This chapter describes how to build a McCabe-Thiele diagram
showing how many stages are required to achieve our separation
defined by:
and in addition:
We will set up a new data file for just benzene and toluene. The
following execution of start403a does this:
Give the name to be used for your data file. No more than 16 characters. bt1 The output file is called:bt1 Give the number of compounds: 2 If you want to see names, formulae or headings used in the 301 data base, execute the FORTRAN program: shownoh. Give the name for compound # 1 benzene Give the name for compound # 2 toluene Give the number of reactions to set or 0 to skip. 0 If you want to set plug flow reactor parms, reply:y
Next we create the outline of our McCabe-Thiele diagram:
>>start403b Copyright 1996 Rice University All rights reserved If you have not run the FORTRAN program start403a to produce a data file, do so now. <-- Specifying bt1 as the file How many streams will there be?3 Here are your compounds' names: benzene toluene
Choosing 110 kPa as our operating pressure, we
find:
>>help mcbt mcbt - McCabe-Thiele Diagram for compounds in data file function mcbt(p,n) p is the total pressure in kPa n is one minus the number of points on the curve. Example: start301a and start301b specify two compounds as toluene and benzene mcbt(200,25) >>mcbt(110,25)
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Next we find the minimum reflux ratio:
>>help rmin Rmin - minimum reflux ratio calculation function R=rmin(p,q,xf,xd) Binary system min R=L/D p is the pressure in kPa q is the quality of the feed: 0 for sat liq., 1 for sat vap. xf is the feed mol frac of the 1st comp. xd if the mol fraction of the 1st comp. in the distillate. >>Rmn=rmin(110,0,0.8009,0.9997) Rmn = 0.8207
Then we find the points on the operating lines
for a reflux ratio 20% higher than the minimum:
>>help oplines Oplines - Calculate vectors for plotting operating lines. function [LoV,DxDoV,LpoVp,BxboVp,xo,yo]=oplines(R,q,xf,xd,xb) Binary Ideal Distillation R is the reflux ratio: L/D q is the fraction vapor in the feed xf, xd, xb are the mol fractions of the 1st compound in the feed, distillate & bottoms. LoV = L/V DxDoV = D*xd/V Top op line: y=LoV*x+DxDoV LpoVp = Lp/Vp BxboVp = B*xb/Vp Bottom op line = y=LpoVp*x-BxboVp xo and yo vectors to plot for operating lines. >>[LoV,DxDoV,LpoVp,BxboVp,xo,yo]=oplines(1.2*Rmn,0,0.8009,0.9997,0.05);
We add the operating lines to our McCabe-Thiele plot:
>>hold Current plot held >>plot(xo,yo)
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It is obvious that we will need a lot of stages
in the top part of our column since the operating line and the
equilibrium line almost coincide. Let's see how many stages will be
required to achieve the separation.
>>[np,xs,ys]=ptop(1.2*Rmn,110,0,0.8009,0.9997,0.05,100); >>np np = 34.5226
We can add the steps to our plot by:
>>plot(xs,ys)
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