Fractional Distillation The McCabe-Thiele Diagram

The feed to be fractionated, which can be a liquid, a vapor, or even a combination of both, enters the column at some central location rather tiian at one of the ends of the cascade as had previously been the case. This results in a division of the column into two parts, the rectification or enriching section above the feed tray, and the stripping or exhausting section below it. The upper section serves to enrich the vapor in the more volatile components, a portion of which is ultimately withdrawn as liquid overhead product or distillate. In the lower section, residual volatile components are progressively stripped off the liquid and conveyed upward as vapor, while the downward flow of liquid becomes enriched in fhe heavier or less-volatile components. 

The two streams entering the ends of the column are generated in separate vaporizers or reboilers and in condensers located at the two outlets. At the top, vapor leaving the column is condensed and returned in part to the column as reflux, while the remainder is withdrawn as distillate. At the bottom, the exiting liquid is partly revaporized in a reboiler and the vapor is diverted back into the column. The remainder of the stream is removed as bottom product or residue. 

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