Lecture 10: Equipment Capacity and
Stage Efficiency.
Countercurrent flows of gas and liquid in a plate or packed column can be
maintained only if the gravitational force driving the liquid down the column is
larger than the force from the pressure gradient that tends to drive the lquid
upward in the column. If the pressure gradient force is larger than the
gravitational force, then no liquid can flow down the column and the column is
said to be flooded.
The pressure
gradient in the column is determined by the superficial gas velocity, so the
diameter of the column needs to be determined such that a critical superficial
gas velocity is not exceeded. Several correlation exist for the critical
superficial gas velocity as discussed in the text. Sometimes these
correlations are presented in terms of the superficial mass flux of the gas,
which is the superficial velocity times the gas mass density.
It is possible to
use a simple mass transfer analysis to determine the stage efficiency for a
plate in a column. In general, the height of liquid on the tray is
equal to the product NTU * HTU where NTU is related to the Murphree vapor
efficiency at a point on the tray and HTU is related to the mass transfer
coefficient and contact area available for mass transfer. Point
efficiencies on a tray can then be averaged depending on the flow configuration
(plug flow or well mixed flow) for the liquid on the plate.
