Effect of Errors in VLE on Efficiency

1 Effect of errors In Vapor-Liquid Equilibrium (VLE) on efficiency 

Errors in relative volatility are the most underrated factor that affects both tray and packing efficiency. The effects are direct when VLE errors affect separation stage requirement at a constant reflux ratio, and indirect when VLE errors affect the reflux ratio requirement (which in turn affects the stage requirement). Since higher relative volatility lowers both stage and reflux requirements (and vice versa), the direct and indirect effects complement each other and do not counteract each other. The discussion below applies to both tray and packed towers. 

Fieur 7.S Direct effect of errors in relative volatility on error in tray effidency. 

Most efficiency data reported in the literature are obtained at total reflux. At total reflux, there are no indirect effects, and Fig. 7.6 shows the overall effect of VLE errors on column efficiency. For measurements at finite reflux ratios, the indirect effects below add to those in Fig. 7.6. 

The indirect effects exponentially escalate as minimum refltix is approached, Small errors in VLE or reflux ratio measurement (this includes column material balance as well as refiux rate) silter Near minimum reflux, even small R/Rmin errors induce huge errors in the number of stages, and, therefore, in tray efficiency. Efficiency data obtained near minimum reflux are therefore meaningless. 

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