The reflux drum pressure is estimated first. This is the bubble point pressure of the top product at the maximum cooling water temperature. The flash zone pressure is then equal to reflux drum pressure plus pressure drop in the condenser overhead lines plus the pressure drop in the trays.
Before fixing the flash zone temperature, the bottom stripping steam quantity and overflash are fixed. The volume percentage of strip-out on crude is calculated using available correlations.8 If D is the sum of all distillate streams, V is percent of vaporization in the flash zone, OF is overflash, and ST is strip out, then
From the flash curve of the crude, the temperature at which this vaporization is achieved at flash zone pressure is determined. This temperature should not exceed the maximum permissible temperature. If it does, the quantity of overflash and stripping steam are changed until a permissible temperature is obtained.
The temperature at which a crude oil begin to undergo thermal decomposition varies from crude to crude, depending on its composition
(naphthenic, paraffinic, or aromatic base) and the trace metals present in the crude. Decomposition temperature can be determined only by actual test runs. For most paraffinic and naphthenic crudes, it is in the range of 650-670°F.
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