methanol water control

[thubelihle]

hi. control loop 1 worked where by l was using reflux ratio to achieve the required distillate purity. i tried to use control strategy 2 to use the distillate rate to control bottoms product, but l get an error that control 2 should be part of the recycle. thanks for your help, attached is the file

[jasper]

The distillate rate cannot be predicted both by ChemSep and by the controller. It is an output of ChemSep, not an input, and therefore is not fed back into ChemSep.

You can however make it an input: change the reflux ratio top spec to distillation rate spec. You can now expose this as input parameter from ChemSep. Then make virtual information port with this input parameter. Then you can hook up the controller. Of course the reflex ratio spec is dropped then. But

- why would you do this and not directly control the reflux ratio instead?
- and why would you even do that, to obtain bottom water purity, and not directly make that the specification of the column?? (you can set a mole fraction purity directly as bottom spec).

[thubelihle]

i am solving the question attached as a picture. one of the requirements are as follows:
Set up DesignSpecs within the column so that there is:
99.95 wt% methanol in the distillate
99.90 wt% water in the bottom
Vary the distillate rate (800-1700 lbmol/hr)
i guess the reflux ratio is put as an input parameter to fix it, as if both the distillate and bottoms purity were put as input parameter, the reflux ratio will vary, correct me if l am wrong. l agree with specifying the bottoms purity, not sure what the author was thinking, maybe your experience may help me. the question is obtained from https://allaboutchem.files.wordpress.co ... adfrac.ppt. thanks

[jasper]

In any case, you cannot fix reflux ratio, distillate flow rate and purity. There aren't that many degrees of freedom.