Modeling fixed triangular valve tray hydraulics using computational fluid dynamics

doi:10.1016/j.cep.2011.11.009

Chemical Engineering and Processing: Process Intensification

Volume 52, February 2012, Pages 74-84

https://doi.org/10.1016/j.cep.2011.11.009 Get rights and content

Abstract

A computational fluid dynamics (CFD) model was developed for describing the flow patterns and hydraulics of triangular fixed valve tray. The gas and liquid phase, as two interpenetrating phases, were modeled in the Eulerian framework. Based on the clear liquid height obtained by experiments, a new correlation for liquid hold-up was adopted, and the interphase momentum transfer source was also calculated. Several simulations were carried out for a triangular fixed valve tray with varying operational conditions. Velocity distributions, clear liquid height, froth height, and phase hold-up were predicted for various combinations of gas and liquid flow rates, and the results were found to be in reasonable agreement with the experimental data.

Introduction

Trayed column is an important type of gas–liquid contact equipments in chemical industry. Fixed valve tray, one of the major research and industrial interest recently, draws more and more attention as a hybrid of floating valve tray and sieved tray. However, the industrial application of the fixed valve tray has been also limited to some degree due to its imperfect mechanical design or insufficient empirical structural optimization [1], [2]. As is well known, the description of the hydrodynamics of fixed valve tray is of great importance in industrial practice. Based on its hydrodynamics, the separation efficiency and overall performance can be predicted for a given set of operating conditions, tray geometry and system properties.

Nowadays CFD is becoming a powerful research and design tool in chemical engineering. There have been many attempts to model tray hydrodynamics by use of CFD [3], [4], [5], [6]. Yu et al. [7] and Liu et al. [8] tried to simulate the two-phase flow behavior with a two-dimensional model, which focused on the description of the hydrodynamics of liquid phase but ignored the variations in the direction of gas flow along the height of the dispersion. Fischer and Quarini [9] simulated the three-dimensional transient gas–liquid hydrodynamics; they assumed a constant drag coefficient of 0.44, which was appropriate for uniform bubbly flow. Yet, this drag coefficient was not appropriate for description of the hydrodynamics of trays operating in either the froth or spray regimes. Krishna and Van Baten [10] described the hydrodynamics of sieve trays by estimating a new drag coefficient correlation for the liquid hold-up on the basis of the correlation of Bennett et al. [11], which was appropriate for a swarm of large bubbles. Because the correlation of Bennett et al. over-predicted the liquid hold-up fraction in the froth regime, Gesit et al. [12] chose the correlation of Colwell [13] to predict the flow patterns and hydraulics of a commercial-scale sieve tray, which worked well in the froth regime. As a result of the previous researches focusing on the sieve tray with a simplest quasi two-dimensional structure, Li et al. [14] attempted to investigate the hydrodynamics of a full-open valve tray which had more complicated three-dimensional cubic structure for the first time. Their work extended the scope of application of the CFD model.

In this work, a three-dimensional transient CFD model was developed for hydrodynamics of a new fixed valve tray within the two-phase Euler framework, whose three-dimensional structure was shown as Fig. 1. The fixed valve opening area is 1.746 × 10⁻³ m², and the height of valve cap was 8 mm, and prop-legs inclined to the surface of valve tray at an angle of 30 degrees. Some other dimensions of the fixed valves are shown as Fig. 2, as well as the configuration of different valves. Several simulations were carried out with varying superficial gas velocity, weir height and liquid strength. Finally, a simulative comparison of correspondent liquid velocity components between two different arrangements of trays, with arrays of fixed valves in respectively positive and reverse directions, was done to study their influence on liquid flow near the arcuate region.

Section snippets

Experiment

The macroscopic hydrodynamic performances of this fixed valve tray, such as pressure drop, entrainment, weeping and clear liquid height and so on, were investigated by use of air-water system in a plexiglass column with an inner diameter of 600 mm. The experimental facilities were shown in Fig. 3, and the data of clear liquid height were obtained by the method of installing a static pressure tube (communicating pipe with scale). What is more, five pressure tappings were mounted at different

Model equations

The model includes the flow of gas and liquid in the Eulerian-framework in which each phase is treated as interpenetrating continuum with separate transport equations. With the model focusing on the froth region of the fixed valve tray, the gas phase has been treated as the disperse phase, while the liquid phase as the continuous one. Like other authors, energy and mass transfer are omitted to facilitate hydrodynamic behavior research. The transport equations of gas (subscript G) and liquid

Results and discussion

A commercial CFD package FLUENT was used to solve the equations of continuity and momentum for the two-fluid mixture. This package is a finite volume solver, and all variables are evaluated at the cell center. The pressure-velocity coupling was obtained using the Phase Coupled SIMPLE algorithm. A fully implicit backward differencing scheme was used with the time step size of 0.005 s. Considering of the accuracy required and the simulation time, such time increment adopted in the simulation is

Conclusions

In this paper, the flow hydraulics of novel fixed valve tray was investigated experimentally by means of CFD; a three-dimensional two-phase CFD model was developed in the Eulerian framework based on the experimental data. A new correlation of gas hold-up was obtained.

The simulation results, in good agreement with the experimental counterparts, exhibit some known features of the two-phase flow field on trays.

In the present work, the simulative investigation on flow patterns in the arcuate

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Modeling fixed triangular valve tray hydraulics using computational fluid dynamics

Abstract

Introduction

Section snippets

Experiment

Model equations

Results and discussion

Conclusions

Trans. Inst. Chem.

Chem. Eng. Res. Des. Trans. Inst. Chem. Eng.

Chem. Eng. J.

Chem. Eng. Res. Des. Trans. Inst. Chem. Eng.

Chem. Eng. Sci.

Trans. Inst. Chem. Eng.

Chem. Eng. Prog.

Chem. Eng. Res. Des. Trans. Inst. Chem. Eng.

Should you switch to high capacity trays

Chem. Eng. Prog.

Efficiencies of sieve tray distillation columns by CFD simulation

Chem. Eng. Technol.