by jasper » 04 April 2016, 16:28
A recycle tolerance of 4e-3 is rather high, but still, if you evaluate all units in the flowsheet, starting at the cut stream, you should - at solution - not find any component flow, ln(T) or ln(P) (these are COFE's internal variables for a stream while solving) that change by a factor more than 1 plus or minus 4e-3 when evaluating all units in the recycle.
This does of course not imply that you are within a relative error of 4e-3 from the solution. The solver cannot determine this as it does not know the solution. As with most iterative solver the error is specified on the function evaluation, not on the solution.
Here's how to control stream 75, presuming you want to have it below the flow rate of stream 7:
- delete streams 201(2), 203(2) and the make-up mixer
- place a splitter, connect 7(2) as feed, and 8(2) as outlet 1
- create another stream to connect as outlet 2
- disconnect any of the streams between the start of 8(2) and 75(2). As these all have the same mass flow, it makes sense to disconnect close to the mixer to make the calculation recycle small. E.g. disconnect 8(2) from the pump.
- insert a measure unit, connect 8(2) as the feed, connect the product to the pump with a new stream.
- edit the measure unit, on the Measure tab, hit Add, add Total Mass Flow (or total mole flow, depending on your interest)
- insert a controller
- use an information stream to connect the measured mass flow to the controller
- double click the mixer. On the Ports tab, Port Operations, Add Virtual Information Port. Select Input, select Split Factors. As this is an array, you must specify the index of desire. Accept the default 0.
- connect the contoller variable of the controller to the new split factor info port of the splitter using an information stream
- double click this stream, set the initial guess of the split factor
- double click the controller to set the flow rate set point
- hit solve
It is not straight forward to specify something that makes sense; the system easily diverges to very high flow rates.