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Optimisation Of Distillation Column On Aspen Plus

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#1 rosesdaf

rosesdaf

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Posted 05 April 2022 - 09:25 AM

I am designing a distillation column and am optimising the total annualised cost by varying the number of stages and by varying the R/Rmin and by varying the inlet feed tray. I am very confused as to how I should go about with looking for the best combination of stages, R/Rmin and inlet feed tray. These variables are all dependent on each other.

 

I have started with the short cut method by hand, and I have obtained a certain reflux ratio but when I run it in rigorous simulation on Aspen plus, the purity in the distillate and bottom product differed from my target. Hence, I used design specification to fix my target purity by varying the reflux ratio. I got a reflux ratio much higher than the one I calculated from the short cut method, which is 2.15Rmin calculated from the short cut method. This deviated from the optimum R/Rmin range which is 1.1-1.5. I don't know why it is much higher.

 

The problem I have in looking into how the no. of stages affect the cost is that when I change the no. of stages on aspen, I am also required to input the position of the feed tray.  The position of the feed tray for a given number of stage can greatly affect how effective the separation is hence the operating and capital costs. f I keep the feed stage constant, the no of stages with the wrong feed stage would perform worse which of course would increase the cost. Hence I dont think it's fair to keep the feed position constant. But I dont know how I can figure out the inlet feed position for every no. of stage. Would it be appropriate if I use the kirkbride correlation to calculate the new feed tray location? Someone suggested that I could increase or decrease the feed tray by the same amount as when i increase or decreases the no. of stages from my original no. of stages. However, I feel like this is an unreliable way that I could not justify why this is appropriate. Someone suuggests that I divide my no. of stages by half to get the feed point which I dont think it's appropriate too. How can I figure out the feed location when I vary the no. of stage?



#2 Pilesar

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Posted 05 April 2022 - 10:55 AM

Your approach sounds reasonable. Short cut methods have limited use. Ignore the short cut method at this point as it has served its purpose to find a starting point for the rigorous model. The manually iterative method you are using to find the optimal configuration is legitimate and perhaps the fastest in the end. There are additional tricks for automation you can use. You can set tray efficiency as a variable. You can set feed tray location as a variable. These variables can be automatically adjusted within the AspenPlus optimizer to find a solution to the objective function. I think it wise to examine the automatically optimized solution carefully and verify it with additional manual iterations. Another option is to use the Sensitivity tool to define multiple case studies. You can graph the results and visually determine optimums. I learn much from case studies as I can view trends and influences better than by looking at a single answer provided by the optimizer.



#3 rosesdaf

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Posted 06 April 2022 - 04:55 AM

I was told that a way to figure out the optimum feed tray location is by looking for the tray with the same inlet feed temperature. The optimum feed tray location in terms of having the less condenser and reboiler duty is always at a higher temperature tray than the tray with the same temperature as the inlet feed. When I tried to use the stage with the same temperature as the inlet feed, aspen tells me that Design spec is not satisfied because one or more manipulated variable is at it’s bound. I guess it means that the feed tray I set would not be able to achieve the purity I fixed on design spec.

 

i have also tried to account for a plate efficiency of 70%. But the tray with similar temperature as the inlet feed temperature remains to be more or less the same no matter how many the no. of stages I have increased by. Can anyone guide me as to how I can match the feed tray and inlet feed temperatures?



#4 Pilesar

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Posted 06 April 2022 - 07:22 AM

You were "told that a way to figure out the optimum feed tray location is by looking for the tray with the same inlet feed temperature." Now you have to figure out if you are going to believe what you were told or believe conflicting evidence from your simulation model. Try to understand the underlying principles. A few votes from internet commentators won't be the same engineering prep. Once you have a valid simulation model, you can really learn a lot by observing effects from changing parameters.






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