HW 4 - p 5.18 - Absorption of Light Hydrocarbons in n-Hexane - 6 pts
One million pound-moles per day of a gas of the following composition is to be absorbed by n-heptane at -30oF and 550 psia in an absorber having ten theoretical stages so as to absorb 50% of the ethane. Calculate the required flow rate of absorbent and the distribution, in lbmol/h, of all the components between the exiting gas and liquid streams.
posted by Dr. B at 2:35 PM
8 Comments:
You give us the figure (link doesnt work BTW) but since they give us the K values in the problem, cant we just use those and not use the nomagraph?
jeff 11:54 PM:
You are correct, you do not need the figure.
Thanks for the heads-up. I fixed the link.tl
where did the equation l3=A3*(v1+l2) come from on slide 6 of the 10-19 lecture presentation?
nevermind that last question
do you assume that there is no stripping at the top of the absorber?
I solved for Ae for ethane, and when I used it to solve for L with the equation A = L/(KV)in a mass balance over the 10 stages, I get the same number as the one given in your hints. However, I used V=the entering vapor instead of the exiting vapor which is what the definition calls for(since it was unknown). How can you use the entering vapor in an equilibrium situation in this assumption?
karen:
1-
l3=A3*(v1+l2) is the combination of the material balance on stages 1+2 and the equilibrium eqn for stage 3.
2-
OK, nevermind, but it was a reasonable question.
3-
There is no stripping anywhere in the absorber. Assume the nC7 is non-volatile and remains in the liquid phase.
jeff 3:45 PM:
You did the problem correctly.
EXCELLENT question.
We are assuming that A is constant throughout the column at its average value. So, for our first approximation, we assume L & V are both constant as well (just to estimate the As.
In principle, after you have gottent he answers that I posted, you should calculate the As at the top and again at the bottom and AVERAGE them. Then, calculate a new set of phi values and component molar flow rates.
But in this HW problem, I didn't ask/want you to go through all of that. But, in practice, you SHOULD.
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