Process Manufacturing Puzzler: Stop solvent losses

A reader is sufferring significant solvent losses during his batch process. Read about how his peers might solve his problem.

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 QUESTION FROM JANUARY’S CHEMICAL PROCESSING

We make adhesives by batch processing solvents and about 1,500 lbs of solids in 3,000-gal. tanks. Our base solvents are hexane, acetone and isohexane. The formula calls for a three-hour mix cycle on high speed to dissolve the solids -- at this time we lose about 15% of the solvent volume. Although the tanks have fixed lids, they are not pressure vessels. Is there a way to reduce the solvent losses by altering our mixing procedure or tank configuration? Is there any other way?


Jacket the lid of the vessel
We have similar vessels on which we installed dimple jackets on the bottom side of the lid. There is cooling water in the jacket so the solvent condenses and falls back into the vessel. This solution was inexpensive and is easy to use.
Philip Howard, vice president of operations
Ethox Chemicals LLC, Greenville, S.C.

Reduce evaporation by cooling
Are you losing 15% of the solvent weight? Perhaps the volume or density is changing as the solids and solvent are dispersed, like mixing salt and water. If you are indeed losing solvent as measured by a change in weight, you should focus on minimizing the forces causing the solvent to evaporate (i.e., temperature and vapor pressure).

Keeping the mixture cooler will slow the rate of evaporation. Another force that drives evaporation is the partial pressure of the solvent in the head space. You can reduce evaporative losses by switching from a constant inert purge to an inert blanket <em dash>-- if the vessels can hold blanket pressure.
This assumes there are no places where liquid solvent is escaping from the vessel, such as through a valve that is not quite closed. A solvent odor in the air would suggest evaporative losses through the agitator shaft seals, flanges or other points.
David S. Tascarella, P.E., process engineer
Dow Corning Corp., Midland, Mich.

Cut back the mixing time
The description suggests it takes three hours to dissolve the solids in the solvent mixture, which is part of the problem. Review the dissolution process to see if the time can be reduced, which will result in lower solvent losses.

In addition to reducing the cycle time, it is necessary to keep the batch temperature as low as possible. This can be achieved by installing a heat exchanger that will maintain the batch temperature close to ambient, thereby minimizing solvent loss.

Two types of heat exchangers can be used to control the temperature. Options are an immersion-plate heat exchanger in the tank, or jacket or external-plate heat exchanger attached to the straight side of the reactor. Water can be used as the coolant in both cases.

If the batch tank does not have any baffles, a properly designed immersion-plate heat exchanger will also act as such and improve the dissolution time. If a jacket or external-plate heat exchanger is used and the tank has no baffles, you should consider installing some.

You will have to evaluate the economics of each temperature-control method to select the best option for your plant.
Girish Malhotra, P.E., president
EPCOT International, Pepper Pike, Ohio

Seal the reactor
Make certain the vessel is sealed and add a vent for the process vapors to escape. The vent line should pass through a chiller so you can drain the condensates back into the vessel or a pump suction. The solvents will condense because they are in equilibrium with the vessel contents. Add a riser pipe with a liquid seal to allow non-condensibles to vent via the liquid seal, to atmosphere or another safe location.
W. Brown, retired engineer
Katy, Texas

Use dry ice to great effect
You might want to strategically hang blocks of dry ice in baskets around the inside rim of the tank. The solvents will condense and fall back into the tank. This solution might sound low-tech, but I have seen it work.
Hans G. Schroeder, Ph.D.
International Consultants Association, Encinitas, Calif.

Cool the reactor
If your solvent losses are due to evaporation, they can be reduced by lowering the temperature, and thus the vapor pressure of the solvent. To reduce vapor losses, the colder, the better! You should use a jacketed mixing tank and circulate a heat-transfer fluid that has been cooled using cooling water, chilled water or possibly liquid nitrogen. However, the process then might need longer mixing times or intermittent heating.
Bill Kubik, business unit manager
Graham Corp., Batavia, N.Y.

Cool the vent line
It sounds like the high-speed mixing is putting enough heat into your solution to vaporize some of the solvent. If that is the case, the solvent vapor must be leaving the vessel through some sort of vent piping.
One way to recover this solvent and limit the rate at which it vaporizes is to install a condenser in the vent line. Have the condenser drain to a knockout pot with a drain line back to the tank. Have a liquid seal in the drain line so the vapor is forced to go through the condenser instead of bypassing it and flowing directly to the knockout pot. The cooled solvent you return to the tank will help offset some of the heat input from the agitator.
Leonard Riker, Powders Production Manger
Henkel Corp., Olean, N.Y.

Reduce the mixing volume
Is it possible to reduce the mixing temperature without affecting the product composition? As I understand from your description, you have already looked at the mixing operations. The key is to minimize the time at which the solvents are exposed to high temperatures.

This point might be easier said than done, but you might want to evaluate replacing your solvents with those that have lower vapor pressure.
If part of the solvent loss is through entrainment (liquid entrainment in the vapor exiting the mix tank), consider a number of things: reduce the liquid level in the mix tank, use submerged pipes to add the solvents, and reduce the speed of the agitator.
G.C. Shah, environmental adviser/industrial hygienist
TOTAL, Bayport, Texas

 

 

 MAY'S PUZZLER

We need to pump a reactor effluent stream. The stream is a slurry that is highly aerated by an agitator in the reactor. We estimate that 10% to 15% of the pump suction volume is gas. The total volume at the pump suction is about 1,200 gpm (including gas) at a head of around 70 psi (200 ft). What kind of pump is best for this service?

Send us your comments, suggestions or solutions for this question by Feb. 5. We’ll include as many of them as possible in the February 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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