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[cali1234]

Hello,

I'm working on designing a column that takes dimethyl ether with a small quantity of methanol to "propellant" grade DME with only 1 mg/kg methanol in the product. The output of the column needs to be 100 tons/ month, and feed composition is given below. I picked a reflux ratio = 1.2*Rmin and stepped off 12 stages on a zoomed-in McCabe-Thiele diagram before running an Aspen Plus simulation. From Aspen, I determined that I need 9 equilibrium stages (my drawing skills aren't the best), and have a calculated diameter of aprox 0.3 ft (which is really small, but the hand calculations match up...) so I think this should be a packed column. How do I determine the stage efficiency / HETP of a packed column? I'm using Sep. Process Principles by Seader (2ed) and looking at HTU and NTU equations, but I can't make sense of the HTU equation that depends on a mass transfer coefficient. Does the mass transfer coefficient depend on what type of packing ends up in the column as well as the V/L specifics(so I won't know what it is unless I assume a certain type of packing)? (If anyone has access to this book, I can't figure out what kxa/kya is so that I can solve for the Kya term in the HOG equation.) I'm inclined to use the rules of thumb mentioned in Perry's as a substitute. I would use equation 14-156 (Perry's 8th ed); HETP = 93/ap where ap is equal to surface area/ unit volume. Perry's says that "For smaller packings, use of the ap at 25mm often gives a slightly conservative HETP estimate." I'm assuming that my packing will need to be pretty small because the column diameter is so small. If I look over at Table 14-13 and get the area for 25mm plastic Super Intalox (which are saddles?), the area is 207m^2/m^3 giving an estimated HETP = 0.45 m - so my column would be 4.05m = 13.2 ft tall. Does this sound reasonable?? Given below is the feed and product stream info along with the feed stage and refulx ratio. I can provide other info as necessary. Any help or general guidance would be greatly appreciated! Thanks for your time.


Column Feed Stream:

Mole Flow lbmol/hr
DME 6.95473318
METHANOL 0.00063202
WATER 0.00029170
CO2 0.00027522
Temperature F 104.926433
Pressure psia 130.53

Product Stream:

Mole Flow lbmol/hr
DME 6.60748281
METHANOL 8.5257e-06
WATER 6.8124e-05
CO2 0.00027515

Reflux mol ratio = 0.19591837
feed stage = on stage 7
total condenser
column pressure = 130.53 psia (pressure drop neglected for now)

Edited by cali1234, 18 August 2011 - 06:52 PM.

[luizmoura]

Dear cali1234,

Unfotunately you need to choose a package because they differ from each other in terms of pressure losses.
I recommend you a look in this article or (as a first guess) to try a rule of thumb.

http://www.scielo.br...n3/a17v26n3.pdf

Sincerely,

Luiz