Module: 2 Lecture: 6 Ammonia

Dr. N. K. Patel

N P T E L

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Module: 2

Lecture: 6

AMMONIA

INTRODUCTION

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. Although in wide use, ammonia is both caustic and hazardous.

It is one of the most important nitrogenous material. It is a base from which all

the nitrogen containing compounds are derived. Mostly is produced synthetically,

but during some chemical processes obtained as by product. Either directly or

indirectly, ammonia is a building-block for the synthesis of many pharmaceuticals

and is used in many commercial cleaning products.

Gaseous ammonia was first isolated by Joseph Priestley in 1774 and was

termed as "alkaline air". Claude Louis Berthollet ascertained its composition in 1785.

The Haber-Bosch process to produce ammonia from the nitrogen in the air

was developed by Fritz Haber and Carl Bosch in 1909 and patented in 1910. Prior to

the availability of cheap natural gas, hydrogen as a precursor to ammonia

production was produced via the electrolysis of water or using the chloralkali

process.

MANUFACTURE

(a) Haber and Bosch Process

Raw materials

Basis: 1000kg of NH3 (85% yield)

Hydrogen = 210kg

Nitrogen = 960kg

Catalyst = 0.2kg

Power = 850KWH

Fuel gas for compressors = 3800Kcal

Cooling water = 12,500kg

Module: 2 Lecture: 6 Ammonia

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Sources of raw material

Nitrogen

Nitrogen is taken form air as discussed in Lecture: 3 (Module: 2)

Hydrogen

Hydrogen can be synthesized from any feed stock listed in the table

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 Electrolysis

Catalyst

Most widely used catalyst for ammonia synthesis is iron with added promoters

e.g. oxides of aluminium, zirconium or silicon at about 3% concentration and

potassium oxide at about 1%. These 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.

Purification of raw gases

The Liquid nitrogen wash is mainly used to purify and prepare ammonia

synthesis gas within fertilizer plants. It is usually the last purification step upstream of

ammonia synthesis.

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. Carbon monoxide must be completely removed, since it is

poisonous for the ammonia 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.

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, carbon dioxide, solvent (methanol) are removed in the

adsorber station. Center piece of the liquid nitrogen wash is the so called ―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.

Module: 2 Lecture: 6 Ammonia

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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.

Reaction

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

Manufacture

The method was first developed by Haber and Bosch therefore known as

Haber and Bosch Process. The manufacture of ammonia is carried out by passing

mixture of pure hydrogen and nitrogen in the proportion of 3:1 by volume under

pressure (100-1000atm depending on conversion required). Both the gases are sent

through filter to remove compression oil and additionally through the high

temperature guard converter in which CO and CO2 are converted to CH4, and also

removal of traces of H2O, H2S, P and As. The relatively cool gas is added along the

outside of converter tube walls to provide cooling. Carbon steel is used as material

of construction for pressure vessel and internal tubes.

Figure: Purification of raw gases

Feed gas (cold)

LP-N2

for stripping

LP-N2

Purified gas

Fuel gas

HP-N2

Steam

Absorber unit Cold box

He

at

ex

ch

an

ge

r

Ab

so

rb

er

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The preheated gas flows next through the inside of the tube which contains

promoted iron catalyst at 500-5500C. 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.

Block diagram of manufacturing process

Diagram with process equipment

Animation

Water

Water

Heating coil

Liquid Ammonia

Liquid Ammonia

Water

Wa

ter

Separators

Filter

NH3 GasL

iqu

id N

H3

Recycled Gas 300 atm

300 atm 1 Vol N2 + 3 Vols H2

Figure: Manufacturing of Ammonia by Haber Process

compresor

Co

nd

en

se

r

Condenser

Co

nd

en

se

r

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Cost is greatly influenced by the pressure, temperature, catalyst, purity of raw

materials and most importantly heat recovery and reuse. For achieving quality

material at lower cost modification in Haber and Bosch Process are initiated.

(b) Modern method/ Killogg ammonia process

Raw material

Natural gas

Air

Reaction

2CH4 + O2 2CO + 4H2

2CO + O2 2CO2

N2 + 3H2 2NH3

Manufacture

Block diagram of manufacturing process

Diagram with process equipment

Animation

In the process natural gas is used for production of nitrogen and hydrogen.

The purified nitrogen and hydrogen is thus reacted to give ammonia gas. In

commercial production sulfur free natural gas is mixed with steam in the volume

H2

Reactor for organic sulphur hydrogenation

Naturalgas

Natural gas heater

H2O

Hydrogen sulphide adsorber

Heatexchanger

Natural gas

Air

Tube- type furnace(methane convertor)

Furnace

Air

Shaft methaneconvertor

2nd - stage CO

convertor

Steam boiler

Ste

am

b

oile

r

Heatexchanger

Air cooler

Steamturbine

1st - stage CO convertor

CO2

regenerator

Methanator

COabsorber

Tu

rbo

c

om

pre

ss

or

wit

h g

as

tu

rbin

e

NH3

NH3

primaryseparator

Co

ld h

ea

t e

xc

ha

ng

er

Ammoniacooler

Secondaryseperator

Air Cold

Ho

t h

ea

t e

xc

ha

ng

er

Ste

am

bo

ile

rw

ate

r h

ea

ter

Pla

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sy

nth

es

is c

olu

mn

Figure: Manufacturing of Ammonia By Kellogg Process

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

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.

(c) Modified Haber Bosch process

The manufacture of ammonia may be carried out by the partial oxidation of

hydrocarbon derived from naphtha, natural gas or coal by oxygen enriched air in

the presence of catalyst. CO is removed by passing through ammonical solution of

cuprous formate. The remaining N2 and H2 gas are utilized for the manufacture of

ammonia by Haber process.

Modified Haber Bosch process has following steps

a) Manufacture of reactant gases

b) Purification

c) Compression

d) Catalytic reaction

e) Recovery of ammonia and recycle of reactant gases

a) Manufacture of reactant gases

Water gas as source of H2 is prepared from coke and steam at 10000C-

14000C. It is cooled and purified by passing through lime and iron oxide coated

wood shavings.

C + H2O CO + H2 ∆H = -38900cal

Producer gas is prepared by passing air through heated coke or coal bed

at10000C-14000C. Resulting CO2 passed through the hot bed of the fuel which

reduced it to carbon monoxide, 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

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C + 1/2O2 CO ∆H = -28900cal

b) Purification

Both water gas and producer gas are mixed in such a ratio so that after

purification concentration of nitrogen and hydrogen by volume becomes one is to

three (1: 3). The cold mixed gas is mixed with excess of steam, then the hot gases are

passed through horizontal converters containing catalyst consisting of iron oxide

promoted with Cr2O3 and CeO2. The exothermic conversion of CO to CO2 by steam

is carried out at an optimum temperature 4500C by the heat of reaction.

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

The hot mixture of CO2, H2, N2 and CO is cooled by passing through the heat

exchanger then the cooled gas is stored. CO2 is removed by any one method which

is described (Module: 2, Lecture: 2) as method of recovery of CO2

The gases after removal of CO2, are compressed to 200atm 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 (3vol: 1vol).

The cuprous formate solution after stripping of carbon monoxide is recycled back to

the process.

c) Compression

The purified N2 and H2 mixture at 200atm 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.

d) Catalytic reaction

The gas mixture then passes into the converter which is made of nickel,

vanadium, chromium steel having 7ft. height and 21 inches diameter. The seamless

cap having 3 inch wall thickness is held by bolts of nickel steel. The converter is fitted

with double coil acting as heat interchanger 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.

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

After passing through the interchanger the gas is heated electrically by the

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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.

e) Recovery of ammonia and recycle of reactant gases

The mixed outgoing gas containing19% NH3 and rest N2 and H2 going out of

the converter is cooled by cold water in the condenser. 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. The temperature

in the contact chamber is 5500C.

Kinetics and thermodynamics

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

The highest yield of above reaction can be obtained at high pressure and

low temperature which can be expressed as follows

√

Where, the equilibrium constant is an inverse function of the absolute

temperature

∆F= -RT ln Kp = -19000 + 9.92T ln T + 1.15 X 10-3T2 - 1.63 X 10-6T3 - 18.32T

The reaction is exothermic and similar to oxidation of SO2 is favoured by low

temperature from equilibrium stand point but reaction kinetics dictate a

compromise temperature at some higher value like 500 - 5500C in single stage

convertor.

The cost of high pressure reaction system is higher so multistage operation as

used with SO2 oxidation is not economically feasible for ammonia production.

The design problem thus reduces to an optimization of space velocity based

on the following considerations.

The fraction of NH3 (x) in the exit gas decreases with increase in flow rate or

space velocity by equation

x = fV-n

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Where,

n<1 if bed is at correct temperature and mass transfer rates are improved

n>1 where bed is at too low temperature because of high velocity gas

cooling

The space time yield (Y) is

( )( )

Y = V.V-n = V(1-n)

In addition to very high space velocity, cooling the bed will increases the cost

of NH3 recovery because x is lower and also increases the pumping cost hence

based on these considerations an optimized cost is calculated.

Catalyst development

Iron oxide promoted by alkali is widely used as catalyst or nonferrous metal

oxides such as K2O (1-2%) and Al2O3 (2-5%). The iron oxide is fused in an electric

furnace and the promoters added. The solidified mass is ground to desired particle

size. The iron oxide is reduced to porous iron in the start-up phases of operation in the

synthesis reactor. There is a maximum operating temperature of about 6200C, above

which the catalyst fuses.

A promoted iron catalyst has recently been developed in Europe (Mont Cenis

process) which allows for very low temperature (4000C) and low pressure operation

(100atm). The life of the catalyst is not firmly established.

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, 50000C, 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

Module: 2 Lecture: 6 Ammonia

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using centrifugal compressors and having capacities as high as 1000 tons/day from

a single reactor at low production cost are used widely.

PROPERTIES

Molecular formula : NH3

Molecular weight : 17.031gm/mole

Appearance : Colourless gas

Odour : Strong pungent

Boiling point : -33.340C

Melting point : -77.730C

Density : 681.9kg/m3 at −33.30C (liquid)

Solubility : Soluble in water

USES

Ammonia is major raw material for fertilizer industries

It is used during the manufacture of Nitro compounds, Fertilizers e.g. urea,

ammonium sulfate, ammonium phosphate etc.

It is also used in manufacture of 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