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Improving cooling jacket efficiency

Improving cooling jacket efficiency

Improving cooling jacket efficiency

(OP)
Hello,



I have a mixing tank 1.7 m3 half covered with plain cooling jacket (heat transfer area 2.44 m2).
I tend to use the mixing tank for a reaction that is exothermic (ca -10kW), for that reason I want to check if cooling duty of chiller and jacket is sufficient.
We carried out some tests and the results showed that jacket had a cooling rate of 2.3 oC/h dropping temperatures of the reactor content (400L) from 15 to -10oC in 11hours. That seems extremely slow.

Notes:
  • Reactor content: 400L of 30% IPA
  • Reactor size 1.7 m3 and filled to 0.4 m3.
  • Tank contents were mixed during cooling and process was batch.
  • Jacket was insulated, pipes of 4 m were not insulated.
  • Coolant: 50% glycol
  • Coolant rate 38 m3/h
  • Coolant initial temperature -2 oC and cooled down down to -20 oC.
  • Coolant buffer 1 m3 was used. The coolant was circulated from buffer to chiller to jacket and back to buffer.
  • Jacket coolant velocity is 0.2 m/s (seems low).
  • We found out that chiller has a cooling duty of 12 kW at temperatures input of 15oC and drops to 5 kW at temperatures input around -5 oC. It showed that cooling duty decreases as temperature input to chiller decreases. Kind of makes sense.
Then we tested the cooling efficiency of the jacket, showing very bad results, of cooling duty ranging from 2 kW (at temperatures around 15 oC) to 0.3kW at temperatures below 0 oC. This duty was calculated measuring the temperature content of the reactor over time, taking into account the mass and specific heat capacity of reactor content.

The temperatures input and output to the jacket were measures but due to calibration errors they were not showing any reasonable temperature difference to be used to calculate duty. In order to get such a low cooling duty the temperature difference should be around 0.01 oC.

In order to understand if this is reasonable, I calculated the U-value of the cooling jacket. I used the following tool: CheCalc ‐ Jacketed Vessel Heat Transfer and I came up with a U of 140W/m2 K (quite reasonable for a cooling jacket).

The effective heat transfer area of the jacket was measured to be 2.44 m2 (the liquid in the reactor was touching half of the cooling jacket. With an estimated LMDT of 10-12 oC (that is the difference throught the tests between cpolant input temperature and reactor temperature), the estimated duty was calculated at 3.42 kW. So I don't understand why in reality the cooling of mixing reactor is a lot less efficient.

My questions are:
  • What do you think are the reasons of this inneficiency and how can I improve it on an existing plant without changing the design.
  • Is the LMΔΤ for jacket of a batch reactor correctly assumed that is the difference of inlet temperature to reactor content temperature?
  • To me it seems that bigger chiller won't make any difference. What do you think?
  • Do you think that low coolant velocity in the jacket is affecting this? Higher flow though won't give higher DT because it's already too small difference.

RE: Improving cooling jacket efficiency

What are you making? Solution acrylics?
In-process viscosity profile of reactor contents?
Desired temperature profile of reactor contents?
More details on "plain cooling jacket" please.
Details on mixer.

Good Luck,
Latexman

RE: Improving cooling jacket efficiency

A drawing or photo or schematic would help. what is the attachment / surface contact between the cooling pipes and the vessel?
Is the mixture circulated / mixed whilst cooling is ongoing?
Have you measured inlet and outlet temps of the cooling circuit? What are they if you have?
LMDT for something like this isn't that simple as there is no counter flow. Hence why you need to know inlet and outlet temp of the coolant. This will lower your LMDT.
Doesn't look like chiller capacity is your issue at the moment.
What temp do you want to have your 10kW heat removal taking place?

Seems rather slow, but you need to identify the coolant outlet temp. The velocity should be such that the outlet temp is still 5-7 C below the contents temperature to be effective. Or do a few trials

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Improving cooling jacket efficiency

(OP)
LittleInch,

My inlet and outlet temp of the cooling circuit is almost the same. No difference which I can't say a number due to error of instruments.
I would like to keep the reaction below 5-6 oC with a coolant that will be at -20oC.

The temperature outlet with content temperature has a difference of 10oC. DO you say that I have to lower the flow rate of coolant?
Trials will be done. Is just for safety purposes I need to ensure that reaction don't runaway.

RE: Improving cooling jacket efficiency

Can you answer the other questions as well please.

Looks like the cooling tubes are not making good contact with the vessel but we can't see what you can see unless you show us

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Improving cooling jacket efficiency

(OP)
The jacket is plain, there are no cooling tubes. Drawing of the jacket is not available.

RE: Improving cooling jacket efficiency

Then sketch one please.

Do you mean this a double skin and the cooling water flows around it?

How is the flow set to flow? Sounds like the water flow is short circuiting?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Improving cooling jacket efficiency

Many materials will freeze or get very viscous when in contact with cold walls, thus impeding heat transfer. The best solution to this problem is to have a mixer blade with wall scrapers in addition to an off-center dispersed blade. Absent wall scrapers, increasing the cooling water temperature may improve heat transfer. Also, higher mixer speed can improve the heat transfer coefficient on the inside wall.

RE: Improving cooling jacket efficiency

(OP)
Compositepro, will this work also if the tank will be used for crystallization purposes?

RE: Improving cooling jacket efficiency

Quote (KangCy)

Then we tested the cooling efficiency of the jacket, showing very bad results, of cooling duty ranging from 2 kW (at temperatures around 15 oC) to 0.3kW at temperatures below 0 oC.

You said your coolant in the jacket goes down to -20 C. 30% IPA in water freezes at -15 C. Pure IPA also gets viscous as it approaches freezing point - I'm not sure if a 30% solution will do that as well.

You have two possible issues:

  • Your batch is thickening as temperature drops - test this out in a lab to determine viscosity vs temperature.
  • Your heat transfer coefficient is wrecked when your jacket temperature drops to -15 C or under, as you will begin to thicken, then freeze, at the jacket wall. If you can, try setting minimum jacket temperature at -12 C to see if this improves performance.

RE: Improving cooling jacket efficiency

Based on your number provided it seems pretty clear that as you get colder, the contents of your vessel are inhibiting the transfer of heat from the majority of the fluid through a thick viscous / frozen layer on the wall of the vessel. So you need to scrape the wall and get some good circulation going.

The flow of cooling liquid could also maybe improved, but without a photo or sketch difficult to see what is going on.

Without adding either fins ( which will probably clog up), or cooling pipes within the vessel itself it is difficult to see how to improve things.

Maybe circulate some of the liquid through the coolant in pipes, but again if it goes viscous that may not work so well.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Improving cooling jacket efficiency

(OP)


This is the cooling rate profile. The duty decreases as temperature decreases and probably is due to loss of thermal coefficient on the wall. However, at over 0oC we can still see that the cooling duty is quite low compared to chiller duty of 10kW.

RE: Improving cooling jacket efficiency

Even at 140 W/m2/K you only seem to be about 3.4kW. I think 140 is quite high and hence to get omre you either need to raise that somehow or increase the area.

There is definitely a drop off in cooling rate below 5C in your vessel.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Improving cooling jacket efficiency

(OP)
Based on the actual experimental data, U-value is at 41 W/m2 K. Is this reasonable for a cooling jacket?

RE: Improving cooling jacket efficiency

KangCy,

41 W/m2K is very low for normal heat transfer in a jacketed vessel. I've seen 300-500 W/m2K in a clean application for heating or cooling water (both vessel and jacket mediums are water). For high viscosity batches, I've seen this drop off significantly to perhaps 100 W/m2K for viscous batches (5,000-15,000 cP). Your "U" value seems quite in line with thickening/freezing causing a "foulant" layer on the Hx surfaces.

Multiple posters here have noted the most likely cause is thickening/freezing at the vessel wall, and have also suggested the most common solution. You haven't addressed their suggestions, as of yet. Is that solution not acceptable? The other alternative is not cool down so far on either the jacket or vessel side.

RE: Improving cooling jacket efficiency

(OP)
Changes to equipment is not possible due to budget. And pilot plant test is coming up soon. That's why change needs to be in process conditions.

RE: Improving cooling jacket efficiency

It sounds like one needs to find a different solvent such that the mixture does not freeze or get viscous near the coolant temperature.

Good Luck,
Latexman

RE: Improving cooling jacket efficiency

So is the actual test giving out 10kW but you want to keep it at 5C?

Was this a test for the cooling?

I think you really need to postpone the real thing or use a lot less initially.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Improving cooling jacket efficiency

Plain jacketed vessels are notorious for short circuit issues and very poor distribution of cooling.
Sometimes you can cheat this by modifying the inlet with a nozzle or short length of pipe inside the jacket to help direct the flow where it is needed.
This will not handle your undercooling issue though. The suggestions already made address that.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Improving cooling jacket efficiency

Could you pipe the cooling fluid through a set of small pipes in the reactor contents itself instead or as well as the jacket?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Improving cooling jacket efficiency

A blast of cold air on the liquid surface in the mixer is surprisingly effective in increasing cooling rate in viscous materials. While air is not great at heat transfer, the liquid surface film cools rapidly and is then incorporated quickly into the bulk fluid, resulting in good overall heat transfer. I have also heard of dry ice pellets being poured into mixers for rapid cooling. One caution though, is that dry ice may contain traces of oil from the compressors.

RE: Improving cooling jacket efficiency

That does not appear to be high his viscosity at all. why are your blades pumping upward? There appears to be a frozen or crystallized white mass on the bottom of the mixer, under the clear liquid.

RE: Improving cooling jacket efficiency

(OP)
What do you mean by pumping upward?

RE: Improving cooling jacket efficiency

The blade angle and direction of rotation of the shaft are such that they are pumping the fluid upwards in the vessel. This is unusual. It will result in less fluid velocity and turbulence on the bottom surface. The motor is almost certainly three-phase, and it may have been connected in the wrong phase rotation. This is a very common problem.

RE: Improving cooling jacket efficiency

Is it just me or is that a very small agitator for the vessel size?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Improving cooling jacket efficiency

Compositepro - are you sure? I saw the video a few days ago, and it's archived now and I cannot re-look at it, but my recollection was it was turning clockwise viewed from above, and the P4's were pumping down in the center and up at the tank diameter. We use Chemineer agitators, and that's their standard set-up.

Ed - OP said agitator diameter/tank diameter was 0.6, and when I looked at it a few days ago that looked right. D/T = 0.6 for a low viscosity application is on the big side in my experience.

The biggest issue I saw was the agitator was only turning 30 RPM. Maybe that was done just for the movie, but 30 RPM in this case is slow.

Good Luck,
Latexman

RE: Improving cooling jacket efficiency

(OP)
Latexman, if we agitate faster, we will add more heat to the system right? Do you think it will solve this issue? Because the solution is anyway non viscous.

RE: Improving cooling jacket efficiency

The mechanical equivalent to heat, 1 Btu = 778 ft-lbf or 1 g cal = 4.184 J, results in lots of work for very little heat. I ignore the heat from mechanical energy in my agitation calculations. I've never seen the mechanical equivalent to heat get significant, compared to the heat removal required from a polymerization reaction, but I'm not familiar with your system. Do the calcs; numbers is hard.

Good Luck,
Latexman

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