(62c) Incorrect Definitions and Uses of Tray/Packing Efficiencies in Process Simulator Codes

The Murphree efficiency concept (including Murphree component efficiencies and the Murphree tray efficiency, wherein all component efficiencies are equal) as well as the component vaporization efficiencies concept, were developed to improve upon the “equilibrium stage” assumption often used in the preliminary evaluation of a column’s separation performance. For packed columns, the HETP (or NTSM) is used to as a link between equilibrium stage calculations and the column packed height. Even with the availability of powerful and fast personal computers and the development of rate-based column calculations which obviate the need for efficiencies altogether, Murphree tray/component efficiencies, component vaporization efficiencies, and HETPs are still used widely in the study of column performance. First, they often supply “good enough” estimates of column temperature and composition profiles. Second, they allow for faster and more stable column calculations compared to those based on rate-based theory and its numerical implementation. And finally, they are often the only option available for carrying out process calculations under some simulation conditions.

Unfortunately a number of well-known and widely used process simulators contain incorrect formulations of these quantities. In particular this talk will focus on the following issues:

Improper use of Murphree and vaporization efficiencies.
Ability to set “N” efficiencies when only “N-1” are independent.
Inclusion of constant tray or section vaporization efficiencies when such a concept is impossible.
Allowing Σ𝐾𝑖𝑥𝑖≠1
HETP for packed columns defined incorrectly for binaries and calculations for multicomponent systems are baseless.