Flash tower

Refinery group" = low pressure naphtha fractionators, gasoline splitters, crude flash towers, etc. 

The ethylene resides in the reactor about two minutes, and as it polymerizes, it remains dissolved in the cyclohexane. To keep the concentration of polyethylene in the cyclohexane at 35-40%, a solution of the feed, solvent, and product are continuously drawn off. Downstream, the ethylene flashes off to be recycled in a flash tower. A precipitator removes the polyethylene from the cyclohexane by centrifuge. The polyethylene is steam-stripped to remove any remaining cyclohexane, then dried, extruded, pelletized, and packaged. 

Notice that there is no reboiler in this flash tower. All the heat input comes from the partially vaporized crude. Both the temperature and the percent vaporization of the crude are fixed. Hence, the external heat input to the preflash tower is constant. The pounds of vapor flowing to the bottom tray must also be constant. 

Steam coils at the top of the tower maintain the specified temperature gradient across the rotating disc contactor. The higher temperature in the top section of the rotating disc contactor results in separation of the less soluble heavier material from the deasphalted oil mix and provides internal reflux, which improves the separation. The deasphalted oil mix leaves the top of the rotating disc contactor tower. It flows to an evaporator where it is heated to vaporize a portion of the solvent. It then flows into the high pressure flash tower where high pressure solvent vapors are taken overhead. 

The raffinate phase containing asphalt and a small amount of solvent flows from the bottom of the rotating disc contactor to the asphalt mix heater. The hot, two phase asphalt mix from the heater is flashed in the asphalt mix flash tower where solvent vapor is taken overhead, condensed, and collected in the low pressure solvent receiver. The remaining asphalt mix flows to the asphalt stripper where the remaining solvent is stripped overhead with superheated steam. The asphalt stripper overhead vapors are combined with the overhead from the deasphalted oil stripper, condensed, and collected in the stripper drum. The asphalt product is pumped from the stripper and is cooled by generating low pressure steam. 

Deasphalted oil containing solvent is heated through a heating furnace, and fed to the deasphalted oil flash tower where most of the solvent is separated under pressure. Deasphalted oil still containing a small amount of solvent is again heated and fed to the stripper, where the remaining solvent is completely removed. 

Asphalt is withdrawn from the bottom of the extractor. Since this asphalt contains a small amount of solvent, it is heated through a furnace and fed to the flash tower to remove most of the solvent. Asphalt is then sent to the asphalt stripper, where the remaining portion of solvent is completely removed. 

Solvent recovered from the deasphalted oil and asphalt flash towers is cooled and condensed into liquid, and sent to a solvent tank. The solvent vapor leaving both strippers is cooled to remove water and compressed for condensation. The condensed solvent is then sent to the solvent tank for further recycling. 

The topped crude is typically reduced by two-thirds of the total naphtha cut. The bottoms from the second pre-flash tower are sent to the charge heater and directed to the main distillation column and produce four product streams heavy naphtha, several kerosene cuts, and bottoms residue. The residue is further processed in a vacuum column and produces VGO and several distillate streams. 

Figure 3. Acetaldehyde from ethylene, two-stage process (a) reactor, (b) flash tower,...

Specialised units are used to simulate complex fractionation processes in petroleum refining. Typical configuration consists of a main column with pump-around and side strippers (Fig. 3.14). Among applications, we may cite pre-flash tower, crude atmospheric distillation, or Fluid Catalytic Cracking (FCC) main fractionator. 

Preheat Train, Crude Heaters, Crude Tower, Atmospheric Flash Tower, Syn Tower, HCC Heaters, Reactor, Kiln, Steam Coils, Air Blower, Cyclone, Main Fractionator, Unsaturated Gas Plant... 

In the extract recovery section severe corrosion can occur, especially where high flow turbulence is encountered. As a result, certain components reqnire selective alloying with Type 316 (S31600) stainless steel. Typically, stainless steel hners are reqnired for the top of the dryer tower, the entire phenol flash tower, and varions condenser shells and separator drums that handle phenohc water. 

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