McCabe-Thiele diagram for rating problem
The minimum reflux ratio (for specified product purities and feed conditions) occurs when an infinite number of trays are required to make the separation. Figure 2.27 shows the normal minimum reflux ratio situation. It occurs when the operating lines just intersect on the VLE curve. An infinite number of trays are required to step past the feed plate because of the "pinch" condition (the converging operating and VLE lines). The actual reflux ratio used must be higher than the minimum.
Increasing reflux ratio requires fewer trays (less capital cost) but increases energy costs. Economic optimization studies have led to the commonly used heuristic (rule of thumb) that the optimum actual reflux ratio is 1.1 to 1.2 times the minimum reflux ratio (see Figure 2.28).
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