Improving the Performance of Process Heaters Through Fireside Modeling

Since 1990, Reaction Engineering International (REI) has been working with industrial clients to help solve difficult problems related to industrial systems encompassing utility boilers, pyrolysis furnaces, gas turbine combustors, rotary kilns, waste incinerators, smelting cyclones and others. The computational tools currently used by REI are based on software developed over the last seventeen years by Dr. Philip J. Smith, vice president of REI. These computer codes use a computational fluid dynamic (CFD)-based turbulent reacting flow solver which couples together the effects of turbulent fluid mechanics, homogeneous gas-phase and heterogeneous particle phase combustion chemistry, and conductive, convective, and radiative heat transfer. These tools simulate reacting and nonreacting flow of gases and particles, including gaseous diffusion flames, pulverized-coal flames, liquid sprays, coal slurries, isothermal and reacting two phase flows, injected sorbents, and other oxidation/reduction systems. In this paper, discussion will be limited to the application of this software to current problems related to process heaters. The operation of process heaters requires exacting control of process fluid coil outlet temperatures and the heat flux distribution to the coils in order to prevent overheating which will accelerate the formation of coke on the inside walls of the process coils. Also, variations in coil outlet temperatures over time and from coil to coil are detrimental to process efficiency. The utility of these computational tools to the design of process heaters lies in their ability to provide full coupling of all relevant physical processes within the radiant firebox. Computer simulations can predict process outlet temperatures under different furnace operating conditions and improve the understanding of the dependence of a particular process on furnace conditions and, thus, lead to improved performance. The use of computer modeling has the potential to substantially reduce the risks associated with operating problems such as localized coil overheating by providing a means for establishing a reliable prediction of burner effects on furnace performance. This allows the designer to determine sensitivities to design changes without relying on simplified empirical models that are often extrapolated beyond the conditions for which their validity has been proven. Process heater case studies presented in this paper include investigation of: 1)the effect of flow pass balancing on variation of coil outlet temperature, 2) the effect of decoke nozzle location on overall firebox conditions and coke burnout during decoking operations, 3) predictions of tube temperatures and heat fluxes within the radiant and convective sections within a xylene splitter unit, and 4) the effect of refractory wall emissivity on conditions within the radiant firebox of a process heater. Case studies 1 and 2 were performed through an alliance between REI and Stone & Webster Engineering Corporation (SWEC), a worldwide leader in the design and manufacture of ethylene cracking furnaces. The remainder of the paper is organized as follows. In section 2, an overview of the modeling approach is given to highlight specific components of the process heater model. This is followed in section 3 by a discussion of the four case studies listed in the previous paragraph. The paper is summarized in section 4 where overall conclusions of this work are discussed