AMMONIA PRODUCTION

  Ammonia is produced by catalytic reaction of H₂ and N₂. Hydrogen can be produced by electrolysis of water (very limited), from steam reforming of natural gas and partial oxidation of naphtha and fuel oil. Nitrogen is obtained from air.

a)         Synthetic Ammonia (NH₃)

As illustrated in fig (7), synthetic ammonia, from natural gas, is produced by reacting hydrogen with nitrogen. Six processing steps are required to produce synthetic ammonia using the catalytic steam reforming method as follows:

1.                  Natural gas desulfurization

In this operation, the sulfur content (mainly as H₂S) is reduced to below 280 micrograms/ m³ to prevent poisoning of the catalyst used in steam reforming step. Desulfurization takes place by adsorption of H₂S gas on the surface of zinc oxide or active carbon. The adsorbent is reactivated by stripping with super heated steam. The feed gas is preheated to 350-400^o C and then treated in a desulphurisation vessel where the sulphur compounds are hydrogenated to H2S using cobalt molybdenum catalyst (CoO and MgO₃) and then adsorbed on palletized zinc oxide.

2.                  Catalytic steam reforming

The desulfurized natural gas is preheated by mixing with superheated steam (to 500- 600^o C) then enters the primary reformer and passes over the Ni catalyst where it is converted to hydrogen, CO and CO₂ according to the following equation:

 

CH₄ + H₂O                    CO + 3H₂

CO + H₂O « CO₂ + H₂

The reaction is highly endothermic and additional heat is required to raise the temperature to 780- 830^o C at the reformer outlet. Only 30- 40 % of the methane feed is reformed in the primary reformer. The gas from the primary reformer is then sent to the secondary reformer, where it is mixed with compressed hot air at around 600^o C and passed over nickel catalyst. Sufficient air is added to produce a final synthesis gas having a hydrogen to nitrogen mole ratio of three to one. The gas leaving the secondary reformer (H₂, N₂, CO, CO₂ and H₂O) is cooled to 360ºC in a waste heat boiler before being sent to carbon monoxide shift.

CH₄ + ½  O₂  + 2N₂                                           CO + 2H₂ + 2N₂

3.                  Carbon monoxide shift

After cooling, the gas, which contains 12- 15 % CO dry gas base, enters high temperature CO shift converter (350-400ºC) where CO converts to CO₂ using iron oxide catalyst and chromium oxide initiator. The following reaction takes place:

CO + H₂O        →      CO₂ + H₂

The exit gas is then cooled in a heat exchanger before being sent to a low temperature shift converter, where CO is converted to CO₂ by a copper oxide/ zinc oxide catalyst. The residual CO content in the converter gas is about 0.2-0.4 % (dry gas base) CO content is important for the efficiency of the process.

4.                  Carbon dioxide removal

The gas from the shift is cooled from 210 to 110º C and steam is condensed and separated from the gas. The shift gas is purified from CO₂ in a chemical or a physical absorption process. The solvents used in chemical absorption are mainly aqueous amine solutions (Mono Ethanolamine (MEA), Di Ethanolamine (aMDEA) or hot potassium carbonate solutions. Physical solvents are glycol dimethylethers, propylene carbonate and others. V₂O₅ is used as a corrosion inhibitor. The MEA process has a high regeneration energy consumption and is not regarded as a BAT process. The condensed steam contains ammonia and methanol, and small amount of amines, formic acid, acetic acid, sodium, iron, copper, zinc, aluminum and calcium. This condensate is sent to the stripper. Trace metals remaining in the process condensate can be removed in waste water treatment plant by ion exchange. The solvent is regenerated by preheating and steam stripping. In the BAT processes the stripped condensate is recycled.

5.                  Methanation

Residual CO₂ and CO, in the synthesis gas, must be removed by catalytic methanation by using Ni catalyst at 400-600ºC according to the following reaction:

CO₂ + H₂         →        CO + H₂O

CO + 3H₂        →        CH₄ + H₂O

Methane is an inert gas with respect to ammonia catalyst, while CO₂ and CO can poison the catalyst.

6.                  Ammonia Synthesis

Exit gas from the methanator is almost a pure. Three to one mole ratio of hydrogen to nitrogen is converted to ammonia according to the following reaction

N₂ + 3H₂          →        2NH₃

First the gas is compressed from 30 atm to a pressure 200 atm, heated against exit gas from converter and entered the converter containing iron promoted catalyst. This results in a portion of the gas being converted to ammonia (15 %), which is condensed and separated in a liquid vapor separator and sent to a let-down separator. The unconverted synthesis gas is further compressed and heated to 180ºC before entering the converter containing an iron oxide catalyst. A newly developed ammonia synthesis catalyst containing ruthenium  on a graphite support has a much higher activity per unit of volume and has a potential to increase conversion and lower operating pressures. Ammonia gas from the converter is condensed and separated then sent to the let-down separator where a small portion of the overhead gas is purged to prevent buildup of inert gases such as argon in the circulating gas system. Ammonia is flashed to get rid of dissolved gas. These gases are scrubbed to remove the traces of NH₃ in the form of ammonium hydroxide and the gases are used as part of the primary reformer fuel. The liquid ammonia can be either stored in pressure storage or in atmospheric double insulated refrigerated tank.

Major Hazards

The major accidents in ammonia plants are explosions and fires. In addition, there is also the potential for toxic hazard due to the handling and storage of liquid ammonia. The credible major hazards identified in an ammonia production plant are:

·               fire/explosion hazard due to leaks from the hydrocarbon feed system,

·               fire/explosion hazard due to leaks of synthesis gas in the CO removal/synthesis gas compression areas (75 % hydrogen)

·               toxic hazard from the release of liquid ammonia from the synthesis loop.

            In ammonia storage the release of liquid ammonia (by sabotage) is a credible major hazard event. Fires and explosions are usually not a hazard or only a minor hazard to the local population although potentially most severe for the plant operators. Appropriate precautions to protect both the operators and the local population are considered in the design and operation of the plants. The toxic hazard of a potential large release of liquid ammonia (i.e. from a storage tank) may be much more serious for the local population. An emergency plan for this event, covering the operators and the local population must be maintained.

 Process Flow Diagram for Ammonia Manufacturing

Inputs	Operations	Outputs
Natural Gas Adsorbent (ZnO/ activated carbon) Catalyst (CoO, MoO3 and/or ZnO)	Desulphurization	H2S, VOCs, emissions Spent catalyst
Steam Nickel catalyst Fuel or purge gas	Primary Steam Reforming	Heat stress Emissions (CO, CO2, H2,CH4) Fuel emissions Spent catalyst
Compressed hot air Nickel catalyst	Secondary Steam Reforming	Heat stress Wastewater (acidic steam condensate) Spent Ni catalyst
Process water		Heat stress
Catalyst (iron oxide/ chromium oxide and copper oxide/ zinc oxide)	W.H.B	Steam condensate to steam stripping (CO, CO2,Na,Ca,Al) Spent catalyst
Catalyst (iron oxide/ chromium oxide and copper oxide/ zinc oxide) Solvent (K2 CO3, MEA and MDEA)	CO shift Converter CO2 Separation	Solvent regenerated and recycled
Nickel Catalyst	Methanation	H2, CH4 emissions Heat stress Spent catalyst
Fe Catalyst	Ammonia Converter	Fugitive ammonia Cooling water to towers Purge gas to primary reformer Spent catalyst
Water	W.H.B	Steam Fugitive ammonia
	Cooling
	Refrigeration   NH3