It’s time to update all of you on my Senior Design Project again! I’m very happy to say my group and I have made some progress since last time. So far, we have the absorption column and distillation column controlled. The Ethylene Oxide reactor is almost finished, and the CO2 removal section is ready to be exported into Aspen for dynamic simulations. Today I want to talk to you about the distillation column. The control objective in the column is to ensure the product purity matches the required 99.5% by mass fraction. Also, the flow rate must be 50,000 metrics tones per year. We have tried many different scheme in order to best accomplish the goal, and we have settled on using a composition controller in concert with the reflux rate.
The most important aspect of this scheme is the incorporation of a composition controller than measures the product purity, and cascade that with a flow controller that manipulates the reflux flux that’s going back to the column. This way, if the product purity is too low, more reflux will be send back to the column for further purification. And if the product purity is creeping up, then the reflux rate will back down. A multiplier (ratio) was used (not shown) to better compare product flow rate to reflux flow rate.
All the other controllers were mostly standard. Since this distillation column is a partial vapor condenser with a total vapor distillate (there’s no liquid distillate, all liquid is sent back as reflux), it makes logically sense to control the column pressure by using the distillate valve. When the pressure of the column starts to build up, the valve will open to let more gas out to relieve that pressure. An alternative for pressure control would be to use the cooling water flow rate for the condenser, however, in this case the cooling water flow valve is used to control the condenser drum level. Since the reflux valve is already being used in cascade with the composition control, the condenser drum level must be controlled by the cooling water flow. More cooling water flow would condense more vapor, thus it will increase the level. When the level gets low, more cooling water will condense the vapor to maintain the level. The column sump level is controlled by the bottoms stream valve. We aren’t really concerned with the bottoms composition so composition control is not necessary. Last but not least, stage 14 temperature is controlled by the heat duty. If the column gets too cool more heat will be supplied to maintain column temperature profile.
It was quite a satisfying feeling after grappling with this column for about 4 weeks and finally be able to get it down. I was able to use a variation of the R/D scheme to control this column. Now I can move on to something more exciting things in this project.