July 20, 2011

When to Use PSD system?


Today there was a comment on Name of specific Industries where we can use PSD (Pressure Swing Distillation) system on my previous article. Out of interest of few visitors I would like to suggest few points.

Also this post onwards, I will try to post very short posts but may be many on the same topic.

Request feedback from all of you on this kind of posts.

See, the pressure swing distillation means - More than one distillation columns in series but at different pressures to achieve the desired purity of one component due to shift in Azeo condition with respect to pressure.

Let me explain again.....with an Example of Ethanol & Ethyl Acetate mixture.
As per literature data, the composition at two different pressures are as below

Pressure Ethanol Ethyl Acetate
760 mm Hg 31% 69%
1500 mmHg 39% 61%
300 mmHg 23% 77%
25 mmHg 13% 87%

Considering the wide difference in Azeo composition in the given system, If I choose say 1500 mmHg pressure of first column & 25 mmHg pressure of second column then the first column top will be richer (Feed is 69% EA & 31% Ethanol at ambient condition which is generally expected from any system) in Ethanol.

This is because top composition will be azeo composition at 1500 mmHg pressure i.e. 39% Ethanol while feed is having 31% Ethanol, so bottom shall be More EA, thus it is possible to get pure EA at the bottom......Wait if you are not clear.....read more below.

Now top of first column which is 39% Ethanol & 61% EA is fed to second column which is operating at say 25 mmHg. Now at this pressure Azeo composition is 13% Ethanol. So most of the Ethanol will settle at Bottom & EA rich top will be (13% Ethanol + 87% EA) coming out from top.

Now this top stream of second column is fed back to first column along with feed. So feed will further get rich in EA. So more EA will settle at the bottom of second column. BUT top will remain same at Azeo of 1500 mmHG & will go to second column again.

Thus with this kind of cycling eventually you will end up with pure EA at bottom of first column and Pure Ethanol at the bottom of second column. All tops will be fed to next column through recycling. This is how PSD works for any system.

However, the limitation is if difference in the Azeo compostion is small then you may need more such columns OR it may not be possible to use PSD at all.

Hope it is more clear now....

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