HW 4 - p 5.38 - Degree of Freedom Analysis on a Distillation Process with Volatile Impurities - 3 pts
When the feed to a distillation column contains a small amount of impurities that are much more volatile then the desired distillate, it is possible to separate the volatile impurities from the distillate by removing the distillate as a liquid substream from a stage located several stages below the top stage. As shown in Figure 5.30, this additional top section of stages is referred to as a pasteurizing section.
(a) Determine the number of degrees of freedom for the unit.
(b) Determine a reasonable set of design variables.
(a) Determine the number of degrees of freedom for the unit.
(b) Determine a reasonable set of design variables.
posted by Dr. B at 2:33 PM
9 Comments:
Part be asks for a reasonable set of design variables. Does this mean we should say things like that each stage is adiabatic, or specify the exit T/P, or things of that nature until we get to 0 DF?
wheelie 3:17 PM:
Yes, you understand the question correctly.
Choose the variables that might be given in a design specification. There is definitely not a single correct set of variables that can/should be fixed. But some are more likely to be fixed by design requirements than others. For example, Qreb is not usually part of a design spec, but the composition of a feed or product stream often is.
I'm getting the # of variables to be 7N + 2NC + 5C + 20. Why does this look exactly like example 5.5? I can't tell what's different.
airhead 4:59 PM:
Hmmm... It is similar to Ex 5.5, but the HW problem has a partial condenser...which counts as an equilibrium stage. The HW problem also does not have a stream splitted on the distillate product because it has a partial reboiler with a vapor product.
Having said all that, I cannot tell what you did wrong. I got ...7N + 2NC + 4C + 16 variables. The reflux goes back on the top stage, so that is not unknown. Feed stage and stage where distillate are withdrawn are unknowns though. Come to office hours, mine or Joseph's and I am sure we can quickly clear this up.
Oh ok, I identified the pictures wrong. I got it now. Thanks.
airhead:
You are welcome.
Dr.B,
I have a question from the answer key about the EQUATIONS part. For external stream Distillate, To = Tm, Po= Pm and Xoi = Xm,i.
Then the total equations are C + 2. I don't understand how you got the C and which the "2" is from? The T,P?
Thanks,
studying_final:
Td = Tm is one equation. Pd = Pm is a 2nd eqn. There are C equations that look xdi = xmi...one eqn for each component. So, there are C of these equations. That gives us a total of C+2 extra equations that tell us that the distillate is the same as the liquid on stage M. I hope this helps. Those little letters are "d" for distillate, not "o". Maybe that helps too ?
thx Dr.B
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