Application of Air/Water Derived Mass Transfer Models to Distillation

Structured packings are often used in distillation, absorption and stripping applications because of their high capacity and mass transfer efficiency with relatively low pressure drop compared to trays. Designers often refer to the packing mass transfer efficiency in terms of its height equivalent to a theoretical plate or HETP. This is a convenient term when working with process simulation output providing the number of equilibrium stages required to perform the separation. Unfortunately the HETP is mistakenly thought to be constant independent of physical properties and flowrates.

In this presentation, two recent structured packing mass transfer models developed by SRP researchers will be presented and compared with packed column distillation efficiency data. The distillation data are based on the cyclohexane/n-heptane test mixture operated at total reflux and four pressures (2.4, 4.83, 24 and 60 psia). An advantage of the test mixture is the pressure variation allows for changes in the liquid film mass transfer resistance which is often mistakenly neglected in distillation design. Both sets of mass transfer models were developed using air/water related test mixtures with relatively minor variations in overall physical properties. The application of the air/water derived models will be challenged by the wide range of distillation physical properties. The presentation also provides a methodology for estimating a packing HETP based on physical property and flow information obtained from a process simulation.