Temperature Gradient per Theoretical Stage

For example, if at theoretical stage 15, temperature is 1.35°C hotter than Stage 16, and Stage 17 is 1.25°C cooler than Tray 16, the average response would be a shift of 1.30°C from the nearest tray. 

The method reported by Tolliver and McCune for finding the most responsive temperature point was also used for this cyclohexane/w-heptane system. A second computer simulation was run using the same reflux/feed ratio for the separation power base while shifting the D/F (distillate/feed) ratio up. The heavy key impurity concentration in the distillate was 

When the distillate heavy key impurity moved up from 0.0700% to 0.1000%, that is, a shift of 0.0300% C7, and the bottoms light key impurity moved down from 0.0700% to 0.0400%, that is, 0.0300% C6, the stage number 16 shifted up 12.4% C7. 

In this case, the stage 16 temperature moved 413 times more than the top temperature due to change in composition. Any significant temperature movement at the top or bottom of the column would be mostly due to changes in pressure. 

In addition to the two methods described for finding the MRT (most responsive temperature) point in a distillation column, there are three additional methods that can be used without a computer simulation. First, if there are temperature points in the column that are monitored, just look at the history record to see which temperature point moves the most. Second, find the temperature point in the column that is nearest to being halfway between the top temperature and the bottom temperature. Third, look in the area of the column where the vapor concentration would be about 50% light key component. For example, if the feed contains 10% light key component, there is a high probability that the MRT would be above the feed point. Similarly, if the feed contains 80% light key, there is a high probability that the MRT would be below the feed point. 

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