No. of Distillation Columns

[nomanfahmi]

Hi All,
I have a small question regarding Distillation Column.
Why is 3 distillation column design (for methanol water system) is more energy savings than 2 distillation column design.

Regards

 

[25362]

I don't know for sure that your statement is right, and would appreciate learning about it from you and other experts.
A known fact, however, is that the relative volatility between methanol and water changes a lot, from 9 at low methanol concentrations to 1.1 at high methanol purities. The latter involves a large number of theoretical trays to achieve methanol separation with water contents in the few ppm range.

 

[nomanfahmi]

The following link talks about having three distillation columns in series rather than two for large methanol plants.

http://www.lurgi.de/lurgi_headoffic...ewsletter/methanol_conventional_reforming.pdf

 

[Montemayor]

nomanfahmi:

Regardless of how badly you, I, and the rest of the world want to save or conserve our energy, it does no good if we can't economically justify it.

If 3 distillation columns are better, maybe 4 columns are best. Who knows? The point here is that it all depends on the market prices for the methanol, the energy used, the capital cost, the minimum acceptable Capital Recovery factor, etc., etc. If I'm not mistaken, that should have been one of the basic and primary lessons learned in your university plant design course.

25362 is telling you the same thing I am: there are a lot of factors involved that determined what is the optimum design - the most important probably being the return on the investment. You can't just generalize a process or design and apply it across the board as a general rule. Every investment has its story and specific economic background. We can't judge which is more cost efficient without the details - of which there are many.

 

[nomanfahmi]

Montemayor

Thats why in my question I mentioned Methanol Synthesis.
For sure I can't generlize what I said of having three columns are more econmical. It is just in methanol synthesis.

 

[mbeychok]

.
nomanfami:

If you will read page 13 of the .pdf file at the web site address to which you referred us, you will find that Lurgi quite clearly explained why they say that the the three column design uses less energy than the two column design:

The distillation principle remains unaffected, but the pure methanol column is divided into two separate columns, one running at elevated pressure, the other at ambient pressure.

Pure methanol is discharged overhead in both columns,
each refining between 40 - 60 % of the product. The
ambient pressure column reboiler utilizes the heat of the
pressure column overhead vapours, which are condensed
simultaneously. This reduces the energy demand for heating
the pure methanol column considerably by approx. 35 %.

I think that, along with the process flow diagram on page 13, the above explanation is quite clear.

Milton Beychok
(Contact me at www.air-dispersion.com)
.

 

[nomanfahmi]

Thanks for all who answered me,
Actually when I posted my question I also was thinking why in a 2 column system I can't increase the pressure in the first column and use the heat from the 1st column condenser in the 2nd column reboiler.
anyhow I found the answers to all of my questions.
Basically the feed to the distillation unit consist of Methanol, Water , non-condesable gases (mainly CO2),
so the first distillation column is just to take off the non-condesable gasses. the size of the that column will be the same in both designs (2 column or 3). It is also small column , so the reboiler and condenser duties are low. so increasing the pressure in the first column doesn't help because the condenser duty is negligable compared to the 2nd column.
the bottom from the first has methanol and water and can be seprated in one column, but increasing the no of column and adjusting the pressures so the first is the highest will help in utilising the energy.
this thing doesnt depend on methanol price, or the other factor that (Montemayor) mentioned. it only depends on 2 factors capital cost of distillation column and energy price. Therfore it might more economical to have 4 distillation column if the energy price is high

again thanks everybody

 

[nomanfahmi]

I have another question to the chemical engineers here.

it is regarding the same distillation columns, which is better using the 2nd and 3rd distillation column in series, or dividing the flow to 2 distillation columns and operating one of them at high pressure to take advantage of the heat.
I think by divididng the flow the diameter of the the first distillation column will be less

Thanks