Wednesday, October 29, 2014

Did you know you can model mass transfer in distillation columns in 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.

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