OCR Text |
Show approaches infinity, heat loss through the furnace walls approaches zero. Using Eq. 1, along with assumed data for refractory wall emissivity, the furnace wall temperature, Ts, can be computed. Finally, thermo-chemical data for all inlets must be specified. Usually this requires specification of temperature, composition, and mass flow rate of a fuel stream and an air stream. W h e n two-phase flows are modeled, as in the simulation of decoking operations inside the firebox of a process heater, the composition, temperature, and mass flow rate of the particulate phase must also be specified. 3.0 CASE STUDIES Computer simulations were recently carried out for three full scale process heaters. Two of these simulations, case studies #1 and 2, were for full scale ethylene cracking furnaces, and the other, case study #3, was for a full scale xylene splitter reboiler (XSR) unit. The primary objectives of the ethylene furnace simulations were to: • Confirm required fuel firing rate to achieve the specified heat transfer to the coils • Predict the coil to coil variation in process fluid outlet temperature • Identify hot spots on process tubes • Quantify the amount of flow pass balancing required to minimize coil outlet temperature (COT) deviations • Predict the overall flow field, coke particle trajectories, and burnout during decoking back to the firebox The simulations of the ethylene furnaces were carried out as part of the design process prior to the manufacture of the units. The simulations of the XSR were conducted to aid in solving problems which were being experienced during normal unit operation. Areas of importance were: • Hot spots on process tubes • High bridgewall/crossover temperatures • Improved thermal efficiency and product yield In addition to these three case studies, results from some preliminary work related to model advancement is presented as case study #4. This work involves the implementation of a two-band, weighted sum of gray gases radiation model to account for spectral dependency of radiation heat transfer. The current radiation model is based on a gray gas approach which does not explicitly account for variation of absorption and transmission through participating media as a function of wavelength. The new approach takes this spectral dependence into account so that the effects of changing furnace wall emissivity on radiant efficiency can be more accurately represented. 6 |