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Ammonia

Zuari Agro Chemicals Limited Ammonia plant is based on the ICI Steam reforming process. Under this process, naphtha is used as feedstock. The manufacturing process for Ammonia results in Carbon dioxide (CO2) being produced as a by-product. Both the Ammonia and the Carbon dioxide generated are essential for the manufacture of urea. Zuari’s Ammonia plant has an installed capacity of 660 metric tonnes per day. There are a number of processes that go into the manufacture of Ammonia, including Hydro Desulphurisation (by IFP Process), Reforming and Shift Conversion (by ICI Process), CO2 Removal (by Benfield Process) and Synthesis (by TEC Process). The result of these processes is liquid Ammonia that is either fed continuously to the Urea plant and NPK A and NPK B plants or sent for storage. All process condensate that is generated from the manufacturing process is fed to a hydrolyser / stripper. The Zuari plant was the first one to adopt this high tech facility in India in 1977. Gas from the stripper is scrubbed and utilised in the NPK A / NPK B plants, while the stripped condensate water is recycled in a water treatment plant

Manufacturing Process

AMMONIA  PLANT

The basic steps involved in the manufacture of Ammonia are as given below :-

The basic steps involved in the manufacture of Ammonia are as given below :-
1.    Naphtha Desulphurisation
2.    Primary Reforming with Steam
3.    Secondary Reforming in an auto thermal reactor using process air to fix hydrogen  to  nitrogen ratio.
4.    2 stage shift conversion of carbon monoxide to carbon dioxide.
5.    CO2 removal using modified Benfield process.
6.    Methanation of residual carbon oxides
7.    Gas compression
8.    Ammonia Synthesis in two numbers parallel , radial  S-200 converters.
9.    Ammonia Separation from converter gases using ammonia refrigeration
10.  High pressure steam generation
11.  Ammonia scrubbing of purge gas including Purge Gas Recovery Unit.
12.  Process Condensate treatment

Naphtha Desulphurisation

Raw Naphtha is preheated / vapourised a fired process heater, mixed with H2 rich recycle gas and then passed over CoMoX  catalyst where sulphur is converted to H2S. The H2S is separated from Raw Naphtha in a Stripper column from which the offgas is routed to Auxiliary Boiler. “Sweet” Naphtha from the bottom of the H2S stripper is cooled and sent to Sweet Naphtha storage.

The Sweet Naphtha is subjected to additional Desulphurising step. Sweet Naphtha is preheated / vapourised a fired process heater, mixed with H2 rich recycle gas and then passed over CoMoX  catalyst where sulphur is converted to H2S. H2S is absorbed in the downstream ZnO catalyst bed. 

Reforming

Desulphurised Naphtha is mixed with process steam and passed through reformer tubes containing nickel based catalyst.  Since reforming reaction is endothermic, heat is supplied through 84 nos. burner. The temperature at the exit of the tubes is 780 deg.C. Most of the heat from the flue gases is recovered in the convection section to preheat process steam, process air, high pressure steam and combustion air. 

The partially reformed gas at 780 deg.C from primary reformer is mixed with process air in the secondary reformer and flows through a bed of nickel based catalyst.  Process air is supplied by a centrifugal compressor given by a condensing turbine.  The quantity of air is controlled to  provide nitrogen in the required proportion for the formation of ammonia.  The gas from the secondary reformer at 945 deg.C is routed through two Waste Heat Boilers in series to generate high pressure steam at  100kg/cm2 (A).  The cooled gas then flows to the high temperature shift converter.

Shift Conversion

Process gas enters HTS at about 350 deg.C.  HTS is charged with Iron oxide catalyst.  At the exit of HTS the CO content is reduced to about 3.0% and the temperature at the outlet of HTS is about 410 deg.C.  This gas is then cooled in process gas heat exchanger and in converted gas economizer to about 205 deg.C  before admitting to LTS guard/LTS. 

In LTS converter CO is further reduced to 0.25% by reaction over copper-zinc catalyst.  The reaction is exothermic and the temperature increases to 230 deg.C.  The gas is cooled  in process gas reboiler where a large part of excess steam present in the gas condenses to supply the regeneration heat for regenerating  aMDEA solution.  The process gas is further cooled in a BFW exchanger to 88 deg.C before being admitted toCO2 absorber.  Process Condensate formed in the two exchangers is separated in a process gas knock out drum and sent for treatment in the condensate stripper. 

CO2 Removal

Carbon dioxide is removed by absorption in Benfield solution in the  CO2 Absorber.  CO2 rich Benfield solution is regenerated in the CO2 regenerator.  The heat for regeneration is supplied in the process gas reboiler by condensing steam from the process gas and in solution exchanger by hot lean Benfield solution exiting the CO2 generator.   CO2 coming out of the regenerator is first cooled in an indirect overhead condenser and subsequently in CO2 direct cooler to 40 deg.C before supplying as a feed to the Urea Plant. 

Methanation

Process gas from the CO2 absorber is preheated in the process gas exchanger to about 290 deg.C and sent to the Methanator.  Methanator is charged with nickel based catalyst. The residual CO & CO2 present in the gas are converted to methane by reaction with hydrogen.  The reaction is exothermic.  The heat is used to  preheat boiler feed water going to the Reformed Gas Waste heat boiler.  The process gas is further cooled in gas final coolers using cooling water and subsequently in a chiller using ammonia  upto 10 deg.C before it flows to  the synthesis gas compressor. 

Ammonia Synthesis and Refrigeration

The synthesis gas which has hydrogen to nitrogen volumetric ratio of 3:1 and a small percentage of inert gases like methane and argon are compressed to the synthesis loop pressure in a steam driven  multistage centrifugal compressor.  Dehydrator which is installed in between the LP & HP casing of the synthesis gas compressor serves to purify the synthesis gas.  Here the final removal of  all oxide impurities including water takes place before the synthesis gas is introduced into the  synthesis loop at the suction of the recirculator, from where it goes into the converter alongwith the recirculating gas. The operating pressure of the synthesis loop is about 115 kg/cm2(A).  In the converters ammonia synthesis takes place over iron catalyst at temperatures ranging from 400~500 deg. C.  The ammonia concentration at the exit of this converter is about 12%. 

Converter effluent gas is progressively cooled in Synthesis Economiser, Synloop Boiler, Synthesis hot exchanger, BFW heater  before being admitted to the refrigeration section involving cold exchangers and ammonia supercoolers.  The gas is thus cooled to minus 20 deg.C and ammonia is separated. Unconverted gases are recycled back to synthesis Convertors. Liquid ammonia is separated in a separator is taken to a product letdown tank where the pressure is let down to 20kg/cm2(A).  The gases dissolved in liquid ammonia are released into letdown tank and are routed to ammonia absorber where ammonia is removed.  Gases free of ammonia are sent as fuel to the auxiliary boiler.  Product ammonia at minus 20 deg. C is heated to 5 deg.C and the chill is recovered in the refrigeration.  Product ammonia at 5 deg.C is sent to urea plant/hortonsphere storage.

Purge Gas Recovery Unit

To control the inert composition in the loop a small stream of purge gas is taken out and inerts, mainly methane and argon are separated in cryogenic purge recovery unit.  Purified hydrogen steam from the PGRU is sent back as make up gas to the synthesis compressor. 

Process Condensate Recovery

Process condensate from knock out drum  is sent to stripper unit where CO2, ammonia and methanol impurities are stripped using live steam in a stripper column operating at  13kg/cm2 (A). The stripped gases are routed to the complex fertilizer plants for recovery and the purified condensate is cooled and used as make up in the cooling towers. 

High Pressure Steam Generation

The high pressure steam requirement (100 kg/cm2(A) ) and 490 deg.C is generated in the Reformed Gas Waste Heat Boiler using the process heat at the outlet of secondary reformer.  The HP steam from the Waste Heat Boiler drums is superheated in the convection section of the primary reformer.  HP steam required over and above generated in the RG Waste heat boiler is generated in the Auxiliary boiler using Naphtha & Tail Gas as fuel.