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w _Li- log (0.995/0.005) (0.336/0.609) fi --log 2.05 " ° °

Ar log (0.609/0.336) (0.744/0.005) 0 9 » -Ï^XSi 9 2

N «■» Nn + Nm = 14.8 theoretical plates vs. 16 found on page 225 by stepwise calculation. Because (a/0)/ and i are near to 1.0, the geometric mean should be more conservative. By Eq. (12-21),

f m av iV„ + 1 - 7.5 TV™ - 11.2 Nn + Nm = 17.7 theoretical plates

The xylene in the distillate can be estimated by Eq. (12-15). For toluene relative to xylene,

1 JL XDX

An approximate check on the assumed feed-plate temperature can be obtained by assuming that the temperature gradient is linear from the still to the condenser.

From pages 222 and 225, tw - 116°C. and tD = 80°C.

as compared to the assumed temperature of 90°C.

The results given by these equations are only approximate, and their accuracy increases as the reflux ratio, O/D, increases. For reflux ratios near the minimum value, pf becomes equal to a, and the equations should be applied with caution because they give too few theoretical plates under these conditions. In fact, owing to the method of obtaining (a/fS)w, they can indicate a finite number of plates at values of the reflux ratio less than the true minimum. For values of (a/P) near to 1.0 a better average is obtained by i +

These averages force the equation to give an infinite number of plates for (a/0) = 1.0.

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