Overview
The final class for BYU Chemcial Engineers is Plant Design, a part of which is a two month project where teams of 4 students design and deliver a report for constructing a chemical plant.
On February 9th of 2021, my team set out to devise a dimethyl ether (DME) production plant that would produce 50,000 metric tons per year of 99.9 wt. % DME from methanol feedstock.
With a 16.5% rate of return on a $25.5M investment, our plant was found to be a profitable investment over 20 years of operation.
My part of the project was safety considerations, designing the reactor, validating the thermodynamics of tertiary system for separation considerations, and simulations (Aspen HYSYS).
Reactor Design
The catalyst-packed reaction vessel was designed to withstand pressures above 10 bar. The kinetics of the reaction followed research conducted by Berićić and Levec. A single pass conversion of ~85% was achieved; this was essential for the separation system. Too high of a conversion would've led to an unwanted phase separation in the 1st distillation column.
Separation Considerations
In conjunction with mass transport, thermodynamics lay at the heart of separation operations. Therefore, having an accurate thermodynamic model was essential to simulating real system dynamics. We sought out published thermodynamic constants for the Non-Random Two Liquid (NRTL) model because of the additional phase possibility between DME and water in the absence of an alcohol.
The separation system was comprised of 2 distillation columns at staggered pressures to bolster product purity and optimize column duty expenses. Rather than adding a refridgeration unit to chill the DME product, the column operating pressure allowed for product storage around room temperature. The bottoms product of column 1 was then fed to another separation unit for waste recycle.
While the simulation predicted a pure water waste bottoms product from column 2, we acknowledged that waste water should be retained and tested for impurities when commissioned prior to environmental release. With treatment, the water could be saved in a water reservoir and cooled for later use.
Considerations for column control involved the temperature profile of the column assuming isobaric operations. Thermocouples placed toward the middle of each column allows for corrective measures via integrated control (see Process Control experience for an example).
