I agree, I think you don't have to memorise it specifically for a Q in the HSC. However, for some people, memorizing the diagram (especially for Solvay) can be really useful when recalling equations. For me, I memorised a flow chart for Solvay, but not for the contact process. I made my own flow chart, using different sources, which was more personalised, but it still took me a while to remember. Also, a way to more easily remember the contact process (For me) is:
Fat Cows Are Dumb
F= furnace C= converter A= absorber D = diluter
Then, it becomes easier as you just have to memorise what occurs in each. I always had trouble just memorising the names
Hope this helps!