324027 CFD Simulation of Bubble Dynamics of Fixed Valve Tray
Mass transfer tray column is an important type of gas-liquid contact equipment widely used in the oil refining and chemical industries, as its features of easy maintainability, low cost, convenient feed and side withdrawing. In industrial practice, however, the performance of the trays always suffers from a loss of separation ability due to maldistributed or abnormal flow regime, e.g. liquid phase circulation which occurs frequently on a tray. It has been long established that the prediction of distillation tray hydrodynamics is necessary for the separation efficiency and overall performance. Besides, the understanding of these two-phase hydrodynamic phenomena on trays is helpful for the fine tuning and optimization of modern tray design.
There have been many attempts to model tray hydrodynamics by use of CFD. Mehta et al.1 analyzed the liquid phase flow patterns on a sieve tray by solving the time-averaged equations of continuity of mass and momentum only for the liquid phase. Interactions with the vapor phase were taken account of by use of interphase momentum transfer coefficients determined from empirical correlations. Yu et al.2 and Liu et al.3 ignored the variations in the direction of gas flow along the height of the dispersion to simulate the two-phase flow behavior, and only the hydrodynamics of the liquid flow was obtained. Fischer and Quarini4 have attempted to describe the three-dimensional transient gas-liquid hydrodynamics. An important key assumption made in the simulations of Fischer and Quarini concerns the interphase momentum exchange (drag) coefficient; these authors assumed a constant drag coefficient of 0.44, which was appropriate for uniform bubble flow. This drag coefficient is not appropriate for description of the hydrodynamics of trays operating in either the froth or spray regimes.
The authors mentioned above have not included any consideration about the bubble dynamics (breakup and coalescence) which is very important in their work. Lift force and other non-drag forces are also sensitive to the bubble diameter distribution. In this work, a three-dimensional computational fluid dynamics (CFD) model was developed to predict the hydrodynamics of a new type of fixed valve tray. The model considered gas- and liquid-flow within the Eulerian framework in which both phases were treated as interpenetrating continuum having separate transport equations. Interphase momentum transfer term was employed for describing the interfacial forces between the two phases, and the related average gas hold-up was obtained via the regression equation from experiment data. Bubble dynamics, such as bubble breakup and coalescence, bubble diameter distribution, was included by use of the Multiple Size Group (MUSIG) Model. Calculations were carried out using the commercial packages ANSYS CFX 12.0. Clear liquid height, gas hold-up, gas and liquid velocity profiles were predicted for various combinations of weir height, gas and liquid flow rates. The predicted clear liquid height was generally in good agreement with measurement. The information predicted by the CFD model can be used in the optimal design of industrial trays.
References:
1. Mehta B, Chuang KT, Nandakumar K. Model for Liquid Phase Flow on Sieve Trays. Chemical Engineering Research and Design. 1998;76(7):843-848.
2. Yu KT, Yuan XG, You XY, Liu CJ. Computational Fluid-Dynamics and Experimental Verification of Two-Phase Two-Dimensional Flow on a Sieve Column Tray. Chemical Engineering Research and Design. 1999;77(6):554-560.
3. Liu CJ, Yuan XG, Yu KT, Zhu XJ. A fluid¨Cdynamic model for flow pattern on a distillation tray. Chemical Engineering Science. 2000;55(12):2287-2294.
4. Fischer CH, Quarini GL. Three-dimensional heterogeneous modeling of distillation tray hydraulics. Paper Presented at the AIChE Annual Meeting. Miami Beach, USA1998.
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