Feedback from our users is what inspires us to keep making CHEMCAD better. Many features, like this one, were added to the software as a direct response to user need. That’s why we consider every CHEMCAD user part of our development team.

From the beginnings of chemical process simulation (computer-based or by hand), ideal stage, VLE-based distillation models have been the workhorses used to design and optimize columns, but often required manipulations to validate against a real column. Users added overall or tray efficiency specifications, optimized BIPs, or simply used more/fewer stages than the "real" column.

Several years ago, we added mass transfer based distillation models to address this challenge, and our users have been reaping the benefits of improved accuracy versus real-world column conditions by calculating non-ideal stages (vapor is not at equilibrium with liquid). However, CHEMCAD had been assuming the temperature of the vapor was equal to the temperature of the liquid in the mass transfer model; in other words, the heat transfer calculation had not been taken into account and the energy balance was performed at each stage at the bulk enthalpy level.

Recently, Chemstations implemented these simultaneous mass and heat transfer calculations in CHEMCAD. For an even more accurate and rigorous model, the heat transfer calculation is done by including the additional variables and equations in the simultaneous correction distillation system (SCDS) solution matrix. With these changes, the energy balance will be calculated for vapor and liquid phases separately, with the conductive and convective effects of the enthalpy flow. The heat transfer coefficient can be derived from the Chilton-Colburn analogy with the mass transfer coefficients. For reference, a detailed analysis can be found in Taylor, Ross; Krishna, R. Multicomponent Mass Transfer, Wiley, 1993. Print.

Our internal testing versus known data and published examples has been extremely positive, as has been the feedback from our users. This model will work with both steady state and dynamic models in CHEMCAD, and it can be used with aqueous electrolyte systems and reactive distillation systems. In addition, it is available in the CC-BATCH module for batch distillation. Users can access calculated mass transfer and heat transfer coefficients for each stage/segment in reports.

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