Ammonia Production Sessions 1

- 1 mole of nitrogen react with 3 mole of hydrogen to produce 2 mole of ammonia- Exothermic reaction (generate heat)- Nitrogen is produced from air and hydrogen is produced from natural gas (by steam reforming)- N2 + 3H2 -> 2NH3 dH = -92.44 kJ/mol- Reversible reaction- Yield is increase as T decreases. Yield is increase as P increase (Le chatelier principle)- Therefore we want low T and high P. But at low T, although high yield, but product is produced

slowly (low rate of reaction). What about P? Too high P is impractical,

Ammonia Production Sessions 2

- Desulphurisation where sulphur is removed from the process gas stream- Feed: desulphurised process gas- Steam is added to the feed stream upstream the primary reformer (heated to about 500 degC)- Addition of steam is according to the steam carbon ratio- Methane and steam are passed thru a nickel catalyst in narrow tubes. The tubes have a thick

walls to withstand high T and P used in the reaction. - In primary reformer, natural gas (HC component) react with steam where H2, CO and CO2 are

produced. For example CH4 + H2O -> CO + H2 / CO + H2O -> CO2 + H2- Primary reforming is endothermic: heat is supplied through burner in the furnace box, where

the burner surrounded tube in which required catalyst is loaded and reaction take place.- After that, process gas stream get into the secondary reformer which still contain 13% methane

that will be further reformed in the secondary reformer (unreacted methane). Composition of outlet of primary reformer are unconverted methane, H2O, CO, H2, CO2.

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- The 1st role of secondary reformer is to introduced nitrogen to the process stream (for the synthesis of ammonia) where air is compressed by compressor and that air is send to the secondary reformer. The 2nd role of secondary reformer is to further reformed the 13% unconverted methane.

- Reaction in secondary reformer: Oxygen is removed where oxygen in air react with some of hydrogen produced in primary reformer to produce steam (this is called auto ignition combustion reaction) and this produce huge amount of heat, T is raised to around 1000 degC. This will leave only nitrogen of the air with some traces of argon. Second reaction: methane is further reformed by the steam.

- Methane slipped out from the secondary reformer is about 0.4 mole%. - The process gas exit the secondary reformer at 980 degC and pressure around 35 bar. This

stream is mainly made up of hydrogen, nitrogen, CO, CO2 and steam (also trace of methane).- No specific technology to capture CO, therefore need to convert CO to CO2 which will be

absorbed in the CO2 removal unit.- Before entering HTS, the process gas high energy content is used to generate a super heated

steam (by the steam superheater heat exchanger). Therefore the process gas entering the HTS reactor is cooled down to around 360 degC thru a cooler.

- In HTS: CO + H2O (steam) -> CO2 + H2, exothermic (therefore there will be increase in the process gas temperature). As the conversion to CO2 increases and the T increases, the rate of shift reaction decreases as the reaction approaches equilibrium. The process gas is cooled in HX be4 entering LTS reactor.

- LTS operated at lower temperature than the HTS. Same shift reaction take place. Almost all of the CO is shifted to CO2. Gas outlet the LTS contain about 0.3 mole% of CO.

- Gas outlet the LTS is mainly hydrogen, nitrogen, CO2 and H2O with traces of argon from air and some CO slipped from the shift reactor. The gas is then cooled to allow all H2O in the stream to condense and separate upstream the CO2 absorber.

- CO2 removal: 2 columns which are CO2 absorber and CO2 stripper. CO2 is absorbed by solution (such as 30% potassium carbonate). The rich solution is stripped of from CO2 in the stripping column so that solution could be used in recirculating manner to allow continuous removal of CO2 from the gas stream. CO2 is removed from the stripper.

- The gas exit from the absorber (in the top section of absorber) is mainly hydrogen, nitrogen with 0.1 mole% of CO2, 0.3 mole% of CO and traces of argon.

- Since any O2 compound are harmful for ammonia synthesis catalyst, CO and CO2 present in the process gas must be either be removed or converted to another compound that is considered inert to the synthesis reaction catalyst. This is why methanation reaction is utilized. CO and CO2 react with H2 in the process gas stream to produce CH4 and H2O. Methanation reaction is the exact inverse of the primary reforming. It is exothermic and thus accompanied by an increase in T.

- Downstream the methanator, the gas is chilled to a temperature low enough to allow for all the H2O produced during methanation to condense and separate. Sometime molecular sieve adsorber is utilized to eliminate moisture from the gas stream. Now the process gas mainly hydrogen and nitrogen with traces of methane and argon is ready for ammonia synthesis (ratio of nitrogen and hydrogen 1:3).

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- The gas is first compressed to the process required pressure then heated to the required T in the inlet effluent HE where the gas outlet the ammonia synthesis reactor heats up the gas inlet.

- In the ammonia synthesis converter, N2 react with H2 to produce NH3. This reaction is exothermic and thus proceed with an increase in T. The converted gas is the cooled in the inlet effluent HE that we mention previously.

- The reactor will has thick wall to withstand high P.- So the gas outlet the ammonia synthesis reactor is cooled and further chilled to -33 degC in the

refrigerant unit that utiutilizesmonia liquid as refrigerant.- The reaction is not 100% conversion reaction where there are unreacted H2 and N2 that is

recycled back to ammonia synthesis loop once ammonia is separated from the stream in the refrigeration unit.

- Purge gas stream must be withdrawn to avoid the building up of inert in the synthesis loop.- The produced ammonia is simply sent to a storage tank at -33 degC and at atm P.

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Haldor Topsoe latest development in ammonia technology

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The pre reformed feed can be reheated to 650 degC before entering the primary reformer. This will result in reduced firing in the primary reformer, and thereby a reduced fuel consumption. When the hot flue gas is used to reheat the reformer feed, the amount of heat available for HP steam production is reduced. This will overall result in a reduced HP steam production in the ammonia plant. In general the reformer size can be reduced up to 25% in a natural gas based plant by incorporating a prereformer.

UHDE

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