16 replies to this topic

[kangas]

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.

Edited by kangas, 23 June 2022 - 06:29 AM.

[breizh]

Hi,

Investigate the agitation ( type , rotation speed)  which  will impact the heat transfer coefficient ,same with the coolant within the jacket (distribution) .

In other words work on the chemical engineering part .

Good luck

Breizh 

[kangas]

Breizh:

 

The agitation parameters does not significantly affect U value for my case. 

Coolant, I guess only the flow rate can be improved but we are limited on that too.

[breizh]

Hi,

What about using an external heat exchanger to cool down with recirculation to the reactor ,in addition or not to the jacket ? Let say spiral heat exchanger .

Calculation can be found in Process heat transfer by Kern and Perry  , other references available.

 

Good luck

Breizh

[latexman]

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.

[kangas]

Latexman:

 

It's a reaction between an extract dissolved in ethanol and NaBH4.

 

Viscosity of the reactor content is hard to say because there will be solids present. In general the liquid will be ethanol (1.7cp)

The reactor content should not exceed temperature of 10oC, otherwise risk of runaway reactions.

Cooling jacket has volume capacity 0.4m3. Total surface area 3.5 m2

Mixer: paddle: 600mm diameter. Two agitators, but fluid is touching only the bottom one.

[latexman]

What is the final viscosity?

Is it a full jacket?  Is there baffling?  Is there jet nozzles in the inlets to create turbulence?

Mixer kW and RPM?  D/T (impeller to tank diameter ratio)?

[breizh]

Hi,

A sketch of your reactor is required with all dimensions and details of internals ( agitator, baffles ) , level of slurry in the vessel during cooling step ,Etc. From my reading above I understand that mixing is quite "poor" , bottom part of agitator (2 stages)  in contact with the product .  The type of agitation is key , Have you ever thought about anchor type ?

Good luck

Breizh 

[kangas]

What is the final viscosity?

Is it a full jacket?  Is there baffling?  Is there jet nozzles in the inlets to create turbulence?

Mixer kW and RPM?  D/T (impeller to tank diameter ratio)?

Half jacket, no baffles in the jacket. 

120rpm on the mixer.

D/T: 0.6

The propeller is quite low on the mixer. Almost at the outlet.

Edited by kangas, 24 June 2022 - 04:13 AM.

[breizh]

Hi ,

To add to my previous replies , documents about heat transfer related to agitated vessels .

Breizh 

[kangas]

Thanks Breizh!

[kangas]

A sample of mixing video: 

https://www.veed.io/...ringWidget=true

[latexman]

It doesn't look vigorous enough for good heat transfer, in my opinion, but the RPM was only about 30 RPM by my count.  You said above the RPM was 120 RPM.  Is there an issue between the RPM indicated and the actual RPM?

[breizh]

Hi ,

To me the agitation is not very efficient , can you operate at higher RPM or do you have constrains with the PSD of the solid ?

Can you add solvent to increase the ratio liquid to solid by recycling mother liquor from a previous batch ? This will improve the mixing and heat transfer . 

 

From your video , the heat transfer area is small . 

note : I did not see deposit on the wall or the agitator .

BTW , Your reactor seems oversized for your batch size . A smaller reactor will perform better in term of heat transfer and hydrodynamic.

 

Let see what others have to say.

Breizh 

[kangas]

Breizh, yes this reactor is oversized because we just have it available. The max volume though that will be used in the reaction will cover the full jacket area. Industrial scale design will need to be optimized.

[latexman]

In the video, the RPM was about 30 RPM.  You said earlier the RPM was 120 RPM.  Is there an issue in this discrepancy, or does the video not represent normal operation?

[kangas]

In the video, the RPM was about 30 RPM.  You said earlier the RPM was 120 RPM.  Is there an issue in this discrepancy, or does the video not represent normal operation?

Video was not representative of the RPM during test.