The development of a relatively simple mechanistic model for an industrial ethylene cracking furnace is described, including the estimation of selected model parameters to improve model predictions. Energy balance equations are developed to account for radiative, conductive, and convective heat transfer in the radiant section, and for convection and conduction in the ultra-selective heat exchanger (USX) and in the transfer line exchanger (TLE). Kinetic schemes by Ranjan et al. and Sundaram and Froment are used to model the cracking reactions. [1,2] The heat transfer model is combined with mass and momentum balances to model gas composition, pressure, and temperature changes as a function of position along the reactor tubes. Initial values and uncertainty ranges are assigned to 44 model parameters based on information in the literature and our industrial sponsor. A sensitivity-based technique and a mean-squared-error (MSE) criterion are used to select the appropriate subset of 22 parameters for tuning. Parameters are estimated and model predictions are validated using industrial data. Model predictions provide a good match to data that were not used for estimation.
An experimental study was conducted on the effects of flow pulsation on the convective heat transfer coefficients in a flat channel with series of regular spaced fins. Glycerol-water mixtures with dynamic viscosities in the range of 0.001–0.01 kg/ms were used as working fluids. The device contains fins fixed to the insulated wall opposite to the flat and smooth heat transfer surface to avoid any heat transfer enhancement by conduction of the fins. Pulsation amplitude xo=0.37 mm and pulsation frequencies f in the range of 10 Hz<f<47 Hz were applied, and a steady-flow Reynolds number in the laminar range of 10<Re<1100 was studied. The heat transfer coefficient was found to increase with increasing Prandtl number Pr at a constant oscillation Reynolds number Reosc. The effect of the dh/L ratio was found to be insignificant for the system with series of fins and flow pulsation due to proper fluid mixing in contrast to a steady finned flow. A decrease in heat transfer intensification was obtained at very low and high flow rates. The heat transfer was concluded to be dynamically controlled by the oscillation.
The development of a model for predicting coke formation in an industrial ethylene cracking furnace is described. Expressions for predicting the rates of catalytic and pyrolytic coke formation are developed and a differential equation is derived to predict changes in coke thickness with time and position. An expression is developed to account for a decline in the rate of catalytic coke formation with increasing thickness of the coke layer. The proposed coke model equations are used to extend a previously developed ethane pyrolysis furnace model that ignored coke. Three model parameters related to coke formation are estimated using industrial data to obtain reliable model predictions. Two of these parameters are coefficients that appear in the catalytic and pyrolytic coke formation rate expressions. The third is a characteristic‐length parameter used to reduce the rate of catalytic coke formation as the coke layer grows. The resulting dynamic model matches the industrial data well and can be used to simulate furnace operation and predict coke thickness profiles over a variety of the operating conditions, thereby helping process engineers who plan the decoking process.
Experimental and CFD studies of the enhanced average crossflow velocity in a laminar flow system were performed. The experiments were carried out using working fluid with a kinematic viscosity of 1.8·10 -6 m 2 /s. A steady flow Reynolds number in the laminar range of 0 < Re < 400 and oscillation Reynolds number in the range 0 < Re osc < 1000 were studied. The range of oscillation amplitude and frequency were 0.2 mm < A < 1.0 mm and 5 Hz < f < 90 Hz respectively. Three experimental configurations were studied, i.e., oscillating finned surface in a fluid at rest, which is similar to a batch configuration, steady finned flow and oscillating finned flow configurations. The acquired images were analyzed using particle image velocimetry (PIV) software. The study is also supported by CFD simulations using the software suit CFX 11.0 from ANSYS GmbH, Germany. The results of the flow visualization and PIV analyses reveal the formation of periodic vortices and increased transverse transport. The maximum enhancement of the average crossflow velocity was obtained at j = 3. The oscillation parameters and shape of the fins have a significant influence on the flow patterns and the crossflow effects. A triangular finned geometry gives better performance considering the enhanced average crossflow velocity. In general, efficient fluid mixing is possible due to the complex flow structures generated.
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