Cure column hiccups

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Another potential reason for this type of problem is insufficient mixing of the feed material. If the alcohol % in the feed becomes too over-concentrated or too diluted then the same type of problem can occur. Proper mixing of the feed and proper temperature of the feed stream is essential to consistent column operation.

Ronnie Dennis, recovery/packing specialist
Martek Biosciences Corp., Kingstree, S.C.

Install side-draw taps
It looks like your ethanol feed stream contains a small amount of substance with volatility that changes with ethanol concentration. At higher ethanol concentration, its volatility is low, and at low ethanol (high water) concentration, its volatility is high. This causes the compound to be trapped in the column because it is driven up at the bottom and down at the top.

An example of such a compound is i-amyl alcohol. This is a well known phenomenon in the fermentation ethanol industry where fusel oil (mostly i-amyl alcohol) accumulates a few trays above the column feed where the ethanol concentration is about 40 mol. %. If the fusel oil is not removed in a side draw, it builds until it affects the column temperature control sensor, causing the column to become unstable, like it is flooding.

In your case, it looks like the material builds until it extends down to the control tray. The temperature drops, due to its lower boiling point, causing the steam valve to open. Eventually the accumulated slug of offending material is burped out the top or bottom of the column and then everything returns to normal until the next cycle begins.

The solution is to install small side-draw taps to remove the accumulated material before it disrupts the column control. The taps are usually installed on three or four trays in a row starting with the third tray above the feed tray. Trial and error will determine which tray is best for tapping off the accumulating material. For fusel oil, the side-draw stream (the flow is small because the amount of material to remove is small) is mixed with cold water, which causes it to separate as a second liquid phase that can be easily decanted. The water phase is returned to the column to recover its ethanol.

Chuck Easley, process engineering department manager
PROCESSPLUS, Cincinnati, Ohio

Rely on a Coriolis meter
You don’t say how you control the flow but it is probably not with a mass flow meter. The density of the stream changes — increases — the flow controller sees more flow and reduces the flow rate (less volume) and the column overflows. The density goes down and flow rate increases, column overflow stops or corrects itself. Put the flow control under a mass-flow Coriolis meter.

Bob Driskell, supervising engineer
WorleyParsons, Texas City, Texas

Identify the contaminant
The likely problem is build up of a minor contaminant in the upper column, where it is getting trapped. A quickie simulation of a ten-stage ethyl alcohol column showed more than an order-of-magnitude build up of concentration for trace amounts of isopropyl alcohol or n-butanol, relative to their minor presence in the feed. Azeotropic behavior allows certain components with boiling points higher than water to do this. The build up would gradually cause the temperature break in the column to migrate downward until it presses against the bottoms temperature and causes the column to go unstable — dumping large amounts of the contaminant into the bottoms, resulting in a vapor surge, which blows most of the accumulation out the overheads. Then all settles back and the long cycle starts over.

Diagnostically, I’d look at samples of the overheads immediately after an incident — the contaminant should be present in significant quantity. If possible, column temperature profile should be monitored to look for the cool zone growing and migrating downward. Also I’d look at the feed gas chromatograph, not the tabular summary but the actual trace. Find that tiny peak of a somewhat higher boiler than ethyl alcohol and, if possible, identify the compound on a gas chromatograph mass spectrometer.

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