Fluid Handling / Ground

Distillation Guidelines: Draw Some Important Lessons

Measuring liquid level on a column draw tray demands care

By ANDREW SLOLEY, Contributing Editor

One way to measure liquid flow inside a distillation tower is to infer the flow rate from a liquid level. Figure 1 shows a typical installation. A partial side draw takes a product from a sump on a draw tray while the remaining liquid overflows a weir and continues down the tower. The level measured on the draw tray goes to the plant’s advanced control system, which uses the value in calculating the 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.

If a weir extends the entire width of the overflow downcomer, the liquid height over the weir is low. In the unit with problems, this height was less than 0.5 in. at the desired flow rate. The level span needed was 0.43 in. at minimum flow to 0.57 in. at maximum flow. Few plant level instruments can measure accurately across a 0.14-in. range. The system failed to work because it required unattainable accuracy. (Small errors in level measurement led to large errors in calculated internal flow rate.)

A much better configuration would use a very high weir with orifices. The orifices would hold a reasonable liquid level. Correct selection of orifice sizes could provide an 8-in. range — for example, by giving a level of 6.4 in. at minimum flow and 14.4 in. at maximum flow — that would allow a level instrument to provide accurate readings.

Use a zero-leak tray. Plants measure internal liquid rates when high accuracy is necessary. Leaks aren’t measured. High liquid heights created to get accurately measurable level changes will increase the leak rates through bolted connections. The collector tray must be leak-tight. This usually requires welding for metal trays or some type of sealant for non-metal trays.

Don’t forget about solids and gunk. The improved configuration in Figure 2 has the orifices above the tray deck. This allows for smoother flow patterns into the orifices at low flow rates. The offset also prevents the orifices from plugging if some scale shows up. The scale will settle on the tray deck. The orifices sizes must be large enough to avoid plugging in systems that might foul.

Ensure the tray can drain. Once the tray is leak tight, the only way to get all the liquid off is to have an external drain through the side draw or to wait for liquid on the tray to vaporize at shutdown conditions.

These aren’t the only points to consider. Proper design and installation will enable many internal metering systems to work properly.

ANDREW SLOLEY is a Chemical Processing contributing editor. You can email him at ASloley@putman.net