Process Puzzler: Deal with a Decanter that Can't

Readers recommend ways to save a standing separator.

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The process design for the vertical decanter is even less promising. Although the residence time for a 3-ft-diameter decanter is more than sufficient to remove the droplets, the calculated entrainment droplet size is about 2,000 μ. This is not the 100 μ desired and explains why carbon is being rapidly consumed. Carbon columns usually capture droplets in the range of about 30 μ.

There is another reason for reducing the overuse of the activated carbon — disposal: roughly $0.50/lb for transportation (within 200 miles) and $0.60/lb for disposal by drying and incineration via a thermal oxidizer (TOX). Once you factor in the disposal, for this example you’re paying $1.60/lb as the true cost of carbon. A more realistic price for activated carbon is about $1.80/lb, including shipping to the site. The final price would then be $2.90/lb. Regeneration is possible and co-generation, typical of a TOX, might reduce costs. Let’s reconsider the design again.

With decanters sometimes efficiency can be improved by increasing the diameter or the temperature. However, a quick check with these parameters shows no improvement: the viscosity of the continuous phase, i.e., the light phase, is too high. We need to consider other options.

If you can add a small trace element, perhaps a surfactant that will not contaminate the organic phase or can be separated easily, this might be the direction to go. Anything that reduces the surface tension may be effective.

Do a drain test in the lab. Cut a circular hole in a small container with about 100 ml of the organic phase. The hole should be smooth but sharp, without a nozzle. Measure the time it takes to drain through the hole after adding the chemical or doing whatever you decide to do. You’ll want to run a standard first with the untreated organic. If you’re making headway, the drain time should decrease.

You must reduce the surface tension of the dispersed light phase. Reducing the aqueous phase by half will cause a phase inversion but this trick won’t improve separation of the two phases. 
Dirk Willard, contract staff engineer
Hemlock Semiconductor, Hemlock, Mich.

Our hydrogenation system, consisting of a column filled with catalyst, an interchanger and a knockout drum under vacuum (Figure 2), produces excess hydrogen. The drum is meant to separate the hydrogen from the product, which is then pumped to the tank farm. The vacuum pump surges constantly, unable to find a sweet spot. The pump vents to atmosphere perhaps as much as 3 std. ft3/min of hydrogen. Our plant manager wants to know if there’s a way to recover the hydrogen and stabilize the level feeding the pump. How can we address this situation? Is there any way to limp along with this process until it’s convenient to shut it down?

Send us your comments, suggestions or solutions for this question by December 10, 2010. We’ll include as many of them as possible in the January 2011 issue and all on Send visuals — a sketch is fine. E-mail us at or mail to Process Puzzler, Chemical Processing, 555 W. Pierce Road, 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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