## Xy diagram showing both stripping and rectifying operating lines

we simply move vertically up a line through xB until the VLE curve is intersected, and then read off the yB value.

A component balance around the reboiler gives:

This is the stripping operating line [equation (2.28)] applied to the reboiler stage. We know yB. We could plug into equation (2.31) and solve analytically for Jii. Alternatively, we can solve graphically for xx simply by moving horizontally on a straight line through yB until the operating line is intersected. Then this FIGURE 2.24

McCabe-Thiele diagram—stepping between VLE curve and operating lines to estimate number of trays required

### FIGURE 2.24

McCabe-Thiele diagram—stepping between VLE curve and operating lines to estimate number of trays required same procedure is applied again to the first tray. yy is determined from (move vertically to the VLE curve). x2 is determined from yv by moving horizontally to the stripping operating line.

This stepping procedure is continued up through the stripping section until the intersection of the operating lines is passed (see Figure 2.24). This determines the number of trays required in the stripping section. Then the rectifying operating line is used, and the stepping is continued until xD is reached. The number of trays in the rectifying section can be determined in this manner. Thus we can design a column (i.e., calculate the total number of trays Nr and the feed tray NF) using this graphical technique, having specified product compositions and operating line slopes. As we will show in the next section, the operating line slopes are both known if the feed thermal condition and reflux ratio have been specified.