Lecture 20 HCl H2SO4

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LECTURE–20: HYDROCHLORIC ACIDSULFURIC ACID

CHEMICAL TECHNOLOGY (CH-206)

Department of Chemical Engineering

http://www.iitr.ac.in/Main/pages/index.html

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HYDROCHLORIC ACID (HCL)

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.

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HYDROCHLORIC ACID (HCL): PROPERTIES

Molecular formula : HCl Molecular weight : 36.5gm/mole Appearance : Colourless liquid Odour : Pungent Boiling point : 85 0C Melting point : 1140C Density : 1.179gm/mL (35.2% HCl ) Solubility : Extremely soluble in water Forms azeotropic mixture with water, containing 20.24% HCl which boils at

1100C. Commercially available in 27.9%, 31.5% and 35.2%wt HCl solution in water. Anhydrous HCl is available in steel cylinders because completely dry HCl is

not very reactive. But dry HCl often reacts only in the presence of catalysts. HCl solution in a polar solvent is strong acid and, therefore, an aggressive

reagent.

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HYDROCHLORIC ACID (HCL): PROCESSES

Synthesis from hydrogen and chlorine From salt and sulfuric acid As by-product from chemical processes From incineration of waste organics Hydrochloric acid solutions

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HYDROCHLORIC ACID (HCL): PROCESSESSYNTHESIS FROM HYDROGEN AND CHLORINE

Raw materials Basis: 1 ton of HCl (98% yield) Hydrogen 28.21 kg Chlorine 999.21 kg

Sources of raw material Both hydrogen and chlorine can be obtained during

electrolysis of brine for manufacturing of NaOH. Hydrogen can also be synthesized from following methods:

Lane process or iron steam process Steam hydrocarbon process Liquefaction of coal gas and coke oven gas Bosch process or water gas-steam process

Reaction H2 + Cl2 2HCl ΔH = 43.9 kcalsTo ensure all the chlorine reacts with hydrogen, excess of 10%

hydrogen compare to chlorine is charged from the bottom of combustion chamber.

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2400 OCBlock diagram of HCl manufacturing

process

Process diagram of HCl manufacturing process

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The combustion chamber is made of structural carbon or lined with silica bricks provided with cooling device which may consist even of cold-water circulation in the shell.

The combustion chamber and length of ducting should be sufficiently specious, which leads the gas to absorber, otherwise hydrochloric acid will contain free chlorine.

The burning of hydrogen is started by igniting the burner with an external air-hydrogen torch.

Dry Cl2 is passed into the combustion chamber, where hydrogen burns in an atmosphere of Cl2 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.

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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.

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Anhydrous hydrogen chloride Hot gases originating from combustion chamber are

passing over anhydrous CaCl2 or washing them with 98% sulfuric acid and then cooled and compressed to 60 atm pressure.

The cooled and compressed gas having 99.9% purity is filled in steel cylinders.

In another process, combustion gas is absorbed into water and distilled to 36% concentration of HCl. If it is desired to obtain 97% HCl at the top of the

column. The 35% acid is cooled to 120C and aqueous liquid containing 50% HCl is left to condense, while residual gases, when they have been de nebulized as compressed to 60 atm are of purity exceeding 99.5%.

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Engineering aspect The combustion chamber and ducting to

absorber should be sufficiently specious for avoiding wall effect. If the walls of reactor in which chain reactions takes

place by their varying nature, development, shape and orientation as to affect the chain carriers is called wall effect.

The wall tends to interrupt the process by promoting the chain breaking reaction (termination reactions).

Physico chemically, chain terminators act as a third body in a system which already consists of the reactant bodies.

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HYDROCHLORIC ACID (HCL): PROCESSESTHE SALT–SULFURIC ACID PROCESS

Raw materials Basis: 1 t Hydrochloric acid Sodium Chloride: 3206 kg (from sea water, salt

lake and sub –soil water) Sulfuric acid: 2688 kg (by Contact process)

Reaction NaCl + H2SO4 NaHSO4 + HCl

NaCl + NaHSO4 Na2SO4 + HCl

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Block diagram of HCl manufacturing process

Process diagram of HCl manufacturing process

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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.

During the 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 hydrochloric acid in cooling tower and absorber, respectively.

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As by-product from chemical processes Over 90% of the hydrogen chloride produced as a by-

product from various chemical processes. The crude HCl generated in these processes is

generally contaminated with impurities such as unreacted chlorine, organics, chlorinated organic and entrained catalyst particles.

A wide variety of techniques are employed to treat these HCl streams to obtain either anhydrous HCl or hydrochloric acid.

Some of the processes in which HCl is produced as by-product is the manufacture of chloro-fluoro-hydrocarbons, manufacture of aliphatic and aromatic hydrocarbons, production of high surface area silica, and the manufacture of phosphoric acid and esters of phosphoric acid.

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From incineration of waste organics Environmental regulations regarding the disposal

of chlorine-containing organic wastes have motivated the development of technologies for burning or paralyzing the waste organics and recovering the chlorine values as hydrogen chloride.

Several catalytic and non-catalytic processes have been developed to treat these wastes to produce hydrogen chloride.

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From hydrochloric acid solutions Gaseous hydrogen chloride is obtained by

partially stripping concentrated HCl acid using an absorber–desorber system.

The stripper is operated at a pressure of 100–200 kPa (1–2 atm) for improved recovery of HCl.

The overhead vapors consisting of 97% HCl and 3% H2O is cooled to remove most of the water as concentrated HCl, and the residual water vapor is removed by drying the gas with sulfuric acid.

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HYDROCHLORIC ACID: APPLICATIONS Anhydrous HCl is consumed for its chlorine value,

whereas aqueous hydrochloric acid is often utilized as a non-oxidizing acid.

Used in metal cleaning operations, chemical manufacturing, petroleum well activation, and in the production of food and synthetic rubber.

Used for the manufacture of chlorine and chlorides, e.g. Ammonium chloride used in dry cell.

In the manufacture of glucose from corn starch. For extracting glue from bones and purifying boneblack. A saturated solution of zinc chloride in dilute HCl is used

for cleaning metals before soldering or plating. It is also used in medicine and as laboratory reagent. Aqua regia used for dissolving metal.

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SULFURIC ACID (H2SO4)

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SULFURIC ACID (H2SO4): PROCESSES

The Industrial manufacture of sulfuric acid is done mainly by two processes The Lead Chamber process The Contact process

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SULFURIC ACID (H2SO4): PROCESSES THE LEAD CHAMBER PROCESS

Raw Materials Basis: 1 ton Sulfuric acid (98% yield) Sulfur 400kg Air 399kg

Reaction S + O2 SO2 ΔH = 70.9kcals

4FeS2 + 11O2 2Fe2O3+ 8SO2

SO2 + NO2 SO3 + NO

SO3 + H2O H2SO4 ΔH = 92.0kcals

NO + O2 2NO2 ΔH = 27.12kcals

NaNO3 + H2SO4 NaHSO4 + HNO3

2HNO3 + 2SO2 2SO3 + H2O + NO + NO2

NO + NO2 + 2H2SO4 2NO.HSO4 + H2O

2ON.O.SO2OH + H2O H2SO4 + NO2 + NO

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SULFURIC ACID (H2SO4): PROCESSESTHE LEAD CHAMBER PROCESS

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Sulfur dioxide is obtained by burning sulfur or by roasting pyrites.

There are two function of burner To oxidize sulfur to maximum extent To produce and constant supply of gas containing maximum

concentration of SO2

The burner of the furnace should expose large surface of melted sulfur and should be provided secondary air in order to burn sublimed burner.

At about 4000C, pyrite (FeS2) decompose in to FeS and sulfur vapour, the later oxidized to SO2 in presence of excess air.

The residual FeS also oxidizes to Fe2O3 and SO2. Iron oxide (Fe2O3) slightly catalyzed oxidation of SO2 to SO3. Burner gas should contain sufficient oxygen for carry out

further oxidation of SO2 to SO3.

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The mixture of SO2, Oxides of nitrogen and air is then passed to series of rectangular vessels made of lead (lead chamber: 15–45 m length, 6–7 m width and 7 m length and 3–6 in numbers).

The chambers are arranged in two parallel rows. Steam from low pressure boiler or pure filtered water

is sprayed from top of the chamber. Mixture of gases is converted into H2SO4 having 65-

70%v strength is collected at the bottom of the chamber.

Dilute sulfuric acid obtained in any of the chamber is called chamber acid.

A part of chamber acid is pumped to Glower tower, and the rest is sent for concentration.

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The burner gases which contain SO2, N2, O2 and dust or fine particle of pyrites are passed through dust chamber followed by Cottrell electrical precipitator or centrifugal separator in order to remove dust or fine particle of ore.

Dust chambers are provided with horizontal shelves or baffles followed by filtration through crushed coke or similar material.

Burner gases further are passed through niter oven made of cast iron in which equimolecular proportion of NaNO3 and H2SO4 is heated.

The nitric acid produced reacts with SO2 to give mixture of nitric oxide (NO) and nitrogen dioxide (NO2) which are carried with burner gases.

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The burner gases they pass through Glower tower (5 m2 and 10–15 m high), which is packed with flint stone, quartz, tile or acid resisting bricks.

The packing in the tower is loosely stacked at the bottom to facilitate mixing of hot gases.

The hot burner gases passed to this tower is at 450–6500C and dilute H2SO4 from the lead chamber and nitrosyl sulfuric acid from Gay-Lussac tower are made to trickle down the Glower tower by means of sprayers.

The burner gases are cooled down to 70-800C, dilute chamber acid is concentrated up to 78% and nitrosyl sulfuric acid (nitrous vitriol) is denitrated by action of water.

The tower acid is drawn off from the bottom of the tower and collected in the container called acid egg.

The acid from base of Glower tower is cooled to 400C by air coolers.

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The unabsorbed remaining gases contain oxides of nitrogen and SO2 from lead chamber are then passed through Gay-Lussac tower at the top of which Glower acid is sprayed to recover oxides of nitrogen.

The oxides of nitrogen recovered in the form of nitroso sulfuric acid are pumped to Glower tower to again regenerate oxides of nitrogen.

When pyrite is used as raw material, the chamber acid may contain arsenious oxide (from pyrite), lead sulfate from lead chamber are removed by treatment of H2S and dilution of acid respectively.

Dilute acid may be further concentrated into Glower tower.

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Reason for obsolesce The overall reaction consisting of number of

partial reactions which takes place in liquid phase, the development of surfaces which are covered in this liquid is a factor of fundamental importance in promoting the synthesis of sulfuric acid.

Maximum strength of sulfuric acid obtained by chamber process is 78%.

However, in manufacture of some dyes and chemical processes require more concentrated H2SO4.

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SULFURIC ACID (H2SO4): PROCESSESTHE CONTACT PROCESS Raw Materials

Basis: 1 ton sulfuric acid (100%) Sulfur dioxide or pyrite (FeS2) 670kg Air 1450–2200 Nm3 Catalyst V2O5

Sources of raw material Sulfur from mines Sulfur or hydrogen sulfide 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

Reactions (Temperature = 450 oC, Pressure = 1.2–1.5 atm)S + O2 SO2 ΔH = 71.2kcals

2SO2 + O2 2SO3 ΔH = 46.3kcals

SO3 + H2O H2SO4 ΔH = 31.1kcals

This process is strongly exothermic

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SULFURIC ACID (H2SO4): PROCESSESTHE CONTACT PROCESS

Burning of sulfur Burning of sulfur in presence of dry air is carried

out in sulfur pyrite burner. As SO2 is needed for the catalytic oxidation and

prevention of corrosion, dry air is used in the combustion process.

If sulfur contains carbonaceous impurities, the molten material has to be filtered to avoid poisoning the catalyst and forming water from burning hydrogen.

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SULFURIC ACID (H2SO4): PROCESSESTHE CONTACT PROCESS Catalytic oxidation of SO2 to SO3

The catalytic reactor is designed as a four-stage fixed-bed unit. The gas has to be cooled between each step.

Four passes, together with "double absorption, are necessary for overall conversion of 99.5-99.8% (three passes, 97-98%).

The temperature rises to over 6000C with the passage of the gas through each catalyst bed.

The doubled absorption consists of cooling the gases between each bed back to the desired range by sending them through the heat exchanger and then back through the succeeding beds.

Between the third and fourth beds, the gases are cooled and sent to an absorption tower. This is to shift the equilibrium to the right by absorbing SO3.

The gases are then sent to the heat exchanger to warm them to 410-4300C and then on to the fourth catalyst bed.

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Hydration of SO3

After the catalytic oxidation process, the resulting SO3 is hydrated by absorption in packed towers filled with 98-99% sulfuric acid.

This is the H2SO4 azeotrope of minimum total vapour pressure. SO2 has a low solubility in 98% H2SO4. At lower acid

concentrations, sulfuric acid and SO3 form a troublesome mist and at higher concentrations emissions of SO3 and H2SO4 vapour become significant.

The absorption acid concentration is kept within the desired range by exchange as needed between the H2SO4 in the drying acid vessel that precedes the combustion chamber with the H2SO4 in the absorption tower.

The acid strength can be adjusted by controlling the streams of H2SO4 to give acid of 91 to 100% H2SO4 with various amounts of added SO3 and water.

The conversion of sulfur to acid is over 99.5%.

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Major engineering problems Design of multistage catalytic convertor 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.

To optimize space velocity in catalyst chamber because it deals with pumping cost or fixed charges of reactor

Thin catalyst beds of 30-50cm 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.

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SULFURIC ACID (H2SO4): PROCESSESPROPERTIES

Physical Properties Molecular formula : H2SO4 Molecular weight : 98.08gm/mole Appearance : Water white slightly viscous liquid Boiling point : 2900C Melting point : 100C Density : 1.840gm/mL (liquid) Solubility : Miscible with water in all proportions Viscosity : 26.7cP (200C) Aq. H2SO4 solutions are defined by their H2SO4 content in weight-

percent terms. Anhydrous (100%) sulfuric acid sometimes referred to as

―monohydrate, which means that it is the monohydrate of SO3. Dissolve any quantity of SO3, forming oleum (―fuming sulfuric

acid‖). The physical properties of sulfuric acid and oleum are dependent on

H2SO4 and SO3 concentrations, temperature, and pressure.

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SULFURIC ACID (H2SO4): PROCESSESPROPERTIES: CHEMICAL PROPERTIES

Dehydrating agent Has a great affinity for water and the reaction is extremely

exothermic. A large amount of heat is produce due to formation of mono and

dehydrates (H2SO4.H2O and H2SO4.2H2O) on mixing acid with water. So while preparing dilute solutions of H2SO4 the acid should be added to water slowly with constant stirring. Never add water to the acid.

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 Also, paper, starch, wood etc. are charred by conc. H2SO4 due to

the removal of water. It is also used in removing water from various substances such as oxalic acid and formic acid.

COOH-COOH H2O + CO + CO2

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SULFURIC ACID (H2SO4): PROCESSESPROPERTIES: CHEMICAL PROPERTIES

Oxidising agent Gives O2 on strong heating, hot conc. H2SO4 also

acts as an oxidising agent. Pickling agent

Finds application in pickling in which layers of basic oxides are removed before electroplating, enameling, galvanizing and soldering.

Acidic nature Strong dibasic acid and forms two series of salts

with alkalis. These are bisulfates (HSO4) and

sulfates (SO4-2).

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SULFURIC ACID (H2SO4): PROCESSESAPPLICATIONS In the fertilizer industry.

In the production of phosphoric acid, which in turn used to manufacture fertilizers such as triple superphosphate, mono and diammonium phosphates.

Used for producing ammonium sulfate. Used as an acidic dehydrating reaction medium in organic chemical

and petrochemical processes involving such reactions as nitration, condensation, and dehydration, as well as in oil refining, in which it is used for refining, alkylation, and purification of crude-oil distillates

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 hydro-metallurgical ore processing, and in the preparation of electrolytic baths for nonferrous-metal purification and plating

Certain wood pulping processes in the paper industry require sulfuric acid, used in textile and chemical fiber processes and leather tanning

In manufacture of explosives, detergents and plastics In production of dyes, pharmaceuticals.

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ACKNOWLEDGEMENT

Slides are developed from the following references: Austin G. T., "Shreve’s Chemical Process Industries",

Fifth edition, Tata McGraw Hill, NY. Kent J.A., "Riegel's Handbook of Industrial

Chemistry,” CBS Publishers. Gopala Rao M. & Marshall Sittig, "Dryden’s Outlines

of Chemical Technology for the 21st Century", Affiliated East –West Press, New Delhi.

Mall I. D., "Petrochemical Process Technology", Macmillan India Ltd., New Delhi.

http://nptel.ac.in/courses/103106108/24

Documents

Lecture 20 HCl H2SO4