(30e) Application of Air/Water Derived Structured and Random Packing Mass Transfer Models to Distillation

Frank Seibert

Separations Research Program

The University of Texas at Austin

Structured and high performance random packings are often used in distillation, absorption and stripping applications because of their high capacity and mass transfer efficiency along with low pressure drop compared to trayed internals. Packing efficiency is often described and modelled in terms of the volumetric mass transfer coefficient (Ka), the product of the overall mass transfer coefficient (K) and the effective mass transfer area (a). While the packing volumetric mass transfer coefficient is easily measured, the direct measurement of either the mass transfer coefficient or the effective area was a significant challenge. In 2002, the Separations Research Program (SRP) performed their first direct measurement of the effective mass transfer area using a structured packing. These measurements enabled the direct experimental determination of packing gas and liquid film mass transfer measurements. In collaboration with the Rochelle Texas Carbon Management Program with application to carbon dioxide absorption and stripping, the SRP developed an extensive Air/Water related packing database which includes measurements of effective mass transfer area, gas and liquid film mass transfer coefficients. Several UT-Austin Ph.D. students including Tsai (2010), Wang (2015) and Song (2017) developed generalized fundamental models based on the database. A question often asked is whether these Air/Water related models can be used to estimate the packing mass transfer efficiency in hydrocarbon distillation service?

In this presentation honoring Dr. Izak Nieuwoudt, the application of the latest generalized models developed by Song (2017) for structured packing and a new set of models for random packing will be tested with distillation data obtained from an equivalent diameter column. 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. Both sets of mass transfer models were developed using air/water related test mixtures with small variations in physical properties. The application of the Air/Water derived models will be challenged by the very different and wider range of distillation physical properties. The presentation also provides a methodology for estimating a packing HETP based on the general packing geometry, physical properties and flow data.

Tsai, R., Mass Transfer Area of Structured Packing, The University of Texas at Austin, Ph.D. Dissertation 2010.

Wang, C., Mass Transfer Coefficient and Effective Area of Packing, The University of Texas at Austin, Ph.D. Dissertation 2015.

Song, D., Effect of Viscosity on Liquid Film Mass Transfer for Packings, The University of Texas at Austin, Ph.D. Dissertation 2017.