HW 8 - 14.3 - Separation of N2 from CH4 Using an Assymetric Polyimide Membrane - 6 pts
Please post all of your questions about this problem as comments on this post. I will respond with my own comments.
posted by Dr. B at 8:25 AM
10 Comments:
Prof. B,
How did you define your driving force? In the book they ask you to define it in terms of partial pressures which adds an additional set of variables to the problem.
kyle:
The driving force is the difference in partial pressure across the membrane. Since these driving forces (for N2 and CH4) are not exactly the same at the inlet and outlet, I used the arithmetic average of the driving force at the inlet and outlet.
Arithmetic mean? You said "I used the arithmetic average of the driving force at the inlet and outlet." Isn't the driving force the the differences in the partial pressures of each species at the inlet and the outlet? And each species has its own driving force, correct? What are we supposed to do about the CH4? We don't know the flow rate at the outlet so we can't find the partial pressures there. What do they mean by "with the permeate-side partial pressures at the exit condition." I'm just confused by what they mean by "driving force" and take the arithmetic mean of what?
yes, the driving force is the difference in the partial pressure across the membrae. Yes, there is a different driving force for each species. Yes, there is a different driving force for each species at the inlet and the outlet. One delta-P is feed-permeate, the other is retentate-permeate. Average these 2 delta-Ps and repeat for the other species.
How are we supposed to determine the CH4 kmol/hr? Or is that one of the unknowns that we're supposed to solve for using the two equations/two unknowns?
Does this problem is a 5 variable 5 equation problem? Or there is the way to do this in easier way?
Anon:
It could be. It depends on how you set it up. I set ip up as 1 eqn in 1 unk...sort of.
I guessed the CH4 flow rate in the retentate. I then solved the material balance on CH4 for its permeate flow rate. Then I did the solver LHS-RHS % error method on the CH4 membrane flux eqn to adjust my guess of R(CH4) until the flux of CH4 based on the guessed value of R(CH4) matched the flux based on the permeance and the driving force.
I hope this helps.
I am using the equation in the book, namely:
Ni=Pbari(driving force)
I am multiplying Pbar by 10^-11 because unlike what you gave us the book says it is in cmHg, not mmHg. I am also dividing be the volume that the gas would take ar STP. Are there any other little tricks that you haven't bothered to mention because I am still getting the wrong answer. Way wrong.
gwydion,
look below that example problem and it gives other unit conversions for a barrier and one is in kpa.kmol,m it might be a little easier to use this one instead of converting.
gwydion:
Unit conversions are not tricks. They are part of life. These unit conversions are unusually nasty. I am not sure what you mean about cmHg and mmHg. Anonymous gave you some good advice for the unit conversions. I hope you figured it out.
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