Batch Cooling: Speed Up Cycle Time

Readers recommend ways to expedite the process

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The jacket area is 220 ft2 for an ASME-standard dished tank 72 in. in diameter with a 15-ft straight side and a working volume of 4,200 gallons at 90% full. I would assume 600 for the plate-and-frame heat exchanger U, an area of 1,000 ft2 and a product pump rate of 100 gpm to the exchanger with a cooling water rate of 400 gpm, to make the contest sporting. I would assume 30 gpm through the jacket. Now, let’s consider how to reduce batch time.

The heat-up with steam is difficult because only a portion of the jacket is exposed. Heating bare wall as the tank is filling is ineffectual. In this situation, the plate heat exchanger could be useful; always consider the limits of plate gaskets: http://goo.gl/4nSvo3. A welded plate may be suitable; 300°F is the limit for most frame gaskets. Typically, a plate-and-frame exchanger offers a 3–4 times improvement in U compared to a shell and tube; a modest improvement in minimum temperature approach also may be possible: 5°F compared to 10°F. With only the jacket or a coil, the solution involves an iteration; ideally, set up an Excel file to do all calculations in a single row, then repeat using delta time and level as parameters.

Cooling is a clear winner for the jacket as I will demonstrate. For a jacket or coil, Kern (Eq. 18.11) developed the following approximation for cooling time:

q = ln[(T1 - t1)/(T2 – t1)]/[(wc)/(MC)×{(e(UA/(wc) -1)/e(UA/wc)}], where T1, T2 are the initial and final temperatures of liquid in the reactor, t1 is the initial temperature of the cooling liquid in the coil or jacket, w is the mass rate for the cooling medium in the jacket or coil in lb per hour, M is the mass of reactor liquid in lb, c and C are the heat capacities of cooling liquid and reactor liquid, respectively, A and U are for the jacket or coil, and q is in hours. For an external heat exchanger, Kerns equation 18.17 applies:

q = ln[(T1 - t1)/(T2 – t1)]/[ (K4 -1)/M×{W(wc)/(K4(wc) – WC)}]

K4 = eUA[1/WC – 1/wc]; w and t1 now refer to the cooling medium going through the exchanger.

Comparing the two cooling methods — using the terms assumed above — shows a cooling time of 4.5 hours for the jacket and 11.5 hours for the exchanger. Of course, you could increase the exchanger’s area and the product flow to it. For example, using the Kern equations, I come up with 17 minutes for cooling using an area of 1,200 ft2. You also could argue that the jacket flow rate is high: I get about 9 hours cooling time at 15 gpm. This analysis has several problems: fouling could increase as the product is cooled; and U changes slightly as the product temperature drops.

The real question is whether management should expend money for a new exchanger and whether sufficient cooling water is available. These decisions only can be addressed based on product demand. This is out of the realm of engineering.
Dirk Willard, senior process engineer
Ambitech Engineering, Joliet, Ill

CONSIDER SEVERAL FACTORS
We propose a few things that may be considered regarding cycle time:

1.    When in the process do you heat up the batch? If it’s at the beginning, then heating will have to be maximum at 125oF to add any extra material such as the waxy material you use in the batch.

2.    Have you ever used antifoam that is specifically made to apply in food products? If not, then the solution is to use antifoam. Do not exceed 300oF. Above this temperature antifoam becomes toxic.

3.   The cooling process is not efficient.  Do you use a water reservoir to circulate and recycle the tank to cool off? If yes, then this process is not the right one. Because, if the same water that touches a very hot tank comes back to the reservoir, it will increase the temperature of the rest of the water ,extending the time to achieve 125oF degrees. Instead, we suggest running cold water thought the tank jacket from bottom to top without recycling the water to the reservoir. We estimate this process will only take a  maximum of 4 to 5 hours to cool off.
Miguel Ortiz, Fabricio Barrera, Washington Miranda, supervisors,
Pharmachem Laboratories, Kearny, N.J.

SEPTEMBER’S PUZZLER
We are suffering an excessively high level of rejects from our process to coat corn flakes with aspartame (Figure 1). Instead of individual flakes, we see clumps coming out of our dryer. Only about a third of our product gets packaged — the weigh cells on the dryer exit conveyor reject the rest of the flakes as over-weight because of clumping. I’m convinced we have a mixing problem. An instrument engineer says the Coriolis meter is plugged. The plant engineer is certain the spray nozzle is fouled. And a production foreman says we’re not properly sifting the aspartame added to the mixer. Aspartame is expensive, more than $800/lb. What do you think is causing our problem and how do we fix it?


Send us your comments, suggestions or solutions for this question by August 15, 2014. We’ll include as many of them as possible in the September 2014 issue and all on ChemicalProcessing.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to Process Puzzler, Chemical Processing, 1501 E. Woodfield Rd., Suite 400N, Schaumburg, IL 60173. 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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  • <p>I would look first at the plumbing. My experience has been that a "pfaudler nozzle" on the jacket inlet to swirl the incoming fluid helps a great deal. It is usually easy to look at the inlet nozzle to see if the swirl nozzle exists. Next, inquire as to the last time the jacket was chemically cleaned. If you have past data on process cycle times look to see if the time to cool has been drifting up, that would indicate fouled jacket. These two items have saved the cost of a water refrigeration unit in at least one instance. I presume that the cooling water is once through, it would take a study or testing to see if once through flow or a larger pump would be more effective. </p>

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