Cure column hiccups

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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

 

 

 

 

 NOVEMBER'S PUZZLER

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.


The process worked very well. Recently, due to increased production, the solvent recovery runs almost every day and the heat exchanger plugs up before the batch is finished. However, if the operators catch the fouling in time and let the solids settle, the run often can be extended. The solids content exceeds 10% though, and the solids settle quickly, in under 15 minutes in a graduated cylinder. The incinerator can handle the higher solids content but only has limited spare capacity. Also, a boost in production is anticipated and the solvent recovery may need to be nearly continuous. We know that increasing the amount of concentrate to the incinerator is not desirable due to solvent losses and environmental regulations. Can you suggest a solution?


Send us your comments, suggestions or solutions for this question by May 30. We’ll include as many of them as possible in the July 2005 issue. Send visuals, too — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to ProcessPuzzler, Chemical Processing, 555 W. Pierce Rd., Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication.

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