Distillation overflash

TBP cut point between the heavy distillate and the bottoms = 650°F. Percent overflash = 2 volume percent of feed. 

Column pressure at the reflux drum is established so as to condense totally the overhead vapor or some fraction thereof. Flash-zone pressure is approximately 69 kPa (10 psia) higher. Crude oil feed temperature at flash-zone pressure must be sufficient to vaporize the total distillates plus the overflash, which is necessary to provide reflux between the lowest sidestream-product drawoff tray and the flash zone. Calculations are made by using the crude oil EFV curve corrected for pressure. For the example being considered, percent vaporized at the flash zone must be 53.1 percent of the feed. 

The intention of defining key indicators is to describe the process and energy performance with a small number of operating parameters. A key indicator can be simply an operation parameter. Some examples of key indicators are reaction temperature, distillation temperature and pressure, column overhead (ovhd) reflux ratio, column overflash, spillback of a pump, heat exchanger U value, and so on. The parameter identified as a key indicator is important due to its significant effect on process and energy performance. 

Heater Outlet Temperature This temperature can affect the lift of diesel out of UCO. A too low heater outlet temperature would cause a slump of diesel into UCO, which degrades the value of diesel into fuel oil. Too high of a heater outlet temperature will cause unnecessarily high reflux rate at the expense of extra heater duty. The heater outlet temperature is mainly a function of the hydrocarbon partial pressure in the flash zone because the distillation cut point is a function of the diesel distillation specification and fractionation efficiency. An optimal heater outlet temperature could be determined by the fractionation overflash, which measures the internal reflux rate. 

The wash section cleans entrained liquids from the flash zone vapor phase. Vapor in excess of the amount needed to meet distillate requirements is referred to as overflash. The wash section condenses the overflash. It also provides some fractionation between the heavy lube sidestream and the vacuum resid stream. 

Because of the continuously increasing temperature of crude oil as more and more of the material is vaporized and because the charge is usually heated to its maximum allowable temperature prior to entering the tower, it is not feasible to use a reboiier in crude distillation. Thus, all distillate products plus the overflash must be vaporized up from the flash zone. Another way of stating the same principle is that the total heat input required by the process must be contained in the feed as it leaves the crude furnace. 

Total distillate yield is the total hydrocarbon vapor leaving the flash zone minus the overflash. 

Now, the ways in which one can define yields and, hence, the overall material balance are to be considered. It should be remembered that, to this point, the hydrocarbon vapor leaving the flash zone and the overflash have been calculated. Thus, the total distillate yield is known. In discussing the various ways for estimating yields, the following terminology will be used. F ure 2 r4 illustrates the physical significance of these terms. 

This process will be cation distillates, ligh heavy naphtha, light dis as sidedraws, Atmospher as a sidedraw liquid as crude vaporization minus + overflash). To insure screams, product strippi tower bottoms is specifi the tower bottoms will b steam at a rate of 10 lb Heavy distillate and all reboil-stripped removing equivalent to steam stri net stripped liquid. 

XO= TOTAL DISTILLATE PRODUCTS EXCLUSIVE OF OVERFLASH, Lo= OVERFLASH. 

Vapor leaving the flash zone including total distillate products, overflash, steam, air and hydrocarbon decomposition gas. 

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