The other common distillation economic heuristic is to select a reflux ratio that is 20% larger than the minimum reflux ratio. The minimum reflux ratio found in Chapter 3 from the simulation was 2.9. Multiplying this by 1.2 gives an actual reflux ratio of 3.48. This is very close to the reflux ratio (3.49) that we found was necessary in Chapter 3 for a 32-stage column.
It is interesting to compare the minimum reflux ratio found in the simulation with that predicted by the Underwood equations. These equations are derived assuming constant relative volatilities. As we have seen in the previous section, the relative volatility between propane and isobutane is almost constant. It varies from 1.85 to 1.76. Therefore, the Underwood equations should predict the minimum reflux ratio quite well.
As discussed in Chapter 2, there are two equations. The first is solved for a parameter 0 that is one of the roots of this equation
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