SHARANAPPA GURAPPA GEDIGERI,
KARANAM MANORANJAN BABU
Oman India Fertiliser Company S.A.O.C. Sur, Oman
This article was first published at Nitrogen and Syngas Conference, 24-27 February 2014, Paris
Urea Production technologies worldwide had been proven to be energy intensive and were mostly set up under the disbursement of sparsely available funds for capital expenditure. Though the process licensors put up their best synergic efforts to get to the as low as reasonably practicable energy numbers, still it had been observed that the actual operated plants provide opportunities for bringing in numerous changes for reducing the specific energy consumption and cut down the flare losses. These changes though initially sound like trivial in nature but in actual practice they prove to be highly beneficial and advantageous. Their implementation had been possible by proper evaluation of the systems based on the site specific conditions and the design margins available in the installed equipment.
This paper highlights an innovative concept of its kind for the first time configured in urea plants wherein a totally in-house designed scheme for recovering Ammonia from the LP section of the Urea plants was successfully implemented. The implementation of the scheme has been a major milestone in OMIFCO as it involved meticulous introspection and integration of the existing facilities of all the Ammonia, Urea and offsite utility plants together. After implementation of the scheme in both the Urea plants a total of about 55 MT per day of Ammonia product is being recovered which otherwise was going to the flare stacks adding to the emissions. The scheme had an attractive payback period of less than a quarter year with significant benefits realized in production and in the reduction of environmental emissions.
OMAN INDIA FERTILIZER COMPANY S.A.O.C. (OMIFCO) was set up as a joint venture project under the initiative of Government of Sultanate of Oman and Government of India. OMIFCO is owned 50% by Oman Oil Company, 25% by Indian Farmers Fertilizer Co-Operative Ltd (IFFCO) and 25% by Krishak Bharati Co-Operative Ltd (KRIBHCO). OMIFCO was registered in the Sultanate of Oman as a closed joint stock company in the year 2000.
The Ammonia Urea complex comprises two trains, each with a design capacity of 1750 MTPD Ammonia and 2530 MTPD granulated Urea, along with all supporting Utilities. It is designed to produce 1.65 million tones of granulated Urea and 0.25 million tones of surplus liquid ammonia annually for export, using natural gas as the raw material. Storage facilities for Urea (2X 75000 MT) and Ammonia (2X30000 MT) as well as jetty with ship loaders are part of the project.
The complex has two service Boilers of capacity 2 X 70 MT/hr and two HRSG boilers of capacity 2 X 110 MT/hr. Also the complex has its own captive power plant with two 30 MW Frame 6B Gas Turbine Generators and Import power connectivity with the national grid for backup power.
UREA PROCESS SALIENT FEATURES
OMIFCO had set up two Urea production trains each of 2530 MTPD production capacity. The Urea manufacturing process is based on the well established Ammonia stripping process designed by M/S SAIPEM Italy.
The Urea manufacturing process is characterized by the following main process steps:
a) Urea synthesis and Ammonia, CO2 recovery at high pressure.
b) Urea purification and Ammonia, CO2 recovery at medium and low Pressures
c) Urea concentration
d) Waste Water treatment.
Compared to the earlier designs some heat recovery features were integrated in the OMIFCO Urea plant design to give benefits in the overall steam and fresh water consumption. These features are:
a) Preheating of Urea Reactor feed Ammonia with off-gases from the L.P decomposer and overhead
gases from the process condensate treatment section.
b) Preheating of H.P carbonate recycle stream to H.P loop with process condensate.
c) Heat supply to vacuum pre-concentrator with overhead off-gases from the M.P.de-composer.
d) Complete recovery of process condensate as Boiler feed water make up.
The Urea melt unit is further supported by
a) The requisite auxiliary installations.
b) Four levels of Steam network involving
• HS Steam at 42 Bar G & 385°C
• MS Steam at 23.5 Bar G & 320°C
• Boosted LP Steam at 5.5 Bar G & 165°C
• L S Steam at 3.4 Bar G & 147°C.
c) Flushing network
d) Independent Flare Systems for MP and LP Sections.
The OMIFCO Urea process schematic flow diagram is shown as Figure-01.
DESCRIPTION OF THE PROBLEM
OMIFCO plants were commissioned in the second quarter of the year 2005.
After successful commissioning and accomplishment of the entire project guaranteed norms with regard to plant capacity, product quality, environmental parameters, specific ammonia consumption, specific energy consumption OMIFCO plants progressively went on to improve their capacity utilization and on-stream days from the second year of operation onwards. Plants achieved consistently capacity utilization at around 118 to 120 percent of the name plate capacity. The achieved on stream factor also has remained on higher side and the overall performance has been quite satisfactory. While sustaining with the impressive performance of the plants, on introspection of the process conditions it has been noticed that the Ammonia vapor losses through the LP section inert gases vent to the flare stack were remaining on higher side than the design. Around 1400 to 1600 KGS/hr of Ammonia rich inert gases stream was being released to the flare stack on continuous basis. This had not only resulted in increased Ammonia Specific consumption for Urea production but also had increased NOX emissions through the flare stack. Many efforts were made to optimize around the plant process parameters in order to reduce the Ammonia losses but could not reduce them to a satisfactory level.
As per the process design in the LP section of the Urea plant on the top of the Carbonate solution accumulator one LP condensate scrubber had been provided for scrubbing of traces of Ammonia from the rest of the inert gases stream before sending them to the flare stack.
This LP condensate scrubber has been fitted with four Ballast type trays for facilitating the mass transfer operations within the wash column for Ammonia scrubbing.
FACTORS RESPONSIBLE FOR HIGHER AMMONIA LOSSES
1) Urea plants had been operating at loads higher than the name plate capacity. Most of the plant operated time the average Urea plant load remained at around 118% to 120% of the name plate capacity. The design margins available with the major equipment provide the opportunity to operate at higher throughputs but the same might not be the case with the auxiliary equipment. This in turn reduces the efficiency of operations and increases the losses from the auxiliary equipment.
2) All these years supply of the fresh cooling water to Urea plants remained lower than the design flows. Due to frequent leakages in the underground RTRP (Reinforced Thermal Resin Pipe) make Fresh cooling water headers the header pressure is being operated at lower value than design and this resulted in less supply of cooling water flows to Urea plants. Due to less supply of cooling water flows than design the Urea plants performance could not reach their peak performance particularly during summer periods.
3) As per the design 715 KGS/hr of LP condensate is to be added to the inert gases washing tower installed above the carbonate solution accumulator for scrubbing of ammonia from the inert gases stream before flaring. But it was noticed that addition of external water was not benefitting the Urea synthesis operations due to reduced CO2 conversion and the plant load was also getting restricted. Due to this reason OMIFCO has been operating the Urea plants without adding the LP condensate for Ammonia scrubbing in the wash column.
INITIAL OPTIMIZATION EFFORTS IMPLEMENTED
OMIFCO tried different optimization efforts to reduce the Ammonia losses to the flare stack. The efforts continued till an effective solution was reached which finally culminated into the development and installation of the Ammonia chiller for condensing and recovering the Ammonia vapors going to the flare stack.
1) Installation of Split range operated Control Valves
In the original plant design the pressure control valve provided for maintaining the Carbonate accumulator overhead LP condensate scrubber pressure was of 6 Inch size. This control valve was primarily provided for quick release of vapors in case of any process upset condition. For maintaining a small flow of 1000 to 1500 NM3/hr of inert gases to flare stack this 6 Inch control valve was proving to be big in size and causing more losses of Ammonia due its intermittent opening.
It was then decided to install a split range controller to operate the exiting high capacity valve and a newly installed small capacity 2 Inch control valve for smooth and close control of the Ammonia rich inert gases flow. The newly installed small 2 Inch control valve is designated as A Valve (Operating signal range 0% to 40%) and the existing old 6 Inch control valve as B valve (Operating signal range 41% to 100%). This modification helped to reduce the losses associated with the mismatch of the control valve size compared with the flow being controlled. But the Ammonia losses arising out of the thermal equilibrium condition of the Carbonate solution in the accumulator were not reduced.
2) Addition of Condensate at Up stream of LP condenser
Another optimization effort by adding the LW condensate at the upstream of LP condenser was implemented to see the impact on the Ammonia losses along with the inert gases. At steady Plant load conditions plant parameters were recorded with and without the addition of the LW at the inlet of the LP condenser. The vent losses reduced considerably after the addition of the LW to the Carbonate solution accumulator.
The LP vent valve opening reduced from the previous 13.3% to 3.2% but unfavorable changes in other parameters indicated that overall this operation would not be beneficial. The average plant load reduced by about 1% and the plant overall Steam consumption by 1 to 1.5 tons. The external water added to the system was impacting the Urea HP loop performance and thus load was getting reduced causing disadvantage.
OPTIONS STUDIED FOR REDUCING THE AMMONIA LOSSES
1) Chilled Water System for more Ammonia Scrubbing
The LW condensate used for scrubbing the Ammonia from the LP vent inert gases is available at around 45 Deg. C. Instead of using this warm LW condensate it was envisaged to use chilled water as scrubbing medium. Chilled water could be produced at about 5.0°C by installing Lithium Bromide based Vapor Absorption system, which consumes low pressure steam. This option was considered as surplus LP steam was available in the OMIFCO complex. This chilled water scrubbing method would have reduced the Ammonia losses significantly.
But due to the disadvantage of external water used as scrubbing medium entering into the Urea synthesis section this scheme was not pursued forward.
2) Installation of Separate Ammonia Scrubber
A separate scrubber with water as the scrubbing medium was studied for recovering the Ammonia from the LP vent inert gases. The Ammonia-cal water from the scrubber would be integrated with the Ammonia distillation column of the Hydrogen recovery plant for Ammonia recovery in HRU plant. As the distillation column of Hydrogen recovery unit needs extensive simulation study and prima facie based on the current operated load doesn’t have much margin to take additional feed this option was not studied further.
3) Installation of falling film type heat exchanger
In many Urea plants LP section vent vapors are scrubbed by cold condensate and then cooled through a falling film type vertical heat exchanger positioned at the top of the Carbonate Accumulator of the LP Section of the Urea plant. This helps to remove the heat of dissolution of Ammonia and keeps Carbonate solution temperature in the accumulator on lower side.
This feature will help the plant operations even if cooling water flows are on a lower side due to any the existing infrastructure and with significant cost OMIFCO envisaged evaluating this feature during the upcoming proposed Urea plant revamp.
4) Diverting the LP vent gases to the suction of Ammonia compressor in the Ammonia plant
Under this option it was envisaged to divert the inert gases to the suction of the Ammonia Booster compressor located in the Ammonia plants through micro filtering arrangement. This was possibly the easiest option as the inert gases relieving control valves of Ammonia plant refrigeration system have margin to relieve more inert gases.
But considering the risk of exposing Ammonia plant equipment to CO2 and the carbonate formation possibilities during the Urea plants up set scenario, this option was not considered for implementation.
5) Possibility of using the Ammonia refrigeration compressor of Granulation unit
A new Ammonia chiller shall be installed for cooling and condensing the Ammonia present in the LP vent gases before sending to the flare stack. This Ammonia chiller shall be integrated with the Ammonia refrigeration compressor of the Air chilling unit located in the Granulation section of the Urea plant.
But this option was found to be technically less supportive due to the fact that the compressor minimum operated suction stage pressure is at about 4.0 Bar Abs which corresponds to an Ammonia saturation temperature of -2.1°C.
This higher pressure and higher saturation temperature doesn’t permit the new Ammonia recovery chiller to operate below -2.1 Deg. C. Thus the available approach temperature (LMTD) for Ammonia condensing would be much less compared to the case where the chiller would be operated at 1.7 Bar Abs. pressure with a corresponding saturation temperature of -23.0°C.
Due to this limitation of lower available LMTD other options to facilitate Ammonia chiller operation at a relatively lower pressure were explored.
IMPLEMENTED SCHEME AS A FINAL SOLUTION
After examining various options subsequently based on the OMIFCO site specific conditions use of an Ammonia chiller for cooling and condensing the Ammonia from the inert gases stream of LP section going to the flare stack was envisaged to be more effective and efficient from process safety point of view.
In Urea plants H. P. Carbonate Pre-heater E-113 had given frequent problems during the initial two years of operation due to its channel cover leakages and had forced unexpected plant shutdowns.
Subsequently it had been taken out of service and kept in isolated condition by foregoing the low grade heat that was being recovered. As LP steam had been surplus in Urea plants this decision was taken without any disadvantage.
This DEU type heat exchanger was examined thoroughly and found to be suitable for its usage as an Ammonia chiller. Simulation study indicated that it would meet the requirements of new process conditions and the surface area with requisite pressure drop profile. The provided material of construction for shell was SS-304 and for tubes & channel was SS-316 L. These materials were confirmed to be safe for new operated conditions. The readily available redundant E-113 also had transpired to work in the direction of the Ammonia chiller option.
By using this DEU type exchanger a scheme was developed for cooling and condensing the Ammonia vapors to facilitate their recovery from the Ammonia rich inert gases stream. The schematic drawing of the implemented Ammonia recovery scheme is shown as Figure-2.
List of Equipment Installed
A) Two motor driven plunger pumps. (1+1).
B) One Ammonia Liquid Knock out Separator.
C) Three control valves.
D) Piping & Fittings as per requirement.
E) DEU type exchanger (Already available)
The LP section vent gases of about 1400 to 1600 Kgs/hr flow (@ 3.9 Bar G & 36°C) are passed through the tube side of the Ammonia chiller and the refrigerant Ammonia drawn from the warm Ammonia header of the Urea plant is introduced on the shell side of the chiller. The condensed Ammonia from tube side is taken to a knock out separator for separating the liquid Ammonia and the balance inert gases with traces of Ammonia are sent to the flare stack. From the knock out separator the liquid Ammonia is pumped (@19Bar G & -8.7°C) to the Ammonia receiver of Urea plant located in the MP section.
The refrigerant vapors generated from the Ammonia chiller (E-113) are sent to Ammonia plant’s Ammonia Booster compressors common suction drum which operates at about 0.03 Bar G Pressure. Ammonia storage tanks boil off vapors are transported to the Ammonia plants Refrigeration Booster compressors common suction drum through an 8 Inch header by the Ammonia vapors Blower located at the Ammonia Storage tanks. Ammonia vapors from the shell side out let of E-113 exchanger are connected to this already existing 8 Inch size Ammonia Storage Blower discharge header at the common pipe rack nearer to the Urea plant Battery limits. This option helped to save around 300 meters of Process piping for two Urea plants.
This had been possible due to the available margins in the Ammonia Storage Tanks Boil off vapors transfer blower discharge pressure and the existing pressure drop profile in the interconnecting piping.
The piping interconnections made with the existing piping network of OMIFCO complex to facilitate the installation of the Ammonia recovery scheme are shown as Figure-03.
The quantity of Ammonia condensed and recovered through the newly established Ammonia recovery system was verified by the laboratory analysis of the streams involved and by measuring the volume of Ammonia accumulated in the knock out separator. The volume measurement was done first by reducing the level to the minimum tractable level reading of K.O. separator and then the Ammonia transfer pump was stopped to let the level accumulate in the separator.
The level was allowed to reach to a possible maximum value and then again pump was started back. The time consumed for the level accumulation from minimum level to the maximum level had been noted.
Please refer to the Graphics & Trend lines with timings spotted on Figures-04 & 05
As the separator dimensions and the temperature of the Ammonia accumulated in the separator are known, using these details the quantity of Ammonia recovered per day was estimated. This exercise was repeated for several times to get to the average value. Please refer to the Table-01 for daily recovered Ammonia quantity estimation calculation details.
Cost & Benefits of the implemented Scheme
After utilizing the already existing HP Carbonate Pre-heaters (E-113) of both the Urea plants a total of about 0.32 million USD was spent on establishing other features for completely installing the Ammonia recovery scheme in both the Urea plants
A) 55 MTPD of Ammonia being recovered from both the Urea plants LP vent gases.
B) Ammonia flaring reduced and equivalent NOx generation reduced. As one KG Mole NH3 burnt produces one KG Mole of NO, an overall 4041 KGS/hr of NO generation has been reduced.
C) Urea Specific energy consumption reduced by 0.065 Gcal/MT.
D) Other than the economic considerations, the Ammonia recovered indirectly saved the quantum of fuel that would have been spent on producing the equivalent Ammonia being supplied for Urea production. The fuel saved had saved equivalent of GHG emissions which otherwise might have been generated.
E) Cold Ammonia added to Ammonia receiver has improved the performance of the HP Ammonia feed pumps.
F) Additional Urea production of 96 MTPD could be achieved due to availability of 55 MTPD extra Ammonia.
G) Pay Back of the scheme is achieved in less than a quarter year.
The DCS face plate showing the Ammonia recovery scheme details are shown as Figure-06.
Table-01: Daily Recovered Ammonia quantity estimation Calculation details
AMMONIA RECOVERY FOR UREA-21 UNIT AS MEASURED ON 27.02.2013:
The best designed new generation plants with the latest knowhow of the process features incorporated in the overall plant design and As-built conditions do provide opportunities for initial optimization to improve the performance. However depending on the specific site conditions and the operating conditions of the plants the end user can explore further opportunities more objectively and devise new criteria for harnessing the synergies of the state of art as built systems and equipment.
In this pursuit OMIFCO without becoming complacent always remains circumspect and keeps continuous introspection of their systems for further optimization both from short term gains and long term perspectives as well without compromising on the Safety, Productivity, Environment, Health of the employees and the society at large.
OMIFCO has isolated the problematic HP carbonate pre-heaters based on the site specific conditions to alleviate from the problem of frequent shutdowns and to save the water lost through the LP steam venting. The same redundant Carbonate pre-heaters have been reused in a new role to derive the benefits out of it and to solve another existing problem in an effective manner.
Figure-01: OMIFCO-Urea process schematic flow diagram
Figure-02: Schematic drawing of the implemented Ammonia recovery scheme
Figure-03: Piping Interconnections made in the existing piping network as part of Ammonia recovery scheme
Figure-04: Time & Level Readings when Ammonia pump was stopped. (At Min. Level of Separator)
Figure-05: Time & Level Readings when Ammonia pump was restarted. (At Max. Level of Separator)
Figure-06: DCS Face Plate showing details of the Ammonia Recovery Scheme