8 replies to this topic

[NikyleMoodley]

Hi all,

I'm working on my final year design project, which involves a distillation/stripping column (MEA-based system) operating under vacuum. For diameter sizing I've been using the standard Coulson & Richardson (Sinnott) approach — flooding correlations (Fair's method / Souders-Brown), net area, downcomer allowance, etc.

My professor has told me this won't give the correct design for a vacuum column and that I need a different design method / code. Is there a specific named method/correlation that's considered "the" standard for vacuum column diameter sizing (as opposed to atmospheric), or is it really just the same flooding correlations with a pressure-drop-driven design philosophy layered on top?

[Pilesar]

Vacuum distillation columns work best with very little internal pressure drop. Because trays keep liquid on the tray by design, the static head of this liquid adds to the pressure drop. Packing is often used since it results in a lower amount of liquid held up in the active area and therefore lower pressure drop. This is not intended to be the direct answer to your question, but is something for you to consider. 

[NikyleMoodley]

Thanks Pilesar , I'll keep this in mind and look into it . I've only designed DCs with sieve trays before so I have my work cut out for me . 

[Pilesar]

Acid gas removal columns also tend to cause the MEA/water mixture to foam. Designers take this into account when sizing the columns. There is much information on the web if you search for "gas sweetening". Here is a list of technical articles you might find useful: https://www.bre.com/...l-Articles.aspx 
I would start with https://www.bre.com/...with-Amines.pdf

[breizh]

Some basics about distillation column design.

Breizh

Attached Files

•  separation tower design.pdf   1.41MB   1 downloads
•  ENGINEERING DESIGN GUIDELINES - distillation column - Rev 04 web.pdf   2.07MB   1 downloads
•  Bulletin-4900.pdf   4.88MB   1 downloads
•  distnprs.pdf   895.14KB   0 downloads
•  Info RASCHIG Column Internals-1101.pdf   2.91MB   1 downloads
•  Internals_for_packed_columns.pdf   2.98MB   1 downloads
•  Amine Plant Troubleshooting & Optimization.pdf   112.24KB   3 downloads

[NikyleMoodley]

Thanks breizh

[NikyleMoodley]

Some basics about distillation column design.

Breizh

oh and btw, Breizh. We also have to use aspen plus to optimize our column . I have only done this with a tray column where the number of trays were determined by taking the number of stages and dividing it by the tray efficiency. In a packed column, there's no efficiency formula. So does that mean , the number of stages correlates to the number of transfer units (NTU) ?

[latexman]

IIRC, there is a Height of a Transfer Unit (HTU) that correlates directly with the gas or liquid mass transfer coefficient (whichever controls) and the effective interfacial area of packing.  Then, the total packed height (Z) of a separation column is calculated using the fundamental relation:

 

Z = HTU x NTU

[breizh]

Hi,

If your solutions are clean, yes packing is the best option for distillation under vacuum. I don't know much about your system except its tendency for foaming.

I've attached document from Sulzer for Internals and structured packing materials. I've good experience with BX packing for the distillation of diphenol under vacuum. A train of 10 columns with steam ejectors for the vacuum. 

 

As you will see in the brochure, Sulzer and other big players use HETP /m or Height equivalent to a theoretical plate per meter of structured packing.

In this case           Z =HETP * Number of stages 

 

As a student you should get access to Perry's Chemical engineers' handbook.

 

About simulators, you should find tutorials using Youtube channel and/or searching using Google or other engines.

 

Good Luck.

Breizh

Attached Files

•  Internals_for_packed_columns.pdf   2.98MB   0 downloads
•  PackdCol.pdf   663.29KB   0 downloads
•  Structured_Packings.pdf   4.19MB   0 downloads
•  Multicomponent_distillation_Grisafi.pdf   1.4MB   0 downloads
•  liptak_distillation_ebook.pdf   1.09MB   0 downloads