Productivity Enhancement at the Shahajahanpur Fertilizer Plant Through CFD based Combustion Air Draft Flow Optimization of Primary Ammonia Reformer

Transcription

1 Productivity Enhancement at the Shahajahanpur Fertilizer Plant Through CFD based Combustion Air Draft Flow Optimization Primary Ammonia Reformer Introduction: Kribhco Shyam fertilizer Ltd (KSFL) had been producing maximum 1740 TPD Ammonia at Shahajahanpur plant. This unit was experiencing certain process bottlenecks to enhance the productivity. One the major bottlenecks was combustion air pre heater draft system primary ammonia reformer. Due to inadequate sizing Air Pre Heater (APH) modules /passes poor duct design caused capacity limitation the existing FD fan. FD fan was unable to pass through more than 185,000 Nm3/ Hr combustion air from APH modules. About 12.5% air was bypassed before air heater to meet desired air. Further there was an additional bypassing air after module/pass III the air heater due to excessive pressure loss across APH module/ pass -IV. Bypassing combustion air was impacting on operational efficiency the operational cost. In today s competitive market, the requirement for efficient plant operation is paramount importance performance expectations are quite deming than a decade ago. Therefore a quick action was required to find a feasible solution to overcome above process bottleneck. Challenges & Approach to Problem solving: There were following options considered to address above mentioned operational constraints. 1) To replace existing FD fan with new FD fan Turbine drive mechanism: This option was simple to configure but very expensive due to higher capital cost, higher plant down time new foundations. Moreover higher rate from same duct APH would have increased induced vibrations turbulence in the system. This may also resulted in an extra stiffening the existing ducts. Further higher power will be consumed by FD fan to deliver required air due to higher duct velocities resulting in inefficient operations. Therefore this option was ruled out by KSFL. 2) To replace existing air heater with larger surfaces area air heater modules: This option was very expensive technically unattractive due to major changes in inlet outlet combustion air duct lay out. These changes would call for longer plant shutdown period. Hence this option was also ruled out by KSFL. Since above options were not a feasible solution, we were forced to think outside the box required to explore innovative & unique theories practices that have already been expounded relied upon in fluid mechanics duly supported by aerodynamics to address above operational challenges. We approached M&I Power Technology Inc, Canada through M/s Triton Synergies (P) Ltd, New Delhi, to fer optimization solutions. Working closely with Mr. Pradeep Gaur M&I Canada & Triton Synergies, we were able to find unique cost effective solutions. We found these solutions quite innovative conducive to eliminate our operational bottleneck without making significant changes to the existing system. Further we are able realize energy savings reducing carbon foot print through reduction in fuel power consumption. The Way Forward: Based on the preliminary assessment technical proposal from M&I & Triton Synergies, we decided to proceed with them for this job. Their innovative combustion air draft optimization through retritting aerodynamic diffusers partial duct modifications at the strategic locations in the existing system would facilitate to realize more output from the existing system. The target monitoring improvement was to accommodate 12.5 % bypass air through exiting air heater modules without changing FD fan & drive turbine APH design.

2 M&I & Triton proposed following improvement in the parameters to eliminate 12.5 % bypass situation in the existing plant: 80mm WC pressure drop reduction across existing primary reformer combustion air heater draft system. This reduction in back pressure would facilitate existing FD fan to push 12.5 % air through existing APH. Project Details: Flaws in Existing Design:Based on M&I s expertise in control their computational simulation (CFD) for air dynamic modelling, we were able identify following abnormalities (Figures 1A to 3) across various elbows, APH inlet transitions the existing duct layout.

3 Design Improvements developed, manufactured installed by M&I & Triton Synergies: Project Schedule: The order for CFD air dynamic modelling, design engineering, Manufacture, supply, dismantling erection was placed in September The unit commissioned April, on was 4th Fabrication & Installation was done by Triton Synergies completed in a very short span time ahead schedule. New innovation to old concepts made this a real option for improving the performance existing system. By leveraging integrating aerodynamic principles, we are able to improve performance FD fan air heater without any modification to original equipment. This energy savings draft loss reduction is achieved by virtue converting unproductive velocity pressure to static regain, which otherwise was lost in higher turbulence, local separation, vortex induced noise.

4 Acceptance Test: Acceptance test measurements guaranteed test run was carried out after 4weeks plant operation, which was adequate to establish benefits design improvements. The pro total pressure drop savings was provided through actual before after the modification as well as readings from local pressure port upstream Air Heaters. Performance Comparison & Summary Results: It is indeed interesting to note that design improvements exceeded guarantee values. Table 1: Site measurement before after modification Values Sl. Description No. Unit Before modification After modification PM 3 PM Deg C Combustion Air Outlet Temp from APH (TI 14) Deg C Combustion Air Flow Nm3/Hr 182, , Combustion Air Pressure at APH inlet (PI-02) Combustion Air Pressure at APH outlet (PIC-03) Pressure drop across APH Combustion Air temp. at APH inlet ( TI 13) 2 3 The table shows that the pressure drop across APH increased from 469 mm WC to 501 mm WC, with the corresponding increase in the from 182,000 Nm3/Hr to 212,000 Nm3/Hr. However, if the rate in the original system was to increase from 182,000 to 212,000 Nm3/Hr the equivalent pressure drop across the system can be computed from formula (1), contained in reference [1] p=.241lρ¾ µ¼d-4.75q1.75 (1)

5 Where p - pressure drop through the system (Pa) L geometric parameter (m) ρ density (kg/m3) µ - viscosity (Ns/m2) d geometric parameter (m) Q- Rate (m3/s) The fluid conditions density viscosity are taken from reference [1] are summarized in the table below Average Temperature (C) Original System Original System (same conditions as during the test after modifications) Viscosity (Ns/m2) 2.58E E-05 Density (kg/m3) L, d are unknown, however they always stay the same for a given system therefore they cancel out when using (1) to compute the pressure drop ratio for a given geometry with different rates: p2/ p1 = ((0.746)¾ *(2.57E-05)¼ *(58.89)1.75) / (( )¾ * (2.58E-05)¼ * (50.56)1.75) = 1.31 Pressure drop in the original system with new rate 212 KNm3/Hr (58.89 m3/s) would be p2 = p1 *( p2/ p1) = 469 * 1.31 = 615 mm WC Therefore the pressure drop savings provided by M&I modifications through the APH is ps = =114 mm WC Summary Benefits: According to the guaranteed test run along with operating data, the following benefits were achieved mm WC pressure drop reduction 16% more air is passing through air heater modules with 0% bypass Additional heat recovered by exchanger after modification due to uniform distribution: 1.97 Gcal/hr Increase in fertilizer productivity Payback less than 6 months on account additional heat recovery Technology Prospects: The optimization carried out at the KSFL, Shahajahanpur plant demonstrate that the application aero-acoustic principles combine with experience, expertise understing induced problems can lead to significant reduction draft loss(pressure drop), improvement in wind boxes performance improvement rotating machines like ID & FD fans. There is a great potential for optimization existing fertilizer plant combustion air, Flue gas ID fans process air compressor draft system since optimization is not given priority during design stage the new plants. The aero-acoustic diffusion system developed for such application also provides great potential for optimization in other air & flue gas application in critical & non critical areas. By using this technology, abnormalities / problem can be solved energy savings & improvement can be realised along with reduction in CO2 emissions. References: [1] Frank M. White, University Rhode Isl, Fluid Mechanics 5th edition, McGraw Hill 2003