Get out of the Soup

Readers clarify a consistency complication.

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Alison Springer-Wilson, co-op student
Mosaic Phosphates, Plymouth, Minn.

SWAP THE PUMP
Add the polymer agent with a positive displacement metering pump.
Wally Marshall, contract analyzer specialist
Dow Chemical Co., Freeport, Texas

OPT FOR A LOW SHEAR PUMP
Change the pump. You are busting up the medium with high shear being introduced into it. My take is the coagulant may be susceptible to shear. Try a low shear pump. Would a diaphragm pump, tube pump or lobe pump work?
Michael Waugh, consultant
Lutz, Fla.

SLOW IT DOWN
It seems that the shear forces created by the impeller negatively affect the polymer agent, breaking down the actual polymer-to-polymer molecular forces. Consequently, this prevents the thickening (coagulation) of the product; the swellability index of the polymer coagulant has decreased markedly. One solution would be to obtain a slower impeller pump with lower shear forces being imposed on the actual polymer agent coagulant. (Assuming these changes can be supported will probably mean a larger motor and other process changes.)
Deepesh Ghela, technical development director
Hardeep Chemical Co., Kwazulu, Natal, South Africa

IT’S THE IMPELLER
We are not even sure that the pump is shredding the polymer but it is a good starting point. In engineering, money is everything. With pumps it often costs almost as much to change the impeller as to change the whole pump. A closed impeller may be the cheapest answer provided that the change in the pump curve is acceptable. Being a new project, the vendor may be willing to accept the change with a minor cost. This may be the solution if you can experiment with a batch to see if the closed impeller works well. If you must bid a replacement pump, it is best to suggest an open bid not a sole source. This may provide the leverage you need with the pump vendor. He wants to keep his customers to himself, not share them.
Another issue is agitation in the tank itself. You may want to look at ways to minimize mixing time while balancing shear. Shear is a direct function of agitator speed, so reducing the shaft speed may help — if you can live with the negative effect on mixing time. Changing the batch recipe may help.
Knowing how the polymer works is important. The shaft could be run at reduced speed while the polymer is blending but at high speeds before that. All the minor ingredients should be mixed early while delaying the polymer addition for as long as possible because mixing efficiency and time will increase once the polymer is added. Unfortunately, laboratory testing won’t do you much good. With many processes, it’s best to take to a scale close to your production size and fix the problems there.
Dirk Willard, process engineer
CITGO Petroleum Corporation, Lemont, Ill.

February’s Puzzler
A contractor installing a new motor starter for a crude pump in a motor control center (MCC) received permission to trip the breaker controlling the old bucket. With the trip, we lost the crude pump, the gas oil pump and several recirculation pumps in an atmospheric crude distillation unit. This breaker was supposed to go only to the pump being replaced. The operator responded quickly, bringing the unit back up although there were severe swings in column temperature and pressure. An investigation later found no loop sheets or inspection records. The wiring diagram for the MCC didn’t show any common wiring for the starters, but then no as-built drawing was produced and no commissioning documentation exists. What can be done to avoid this problem in the future and address what may be a refinery-wide problem?

Send us your comments, suggestions or solutions for this question by January 9, 2009. We’ll include as many of them as possible in the February 2009 issue and all on CP.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net 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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