ammonia industries

Ammonia

Introduction

• Ammonia (NH3) is a compound of nitrogen and hydrogen. It is a colorless gas with a characteristic pungent smell of 'wet nappies' − this is because urine can decompose to ammonia.

Year Million tonnes of Ammonia

2005 148

2006 153

2007 159

2008 158

2009 158

2010 159

World consumption of Ammonia - 2011

China 33%

Russia 11%

India 9%

Pakistan 1%

America 11%

Others 35%

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

Uses

20% is used in manufacturing: • Plastics • Synthetic fibres • Explosives • Dyes • Pharmaceuticals • Industrial refrigerant • Industrial and domestic cleaning agent

History

• 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.

Production of Ammonia (kt) in Pakistan

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2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Process

• Ammonia is synthesized from nitrogen and hydrogen by the following reaction: N2 + 3H2 ⇌ 2NH3

• The best available source of nitrogen is from atmospheric air. The hydrogen required can be produced from various feedstock but currently it ias derived mostly from fossil fuels.

• Depending of the type of fossil fuel, two different methods are mainly applied to produce hydrogen for ammonia production: – steam reforming CnHm + nH2O nCO + (n+ m/2) H2 H > 0 – partial oxidation CnHm + n/2 O2 nCO + m/2 H2 H < 0

• Steam reforming of light hydrocarbons is the most efficient route, with about 77% of world ammonia capacity being based on natural gas.

Steam reforming of NG

A) Synthesis gas production

– Feedstock pretreatment and gas generation

– Carbon monoxide conversion

– Gas purification

B) Compression

C) Synthesis

Hydro-desulfurization

Raw materials • Air, natural gas (or other

hydrocarbons) and steam. • Removal of sulfur from feedstock,

e.g. natural gas, involves these chemical reactions:

R-CH2SH + H2 H2S + RCH3

H2S + ZnO ZnS + H2O

• zinc oxide beds are typically not regenerated but are replaced with new absorbent once exhausted

Primary reformer

• After the removal of sulfur compounds, the gas is mixed with superheated steam and fed into a primary reformer.

• To achieve the required stoichiometric hydrogen/nitrogen ratio for ammonia synthesis, the reforming reaction is split into two sections.

• In the first section, the primary reformer, the reaction proceeds to achieve a partial conversion only [in conventional plants 65 % based on methane feed, leaving around 14 mol % methane (dry basis) in the effluent gas].

• In the secondary reformer— the gas is mixed with a controlled amount of air.

• The steam/carbon ratio used in modern commercial primary reformers for natural gas is between 2.8 and 3.5.

Primary reformer The burners primary reformers can be classified as top-fired, side-fired, terraced-wall, or, less common, bottom-fired reformers

Primary reformer

• The mixture is heated to 770oC in the presence of a nickel catalyst and these reversible reactions occur:

CH4 + H2O ⇌ CO + 3H2 ∆H=+206kJmol−1

CnHm+ nH2O ⇌ nCO + (n+ m/2) H2

CO + H2O ⇌ CO2 + H2 ∆H=−41kJmol−1 Overall reaction: CH4 + 2H2O ⇌ CO2 + 4H2 ∆H=+165kJmol−1

• The mixture that emerges is called synthesis gas

• The overall reaction (CH4 and steam to CO2 and H2) is highly endothermic.

• Therefore, maintaining a high reaction temperature moves the position of equilibrium to the right.

Primary reformer • Primary reformer has been identified as bottle-neck in increasing

the capacity of existing natural-gas-based ammonia plants. • This can be addressed by

– having pre-reforming, installed at the up-stream

• to transfer part of the conversion duty to the secondary reformer with application of an superstoichiometric amount of air.

Secondary reformer

• Only 30-40% of the hydrocarbon is reformed in the primary reformer because of the equilibrium reactions. The synthesis gas is cooled slightly to 735 oC, mixed with air and passed into the secondary reformer. Highly exothermic reactions happen such as:

CH4 + 2O2 2H2O + CO2 ∆H=−82kJmol−1

• With the energy released and further

heating a temperature of about 1000oC is reached and up to 99% conversion to methane to hydrogen achieved.

• Nitrogen (from the air) is used later in the synthesis of ammonia.

Uhde combined autothermal reformer (CAR)

a) Sandwich type tubesheet; b) Enveloping tube; c) Reformer tubes; d) Tubesheet; e) Refractory lining; f) Water jacket

Alternate technologies

Kellogg reforming exchanger System (KRES)

exchanger reforming + noncatalytic partial oxidation

Shift conversion

• Any remaining carbon monoxide in the gas mixture is converted to carbon dioxide in the shift section of the process:

CO + H2O ⇌ CO2 + H2 ∆H = −41 kJ mol−1 • The shift conversion happens in two stages:

–HTS | High temperature shift (iron oxide/chromium oxide based catalyst at about 400 oC) that lowers the CO content from 12-15% to about 3%.

– LTS | Low temperature shift (copper oxide/zinc oxide -based catalyst at about 200-220 oC) which lowers the CO content further to about 0.2-0.5%.

Purification

• CO2 containing synthesis gas is scrubbed under pressure with a solvent capable of dissolving carbon dioxide in sufficient quantity and at sufficient rate, usually in countercurrent in a column equipped with trays or packing

• The CO2-laden solvent is flashed, often in steps, to around atmospheric pressure, and the spent scrubbing liquid is subsequently heated and regenerated in a stripping column before being recycled to the pressurized absorption column

• CO2 is removed by chemical or physical absorption.

• Chemical Absorption – Tertiary amines methyldiethanolamine (MDEA) together with an activator. Approximately 80

percent of the ammonia plants use this process to aid in removing CO2

– The potassium carbonate with an activator, corrosion inhibitor • Benfield LoHeat System (UOP)

• Catacarb Process

• Giammarco -Vetrocoke process

• LRS 10 of British Gas

• Exxon's Flexsorb process

Abdul Samad

Highlight

Purification

• Physical solvent e.g. polyethylene glycol dimethyl ether, polyethylene glycol methyl isopropyl ether, polypropylene carbonate etc – Selexol Process (UOP) – Sepasolv MPE process (BASF) – Fluor Solvent Process

• After bulk removal of the carbon oxides has been accomplished by shift reaction and CO2 removal, the typical synthesis gas still contains 0.2 – 0.5 vol % CO and 0.005 – 0.2 vol % CO2. These compounds and any water present have to be removed down to a very low ppm level, as all oxygen-containing substances are poisons for the ammonia synthesis catalyst

• The gas mixture is further cooled to 40°C, water condenses out and is removed.

Purification

Methanation • Any remaining amounts of CO and CO2 must be removed before the NH3

synthesis stage as they would poison the catalyst. Methanation is the simplest method to reduce the concentrations of the carbon oxides well below 10 ppm and is widely used in steam reforming plants.

• The reaction is carried out over a supported nickel catalyst at a pressure of 25 – 35 bar and a temperature of 250 – 350 °C.

CO + 3H2 ⇌ CH4 + H2O ∆H = −206 kJ mol−1 CO2 + 4H2 ⇌ CH4 + 2H2O ∆H = −165 kJ mol−1

• The emerging gas must be completely dry so water produced in these

reactions is removed by condensation. This is accomplished by passing the makeup gas through molecular sieve adsorbers, which can be positioned on the suction side or in an intermediate-pressure stage of the synthesis gas compressor.

Effect of catalyst on reaction rate

• A catalyst has no effect on equilibrium. It speeds up both forward and backward reactions equally.

• Provide an alternate pathway with lower activation energy

• Increase rate of reaction

Effect of pressure on reaction rate

• Collision theory

– High pressure - More successful collisions

• Increase rate of reaction

Le Chatlier’s Principle

• If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or partial pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established.

N2 + 3 H2 ⇌ 2 NH3 + 92kJ

– Decreasing the temperature of a system in dynamic equilibrium favours the exothermic reaction.

4 volumes ⇌ 2 volumes

– an increase in system pressure due to decreasing volume causes the reaction to shift to the side with the fewer moles of gas.

Compression

• Centrifugal compressor for make-up and recycle gas compression of an ammonia plant

a) Air cooler; b) Separator; c) Silencer; d) Water cooler

Compression

Advantages over reciprocating compressors:

– Lower investment (single machines even for very large capacities)

– Lower maintenance cost

– Less frequent shutdowns for preventive maintenance

– High reliability (low failure rate)

Other compression duties in the plants are also performed by centrifugal compressors.

Ammonia synthesis

Ammonia synthesis

• The synthesis of NH3 takes place on an iron catalyst at pressures usually in the range 10-25 MPa and temperatures in the range 350-550oC

N2 + 3H2 ⇌ 2NH3 ∆H = –92 kJ mol−1

• The conditions used are a compromise between yield, speed & energy demands.

Mittasch

Ammonia synthesis

• The synthesis reaction is limited by the unfavorable position of the thermodynamic equilibrium, so that only partial conversion of the synthesis gas (25 – 35 %) can be attained on its passage through the catalyst.

• Ammonia is separated from the unreacted gas by condensation, which requires relatively low temperatures for reasonable efficiency.

• The unconverted gas is supplemented with fresh synthesis gas and recycled to the converter.

• The concentration of the inert gases (methane and argon) in the synthesis loop is controlled by withdrawing a small continuous purge gas stream.

• These basic features together with the properties of the synthesis catalyst and mechanical restrictions govern the design of the ammonia synthesis converter and the layout of the synthesis loop.

Ammonia synthesis

Schematic flow diagrams of typical ammonia synthesis loops

A) Synthesis loop for pure and dry make-up gas; B) Product recovery after recycle compression; C) Product recovery before recycle compression (four-nozzle compressor design); D) Two stages of product condensation a) Ammonia converter with heat exchangers; b) Ammonia recovery by chilling and condensation; c) Ammonia recovery by condensation at ambient temperature; d) Synthesis gas compressor; e) Recycle compressor

Molecular sieve drying of make-up gas has increasingly been applied in order to realize the energy-saving arrangement of the synthesis loop corresponding to Figure A.

Ammonia synthesis

a) Locus of temperatures resulting in maximum reaction rate

space velocity (per hour) 1/ contact time

reaction rate v in m3 NH3 / (m3 catalyst · s)

Influence of the different variables on the Synthesis Loop

• Pressure: increasing pressure will increase conversion due to higher reaction rate and more favourable ammonia equilibrium.

• Inlet temperature: there are two opposed effects as increasing temperature enhances reaction rate but decreases the adiabatic equilibrium concentration.

• Space velocity: increasing the space velocity normally lowers the outlet ammonia concentration, but increases total ammonia production.

• Inert level: increasing the inert level lowers the reaction rate for kinetic and thermodynamic reasons

Ammonia converter

Commercial Ammonia Converters • Commercial converters can be classified into two main

groups: – Internally cooled with cooling tubes running through the

catalyst bed or with catalyst inside the tubes and the cooling medium on the shell side. The cooling medium is mostly the reactor feed gas, which can flow counter- or cocurrently to the gas flow in the synthesis catalyst volume (tube-cooled converters).

– The catalyst volume is divided into several beds in which the reaction proceeds adiabatically. Between the individual catalyst beds heat is removed by injection of colder synthesis gas (quench converters) or by indirect cooling with synthesis gas or via boiler feed water heating or raising steam (indirectly cooled multi bed converter).

Ammonia converter

• Assignment

– Historical evolution of ammonia converter technology.

• 15 hand written pages

• Deadline : 8 November 2013

Complete Ammonia Production Plants

Modern integrated single-train ammonia plant based on steam reforming of natural gas (Uhde process)

a) Sulfur removal; b) Primary reformer; c) Steam superheater; d) Secondary reformer; e) waste-heat boiler; f) Convection section; g) Forced draft fan; h) Induced

draft fan; i) Stack; k) HT and LT shift converters; l) Methanator; m) CO2 removal solvent boiler; n) Process condensate separator; o) CO2 absorber; p) Synthesis

gas compressor; q) Process air compressor; r) Ammonia converter; s) High-pressure ammonia separator; t) Ammonia and hydrogen recovery from purge and

flash gas


M.W. Kellogg's low energy process


Haldor Topsøe's low energy process a) Desulfurization; b) Primary reformer; c) Secondary reformer; d) Shift conversion; e) CO2 removal; f) Methanation; g) Main compressor; h) Recycle compressor; i) Heat recovery; j) Converter


The Braun purifier ammonia process a) Sulfur removal; b) Primary reformer; c) Convection section; d) Secondary reformer; e) waste-heat boiler; f) Process air compressor; g) Gas turbine; h) High- and low-temperature shift converters; i) CO2 removal solvent reboiler; k) CO2 absorber; l) CO2 desorber; m) CO2stripper; n) Methanator; o) Driers; p) Purifier heat exchanger; q) Expansion turbine; r) Purifier column; s) Synthesis gas compressor; t) Synthesis converters; u) waste-heat boiler; v) High-pressure ammonia separator; w) Ammonia letdown vessel; x) Ammonia recovery from purge gas


ICI AMV process a) Desulfurization; b) Natural gas saturation; c) Process air compression; d1) Primary reformer; d2) Secondary Reformer; e) Boiler; f) High temperature shift; g) Low temperature shift; h) Selexol CO2 removal; h1) CO2 absorber; h2) Regenerator; i) Single stage compression; j) Methanation; k) Cooling and drying; l) Circulator; m) Hydrogen recovery; n) Ammonia converter; o) Refrigeration system


ICI LCA process (core unit) PAC, purified air compressor; HDS, hydrodesulfurization; IP, intermediate pressure; LP, liquefied petroleum; BFW, boiler feed water


KBR KAAPplus Process a) Air compressor; b) Sulfur removal; c) Process heater; d) Automatic reformer (ATR); e) Reforming exchanger (KRES); f) Condensate stripper; g) CO2 absorber; h) Methanator; i) CO2 stripper; j) Dryer; k) Expander; l) Feed/effluent exchanger; m) Condenser; n) Rectifier column; o) Synthesis gas compressor; p) KAAP ammonia converter; q) Refrigeration compressor; r) Refrigeration exchanger


Ammonia production based on heavy fuel oil (Linde flow scheme with Texaco gasification) a) Air separation unit; b) Soot extraction; c) CO2 absorption; d) Methanol/H2O distillation; e) Stripper; f) Hot regenerator; g) Refrigerant; h) Dryer; i) Liquid N2 scrubber; j) Syngas compressor; k) NH3 reactor

ammonia industries

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