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Cure column hiccups
Readers suggest how to avoid sporadic flooding of a distillation column.
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THIS MONTH'S PUZZLER |
| A small part of our operation is a distillation column where ethyl alcohol is recovered from a weak aqueous solution. Live steam is injected into the column base to control temperature on the third tray up — there is no reboiler. Reflux is under flow control, so that whatever alcohol is boiled up in excess of the reflux overflows the reflux drum. It is a pretty simple system and works well enough, most of the time. But every once in a while, the column inexplicably floods, indicated by abnormally high differential pressure. Base level falls and the flow of overhead product increases markedly — most of it water. At the same time, temperature at the control point falls, driving the steam valve open, which adds to the flooding. It doesn’t seem to matter whether we go to manual control or just leave the column alone — the problem goes away in a few minutes, and normal operation gradually returns. These incidents are short-lived and weeks apart, so that we haven’t been able to determine the cause. What is going on here and how do we fix it? |
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Address an azeotrope
During the production of ethanol, trace quantities of higher alcohols are formed — including butanols — collectively known as fusel oil. These alcohols form azeotropes with water, boiling at lower temperatures than water but at higher temperatures than ethanol. Being boiled in the base of the column and condensed by the reflux, they are unable to escape during normal operation. Most ethanol/water columns have a small side-draw below the feed for removing fusel oil. If you do not have a side draw or its flow is too small or blocked, fusel oil can accumulate until it reaches its limit of solubility in water. At this point, a second liquid phase forms, exerting as much vapor pressure as the existing liquid phase. This sudden doubling of the vapor pressure momentarily doubles the rate of boiling, flooding the column with vapor — like a pot boiling over. The heat required for the extra boiling is taken from the sensible heat of the liquid, causing its temperature to fall and the temperature controller to open the steam valve and maintain the flooding condition. Because the flooding prevents the liquid from returning down the column, the fusel oil is driven out the top — “up-chucked” so to speak. Once it leaves the column, the second phase disappears and operation returns to normal, allowing more to accumulate over time.
Drawing a slipstream from the side of the column where the fusel oil accumulates is necessary to keep the flooding from repeating. To minimize ethanol loss there, the stream should be cooled to lower the solubility of the fusel oil and decanted, with the aqueous bottom layer returned to the column. Other columns featuring mid-boiling heterogeneous azeotropes pose the same danger and can be treated the same way.
Greg Shinskey, process control consultant
North Sandwich, N.H.
Adjust steam flow
It sounds like a jump in alcohol concentration in the feed. Increase the height of the reboiler vessel or install a mass flow meter so that if the specific gravity falls, the steam flow could be reduced. The control algorithm for the steam flow valve could be based on feed enthalpy. It could rely on a specific-gravity/heat-of-vaporization table and regulate the steam flow so as to not flood the column but maximize distillate.
As soon as the column floods, all the distillate on the reflux drum is contaminated and possibly the whole receiver.
Karl Hertel, senior commissioning engineer
Abbott Laboratories, North Chicago, Ill.
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