5 replies to this topic

[marcus]

Hi I was just wondering how the thermal state of feed, the q value and the q line for Mccabe, is calculated for given feed conditions other than those generally assumed by the typical q lines. e.g. q=1 or q=0 0<q<1 etc.

I understand q=(heat to vapourise feed)/(molar latent heat of feed)

So if given liquid of benzene at 15C in toluene with mole fraction 0.40 benzene in toluene is it possible to calculate q? I think i need to calculate q for heating to boiling point then q for vapourising, but im not entierly sure how. Also what would the boiling point be of this 0.40 feed mixture?

any help appreciated
thanks

[Zauberberg]

For a mixture, there is no single boiling point. There is a temperature range within which vaporization occurs, and it is defined by the initial boiling point (beginning of vaporization) and the final boiling point (end of vaporization).

If the feed is a mixture if liquid and vapor, "q" is the fraction that is liquid, i.e. q = 1-y, where y is molar fraction of vapor phase in the feed stream. That is visible from the q-equation you showed in your post.

To cut the story, if you want to know the q-value for your particular case, you need to calculate molar fraction of vapor at given conditions (15C, 0.4mol% benzene, 0.6mol% tolene, pressure is missing). Unless you are in deep vacuum conditions (e.g. P<5 kPa abs), your feed is subcooled liquid.

[flynner]

Hi there, to the best of my knowledge: First you need to calculate q. This is done using the following equation q = latent heat + (Specific heat capacity(boiing boint for Xf - Feed temperaure)) / latent heat
(temperatures are in deg Kelvin)


To draw the q line, your first point can be found where a vertical line from Xf intersects with the 45o line. To find a second point to draw the line use the following equation

Y = (q/(q-1))X - (Xf/(q-1)) Pick an arbitary value for X just a little to either side of Xf. You now have two points to draw the q line.

You will need to get your hands on a boiling point diagram for the benzene toulene mix at a specific pressure, but assuming a pressure of 101.325 kPa Xf at 0.4 would be about 93 oC or 366 kelvin

hope this helps, regards chris

[Andres Valencia Michaud]

Well, both replies are what you need. Anyways, you should take a look at "Unit Operations of Chemical Engineering" by McCabe-Smith chapter distillation (there are better books for this, but this one should do, plus is easy to get). I'm attaching an excel file I made for the same components some time ago. It's in spanish, but you'll get it very quickly.
Good luck!

Attached Files

•  ELV Benceno-Tolueno.xls   47.5KB   137 downloads

[mbeychok]

marcus:

You might also find it useful to read this article in the online Citizendium encyclopedia:

Click here ==> McCabe-Thiele method

[sheiko]

Hi there,

There is an interesting article in the December issue of Chemical Engineering Progress, that will certainly amswer your questions. Its name is:

Visualizing the McCabe-Thiele Diagram

Use this spreadsheet-based visualization and interactive analysis of the McCabe-Thiele diagram to understand the foundations of distillation engineering.

Paul M. Mathias
Appendices -— Visualizing the McCabe-Thiele Diagram

Appendix A. Function Interp; Appendix B. Details of Excel Calculations

Paul M. Mathias
Spreadsheet — Visualizing the McCabe-Thiele Diagram

This spreadsheet produces McCabe-Thiele diagrams for binary systems so that the interacting effect of process variables can be visualized easily and interactively.