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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? |
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.
Use an anti-foam
The article does not state what the feed solution is comprised of, so it is hard to make a determination, but it sounds as though there could possibly be a problem with foam in the column. If the product has a tendency to foam and no type of anti-foam is injected into the feed stream to counteract it, then foaming can occur in the distillation column and this will cause the type of situation that is described. If the column has sight glasses, a foaming condition should be able to be observed. If the problem is foaming, then addition of anti-foam at the correct ppm to the feed stream would solve the problem.
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.
Jack Hamshar, senior process engineer
Great Lakes Chemicals, El Dorado, Ark.
Check temperature and pressure
I would put temperature indicators and pressure indicators at every tray to see what the mole fraction was doing to my temperature and pressure. (See Figure 13-63 on p. 13-61 of the 6th edition of Perry’s Handbook.) Then, I would plot those points during normal operation and flooding, to decide what is the easiest to change — reflux, steam, et al. Probably, I would change from trays to high liquid/gas-ratio structured packing and be done with it.
Tom Murphy, ceo
Puritrol, Inc., Centerville, Mass.
Assume You Have Side Draw Points For Fusel Oil
• Fusels have a fixed boiling point, for a given pressure. Therefore they will accumulate at the point in the column that is at their boiling point.
• If the temperature of the column is on the high side (to prevent dropping ethanol out the bottom), the fusels will be at a relatively higher tray, very possibly above their draw point.
• They will continue to build in volume, since they can’t rise, and they can’t descend. When the expanded volume of fusels reaches the draw point, they start exiting through the draw point pipe(s). The temperatures in the column drop.
• After the burp, the temperatures return to their previous high levels, and the fusels begin to accumulate again. If you are charting your temperatures, you have probably seen all of the column temperatures take a sudden dip, and after a period, slowly return to their previous levels.
• The solution is to lower the column temperature profile so that the boiling temperature of the fusel oils now occurs at a lower tray, ideally at the fusel oil side draw point. They will then exit the column in a continuous fashion.
• To lower the temperature profile of the column, you need to raise the “inventory” of ethanol in the column without disrupting the steady state balance of flow in and flow out. Do this as follows:
1) Lower the flow of Ethanol vapor to the Sieves.
2) Temperatures should start to drop in the column.
• When the temperature at the probe located around the draw points gets to 137C (boiling point of fusels in a particular system that I was working with) or whatever temperature applies in your case, increase the flow to the sieves back to its original value to maintain steady state flow. The temperature profile will remain at the lower level, because you now have more ethanol in the water-ethanol mix.
• The fusels should now be free to exit the draw points with very little accumulation.
• I determined the boiling point of the fusels by noticing that, when the probe at the side draw reached 137C, the level in the fusel oil decanter started rising abruptly. If you don’t have a decanter, you may have to use trial an error to home in on the correct temperature profile.
• You may have to do some tweaking, but this is the basic approach.
Thomas Jansen
Retired Mechanical Engineer
Ban Pong, Ratchaburi, Thailand
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NOVEMBER'S PUZZLER |
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Our solvent recovery system uses a batch evaporator to concentrate the mother liquor from a crystallization process. The evaporator is a falling-film unit with a recirculation pump and a shell-and-tube heat exchanger. The concentrated solution (3% solids) is sent to an incinerator. At the end of a batch, the recirculation pump is used to transfer the concentrated solution to the incinerator feed tank. When the process originally was started up, the batch evaporation was run every 3-4 days, whenever enough mother liquor had accumulated. |