I don't know anything about the process you are looking at but I do know a couple of things about HYSYS. I'm guessing that you are trying to absorb and react H2S in a solution of caustic. This means that you have an electrolyte system. The basic version of HYSYS will not simulate this. I believe there are options (at a cost) for simulating electrolytes but I really doubt HYSYS is the best software for this type of chemistry.