While setting up the reaction we put the phase as a mixture(i.e. Liquid and vapor). so the null stream 1 being a vapor stream so my doubt is how does it have zero mass flow in it?
Do the two outlets from a conversion reactor correspond to Liquid and Vapor?
DWSIM Creating-Multiple-Reactions-set 12-13 min 0-10 sec
To answer your first question is that although you specify the phase, however, the final phase of the product formed is re-calculated at the outlet temperature. So, if there is an existence of two phases at the outlet temperature, then only you get flows coming out of both streams.
For your second question, it is correct that two outlets from a conversion reactor correspond to Liquid and Vapor.