Steady-State Design of Preflash Column

TBP Liq. Vol% Distillation Temp. C F) Light Ends Component Analysis Liq. Vol. Frac. API Curve Mid. Vol% Gravity 

The total crude feed to the preflash column is 100,000 B/D, with each of the two crudes (OIL-1 and OIL-2) set at 50,000 B/D. Their temperatures are 200 °F. The furnace outlet temperature is specified to be 450 °F, which requires a heat input of 203MMBtu/h (203 X 10 Btu/h). The crude is about 30 wt% vaporized in the furnace. 

Both a gas stream and a liquid distillate are removed from the reflux drum. In addition, since live steam is fed into the bottom of the column and separates into an aqueous phase in the reflux dmm, a water stream is removed from a small boot at the bottom of this drum that serves as a decanter. The stripping steam flow rate is 5000 Ib/h and its temperature is 400 °F. Both the gas and the liquid products contain some water. The water decanted is 2441b/h. 

The distillate rate is set at 20,000 B/D. This will be adjusted later to obtain a desired ASTM 95%pointof 375 °F for the liquid distillate product, which is a fight naphtha stream. Note that there is only one degree of freedom in this rectifying column since there is no reboiler. All of the vapor coming up the column comes from the partially vaporized furnace effluent. 

Opening the Pressure page tab (shown at the top of Fig. 11.21) permits setting pressures in the column. The pressure in the reflux drum is specified to be 39.7 psia. The condenser pressure drop is 2psi, so the pressure on Stage 2 is set at 41.7 psia. The pressure at the bottom of the column is specified to be 44.7 psia. 

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STEADY-STATE DESIGN OF PREFLASH COLUMN 327 65.9 MMBtu/h... 

The steady-state design is now complete. Figure 11.25a gives the flowsheet of the PREFLASH column with conditions and properties of the various streams. All of the flow rates are given in B/D at standard conditions. 

The 20% increase in feed flow resulted in the saturation of several control valves when the original design size was used (valves 50% open). The valve sizes were doubled to remove these limitations. The valves modified were V14 (LIGHTS from the preflash column), V25 (HNAPH), and V22 (AGO). The steady-state positions of these three valves are now about 25% open, as can be seen on the faceplates of the BChvynaph 95% boiling point controller, the PCI pressure controller in the preflash column, and the FCwash flow controller, which manipulates the AGO to hold constant the Stage 19 liquid flow rate (see Fig. 11.51). 

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