Energy Integration of Fired Heaters
James Varghese, Ph.D, 07
Supervisor(s): Santanu Bandyopadhyay
Fired heaters, also known as process heaters or process furnaces, are important equipment in many chemical process plants. Fired heaters supply heat to process streams at elevated temperatures. Over 60\% of the energy utilized in a refinery is obtained by burning fuel in fired heaters. Other than refineries, fired heaters are also used in many chemical process industries such as ethylene, ammonia, fertilizers, etc. Energy conservation in every fired heater integrated process is imperative with increasing cost of primary energy. This study aims at developing a systematic energy integration procedure useful for energy conservation and design optimization of fired heater integrated processes. Targeting tools are useful to optimize and rapidly reduce the potentially large space of design alternatives to a small set of candidate designs that merit more detailed attention. Techniques based on pinch analysis of a fired heater and of the background process have been adapted for the present study. Algorithmic as well as graphical techniques are proposed to predict the energy requirements prior to the detailed design of a fired heater integrated process. Based on a simplified model of a fired heater, an analytical procedure is developed for integrating a fired heater into the overall process. This procedure helps in estimating the minimum fuel requirement. The minimum and the maximum air preheat temperaturesfor energy efficientintegration of a fired heater are also identified. It is shown that forfired heaters with air preheating using flue gas heat only, the minimum fuel requirement is independent of the air-preheat temperature as long as it is within the minimum and maximum range. Process heat below the pinch can be utilized for further reduction in the fuel requirement. In this case, it is demonstrated that there exits a maximum limit up to which process heat below the pinch can be utilized. In certain processes, multiple fired heaters are required to satisfy the total hot utility demand. A methodology is proposed to target the minimum number of fired heaters required to satisfy the energy demand. Fuel requirement and air preheat temperature for every fired heater may have to be re-estimated to set achievable targets. With multiple fired heaters, duties of different fired heaters can be varied to simplify the design of the overall heat exchanger network. Through a novel site grand composite curve the maximum potential of indirect inte- gration among multiple plants is established. The site grand composite curve represents the total utility requirement at any temperature after achieving maximum integration between dierent processes through intermediate fluids. The importance of selecting the intermediate fluid parameters for indirect integration of processes involving fired heaters is demonstrated. effect of steam as an intermediate fluid has been studied for improved integration between plants. The detailed design of a fired heater helps in estimating duty split between the convection and the radiation sections of a fired heater. A procedure for detailed design of a fired heater is also reported in this thesis. The application of the proposed targeting tools for developing and designing an energy efficientfired heater integrated process is demonstrated through a case study of a crude distillation unit of a refinery.