(62c) Application of the New FRI Valve Tray Efficiency Model in the Simulation and Design of Distillation Columns

Models for the estimation of the efficiency of distillation trays have been part of the chemical engineers toolbox for many decades. Efficiency models fall into a number of distinct classes that include:

1. Empirical correlations of tray efficiency data

2. Semi-empirical that combine correlations for Numbers of Transfer Units with a mechanistic model of mass transfer.

3. Fundamental models based on a simplified picture of the hydrodynamics on distillation trays.

In this paper we are concerned with the latter class of model.

There are two quite different regimes of operation of distillation trays, the froth regime and the spray regime. Detailed models of the hydrodynamic and mass transfer character of these regimes have been developed by several investigators (Lockett and Plaka, 1983; Lockett, 1986; Kaltenbacher, 1982; Hofer, 1983; Prado and Fair, 1990, Syeda et al. 2007). In this class of model the froth regime on a distillation tray typically consists of three zones:

1. Zone I - Jetting-bubble formation region.

2. Zone II - Free bubbling zone.

3. Zone III - The splash zone.

In Zone I, the jetting-bubbling formation zone, the vapor issues through the perforations in the tray in the form of jets, breaking into bubbles. In the free bubbling zone a distribution of bubble sizes is assumed. Available experimental data show that the assumption of a bimodal bubble size distribution is a good approximation. Thus, we have fast-rising "large" bubbles and slowrising "small" bubbles in the dispersion and this picture has been incorporated in the class of model inspired by this picture. The splash zone above the free bubbling zone consists of entrained droplets. This zone – if it is considered at all – may be modeled as being made up of droplets of uniform size rising in plug flow through the splash zone.

Recently, a new member of this class of fundamental model has been developed by Fractionatation Reesearch Incorporated.

The FRI model has been implemented in the rate-based column simulation program ChemSep.

In this paper we use this new model to model distillation columns for which performance data is available. We show how a column performance model can be tuned to match operational data by fitting model parameters that have physical significance. Column profiles calculated with the FRI model are compared to profiles calculated with standard efficiency models such as that published by the AIChE.