(200e) 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.