Hello everyone. I am trying to find the min. no. of trays in a short-cut distillation column (DSTWU) using Aspen Hysys for a column with 6.42 atm pressure, but after simulation run gets completed, it shows a message that "A min. reflux ratio less than zero has been calculated". Can anybody please help me in understanding this.
The attached document from Dupont may be useful to you.
I am not familiar with this particular shortcut distillation method but it is often difficult to get distillations to converge the first time they are set up. Using easy constraints like volume flows to get the simulation to converge on any solution and gradually replacing constrains with real constraints.
Dear Asma,
DSTWU operates on the assumption of constant molar flow, and constant relative volatility between light and heavy keys. Aspen Utilizes the method of wind, underwood, to calculate min, no. of stages, the minimum reflux and the appropriate number of stages for a given reflux, or the reflux for a given no. of stages for separation. Assume you want a reflux ratio of 2.5 times of minimum reflux ratio. you will enter -2.5 in the reflux ratio box for the calculation of min no of stages.
Thanks,
@Rick Manner Sir. Thanks for your effort, but I am practically trying to find min. no. of stages in Aspen Hysys which is having an issue of reflux ratio less than zero.
Dear Asma,
When you have to set actual reflux ratio, which is Ractual=xRmin. The Aspen takes that min reflux as negative by default just to make a difference between actual reflux and minimum reflux.
Thanks,
Try to change a bit the number of RecovL or RecovH, then it will run!! If you want send me your file and I can do it for you!!!
@Panagiotis Kagioulis. Thank you so much sir for your suggestion. It worked.
A negative minimum reflux ratio means that your feed composition is GREATER Than the distillate purity. e.g. the light component in the feed is maybe 80% but you entered a distillate composition of 70%. The same applied for the bottoms composition. Maybe your feed is at 50% but you entered a bottoms purity of 60%.
Check your input for a typo.
Dear Asma,
I recommend using the module for the conceptual design of distillation columns of Aspen Plus (ConSep).
If you wish, I can send you some exercises that I developed in advanced separation operations course with Dr. Carl Lira as the teacher. Write me at [email protected].
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