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Vacuum Requirement Calculations In Evaporation Vessel

7 replies to this topic
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#1 noobeng

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Posted 17 August 2018 - 08:04 AM

Hi,

I need to calculate the vacuum requirement for a 100 L vessel used for boiling a solvent (DMSO). The vessel is to operate under 0.2 bar. The jacketed vessel will be heated using a oil heater (~20 kW). The solvent will boil at 100 C at that pressure. The evaporation rate will be ~0.6 mol/s.

I have done a simple ideal gas equation, however it doesn't look right to me:

Volumetric flow rate = (0.6 (mol/s) x 0.08314 (bar L/mol.K) x 373 (K))/0.2 bar = 93 L/s

Based on the above I will require a vacuum pump with a capacity of 5580 LPM which seems quite high.

#2 thorium90

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Posted 17 August 2018 - 08:37 AM

Have you considered just using a smaller heater? Or turn down the heater dial...

#3 noobeng

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Posted 17 August 2018 - 08:48 AM

Have you considered just using a smaller heater? Or turn down the heater dial...

Yes! However as part of the R&D process the diffusion at higher evaporation rates are to be evaluated (so the Q will be varied).
Would you say the above calculation is correct then?

Edited by noobeng, 17 August 2018 - 08:48 AM.

#4 sgkim

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Posted 04 September 2018 - 01:59 AM

noobeng,

The volume rate you calculated is the evaporating DMSO volume rate.  If all of the vapor pass through the exhausting vacuum pump, then the figure would be the capacity of the vacuum pump.  But if all the evaporating vapor (with no leaking air) condense in the condenser, then there would be no vacuum duty.

So, to minimize the capacity of the vacuum pump you have to install a condenser in front of the vacuum pump. Then, the vacuum duty shall be based on the leaking air rate in the system.   The volume flow to the vacuum pump shall usually be the "leaking" air rate plus saturated DMSO vapor rate leaving at the same temperature and pressure on the condenser outlet.

Evaporating rate of DMSO = (Heat absorbed in the jacketed vessel, kcal/h)/(Latent heat of DMSO under vacuum, kcal/kg)...(1)
Leaking Air rate  in the system (under vacuum) = L    m3/h  ...(2)
Vacuum Pump inlet volume rate under vacuum= L* (1+ vapor pressure at condensed liquid, mmHg/vacuum pressure mm Hg)....(3)

The problem is how to estimate the leaking air rate "L" in the system.
One of 'rule-of-thumbs' estimation can be found here:  http://thermosysinc.com/inquiry1.html  Other good examples can be found elsewhere.

~Stefano Kim

#5 AndyChemEng

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Posted 04 September 2018 - 09:17 AM

How quickly do you want to reach your desired vacuum at the start? What is your initially gas volume? Is the 100 litre pot full?

This is most likely your dominating factor for sizing your vacuum pump and not the leakage rate, particularly at an operating pressure of 0.2bara.

#6 breizh

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Posted 08 September 2018 - 08:44 PM

hello,

thanks to share with us a sketch of your set up ?

Probably you have a total reflux condensor prior to your vacuum system , to remove  condensable material.

hope this is helping you .

Breizh

Attached Files

Edited by breizh, 30 August 2019 - 10:34 PM.

#7 bkultgen

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Posted 30 August 2019 - 12:04 PM

Hello Breizh,

Thanks for the attached files, these have helped me greatly. However, I have a question regarding the "Calculate Saturated-Gas Loads for Vacuum Systems" document that I am hoping you can comment on...

When I get to equation 15 I still have two unknown variables P_t and M_t. I can define P_t but this will significantly impact the total moles of condensable vapor (the closer Pt is to atmospheric pressure the less total moles I get). Am I correct in assuming that I have two unknown variables? If so, might it be best to try and find a vacuum pump curve and fit my calculations as possible into this curve?

Thanks again,

Ben

#8 breizh

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Posted 31 August 2019 - 12:08 AM

Hi,

Let you prepare a spec (data sheet) with your process conditions and ask the vendor to supply the equipment .

My view.

Breizh

Edited by breizh, 01 September 2019 - 02:43 AM.