 2. Filtration ▪ ▪ Used for non- settleable suspended solids Sand filter is used as a filter (Porous) medium then , water containing the non- settleable suspended solids percolates down through fine sand to a coarse sand in a perforated plate. 35 Sewage Waste Treatment • Typical municipal sewage contains oxygen-demanding materials, sediments, grease, oil, pathogenic bacteria, viruses, salts, algal nutrients, pesticides etc. • Current processes for the treatment of wastewater can be divided into three main categories of primary treatment, secondary treatment and tertiary treatment. 36 Primary Treatment of Wastewater Pre-treatment process(10) involves : a) Screening —remove floating materials from impure water b) Storage —kill pathogenic bacteria when exposed to sunlight c) Aeration —remove Fe2+, Mn2+, CO2, H2S etc d) Pre-chemical treatment —E.g. Pre-chlorination to remove odor & color problems 37 𝐒𝐞𝐜𝐨𝐧𝐝𝐚𝐫𝐲 𝐖𝐚𝐬𝐭𝐞 𝐓𝐫𝐞𝐚𝐭𝐦𝐞𝐧𝐭 • Secondary wastewater treatment is designed to remove biochemical oxygen demand (BOD) usually by taking advantage of biological processes. • The action of microorganisms provided with added oxygen degrading organic material in solution or in suspension until the BOD of the waste has been reduced to acceptable levels. Trickling filter for BOD removal —The wastewater is sprayed over rocks or solid support material covered with microorganisms. —The structure of the trickling filter allow contact of the wastewater with air and degradation of organic matter occurs by the action of the microorganisms 38 𝐒𝐞𝐜𝐨𝐧𝐝𝐚𝐫𝐲 𝐖𝐚𝐬𝐭𝐞 𝐓𝐫𝐞𝐚𝐭𝐦𝐞𝐧𝐭 (cont’d) Rotating biological reactors (contactors) —consists of groups of large plastic discs mounted close together on a rotating shaft — half of each disc is immersed in wastewater and half exposed to air —When the shaft (polyethene or polystyrene plastics) rotate it accumulate thin layers of attached biomass which degrades organic matter in the sewage. 39 𝐒𝐞𝐜𝐨𝐧𝐝𝐚𝐫𝐲 𝐖𝐚𝐬𝐭𝐞 𝐓𝐫𝐞𝐚𝐭𝐦𝐞𝐧𝐭 (cont’d) Activated Sludge Process —Probably is the most versatile and effective of all wastewater treatment processes. —Microorganisms in the aeration tank convert organic matters into microbial biomass and CO2. —Organic nitrogen is converted to ammonium ion or nitrate. —Organic phosphorus is converted to orthophosphate. 40 Tertiary wastewater treatment • The treatment of water beyond the biological method is called as advanced/ tertiary treatment • can be treated by a variety of chemical processes, including acid/base neutralization, precipitation, and oxidation/reduction. • Cyanide in the wastewater may be oxidized with chlorine • organics with ozone, hydrogen peroxide promoted with ultraviolet radiation, or dissolved oxygen at high temperatures and pressures. • Heavy metals may be precipitated with base, carbonate, or sulfide. • Wastewater can be treated by several physical processes. • density separation, sedimentation, filtration etc • Wastewater solutes can be concentrated by evaporation, distillation, and membrane processes, including reverse osmosis, hyper-filtration, and ultrafiltration. Organic constituents can be removed by solvent extraction, air stripping, or steam stripping. • Synthetic resins are useful for removing some pollutant solutes from wastewater. • Organophilic resins used to remove alcohols, aldehydes, ketones, pesticides etc. • Cation exchange resins are effective for the removal of heavy metals 41 3.Reverse - Osmosis Two solutions of unequal concentration are separated by semi-permeable membrane. The membrane does not allow the solute to pass. Excess external pressure is applied on the solution of higher concentration side,the solvent molecules starts flowing from solution of higher conc. To lower conc. Is known as reverse osmosis. The membrane comprised of thin film of cellulose acetate, poly metacrylates & polyamides (recent semi-permeable membranes with long life). The water obtained by this process is absolutely free from all impurities like ionic, non-ionic and microorganisms. ▪Fig1. reverse osmosis cell 42 4. Ion-Exchange Process Exchangers can categorized into Natural (Zeolite) and synthetic ion exchangers. Synthetic resins like styrene-divinylene benzene copolymer with a functionalities of –SO3H for cation exchanges and -NH4-OH (quaternary ammonium for anions exchange) Natural exchangers 43 Cont’d.. 44 Cont’d... Dissolved solids containing ionic impurities are pulled out by this mechanism. This process uses ion –exchange resins which exchange the cations and anions present in the impure water. Cation exchange resins(RH+) like sulfonic acid of styrene – divinyl benzene cross –linked co-polymer exchange its H+ with the cations of impure water ( Ca2+, Mg2+) Anion –exchange resins(ROH-) : styrene – divinyl benzene cross –linked co-polymer containing quaternary ammonium or phosphonium hydroxide group.exchange ions like Cl-,SO42-,NO3etc. 45 Cont’d… 46 Regeneration of Ion - Exchangers 47 Cont’d… 48 Electrodialysis Dissolved inorganic salts in the water are in the form of electrically charged ions. The process uses electric field to remove/pulled out the electrically charged impurities from water. Positive ions move towards cathode and negative ions move towards the anode. As a result of this ionic impurities are pulled out from the central compartment as shown in figure below . 49 Simple electrodialysis Has three compartments using thin, rigid plastic membrane Anode and cathode placed at the two extreme ends of the cell and connected to positive and negative terminals of the DC source Draw backs Concentrations of all cations and anions surround their respective electrode and therefore, setting back emf and slow down the electrodialysis process. Then, it can be overcome by using a cell with ion-selective membrane. 50 Ion selective membrane Cation selective membranes: −SO3‾H⁺, −COO‾H⁺ allows only the cations to pass through. Anion selective membranes: are like R4N⁺OH‾ allows only the anions to pass through. 51 Wastewater stabilization ponds (WSP) • WSP systems consists of a single series of anaerobic, facultative and maturation ponds or several series in parallel • Anaerobic and facultative ponds are mainly designed for BOD removal and maturation ponds mainly for pathogen removal 52