4 replies to this topic

[Art Montemayor]

Dayita:

There is a very, very important point that is at the heart of your exercise. I have to assume that you are a Chemical Engineering university student taking your first university course in Unit Operations. If that is correct, then you must have have had (or still having) Unit Operation – Distillation lectures and text book assignment lessons on the various ChE principles that lead to the process design of a distillation column. One of the results of such an exercise is that you predict the correct diameter, the number of mass transfer stages (trays or packing), and the height required for the indicated distillation column.

Therefore, it is not logical – nor credible – that you have been taught how to apply the McCabe Thiele to determine the theoretical number of transfer stages required without being instructed on how to convert this value to a realistic value. Is my thinking and logic all messed up here? Is there something missing from your basic question? To respond directly to your questions:

• How do I compare this number with values from the short cut method in hysys?
  Why compare? There is no logical reason to compare --- unless you are in possession of the total programming algorithm and procedure used by Hysys (which I seriously doubt!) No process engineer that I’ve ever worked with has the slightest accurate idea of what HySys has programmed as their procedure for the calculation – including all the factors and equations employed. So you can’t arbitrarily state that HySys is correct and McCabe-Thiele is not. You have no basis. At this moment, a lot of process engineers are putting their “trust” in Hysys – and that is all they can do because all they have from Hysys is the resulting printout, and no detailed explanation as to how the result was achieved.
• How do I determine the actual number of plates?
  You do this by applying the HETP method or use empirical factors – which ever method you have been taught or what the text books you have been assigned state.
• For calculating the height of the column, do I start from the HETP
  That’s a good start. I assume you mean the actual, real, physical required height for the overall column. If that is so, then you have to add heights due to practical, real-life necessities: manways, maintenance, 24” between trays for access, different feed points, vapor disengagement spaces at the top, feed points, and sump, bottom height to accommodate a reboiler and pumping equipment. In your case you have to furnish A COLD BOX, so you better allow for enough insulation – if you are smart.
• Does the HETP represent the efficiency of the column?
  Do your homework and read about the distillation theory and application. If you have learned how to apply the McCabe-Thiele method, then you should already know about what the HETP is and why, how is applied. The HETP doesn’t involve any energy input. The efficiency of the column is all about energy.
• Where do I get the packing efficiency?
  Again, do your homework and read about the distillation theory and application. The manufacturer is where I’ve always obtained this information.
• Does the HETP depend on the type of packing?

My fellow Texan, Ernest E. Ludwig, in his great and practical work, “Applied Process Design for Chemical and Petrochemical Plants – Volume 2”, explains the principle of HETP in detail and even gives examples on how it is applied. He writes, for example:

“Distillation operations can best be expressed in terms of equilibrium relations and theoretical plates. Therefore, one of the correlating factors for various packings is the height of packing equivalent to a theoretical plate for the separation. Data for effectively using this concept is extremely meager and apparently contains many uncertainties as far as general application is concerned. For this reason, the use of HETP is not popular. When good correlations are developed to predict HETP without test data, then this can be an effective means of expressing packing heights in distillation. Most HETP data has been obtained on small diameter (often laboratory size) columns using very small packing, and operated at essentially total reflux. The scale-up of such data to industrial sizes is questionable.”

I strongly urge you and all other Chemical Engineering students to obtain a copy of Ernie’s famous 3 volumes – all dealing with the practical design of chemical processing equipment. I know of no one who has done a better, more practical and realistic presentation of how to do it. Ernie is the EXPERT in this area because HE HAS BEEN THERE, and DONE IT – many times. He has the license to speak authoritatively on the subject. University professors and other text book authors have seldom – if ever – had the “hands-on” experience that Ernie amassed.

[breizh]

I've added a Xcel sheet to support your query . As Art mentions , here HETP is calculated using a correlation (NORTON) .
http://sol.rutgers.e...om20Packing.pdf



An other good reference is Wuithier "Raffinage et Genie Chimique"

Hope this helps

Breizh

Edited by breizh, 18 March 2012 - 06:33 PM.

[clarenceyue]

Dear Art & Dayita,

I would just like to add a short comment regarding Aspen-HYSYS's short-cut method which I believe Dayita was making reference to. If I remember rightly since the last time I used it (some 2 years ago), the short-cut column makes use of the FUG (Fenske-Underwood-Gilliland) method to determine the no. of theoretical stages. This might be of some help to you (Dayita), so you might actually be able to use the FUG equations and check with what HYSYS throws out to see if you can get similar answers.

Hope this helps.

Cheers,
Clarence

[Teck Guan Wan]

Hey Dayita, are you from nanyang technological university? Cryogenic air seperation huh