Figure 1. Measurement of level on the draw tray allows the control system to infer the column’s internal liquid flow rate.
In this case, the purpose of obtaining the internal liquid rate was to ensure the trays below the liquid draw had sufficient liquid for proper operation. Trays have a mechanical limit on liquid rates. Below that limit, efficiency may drop dramatically. The tray type and geometry as well as the physical properties of the system determine the minimum acceptable liquid rate.The plant found controlling the distillation specifications of the side-product draw extremely difficult. Purity, especially of heavy components, would vary dramatically for the same calculated liquid internal rate. (Sudden spikes in heavy components in the side draw are strong signals that the internal liquid rate is too low.)Most installations like this fail to work properly. Indeed, success is so rare that you should view with great suspicion any control system that uses an internal liquid level to infer a flow rate. The problem is not the concept but how the concept is executed.If you must use liquid level inside a column to infer flow rate, take steps to address the most common reasons for failure. Follow these basic guidelines:Never use an active tray.
Active trays mix vapor and liquid. Liquid inventory on the tray varies with tray pressure drops. Tray pressure drops change strongly with vapor rates. Accurate prediction of internal liquid rates based on liquid level on an active tray is essentially impossible.Allow for vapor/liquid disengagement.
The liquid entering the metering tray often contains entrained vapor. Accurate flow measurement depends upon reliable liquid densities. So, ensure that the liquid being measured and the liquid leaving the tray both are vapor-free.Select a tray layout with reasonable height-to-flow response.
Figure 2 shows the layout of the installed tray from the unit in Figure 1 (left side) and an improved design that would enable the unit to work properly (right side). Instead of the long overflow weir found on the installed tray, the new tray’s outlet could use orifices, weirs (or slots) and notches. The flow rate will vary with each: f ≈ h0.5
(orifices), f ≈h1.5
(weirs), and f ≈ h2.5
× sin θ (notches), where h
is the height of liquid and θ is the angle of the notch.