Pfsteam I

Figure 11.16 Preflash column.

Figure 11.16 Preflash column.

Figure 11.17 Types of petroleum (PetroFrac) columns.

Figure 11.18 Attaching feed to furnace.

Q Results d EO Variables

¿SI CRUDE2

frg NAPHTHA

B PF-STEAM

Q Results d EO Variables

¿SI CRUDE2

frg NAPHTHA

B PF-STEAM

D Utilities B gi Blocks

-Operating specifications

-Operating specifications

B-Qfl PREFLASH

Setup

Pumparounds Strippers Heaters Coolers

Setup

Pumparounds Strippers Heaters Coolers

Figure 11.19 Column setup.

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 drum, a water stream is removed from a small "boot" at the bottom of this drum that serves as a decanter. The stripping steam flowrate is 5000 lb/h, and its temperature is 400oF. Both the gas and the liquid products contain some water. The water decanted is 244 lb/h.

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

Clicking the Stream page tab opens the window shown in Figure 11.20, where the combined crude feedstream is specified to be fed to the furnace by using the dropdown menu. The stripping steam is fed on stage 10.

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 2 psi, 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.

Clicking the Condenser page tab opens the window shown at the bottom of Figure 11.21, where the condenser temperature is specified to be 170OF. This is high enough to permit the use of air-cooled condensers, which conserves the use of cooling water.

Clicking the Furnace page tab opens the window shown in Figure 11.22, on which the temperature is set at 450OF and the pressure is set at 44.7 psia. This is the same pressure as that at the bottom of the column. If the pressure is set at a higher value, the steady-state simulation will run, but an error will occur when exporting a pressure-driven dynamic simulation file.

The final job in the steady-state design is to achieve the desired specification of an ASTM 95% point of 375OF (ASTM D86). An initial guess of 20,000 b/d for the liquid distillate flowrate gives an ASTM 95% point of 353OF. This is lower than the specification,

Figure 11.20 Stream locations.
Figure 11.21 Setting pressures.
Figure 11.22 Setting furnace conditions.

which indicates that more material can be taken overhead. Increasing the flowrate carries more higher-boiling material into the naphtha product.

A "Design Specs" function can be used to achieve the specification. Clicking Design Specs under the PREFLASH column block, clicking New, and giving an identification label open the window shown at the top of Figure 11.23. The Type is specified at ASTM D86 temperature (dry, liquid volume basis). The Target is 375OF at a Liquid % of 95%. Click the Feed/Product Streams and select NAPHTHA as the Selected Stream (see the bottom of Fig. 11.23). Finally the Vary page tab is clicked and Distillate flow rate is

Figure 11.23 Design specs.
Figure 11.24 Set Vary in design specs.

selected as the Type (see Fig. 11.24). Running the simulation by clicking the blue N button produces a naphtha flowrate of 21,040 b/d to achieve the 375 °F target.

One source of confusion is a difference between the b/d flowrates specified in the original Setup and the Liq vol 60F bbl/day reported in the Stream Results. The specified 20,000 b/d is at the actual flow conditions (temperature and pressure). The results given in the Stream Results are at standard conditions (60°F and 1 atm).

The diameter of the column is calculated in the normal way by using the Tray Sizing feature. The result is a diameter of 11.1ft. The composition of the LIGHTS vapor stream from reflux drum is given in Table 11.5. Most of the light hydrocarbons that are in the crude oil feedstreams are removed in this vapor stream. They are sent to downstream units for separation into individual components.

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 the flow-rates are given in barrels per day at standard conditions.

Since English petroleum units are not available in the current version of Aspen Dynamics, we will switch to metric units when we look at dynamic control. Figure 11.25b gives the flowsheet of the PREFLASH column in metric units (1T = 1000 kg).

TABLE 11.5 LIGHTS Composition

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