(38d) Slicing and Dicing: Revising the Thickness of Packing Slices to Use in a Rate-Based Model

At the 2024 AIChE Spring meeting, we posed this question: “How do you slice a column filled with packing?” Our main goal was to recommend the minimum stage height for the depth of packing in a non-equilibrium model of a packed column. Our standard suggestion prior to that investigation had been to use three rate-based stages for each equilibrium stage with a depth of packing in each one equal to one-third of the expected HETP. However, the dependence of this answer on HETP was unsatisfactory, especially since this required a priori knowledge of the HETP.

To answer this question, we had conducted an in-depth study to provide an initial recommendation for the value of the minimum stage height to be used in non-equilibrium column simulations of packed columns, a quantity we named the Height Equivalence of a Rate-Based Stage or HERBS. We highlighted that the use of a plug-flow model allows the use of a larger HERBS value. The results of our prior study led us to recommend that HERBS should be approximately 200 mm, with a reduction in that number for high-pressure systems.

Since that meeting, we have extended our study by including a wider range of chemical systems with different relative volatilities and densities to provide a broader basis for our minimum HERBS recommendations. In addition, we highlight the importance of using a theoretical plug-flow model for packed columns over approximate plug-flow models. This larger study explores the parameters investigated earlier, such as packing type and size, column pressure and ratio of the internal flow rates, as well as mass transfer coefficient models. We comment on the relationship between HERBS and these parameters and why we revise downwards our recommended value of HERBS.