6 replies to this topic

[riwaldron1]

Hi all,

 

I'm designing a distillation column to purify methanol, with the primary components of my feed mixture being methane, carbon dioxide, methanol & water. How do I treat this column considering 2 of the components are non-condensable gases? Can this be treated as a binary mixture and neglect the impacts of CO2 & methane, or should I only consider them in calculations for bubble & dew points (and therefore only in calculations & design of the condenser)? I initially designed it in a similar way to multicomponent distillation however, I'm not sure that's the correct approach considering they're not specifically part of the vapour-liquid mixture.

 

Any advice would be really appreciated!

Thank you.

[latexman]

If you have some design latitude, and you may not, a flash of the feed before the distillation column may get most of the non-condensables out of the feed before introducing it to the column. A flash pot could be an alternative to a larger condenser caused by the presence of non-condensables.

[riwaldron1]

If you have some design latitude, and you may not, a flash of the feed before the distillation column may get most of the non-condensables out of the feed before introducing it to the column. A flash pot could be an alternative to a larger condenser caused by the presence of non-condensables.

No, there's no flexibility for adding units to change the feed composition. I believe the methane is probably low enough to be considered negligible and vented in the condenser, however I'm concerned there's too high a flowrate of CO2 to treat it in a similar way and I'm not sure how the CO2 changes the approach to the design calculations.

[latexman]

Do you have access to a simulator with physical properties? HYSYS, Aspen Plus, or ? That, with a good choice of thermo model, would be the most rigorous way. It would be a multi component design, as you said.

[shvet1]

Can this be treated as a binary mixture and neglect the impacts of CO2 & methane, ... )? 

 

No as CO2 reacts with water and interferes the equilibrium of H2O-MeOH and CH4 affects H2O&MeOH vapor pressure and therefore equilibrium temperature.

CH4 should be treated as Henry's gas

CO2 should be treated as a water-soluble compund CO2+H2O <=> CO3- + H+

 

If you ignore those the temp-concentration profile of trays will be different.

 

A tough task to model I should say.

Edited by shvet1, 10 April 2025 - 11:36 PM.

[breizh]

Hi,

I believe the answer given by latexman about installing a flash prior to distillation if the best but,

 we have no idea about the composition of the feed stream.

Flash is not necessary a big piece of equipment.

Breizh

[shvet1]

MeOH is volatile and toxic. Flash means losses of MeOH with non-condensables and challenges in MeOH dispersing.

Is it a MeOH production or a water stripping? What losses this process is able to tolerate?

 

Given this is a study task you should identify&compare options and justify your choosed one. Flashing is one of those.

 

I believe there are a lot of discussions of a preferred H2O-MeOH model and accompanying impurities in the publications. Did you try any search engine before posting?

Edited by shvet1, 11 April 2025 - 01:01 AM.