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Chemical Process Industries

Chemical Process Industries

Chemical Process Technology

Module-I CHEMICAL PROCESS INDUSTRIES INTRODUCTION is one of the oldest industries and plays an important role in the social, cultural and economic growth of a nation. Food, shelter and clothing have become an indispensable part of our lives. It is one of the most diverse of all industrial sectors, covering thousands of products. The chemical industry comprises basic chemicals and their derivatives such as petrochemicals, fertilizers, paints, varnishes, vegetables, perfumes, pharmaceuticals and encompasses thousands of items that are user in our everyday lives from manufacturing to household goods.

The basic layout of chemical process industry is shown in Figure 1. The various product of chemical industries are used in the various fields such as packaging, agriculture, automobiles, telecommunication, construction, home appliances, health care, explosive, pesticides, textile and pharmaceuticals [Table 1]. Indian chemical industry plays a significant role in Indian economy’s overall growth and contributes to the country’s GDP by around 3%. It consists of large, medium and small units. The chemical industry is a major contributor to the glob al economy and produces more than 8,000 products, which is a crucial part of India's agricultural and industrial growth and has important ties with many other downstream industries such as automotive, consumer protection, electronics and food processing [Chemical Engineering World, 2004]. Table 1: Major Products of Chemical Industries and their Area of Application

Group of Product Areas

Plastics and Agricultural water management, packaging, automobiles, Polymers telecommunications, health and hygiene, education Synthetic rubber Transportation Industry, Textile, Industrial equipment lining

Synthetic fiber Non-woven and woven fibre in automobile, hosiery, textile

Soap and Synthetic Health and hygiene domestic as well as industrial detergents

Industrial chemicals Drugs & pharmaceuticals, pesticides, explosives, surface loading, dyes, lube additives, adhesive oil field, antioxidants, chemicals, metal extraction, printing ink, paints Sugar & Alcohol Food, Alcoholic Beverage, Chemical Feed Stock, Ethoxylate, biofuel

Pulp & Writing & Printing Paper, Culture Paper, News Printing Paper, Tissue Paper, Packaging Paper Fertilizer Agriculture, Chemical industry ( and urea)

Agrochemicals Pesticides

Mineral Chemical industry- organic and inorganic

Figure 2: Structure of Organic Chemical Industry GLOBAL AND INDIAN CHEMICAL INDUSTRIES The chemical Industry of India is considered of the largest chemical sector in world. In 2008, its sales exceeded 3 trillion USD and over 20 million people around the global have employed in chemical Industry. The details of world chemical market is given in Figure 3.

Chemical Market: 2 tr. USD Global Chemical Market: Growing @ 1.5 times GDP Petrochemical dominate with Share 40percent

Figure 3: World Chemical Market

Sources: Nanvaty, 2008

Indian chemical industry is an important constituent of the Indian economy. Its size is estimated at around US $ 35 billion approx. which is equivalent to about 3% of India’ GDP. The total in Indian Chemical industry is approx. 60 billion and total employment is about 1 million. The Indian chemical sector accounts for 13-14% of total exports and 8-9% of total imports of the country. Export of chemicals and related products was 24.066 billion USD in 2008-9 accounting to 13.63 of total imports. In terms of volume, it is 12th largest in the world and 3rd largest in Asia. Currently per capita consumption of products of chemical industry in India is about 1/10th of the world average. Over the last decade, the Indian chemical industry has evolved from being a basic chemical producer to becoming and innovative industry. The total production of organic chemicals during 2008-09 works out to 1.25 million with value of 0.9717 billion. The size of the petrochemicals segment was estimated at 13.96 billion. Total size Dyestuff industry is estimated 4 billion USD. There are 50 organized industries and over 900 small-scale industries.

India has 8.5-9% global market share. The India pharmaceutical industry is the fourth largest volume terms and 15th largest market in value terms. The market will reach 20 billion USD by 2015 and 30 billion USD by 2020. The size of the Agrochemicals industry estimated at over 1billion USD.

Fertilizer capacity N and P as (P3O5) is 12.29 and 10.90 million tones and production 10.9 and 3.4 million tons respectively. The turnover during 2008-09 was around 28.49 USD. Chemical and fertilizer sector in India presently constitutes 14% of the domestic industrial activity [Lokhapare, 2011, Annual report 2010-11]. Segments of the Indian chemical industry are given in Table 2.

Table 2: Segments of the Indian Chemical Industry Basic Chemicals (49.05%): Market value: 32.78 USD  Inorganic chemicals:(Caustic , soda ash, , black, titanium oxide, sulphuric , etc.)  Organic chemicals (acetic acid, acetic anhydride, acetone, phenol, methanol, formaldehyde, nitrobenzene, malice anhydride, aniline, chloromethanes, acetaldehyde, ethanol amines, ethyl acetate etc.  Petrochemicals( Olefins, aromatics-benzene, toluene, xylenes, fibre intermediates MEG, PTA, acrylonitrile, propylene, caprolactam, adipic acid, hexamethylene diamine, phthalic anhydride, methanol, LAB, polymers, synthetic fibre etc)  Fertilizers( Nitrogenous and Phosphatic)  Other industrial chemicals Specialty Chemicals (24.69%): Market value:16.50USD  Paints and varnishes,  Textile chemicals  Dyestuffs and intermediates  Catalysts  Plastic additives  Adhesive sealants  Industrial gases Knowledge Chemicals (26.6%): Market value:17.55USD  Pharmaceuticals  Biotechnology

Characteristic of the Indian Chemical Industry [Lokhapare, 2011]

 High domestic demand potential as the Indian markets develops and per capita consumption levels increases.  High degree of fragmentation and small scale of operations  Limited emphasis on exports due to domestic market focus  Low cost Competitiveness as compared to other countries due to the high cost of feed stocks and power  Low focus on R & D despite initiatives to innovate processes to synthesis products effectively  Process development, Low R &D investment  Mindset

TYPICAL ISSUES FOR CHEMICAL INDUSTRIES Due to various technological and engineering developments, chemical industry has been able to reduce the cost of production. Changes in technology and raw materials have shifted regularly and frequently towards Lower costs and competitive, better conversion and efficiency, high productivity, less energy consumption, Broader spectrum of product grades. However, due to increasing cost of raw materials and stringent environment issues, chemical industry is facing major challenges in future. Typical issues in chemical industry to meet the future challenges are shown in Figure 4.

Figure 4: Typical Issues in Chemical Industry

Basic Principles of Chemical Processes: UNIT PROCESSES AND UNIT OPERATIONS

Chemical processes usually have three interrelated elementary processes

 Transfer of reactants to the reaction zone

 Chemical reactions involving various unit processes

 Separation of the products from the reaction zone using various unit operations Processes may involve homogeneous system or heterogeneous systems. In homogeneous system, reactants are in same phase-liquid, gases or solids while heterogeneous system include two or more phases; gas liquid, gas–solid, gas-gas, liquid–liquid, liquid solid etc. Various type reactions involve maybe reversible or irreversible, endothermic or exothermic, catalytic or non-catalytic. Various variables affecting chemical reactions are temperature pressure, composition, catalyst activity, catalyst selectivity, catalyst stability, catalyst life, the rate of heat and mass transfer. The reaction may be carried out in batch, semi batch or continuous. Reactors may be batch, plug flow, CSTR. It may be isothermal or adiabatic. Catalytic reactors may be packed bed, moving bed or fluidized bed. Along with knowledge of various unit processes and unit operation following information are very important for the development of a process and its commercialization [Austin,1984]

Basic Chemical data: Yield conversion, kinetics

 Material and energy balance, raw material and energy consumption per ton of product, energy changes

 Batch vs Continuous, process flow diagram

 Chemical process selection: design and operation, pilot plant data, Equipment required, material of construction

 Chemical Process Control and Instrumentation

 Chemical Process Economics: Competing processes, Material and, Energy cost, Labour, Overall Cost of production

 Market evaluation: Purity of product and uniformity of product for further processing

 Plant Location

 Environment, Health, Safety and Hazard

 Construction, Erection and Commissioning

 Management for Productivity and creativity: Training of plant personals and motivation at all levels

 Research, Development and patent

 Process Intensification In order to improve productivity and make the process cost effective and for improving overall economy, compact, safe, energy efficient and environmentally sustainable plant, process intensification has become very important and industry is looking beyond the traditional chemical engineering.

UNIT PROCESSES AND UNIT OPERATIONS IN CHEMICAL PROCESS INDUSTRIES Chemical process is combination of unit processes and Unit operation. Unit process involves principle chemical conversions leading to synthesis of various useful product and provide basic information regarding the reaction temperature and pressure, extent of chemical conversions and yield of product of reaction nature of reaction whether endothermic or exothermic, type of catalyst used. Unit operations involve the physical separation of the products obtained during various unit processes. Various unit processes in chemical industries are given in Table 3.

NITRATION

Nitration involves the introduction of one or more nitro groups into reacting molecules using various nitrating agents like fuming, concentrated, aqueous nitric acid mixture of nitric acid and sulphuric acid in batch or continuous process. Nitration products find wide application in chemical industry as solvent, dyestuff, pharmaceuticals, explosive, chemical intermediates. Typical products: TNT, Nitrobenzene, m- dinitrobenzene.

Table 3: Unit Processes in Chemical Process Industries Alkylation and Hydro delkylation Decomposition Acylation Fermentation Ammonoxidation Halogenation Amination by reduction Hydsogenation Amination Hydrohenatlysis Aromatisation Hydroformylation Amination by ammonalysis Hydrolysis Calcination Hydration Carbonation Hydroammonalysis Causticisation Isomerisation Chlorination and Oxy chlorination Neutralistion Condensation Nitration Biomethhanation Methanation Carbinisation Disproportination Oxidation and partial oxidation Cracking; Thermal, steam cracking, catalytic Pyrolysis cracking Dehydration Polymeristion: Addition and condensation Chain growth and step growth,Bulk, Emulsion, suspension, solution, Radical and coordination polymeristion Dehydrogenation Reduction Ditozitation and coupling Reforming: Steam reforming Catalytic reforming Gasification of and biomass Sulphidation Desulphurisation and hydro desulphurisation Sulphonatiomn Electrolysis Sulphation Etherification Xanthation Estertification and Trans Estrerificartion

SULPHONATION AND SULPHATION Sulphonation involves the introduction of sulphonic acid group or corresponding salt like sulphonyl halide into a organic compound while sulphationinvolves introduction of -OSO2OH or -SO4-. Various sulphonating agents are sulphur trioxide and compounds, sulphurdixide, sulphoalkylating agents. Some of the sulphaming agents are sulphamic acid. Apart from sulfonation and sulphamate sulpho chlorinated, sulfoxidation is also used.

Typical application of sulphonation and sulphation are production of lingo sulphonates, linear alkyl benzene sulphonate, Toluene sulphonates, phenolic sulphonates, chlorosulphonicacd, sulphamates for production of herbicide, sweetening agent (sidiumcyclohexysulphamate). Oil soluble sulphonate, saccharin

Preparation of Saccharin

The industrial synthesis entails the reaction of with a solution of trioxide in . Sulfonation by chlorosulfonic acid gives the ortho and para substituted chlorosulfones. The ortho isomer is separated and converted to the sulfonamide with ammonia. Oxidation of the methyl substituent gives the carboxylic acid, which cyclicizes to give saccharin.

HCl + SO3 → ClSO3H

OXIDATION Oxidation used extensively in the organic chemical industry for the manufacture of a large number of chemicals. Oxidation using oxygen, are combinations of various reactions like oxidation via dehydrogenation using oxygen, dehydrogenation and the introduction of oxygen and destruction of carbon, partial oxidation, peroxidation, oxidation in presence of strong oxidizing agent like KMnO4, chlorate, dichromate, peroxides H2O2, PbO2, MnO2; nitric acid and nitrogen tetra oxide, oleum, ozone. Some of the important product of oxidation are aldehyde, ketone, benzyl alcohol, phthalic anhydride, ethylene oxide, vanillin, bezaldehyde, acetic acid, cumene, synthesis gas from hydrocarbon,, propylene oxide, benzoic acid, maleic acid, benzaldehyde, phtathalic anhydride. Oxidation maybe carried out either in liquid phase or vapor phase.

HYDROGENATION

Hydrogenation involves the reaction of a substance with hydrogen in the presence of a catalyst. Some of the other reaction involving hydrogen are, hydrodesulphurisation, hydrcracking, hydro formylation, oxosynthesis, hydroammonylsis, synthesis of ammonia.

ESTERIFICATION Esterification is an important unit process in the manufacture of polyethylene terephathalate, Polymethyl-methaacrylate (PMMA), , viscose rayon manufacture, nitroglycerine.

HYDROLYSIS Hydrolysis is used both in inorganic and organic chemical industry. Typical application is in oil and fats industry during soap manufacture where hydrolysis of fats are carried out to obtain fatty acid and glycerol followed by addition of sodium to form soap. Other application is in the manufacture of amyl alcohols. Some of the major product using hydrogen is ethylene from acetylene, methanol, propanol, butanol, production of alcohol from olefins (eg. Ethanol from ethylene). Various types of hydrolysis reaction may be pure hydrolysis, hydrolysis with aqueous acid or , dilute or concentrated, alkali fusion, hydrolysis with enzyme and catalyst. POLYMERIZATION Polymerization is one of the very important unit processes which find application in manufacture of polymer, synthetic fibre, synthetic rubber, polyurethane, paint and petroleum industry for high octane gasoline. Polymerization maybe carried out either with single monomer or with multi monomer (comonomer). Polymerization reaction can be addition or condensation reaction. Various Polymerization methods may be bulk, emulsion, solution, suspension. Typical important product from polymerization are, Polyethylene, PVC, poly styrene, nylon, polyester, poly butadiene, poly styrene, phenolic, urea, melamine and alkyd resins epoxy resin, silicon polymers, poly vinyl alcohol etc.

Preparation of Polyethylene or polythene It is the most common plastic.

n CH2 = CH2 -CH2-CH2 - n

UNIT OPERATIONS IN CHEMICAL INDUSTRIES Unit operations are very important in chemical industries for separation of various products formed during the reaction. Table 4 give the details of unit operation in chemical process industries.

Table 4: Unit Operations in Chemical Process Industries Absorption and stripping Membrane Process: Reverse osmosis, Ultrafiltration, Dialysis, Electro dialysis, Per evaporation Adsorption and desorption Crushing Grinding, Pulverizing and Pressure Swing adsorption Screening Chromatography Distillation: Batch distillation Solid liquid extraction Flash distillation, Azeotropic distillation, Extractive distillation Reactive distillation Evaporation Striping Fluidization Sublimation Crystallization Solvent extraction Liquid- Liquid extraction

DISTILLATION

Distillation has been the king of all the separation processes and most widely used separation technology and will continue as an important process for the foreseeable future [Olujie et al., 2003]. Distillation is used in petroleum refining and petrochemical manufacture Distillation is the heart of petroleum refining and all processes require distillation at various stages of operations. MEMBRANE PROCESSES Membrane processes have emerged one of the major separation processes during the recent years and finding increasing application in desalination, wastewater treatment and gas separation and product purification. Membrane technology is vital to the process intensification strategy and has continued to advance rapidly with the development of membrane reactors, catalytic membrane reactor, membrane distillation, membrane bioreactors for wide and varied application [Sridhar, 2009]. Based on lower operating costs, comparable capital cost and only slightly product loss (including fuel), membranes have demonstrated a flexible, cost, effective alternative to amine treating for some natural gas processing applications [Cook & Losin, 1995].

Membrane distillation is a membrane separation process, which can overcome the limitation of more traditional membrane process. Membrane distillation has significant advantage over other processes, including low sensitivity to feed concentration and the ability to operate at low temperature [Patli and Patil, 2012].

ABSORPTION

Absorption is the one of the most commonly used separation techniques for the gas cleaning purpose for removal of various gases like H2S, CO2, SO2 and ammonia. Cleaning of solute gases is achieved by transferring into a liquid solvent by contacting the gas stream with liquids that offers specific or selectivity for the gases to be recovered. Unit operation and is mass transfer phenomena where the solute of a gas is removed from being placed in contact with a nonvolatile liquid solvent that removes the components from the gas.

Solvent: Liquid applied to remove the solute from a gas stream. Solute: Components to be removed from entering streams.

Some of the commonly used solvents are: Chemical Absorption

Amine Processes: Mono-ethanol amine (MEA), di-ethanol amine (DEA), tri-ethanol amine (TEA), diglycol amine (DGA), methyl diethanol amine (MDEA)

Carbonate Process: K2CO3, K2CO3+MEA, K2CO3 +DEA, K2CO3+arsenic trioxide

Physical Absorption Polyethylene Glycol Dimethyl Ether (Selexol), N-methyl pyrrolidine,NMP (Purisol), Methanol (Rectisol), Sulphonane mixed with an alkanolamine and water (sulfinol).

ADSORPTION Adsorption technology is now used very effectively in the separation and purification of many gas and liquid mixtures in chemical, petrochemical, biochemical and environmental industries and is often a much cheaper and easier option than distillation, absorption or extraction. Some of the major applications of adsorption are gas bulk separation, gas purifications, liquid bulk separation, liquid purifications [Keller II, 1995]. One of the most effective method for recovering and controlling emissions of volatile organic compounds is adsorption Some of the commercial adsorbent s are silica gel, activated carbon, carbon molecular sieve, , zeolites molecular sieves, polymer and resins, clays, biosorbents. Some of the key properties of adsorbents are capacity, selectivity, regenerability, kinetics, compatibility and cost [Knaebel, 1995]. Some of the methods used for regeneration of adsorbent are thermal swing, pressure swing, vacuum (special case of pressure swing), purge and gas stripping, steam stripping [Crittenden, 1988]. Commercial adsorption processes is given in Table 5. Some of the important criteria of good adsorbent are [Keller II, 1995].

(1) it must selectivity concentrate one or more components called adsorbate to from their fluid phase levels (2) the ability to release adsorbate so that adsorbent can be reused, (3) as high as possible delta loading the change of weight of adsorbate per unit weight of adsorbent between adsorbing and desorbing steps over a reasonable range of pressure and temperature

Table 5: Commercial Adsorption Processes

Sorbex process Application

Parex Separation of paraxylene from mixed C8 aromatics isomers

MX sorbex Meta xylene from mixed C8 aromatics isomers Molex Linear paraffins from branched and cyclic hydrocarbons Olex Olefins from paraffins Crsex Para cresol or meta cresol isomers Cymex Para cymene or meta cymene from cymene isomers Sarex Fructose from mixed sugar UOP ISOSIV separation of normal paraffins from hydrocarbon mixture processor Kerosene Isoiv For separation of straight chain normal paraffins from the process kerosene range(C10-C18) used for detergent industry

Pressure swing adsorption (PSA) is based on the principle of relative adsorption strength, is a milestone in the science of gas separation [Shiv Kiran and Chakravarty, 2002]. Some of the commercial application of PSA are air drying, hydrogen purification, bulk separation of paraffins, air separation for oxygen and nitrogen production,

Chromatography is a sorptive separation process. In choromatography feed is introduce in column containing a selective adororbent (stationary phase) and separated over the length of the column by the action of a carrier fluid (mobile phase)that is continually supplied to the column following the introduction of the feed. The separation occurs as a result of the different partitioning of the feed solutes between the stationary phase. The separated solutes are recovered at different time in the effluent from the column [Rangrajan,2010].

CRYSTALLIZATION PROCESS Crystallization processes are used in the petroleum industry for separation of wax. The process involves nucleation, growth, and agglomeration and gelling. Some of the applications of crystallization is in the separation of wax, separation of p-xylene from xylenes stream. Typical process of separation of p-xylene involves cooling the mixed xylene feed stock to a slightly higher than that of eutectic followed by separation of crystal by centrifugation or filtration.

Principles applied for studying Chemical Industry

The chemical industry is studied by following important steps

1. Brief introduction (market and sales)-Justification of the Industry 2. Properties-physical and chemical properties 3. Methods of production (a) Chemical reactions (b) Process flow diagram (c) Materials requirement (i) Raw material (chemicals and others) -Quantitative requirement (d) Process description 4. Chemical Engineering Problems (a) Manufacturing section-Major engg. Problem and its potential solution (b) Economics 5. Uses or Important application HYDROCHLORIC ACID

INTRODUCTION: Hydrochloric acid (HCl), also known as muriatic acid, is a solution of hydrogen chloride in water. HCl exists in solid, liquid, and gaseous states and is water soluble in all proportions. The first hydrochloric acid was prepared through heating common salt and sulfuric acid by Benedictine Monk and Basil Valentine in 15th century. Also, Libavius prepared free hydrochloric acid by heating salt in clay crucibles in 16th century.

In the 17th century, Johann Rudolf Glauber used NaCl and H2SO4 for the preparation of in the Mannheim process, releasing hydrogen chloride gas as a by-product. Joseph Priestley prepared pure HCl in 1772, and chemical composition includes hydrogen and chlorine was proven by Humphry Davy in 1818. Demand for alkaline substances increased during the in , developed cheap large-scale production of (soda ash). Using common salt, sulfuric acid, and coal, which releases HCl as a by-product. Until the British Alkali Act 1863 and similar legislation in other countries, the excess HCl was vented to air. After the passage of the act, waste gas is absorbed in water, producing hydrochloric acid on an industrial scale. In the twentieth century, the was effectively replaced by the without hydrochloric acid by-product. Since hydrochloric acid was already fully settled as an important chemical in numerous applications, the commercial interest initiated other production methods, some of which are still used today. After the year 2000, hydrochloric acid is mostly made by absorbing by-product hydrogen chloride during a chemical manufacturing process such as chlorination of hydrocarbons. Since 1988, hydrochloric acid has been listed as a Table II precursor under the 1988 United Nations convention against illicit traffic in narcotic drugs and psychotropic substances because of its use in the production of heroin, cocaine, and methamphetamine.

MANUFACTURING PROCESS HCl is manufactured by various methods as follows 1. Synthesis from hydrogen and chlorine

2. From salt and sulfuric acid

3. As by-product from chemical processes

4. From incineration of waste organics

5. Hydrochloric acid solutions

1. Synthesis from Hydrogen and Chlorine There is large demand in the market for water white acid. Such acid is obtained by synthetic method, and most of the plants are based on this process. Raw materials Basis: 1000kg of Hydrochloric acid (98% yield) Hydrogen = 28.21kg Chlorine = 999.21kg Sources of raw material Both hydrogen and chlorine can be obtained during electrolysis of for manufacturing of NaOH. Also, hydrogen can be synthesized from any one methods of following. 1. Lane process or iron steam process

2. Steam hydrocarbon process

3. Liquefaction of coal gas and coke oven gas

4. Bosch process or water gas-steam process

Reaction H2 + Cl2 2HCl ΔH = - 43.9kcals

Flow Sheet:

Process Description:

The plant consists of combustion chamber of structural carbon or lined with silica bricks provided with cooling device which may consist even of cold-water circulation in the shell. To ensure all the chlorine reacts with hydrogen, excess of 10% hydrogen compare to chlorine is charged from the bottom of combustion chamber. Also, care should be taken that the combustion chamber and length of ducting which leads the gas to absorber should be sufficiently specious, otherwise hydrochloric acid will contain free chlorine. The burning of hydrogen is started by igniting the burner with an external air-hydrogen torch. Dry chlorine is passed into the combustion chamber, where hydrogen burns in an atmosphere of chlorine to produce HCl. The exothermic nature of the direct combination of both gases (H2 and Cl2) is such as to raise the temperature of the reagents, and the reaction products to a point where they are incandescent. The reaction is carried out at 24000C with greenish flame. The gases are always kept above dew point to avoid corrosion. The combustion chamber is then cooled externally by water and gas tight lid is fitted at the top of the reactor which suddenly opens to allow the gases to escape in case of emergency. Hydrochloric acid gas is cooled absorbed in water or dilute HCl solution by passing through cooler and absorber through the connecting pipe. The strength of acid produced is generally 32-33 %. The heat of absorption of HCl in water is removed by spray of cold water outside the absorber. The solution of HCl flows into a storage tank.

Initiation Cl2 + hυ 2Cl* Propagation + Cl*+ H2 HCl + H * H* + Cl2 HCl + Cl Termination Cl* + Cl* Cl2 + heat H* + H* H2 + heat H* + Cl* HCl +heat A large amount of heat is developed both from chain propagation reactions and from chain termination processes, the continued renewal of the chain propagators by thermal route is ensured over the long term. In brief reaction between hydrogen and oxygen to produce hydrogen chloride is a chain reaction with a high quantum yield. 2. The Salt–Sulfuric acid process The reaction between NaCl and sulfuric acid occurs in two endothermic stages. Raw materials Basis: 1000kg Hydrochloric acid = 3206kg Sulfuric acid = 2688kg Sources of raw material Sodium chloride can be obtained from sea water, salt lake and sub –soil water. Sulfuric acid can be obtained by contact process. Reaction

NaCl + H2SO4 NaHSO4 + HCl

NaCl + NaHSO4 Na2SO4 + HCl Flow Sheet:

Process Description: Salt (NaCl) and sulfuric acid are charged to the furnace. It is desirable to keep one of the components in the reaction mixture in a liquid form in both steps. The first step is carried out at the lower temperature compare to second step. Even so, for liquefaction of NaHSO4, which is required to carry out in second step, material is heated up to 4000C. Sodium sulfate in form of sludge is collected from the bottom of the furnace. The product and unconverted sulfuric acid is sent to further processing in which recovery of sulfuric acid and nitric acid in cooling tower and absorber respectively.

CAUSTIC SODA

Properties of Chlorine (Cl2)

Molecular weight-70.9, Melting point: -101.6°C, Boiling Point: -34.6°C, Liquefaction point-5.7 atms. & 15°C Properties of (NaOH) Molecular weight: 40, Boiling Point: 1390°C, Melting Point: 318°C, Solubility: Very soluble in water with high exothermic heat of solution. Grades: Available in solid form of flakes, granules, lumps, pellets and aqueous solution (50 and 73% NaOH) Principle: Sodium hydroxide has diverse uses and is a reactant in organic and inorganic chemical manufacturing processes. It is also used in the petroleum, pulp and paper, textile, and alumina industries. Sodium hydroxide (caustic soda, caustic) was made for many years by the lime causticization method, which involves reaction of slaked lime and soda ash.

Na2CO3 + Ca(OH)2 → 2NaOH + CaCO3 In 1892, the electrolysis of brine was discovered as a method for making both sodium hydroxide and chlorine, and since the 1960s it has been the only method of manufacture of sodium hydroxide.

2NaCl + 2H2O → 2NaOH + H2 + C12 The brine that is used for the electrolysis must be purified, and calcium, magnesium, and sulfate ions are removed by precipitation reactions.

Na2CO3 + CaCl2 → CaCO3 + 2NaCl

2NaOH + MgCl2 → Mg(OH)2 + 2NaCl Liquid caustic soda is manufactured by electrolysis of purified and concentrated brine using mercury, diaphragm or membrane cell technology as follows:

+ - Na + Cl + H2O + electricity = ½ Cl2 + ½ H2 + NaOH Sodium chloride solution (i.e. brine) is decomposed electrolytically to chlorine at the anode of the cell and to a sodium hydroxide solution (i.e. liquid caustic soda) and hydrogen at the cathode of the cell.

The Electrolysis of Aqueous NaCl

An idealized cell for the electrolysis of sodium chloride is shown in the figure below. A source of direct current is connected to a pair of inert electrodes immersed in molten sodium chloride. Because the salt has been heated until it melts, the Na+ ions flow toward the negative electrode and the Cl- ions flow toward the positive electrode. The figure below shows an idealized drawing of a cell in which an aqueous solution of sodium chloride is electrolyzed.

Once again, the Na+ ions migrate toward the negative electrode and the Cl- ions migrate toward the positive electrode. But, now there are two substances that can be reduced at the cathode: Na+ ions and water molecules. Cathode (-): + - o Na + e Na E red = -2.71 V - - o 2 H2O + 2 e H2 + 2 OH E red = -0.83 V

Because it is much easier to reduce water than Na+ ions, the only product formed at the cathode is hydrogen gas.

- - Cathode (-): 2 H2O(l) + 2 e H2(g) + 2 OH (aq)

There are also two substances that can be oxidized at the anode: Cl- ions and water molecules.

Anode (+): - - o 2 Cl Cl2 + 2 e E ox = -1.36 V

+ - o 2 H2O O2 + 4 H + 4 e E ox = -1.23 V

The standard-state potentials for these half-reactions are so close to each other that we might expect to see a mixture of Cl2 and O2 gas collect at the anode. In practice, the only product of this reaction is Cl2.

- - Anode (+): 2 Cl Cl2 + 2 e o - o At first glance, it would seem easier to oxidize water (E ox = -1.23 volts) than Cl ions (E ox = -1.36 volts). It is worth noting, however, that the cell is never allowed to reach standard-state conditions. The solution is typically 25% NaCl by mass, which significantly decreases the potential required to oxidize the Cl- ion. The pH of the cell is also kept very high, which decreases the oxidation potential for water. The deciding factor is a phenomenon known as overvoltage, which is the extra voltage that must be applied to a reaction to get it to occur at the rate at which it would occur in an ideal system.

Under ideal conditions, a potential of 1.23 volts is large enough to oxidize water to O2 gas. Under real conditions, however, it can take a much larger voltage to initiate this reaction. (The overvoltage for the oxidation of water can be as large as 1 volt.) By carefully choosing the electrode to maximize the overvoltage for the oxidation of water and then carefully controlling the potential at which the cell operates, we can ensure that only chlorine is produced in this reaction. In summary, electrolysis of aqueous solutions of sodium chloride doesn't give the same products as electrolysis of molten sodium chloride. Electrolysis of molten NaCl decomposes this compound into its elements. electrolysis

2 NaCl(l) 2 Na(l) + Cl2(g)

Electrolysis of aqueous NaCl solutions gives a mixture of hydrogen and chlorine gas and an aqueous sodium hydroxide solution.

electrolysis

+ - 2 NaCl(aq) + 2 H2O(l) 2 Na (aq) + 2 OH (aq) + H2(g) + Cl2(g)

Because the demand for chlorine is much larger than the demand for sodium, electrolysis of aqueous sodium chloride is a more important process commercially. Electrolysis of an aqueous NaCl solution has two other advantages. It produces H2 gas at the cathode, which can be collected and sold. It also produces NaOH, which can be drained from the bottom of the electrolytic cell and sold. The electrolysis of sodium chloride to produce sodium hydroxide and chlorine can be carried out in 3 types of electrolytic cells-mercury, diaphragm and membrane cells. The diaphragm cell is used most, but the membrane cell is becoming more common as it is used in most plants. In all three cell, the step involved are:

A saturated brine (sodium chloride) solution has impurities removed by precipitation. Water used to dissolve the salt and make the brine must be purified and softened. Calcium ions are removed by adding sodium carbonate-forming insoluble CaCO3. Magnesium ions are removed by adding sodium hydroxide-forming insoluble Mg(OH)2. Iron ions are removed by adding the sodium carbonate and sodium hydroxide-forming

FeCO3 and Fe(OH)2. Sulfate ions are removed by adding -forming CaSO4. Electricity is passed through the brine solution: The electrolyte surrounding the cathode is called a catholyte, the electrolyte surrounding the anode is called an anolyte. Products are separated out-these include chlorine, sodium hydroxide, hydrogen and wastes. Manufacture of Caustic Soda and Chlorine using Diaphragm cell

Raw material: the main raw material for the manufacture of caustic soda by electrolytic method is the common salt of required purity. In India, this salt is found in various parts, such as Sambhar, Khragods, Saurashtra, Tutecorin, Adhirampatanara etc.

Brine purification: Commercial sodium chloride usually contains impurities such as calcium, magnesium and iron compounds. So, the brine purification is necessary for getting pure caustic soda and to decrease clogging of the cell diaphragm by insoluble formed during electrolysis. These impurities are removed by adding lime and soda ash, when insoluble carbonates and hydroxides are precipitated. Sometimes sulfate are 2- removed with BaCl2 or the hot brine is treated with OH- and CO3 ions. The clear brine is neutralized with hydrochloric acid. After treatment for purification the brine is allowed to settle. The brine saturated with NaCl at 600°C is fed into the cell containing 324 gms/liter of NaCl. The electrolysis is carried out in diaphragm cells; each cell usually required 3-4.5 volts. A number of them are put in series to increase the voltage of a given group. Brine electrolysis is carried out with an anode current density of 0.07 amp/cm2. As a result of electrolysis Na+ ions move to the cathode, where H+ ions and OH- ions are formed as a result of reduction of water. On the other hand Cl- ions are directed towards the anode, where they lose one electron each and are thus liberated as chlorine atoms, which unite immediately to form chlorine molecules and hence chlorine gas at the anode. Since the discharge potential of chlorine ion is lower than that of OH- ions, Cl- ions are discharge at the anode and OH- ions are remin in solution. Similarly the discharge potential of Na+ is greater than H+ ions hence H+ ions are dischare at the cathode, while Na+ ions remain in the solution.

Electrolysis reaction: Nacl=Na+ + Cl- At cathode: - - 2H2O + 2e H+ + 2OH Na+ + OH- = NaOH At Anode Cl—e- Cl- - - Cl + Cl =Cl2 Since chlorine attack caustic soda solution even in the cold, formed sodium chloride and hypochlorite, it is necessary that the construction of the cell should be such that NaOH and Cl- once formed do not come in direct contact with one another.

2NaOH + Cl2 =NaCl + NaClO + H2O Evaporation and salt separation: The caustic soda solution obtained from the cell contains about 10 to 15% caustic soda and some unconverted NaCl. The decomposition efficiency of the cells being in the range of only 50%, about half of NaCl remains unconverted and is recovered by reason of its low solubility in caustic soda solution after concentrations. Hence the weak caustic soda obtained from the electrolyte cell is first concentrated to 50% in a double or triple effect evaporator so than NaCl being less soluble and almost completely separated, particularly in the presence of caustic soda. The salt so recovered is used again. The liquid obtained from the salt separator is 50% caustic soda solution containing 2% NaCl and 0.1 to 0.5% NaCl on dry basis. Final Evaporation: The 50% NaOh solution is concentrated in huge cast iron pot on open fire. About 99% water is removed and molten caustic soda is formed. The final temperature is 500 to 600°C. These pots have now been replaced by dowtherm heated evaporators for caustic evaporation about 50%.

Another method of dehydrating 50% caustic soda is the precipitation of sodium hydroxide mono hydrate which contains much less water than the original solution. The precipitation may be carried out by adding ammonia to the 50% solution. These also purify the caustic soda. If 50% caustic soda is treated with anhydrous ammonia in pressure vessel in a counter current manner, free flowing anhydrous crystals of NaOH separate out from the resulting aqua ammonia. The hot anhydrous caustic is treated with sulfur to precipitate iron and then allowed to settle. Then a centrifugal pump is lowered by crane in the molten NaOH and the liquid is pumped out in to thin steel drums. Purification of caustic soda: 50% caustic soda solution still contains impurities such as colloidal iron, NaCl,

NaClO. Iron is removed by treating with 1% by weight of 300mesh CaCO3 and filtering the resulting mixture through a filter on a CaCO3 per coat. Sodium chloride and hypochlorite are removed by dropping the 50% caustic solution through a column of 50% NH4OH. Chlorine Drying: The hot chlorine evolved from the anode compartment contains much water vapour. It is therefore, cooled to condense most of the water vapour and further dried in a sulfuric acid scrubber. A stoneware tower or stainless steel tower with acid proof packing should be used. Wet chlorine is also handled in polyster, PVC or similar resistance material.

Rotary compressor with H2SO4 seals have been used for liquefaction process. The heat of compression is progressively removed by water and finally by refrigeration to about -20⁰F, when all the chlorine should be liquefied. It is further cooled -50⁰F and the liquid chlorine is led to a steel storage tank and then filled in steel cylinder of 50-100 kg capacity for sale. Hydrogen: Hydrogen evolved at the cathode is either burnt for boiler fuel or used as hydrogen source. Membrane cell The anode chamber is fed with purified and concentrated brine. Sodium chloride solution (i.e. brine) is decomposed electrolytically to chlorine at the anode of the cell and sodium ions migrate to the cathode chamber through the membrane. Depleted brine exits the anode chamber. 2 NaCl = 2 Na+ + 2 Cl-

- + 2 Cl = Cl2 + 2 e

The cathode chamber is fed simultaneously with diluted liquid caustic soda. Water is decomposed electrolytically to hydrogen at the cathode of the cell and hydroxide ions are combined with sodium ions. Concentrated liquid caustic soda (i.e. sodium hydroxide solution) exits the cathode chamber.

- - 2 H2O + 2 e H2 + 2 OH

2 Na+ + 2 OH- 2 NaOH

The exchange membrane separating the anode and the cathode chambers consists of a substrate with carboxylic or sulfonic acid functional groups; this membrane is permeable to sodium ions but impervious to chloride and hydroxide ions.

Fig 5: Membrane cell for caustic soda production Advantages:  Pure sodium hydroxide  Electrical energy consumption only about 77% of that of the mercury process  No mercury or asbestos used Disadvantages:  Chlorine gas contains oxygen  Very high purity brine required  Present high cost and short lifetime of the membrane

Fig. 6: Manufacturing of soda ash by different process Mercury Process: Advantages  Pure 50% sodium hydroxide solution ( without evaporation)  Pure chlorine gas Disadvantages:  Higher voltage than with the diaphragm process and hence 10 to 15% higher electrical energy consumption.  More stringent brine purification requirements  Stringent mercury contamination avoidable measures required Diaphragm Process: Advantages  Utilization of less pure brine  Lower voltage than in the mercury process Disadvantages:  Sodium hydroxide produced is both dilute and chlorine-contaminated, evaporation required  Chlorine gas contains oxygen  Rigorous measures required to avoid asbestos emission

Uses of caustic soda  Chemical Production-The chemical industry consumes nearly 40% of the caustic soda produced as a basis reagent for a multitude of general industrial applications.  Pulp and Paper- Both sulfate and sulfite pulps are purified by removing lignin compounds in the caustic extraction stages of multistage plants. In some kraft mills, caustic soda is also used as a makeup chemical. It is also used as the initial treatment in deinking secondary fibers.  Rayon and Cellophane-Fiber production by the viscose process requires caustic soda at two main stages. Cellulose is treated with caustic soda solution to mercerize it and form alkali cellulose, which is then dissolved in dilute caustic soda solution to form viscose prior to extruding rayon fibers and cellophane films.  Alumina Extraction- Caustic soda is used to digest bauxite ore, precipitating alumina (aluminium oxide). It is also used as an etchant in the finishing and chemical milling of aluminium products.  Soapmaking-Caustic soda saponifies fats into water soluble sodium .  Textiles- Used in scouring, bleaching, desizing, lustering and merccerizing.  Petroleum Production and Refining- Caustic soda is used as an absorbent for in light petroleum fractions; as an absorbent for sulfides in the purification of various fractions; and with chlorine for hypochlorite sweetening, a treatment step in the removal of various sulfur compounds. Chlorine - disinfectant and purifier, manufacture of hydrochloric acid and making plastics Hydrogen - manufacture of hydrochloric acid and potential as a pollution-free fuel Environmental impacts of the electrolysis of brine on a large scale must be considered. The process uses a lot of electricity that is mainly produced by the burning of fossil fuels. During the actual process of electrolysis, metal must be in contact with the solution of brine. A metal commonly used is mercury which is toxic. Some mercury escapes into the solution and into the environment.

MANUFACTURE OF SULFURIC ACID

Properties: Physical Properties: Molecular formula-H2SO4 Molecular weight-98 gm/mole Appearance-Water white slightly viscous liquid, Boiling point-290⁰C, 340⁰C (decomposes), Melting point-10.5⁰C Solubility-Miscible with water in all proportions. Aqueous sulfuric acid solutions are defined by their H2SO4 content in weight percent terms.

Dissolve any quantity of SO3, forming oleum (fuming sulfuric acid). The physical properties of sulfuric acid and oleum are dependent on H2SO4 and SO3 concentration, temperature, pressure.

Chemical Properties: 1. Dehydrating agent  Has a great affinity for water and the reaction is extremely exothermic

 Used for drying almost all gases, except NH3 and H2S.  Its corrosive action on skin is also due to dehydration of skin which then burns and produces itching sensation.  Due to dehydrating property, it chars sugar to give carbon.

C12H22O11 12C + 11H2O  It is also used in removing water from various substances such as oxalic acid and formic acid.

COOH-COOH H2O + CO + CO2 1. Oxidizing agent

Gives O2 on strong heating, hot conc. H2SO4 also acts as an oxidizing agent. 2. Pickling agent Find application in pickling in which layers of basic oxides are removed before electroplating, galvanizing, enameling and soldering. Methods of Production 1. Contact process 2. Chamber process Both contact and chamber process are based on SO2. Chamber process was developed first (1746) but produces acid concentration of less than 80%. Contact process yields 98% H2SO4 and higher which can be diluted, if necessary. : (a) S(g) + O2(g) SO2 ΔH=-71.2 Kcal/mole

(b) SO2 + 1/2O2 SO3 ΔH=-46.3 Kcal/mole

(c) SO3 + H2O H2SO4 ΔH=-31.1 Kcal/mole

(d) H2SO4 + SO3 H2S2O7 (Oleum) Raw Materials The source of sulfur and sulfur oxide are as follows  Sulfur from mines  Sulfur or recovered from petroleum desulfurization  Recovery of sulfur dioxide from coal or oil burning public utility stack gases  Recovery of sulfur dioxide from the smelting of metal sulfide ores

2PbS + 3O2 2PbO + 2SO2

 Isolation of SO2 from pyrite Quantitative requirement: (a) Basis: 1 ton of 100% H2SO4

SO2 0.67 ton Air 1450-2200 Nm3 (b) Plant capacities: 50-1000 tons/day of 100% acid Catalyst: Most widely used catalyst is Vanadium pentoxide dispersed on a porous carried in pellet form. Platinum catalyst was previously used but suffers from easy poisoning, rapid heat deactivation, and high initial investment. Characteristics of catalyst i) Porous carrier having large surface area, controlled pore size and resistance to process gases at high temperature ii) Active catalytic agent iii) Promoter-alkali and/or metallic compounds added in trace amounts to enhance activity of catalytic agent.

Difference between V2O5 and Platinum catalyst used in H2SO4 Industry: Aspect Vanadium pentoxide (V2O5) Platinum catalyst Conversion Higher Lower and decreases with use Investment Initially less, 5% replacement is High, lower life and highly fragile required per year Catalyst Poisoning Relatively immune to poison Poisoned, especially by arsenic Handling of SO2 Less (7-8%) High (8-10%) Requirement per 1000kg (100% 14kg catalyst mass containing 7- 189gms acid)/day 8% V2O5

Process Description: The starting material for sulfuric acid manufacture is clean, dry sulfur dioxide gas. This can be obtained by burning molten sulfur from metallurgical off gases or by decomposing spent sulfuric acid. The basic steps involved in contact process are as follows 1. Burning of sulfur

2. Catalytic oxidation of SO2 to SO3

3. Absorption of SO3 to form sulfuric acid Step 1 – Burning of sulfur This reaction is described by the equation: -1 S + O2 → SO2 ΔH = -300 kJ mol Elemental sulfur is purchased on the international market having been recovered as a by-product of the oil refining process. This sulfur is melted by steam coils at 140°C in brick lined tanks. The molten sulfur is filtered to remove any impurities (usually iron or organic compounds). Lime is added to reduce the acidity of the molten sulfur therefore reducing its corrosivity. The molten sulfur is pumped to the burner where it is burnt in an excess of dry air. The gas exiting the burner is maintained at 8 - 9%v/v sulfur dioxide and approximately 830°C due to the heat produced by the exothermic reaction. The sulfur dioxide/air gas mixture is then passed through the hot gas filter, where any ash contamination is removed. After that gas is cooled, SO2 enters washing tower where it is sprayed by water to remove any soluble impurities. In the drying tower, H2SO4 is sprayed on the gas to remove the moisture content.

Finally arsenic oxide is removed when the gas is exposed to ferric hydroxide, Fe(OH)3.

Step 2 – Catalytic oxidation of SO2 to SO3 The sulfur dioxide is converted to sulfur trioxide by reacting with oxygen over a catalyst. This reaction is described by the equation:

SO2 + ½O2 → SO3 ΔH = -100 kJ mol-1 This reaction occurs in the converter, a four-stage reaction vessel with each stage consisting of a solid catalyst bed through which the gas is passed. The catalyst used is vanadium pentoxide (V2O5) and potassium sulphate dispersed on a silica which forms a porous support, giving a large surface area for reaction. It is believed that the V2O5 increases the rate of the overall chemical reaction by oxidizing the SO2 to SO3 and being re- oxidized itself by the oxygen in the gas stream. This reaction is exothermic and its equilibrium constant decreases with increasing temperature (Le Chatelier.s Principle). Figure 7(a) shows the percentage conversion of SO2 to SO3 that would be reached at an SO2 concentration of 8% v/v and a range of gas temperatures. However, the reaction rate is also temperature dependent, so that if the temperature becomes too low the

equilibrium point will not be reached. In practice, the gas temperature must be maintained between 400 - 500°C to maintain a high reaction rate and also a high conversion equilibrium.

As the reaction is exothermic, heat is generated across each of the catalyst beds. This heat must be removed between each stage to maintain the optimum reaction temperature into the following stage. The temperature rise through each catalyst bed and the inter-stage cooling is shown in Figure 7(a).

(a) (b) Figure 7 (a): Effect of temperature on the conversion of SO2 into SO3 (b) Vapor pressure above sulfuric acid

The greatest degree of cooling is required between the first and second stages. Cooling after the second and third stages is by injection of dried air. The gas exiting the converter is used to pre-heat the boiler feed water. A simplified process flow diagram is shown in Figure 8.

Step 3 - Absorption of SO3 to form sulfuric acid

The gas is passed to the absorption tower, a packed tower where SO3 is absorbed into a counter-current flow of 98 - 99% sulfuric acid. The overall reaction can be described by the following equation, where sulfur trioxide reacts with the free water to produce sulfuric acid: -1 SO3 + H2O → H2SO4 ΔH = -200 kJ mol

The circulating sulfuric acid must be maintained at about 98% concentration and 70°C to maximize the absorption efficiency. The acid strength is important because the vapour pressure of sulfur trioxide above sulfuric acid is at a minimum at an acid strength of 98% (see Figure 7(b)). At higher concentrations the increased vapour pressure is caused by SO3 and at lower concentrations the water vapour pressure increases sharply and the resultant acid mist is not readily re-absorbed and escapes to the atmosphere.

Fig. 8: Detail layout of H2SO4 Production

The sulfuric acid is circulated at such a rate that there is only a very small increase in concentration through the absorber tower. Dilution water is added to the circulating acid tank and also as atmospheric water absorbed in the drying tower. A stream of sulfuric acid is continuously bled off and cooled through a plate heat exchanger before being passed into the storage tanks. The overall conversion from sulfur to sulfuric acid is greater than 98.5%. The plant operates under an air discharge permit which controls emissions of sulfur dioxide and total acidity. Traditionally mild steel has been used as the primary material of construction for process equipment containing 98% sulfuric acid. The corrosion rate is reasonably low, except at the air/liquid interface where atmospheric moisture encourages corrosion. The trend is now towards more sophisticated materials including Teflon-lined steel pipe work to reduce iron contamination of the sulfuric acid.

Kinetics and thermodynamics: The crucial step is the oxidation is the oxidation of SO2 to SO3. At normal condition, the equilibrium lies far to the left and the amount of SO3 formed is very small. To improve the yield of SO3, the reaction is carried out at around 450⁰C and 1.5-1.7atm pressure in presence of V2O5 or Pt as catalyst.

2SO2 + O2 2SO3 ΔH=-46.9Kcal/mole

These conditions are chosen by applying Le Chatelier’s principles as explained below

Effect of Temperature: Since the forward reaction is exothermic, at higher temperatures the backward reaction i.e. the dissociation of SO2 is more favored. However, at very low temperature, the rate of combination of SO2 and O2 is very slow and at higher temperature of about 450⁰C, the rate of formation of SO3 is high and rate of decomposition of SO3 is minimum. Hence, the temperature range which best meets kinetics and thermodynamics requirements for higher yield in the synthesis of SO3 is located in between 400⁰C to 500⁰C, with optimum temperature at about 450⁰C.

Effect of pressure: In the forward reaction i.e. formation of sulfur trioxide, the number of moles of gaseous components decreasing.

Δng= (2)-(2+1) = -1

The formation of SO3 takes place with decreasing in volume and hence increasing in pressure is expected to increase the rate of formation of SO3, i.e., rate of forward reaction. However, it has been observed that there is no appreciable change in the yield at higher pressure. Also, higher pressure of 15-1.7atm is usually satisfactory.

The equilibrium constant in terms of partial pressure is given by

2 푝 푆푂3 Kp= 2 푝 푆푂2푝푂2

Major engineering problems:

 Design of multistage catalytic converter for highly exothermic reaction. Earlier two stage converter is used but nowadays the design of three or four stages rather than conventional two stage operation are developed.  Thin catalyst bed of 30-50 cm height used to avoid above difficulties. Yield can drop due to longitudinal mixing if the convective gas velocity through the bed is low.

 Removal of heat of absorption of SO3 in acid. Pipe coolers with water dripping over external surface have been replaced by cast iron pipe with internal fins to promote better heat transfer.  Pressure drop must be low, so, 8cm stacked packing is often used.  Optimization of space velocity in catalyst chamber: Pumping cost versus fixed charges of reactor  Adaption of process to various types of gas feeds Important Uses:

 The largest single use is in the fertilizer industry  Mostly in production of phosphoric acid, which in turn used to manufacture fertilizers such as triple superphosphate, mono and diammonium phosphates.

 In the inorganic chemical industry e.g. in the production of TiO2 pigments, hydrochloric acid and hydrofluoric acid.  In the metal processing industry e.g. for pickling and descaling steel, for leaching copper, uranium and vanadium ores in hydrometallurgical ore processing and in the preparation of electrolytic baths for nonferrous metal purification and plating.  Certain pulping processes in the paper industry requires sulfuric acid.  Used in textile and chemical fibre processes and leather tanning.  In manufacture of explosives, detergents and plastics.

SODIUM CARBONATE INTRODUCTION

Sodium carbonate (Na2CO3) also known as washing soda or soda ash, is a sodium salt of carbonic acid. Most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate. Sodium carbonate is domestically well known as a water softener. It can be extracted from the ashes of many plants. It is synthetically produced in large quantities from salt and limestone in a process known as the Solvay process. Soda ash is the most important high tonnage, low cost, reasonably pure, soluble alkali available to the industries as well to the laboratory. Properties: Molecular weight-106, Melting Point 851°C, Boiling Point-Decomposes, Solubility: Soluble in water (8.9gm per 100g at 20°C)

Grades: 99% sodium carbonate (58% Na2O) as light (solids density 1.86, bulk density 0.6) and dense (solids density 1.91, bulk density 1.0) grades of granular product

Washing soda (Na2CO3.10H2O) MANUFACTURING PROCESS Sodium carbonate is manufactured by following process. 1. Leblane process.

2. Solvay’s ammonia soda process.

3. Dual process (modified Solvay’s process)

4. Electrolytic process.

Leblane process

The process has only historical importance, because is now been replaced completely by Solvay process or modified by Solvay process. Raw materials Basis: 1000kg Sodium carbonate (98% yield) Common salt = 1126kg Sulfuric acid = 945kg Lime stone = 963kg Coke = 463kg Sources of raw material Common salt can be obtained from sea water, salt lake. Sulfuric acid can be obtained by contact process. Lime stone is obtained from mineral calcite or aragonite, which can be used after removal of clay, slit and sand (silica). Chemical Reactions NaCl + H2SO4 NaHSO4 + HCl NaHSO4 + NaCl Na2SO4 + HCl Na2SO4 + 4C Na2S + 4CO

Na2S + CaCO3 Na2CO3 + CaS (Black ash sludge) CaS + H2O + CO2 CaCO3 + H2S CaS + H2S Ca(HS)2 Ca(HS)2 + CO2 + H2O CaCO3 + 2H2S H2S + O H2O + S

Manufacturing Process

Common salt is first mixed with the conc. H2SO4 in equivalent quantities and heated in a cast iron salt cake furnace by flue gases from adjacent coal of fire. NaHSO4 along with HCl gas is formed. HCl is passed to tower packed with coke and is absorbed through a spray of water comes down in the tower. The paste of NaHSO4 is taken out and heated to a high temperature on the hearth of a furnace along with some more common salt.

NaHSO4 is thus converted into sodium sulfate, known as salt cake. The salt cake is broken or pulverized, mixed with coke and limestone and charged into black ash rotary furnace consisting of refractory lined steel shells. The mass is heated by hot combustion gases entering at one end and leaving at the others. The molten porous gray mass thus formed known as black ash is separated from the calcium sludge and then crushed and leached with water in absence of air in a series of iron tank.

The extract containing Na2CO3, NaOH, and other impurities is sprayed from the top of a tower in counter current to flow of hot gases from the black ash furnace. The sodium carbonate thus obtained is concentrated in open pans and then cooled to get sodium carbonate. The product is calcined to get soda ash which is re- crystallized to Na2CO3.10H2O. The sludge containing mostly CaS is left behind as alkali waste.The liquor remaining after removal of first batch of soda ash crystals is purified and then causticized with lime to produce caustic soda.

Recovery of sulfur from alkali waste

Alkali waste is charged into cylindrical iron vessels arranged in series and CO2 delivered from lime kilns is passed through it, the H2S gas thus obtained is then conduced together with a regulated amount of air in a Claus kiln containing iron oxide as catalyst. The exothermic reaction proceeds without further external heat. Recovered sulfur is used in the manufacture of sulfuric acid.

Solvay's ammonia soda process Raw materials: Quantitative requirement: Basis: 1000kg sodium carbonate Salt = 1550kg Limestone = 1200kg Coke = 90kg Ammonia as a catalyst = 1.5kg (Loss) High pressure steam = 1350kg Low pressure steam = 1600kg Cooling water = 40000 - 60000kg Electric power = 210KWH Sources of raw material Common salt can be obtained from sea water, salt lake. Lime stone is obtained from mineral calcite or aragonite, which can be used after removal of clay, slit and sand

(silica). NH3 make-up in the recirculation load amounts to about 1.5 kg/ton of Na2CO3. Chemical Reactions:

CaCO3 CaO + CO2 ΔH = + 43.4kcals

C(s) + O2 (g) CO2 (g) ΔH = - 96.5kcals

CaO(s) + H2O (l) Ca(OH)2 (aq) ΔH = - 15.9kcals

NH3(aq) + H2O(l) NH4OH(aq) ΔH = - 8.4kcals

2NH4OH + CO2 (NH4)2CO3 + H2O ΔH = - 22.1kcals

(NH4)2CO3 + CO2 + H2O 2NH4HCO3

NH4HCO3 + NaCl NH4Cl + NaHCO3

2NaHCO3 Na2CO3 + CO2 + H2O ΔH = + 30.7kcals

2NH4Cl + Ca(OH)2 2NH3 + CaCl2 + 2H2 ΔH = + 10.7kcals

Overall reaction

CaCO3 + 2NaCl Na2CO3 + CaCl2

Ammonia is dissolved in a salt solution and ammoniate brine solution is allowed to react with CO2 which is obtained by calcining lime stone with coke. A precipitate of NaHCO3, thus obtain is then calcined to produce high purity Na2CO3.

Fig. 10: Manufacturing of Soda ash by Solvay process

Preparation and purification of brine Saturated solution of NaCl is used. Brine contains impurities such as calcium, magnesium and iron compounds. To remove calcium sulfate, magnesium and iron salts sodium carbonate and sodium hydroxide are added. The precipitated carbonates and hydroxide are removed by filtration. Sometimes sulfate are removed with BaCl2 or 2- the hot brine is treated with OH¯ and CO3 ions. The calcium, magnesium and iron salts from saturated brine may be precipitated by dilute ammonia and CO2 in a series of washing towers. The brine is purified by allowing it to settle in vats, as a result of which precipitated CaCO3, MgCO3, Mg(OH)2 and iron hydroxide settle down and pure brine solution is pumped to the ammonia absorber tower, where it dissolve NH3 with the liberation of heat. The production process is completed by following five steps -

I. Ammoniation of brine: The purified brine is allowed to percolate down the ammonia tower in which ammonia gas is passed through the bottom in a counter current fashion. The brine solution thus takes up the necessary amount of ammonia and liberates heat. The gas which escapes solution in the tank is absorbed by the brine falling down the tower. Some carbon dioxide is also absorbed by ammonia, as a result of which some insoluble carbonate is also precipitated. The ammoniated brine is allowed to settle, coded to about 30°C and pumped to the carbonating tower.

II. Carbon dioxide formation: Limestone is calcined to get CO2 in a lime kiln filled with coke. As a result of burning of coke necessary heat required for the decomposition of lime stone is generated. CaO obtained from the lime kiln is converted into slaked lime and pumped to the ammonia recovery tower.

III. Carbonation of ammonium brine: CO2 from the lime kiln is compressed and passed through the bottom of carbonating tower down which ammoniated brine percolates. Carbonating towers operated in series with several precipitation towers are constructed of cast iron having 22-25meter height, 1.6-2.5meter in diameter. During the precipitation cycle, the temperature is maintained about 20-25°C at the both ends and 45- 55°C at the middle by making use of cooling coils, provided at about 20ft above the bottom. The tower gradually becomes flooded as sodium bicarbonate cakes on the cooling coils and shelves. The cooling coils of the foulded tower are shut off. Then the fresh hot ammoniated brine is fed down the tower in which NaHCO3 are dissolved to form ammonium carbonate solution. The solution containing (NH4)2CO3, unconverted

NaHCO3 is allowed to fall down a second tower, called making tower. The making, towers are constructed with a series of boxes and sloped baffles. Ammoniated brine and CO2 gas (90-95%) from the bicarbonate calciner is recompressed and pumped to the bottom of the making tower. The ammonium carbonate first reacts with CO2 to form ammonium bicarbonate and the latter reacting with salt, forms sodium bicarbonate. The heat of exothermic reaction is removed by cooling coils.

IV. Filtration: NaHCO3 slurry is then filtered on a rotary vacuum filter which helps in drying of bicarbonate and in recovering ammonia. The filter cake after removal of salt and NH4Cl by washing with water, sent to a centrifugal filter to remove the moisture or calcined directly. During washing, about 10% NaHCO3 also passes into filtrate. The filtrate containing NaCl, NH4Cl, NaHCO3 and NH4HCO3 is treated with lime obtained from lime kiln to recover NH3 and CO2.

V. Calcination: NaHCO3 from the drum filter is calcined at about 200°C in a horizontal calciner, which is either fired at feed end by gas or steam heated unit. The heating being through the shell parallel to the product, which prevent the formation of bicarbonate lumps.

The hot soda ash form the calciner is passed through a rotary cooler and packed in bags. The exit gases (CO2,

NH3, steam etc.) are cooled and condensed to get liquid ammonia; the rich CO2 gas is cooled and returned to the carbonating tower. The product from the calciner is light soda ash. To produce dense soda ash, sufficient water is milled with it to form more mono hydrate Na2CO3.H2O and the mixture is recycled. Recovery of ammonia: The ammonia is recovered in strong ammonia liquor still, consisting of two parts. The parts above and below the lime inlet is called as heater and lime still respectively. The filtrate obtained from washing of NaHCO3 from the pressure type rotary filter is fed into the heater, where free ammonia and carbon dioxide are driven off by distillation. Dry lime or milk of lime (slaked lime) obtained from lime kiln is fed through the lime inlet and mixed with the liquor from the heater. As the liquor flows down the column, calcium chloride and calcium sulfate are formed and NH3 gas is released.

NH4Cl + Ca(OH)2 CaCl2 + 2NH3 + H2O

(NH4)2SO4 + Ca(OH)2 CaSO4 + 2NH3 + 2H2O The liquor from the bottom of the lime still is free from ammonia and contains unreacted NaCl and largely

CaCI2, which is disposed of. The liquor is, therefore allowed to settle in settling ponds and the clear liquid is evaporated till the salt separates out and is sold as such for calcium chloride or further evaporated.

Fig. 11: Recovery of Ammonia Advantages of Solvay process:  Can use low-grade brine  Less electric power  Less corrosion problems  No co-products to dispose of

 Does not require NH3 plant investment

Disadvantage of Solvay process:  Higher salt consumption

 Higher investment in NH3 recovery units versus crystallization units for NH4Cl

 Waste disposal of Cacl2-brine stream  More steam consumption  Higher capacity plant for economic break-even operation

 With current fertilizer shortages, all of the NH4Cl will be used as a mixed chemical fertilizer ingredient, so co-product disposal no problem.

Major engineering problems

(a) Development of suitable calcining equipment Moist NaHCO3 will cake on sides of the kiln, preventing effective heat transfer through the shell. The kiln must be equipped with heavy scraper chain inside and wet filter cake must be mixed with dry product to avoid caking. These problems can be avoided by using fluidized bed calciners in newer installations. (b) Economic balance on tower design The tower height, pressure and temperature are optimized, giving approximately 75% yield of

NaHCO3 from NaCl.

(c) Ammonia recovery NH3 inventory costs 4-5 times that of Na2CO3 inventory so losses must be kept low. By proper choice of equipment design and maintenance, losses are less than 0.2% of recycle load (0.5 kg/kg product) or about 1 kg/ton of Na2CO3.

(d) Plant modernization Three solvay plants were built prior to 1947 in India. These had to be modernized:  Substituting better materials of construction in replacement maintenance  Use of automatic control. (e) Waste disposal: Find uses for large quantities of CaCl2-NaCl liquor or dispose as waste.

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