HW 6 - p 7.11 - McCabe - Thiele Analysis of Benzene-Toluene Distillation - 12 pts
A saturated-liquid mixture containing 70 mol% benzene and 30 mol% toluene is to be distilled at atmospheric pressure to produce a distillate of 80 mol% benzene. Calculate and tabulate:
(a) Moles of distillate per 100 moles of feed,
(b) Moles of total vapor generated per mole of distillate,
(c) Mole percent benzene in residue, and
(d) For each part, construct a x-y diagram. On this indicate the compositions of the overhead product, the reflux, and the composition of the residue.
(e) If the objective is to maximize total benzene recovery, which, if any, of these procedures is preferred?
Note: relative volatility equals 2.5.
(a) Moles of distillate per 100 moles of feed,
(b) Moles of total vapor generated per mole of distillate,
(c) Mole percent benzene in residue, and
(d) For each part, construct a x-y diagram. On this indicate the compositions of the overhead product, the reflux, and the composition of the residue.
(e) If the objective is to maximize total benzene recovery, which, if any, of these procedures is preferred?
Note: relative volatility equals 2.5.
posted by Dr. B at 2:45 PM
7 Comments:
Since the feed enters stage 1, you switch from the rectifying section operating line to the stripping section operating line on stage 1, not necessarily where the q-line and the operating lines meet. In this case, the feed has been put into the column on the wrong stage. Where SHOULD the feed enter the column ?
Scheme 5 is a bit tricky because you do not know x{b} and that makes it tough to locate the stripping section operating line.
You can find the equation for the stripping section operating line in the text or in my powerpoint. Use material balances on the feed stage and the condenser (use the known value of R as well) to eliminate Lbar and Vbar in the stripping section operating equation. The 4 unknowns I solved for were D, B, x{B} and y{B} (which is the y value of the vapor leaving the reboiler). Once you have used SOLVER to determine these 4 unknowns, you can step off all the stages. Be sure to minimize the SUM{%error^2} for your 4 equations by changing your 4 unknowns. It may help to set the precision to 1e-15 and the convergence to 1e-12 on the options tab in SOLVER.
I'm stuck on Scheme four, I don't know what you mean in your hint and I can't seem to figure it out. Can you use volitility to find y1 going into the partial condenser because it doesn't seem to be in equilibrium with x1 since xr is also flowing into stage 2. I'm confused...
Scheme #4 is best left thinking about conceptually. Think about it: is stage 1 (using the diagram from class) doing anything? We're adding liquid onto Stage 2. Is it possible for any liquid to acutally be coming off of stage 1? Think about what's going into stage 1. In light of that, Scheme 4 reduces down to a scheme that's already been solved for.
How can i find the alpha to put in 7-3? Can i read the X&Y at the equlibrium on the graph?
Or i suppost to assume the pressure and temperature to find K value of benzene and toluene and use the equation 7.1 to find alpha?
Anon:
Purple spelled this out correctly. Scheme 4 is really the same as scheme 2. The top stage in the column accomplishes NOTHING. No liquid will flow down from stage 1 to stage 2. Where would sucha liquid come from ? Assuming no heat transfer at stage 1 so the vapor cannot condense, there will be NO liquid on tray 1. It is just sitting there dry with hole in it and vapor flowing through. It is NOT an equilibrium stage or even a fraction of an equilibrium stage.
a 3:42 & 3:45
Alpha is given in the problem statement: alpha = 2.5.
Post a Comment
<< Home