Non-key component

Minimum total reflux (lbs or mols/hr) corresponding to given total feed will be greater than if only the actual total mols of heavy and light key components were present. Reflux need will be less than if the actual total mols of feed were present, but composed only of light and heavy keys. The more closely non-keyed components are clustered to volatilities of the keys, the nearer are reflux needs to that calculated for the binary and total feed volume. 

Split keys key components that are separated in volatilities by a non-key component, i.e., the system of components contains one or more whose volatilities fall between the volatilities of the designated keys. 

The recoveries of the non-key components are estimated by first calculating the correlation constants ... 

Repeating Equations 8-126 and 8-127 for each of the other non-key components in the feed mixture gives the results shown in Table 8-4. Good agreement was demonstrated. 

Plot relative volatility (cq) and % desired recovery for LK and HK Draw a straight line through these two points. The non-key component points will also be on this straight line. 

Using Qi and the component distribution line, estimate % recovery of non-key components in distillate and bottoms. 

Figure 8-50. Estimation of recovery of non-key components using short-cut method of Yaws, et al. Used by permission. Yaws, C. L. et al., Chem. Eng., Jan. 29 (1979), p. 101.

The non-key components that appear in both top and bottom products are known as distributed components and those that are not present, to any significant extent, in one or other product, are known as non-distributed components. 

If the presence of the other components does not significantly affect the volatility of the key components, the keys can be treated as a pseudo-binary pair. The number of stages can then be calculated using a McCabe-Thiele diagram, or the other methods developed for binary systems. This simplification can often be made when the amount of the non-key components is small, or where the components form near-ideal mixtures. 

But the composition of the non-key components on the plate does not match the feed composition. 

It will be necessary to estimate the concentration of the non-key components in the bottom product as a first trial take ... 

Component splitters are sometimes used in place of distillation columns when building simple models to provide initial estimates for processes with multiple recycles. There is little advantage to this approach compared with using shortcut distillation models, as the component splitter will not calculate the distribution of non-key components unless a recovery is entered for each. Estimating and entering the recoveries for every component is difficult and tedious, and poor estimates of recoveries can lead to poor estimates of recycle flows, so the use of component splitters in this context effectively adds another layer of iteration to the model. 

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