Your application sounds similar to Perchloric Hood application. FRP duct is an appropriate system for this application. Your should have one dedicated exhaust ductwork per hood (cannot combine exhaust ductwork into one main). Your duct system should be run vertically with only one horizonal offset allowed. Ductwork should be slopped back to the hood (in case you have the horizonal section). You must have sprinkler head for ductwork washdown. PP is not good enough for this application where requirements are for temperature, fire-retardant, corrosiveness together. Exiting velocity should be at 3000 feet per minute. You should be careful with entrainment issue so that you plan your exhaust stack high enough. Good luck.