In our most current Ethylene Oxide production process, the fresh ethylene stream is first mixed with recycled ethylene stream. The mixed ethylene is then combined with fresh oxygen feed at 25C to form a final feed stream of the mixture at 22.7C. Since the feed needs to be heated up to reaction temperature before enter the reactor, and reactor effluent needs to be cooled down, we decided to use a unit called the Feed Effluent Heat Changer (FEHE).
Basically, the way FEHE works is that it preheats the feed by using the heat generated from the exothermic reaction in the reactor. It then cools the hot effluent from the reactor by using the cold fresh feed. FEHE is widely used in high-temperature exothermic adiabatic tubular reactor systems to conserve energy. It applies very well to our system because the partial combustion of ethylene to form EO is very exothermic(generate a lot heat). The reaction occurs in the gas phase, so a tubular Plug Flow Reactor(PFR) is chosen. From a capital-investment and energy standpoint, FEHE is a lot efficient than a network of heater and coolers.. Since it’s a gas to gas heat transfer, the overall heat transfer coefficient is 0.17 kWm-2K-1.
Also, the reason why we need to preheat the inlet stream to a certain minimum temperature is that with a tubular reactor, if the feed temperature is too low, the reactor will quench. Quench means that the reaction will move a low-conversion steady state. However, separate heaters and coolers are easier to control from the controller design standpoint. FEHE is generally harder to control and can give us dynamic control problems. In next semester, we’re going to have to implement the system control concepts into out process since it’s going to be dynamic system with disturbances being introduced. This semester we’re only worrying about the steady state behavior of our process.
Here a picture comparing independent heating and cooling vs. FEHE.
And here’s what we have in our EO process flow diagram.