In order to fractionate the crude oil into the various products, it has to be heated to a temperature between 330 and 385 C (626 and 725 F), depending on the crude composition. The partially vaporized crude is transferred to the flash zone of the column located at a point lower down the column.
The furnace outlet temperature should be enough to vaporize all products withdrawn above the flash zone plus about 3–5 vol% of the bottom product. This over flash has the function of providing liquid wash to the vapors going up the column from the flash zone, and improving fractionation on the trays above the flash zone, thereby improving the quality of the HGO and reducing the overlap with the bottom products below the flash zone. This necessitates that there must be few trays in the region between the flash zone and theHGO drawoff.
The overflash provides heat input to the column in excess to that needed to distill the overhead products. It also prevents coke deposition on the trays in the wash zone.
The furnace outlet temperature is controlled to keep coking inside the furnace tubes and in the column flash zone to a minimum. However, the composition of the crude plays a part in determining the maximum temperature allowed.
Paraffinic crude oils cracks more readily than an aromatic or asphalt-base crude. Therefore, the furnace outlet temperature for paraffinic crude oils is lower than that for other crude types.