HW 5 - p 6.16 - Absorption of C1 and C2 from H2 Fuel Gas Using Oil- 9 pts
Several hydrogenation processes are being considered that will require hydrogen of 95% purity. A refinery stream of 800,000 scfm(at 32oC and 1 atm), currently being used for fuel and containing 72.5 mol% H2, 25 mol% CH4 and 2.5 mol% C2H6 is available. To convert this gas to the required purity, oil absorption, activated charcoal adsorption and membrane separation are being considered. For oil absorption, an available n-octane stream can be used as the absorbent. Because the 95% H2 must be delivered to a hydrogenation process at not less than 375 psia, it is proposed to operate the absorber at 400 psia and 100 oF. If at least 80% of the H2 fed to the absorber is the leave in the exit gas, determine:
(a) The minimum absorbent rate in gallons per minute.
(b) The actual absorbent rate if 1.5 times the minimum amount is used.
(c) The number of theoretical stages.
(d) The stage efficiency for each of the three species in the feed gas, using O’Connell correlation.
(e) The number of trays actually required.
(f) The composition of the exit gas, taking into account the stripping of octane.
(g) If the octane lost to the exit gas is not recovered, estimate the annual cost of this lost oil if the process operates 7,900 h/year and the octane is valued at $1.00/gal.
(a) The minimum absorbent rate in gallons per minute.
(b) The actual absorbent rate if 1.5 times the minimum amount is used.
(c) The number of theoretical stages.
(d) The stage efficiency for each of the three species in the feed gas, using O’Connell correlation.
(e) The number of trays actually required.
(f) The composition of the exit gas, taking into account the stripping of octane.
(g) If the octane lost to the exit gas is not recovered, estimate the annual cost of this lost oil if the process operates 7,900 h/year and the octane is valued at $1.00/gal.
posted by Dr. B at 2:40 PM
11 Comments:
I updated the hints for this problem on the course website on 11/2 at 9:22 AM. Here is what I added.
Part (a)
Methane is the key component in this problem. This is equivalent to using a methane XY Diagram to determine Lmin (see Figure 6.9 in the textbook).
Part (c)
I used Solver in Excel to determine A(CH4).
Part (f)
I used K(C8) = 0.003.
I hope this helps.
For the O'Connell correlation, is there someplace special we are supposed to get the data for density and viscosity for H2, CH4 and C2H6? I cant find anything in Perry's for them.
bloop 2:09 PM:
Good question.
I used the the properties of liquid n-octane for the liquid phase since it is pure at the inlet and mostly nC8 at the outlet. I think you can find density and viscosity of nC8 in Perry's. I even used the MW of nC8 as the average liquid MW ! Let me know if you have trouble finding the properties of nC8.
Part d says to do the O'Connell correlation to find "the stage efficiency for each of the 3 components in the feed gas", which are H2, CH4 and C2H6. It doesnt say anything about the liquid feed (nC8). Is this one of those book edition contradictions again?
Bloop:
I don't think it is a problem between editions. You need the properties of the liquid to estimate the overall efficiency using O'Connell. Since the liquid is MOSTLY nC8, you can use the properties of nC8 in O'Connell to estimate the efficiencies for the solutes.
Good luck !
Are the % compositions in the feed gas vol% or mol5? Then, likewise, for exit gas being 95% H2, does that mean mol or vol %?
I figured out what I was doing wrong, sometimes it helps to do more than just glance at the eqns! :)
getting there:
For ideal gases vol% and mol% are the same thing.
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bloop:
Yes, I have been burned that way a few times myself. Glad you are good to go now.
trista:
That is a good question!
Use the KEY component, CH4.
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