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Show AFRC 2017 INDUSTRIAL COMBUSTION SYMPOSIUM September 20th, 2017Houston, Texas, USA Fired Heater Optimization using Dynamic Simulation for Transient Plant Operations Sultan Ahamad, Mohit Thakur, Rimon Vallavanatt Bechtel Corp., Houston, TX Introduction Hot oil is the most common heat transfer fluid in process heat transfer systems. The hot oil system is an integrated closed loop system with heat supplier (WHRU & fired heater) and users all over the plant. Availability of reliable hot oil system is essential for continuous and steady operation of an LNG plant. Hot oil is the only heat source for various unit operation in the plant. Figure-1 illustrates a typical hot oil system flow diagram. Modern petro-chemical plants utilize Waste Heat Recovery Units (WHRUs) to achieve energy demands and increase the overall plant energy efficiency. In a typical LNG plant, WHRUs are installed at Gas Turbines exhaust to recover the residual heat from flue gases. Waste heat recovered in WHRUs is used for various plant operation. WHRUs cannot provide energy requirements during critical transient operation (for example - startup of plant or process units, turbine trips, WHRU trip or maintenance, etc.). Therefore, a fired heater is used in combination with WHRUs to provide heating requirements during transient operation and/or supplemental heat demand. In the hot oil system, the heat energy from turbine exhaust flue gas is transferred to the hot oil in WHRU and is distributed to the process users (heat exchangers). The hot oil after exchanging heat with process fluid is cycled back to the WHRUs through Figure-1: Typical Hot Oil System Flow Diagram 1 Fired Heater Optimization using Dynamic Simulation for Transient Plant Operations Figure-3: Typical Waste Heat Recovery Unit (WHRU) in an LNG Plant This paper analyzes and provides insight into the following: • Optimizing the fired heater design and operation to minimize start-up and ramp-up time. Various design and operation optimization have been analyzed to bring the hot oil fired heater online at the fastest possible pace without adversely affecting any critical components. • Hot oil system response due to WHRU failure and start of fired heater to full capacity. A dynamic simulation was developed to analyze the overall plant performance to verify whether the system can ride through WHRU upset without operator intervention or exceeding any key process parameters. • In dynamic simulation, WHRU's and startup / backup hot oil heater are modeled including the rate of change in heat duty and ramping up of fired heater along with other equipment performance. Figure-2: A Typical Hot Oil Heater in an LNG Plant the hot oil surge drum. Figure-2 & 3 illustrates a typical hot fired heater and WHRU respectively in an LNG plants. Transient operation Any upset in hot oil system will affect the performance of equipment serviced by hot oil and ultimately affects the plant operation. When an operational WHRU is tripped, the fired heater shall start and ramp-up to compensate for loss in heat duty. To avoid any upsets in heating medium (Hot Oil system) due to the loss of WHRU, the start-up of fired heater and time required to reach its full capacity is critical. The purpose of the system optimization is to ensure that the system operation and controls are stable over the entire upset scenario. The system response over the entire range of operating scenarios must be able to shift from tripped WHRU's to Fired Heater and prevent exceeding any key process performance parameters. In this paper, an integrated hot oil system has been analyzed to ensure that hot oil system design is robust during various process upset scenarios. The dynamic response of the system is analyzed in details to study the plant's integrity and transient behavior during trips and upsets. WHRU Failure The integrated hot oil system is similar to the system shown in figure-1. When both WHRU's are in 2 Fired Heater Optimization using Dynamic Simulation for Transient Plant Operations operation, failure of one WHRU shall immediately result in 50% reduction in hot oil heat availability in the system. rain water falling into the heater cavity. The rain-hat can be designed to prevent the rain water into the heater. At the same time, it can allow the flue gases to go vertically up when fired heater is in operation. The fired heater is the only supplemental source of heat. However, it requires to start up and reach its full capacity quickly to prevent any upsets in the process users and potential of plant shutdown. Once started, the startup heater will compensate for loss of heat duty from the tripped WHRU. Fired Heater Design and Optimization for faster ramp-up StackStack is generally designed with a relatively high flue gas velocity. It is not practical to use a ceramic fiber blanket or module due to erosion. Therefore, ceramic fiber blanket with a liner plate (generally SS-409 material) shall be used in stack. The liner plate, in contact with flue gas, shall protect any erosion of ceramic fiber blanket. Operation As soon as WHRU is tripped, the back-up fired heater must start and reach to full load in minimum possible time. The refractory dry-out and purge time along with many other parameters will result into a very long start-up which will defeat the purpose of providing a backup hot oil heater. Following are some of the major design features & operation optimization implemented to achieve faster start-up. Stack also can be lined with ceramic fiber with closely spaced refractory anchors and keeping the flue gas velocity limited to no more than 40 ft/s at design duty. Convection SectionConvection Section is also designed with a relatively high flue gas velocity (though lower than stack section). At the same time, convection section (especially shield section) operates at a higher flue gas temperature which will restrict the use of liner plate. (a) Refractory Design Considerations Sudden heat input to the heater at design rate is not recommended for a heater with castable refractory due to concern of refractory cracking. For this reason, minimize the castable refractory as much as possible in the heater sections. Ceramic fiber module with a backup layer of ceramic fiber blanket can be used in convection section. The convection breeching section and lower temperature section of convection section can use ceramic fiber blanket with a liner plate. The header box can be ceramic fiber lined. The tube sheet can be made of same material as intermediate tube sheet without any refractory lining. Alternatively, tube sheet can be insulated with ceramic fiber board. If the heater is down for a long period of time, the refractory needs to be dried out before the heater can be brought to full operation. The drying out process is important to remove moisture from refractory. The refractory dry-out is a slow heating process with multiple hold-up in-between at various temperature. The slow heating and multiple hold ups are important to safely remove moisture without damaging refractory. This will take many hours or days. It is important to check flue gas velocity in each zone of convection section and ensure velocity is within allowable limit for selected lining system. Refractory dry-out is the biggest limitation for fired heater start-up. It is desirable to provide a refractory system which essentially do not require any dry-out. There are various sections / components in fired heater. Each section requires a different set of consideration for refractory design. It is possible to use ceramic fiber for most of the fired heater. However, each section will have different set of requirements which will result in different set of refractory system. Radiant SectionRadiant section has a very low flue gas velocity. A ceramic fiber blanket lining (without any liner) can be used in shielded wall and arch. Ceramic fiber blanket cannot be used on heater floor. However, using a castable lining on floor will not limit the heater start-up sequence. (b) Purge time and Pilot operation Purging of fired heater ensures that the concentration of any combustible hydrocarbon mixture present in It is also recommended to have a rain-hat to minimize 3 Fired Heater Optimization using Dynamic Simulation for Transient Plant Operations 420 420 410 410 Hot Oil Temperature to users, °F Hot Oil Outlet Temperature, °F Target Temperature 400 390 380 370 360 350 340 0 2 4 6 8 10 Target Temperature 400 390 380 370 360 350 12 0 Time, Minutes 10 20 30 40 50 60 Time, Minutes Figure-4: Fired Heater Ramp-up Curve Figure-5: Hot Oil Temperature to Users fired heater is reduced to a safe level. Per API-560, Fired heaters for general refinery services, purge should continue for a minimum of 3 firebox volume changes within 15 minutes to consider the combustion cavity free of hydrocarbons. If the heater is not sufficiently purged, an explosion may occur. understanding the transient nature of processes. Also, the capability of dynamic simulation to predict the design limitations within different operating envelops helps to validate the design. Usually process plants are designed based on the maximum operating flow or a worst case temperature and so on. But they are not studied for different possible combination of these operating cases. Dynamic simulation is an appropriate tool to validate such designs. Furthermore, identification of any design change during early engineering phase can provide significant savings for the plant over long term. In summary, this purging step takes at least 15 minutes. Purge time is another parameter which is a significant time consuming step. To avoid this time consuming step and keep heater ready for start-up, the pilots are kept under operation even when the fired heater is not in operation. Keeping the pilots in operation shall prevent any hydrocarbon accumulation in the heater firebox. This further eliminates the need of fired heater purging as well as ignition cycle typically required before the startup of a fired heater. Also, most of the moisture is kept away by keeping the heater pilots always in operation. The loss of energy due to pilot in continuous operation is negligible. A detailed dynamic model of the integrated hot oil network was developed and employed to study the overall hot oil system performance. The model was developed in commercially available dynamic simulation software. All equipment in the system were modeled based on equipment specific design data provided by equipment suppliers. The controllers were configured and tuned to respond to upset scenario. With these two major changes and many other minor changes, heater can be started and ramped-up very fast. The ramp-up rate curve of fired heater in case study is provided in Figure-4. In the case study, dynamic simulation model was developed and used for analyzing the overall plant performance to verify whether the system can ride through WHRU upset without operator intervention or exceeding any key process parameters. The dynamic response of the system is analyzed in details to study the plant's integrity and transient behavior during trips and upsets. WHRU's and startup / backup hot oil heater are modeled including the rate of change in heat duty and ramping up of fired heater Dynamic simulation Dynamic simulation is an effective engineering tool which can be used during the engineering phase of a project for study of plant startup, shutdown, revamp and troubleshooting. This can be used especially for 4 Fired Heater Optimization using Dynamic Simulation for Transient Plant Operations along with other equipment performance. Dynamic simulation was run with following sequence: • Starting with both WHRU's in operation at 50% load, one WHRU fails and startup heater provide the required heat load. • The fired heater optimization ensures that the system operation and controls are stable over the entire upset scenario. The system response times over the entire range of operating scenarios is able to shift from tripped WHRU's to Fired heater and prevent exceeding any key process performance parameters. Startup heater temperature controller and other hot oil controllers are kept in auto control mode. However, no additional heat was absorbed up by the WHRU remaining in service. Sultan Ahamad is a Sr. Fired Heater Specialist at Bechtel Corp. in Houston, Texas. He has more than 19 years of experience in the design, engineering and troubleshooting of fired heaters and combustion systems for the refining, petrochemical and LNG industries. He graduated from the Indian Institute of Technology in Roorkee, India, with a degree in chemical engineering. He is a member of API Sub-Committee on Heat Transfer Equipment and contribute to the development of API standards and recommended practices. He is also a member of American Institute of Chemical Engineers (AIChE), American Society of Mechanical Engineers (ASME) He has published and presented several papers on fired heaters and related subjects. He can be reached by email: sahamad@bechtel.com Dynamic Simulation Results • • Sudden loss of one WHRU will result in decrease of the overall hot oil temperature to users. To mitigate this decrease in oil temperature, startup heater will start and pick up the load and compensate for the heating loss in WHRU. Figure-4 shows the response of startup heater temperature controller. The plot shows that startup heater in auto control mode responds quickly and maintains the temperature at set point of 403°F. Figure-5 shows the response of overall temperature controller to users. The plot shows that controller responds quickly to mitigate the decrease in overall hot oil temperature and maintains it at set point of 403°F. Mohit Thakur is a Sr. Process Engineer at Bechtel Corp. in Houston, Texas. He has more than 10 years of experience in Process and equipment design, engineering, and dynamic simulations in the Oil, Gas and Chemicals industries. He graduated with a thesis in Chemical Engineering from Lamar University, Beaumont, TX. He can be reached by email: mthakur@bechtel.com Conclusion An integrated analysis of heat recovery and fired heater units is crucial to ensure that hot oil system design is robust during various process upset scenarios. The fired heater design optimization is critical to ensure that the back-up fired heater starts and reaches the full load in minimum possible time. Rimon Vallavanatt is a Sr. Principal Engineer at Bechtel Corp. in Houston, Texas. He has more than 40 years of experience in the design, engineering and troubleshooting of fired heaters, thermal oxidizers, boilers and flares. He graduated from the University of Kerala in India with a degree in mechanical engineering. He also received a degree in industrial engineering from St. Mary's University in San Antonio, Texas. Mr. Vallavanatt is a registered professional engineer in the state of Texas, and he has served on the American Petroleum Institute's subcommittee on heat transfer equipment for the past 30 years. He can be reached by email: rvallava@bechtel.com The dynamic response of the system is analyzed in details to study the plant's integrity and transient behavior during trips and upsets. The dynamic simulation results show that the loss of one operating WHRU and consequently drop in hot oil temperature was mitigated by the startup heater (operating in auto control mode). The start-up hot oil heater temperature controller response brings the hot oil temperature to normal operating conditions within required time limit for the case study. The overall hot oil temperature deviation to users was maintained within allowable deviation from the normal operating set point of 403°F. 5 |
