Precondenser outlet temperature

Regardless of ejector capacity, if the vacuum tower uses stripping steam, the pressure at the top of the tower cannot be lower than the vapor pressure of water at the precondenser outlet temperature. 

The colder the vapor outlet temperature from the precondenser, the better the vacuum he could pull, because of reduced vapor flow to the jet. Agreed ... 

The chief operator also insisted that lowering the vacuum tower-top temperature too much would hurt the vacuum. But why There is no doubt that the colder the tower-top temperature, the less the heat-duty load for the precondenser to absorb. Hence, cooling the vacuum tower-top temperature should, and did, reduce the precondenser vapor outlet temperature. This should have reduced the vapor load to the downstream jet. But it didn t. Here is why ... 

Of course, if the vacuum tower-top temperature became too high, the increase in the precondenser vapor outlet temperature would increase the vapor pressure of water. This factor would then limit the minimum pressure in the precondenser. 

Increasing cooling-water flow to the precondenser decreases the vapor outlet temperature." Agreed ... 

Low flash-zone temperature. Have the instrument mechanic check the furnace outlet thermocouple. The optimum tower top temperature for a vacuum tower equipped with a precondenser is usually not the minimum temjjerature. As the tower top temperature is raised, heavy naphtha boiling-range materials are flashed overhead into the precondenser. Acting as an absorption oil, they absorb a portion of the light hydrocarbons that would otherwise overload the jets. However, getting the vacuum tower top too hot can overload the precondensers. By field trials, find the tower top temperature (usually 230°F to 280°F), that minimizes flash-zone pressure. 

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