Modular Shortcut Methods

The concepts of this generalized shortcut approach are outlined in this section 

As in other shortcut methods, the liquid and vapor molar flow rates are assumed constant in the column section  

In addition, the method assumes a constant equilibrium coefficient for each com- 

This assumption is more restrictive than the assumption of constant relative volatilities, or relative X-values, that is used in the Fenske and Underwood methods. The payback for this assumption is the ability to generalize the model to different degrees of column complexity. The success of the method is dependent on proper evaluation of effective /C-values or other model parameters that would represent actual behavior of the column section. The equilibrium coefficient is commonly lumped with the vapor and liquid molar flows in the column to define the stripping factor. 

The assumption of constant liquid and vapor molar flow may, in many situations, be grossly in error, especially in processes where mass transfer between the phases takes place mostly in one direction, such as in absorption and stripping. The method may, nevertheless, be used satisfactorily in such situations if appropriate stripping factors can be determined. This topic is addressed in more detail further along in this section. 

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In spite of the simplifications, these methods are still quite computation-intensive and in most cases must be solved using computer programs. However, due to the much shorter computing time, these methods are useful in situations where computing time is crucial, such as in online, real-time applications. Especially suited for these applications are the modular shortcut methods based on column sections— another topic discussed in this chapter. 

The modular shortcut column section method can also be applied to extractors, as described in Chapter 12. 

Shortcut computation methods, including modular techniques for online real-time applications, are discussed, followed by a discourse on the major rigorous algorithms in use for solving multi-component separations. The application of these methods is detailed for the various types of multistage separation processes discussed earlier. The models are also expanded to cover column dynamics. 

Thonpson, R. E., Shortcut Design Method-Minimum Reflux, AIChE Modular Instructions, Series B, Vol. 2, 5 (1981). 

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