Two liquid phases can form inside distillation towers in specific systems. Typically one phase is ionic, usually aqueous, and the other nonionic, generally oil or hydrocarbon-based. Either phase may be the major component. Water entrapment in hydrocarbon columns is a common example of an ionic liquid phase as the second phase. Fusel oils in ethanol distillation (see: "Cure Column Hiccups") typifies a relatively nonionic liquid phase occurring in a mostly ionic system.
The leading cause of the second phase is concentration of non-key components. This may stem either from a local composition profile or outright entrapment ("Watch Out for Trapped Components in Towers"). Multiple operating problems can occur with an unwanted liquid phase.
The cheapest method to draw out an unwanted phase usually is a simple liquid sump from a tray. Unfortunately, many towers have ineffective sumps. For the sump to work, the unwanted phase must be the higher-density phase and the sump must be correct.
Often, standard sump layouts are so ineffective that the operating department may simply not believe that a second phase exists — in spite of clear evidence from tower upsets, experience from other plants, and analytic data. After all, if the heavy phase can't be drawn off, it must not be there.
The cause for many of these ineffective sumps seems to be engineering standards based on one article from 1969. Figure 1 illustrates the basic configuration that reference recommends, as applied by one plant to separate an entrapped water phase. The plant made three increasingly convoluted attempts to make the water draw work — first, adding the initial draw sump, then installing a draw at a second location, and finally adding an external drum on the draw stream. After that, the plant gave up and assumed no water was present.
The root of failure all three times was the water draw. Fancier separation techniques to remove the water outside the tower won't accomplish anything when the water doesn't get out of the tower in the first place.
On a 24-in. tray spacing and with high liquid rates, the liquid falling into the downcomer continuously stirs the liquid in the sump. Even with the perforated plate, virtually no phase separation will occur. Additionally, residence time is very low — seconds, not the minutes seen in external separator drums. The result: no separation.
The system in Figure 1 only works with large quantities of a second, heavier phase. With more modest amounts of the heavier phase or with a lighter second phase, something else is needed. The ultimate solution uses a drum outside the tower as a classic liquid/liquid separator.
However, in the example here, a cheaper option would work. Figure 2 shows a collector tray that stills the liquid and adds residence time. As long as 100% water removal isn't required, this can allow for a reasonable draw rate of a stream that's mostly water. With the external drums the plant already had, it could easily separate the water from the oil outside the column.
The chimney tray eliminates one active tray in the column. However, the benefit of removing the water easily outweighs the loss of an active tray.
The lesson here is that published material isn't always right. Rules-of-thumb and standard practices can be slavishly copied out of context, or even be wrong from the very beginning. Apply engineering fundamentals and always ask: "Does this make sense?"
ANDREW SLOLEY is a Chemical Processing Contributing Editor. You can e-mail him at ASloley@putman.net