Top-PA
Naphtha
Water
Mid-PA
Steam
Kerosene
Bottom-PA
Steam
Diesel
Desalter
Crude atmospheric column
Preflash drum
Steam
Gasoil
Water
Crude preheat
Crude oil
Steam
Atmospheric residue
Crude heater
Figure 2 Preflash drum scheme with the drum overhead routed to a higher point in the atmospheric tower
rapid coking, and even tube overheating and rupture. The alternative is to use high-pressure booster pumps with expensive high-pressure piping and exchangers to prevent flashing upstream of the valves. In addition, vapourisation in the crude train dramatically increases the pressure drop, which may restrict the crude feed rate. Depending on the configuration, preflashing may also be valuable in debot - tlenecking the furnace and/or the atmospheric tower, espe - cially when processing lighter crudes (>30° API). Preflash devices can be located anywhere in the preheat train, with temperatures typically varying from 300°F to 500°F.1 , 2 Higher temperatures give higher preflashing rates. Preflash device pressure often ‘rides’ on the atmospheric tower pressure, but in some cases preflashing is performed at higher pressure by adding a control valve in the drum overhead vapour line. Preflash towers with condensers have their own pressure control systems. A key consideration is where the preflash drum overhead vapour is routed. In most crude trains, it is routed to the flash zone of the atmospheric tower (see Figure 1 ). In this configuration, it debottlenecks neither the furnace nor the tower. Its only merit, then, is to permit lower pressures to be used upstream of the furnace control valves. Any unload - ing it does on the furnace is countered by the need to add heat in the furnace to make up for the cooler drum over - head vapour bypassing the furnace into the flash zone of the atmospheric tower. The bypassing of lights raises the coil outlet temperature, raising the potential for coking or encountering metallurgical limitation.
Golden 3 presents a case of a unit processing 26.3° API crude, with the heater coil outlet temperature maintained at 700°F. Raising the preflash temperature from 275ºF to 400°F reduced the heater duty by 12% but increased the resid yield on the crude from 48.2% to 51.1%. The signif - icant loss of distillates to resid was because no heat was added in the furnace to make up for the cooler drum over - head vapours entering the atmospheric tower flash zone. If one wanted to keep the resid yield unchanged for the same increase in preflash temperature, the heater coil out - let temperature would have needed an increase of 22°F (to 722°F). An alternative configuration to debottleneck the furnace and atmospheric tower is to have the preflash drum over - head routed to a point further up in the atmospheric tower, as shown in Figure 2 . In this configuration, the preflash drum unloads the furnace and the section of the atmos - pheric tower below the point of entry of the preflash drum vapour into the atmospheric tower, which in Figure 2 is above the diesel draw. The maximum unloading is achieved with a preflash tower (or pre-fractionator), as shown in Figure 3 . This arrangement gives a large unloading both on the furnace and the entire atmospheric tower. With light crudes ( >30º API), debottlenecking of 10-20% can be achieved using a preflash tower. In some cases, some ker - osene can also be drawn from a preflash tower a few trays above the feed. It has been estimated that approximately 20% of the crude distillation units in North America include an independent crude preflash tower.2
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PTQ Q1 2024
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