Gujarat Narmada Valley Fertilizers Company (GNFC) located at Bharuch in Gujarat has fuel oil based single stream Ammonia 1350 MTPD, Urea 1800 MTPD complex with other Phosphatic Fertilizers - ANP, CAN, AN MELT and industrial chemical manufacturing plants for Formic Acid, Methyl Formate, Acetic Acid, Methanol and Weak & Concentrated Nitric Acid. The Ammonia plant, was commissioned in 1981, is based on high pressure (80 barg) Texaco Gasification of fuel oil, which is used as the feedstock. Points of progressive developments in operation of Ammonia plant with complex fuel oil based technology to reach a new height of capacity utilization are summarized in this paper. To achieve the desired result of minimum production losses, emphasis has been laid on efforts to maintain better health of plant.
Due to fuel oil based technology, specific energy requirement is much higher in comparison with that of reformer plants. Thus implementation of innovative ideas is vital in order to achieve optimum energy level and maintain low operating cost. Considering the higher cost of grass-root project for Ammonia production, which results in an increased production cost, makes the project less viable in the changed scenario and hence retrofits and revamps become attractive options. GNFC has implemented such major retrofits and revamps which include addition of third Gasifier Train, replacement of Ammonia Synthesis Catalyst Basket from S100 to S200, Change in flow pattern of CO Shift Reactors, ASU revamps etc.

Introduction
Ammonia plant is based on partial oxidation of fuel oil technology of M/s. Texaco, USA. The detail engineering of the project was carried out by M/s. Linde AG, Germany and the best available technology was selected for different sections of Ammonia plant. The process involves Air Separation Unit (ASU) of M/s. Linde AG, Fuel Oil Gasification and Carbon Recovery Unit of M/s. Texaco, USA, CO Shift by BASF, Germany whereas  H2S and CO2 removal of M/s. Linde AG and Ammonia synthesis of M/s. Haldor Topsoe.
TEXACO'S high pressure partial oxidation technology using oil as feedstock for producing raw gas and its purification processes need different approach to operate the plant as compared to conventional reforming plants. Knowledge and understanding the problem and search for right solution to handle different situations required in-depth and analytical study to ensure higher loads of operation. After establishing confidence of operation in such technology and adequate absorption of technicality, we could reach to the capacity utilization of 126 percent maximum on day base and yearly at 117 percent. Details of performance are depicted at Table: 1.

Design capacity: 4, 45,500 MTPY
Exploring the possibilities of increasing the plant load by judging the inbuilt capacity and implementation of small innovative ideas was the continuous exercise throughout the operation period of last 28 years.  Close monitoring of all equipments and all machineries for ensuring their reliability has paid rich dividends for increasing production and continuous running of the plant.
 
Improvements without Major Investment

• Additional air input to air separation unit from utility plant air compressor to increase oxygen production by 1000 Nm3/hr and in turn Ammonia production by @ 40 MTPD.
• Removal of third stage suction strainer of synthesis compressor, which was having high pressure drop. This resulted in lower loop pressure by 3 bar and giving margin for 20 MTPD rise in Ammonia production.
• Use of LSHS as feed stock along with Fuel Oil giving higher production due to low sulphur (3.0 percent w to <1.5 percent w).
• Conversion of hollow pre-cooling tower to packed bed tower at Air Separation Unit using structured packing which has helped to reduce the tower outlet air temp by 13 deg C before inlet to revex. Moisture load on revex has reduced to half on reduction in air temperature. Cooler air resulted to higher oxygen product yield and thus higher production.
• Use of 3degc. refrigeration instead of 10 degc. for air separation unit. This helped in reducing air temperature by 2 degc. Giving more oxygen output which is one of the limiting factor for higher load.
• Retubing of 4 nos of turbine condensers, synthesis gas cooler and compressor inter stage coolers tubes with SS tubes to eliminate problems of leakage and building reliability.
• Provision of dedicated booster pump for higher elevated cooling water exchanger thereby reduction of C W circulation pressure by 1 bar and stoppage of sixth pump. Saving of energy by 10 MWh per day. Replacement of solid blades to hollow FRP blades in all 12 Nos of cooling tower fans reduced energy by 25 percent, saving of energy by 6 MWh per day.
• Installation of additional equipment like Gasifier feed oil pump, grey water pump, recycle gas compressor, 15,000 m3 capacity second ammonia storage tank, third refrigeration compressor has provided redundancy in operation. Production loss on account of limitation of running one has reduced considerably.
• Cooling water system was a major limitation during the peak summer and also sometimes during the normal running of the plant. We provided additional 12th cell to eliminate the limitation. 

Revamps and Retrofitting
Looking at the limitations after exploring the inbuilt capacity of the plant, it was observed that synthesis loop, air separation unit, and Rectisol wash unit were the major load limiting contributors along with soot extraction unit which played a major role in recycling impurities in feed oil. At every four months forced shutdown was required for catalyst replacement of CO shift reactor on account of higher pressure drop. With two available gasifiers, production loss on account of its schedule stoppage for maintenance was on higher side. Additional ammonia requirement for downstream product after Nitrophospate addition and use of CO rich gas from Gasifier outlet for Acetic acid, Formic acid, Methanol production compelled to enhance the ammonia production. Each of these factors was studied in detail and best possible revamp or retrofitting options were thought off. The brief details of such revamps is summarised below:
 
De-bottlenecking of Synthesis Loop
After detailed study of the ammonia synthesis loop supplied by M/s.Haldor Topsoe with S-100 basket and running at 240 bar pressure  at the plant load of 110 percent.Two major decisions were taken which are as follows :-
1) Installation of modified higher diameter impeller in recycle stage of  synthesis compressor.
It was decided to increase the synthesis gas circulation through the ammonia reactor to achieve overall higher conversion. The recycle stage impeller diameter was increased as per the margin available to accommodate in the same casing. Increase in circulation flow of 80000 Nm3/hr was achieved. This  reduced the loop pressure and rise in production level by 40 MTPD.
2)  Installation of Topsoe S-200 basket in place of S-100 basket.
The iron catalyst basket S-100 was in operation since plant commissioning(1981). After a decade of the operation, its activity got affected. Lower per pass conversion was causing high loop pressure of synth section, hence energy consumption was remaining high. Also it was the major bottleneck for higher production. Modified version of catalyst basket S-200 was available having inter bed heat exchange with better efficiency of catalyst bed, therefore S-100 basket replaced with better efficient S-200 basket in 1992. With the installation of S-200 basket,

• Per pass ammonia conversion increased from 25 percent to 30 percent.
• Loop pressure decreased from 240 barg to 218 barg for same load.
• 1500 kw energy saving achieved. Operation of synthesis compressor became smooth as higher loop pressure was causing higher load on synthesis gas compressor.
• Ammonia production level increased by @ 100 MT/day.
  Execution of both the retrofitting plans helped us in reducing loop pressure along with energy consumption and increase in plant load to 125 percent.

Gasifier Quench Ring Modifications
Original design of Quench Ring having 'C ' shape was failing in short time span of 3 months, resulting in unit  stoppages and huge production loss.  A series of modifications were carried out in step by step way in consultation with M/s Texaco - the technology supplier to increase its life. Ultimately, semi-circular shape Quench Ring with increased diameter from 18" to 23" proved to be more reliable. With this modified Quench Ring, extent of damages have reduced. At some occasions, after matured period of operation, its condition found good. Sudden failure of Quench ring and refractory leading to gasifier stoppage and major repair have considerably reduced. Production level has improved due to reliability of gasifier operation.

Installation of Vertical Decanter and Gasifier Bottom Catch Pot
Soot water outlet of Gasifier containing metal impurities like Ni and Va. Soot is removed from water stream during extraction operation in Decanter and is recycled back to Gasifier mixing with fresh oil. The metallic impurities preferentially remain in water phase and removed by blow down. In Horizontal decanter, the extraction and metal removal efficiency was poor resulting to more recycle of metal impurities along with oil back to the Gasifiers. Higher metal contents with oil were causing slag formation on the Gasifier refractory. Refractory damages, higher pressure drop at CO-Shift convertor due to soot/metal deposition, choking at soot water outlet lines and exchanger, quench ring damages etc. were the major problems due to metallic impurities going to gasifiers. In 1996, Horizontal decanter was replaced with better design Vertical decanter. Also catch pots provision made in bottom of gasifiers for holding more quantity of slag. With Vertical decanter and catch pot  installation, continuity of plant increased and operation stabilised. 

Third Gasifier Installation
Ammonia Plant had two units for gasification, which is the main gas generating section of the plant. The Texaco Quench type Gasifiers requires intensive maintenance at the regular interval of 3 - 4 months. However the plant is designed with two Gasifier trains, such that when one unit is down the other can operate at 80 percent of the plant load. With this little flexibility, continuity of the plant is ensured but production level comes down to 80 percent. Accordingly ammonia production loss was occurring by @ 30000 MT per year on account of gasifier stoppages. Apart from this, other plants i.e. Urea, Ammonium Nitro Phosphate, Formic acid, Methanol, Acetic acid; TDI etc are also affected due to such stoppages, affecting the profitability of the whole complex. Hence to reduce the loss of production, third gasification train is added to existing set up as a spare train. It was commissioned in May -2001. Average saving in production with addition of Third gasifier works out to be about 26000 MT per annum ( Table:3).

R-401/402 Parallel in Place of Series Operation 
Higher pressure drop across the catalyst bed due to soot/metal deposition at CO-Shift-Convertor R-401 was posing the load limitation. Also due to higher pressure drop, plant shutdown was required at interval of 4~6 months to replace and screen the catalyst. Various modifications like change in catalyst size and shape to increase the catalyst strength, change in steam by Gas ratio at convertor inlet, horizontal decanter to Vertical decanter resulted in marginal improvement.  As an ultimate measure, in 1999, the original gas entry in series of convertors (R-401 followed by R-402) changed to parallel entry to both R-401 & R-402. Load on both the converters has been divided to half with parallel entry which has reduced the velocity across the bed. Now the problem of higher Delta-P has been eliminated. Catalyst replacement is done on opportunity base in annual turn around. The modification became beneficial in reduction of down time as well as saving in costly catalyst. 

Replacement of Spiral Wound Exchanger in Rectisol Wash Unit
In Rectisol Wash Unit, CO2 and H2S are removed from the crude gas by absorption in methanol. The unit is designed and supplied by M/s Linde and has unique feature of spiral wound heat exchangers for large heat transfer requirement. Over a period, high pressure drop developed across the shell side of these exchangers and finally leakage started due to corrosion. Cleaning of such exchangers was also difficult due to very compact design, however chemical cleaning was attempted, which gave temporary relief. We decided to change all the exchangers with modified maintenance friendly design and better material of construction to overcome these problems. Accordingly Spiral wound exchangers as listed in Table: 2 were replaced with SS material from CS/LTCs. This change has increased the operation reliability, eliminated load limitation & minimized the downtime.

CO2 Enhancement Scheme
Ammonia plant was designed to supply pure CO2 to Urea plant. After added requirement of CO2 for Nitropohsphate Plant and Methanol-II Plant, pure CO2 shortage was limiting the load of Urea Plant. Hence CO2 enhancement scheme was conceived by modification in the existing rectisol wash unit and installed in 1998. With this innovative idea, left out CO2 in MeOH after stripping at first stripping tower (T-502) is generated by lowering the pressure in a drum. 3000 Nm3/hr pure CO2 at lower pressure of 0.3 barg is produced which is suitable for Nitrophospate plant.  

Air Separation Unit (ASU) Revamp
The Ammonia plant is operated at peak load on consistent basis with limiting factor being Oxygen produced in air separation unit up to 40000 Nm3/hr. Increasing demand of CO rich process gas to Acetic acid, Formic acid and Methanol plant has resulted in reduction of ammonia production. For achieving higher Ammonia production and supply of gas to Methanol / Acetic acid plants, ASU revamp was planned to get additional 6000 Nm3/hr of oxygen.  2.6 MW steam turbine driven N2 compressor and expansion turbine are added to the existing set up of ASU to get additional cold. ASU revamp hook up and other mechanical work is completed in April-05 shutdown. Pre-commissioning part is also over. Commissioning and lining up with ASU is expected in July - 05. With ASU revamp, air to oxygen ratio will improve from 6.8 at present (6.6 design) to 6.24. Also energy requirement per Nm3 of Oxygen production will reduce by 5 percent. Rise in ammonia production is expected by 180 MTD.

Refrigeration Compressor Revamp
The refrigeration requirement of the ammonia plant is catered by the GHH make four stage refrigeration compressor, providing -40°C, -20°C, 3°C and 10°C levels of refrigeration. Due to consistent high load of operation, machine had to be operated at high speed above maximum continuous speed. The distribution of the load on compressor was evaluated and limiting stages were identified for capacity enhancement. To overcome this limitation and modernize the control system for reliable and safe controls, the revamp is done by replacement of third stage rotor and stator parts. The control system is also converted to Turbolog electronic system.

Pneumatic Control System to DCS
Ammonia plant was having old pneumatic control system since inception which had become obsolete. Increasing maintenance with old system was more difficult to handle due to non availability of spares. New units added like Third gasification train had Distributed Control System. Considering fact of obsolesce and part of the modernisation with benefits like reliability, better controllability & problem diagnosis tools etc., it was planned to convert the pneumatic control system to DCS in three phases during the yr 2002 and 2003. Total 450 close loops, 1611 open loops and 1100 digital I/O taken on DCS. Conversion was done 'in house and on line' without losing a single day of production loss. After DCS control, plant operation became smooth with better control. Some of the perennial problems like super heater F1301 temperature control, de-aerator level control, faulty trip of gasifiers & compressors etc. could be eliminated. With DCS, Triple module redundancy (TMR) concept is adopted in critical parameters which have paid a reliable operation. Timer base Revex switching system in Air separation unit also taken on DCS through PLC and working smooth since last one year. All the local control panels for compressors and some critical pumps taken on DCS by which close monitoring became possible. 

Future Plans
Feed Stock Conversion from FO/LSHS TO LNG.
In view to reduce subsidy burden and to encourage the energy efficiency, Government has advised (as a part of Fertilizer policy) the Naphtha and FO/LSHS based Ammonia-Urea producers to switch over to Natural Gas as feed stocks. The GOI has also announced the long term pricing policy regarding the investment for feed stock conversion from FO / LSHS to LNG. In view of Government policy and also seeking the energy efficient option of NG based Ammonia - Urea, the study to switch over from FO / LSHS to LNG as feed stock was carried out through technology supplier and is under the final stage of Approval. 
CO2 Production Enhancement
Other than pure CO2 requirement in Urea plant, CO2 is supplied to Nitrophosphate and  is also added in synthesis gas for Methanol production. On increasing demand of CO2 with capacity enhancement at down stream plants, shortfall of pure CO2 will become limiting factor for higher loads. As explained in replacement of Spiral wound exchanger in rectisol wash unit) above, 3000 Nm3/hr pure CO2 as additional  production at Rectisol unit implementing CO2 enhancement scheme is in operation since 1998. On the same line, schemes for 7000 Nm3/hr additional CO2 generation at Rectisol wash unit have been worked out and is planned for implementation. Study is being carried out and it is under active consideration.

Installation of S 50 Reactor Loop with Topsoe S 200 Convertor
Additional S 50 reactor along with WHB is being added to synthesis loop to reduce energy level  by increasing per pass Ammonia Conversion. Ammonia Conversion will increase by @ 4 percent and energy get reduced by 0.21 MKcal/Ton of Ammonia.
 
Installation of Molecular Sieves Bed in Air Separation Unit 
Present Revex System will be added by Molecular Sieve Bed in Air Separation Unit for increasing the reliability of plant operation. It will also increase oxygen production marginally.
Conclusion
Being the core plant of GNFC, reliable operation of Ammonia plant at higher capacity is essential. To sustain the production level and with dependency of down stream plants  on Ammonia plant, reliability of gas supply to those plants is of prime importance. Continuous efforts to improve the production capacity and steps towards downtime reduction have been found to be encouraging. Retrofits and revamps at GNFC have become more attractive techno-economical option in improving profitability of the company. Reduction in energy level is today's need. Significant improvements are made in reduction of energy figure, however option to switch over in feed stock from FO / LSHS to NG is seriously considered.

P M Patel is Ammonia Plant, GNFC Ltd

G C Shah is Chief Manager Ammonia Plant, GNFC Ltd