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OPTIMIZATION OF FLUIDIZED HORIZONTAL HEAT EXCHANGER WITH LENGTHWISE DISPERSION

Artur Poswiata
Warsaw University of Technology, Faculty of Chemical and Process Engineering, Poland

Sinopsis

In this study, the author considers optimization (minimization of process cost) of solid particles being heating in a fluidized bed. The solid particles flow along the bed and are heated by hot gas which enters from the bottom of the fluidized apparatus. However, temperature distribution causes dispersion of solid particles to occur. Therefore the influence of the dispersion on an optimal run of the process is investigated in this work. The hydrodynamics of the fluidized bed is described by the two-phase Kunii – Levenspiel model. Moreover, in modelling this horizontal fluidized heat exchanger, it was assumed that the gas flows only vertically, whereas because the solid particles which have been ideally mixed throughout the cross-section of the bed result in a dispersive flow along the bed length.
The profile of inlet temperature of the heating gas and total gas flow rate, which minimize the total process cost, are searched during optimization. Because economic values are subject to local and time fluctuations, the accepted objective function describes the cost of the process expressed in exergy units. The continuous optimization algorithm, developed by Pontryagin, is used in the calculation. The adaptation of model equations to the algorithm and determination of the boundary conditions for optimization are presented. The results of the optimization, as optimal trajectories for temperatures of inlet gas and solid are presented. The influence of heat transfer kinetics and dispersion coefficients on optimal runs of the heating process is discussed. Our investigations indicated that for processes with dispersion, the optimal profiles of temperature for normalized time also are independent on process kinetics and hydrodynamics if the Peclet Number is constant. The process conditions influence only the total gas flow rate.

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