Evaporation under vacuum; surface area

In summary, the conversation discussed the operation of a vacuum distillation unit used to distill ethylene glycol. The unit has a diameter of 36 inches and is 124 inches long, and the old set up involved filling it half full, 18-20 inches high, while the new set up involves filling it to 31 inches high. The speaker mentioned that the lower surface area may result in collecting evaporation at a slower rate, and asked if there is a formula to prove this assumption. In response, the other person explained that the evaporation rate is proportional to the surface area, with a constant factor depending on pressure, temperature, and chemical composition. The vacuum pump used is a constant 24-27Hg and
  • #1
Jay167
I run a vacuum distillation unit that is used to distill ethylene glycol. The old glycol is subjected to 25Hg of vacuum at a temp of 275 degrees F. There are burner tubes submerged in the glycol to heat it up. The vessel has a diameter of 36 inches and is 124 inches long. We fill the vessel half full, 18-20 inches high, heat it under vacuum and collect the evaporation. The new set up has us filling the vessel to 31 inches high. I stated that the lower surface area would make it so we are collecting evaporation at a slower rate. Am I right? Is there a formula I can use to prove my assumption. Thanks
 
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  • #2
If the pump is strong enough to keep pressure significant below the pressure where the liquid boils, evaporation will be proportional to the surface area, the proportionality constant will depend on pressure, temperature and chemical composition.
R=k*A if you absolutely need a formula (with the evaporation rate R, the area A and k as constant).

If the pump is weak, then the surface area won't matter, and just the pumping rate matters.
 
  • #3
Thank you for taking the time to send a reply. here are the answers to your questions if it helps to get a better answer. I explained that in the old set up, we filled the machine 20 inches high so we had a surface area of 34 inches wide by 124 inches long. We would collect 25 gallons of evaporate (93% glycol 6.2% water .8% light oils) per hour. Now that we fill to 31 inches we only get 16 in cross section by 124 long. I say we will collect slightly under half of what we used to.
Our vacuum pump is a constant 24-27Hg of Vacuum. The temp fluctuates between 270F (burner turn on point) and 275F (burner high limit setting). The usual composition of the glycol mixture is 40% glycol 52% water 8% light oils, other automotive fluids some suspened solids etc. We have a level sensor hooked to a pump that feeds the vessel to keep the fluid level a constant as well.
 

1. What is evaporation under vacuum?

Evaporation under vacuum is a process in which a liquid is transformed into a gas through the removal of air pressure. This causes the liquid to evaporate at a lower temperature than it would under normal atmospheric conditions.

2. How does surface area affect evaporation under vacuum?

The larger the surface area, the faster the evaporation under vacuum will occur. This is because more molecules of the liquid are exposed to the reduced air pressure, allowing them to escape and turn into gas.

3. What is the purpose of evaporation under vacuum in scientific research?

Evaporation under vacuum is used in scientific research to study the properties and behavior of substances at low pressures. It can also be used to remove solvents or other substances from a sample, allowing for more accurate analysis.

4. How is evaporation under vacuum different from regular evaporation?

Regular evaporation occurs at normal atmospheric pressure, while evaporation under vacuum occurs at reduced air pressure. This means that evaporation under vacuum can happen at lower temperatures and at a faster rate than regular evaporation.

5. What are some common applications of evaporation under vacuum?

Evaporation under vacuum is commonly used in industries such as food, pharmaceuticals, and chemicals for processes such as freeze-drying, distillation, and concentration. It is also used in laboratories for sample preparation and analysis.

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