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LECTURE–14: AMMONIA, NITRIC ACID, NITROGEN BASED FERTILIZERS

CHEMICAL TECHNOLOGY (CH-206)

Department of Chemical Engineering

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AMMONIA

Ammonia (NH3) or azane is a compound of nitrogen and hydrogen.

It is a colourless gas with a characteristic pungent smell.

Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers.

Ammonia is both caustic and hazardous.

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AMMONIA: PROPERTIESProperties

Molecular formula NH3

Molar mass 17.031 g/molAppearance Colourless gasOdor strong pungent odorDensity 0.86 kg/m3 (1.013 bar at boiling point)

0.769  kg/m3 (STP)0.73 kg/m3 (1.013 bar at 15 °C)681.9 kg/m3 at −33.3 °C (liquid)817 kg/m3 at −80 °C (transparent solid)

Melting point −77.73 °C (−107.91 °F; 195.42 K)

Boiling point −33.34 °C (−28.01 °F; 239.81 K)

Solubility in water 47% w/w (0 °C), 31% w/w (25 °C), 18% w/w (50 °C)Solubility soluble in chloroform, ether,ethanol, methanolVapor pressure 8573 h PaAcidity (pKa) 32.5 (−33 °C)

Basicity (pKb) 4.75

Refractive index(nD)

1.3327

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AMMONIA: PROCESSES

Haber Bosch process Modified Haber Bosch process Killogg ammonia process

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AMMONIA: HABER AND BOSCH PROCESS

Raw materials Basis: 1000kg of NH3 (85% yield) Hydrogen 210kg (synthesis gas) Nitrogen 960kg (air or air

liquefaction process) Catalyst 0.2kg Power 850KWH Fuel gas for compressors 3800Kcal Cooling water 12,500kg

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AMMONIA: HABER AND BOSCH PROCESS Raw material sources:

Nitrogen – it is taken form air as discussed Hydrogen – It can be synthesized from any feed

stock listed below:

Feed stock Process or techniques to produce H2

Natural gas Partial oxidation / steam reforming

Coke oven gas Partial oxidation/ low temperature separation

Fuel oil or low sulfur heavy stock

Partial oxidation

Coal Partial oxidation

Water Eletrolysis

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AMMONIA: HABER AND BOSCH PROCESS

Catalyst Iron with added promoters e.g. oxides of

aluminium, zirconium or silicon at about 3% concentration and potassium oxide at about 1%. Promoters prevent sintering and make the catalyst

more porous. Iron catalysts lose their activity rapidly, if heated

above 520°C. Also, is deactivated by contact with copper,

phosphorous, arsenic, sulfur and CO.

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AMMONIA: HABER AND BOSCH PROCESS

Purification of raw gases

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AMMONIA: HABER AND BOSCH PROCESS

Purification of raw gases The liquid nitrogen wash is mainly used to purify

and prepare ammonia synthesis gas within fertilizer plants.

The liquid nitrogen wash has the function to remove residual impurities like CO, Ar and CH4 from a crude hydrogen stream and to establish a stoichiometric ratio H2/N2 = 3:1.

CO must be completely removed, since it is poisonous for the NH3 synthesis catalyst.

Ar and CH4 are inert components enriching in the ammonia synthesis loop. If not removed, a syngas purge or expenditures for purge gas separation are required.

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AMMONIA: HABER AND BOSCH PROCESS

Purification of raw gases If partial oxidation of coal, heavy oil or residue oil

were selected as feedstock basis for ammonia production then liquid nitrogen wash is typically arranged to downstream of the scrubbing process.

Traces of water, CO, solvent (methanol) are removed in the adsorber station.

Center piece of the liquid nitrogen wash is referred as coldbox.

The process equipment of the cryogenic separation is installed close-packed in the coldbox, which is covered with a metal shell.

The coldbox voidage is filled with insulation material (perlite) to prevent heat input.

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AMMONIA: HABER AND BOSCH PROCESS

Raw hydrogen and HP nitrogen are fed to the liquid nitrogen wash unit.

Both streams are cooled down against product gas.

Feeding raw hydrogen to the bottom of the nitrogen wash column and some beforehand condensed liquid to the top.

Trace components are removed and separated as fuel gas.

To establish the desired H2/N2 ratio, HP nitrogen is additionally admixing inside and outside the coldbox.

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AMMONIA: HABER AND BOSCH PROCESS (1910)

Reaction

N2(g) + 3H2(g) 2NH3(g) ΔH = 22.0 kcals

Ammonia is manufactured by passing mixture of pure H2 and N2 in the proportion of 3:1 by volume under pressure (100–1000 atm depending on conversion required).

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AMMONIA: HABER AND BOSCH PROCESS

Ammonia converter: Carbon steel (CS) is used as material of construction (MOC) for pressure vessel and internal tubes.

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AMMONIA: HABER AND BOSCH PROCESS

N2 and H2 are first passed through filter to remove compression oil and additionally through a high temperature guard converter (convert CO and CO2 into CH4 and remove traces of H2O, H2S, P and As).

The relatively cool gas is added along the outside of converter tube walls to provide cooling.

The preheated gas flows next through the inside of the tube which contains promoted iron catalyst at 500-550 0C.

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AMMONIA: HABER AND BOSCH PROCESS

The NH3 product, with 8-30% conversion depending on a process conditions, is removed by condensation, first with water cooling and then NH3 refrigeration.

The unconverted N2–H2 mixture is recirculated to allow an 85-90% yield.

Economy of the process is greatly influenced by the pressure, temperature, catalyst, purity of raw materials and most importantly heat recovery and reuse.

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AMMONIA: MODIFIED HABER–BOSCH PROCESS

The NH3 is also manufactured by the partial oxidation of hydrocarbon derived from naphtha, natural gas or coal by oxygen enriched air in the presence of catalyst.

Modified Haber Bosch process has following steps Manufacture of reactant gases Purification Compression Catalytic reaction Recovery of ammonia and recycle of reactant

gases

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AMMONIA: HABER AND BOSCH PROCESS (1910)

Reaction

N2(g) + 3H2(g) 2NH3(g) ΔH = 22.0 kcals

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AMMONIA: HABER AND BOSCH PROCESS

Ammonia converter: Carbon steel (CS) is used as material of construction (MOC) for pressure vessel and internal tubes.

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AMMONIA: MODIFIED HABER–BOSCH PROCESSMANUFACTURE OF REACTANT GASES

Water gas as source of H2 is prepared from coke and steam at 1000 –14000C.

It is cooled and purified by passing through lime and iron oxide coated wood shavings.

C + H2O CO + H2 ΔH = –38900 cal Producer gas is prepared by passing air through

heated coke or coal bed at1000–14000C. Resulting CO2 passed through the hot bed of the fuel

which reduced it to CO, the nitrogen of the air remains mixed with CO.

The gas is cooled and purified. In both the cases sensible heat of the gases is utilized

by raising steam in waste heat boiler C + 1/2O2 CO ΔH = –28900 cal

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AMMONIA: MODIFIED HABER–BOSCH PROCESSPURIFICATION

Both water and producer gases are mixed in such a ratio so that after purification concentration of N2 and H2 by volume becomes 1:3.

The cold mixed gas is mixed with excess steam, then the hot gases are passed through horizontal converters containing catalyst consisting of iron oxide promoted with Cr2O3 and CeO2.

The CO is converted into CO2 by steam at 4500C temperature by the heat of reaction.

CO + H2 + H2O CO2 + 2H2O ΔH = 98,000 cals.

The hot mixture of CO2, H2, N2 and CO is cooled by passing through the heat exchanger then the cooled gas is stored.

The gases, after removal of CO2, are compressed to 200 atm pressure, cooled, and treated in a pressure tower with ammonical solution of cuprous formate (HCOOCu) which absorbs CO.

The resultant gas is mixture of H2 and N2 (3:1).

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AMMONIA: MODIFIED HABER–BOSCH PROCESSCOMPRESSION

The purified N2 and H2 mixture at 200 atm pressure is further compressed to 300atm pressure mixed with recycling gas at the same pressure and passed through oil filters.

The compressed gas mixture is then cooled by cold water followed by refrigeration by liquid ammonia.

The recycling gas in the mixed gas contained some ammonia.

This ammonia is liquefied by pressure and refrigeration hence before allowing the gas mixture to enter into the converter, the liquid ammonia is separated.

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AMMONIA: MODIFIED HABER–BOSCH PROCESSCATALYTIC REACTION

The gas mixture then passes into the converter which is made of Ni, V, Cr steel having 7’ height and 21” diameter.

The seamless cap having 3” wall thickness is held by bolts of nickel steel.

The converter is fitted with double coil acting as heat exchanger through the inner tube of which cold gas mixture passes, and through the outer tube of which passes the hot outgoing gas mixture.

At the base of the coil there is resistance coil for electrical heating.

In the converter there is the contact catalyst chamber consist of three concentric tubes which contain the granular catalyst.

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AMMONIA: MODIFIED HABER–BOSCH PROCESSCATALYTIC REACTION

The compressed gas enters through the inner coil of the heat interchanger.

After passing through the interchanger the gas is heated electrically by the resistance coil and then goes up 1st catalyst chamber, and then down through the 2nd, and lastly up through the last.

It then enters the outer coil of the central heat exchanger, gives up the heat to the incoming gas, and then goes out of the converter from the top.

The temperature in the contact chamber is 5500C.

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AMMONIA: MODIFIED HABER–BOSCH PROCESSRECOVERY OF NH3 AND RECYCLE OF REACTANT GASES

The mixed outgoing gas containing 19% NH3 and rest N2 and H2 going out of the converter is cooled by cold water in the condenser, where major portion of ammonia liquefies.

The liquid NH3 is separated and the unconverted gas mixture containing some unliquefied NH3 is compressed to 300atm pressure and then mixed with fresh compressed gas mixture and recycled.

A part of the recycled gas is rejected from time to time to prevent the accumulation of argon and methane.

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AMMONIA: KILLOGG AMMONIA PROCESS

Raw material Natural gas Air

Reaction 2CH4 + O2 2CO + 4H2

2CO + O2 2CO2

N2 + 3H2 2NH3

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AMMONIA: KILLOGG AMMONIA PROCESS

Natural gas is used for production of hydrogen. The purified nitrogen and hydrogen is reacted to give ammonia gas.

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AMMONIA: KILLOGG AMMONIA PROCESS

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AMMONIA: KILLOGG AMMONIA PROCESS The sulfur free natural gas is mixed with steam in the

volume based ratio of 3.7:1 and compressed to 40 atm.

The mixture is preheated with the recycled flue or effluent gases and fed into the furnace.

At 800-8500C in the presence of iron catalyst promoted with other metal oxides conversion of methane takes place with the formation of CO.

The residual gas is mixed with air and fed into shaft converter to get complete conversion.

The waste heat is utilized for the steam generation and ethanolamine which are used in CO2 and H2S removal.

The exit gas containing poison was regenerated in the methanator at 280-3500C which ultimately used for heating the feed water.

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AMMONIA: KILLOGG AMMONIA PROCESS

Purified N2 and H2 mixture was compressed to 300atm at 320 to 3800C in the presence of catalyst converted to NH3.

14-20% conversion per pass was achieved. NH3 condensed and separated from exit gas,

whereas unconverted N2 and H2 gases were recycled along with the fresh gases.

Ammonia synthesis is being exothermic the process requires an effective temperature control system at every stage of reaction.

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AMMONIA: PROCESS DESIGN MODIFICATIONS The pressure affects conversion, recirculation rates and

refrigeration of the process. The various process used with different process parameter

are as follows Very high pressure (900-1000atm, 500-6000C, 40-80%

conversion) — Claude, Du pont, L‘air liquide High pressure (600atm, 5000C, 15-25% conversion) — Casale Moderate pressure (200-300atm, 500-5500C, 10-30%

conversion) — Haber Bosch, Kellogg, Fauser, Nitrogen Engineering Corporation

Low pressure (100atm, 400-4250C, 8-20% conversion) Mont Cenis: uses a new type of iron catalyst promoted iron

cyanide. The modern trend is towards lower pressure and increased recirculation loads because of the relatively high cost of pressure vessels. The large single train plants using centrifugal compressors and having capacities as high as 1000 tons/day from a single reactor at low production cost are used widely.

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AMMONIA:

USES Ammonia is major raw material for fertilizer

industries It is used for the manufacture of

Nitro compounds, Fertilizers e.g. urea, ammonium sulfate, ammonium phosphate etc.

Nitric acid, Hydroxylamine, Hydrazine, Amines and amides, and in many other organic compounds

It is also used in heat treating, paper pulping, as explosives and refrigerants

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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