Feed stage alternate points

Alternatively, results from a computer simulation can be plotted to determine the optimum feed stage. Simulation runs ere performed at several different feed points, keeping the material balance, reflux ratio, and total number of stages constant. Ksy component concentrations in the product streams are plotted against the feed stage number (Fig. 3.7). The minimum is at the optimum feed stage. 

For binary systems, the pinch point usually occurs at the feed plate. When this occurs, an analytical solution for the limiting flows can be derived fKing. 19801 that is also valid for multiconponent systems as long as the pinch point occurs at the feed stage. Unfortunately, for multiconponent systems there will be separate pinch points in both the stripping and enriching sections if there are nondistributing conponents. In this case an alternative analysis procedure developed by Underwood (1248) is used to find the minimum reflux ratio. 

A limitation of the Erbar-Maddox, and similar empirical methods, is that they do not give the feed-point location. An estimate can be made by using the Fenske equation to calculate the number of stages in the rectifying and stripping sections separately, but this requires an estimate of the feed-point temperature. An alternative approach is to use the empirical equation given by Kirkbride (1944) ... 

Continuing with the procedure of defining equilibrium stages down the column, the pivot point is switched from D to B when the feed tray is crossed. Alternatively, the procedure could be started at the reboiler, defining stages up the column and switching from B to D as the feed tray is crossed. 

The idea of the simpler single-temperamre loop-control strategy is to use an aqueous reflux flowrate to hold some tray temperature inside the column and to ratio the other two manipulated variables (reboiler duty and entrainer makeup) to the feed flowrate. From the earlier sensitivity analysis in Figure 9.9a, the most sensitive control point inside the column is Stage 6, which is close to the top of the column. Since the main acetic acid product is drawn out from the bottom of the column, an alternative control point is to select the second most sensitive control point, which is closer to the bottom of the column. This alternative control point will be Stage 16. Thus two single-loop-control stmctuies will be evaluated using closed-loop dynamic tests. They are ... 

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