Besides inducing false level readings with DP cells (CP, Jan., p. 54), foam causes lots of real operating problems in distillation columns. While we may suspect foam, how do we prove it? The critical first step is to check if foaming actually is occurring. Five techniques are most commonly used to identify foaming in an operating unit. Four are low-tech approaches while the fifth is a high-tech solution. Depending upon your unit, one of these might be right for you.
A constant level reading is the key to the first method. Sight glasses only show average-density changes across their taps. Liquid or foam can back up the tower all the way to the overhead vapor line, yet a sight glass still shows a liquid level and a DP-cell level controller shows a liquid level. So, if you dramatically change (on manual) the draw rate from the bottoms of the tower and do not see a level change <ital>and<end ital> the reported level is within the span of the instrument, then you very likely have foaming (or another type of liquid-level measurement problem).
The second method suits towers with multiple sight glasses. Figure 1 illustrates foaming in a petroleum crude pre-flash tower that has multiple sight glasses and taps installed due to the level range (over 40 ft.). Pre-flash towers in this service should have large liquid inventories to break the foam to prevent entrainment and pump problems. The still liquid in the sight glasses has a lower height than the foam inside the tower. Multiple sight glasses show liquid levels at different elevations. Each liquid level is really a measure of foam density at that elevation. This is a clear indication of foaming and is a powerful troubleshooting tool.
Liquid flowing down the sight glass is a third method to spot foaming. In units with heat loss condensing vapor inside the sight glass, some liquid always flows down the sight glass. However, foam entering the top of the sight glass is at a much higher liquid rate than seen with normal condensation. In units with heat gain through the sight glass, the vapor space above the sight glass should be dry. Liquid falling inside the vapor space is a very strong indication of foaming in these services.
The fourth low-tech method involves taking a sample, either in an open container or through a sample bomb, from a location that should be dry. If the sample contains liquid, then the vessel has foam or liquid inside at that point (Figure 2).
In all of these methods, you must check how the sight glass or sample connects to the vessel. Stilling wells inside the vessel can confuse the observations, because they prevent foam from getting to the sight-glass connections or the sampling taps.
If appropriate connections are unavailable, you may require the high-tech approach — radio-scanning. Essentially, a gamma-ray source and detector determine the average mass density across the vessel. A plot of relative mass density versus elevation results. Properly done, this can identify foaming.
Foaming is a major cause of tower problems. Proving that foam is present (or absent) is one of the more challenging field jobs in process engineering. The techniques here are a start to finding out if you have foam or not.
By Andrew Sloley, contributing editor