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Show and weight providing the main driving force for mean particle position change and the fluctuating component of velocity determining the variance about this value. Further details describing the methodology used for modeling turbulent particle transport are available elsewhere (Baxter, 1989; Jain, 1996, Smoot, 1985). Within the model, the three mechanisms for mass loss of each particle are vaporization, devolatilization, and heterogeneous reaction. The overall rate of reaction of each type of particle is found by solution of governing continuity equations for each of these processes. The kinetic parameters that are used are based on independent experimental observations and kinetic parameters deduced from these observations. In the context of coke particles in a process heater, the particle is considered to consist of four components: liquid, raw coke, char, and ash. Reaction rates for these components are computed based on local gas properties which affect heat transfer and mass transfer to the particle. O n the other hand, mass evolved from each particle due to vaporization, devolatilization, or char oxidation is locally incorporated into the Eulerian gas phase computations. In particular, this approach leads to a comprehensive strategy for representing coke particle transport and burnout within the radiant firebox of a full-scale process heater. Further specific details concerning particle phase reactions are available elsewhere (Baxter, 1989; Smoot, 1985). 2.3 Process Side Modeling To accurately predict heat transfer to the process side fluid, the fireside conditions must be coupled with the process side. The energy absorbed into the process coils acts to heat up the fluid and provide energy for the endothermic cracking reactions. This process temperature affects the convective heat transfer on the inside tube wall as well as the outside tube metal temperature and finally, the net heat flux to the coil. Thus, the process and fireside conditions are tightly coupled and must be modeled as such. The approach used for providing this coupling in the simulations discussed in this report was as follows. Using a detailed simulation of the kinetics and heat transfer inside the tubes coupled with a relatively simple model of external heat transfer to the tubes, S W E C obtained tabulated data of process temperature and heat transfer coefficient versus cumulative absorbed duty for each firing condition. Since the overall heat transfer coefficient is dependent on tube diameter, this correlation was also tabulated as a function of tube diameter. This relationship was then used within the comprehensive model to provide the link between absorbed duty, heat transfer coefficient, and process temperature. In other words, given the absorbed duty as predicted by the comprehensive model, this relationship provided the appropriate process temperature and heat transfer coefficient at each grid location along the process coil. In principle, the process side chemistry model should be fully coupled with the fireside combustion and heat transfer model since variation in the external heat transfer to the process coils, as predicted by the comprehensive model, could potentially affect the relationship between absorbed duty and process temperature. However, calculations performed by S W E C indicate that this level of coupling is not necessary and that the simplified model of external heat transfer, based on a given heat flux profile, is sufficiently accurate to identify the relationship between absorbed duty and process temperature. This is not to say that the external heat flux profile is not important with respect to its effect on process temperature, but that the distribution of heat transfer 4 |