5 replies to this topic

[Sadananda Konchady]

The following 3 mass transfer options are available in my distillation simulation package.

Mass transfer coefficients for Tray columns -

1. AIChE (1958)

(Is this generally applicable to sieve trays, bubble caps trays, and valve trays?)

2. Chan and Fair (1985)

(Is this applicable only to sieve trays?)

3. Zuiderweg (1982)

(Is this applicable only to sieve trays?)

While applying the AIChE model for a multicomponent distillation column I happened to get one negative value for the Murphree stage efficiency for one of the components on the bottom stage of a column. This does not seem right. Is the negative value logical? All other values of stagewise efficiency for the components were positive.

Thank you for your time.

[Sadananda Konchady]

Thanks for your feedback. Yes, Dave, you are right about the Murphree stage efficiency. The following weblink suggests that it could be typically in the range of 60% to 120%.

http://www.euronet.n...l/TrEffGnl.html

I attempted the Murphree stage efficiency calculation using the AIChE model as well as the Chan and Fair models. In both the cases, I got negative values for the stage efficiency at the bottom of one of the distillation columns (with bubble cap trays) during the simulations. I will check with the software vendor to determine whether this is a program bug (all the other values were at least positive).

--- escape

[siretb]

Efficiency cannot be negative. Can exceed 100%
As a rough estimate consider the O'Connel correlation
Eff=0.492 * ( viscosity * alpha ) ^ -0.245

This has been done for bubble cap trays, conservative for sieve trays.
Alpha is the relative volatility of the light/heavy key.

[Sadananda Konchady]

Hard to believe!

But the software vendor has replied that in fact the Murphree stage efficiency could be negative and has mentioned that there are some references in the technical literature about such cases, especially for multicomponent distillation.

[Sadananda Konchady]

Negative Murphree stage efficiencies may indeed be possible in multicomponent distillation.

See the ternary example involving ethanol, water, and butanol in Mass Transfer by J. A. Wesselingh and R. Krishna, Ellis Horwood Series in Chemical Engineering (1990) given on pages 45 to 49

Alternatively you can click on this weblink which is a sample chapter in

http://130.161.129.68/hlf/d004ch06.pdf

Mass Transfer in Multicomponent Mixtures
prof.ir. J.A. Wesselingh and prof.dr. R. Krishna

2000 / 329 p. + CD-ROM / ISBN 90-407-2071-1 / Euro 35.85
Details of this publication are given at:

http://www.vssd.nl/hlf/d004.htm