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This Example shows that the number of plates falls off rapidly at first, though more slowly later, and a value of R ~ 2 is probably the most economic.

For calculations of the type illustrated by Example 11.18 a convenient nomograph has recently been produced by Zanker(41) which relates R, Rm, n, and nm so that any variable may be quickly found if the other three are known.

Example 11.19

In the separation of a mixture of 100 kmol of hexane, heptane and octane, the flows and concentrations are:

hexane heptane octane

Relative volatility 2.70 2.22 1.0 Feed:

kmol 40 35 25

mole fraction 0.40 0.35 0.25 Overheads:

kmol 40 34 1

mole fraction 0.534 0.453 0.013 Bottom product:

kmol 0 1 24

mole fraction 0 0.04 0.96

Assuming operation for 8000 h/year, a plate efficiency of 0.95, allowable vapour velocities of u' = 2 x 10-3 and N'' = 1.35 x 10-3 kmol/m2 s respectively and the following incremental costs:

cb = £25/m2 year, both of which allow 50 per cent/year for depreciation, interest and maintenance.

and cd = £0.05/kmol based of an overall coefficient of 0.5 kW/m2 deg K and a temperature difference of 15 deg K in both the condenser and the reboiler.

Estimate the optimum reflux ratio.

Solution

The minimum reflux ratio, Rm is calculated using Underwood's method (Example 11.16) as 0.83 and, using Fenske's method, Example 11.17, the number of plates at total reflux is nm = 8. The following data have been taken from Figure 11.42, attributable to Gilliland(30) :

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