CAPE-OPEN

Determining the best feed stage
There are two methods of doing this task. For each case, it is necessary to be sure that that the column performance criteria are being met. The two methods are the constant reflux method and the constant performance method. The major difference between the methods is that with the constant reflux method, the reflux is held constant and the feed stage is varied. The feed stage that gives the best performance is selected as the feed stage. In the constant performance method, the Design-Specs are used to hold the column performance constant. The reflux (and perhaps some other variables such as the D:F ratio) is varied to maintain the constant performance. The feed stage with the minimum reflux ratio or the lowest energy requirement is selected as the best feed location. An example of the results from a constant performance study is shown below.
1. Does column performance criteria mean column design specifications like product purity or distillate as an example.
2.in the constant reflux method, can l specify reflux ratio and distillate purity and vary feed stage or feed stage is used as one of column specifications
3. What is meant by column performance constant
Thanks for your help, l am confused

You seem to be quoting from somewhere. It'd be better to say this clearly when you do that, and to state where you are quoting from (with a full reference). Without having the full reference, I can only indicate as to what was likely meant with keeping the column performance constant: obtaining a top (and/or bottom) product at a specific rate and purity. Since on a ordinary 2-cut column with condenser and reboiler you can only make two specifications you could try to do the parametric study while specifying a key components purity as well as the distillate (or when your feed varies in flow the distillate/feed rate). Note with two cut splitter a D/F rate spec can also be accomplished by means of a B/F spec, given that 1=B/F+D/F. For your information, I always do my column optimizations with the parametric study inside ChemSep, and only later optimize at the flowsheet level. Reason is that by looking at the McCabe-Thiele diagrams you can see what is happening and why performance is better or worse. I recommend you read chapter 13 in the 7th Ed. of Perry's handbook of Chem. Eng. on Distillation.

Googling the text, the quote is from here

www.che.udel.edu/pdf/Modeling_Separatio ... n_Plus.doc