AMMONI A PLANT
There are two streams of ammonia plant, each of 1350 MTD capacity based on the renowned KELLOGG High Pressure Reforming Technology. The gas obtain from Bombay High/South Basin, will be desulphurised, mixed with steam and passed on to the battery of tubes in a furnace called primary reformer where in the gas steam mixture passed to through the bed of nickel catalyst as well as heated externally to a temp. Of 818oC to reform the Hydrocarbons in to CO, CO2 and H2. This partially reformed gas mixture is passed on to the secondary reformer where in air is added in stochiometric proportion to supply the N2 as well as to complete the reforming .The reformed gas pass through the high temp. and low temp. shift converters wherein the CO is converted into CO2. The CO2 is then removed from the gas mixture in the CO2 Removal Section employing the latest “Modified Ben filed” process. The gas mixture is then Methanated to remove the final traces of CO, CO2. The gas mixture contains N2 and H2 in the ratio 1: 3 is then compressed in sys. Gas compressor to a pressure of 220 atm and passed over the catalyst filled Ammonia Converter to Synthesize Ammonia. The Ammonia formed is refrigerated to –30oC and stored in the Atmospheric Storage Tank. The waste heat generated at the various stages of exothermic reactions utilized to produce steam at 105-ata pressures. This steam coupled with that from an Auxiliary Boiler situated in the Main Reformer furnace provide power for all the drives in the Ammonia Plants as well as satisfied
the process steam requirement, making the plant self-sufficient, independent, and reliable and energy efficient.
PROCESS DESCRIPTION The Hazira Ammonia plant uses the “Kellogg Process” and has many features in common with the other Kellogg plants even though the feed stock used is Nature gas.
The manufacture of the Ammonia involves the following basic steps namely,
1. DESULPHURISATION: - Pretreatment of Natural gas feed stock for removal of sulphur,
which is a poison for catalysts used in Ammonia plant. This is accomplished in de-sulphuriser. Sulphur compounds present in the natural gas feed are assumed of the reactive type, that is they can be all be removed by hot zinc oxide alone. Provision is made to install a bed of carbon molybadate catalyst in the desulphuriser reactor, should this ever be required to hydrogenate organic sulphur compounds that might be found in natural gas which might not be eliminated by zinc oxide alone. The preheated natural gas enters 102-D from the top and passed through a bed of zinc oxide. All sulphur in the natural gas is absorbed by zinc oxide. natural gas exiting the desulphuriser is expected to contain less than 0.25 ppm sulphur. Other materials detrimental to long zinc oxide are: (1) Gum forming substances such as dienes and oxides of nitrogen. (2) Amine solution (3) Admission of air. (4) Aromatic compounds such as benzene (5) Introduction of alkaline constituents which convert organic sulphur compounds to unregenerated sulphate of elemental sulphur. The zinc oxide will absorb sulphur compounds with increasing ease in the following order: (1) Hydrogen sulphide (2) Carbonyl sulphide (3) Carbon disulphide. (4) Mercapton sulphur (5) Disulphides
(6) Thiophenes
2. REFORMING: - Reforming of the desulphurised Natural gas mixture of Hydrogen and Carbon
Oxides and addition of air in-between two stages of reforming. PRIMARY REFORMER : The primary reformer consists of 504 tubes suspended in 12 rows of
42 parallel tubes each, in the radiant section. Each row of tubes terminates in a manifold placed within the radiant section of furnace. There are twelve centrally located risers on each of these manifolds. These risers lea the gas flow to a water jacketed transfer line located over the top of primary reformer furnace. As the reforming reaction is endothermic heat is supplied externally to the tubes. The furnace operates with a down firing of natural gas between the rows of tubes to develop a process gas temperature of 818°C at the catalyst tube outlet. There are 234 arch burners arranged in 13 rows of 18 burners each. Natural gas, purge and flash gases from ammonia synthesis loop and absorber K.O. drum are used as fuel for the reformer furnace. Inside the catalyst tunes the natural gas-Steam Reforming reaction takes place. the transfer line directs the reformed gas into the secondary reformer at the following conditions: Pr.
: 33.1 Kg/cm²
Temp. : 824°C
REFORMING REACTIONS:
The following reactions take place producing a mixture of H2, CO, CO2, CH4 and excess H2O when hydrocarbons undergo steam reforming over nickel catalyst: CnH2n + 2nH2O = nCO + (2n+1) H2 CH2 + H2O + heat=CO + 3H2 CO + H2O = CO2 + H2 + heat
Carbon formation is a serious problem in catalyst tubes when higher hydrocarbons are used as a feed stock. Carbon deposition will hinder reforming and reduce heat transfer so that the tube wall temperature will rise in that zone producing “hot bands” and “hot tubes”. Carbon formation also causes disintegration of catalyst resulting in higher pressure drop in reformer tubes, loss of catalyst activity, low steam/ carbon ratio contribute to carbon formation. Precautions should be taken to prevent carbon formation on reforming catalyst for successful performance of reformer operations. SECONDARY REFORMING:
Partially reformed gases from the water jacketed transfer line (107-D) are directed to refractory lined and water jacketed secondary reformer (103-D) tangentially. Once it enters the secondary reformer the flow is downward around a centrally located Air inlet pipe of an air burner and passes through a diffuser to enter a mixing zone where the gas and air are mixed before entering a catalyst bed of secondary reform. Preheated air steam mixture from the coil enters the secondary reformer vertically through the centrally located Air burner. Instantaneous mixing and rapid combustion of the air reformed gas take place at the burner tip as per the following equation at a temperature of 1238°C. 2H2 + O2 +Air 3.8 N2 = 2H2O + 3.8N2 + heat CH4 + O2 + air3.8 N2 = CO2 + CO + H2O + N2 + heat
After combustion the gas mixture flows through a bed of secondary reforming catalyst. The catalyst in the secondary reforming comprises of a layer of high temperature chromia at the op and nickel reforming catalyst at bottom. The shallow layer of chromia has a high fusion temperature and protects the lower layer of nickel catalyst against excessive temperature. A layer of alumina target bricks is laid over the chromia catalyst for protection of catalyst and also proper distribution of gas in the catalyst bed Methane reforming reaction in secondary reformer is as follows: 2CH4 + 3H2O + heat = CO + CO 2 + 7H2
The reformed gas leaves the reactor at 996°C.
3. CO SHIFT CONVERTER : - Conversion of Carbon monoxide to Carbon dioxide.
Shift conversion is actually a two-converter system, high and low temperature. The reformed gas flow enters the HT shift converter (104D1) at 371°C temperature and about 31.3 Kg/cm² pressure and flows through the catalyst bed. The catalyst here is iron chromia catalyst. The following reaction takes place: CO + H2O = CO2 + H2 + heat
As indicated by this equation, we are converting most of carbon monoxide to carbon dioxide gaining an additional mole of hydrogen per 1 mole of carbon monoxide.
Above reaction is reversible one, with “shifting” carbon monoxide favored by low temperature. However, the rate of reaction is favored by high temperature. The gas leaves the HT shift converter at 437°C temperature and 31 Kg/cm² pressure. In passing through the LTS section the 3.3% content of the gas is reduced to appr. 0.3%. the amount of CO converted to CO2 in LTS section is about 30% of that of HTS section the temperature rise will be small with an inlet of 3.3% CO it will be around 27°C. The catalyst provided in the LTS section is Copper-zinc which is highly for sulphur and chloride poisoning and high temperature. Hence extreme care is to be exercised while operating with LTS.
4. CO2 ABSORPTION AND REMOVAL OF CARBON DIOXIDE : - By absorption in
Alkaline absorbent. The raw synthesis gas from 102-F at a pressure of 28.3 Kg/cm² and 93°C containing @19.25% dry volume of CO2 is introduced at the bottom of CO2 absorber tower (101-E). absorber is a cylindrical tower and has four beds packed with Hypack rings for large surface area and to increase the contact time between the gas and the benfield solution while flowing counter currently to the gas. A 30% (by wt) potassium carbonate solution enriched with 3% (wt) of diethanol amine (DEA) as an activator and 0.3 to 0.5% (wt) of vanadium pentoxide as corrosion inhibitor is called the BENFIELD solution and is a good reagent for CO2 absorption and the process is known as BENFIELD process. In the benfield process the removal of CO2 is accomplished in two absorption stages each having two beds of packing. In the lower of primary absorption stage partially regenerated benfield solution (semi lean solution) is made to contact with the CO2 containing gas in the I and II bed of absorber and bulk of the CO2 gets absorbed. Gas leaving the primary absorption stage obtains 1% CO2 .in the secondary
absorption stage fully regenerated and
cooled benfield solution (lean solution) is contacting the gas in the next two beds. The CO2 in the two raw syn. Gas is reduced to less than 1000 ppm (0.10%) by volume before it leaves the absorber tower. The absorption of CO2 by benfield solution involves the following reactions: K 2CO3 + H2 + CO2 = 2KHCO3
The rich benfield solution containing all absorbed CO2 from the bottom of the absorber is pressure reduced and flashed in the upper portion of CO2 strippers (102-E) either through hydraulic turbines or control valves. The regeneration of benfield solution is carried of in the strippers. The strippers contain three packed with hypack rings. The stripper operates with two solution levels one below the second bed from the top and the other at the vessel bottom below III bed. In the upper two beds rich solution is partially stripped of carbon dioxide. Part of solution from second bed bottom is withdrawn at about 125°C and serves as partially regenerated (Semi lean) stream after using in four stage flash tank (132°F), where it cools to 112°C and delivered to third bed top of the absorber by semi-lean solution pump 107-J. In the bottom bed most of the remaining CO2 is stripped from the remainder of the partially regenerated solution the “lean “ solution withdrawn from the bottom of the stripper (102-E) is cooled to 70°C in the tube side of air cooler (109-C) and delivered to top of absorber by lean solution pump(108-J). The stripper is operated at 130°C temperature and a Kg/cm² pressure at the bottom of the stripper column. The Reboiling heat requirement for the stripper is obtained from: 1. Reboiling benfield solution with LTS effluent gas in 105-C. 2. Reboiling reflux condensate from water wash trays with LTS effluent gas in 160-C. 3. Ejector steam required for the flash tank is produced in III-C by reboiling some of the condensate from the water trays with LTS effluent gas. In the first two beds from top the down coming rich solution comes in contact with hot liberated CO2 and water vapor flowing upwards and bicarbonate gets converted to carbonate under the action of heat and low CO2 partial pressure. 2KHCO3 = K 2CO3 + CO2 + H2O
A major portion of the partially regenerated solution is withdrawn from the level hold up maintained below the second bed is withdrawn in flash tank by four ejectors. The remainder of the partially regenerated benfield solution flows downward to the stripper bottoms through the III bed and gets regenerated to lean condition and collects at the stripper bottoms. The solution is withdrawn from the stripper and is cooled to 81°C in the shell side of the lean solution BFW exchanger. The partly cooled lean benfield solution is pumped by a steam driven lean solution pump(108-J) and is cooled with air in the lean
solution cooler(109-C) to 70°C. it then flows to the top of CO2 absorber. Part of lean solution is pumped to benfield filter (105-L) and return to the stripper bottom. The total flow of lean solution to absorber is around 188132 Kg/Hr. carbon dioxide enriched with water vapor is given a wash in the three water wash trays provided at the top of atrippers to wash down the carry over of carbonate solution. Reflux condensate from 109-J pump discharge is used for this purpose. After the wash, water flows down to a condensate drum (133-F) and then pumped by 116-J to condensate boilers III-C and 160-C.
5. METHANATION: -Final purification of the gas in a methanator to give a pure synthesis
gas of H2:N2 in a volumetric ratio of 3:1. Methanation reactions are given below: CO + 3H2 = CH4 + H2O + HEAT CO2 + 4H2 = CH4 + 2H2O + HEAT
Both these reactions are highly exothermic, and hence,extreme care is to be taken while operating methanator. The design temperature of the methanator (vessel) is 454°C which provides a large margin of safety in the event carbon oxides in feed gas to methanator are momentarily above normal conditions. The unit is properly protected with high temperature alarms and shut off systems. Methanator vessel, filled with a single bed of nickel catalyst operates at a temp of around 315°C inlet.
6. AMMONIA SYNTHESIS: -Compression of pure syn. Gas and synthesis of H2 &N2 in
Ammonia converter to form Ammonia. Synthesis gas from the 103-J second case discharge at a pressure of 210 Kg/cm² and 119°C flows through the shell side of syn. Gas compressor after cooler (156-C) it cools to 40°C by cooling water. It is further cooled to 5.6°C in tube side of make up gas chiller (140C) by means of refrigerant ammonia from 110-F. In the ammonia converter synthesis reaction takes place at a temperature of 480°C. N2 + 3H2 = 2NH3 + HEAT
There is also a purge gas system working where argon and methane are inerts. They tend to increase to high values as the process goes on. In order to limit their concentration in synthesis loop to 13.6 vol% a portion of the recycle gas is continuously purged.
7. PROCESS CONDENSATE POLISHING :- Process condensate from raw gas separator,
synthesis gas compressor suction drum, and synthesis gas compressor 1st and 2nd stage separators is steam tripped in process condensate stripper to remove bulk of ammonia carbon dioxide and methanol. Stripped gasses along with the steam are vented to atmosphere through primary reformer stack. Condensate collected from bottom of the stripper is polished in mixed bed polisher and is used as make up BFW. Mixed bed polisher contains 1.67 m3 of cation resins and 3.12 m3 of anion resin. Sulfuric acid and caustic soda are used to regenerate this bed once a day. Approximately 45m3 of liquid effluent is generated per regeneration and is sent to DM plant neutralization pit.
8. REFRIGERATION: Separation and purification of Ammonia to get the final Ammonia
product. The primary purpose of this system is to condense product ammonia for separating it from the converter feed. Further it is applied (a) to cool make up gas for separation at water (at the interstage of compression), to condense and recover liquid ammonia from purge gas and flash gases, and to cool the product run down to -33°C and degassing inerts.
