Hi,

Generally, lower the temperature better the separation. Ideally Liquid nitrogen temperature of -196 degrees is preferred. However, Dry ice temperature -57 degrees cam be used. There are also some eutectic bath  mixtures which can be used.

But, liquid nitrogen is commonly used in laboratory set-up. 

Hi, because the melting point of ethanol is -114 degree C, and the vapor pressure is increasing with temperature, it is better to use liquid N2 trap (-196 degree) for your pervaporation.

I would like to correct the minimum temperature that nitrogen can reach:

Under normal atmospheric pressure, nitrogen exists as a liquid between the temperatures of -210°C and -196°C which equals -346°F and -320.44°F. Below -210°C, nitrogen freezes and becomes a solid. Above -196°C, nitrogen boils and becomes a gas. Liquid nitrogen stored in unpressurized conatainers like the tanks sold here is slowly boiling away, so the temperature is -196°C or -320.44°F.

http://liquidnitrogentank.com/how_cold_is_liquid_nitrogen.php

Rafik

I assume you will also have vacuum applied to the cold trap on the permeate side. One indication of insufficient cooling, is when you see the permeate pressure starts to increase significantly. With liquid nitrogen, you can expect to have a sufficient cooling for a relatively longer time than using dry ice. 

 Dear Worely

Hi, I really appreciate you. So, if the alcohol could not trap and refrigerate in product side, the pressure should be level up? Do I get your answer?

Thank you

Hi Zohreh,

The alcohol will be frozen in the cold trap. I am not sure what is the volume of your cold trap, but if you expect to collect a large volume of permeate, you should watch the permeate pressure. Once you get too much frozen alcohol in your cold trap, the permeate pressure will go up and if you do not switch to another cold trap, the incoming permeate alcohol won't get cooled and trapped and instead they will be taken by the vacuum.