5 replies to this topic

[Dhanya S Nair]

We have a distillation column which seperates Methyl acetate & water.The column operates under atmospheric pressure.The top product is 97% Methyl Acetate & bottom product is 99.7% Water.The column feed is dilute Methyl Acetate feed.For boil up,steam is directly injected.
We wanted to increase the top product purity to 99.5%.Simulations were carried out & it revealed that operating column under vacuum of 0.65BarA with same feed/steam/distillate rate can yield MA purity of 99.5%.I have observed that requirement of steam is less in vacuum operation.But I expected that steam flow requirement would be more as the latent heat is high in vacuum.Why is the steam flow requirement less in my case for vacuum operation.Please explain.
Generally,How do we decide upon the optimum steam flow to distillation column.

[xavio]

Hi,

You said that you maintained the same feed/steam/distillate rate, but later you said steam requirement was less. How did you actually perform the simulation?
If separation difficulty is unchanged with change of pressure (i.e. system is not pressure sensitive), then you will see reflux and boil-up rates requirement are unchanged, too. It means, in vacuum condition you may see higher steam rate due to higher latent heat.
But some systems are pressure sensitive, meaning that their separation might become easier or harder when pressure is changed.
In your case, it seems to become easier when you lower the pressure, so you obtain higher purity with fixed boil-up.

You should check the VLE for both cases and learn more about "pressure swing distillation".

Good luck.

xavio

[Fr3dd]

I don't see the problem here. If the operating pressure is reduced (in this case for vacuum pressure) the boiling points of the components of the mixture drop and you require less heat input to perform the separation.

This is a basic principle in distillation.

Regards,

[xavio]

Fr3dd,

I don't see how lower boiling point automatically means lesser heat input to perform a separation.
In fact, latent heat of liquid increases with decrease in boiling point (lower pressure).
Some systems become harder to separate at lower pressure; separation difficulty depends on thermodynamic property.

Thanks.

xavio

[Fr3dd]

Well, I think it is a general trend. Of course there could be exceptional cases and I don't know anything about the behavior of this particular system.

When pressure decreases, latent heat increases since both enthalpies (liquid and vapor) decrease, but liquid enthalpy decrease is larger than vapor enthalpy decrease, so the difference between them (latent heat) is bigger. As Xavio said, the boiling point of the mixture will be lower. This means that the heat input you need for separation is lower, since you will need to reach a lower temperature to perform your distillation.

Of course the operation is much more complex than that, but I'm just trying to make my point as simply as I can.

Regards,

[sureshrajeshb]

see.......Vaccum distillation is not simply as like atomsphere distillation.
It need lot of design requirement on the tower and vaccum system.
so cost requirement of the plant would became factor of concern..