enter on stage 22, while the stripping steam enters three trays lower (stage 25). Clicking the Pressure page tab gives the window shown at the top of Figure 11.40b, and the appropriate pressure information is entered.
Clicking the Furnace page tab opens the window shown at the bottom of Figure 11.40b. There are several options for setting up the furnace. For Furnace type we select the Single stage flash with liquid runback. For Furnace Specification we select Fractional overflash and specify it to be 0.03. The term "overflash" refers to the fraction of the vapor that is produced in the flash zone of the column that is returned as liquid to the flash zone from the tray above the flash zone. Most of the liquid coming down the column is withdrawn as sidestreams, but some liquid is needed on the trays below the lowest sidestream and above the flash zone to prevent entrainment of heavy liquid up the column with the vapor. Entrainment could drive the color of the gasoil off specification. It could also increase the concentration of metal contaminants in the gasoil because some crude oils contain small amounts of metals. The gasoil is usually fed to a catalytic cracking unit, and the catalyst in this unit is deactivated by metals. Therefore a small "wash" or "overflash" stream is needed.
The final part of the steady-state design is to set up two Design Specs. The first varies the flowrate of the heavy naphtha to achieve an ASTM 95% boiling point of 375°F. The second varies the flowrate of the diesel to achieve an ASTM 95% boiling point of 640°F.
The setups for these are shown in Figure 11.41. The top panel in Figure 11.41a gives the Specification page tab for the heavy-naphtha design spec. The middle panel gives the Feed/Product Streams page tab. The bottom panel gives the Vary page tab. Note that Distillate flow rate is selected.
Figure 11.41b gives the same information for the diesel design specification. Note that the stream selected to vary is Bottoms flow rate from the S-2 stripper.
The ASTM curves for all the products from the pipestill are generated by going to Results Summary on the Data Browser window, clicking Streams, selecting the Vol. % Curves page tab, and selecting the ASTM D86 curve in the Curve view window, as shown in Figure 11.42.
Next, left-click the top of the Vol% column, which highlights this column. Go to the toolbar at the top of the Aspen Plus window, click Plot and click X-Axis Variable. Highlight each of the columns for the products that you want to plot by holding down the Ctrl key and clicking the top of each column. Then go to the toolbar, click Plot, and click Y-Axis Variable. Finally go to the toolbar, click Plot, and click Display Plot. Results are shown in Figure 11.43.
Two of the unique features of a petroleum fractionator are the large changes in temperature and flowrates from the bottom to the column. Figure 11.44 gives these profiles for the pipestill.
The temperature ranges from near 180°F at the top to over 650°F at the flash zone. Note that the temperature decreases slightly in the bottom three trays because of the stripping with 400°F steam.
The molar flowrates of the liquid and vapor increase as we move up the column from the flash zone. At the pumparounds the vapor rate decreases.
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