May 28, 2010

Pressure Swing Distillation - Use it Economically - 2

In the last post we discussed a bit about azeotropes & refreshed ourselves on basics.

So basically because of deviations from ideality (Raoult's Law), liquid mixtures form azeotropes.

Now, the phase envelope at different pressure suggest that the azeotropes shift due to change in pressure and this is our best tool to be utilized in the industry instead of going for extractive distillation where other solvent is used to carry over more of one component than other thus breaking the azeotrope.

Let us understand the conventional azeotropic distillation system, its basic, benefit & limitation.

The conventional method of separation of binary azeotropes have been through addition of another solvent to shift the equilibrium by forming ternary azeotrope. This is called azeotropic distillation.

The common example is to separate ethanol & water using cyclohexane. The ternary azeotrope of 7% Water, 17% Ethanol & 76% Cyclohexane boils off at ~62 C. So in the binary azeotrope of 95.6 % Ethanol and 4.4% Water, just enough cyclohexane is added to remove the water in 7 to 76 ratio which removes all water with some ethanol leaving almost pure ethanol at the bottom.

For example - For 100 Kg of ethanol azeotrope (means 4.4 Kg water) requires 4.4 x 76 / 7 = 47.8 Kg of Cyclohexane. This will carry all water i.e. 4.4 kg of water, 47.8 Kg of cyclohexane & 10.7 Kg Ethanol.

So WHAT????????????

This is just an example of azeotropic distillation where it can be used effectively, but just think if the azeotrope of ethanol water is say 60:40 in place of 95.6 & 4.4. Will it be possible by same system of cyclohexane let us see...

In this hypothetical case, 40 Kg water requires 434 Kg of cyclohexane & how much ethanol it will carry....97 Kg. SO It is not possible because all ethanol is also carried to top distillate & practically no separation.

So now you have fair idea about the benefits & limitations of azotropic distillation.

Therefore, I would like to explain something more which is more beneficial.

Pressure Swing Distillation
As explained earlier, the azeotrope composition varies with change in pressure. This can be understood from the following diagram.

In the above figure, you can clearly see that the azeotrope point is shifting towards right side at high pressure.

So if let us say point B = 30% of component A is at azeotrope now this mixture if pressure is raised will become point C at high pressure & then distillation can continue to point D of another azeotrope at that higher pressure.

So now if I again subject point D to same low pressure then it will become point E at low pressure but distillation can continue. So I can achieve 100% of component A.

This can be applied to any system without any issue and you have following benefits.

1. You do not need any third component.
2. You do not need two more columns for removal of solvent - one for solvent removal from light component & one for solvent removal from heavy component.
3. So more hardware cost.
4. So more energy cost also.
5. PSD (Pressure Swing Distillation) is very simpler to design.
6. No losses of components.
7. No Bulk quantities handling hence very low cost of operation.
8. Many more.........I am not in a mood to loist down all.

Now I will give you one practical example.....which I personally handled.
Ethanol & Ethyl acetate also forms azeotropes & I came across one such system with large quantities. So instead of considering Az Disitll I used PSD.

In this case, We came across an Az mix of ~69% Ethyl acetate (EA) & 31% Ethanol (EtOH). When we ran Aspen Simulation for PSD, we fixed the pressure of first column as 5.5 Bar (After Optimization) & 300 mm Hg for second column.

Now feed was sent to first column & we could recover EA from bottom as 99.9% pure and top was fed to vacuum column at 300 mmHg pressure. Here we coul dseparate 99.9% EtOH from bottom again. the top cut from second column was recycled to first column and thus there was no loss.

I also used it for separation of cyclohexane, EA & EtOH separation later on & my experience shows that slight moderate pressures of 2 to 5 bar can change azeotropes significantly.

So now on try to use it for your benefit & get the recognition from your Boss.

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May 10, 2010

Pressure Swing Distillation - Use it Ecnomically - 1

All the process engineers must have come across the term azeotropic distillation in some or the other way. Most of the time the conventional approach of handling an azeotropic mixture is to carry out extractive distillation using some solvent.

Very easy know. So many lists of such solvents are available in the literature or on internet and then we go ahead with that age old philosophy.

You know, I NEVER follow conventinal things. So Pressure Swing Distillation (which is now becoming more popular) is an effective answer to such problems.

To define it & understand it properly, I would like to first cover up few basics of azeotropes which you may be knowing if you were good in theory during your college days.

Azeotrope - means a mixture of liquid which generates vapor of same composition as that of liquid.

There are two types of azeotropes - Negative azeotropes & Positive Azeotropes.

Negative Azeotropes are those in which the azeotropic boiling point is higher than any of its constituent's boiling point e.g. the mixture of Formic acid & water as given on wikipedia.

Positive Azeotropes are those in which the azeotropic boiling point is lesser than any of its constituent's boiling point e.g. the mixture of Chloroform & Methanol.

Now curves represented in above diagrams are simple T-x-y OR P-x-y curves which are commonly found in distillation textbooks, but what we forget or do not know is that these curves are pressure dependant curves. This means the phase envelope changes with change in system pressure. So if you alter the pressure of the system the azeotrope point or composition will change. You can find it out how? YES...Whenever something goes out of track always remember RAOULT's LAW.

Rest in next post.........

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