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[dannb]

Hello,

 

So I am modeling the distillation of % 99.5 (by mass) ethanol from an ethanol/water mixture that is 8% by mass ethanol. I purified ethanol to the azeotrope (~%93.9 by mass) with one column (Radfrac), and then sent the distillate to a mixer where i added toluene as the entrainer for further distillation in a second column.

 

I have had a few issues with the second column:

 

1. I have determined the appropriate feed composition based on the ternary mixture results (x_water: 0.124, x_Toluene 0.136, x_Ethanol: 0.74) and adjusted the feed flowrates accordingly to obtain this composition, however, when i model the second column in DSTWU, i cannot obtain a positive value for the number of stages, or reflux ratio. Aspen suggests that this occurs when the mixture is separated too easily (always get negative stages / reflux ratio in DSTWU results).

 

2. I can obtain results (without errors) if i switch the component recovery fields (in DSTWU) to water as the light key, and ethanol in the heavy key. intuitively this does not make a lot of sense to me;  My knowledge would suggest that even after adding toluene to the mixture, that ethanol would still remain the light key, but again, I cannot get the simulation to run any other way.

 

3. Results of DSTWU with component recovery: LK (Water) = 0.95, HK(Ethanol) = 0.35, Reflux ratio: -1.5, T = 96.4 C (column 1 distillate temp = column 2 feed temp), P = 2 Bar are:

 

COLUMN 2 PERFORMANCE RESULTS

Actual Reflux Ratio: 4.271

Number of Actual Stages: 30.1

Feed Stage: 3.64

 

STREAM RESULTS:

Mass Fractions: 

Column 2 Feed (column 1 distillate + Toluene Stream): 0.046 (water), 0.257 (Toluene), 0.698 (Ethanol)

Column 2 Distillate: 0.080 (water), 0.472 (toluene), 0.449 (Ethanol) - mass flow = 44 kg/hr

Column 2 Bottoms: 0.004 (water), 380 PPM (Toluene), 0.995 (Ethanol) - mass flow = 88 kg/hr

 

from the stream results i can immediately see that although i am achieving the purity i want (in the bottoms no less), i am still loosing a great deal of ethanol to the distillate so this would definitely not be feasible.

 

4. With water as the LK, and Ethanol as the HK, I can reach the target purity, but the ethanol is in the bottoms, not the distillate (due to switching the component recovery designations).

 

5. The results i obtained (shown in 2.) by switching the component recoveries did not bode well when i transferred them to the inputs for the RadFrac method (the # of stages, and reflux ratio extracted from a DSTWU run did not hold as even rough estimates). 

 

Now, I am just looking for some guidance on where i might have gone wrong in my second column DSTWU configuration set up? particularly with the issue of component recovery designations in DSTWU? Any insight is greatly appreciated, I am a student by the way, forgive me if this problem seems trivial.

 

I have attached the results that I have obtained thus far and a pfd for reference. Thank you!

Attached Files

•  ceng124Prog3.docx   121.11KB   18 downloads
•  pfdProg3.PNG   9.55KB   2 downloads