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LECTURE–19: AMMONIUM NITRATEAMMONIUM PHOSPHATEAMMONIUM SULFATE

CHEMICAL TECHNOLOGY (CH-206)

Department of Chemical Engineering

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AMMONIUM NITRATE

It is produced in solid form as pellets, crystals or flakes by reacting nitric acid and ammonia in different proportions.

It is widely used either as fertilizer or explosive or for the production of chemicals.

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AMMONIUM NITRATE: PROPERTIES

Chemical formula : NH4NO3

Molecular weight : 80.05 M.P. : 170oC B.P. : Decomposes at 200 oC or

higher Solubility : Soluble in water, alcohol,

and NH3

Grades : C.P.: White hygroscopic crystals or granulesFertilizer93-95% containing 33% N

Explosive when mixed with combustible materials or exposed to high temperatures

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AMMONIUM NITRATE: PRODUCTION Chemical reaction

NH3 + HNO3 NH4NO3; H = 20.6 Kcal

Raw materials 57 – 60% HNO3 from oxidation – absorption tower Liquid NH3

Clay such as diatomaceous earth, kieselguhr for coating on end–product to avoid explosions

Quantitative requirements: Basis: 1 ton of ammonium nitrate (98% yield)

Ammonia (NH3) 0.22 ton 60% HNO3 1.38 ton

Plant capacity 100–500 tons/day

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AMMONIUM NITRATE : PROCESSES

Prilling process Crystalization process Stangel process Nitrolime process

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AMMONIUM NITRATE : PROCESS

H2O to waste

Moist airScreen

Steam heated conveyor dryer

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AMMONIUM NITRATE: PRILLING PROCESS

The ammonia vapor is reacted with aq. HNO3 in a stainless steel reaction vessel with agitation.

The heat of reaction causes water to boil off and the final salt solution is 75% and 140 oC when pumped to a vacuum evaporator to concentrate further to 95% solids.

The hot liquor is sprayed from the top of a prilling tower counter–current to conditioned air flow.

The solidified spherical pellets or prills are about 1.5 mm in diameter and must be screened and dried before coating with clay.

Fines and oversized materials are redissolved and sent to the neutralizing reactor for recycle.

The process is further modified using an oil quenching unit for prill production to replace the large and uneconomical prilling tower.

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AMMONIUM NITRATE: CRYSTALLIZATION PROCESS

Upto the point of spraying the process is similar to the prilling process.

The liquor from the vacuum evaporator contains 80–85% solids and is fed to a vacuum crystallizer where crystal growth is controlled at about 40 oC to yield a large grain required for fertilizer use.

The crystal slurry with about 40% (wt.) crystals is centrifuged; the mother liquor is returned to the evaporator and the crystals are dried in a through conveyor or a rotary dryer at about 15 oC.

Dusting with clay (2–4%) is used where explosion hazards in bulk storage are prevalent and free–flowing non – hygroscopic material is specified.

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AMMONIUM NITRATE: STENGEL PROCESS

Preheated HNO3

Preheated NH3

Reactor and separator

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AMMONIUM NITRATE: STENGEL PROCESS Ammonia vapors are preheated to 140–150 OC in an single

stage heat exchanger. 60% HNO3 is preheated first to 95 OC in a stainless steel

unit and then to 160 OC in a tantalum unit to avoid high temperature HNO3 corrosion.

The Molten NH4NO3 and water vapors at 200 OC are passed through the tangential entry of a cyclone separator where air aids in removal of steam from the molten salt.

The latter is removed at the bottom and solidified on a water cooled steel belt.

The solids are crushed, ground to flake size and screened. Oversized are reground and fines are dissolved and

returned to HNO3 preheater stream. The product flakes are coated, and bagged or bulk shipped.

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AMMONIUM NITRATE: NITROLIME PROCESS The 95% melt from evaporator is passed to a pug

mill, which is a screw mixer consisting of a two sets of mixing blades rotating in opposite directions.

Pulverized lime (325 mesh) is admixed here with some heat of reaction evolved.

The product flows to a rotary drum granulator, then to a co–current direct heated flue gas dryer where product moisture is reduced to 0.5–0.6 % H2O.

Screen classifying separates 2 – 4 mm particles which move on to a coating drum, where fine limestone dust is added to yield a free – flowing product.

Oversize material (> 4mm) is recycled. Undersize material (< 2mm) is returned to the pug

mill for size buildup.

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AMMONIUM NITRATE : PROCESS

H2O to waste

Moist airScreen

Steam heated conveyor dryer

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AMMONIUM NITRATE:

Major Engineering problems: Corrosion:

Carbon steel can only be used in the NH3 storage and feed system.

Type 304, extra – low carbon (ELC) stainless steel is used throughout most of the remainder of an ammonium nitrate plant.

In the Stengel process, expensive tantulum metal is used in the heat exchanger.

Crystallization: Typically the proper size and shape, and distribution of

crystals is not obtained. An Oslo–Krystal classifier is used to have adequate

growth of seed crystals to the proper size, shape and strength in a continuous manner.

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AMMONIUM NITRATE: MAJOR ENGINEERING PROBLEMS

Major Engineering problems: Safety:

Safety precautions are required, since NH4NO3 is extremely reactive with combustible and it is also sensitive to explosive decomposition.

Air used in the drying must be free of oil and other combustibles.

Stangel process has the greatest inherent safety, since it is designed for short residence times and low hold–ups.

Air conditioned requirement Both NH4NO3 and nitrolime are hygroscopic.

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AMMONIUM PHOSPHATE

Monoammonium Phosphate (NH4H2PO4) Mol. wt. 115 M.P. Decomposes Density 1.80 gm/cc Solubility in water 32 gm/100cc at 15 OC

Diammonium Phosphate ((NH4)2HPO4) Mol. wt. 132 M.P. Decomposes Density 1.62 gm/cc Solubility in water 131 gm/100cc at 15 OC

Mostly used as a fertilizer

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AMMONIUM PHOSPHATE

Chemical Reaction:NH3 + H3PO4 NH4H2PO4

NH3 + NH4H2PO4 (NH4)2HPO4

NH3 + H2SO4 NH4HSO4

NH3 + NH4HSO4 (NH4)2PO4

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AMMONIUM PHOSPHATEProcess: Neutralization and granulation

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AMMONIUM PHOSPHATE In the first step, neutralization, the phosphoric and

sulfuric acids are added to the first of three continuous sttirred tank reactors.

Anhydrous liquid ammonia is added beneath the slurry level in the first neutralizer in an amount equivalent to 80% neutralization.

Further NH3 is added in the 2nd and 3rd reactor to obtain conversion to the diammonium salt if a higher N fertilizer is needed.

The exothermic reaction heats the slurry nearly to the boiling point (130 OC).

Unreacted and excess NH3 vapors are collected from the top of the each tank and recharge below the liquid level. It cuts the NH3 losses to less than 3%.

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AMMONIUM PHOSPHATE Slurry from third neutralizer is mixed with KCl and

absorbed in a bed of dry recycle fertilizer moving through a rotating drum granulator.

It provides a tumbling action to coat recycle material with a slurry film.

A rotary adiabatic dryer reduces the moisture to less than 1%, with a 10 min contact time with air initially at 150OC.

Dried product is separated into three fractions on a double deck screen.

A portion of the product from the deck of the lower screen ( 6 +12) is sent to bagging operations.

The balance, together with pulverized oversize and fines, is returned to the granulator.

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AMMONIUM PHOSPHATE

Major Engineering Problems: NH3 losses Corrosion - Type 316 SS is used for hot acid and

fume ducts; carbon steel for granulation, drying and scrrening

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AMMONIUM SULFATE

Originally it was made using sulfuric acid to scrub by–product ammonia from coke oven gas.

Nowadays the ammonium sulfate is manufactured by reacting synthetic ammonia with sulfuric acid.

At Sindri, India the ammonium sulfate is produced by the following reaction:

(NH4)2CO3(aq) + CaSO4.2H2O(s) CaCo3(s) + 2H2O + (NH4)2SO4(aq)

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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/Lecture%207.pdf (Acetylene)

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