5 replies to this topic

[suphaklitwd]

Dear All,

 

I have been trying to simulate a distillation column to separate butanol and water. The bottom product should be high purity butanol (>90%), I am able to match the composition, but I couldn't match the operating temp at the bottom (approx. 120 C, the temp indicator is at the column bottom with a meter of liquid above it). In my simulation the bottom product temp it gives is around 110 C.

 

All I can think of now is that the difference could be caused by the static head above the temperature indicator causing a higher bubble point at the bottom of the liquid sump. Can someone please confirm, or if you have experienced something similar, please share.

 

Thank you!

 

 

[breizh]

Butanol + water forms an azeotrope. How do you intend to perform the separation?

High purity butanol @ > 90 %

Share with us a simple PFD.

Breizh

[katmar]

The pressure at the base is also increased by the pressure drops across the trays or beds of packing.

[suphaklitwd]

Thanks all for the feedback

 

 

Butanol + water forms an azeotrope. How do you intend to perform the separation?

High purity butanol @ > 90 %

Share with us a simple PFD.

Breizh

This is one of two columns in an azeotropic distillation, the distillate of this column goes to a decanter and from the decanter, the aqueous phase goes to another column while organic phase gets recycled back to this one. The azeotrope happens at around 23% butanol so from the feed characteristics of this column we can get high purity butanol at the bottoms but not high purity water from the distillate.

 

FYI, a typical PFD of azeotropic distillation.

https://ars.els-cdn....80323911788.jpg

 

 

 

The pressure at the base is also increased by the pressure drops across the trays or beds of packing.

I have tried varying the column pressure drop too but to get this temp, the total column pressure drop would be around 25-30 kPa, is this normal for a packed column? We don't have a pressure indicator at the column bottom unfortunately.

[katmar]

The typical operating pressure drop for a packed column operating at atmospheric pressure would be in the range of 0.5 to 1.0 kPa per metre of packed height.  If your column is 20 m tall you could be in the ballpark.

 

I do not believe there would be a significant increase in temperature at the bottom of the column sump due to static pressure.  The vapor/liquid mixture returning from the reboiler would be at the saturation temperature and if there is an increase in pressure towards the bottom of the sump it would lead to the suppression of boiling rather than an increase in temperature.

 

Do you have any other temperature points on the column where you can check the correspondence of the temperature and expected composition?

 

The base pressure of a column is a very useful fault finding tool and you should try to get a gauge fitted.

[breizh]

Like Katmar said the Bottom pressure is a key indicator as composition to get the boiling temperature.

Note: Make sure your pressures and temperature sensors are well calibrated before use.

You should also question the Thermodynamic model used for the simulation.

 

Control of the Heterogeneous Azeotropic n-Butanol/Water Distillation System | Energy & Fuels | ACS Publications

My 2 cents

Breizh