Hydrogen, Nitrogen and Oxygen Plants

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Nitrogen & Oxygen

INTRODUCTION Properties of Oxygen • Molecular formula : O2 • Molecular weight : 32gm/mole • Appearance : Colourless gas • Odour : Odourless • Boiling point : -182.950C • Melting point : -218.790C • Density : 1.429gm/L (00C,101.325kPa) • Solubility : Sparingly soluble in water • Composition in air : 20.99% 2

Nitrogen & Oxygen

INTRODUCTION •

•

Oxygen was independently discovered by Carl Wilhelm Scheele and Joseph Priestley in 1773 and 1774 respectively, but work was first published by Priestley. Antoine Lavoisier named as oxygen in 1777, whose experiments with oxygen helped to discredit the then-popular phlogiston theory of combustion and corrosion. Oxygen is produced industrially by fractional distillation of liquefied air, use of zeolites with pressure-cycling to concentrate oxygen from air, electrolysis of water.

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Nitrogen & Oxygen

INTRODUCTION Properties of Nitrogen • Molecular formula : N2 • Molecular weight : 28gm/mole • Appearance : Colourless gas • Odour : Odourless gas • Boiling point : -195.790C • Melting point : -2100C • Density : 1.251gm/L (00C,101.325kPa) • Solubility : Slightly soluble in water • Composition in air : 78.01 4

Nitrogen & Oxygen

INTRODUCTION • Nitrogen was discovered by Daniel Rutherford in 1772, who called it noxious air or fixed air. He explains that nitrogen does not support combustion. • Antoine Lavoisier referred nitrogen as inert gas and as "mephitic air" or azote, in which animals died and flames were extinguished. English word nitrogen entered the language in 1794.

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INTRODUCTION USES of Oxygen • It is used to produce oxyacetylene flame in cutting and welding the metals • Used in L. D. process for steel production • Used for artificial respiration in case of patients • Used for mountain climbers and high altitude flights

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INTRODUCTION Uses of Nitrogen • Used in manufacture of synthetic ammonia, nitric acid • Used in manufacture organic nitrates like propellants and explosives, • Synthetically produced nitrates - key ingredients of industrial fertilizers • Used in producing nitrogen oxide. • Applied to create inert atmosphere.

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Nitrogen & Oxygen

INTRODUCTION Critical Temperature and Pressure Nitrogen Critical Temperature

-147.13 C

Critical Pressure

33.49 atm

Oxygen -118.75 C 49.7 atm

Critical temperature Critical temperature is the temperature below which any gas can be liquefied by increasing the pressure. Above the critical temperature any gas cannot be liquefied by compression. Critical Pressure The minimum pressure under which gas liquefies at the critical temperature is called as critical pressure. 8

Nitrogen & Oxygen

INTRODUCTION

Critical Temperature(CT) and Pressure(CP) for gases

No

Gas

CT(0C)

CP(atm)

1

Ethylene

+9.5

50.65

2

Methane

-82.85

45.6

3

Hydrogen

-239.9

12.8

4

Acetylene

+35.5

61.55

5

NH3

+132.5

112.3

6

CO2

-138.7

34.6

7

CO

+31.3

72.9

9

Nitrogen & Oxygen

Liquefaction of air by Joule - Thomson effect

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Nitrogen & Oxygen

Liquefaction of air by Joule - Thomson effect

• CO2 free air is compressed to 200atm and is cooled by water. • The condensed water is removed by passing through activated alumina. • Then air is passed through inner coil of heat exchangers. The valve with nozzle helps in the expansion of air. • The cold air goes out through the outer coil, recompressed to 200atm pressure, cooled by water and then again allowed to transverse the inner coil.

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Nitrogen & Oxygen

Liquefaction of air by Joule - Thomson effect

• The temperature of the incoming air falls due to the presence of cold air in the outer coil. • The cooled air expands through the nozzle, the temperature becomes lower than in the first operation. • The colder air now passes through the outer coil producing an atmosphere of lower temperature. • As the air passing repeatedly through the inner coil it undergoes Joule-Thomson effect- below the critical temperature of O2 & N2. • At this point air liquefies and liquid air is collected in the container.

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MANUFACTURE

Linde's process (O2 and N2)

Basis: 1000kg Oxygen (95%) Raw materials • Air = 3600Nm3 • Steam = 1750kg • Cooling water = 5000kg • Electricity = 450-480KWH

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MANUFACTURE

Linde's process (O2 and N2)

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MANUFACTURE

Linde's process (O2 and N2)

The distillation tower has bubble cap tray double columns arranged one above another. Intermediate distillation is done for effective separation of liquid enriched with O2. The column feed is liquefied air at 200atm pressure introduced at the bottom of the column. The construction of dome includes number of internal pipes so that distillate of the lower column collides to the roof and is returned back to the column as reflux.

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MANUFACTURE

Linde's process (O2 and N2)

Nitrogen with a small oxygen impurity collects at the top of the enrichment column- sent as the reflux in the rectification column. After number of recycling, liquid with 82% concentration of O2 is taken in outer part of dome. Gaseous N2 is the top product of the column and the bottom product is liquid O2.

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Hydrogen

INTRODUCTION

Properties • Molecular formula : H2 • Molecular weight : 2.0gm/mole • Appearance : Colourless gas • Odour : Odourless • Boiling point : -252.870C • Melting point : -259.140C • Density : 0.08988 gm/L (0°C, 101.325kPa) • Solubility : Slightly soluble in water 17

Hydrogen USES INTRODUCTION

• In fertilizer industries to produce NH3 which is converted into (NH4)2SO4, urea and HNO3 • In hydrogenation of oils to make fats or in hardening of fatty oils. • In production of HCl, which is used in large quantity in industries. • For filling in metrological balloons which are essential for upper air observation to guide the air flights. • In making oxy-hydrogen flame used for melting of platinum, quartz and in auto welding of lead. • In producing an inert media and in making tungsten filaments for electric lamps, mixture of nitrogen and hydrogen is used.

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Hydrogen

INTRODUCTION • Anaerobic oxidation of iron by the protons of water at high temperature can be schematically represented by the set of following reactions

• Hydrogen was first liquefied by James Dewar in 1898 by using regenerative cooling in the vacuum flask.

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Hydrogen

MANUFACTURE Methods of Production The various method used for production of hydrogen gas are as follows. • Electrolytic process • Lane process or iron steam process • Steam hydrocarbon process • Liquefaction of coal gas and coke oven gas • Bosch process or water gas-steam process

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Electrolytic Process

MANUFACTURE Pure hydrogen along with oxygen is manufactured by electrolytic process. It is also obtained as a by-product in the production of caustic soda by electrolysis of aqueous solution of sodium chloride. Reactions In acidulated water At cathode

At anode

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Electrolytic Process

MANUFACTURE In KOH Solution At cathode

At anode

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Electrolytic Process

MANUFACTURE • A dilute aqueous solution of acid or alkali is decomposed by passing direct current through them to get high purity hydrogen (99.7%) and oxygen gas by the Electrolytic method. • • Electrolysis is done in a photovoltaic cell using iron cathode and nickel anode with an asbestos diaphragm separating the cells into two compartments. • • A 15% sodium hydroxide solution is used to electrolyze at temperature of about 60-700C.

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Electrolytic Process

MANUFACTURE

24

Lane process

MANUFACTURE Raw material • Iron • Steam

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Lane process

MANUFACTURE Iron oxide and water gases are charged to fire bricks lined generator which is heated externally by burning of producer gas or other gaseous fuel. The temperature is maintained at 6500C, first by heating the iron oxide and second by passing superheated steam through the iron forming from iron oxide.

The spent gas coming out of the generator are sent through the super heater which is a chamber filled with checker work. The hydrogen goes out through the pipe at the bottom of the chamber. 26

Steam Hydrocarbon Process

MANUFACTURE • Catalytic steam-hydrocarbon reforming began commercial

operation in 1930 and by 1965 most of the hydrogen and synthesis gas mixture are produced by this method. • Gaseous hydrocarbons (methane and ethane) and low boiling liquid (propane, butane) and other normally liquid hydrocarbons up to octane are reacted with steam over nickel catalyst at 650-9500C to produce carbon oxides and hydrogen.

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MANUFACTURE Raw material

Steam Hydrocarbon Process

• Gaseous hydrocarbon/liquid hydrocarbon up to octane • Air • Catalyst Reactions Overall reaction

From propane

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Steam Hydrocarbon Process

MANUFACTURE  Coke oven gas free from sulfur compounds is scrubbed with water under pressure and weak alkali solution to remove CO2  Methane extracted from natural gas may be used.  Finely divided nickel supported on carrier of silicate used as a catalyst.  The temperature of the endothermic reaction is maintained at 8150C by partial combustion of methane in presence of oxygen.  In case of propane as the raw material the reaction is endothermic. The temperature is maintained at 8500C either by external heating or internal combustion as in case of methane.  For high-purity hydrogen, the product is reacted catalytically with additional steam to oxidize CO to CO2. CO2 is then removed. 29

MANUFACTURE

Liquefaction of coal gas

 The coke oven gas was first purified from H2S, HCN, NH3, CO2 and light oil.  The gas was compressed to 250 to 300psi and then first scrubbed in a pressure bubble cap tower with dilute ammonia to remove CO2, and HCN.  Then the scrubbing was done with water to remove ammonia, then the gas was scrubbed with a petroleum oil solvent to remove the light oil, and finally the gas was washed with an alkali solution to remove the remaining CO2.  The gas is dried, compressed and subjected to cooling to remove ethylene and other olefins first, then methane and other gaseous paraffin. Then the gas is further compressed and cooled so that nitrogen is liquefied and hydrogen remains in the gaseous condition having very high purity. 30

Bosch Process

MANUFACTURE  Bosch process is used in the industrial preparation of Hydrogen. Raw Materials • Water • Coke  steam is passed over red hot coke(carbon) at a temperature of about 1200C, a mixture of CO+H2is produced. Excess steam + water gas -passed over a catalyst at 450C. The resultant effect is that the carbon(II) oxide in the water gas is converted to carbon(IV) oxide, CO is converted to CO2 with a further yield of hydrogen. CO2is removed from the mixture by dissolving it in water under pressure of 30 atmospheres or in other solvents such as caustic soda solution. 31

Synthesis Gas industry

32

INTRODUCTION • Syngas, or synthesis gas, is a fuel gas mixture consisting primarily of hydrogen, carbon monoxide, and very often some carbon dioxide. • The name comes from its use as intermediates in creating synthetic natural gas (SNG) and for producing ammonia or methanol. • Syngas is usually a product of gasification and the main application is electricity generation. Syngas is combustible and often used as a fuel of internal combustion engines. Syngas is also an intermediate in creating synthetic petroleum to use as a lubricant or fuel. • syngas has 50% of the energy density of natural gas. It cannot be burnt directly, but is used as a fuel source.

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Uses

INTRODUCTION • Electrcity Generation: Electricity can be generated from the power provided by the combustion of syngas at the cost of zero carbon emissions • Gas Engines: Syngas is considered a renewable fuel since its origins mainly come from biological materials such as organic waste. • Intermediate for other compounds: intermediate in generating synthetic natural gas, synthetic petroleum and to create ammonia or methanol.

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Synthesis Methods • Carbon feedstock is reacted with H2O and/or O2 to produce H2 and CO in a process called Gasification

• Types of carbon feedstocks: • Natural gas and Heavy Oil: • Requires purification of methane and higher hydrocarbons respectively

• Biomass and Coal: • Requires pyrolysis prior to gasification • Pyrolysis: decomposition of carbon material by heating in the absence of oxygen

Gasification: Steam Reforming • Steam Reforming: • • • •

Feedstock reacts with steam to produce CO and H2 CH4 + H2O  CO + 3H2 ΔH = +206kJ/mol Results in CO:H2 ratio of 1:3 Highly endothermic reaction • Operating temperature can range from 800K to 1500K • Heat generated by combusting part of feed stock or external heating • Catalysts used to enhance reaction kinetics

Gasification: Partial Oxidation • Partial Oxidation: • Feedstock reacts with oxygen to produce CO and water; generated water reacts with feedstock • CH4 + 0.5O2  CO + 2H2 ΔH = -38kJ/mol • Results in CO:H2 ratio of 1:2, which is desirable for methanol synthesis • Exothermic, so requires less heat generation

Other Gasification Reactions • Autothermal Reforming: • Combines steam reforming and partial oxidation into one process • Can be used with CO2 feed to yield different CO:H2

• Water Gas Shift: • Equilibrium reaction converting between CO and H2 • CO + H2O ↔ CO2 + H2 ΔH = -41kJ/mol • Control T and P for desired CO: H2 ratio

Chlor-Alkali Industry

39

INTRODUCTION • The chlor-alkali industry is the industry that produces chlorine (Cl ) 2

and alkali, sodium hydroxide (NaOH) or potassium hydroxide (KOH), by electrolysis of a salt solution. • The Chlor-Alkali industry in India forms an important component of basic chemicals industry,comprising around 74% of the basic chemicals production in India. Caustic soda, soda ash, chlorine alongside hydrogen and hydrochloric acid comprise the components. • These chemicals find their applications in a number of industries such as textiles,chemicals, paper, PVC, water treatment, alumina, soaps & detergents, glass, chlorinated paraffinwax, among others.

40

INTRODUCTION • The main technologies applied for chlor-alkali production are mercury, diaphragm and membrane cell electrolysis, mainly using sodium chloride (NaCl) as feed or to a lesser extent using potassium chloride (KCl) for the production of potassium hydroxide. • Each of these processes represents a different method of keeping the chlorine produced at the anode, separate from the caustic soda and hydrogen produced, directly or indirectly, at the cathode. Currently, 95% of world chlorine production is obtained by the chlor-alkali process.

41

Mercury Cell

42

Mercury Cell

 The mercury cell has a steel bottom with rubber-coated steel sides, as well as the boxes for brine and mercury feed and exit streams, with a flexible rubber or rubber-lined steel cover.  Horizontal, adjustable metal anodes hang from the top, and mercury flows on the inclined bottom.  The current flows from the steel bottom to the flowing mercury.  Saturated brine fed from the end box is electrolyzed at the anode to produce chlorine, and the depleted brine flows from the top portion of the trough.  The cation, be it sodium or potassium, reacts with the mercury to form an amalgam, which flows out of the end box to a vertical cylindrical tank.  The amalgam reacts with water in the decomposer, packed with graphite particles, and produces caustic soda or potash and hydrogen.

43

Diaphragm Cell

44

Diaphragm Cell • The diaphragm cell is a rectangular box with metal anodes supported from the bottom with copper base plates. • The cathodes are vertical asbestos screens connected from one end to the other end. • Saturated brine enters the anode compartment, and the chlorine gas is liberated at the anode during electrolysis. • The sodium ion from the anode is transported to cathode, where it combines with the hydroxyl ion and forms caustic soda.

45

Membrane Cell

46

Membrane • In this, an Cell ion-exchange membrane separates the anode and cathode compartments. • The separator is generally a bilayer membrane made of perfluorocarboxylic and perfluorosulfonic acid-based films, sandwiched between the anode and the cathode. • The saturated brine is fed at anode where chlorine is liberated at the anode, and the sodium ion migrates to the cathode. • In contrast to the diaphragm cells, only sodium ions and water molecules are transported across the membrane. The unreacted NaCl and other inert species remain in the anode.

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Major CA industries CHLOR-ALKALI INDUSTRIES 3 Major industrial Chemicals • Soda Ash (Sodium carbonate) • Caustic soda (Sodium hydroxide) • Chlorine Sodium Chloride Industries • Oldest Chemical Industry. • India is 4th largest producer of common salt. 48

Sodium Chloride Industry

49

INTRODUCTION • Sodium chloride (NaCl), also known as salt, common salt, table salt or halite,is an ionic compound. • It is responsible for the salinity of the ocean and of the extracellular fluid of many multi-cellular organisms. • All living beings also require salt for their growth. In India, about 70% of the salt is consumed by human being and rest 30% is used in the manufacture of chemicals. • Salt is the basic raw material for the caustic soda and chlorine, soda ash (sodium carbonate), sodium sulfate, hydrochloric acid etc. • Salt is also used in a large number of other industries, such as hydrogenation of oil, manufacture of soap, dyes, textile, food processing etc. 50

INTRODUCTION Properties • Molecular formula : NaCl • Molecular weight : 58.44gm/mole • Appearance : White crystal • Odour : Odourless • Melting point : 8010C • Density : 2.165gm/mL • Solubility : Soluble in water 51

SOURCES OF NaCl 1. Sea Water India has one of the largest seashore in the world, salt manufacture sites are spread throughout the country. Main salt manufacturing centers are Gujarat, Maharashtra, Tamilnadu, Kerala, Andhra Pradesh, Karnataka, Orissa and West Bengal. About 70% of the total salt production comes from sea water. 2. Salt Lakes • There are two important salt lakes in India. Sambhar lake in Rajasthan and Chilka lake on eastern coast. Sambhar lake produce more than 2.5 lakh tones of common salt every year.

52

SOURCES OF NaCl 3. Sea Water • Sub Soil Water • It contains more salt than the sea water. Leading salt manufacture sites form sub soil water are Kharagoda, Didwana, Dharangadhra and Tucticorin. 4. Rock Salt • Rock salt is used during religious festivals mainly produced in Mandi (Himachal Pradesh).

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MANUFACTURE • Salt obtained from above sources 1, 2, or 3 is in solution or liquid form. This form is called as brine. The various methods used for concentrating the brine solutions are 1. Solar Evaporation 2. Artificial Evaporation 3. Freezing method- Not commonly used

54

MANUFACTURE

Solar Evaporation

• It is the cheapest and best method of manufacturing salt from the brines. This method has widely been used in India.

55

MANUFACTURE

Solar Evaporation

Sea brine (3-3.5°) is drawn into reservoirs where it is concentrated to 10° by solar heating The concentrated brine is mixed with recirculating brine from grainer pan  At 7.5° ferrous iron separates out as ferric oxide  At 10°, calcium carbonate precipitates out  At 12-25°, Calcium sulfate, calcium carbonate and other impurities are separated out by Graveler

Further evaporation is carried out in flasher . Crystals are formed in grainer pans which is maintained at 951000C.

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MANUFACTURE

Solar Evaporation

The remaining brine is recirculated to dissolution tank The wet crystal are centrifuged, dried and screened 99.98% NaCl can be obtained, if the incoming brine treated properly The mother liquor is separated for the recovery of other by products. The main constituents are NaCl, MgCl2, MgSO4, KCl and Br2.

57

MANUFACTURE

Artificial Evaporation

Raw material • Saturated brine = 3450kg • Soda ash (58%) = 3.5kg • Caustic soda (50%) = 0.375kg • Steam = 1135kg (for triple effect evaporator

58

MANUFACTURE

Artificial Evaporation

59

MANUFACTURE

Artificial Evaporation

• Brine is first aerated to remove H2S. • Addition of chloride removes the last traces of H2S and oxidize ferrous ion to ferric ion. • Then brine sent to settling tank where it is treated with dilute solution of caustic soda and soda ash to remove most of calcium, magnesium and ferric ions. • Purified brine is pumped to the vacuum pan evaporators which are usually triple effect evaporators. • Salt slurry is continuously drawn from each evaporator and washed with water and the washing solution are recycled to evaporator.

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MANUFACTURE

Artificial Evaporation

• Solution from washer is filtered to remove impurities. • Filtered solution is dried into NaCl and dried salt are removed as fine dry crystals by passing through screens • Free flowing table salts are made by blending 0.5-2% magnesium carbonate, hydrated calcium silicate or tricalcium phosphate with the salt. • Iodized salt after blending contains 0.01% potassium iodide, 0.1% sodium carbonate as stabilizer and 0.1% sodium thiosulfate.

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MANUFACTURE

Artificial Evaporation

Uses • In chlor –alkali industries • In manufacture of chemical like caustic soda and chlorine, soda ash, sodium sulfate, hydrochloric acid etc • In manufacture of soap, dyes • Used in textile, food processing, pharmaceutical industries • Used in fire extinguisher • Used in house hold food preparation.

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Sodium Carbonate Industry

63

INTRODUCTION Sodium carbonate (Na CO ) also known as washing soda or soda 2

3

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.

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INTRODUCTION Properties • • • • • • •

Molecular formula : Na2CO3 Molecular weight : 105.98 gm/mole (anhydrous) Appearance : White solid, hygroscopic Odour : Odourless Melting point : 8510C Density : 2.54 g/mL Solubility : Soluble in water

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INTRODUCTION USES • • • • • • • •

Widely used in the manufacture of glass, Used in manufacture of sodium bicarbonate, caustic soda, Used in soap, pulp and paper, textiles industries Used in petroleum and dyes industries Used in foods, leather and water softening industries. As a photographic film developing agent As an electrolyte As a washing soda in household uses.

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MANUFACTURE Sodium carbonate is manufactured by following process 1. 2. 3. 4.

Leblanc process. Solvay‘s ammonia soda process. Dual process (modified Solvay‘s process) Electrolytic process.

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MANUFACTURE

Leblanc process

• The process has only historical importance, because is now been replaced completely by Solvay process. Raw materials • Basis: 1000kg Sodium carbonate (98% yield) • Common salt = 1126kg • Sulfuric acid = 945kg • Lime stone = 963kg • Coke = 463kg

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MANUFACTURE

Leblanc process

Reactions

69

MANUFACTURE

Leblanc process

70

MANUFACTURE

Leblanc process

Common salt is first mixed with the conc. H2SO4 in equivalent quantities and heated in a cast iron salt cake furnace by flue gasesNaHSO4 & 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 heated on the hearth of a furnace with some more common salt. NaHSO4 is converted into sodium sulfate, known as salt cake. The salt cake is pulverized, mixed with coke and limestone and charged into black ash rotary furnace consisting of refractory lined steel shells.

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MANUFACTURE

Leblanc process

The molten porous gray mass 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.

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MANUFACTURE Solvay's ammonia soda process Raw materials • 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

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MANUFACTURE Solvay's ammonia soda process Reactions

Overall Reaction

74

MANUFACTURE Solvay's ammonia soda process

75

Solvay's process

Preparation and purification of brine

Brine contains impurities such as calcium, magnesium and iron compounds. To remove them sodium carbonate and sodium hydroxide are added. The precipitated carbonates and hydroxide are removed by filtration. 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. Pure brine solution is pumped to the ammonia absorber tower.

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Solvay's process

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, cooled to about 30°C and pumped to the carbonating tower.

77

Solvay's process

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.

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Solvay's process

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, 1.62.5meter 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.

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Solvay's process

Carbonation of ammonium brine

The cooling coils of the foulded tower are shut off. 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. 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.

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Solvay's process

Filtration

NaHCO3 slurry is 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.

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MANUFACTURE

Dual process

Raw materials • Basis: 1000kg Sodium carbonate • Crystalline Salt = 1260kg • Ammonia = 325kg • High pressure steam = 1350kg • Low pressure steam = 100kg • Cooling water = 50000 - 80000kg • Electric power = 450KWH • Co-product (NH4Cl) = 620kg

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MANUFACTURE

Dual process

Reactions

83

MANUFACTURE

Dual process

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MANUFACTURE

Dual process

The liquor from carbonation tower, containing NH4Cl, unreacted NaCl and traces of sodium carbonate is ammoniated in ammonia absorber. The ammoniated liquor is sent to a bed of washed salt in salt dissolver. The resulting liquor is gradually cooled to 00C in refrigerating tank unit, resulting into crystallize out ammonium chloride.  The slurry containing ammonium chloride is thickened and NH4Cl is centrifuged and dried. In this process ammonium chloride is obtained as co-product. NH4Cl is recovered as co-product in this process rather than liberation of the contained ammonia for recycle as in the Solvay process.

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MANUFACTURE

Dual process

The liquor obtained after separation of NH4Cl is charged to series of carbonation towers in which CO2 is passed from bottom in the counter current flow of liquor. The resulting sodium bicarbonate is thickened into thickener and centrifuged. It is then calcined into sodium carbonate.

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Advantage of Solvay process • Less electric power • Less corrosion problem • Use of low grade brine • Not a problem of disposal of co-product • Does not require ammonia plant Disadvantage of Solvay process • Higher salt consumption • Higher investment in ammonia recovery units than crystallization unit of NH4Cl • More steam consumption • NH4Cl will be used as mixed fertilizer ingredient which minimizes the disposal problem of Duel process.

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MANUFACTURE

ELECTROLYTIC PROCESS

Raw materials • Basis: 1000kg Sodium carbonate (98% yield) • Salt = 563kg • Carbon dioxide = 424kg

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MANUFACTURE

ELECTROLYTIC PROCESS

Reactions

At Cathode

At Anode

89

MANUFACTURE

ELECTROLYTIC PROCESS

90

MANUFACTURE

ELECTROLYTIC PROCESS

o Electrolytic cell consists of a perforated steel tube having a thin lining of asbestos on the inside. The steel tube acts as the cathode and is suspended in an outer steel tank. o Brine is placed inside the cathode tube and a graphite rod is immerged in it acts as anode. When an electric current is passed, the salt solution undergoes electrolysis and its ions pass through the diaphragm. o Hydrogen and caustic soda are formed at the cathode and chlorine at the anode. Hydrogen gas is allowed to escape through an opening provided at the top of the cell. o Chlorine liberated at the anode is led away through pipe and compressed into steel cylinders. o The space between the cathode and outer tank is kept full of steam and Carbon dioxide. 91

MANUFACTURE

ELECTROLYTIC PROCESS

o Sodium ions pass through the asbestos and reach the cathode, where H+ ions and OH¯ ions are formed as a result of reduction of water. o Hydrogen escapes through an opening at the top and Na+ ions combine with OH¯ ions to form caustic soda. Sodium hydroxide is reacted with pressurized CO2 yielding Sodium carbonate which is collected from bottom of the cell.

92