(1) The fresh propylene feed steam contains some propane impurity, which is inert in the reactor and must have a place to get out of the process. Since the separation of propylene and propane is difficult, the economics strongly favor designing the reactor for a very high conversion of propylene. The propane and any un reacted propylene are flashed off and burned, so they only have fuel value. High propylene conversion can be achieved by either running at high temperatures or increasing the size of the reactor. The former increases the production of undesirable byproduct. The latter increases capital cost.