Water Purification Methods

Module 7: Industrial Applications

Water Purification Methods

• The module focuses on industrial applications of water purification, fuel combustion, and corrosion prevention.
• Water purification methods are zeolite processes, ion-exchange resins, and reverse osmosis.

Zeolites

• Zeolites soften water through the Permutit process.
• Zeolite is a hydrated sodium aluminum silicate with a general formula of Na₂OAl₂O₃.xSiO₂.yH₂O.
• Zeolites exchange sodium ions (Na+) for calcium (Ca2+) and magnesium ions (Mg2+).
• A common zeolite is Natrolite, with the formula Na₂Al₂Si₃O₁₀·2H₂O.
• Other materials like green sand, containing iron potassium phyllosilicate (Glauconite), can be used for softening.
• Artificial zeolite, known as Permutit, is used for water softening.
• Zeolites consist of porous, glassy particles, offering a higher softening capacity than green sand.
• Zeolites are synthesized by heating china clay (hydrated aluminum silicate), feldspar (KAlSi₃O₈-NaAlSi₃O₈ – CaAl₂Si₂O₈), which is a tectosilicate mineral constituting about 60% of the earth's crust, with soda ash (Na₂CO₃).
• Natural zeolites include Natrolite (Na₂O.Al₂O₃.4SiO₂.2H₂O), Laumontite (CaO.Al₂O₃.4SiO₂.4H₂O), and Harmotome ((BaO.K₂O).Al₂O₃.5SiO₂.5H₂O).
• Zeolites can exchange their sodium ions.
• Zeolites achieve softening through reactions such as Na₂Ze + Ca(HCO₃)₂ → 2 NaHCO₃ + CaZe.
• Zeolite regeneration uses brine solution, e.g., CaZe + 2 NaCl → Na₂Ze + CaCl₂.
• Zeolite process advantages include residual water hardness of approximately 10 ppm.
• Zeolite process advantages also include small, easy-to-handle equipment and minimal softening time.
• Sludge formation is absent, and regeneration is easily achieved using brine.
• Zeolite process advantages include the ability to remove any type of hardness without process modifications.
• Zeolite process disadvantages include not being able to use colored water or water containing suspended impurities without filtration.
• Acidic pH waters will destroy the zeolite.

Ion-Exchange Resins

• Ion-exchange resins are cross-linked long-chain polymers with a microporous structure.
• Functional groups in ion-exchange resins are responsible for ion-exchange properties.
• Acidic functional groups (-COOH, -SO₃H, etc.) in ion-exchange exchange hydrogen ions (H+) for cations.
• Basic functional groups (-NH₂, =NH, etc.) exchange hydroxide ions (OH-) for anions.
• Cation-exchange resins (RH+) include styrene divinyl benzene copolymers.
• Cation exchange resins are capable of exchanging H+ when sulfonated.
• Anion-exchange resins (R'OH) include styrene divinyl benzene or amine formaldehyde copolymers with NH₂, QN+, QP+, and QS+ groups.
• Anion exchange is capable of exchanging of OH- with alkali treatment.
• Ion-exchange facilitates reactions like 2 RH+ + Ca2+/Mg2+ ↔ R₂Ca2+/R₂Mg2+ + 2 H+ and R'OH- + Cl- ↔ R'+ Cl- + OH-.
• Hard water should pass through a cation exchanger before an anion exchanger to avoid forming hydroxides of Ca2+ and Mg2+.

Mixed Bed Deionizers

• A mixed bed deionizer uses a single cylindrical chamber that contains a mixture of anion and cation exchange resins.
• When hard water pass through the deionizer bed, the cations and anions of the hard water engage with two types of resins many times.
• The process is equivalent to repeatedly passing hard water through a series of cation and anion exchange resins.
• Soft water produced through this method contains less than 1 ppm of dissolved salts.
• Exhausted deionizer resins are backwashed using an upward flow of water.
• A light weight anion exchanger moves to the top and forms a upper layer above the heavier cation exchanger.
• The anion exchanger is regenerated from above through passing caustic soda solution, then rinsed with pure water.
• The bottom cation exchanger bed is washed with dilute sulfuric acid solution, then rinsed.
• The beds are mixed using compressed air for reuse.
• Mixed bed deionizers can be used for highly acidic and alkaline water.
• The residual hardness of water can be as low as 2 ppm and is suitable for boilers at high pressure.
• Mixed bed deionizers often involves expensive equipment, chemicals, and needs skilled labor.
• Turbidity should be lower than 10 ppm and may require coagulation before treatment

Reverse Osmosis

• Reverse osmosis occurs when two solutions of unequal concentrations are separated by a semipermeable membrane.
• Solvent naturally flows from the lower to the higher concentration.
• This phenomenon can be reversed by applying pressure to the concentrated side.
• Pressure of 15-40 kg/cm² is applied to seawater.
• Pure water is forced through the semipermeable membrane.
• Semipermeable membranes filter out Bacteria, pyrogens, viruses, pesticides, hydrocarbons, radioactive contaminants, turbidity, colloidal matter, chlorine, detergents, wastes, asbestos, dissolved solids

Fuels and Combustion

• Fuel is a combustible substance that produces a large amount of heat upon combustion.
• That heat can be used for domestic and industrial purposes.
• Combustion involves oxidation of carbon and hydrogen in fuels to form CO₂ and H₂O.
• The difference in energy between reactants and products is released as heat.
• Coal and petroleum oils are the primary fuels.
• Coal and petroleum oils are diminishing, so are called "fossil fuels".

Fuel Classification

• Primary/natural fuels include solid (Wood, Coal, Dung), liquid (Crude oil), and gaseous (Natural gas) types.
• Secondary/derived fuels include solid (Coke, Charcoal, Petroleum-coke, Coal-briquette), liquid (Tar, Kerosene, Diesel, Petrol, Fuel oil, LPG, Synthetic gasoline), and gaseous (Coal gas, Water gas, Oil gas, Biogas, Coke oven gas, Blast furnace gas) types.

Calorific Value and Units of Heat

• Calorific value is the most important fuel property, or its capacity to supply heat.
• The calorific value is the total heat quantity when a unit mass or volume of fuel is completely burned.
• A calorie is the amount of heat required to raise the temperature of one gram of water by one degree centigrade.
• A kilocalorie (kcal) is 1000 calories.
• A British thermal unit (BTU) raises the temperature of one pound of water by one degree Fahrenheit.
• One BTU equals 252 calories or 0.252 kcal.
• One kcal equals 3.968 BTU.
• A Centigrade Heat Unit (CHU) raises the temperature of one pound of water by one degree Centigrade.
• One kcal equals 3.968 BTU or 2.2 CHU.
• Higher or Gross Calorific Value (HCV or GCV) is the total amount of heat when one unit mass/volume of fuel is burnt completely and then cooled to room temperature.
• If the products of combustion are condensed to room temperature (15°C or 60°F), the latent heat of condensation of steam also gets included in the measured heat
• Lower or Net Calorific Value (LCV) is the net heat when one unit mass/volume of fuel is burnt completely at which the products are permitted to escape.
• LCV= GCV - Latent heat of water vapor formed
• Latent heat of steam is 587 kcal/kg or 1060 BTU/lb of water vapor at 15 degrees C.

Characteristics of a good fuel

• The characteristics of a good fuel include High calorific value, Moderate ignition temperature, Low moisture content, Low non-combustible matter content, and Moderate velocity of combustion.
• Products should not be harmful, Low cost, Easy to transport, Combustion should be easily controllable, Should not undergo spontaneous combustion and Size should be uniform (solid fuel)

Fuels Comparison

• Solid fuels are easy to transport and store without spontaneous explosion risks.
• Solid fuels have low production cost and moderate ignition temperature.
• Solid fuel disadvantages include high ash content, large heat waste during combustion, and clinker formation.
• Solid fuels disadvantages also include difficult combustion control, high handling cost, lower calorific value.
• Solid fuels also require a large air excess for combustion and cannot be used in internal combustion engines.
• Liquid fuels possess high calorific value per unit mass and burn without forming ash, clinker, or dust.
• Fire can easily be extinguished by stopping the fuel supply.
• Liquid fuels can be stored indefinitely and transported in pipes without loss.
• Flames can be controlled, and handling is easy.
• Liquid fuels are clean in use, economic in labor, and losses of heat in the chimney is low.
• Liquid fuels require less air excess for combustion, less combustion space, and can be used in internal combustion engines.
• Liquid fuel disadvantages include higher cost, the need for costly storage tanks, and greater fire risks.
• An additional liquid fuel disadvantage is the specially designated burners for spraying the fuel are required with its potential problems.
• Gaseous fuels can also be used in internal combustion engine.
• Gaseous fuel advantages include easy conveyance via pipelines, instant lighting, and high heat content.
• Combustion can be readily controlled, and gas burns without ash and with less smoke.
• Gaseous fuels are clean in use, don't need special burners and burn without heat loss due to convection.
• Gaseous fuels burn in slight air excess, are free from solid/liquid impurities, and enable complete combustion without pollution.
• Gaseous fuel disadvantages include large storage tanks often needed.
• High flammability means chances of fire hazards are high and more costly than liquid and solid states.

Bomb Calorimeters

• Bomb Calorimeters determine the calorific value of a substance.
• Calculation includes m = mass of fuel pellet (g) W = mass of water in the calorimeter (g) w = water equivalent of calorimeter (g) t₁ = initial temperature of calorimeter. t₂ = final temperature of calorimeter. HCV = gross calorific value of fuel.
• Water Equivalent of the calorimeter is determined by burning a fuel of known calorific value (benzoic acid (HCV = 6,325 kcal/kg) and naphthalene (HCV = 9,688 kcal/kg)
• LCV = [HCV – 0.09 H × 587] kcal/kg were H is the percentage of hydrogen in fuel
• Use of standard fuel with known fuel

Calorimeter Calculations

• Various corrections are incorporated for accurate results. Fuse wire correction - heat generated by burning the Mg wire, used as an ignition source.
• Acid correction is when sulphur and nitrogen get oxidised from fuel
• Cooling correction occurs as the chamber rises above room temp, with heat being lost as a result.

Corrosion

• Corrosion is deterioration and loss of solid metallic material via chemical or electrochemical attack.
• Corrosion is the metal's oxidization into metallic compounds plus energy.
• Types of corrosion include dry/chemical corrosion and electrochemical corrosion.
• Dry/chemical corrosion is oxidation, corrosion by gases, and liquid metal corrosion.

Oxidation Corrosion

• Oxygen in the atmosphere attacks the metal, forming oxide layers (2M + n/2 O₂ → 2 Mn+ + n O2-) and the nature of the oxide determines its action.

Factors Influencing Corrosion

• Factors influencing corrosion are both Metallic and Environmental
• Metallic ones include: Position in galvanic series, Overvoltage, Relative areas of anode and cathode, Purity, Physical state of material and the passive character of metal and its solubility.
• Environmental ones include: Temperature, Humidity, Presence of impurities in atmosphere, Suspended particles pH, Silicates.
• Environmental factors also include Conductance, Formation of O₂ concentration cell, Flow velocity

Corrosion Form

• Forms of corrosion include Uniform corrosion (direct chemical attacks), Galvanic corrosion (electrochemical action between dissimilar metals), Concentration Cell corrosion (different concentrations of the same solution in contact).
• Forms of corrosion also include Pitting corrosion (localized corrosion with pits), Crevice Corrosion ( contact with nonmetals)
• Corrosion can also be Filiform Corrosion occurs on painted surfaces after penetration by moisture, Intergranular occurs on grain boundaries of metals.
• Some other corrosion forms include: Stress Corrosion Cracking (applied loads, residual stresses), Corrosion Fatigue (cyclic stress and corrosion), Fretting Corrosion (rapid corrosion with highly loaded metal surfaces), Erosion Corrosion (combination of chemical environment/fluid velocities.

Corrosion Control

• Control of corrosion can be achieved by Proper designing, Proper selection of metal or alloy, Use of pure metals or alloys
• Control can also be achieved through Cathodic or Anodic protection and the use of inhibitors
• Control can also be achieved Changing the environment Application of protective coatings

Cathodic Protection

• Cathodic protection makes base metal the cathode by connecting it to a high anodic metallic plate.
• Two methods: Sacrificial anodic protection, Impressed current cathodic protection
• Sacrificial anodic protection involves connecting the metallic structure through a wire to a more anodic metal.
• This induces corrosion at the more anodic metal, preventing corrosion of underlying material.
• Common sacrificial anodes are Zn, Mg, and Al.
• Sacrificial Anodes are used to protect underground pipelines, ship hulls, and water tanks.
• Galvanization of Steel is a Sacrificial Anode: Dip steel sheet in molten zinc, steel exposed is the cathode.

Impressed Current Cathodic Protection

• Impressed direct current is applied to nullify the corrosion current.
• One battery terminal connects to an insoluble anode (e.g., graphite) immersed in a black fill (coke, gypsum, bentonite, sodium sulphate).
• The method protects underground pipes, oil lines, transmissions and ships.