Vacuum tower-top temperature

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. 

A sudden rise in vacuum tower top temperature, which occurs simultaneously with the onset of cold weather, can be due to freeze-ups of the seal legs. Plugging with waxy deposits is also possible. 

Befineiy Vacuum tower Top tower tempwature was reduced by ISOTby cooling top punqiaroond. T caused an unexpected rise in column pressure. The pressure rise was caused by the flash equililnium behavior of the precondensM-. When temperature was reduced, less liquid was condensed in die precondenser, and less U ts were absorbed. The absorption effect m be important in some systems. 

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. 

Heavy materials remaining at the bottom are called the bottoms, or residuum, and include such components as heavy fuel oil (see fuel oil) and asphaltic substances (see asphalt). Those fractions taken in liquid form from any level other than the very top or bottom are called sidestream products a product, such as propane, removed in vapor form from the top of the distillation tower is called overhead product. Distillation may take place in two stages first, die lighter fractions—gases, naphtha, and kerosene-are recovered at essentially atmospheric pressure next, the reamining crude is distilled at reduced pressure in a vacuum tower, causing the heavy lube fractions to distill at much lower temperatures than possible at... 

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. 

Severe naphthenic acid corrosion (in the form of pitting) has been experienced in the vacuum towers of crude distillation units in the temperature zone of 290 to 345 °C and sometimes as low as 230 °C. Attack is often limited to the inside and the very top of the outside surfaces of bubble caps. Figure 1.5... 

Example 16-7. Diameter of Vacuum Tower. See Example 17-11. Trays at 30 in. Pressure at top, 30 mm. Top tower temperature, 490 F. 

Crude oil fractionators are an example of a more elaborate system. They make several products as side streams and usually have some pumparound reflux in addition to top reflux which serve to optimize the diameter of the tower. Figure 3.13 is of such a tower operating under vacuum in order to keep the temperature below cracking conditions. The side streams, particularly those drawn off atmospheric towers, often are steam stripped in external towers hooked up to the main tower in order to remove lighter components. These strippers each have four or five trays, operate... 

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