Hysys, ProII, and Aspen all have strengths and weaknesses. I am not technical enough to be able to provide a good analysis of the details.To the best of my knowledge, Aspen is widely used for chemicals, electrolytes, and solids. ProII seems to be the simulator of choice for refinery applications, though Hysys certainly has some following here. Hysys seems to be preferred for oil and gas work. This is all very general and no doubt some will disagree with these generalizations, but this fits with what I've seen and heard.