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Figure 11.43 ASTM curves for cuts.

4. The fractional overflash specification is 0.03.

5. The four stripping steam flowrates are fixed (one to the base of the column and one to each of the three strippers).

6. The heat removals in the two pumparounds are fixed.

Let us explore the steady-state effects of changing some of these variables and establish some principles of operating a petroleum fractionator.

Effect of Changing a 95% Specification Suppose that we change the 95% boiling point specification on the heavy-naphtha stream from 375 to 350°F. The other degrees of freedom are unchanged. The result is a decrease in the heavy-naphtha flowrate from 6830 to 5425 b/d. The lower 95% boiling point means that a lower fraction of the crude is taken off as heavy naphtha. There is a corresponding reduction in the 5% boiling point of the kerosene, which changes from 395 to 380°F. This illustrates an important principle in the operation of sidestream petroleum fractionators.

Principle 11.1 The 95% and 5% boiling points of adjacent cuts cannot be independently set. For example, a decrease in the drawoff rate of a product results in a decrease

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