modeling mass transfer in distillation columns Using CHEMCAD
Distillation columns are the ubiquitous workhorses of the chemical processing industry. Design and optimization of these unit operations is done at a much higher level of fidelity than ever before using mass transfer models.
As students, we learned McCabe-Thiele diagrams and shortcut methods to solve simple distillation systems. Often, a little more complexity was added using a spreadsheet to solve harder problems. Then, of course, we were given access to a simulation program which used more rigorous methods, including simultaneous correction and/or inside-out algorithms. When it came to actual columns in the field, however, they didn't always achieve quite what the equilibrium-stage based calculations predicted. You could specify an overall efficiency or even stage-by-stage efficiency profiles to get a better match, but this was a crude way of dealing with the issue.
Enter mass-transfer models, like Bravo & Fair for random packing, Bravo, Rocha & Fair for structured packing, and Billet & Schultes for random or structured packing. CHEMCAD includes these models, along with a database of common packing coefficients. For trayed columns, we include the Chan-Fair, AIChE, and Zuiderweg tray models.
Defining the geometry of such a column requires more user input, but the payoff is vastly improved accuracy and realism in the results for product streams, energy usage, and pressure drop.*
So, take a moment to back up a copy of a simulation with distillation column(s); convert them to mass-transfer by specifying the actual trays/packing you're using; and enjoy the benefits of more accurate answers to your distillation calculations. If you have questions, or if you want to share your successes, give our technical staff a call today!
* I should mention here that from what I've seen in papers delivered by distillation experts over the years, column-internals installation issues can cause significant deviation from expected results in the field. There are a number of companies that specialize in identifying such issues, and I invite you to check out our friends, Dr. Frank Seibert & Dr. Bruce Eldridge, over at the University of Texas Separations Research Program if you're interested in applied research on this topic and more.