In HYSYS, this can be easily done by introducing a "set point" block which would maintain a constant ratio between two streams which can be connected to a "mixer" block to produce a mixed stream. I'm certain that a similar operation should be available in ASPEN as well.

thats it ...

Thanks Subhasis

Use Aspen Plus instead. Moreover, in Aspen Pinch there is a priority module for this task

The easiest way in Aspen plus is to use a calculator block. It's a little bit more complicated than what you should do in Hysys (but still fairly easy). 

Thanks