Distillation Basics/tray Efficiency

[maddocks]

Does anybody recall seeing a pdf paper regarding distillation basics? We're into a big issue regarding murphree tray efficiences (vs overall column efficiency) on demethanizers. Values being discussed are ranging from 20% efficiency up to 46-50% efficiency.....

Vendors to date have proved unhelpful...

 

[dcasto]

In Chapter 19 of the GPSA Data Book (available from

http://www.gasprocessors.com),

in figuire 19-19, demethanizers generally run 45 to 60%. This should get you going on similations, but it still takes the evndors to give you exact values which all depend on composition, throughput, temp and press, and so on.

 

[sshep]

Maddocks,

In about 1999 I did some overall efficency calculations on a ethylene plant demethanizer using the method described in faq124-1340, with VLE per a high pressure modified RKS method that I had used in other parts of the plant. This was a trayed tower, and a good match was achieved at relatively low overall efficiency- meaning I needed few theoretical stages (relative to actual trays) between the top of the tower and upper feed point. I think 40% overall efficiency seems in the ballpark, although I can check my files if this is of interest.

While this result was fine for the energy study we were doing at the time, it was pointed out to me during review that there were some more "accurate" VLE correlations available for methane, hydrogen, and C2 systems. These correlations predicted more stages required, and therefore a higher overall efficiency. As a result of this realization, I caution against getting too serious about column efficiency numbers for which the full context is not known.

best wishes,
sshep