Figure 11.44 Pipestill profiles.

in its 95% boiling point and a decrease in the 5% boiling point of the next heavier product stream.

The reflux flowrate also increases (from 2517 to 2884 lb.mol/h). This and the lower distillate flowrate produce a higher reflux ratio (increasing from 3.875 to 5.41), which has the effect of providing more fractionation between the heavy naphtha and the kerosene. The "gap" between these cuts increases from 395 - 375 = 20oF to 380 - 350 = 30oF.

In a similar way, suppose that the specification on the 95% boiling point of the diesel is reduced from 640 to 6200F. The flowrate of the diesel decreases from 14,363 to 12,112 b/d. This drops more light material into the lower AGO stream, so its 5% point drops from 589 to 5000F and its 95% point, from 782 to 7650F. The flowrate of the bottoms increases from 37,647 to 39,953 b/d, and its 5% point changes from 692 to 6650F.

Effect of Changing a Pumparound Changing a pumparound heat removal affects the vapor traffic in the column above the pumparound and the liquid traffic below the pumparound. Of course, it also affects the furnace firing rate because the temperature of the feed to the furnace changes. For example, suppose that we change the heat removal in the top pumparound (P-1) from 40 to 30 x 106 Btu/h. More vapor flows up through the top part of the column, which increases the reflux ratio from 3.875 to 4.466 and increases the condenser heat removal from 92 to 102 x 106 Btu/h. The higher liquid to vapor ratio provides better fractionation above the pumparound. The gap between the heavy naphtha and the kerosene increases from 395 — 375 = 20°F to 398 — 375 = 23°F. Thus there is a slight improvement in this separation.

The downside of this change is that more heat is rejected to cooling water in the condenser instead of being recovered by feed preheating. The effect on furnace firing depends on the configuration of the heat exchanger network used, which is not modeled in the simulation considered in this chapter.

Principle 11.2 Pumparounds affect separation between cuts and furnace firing in opposite ways. Reducing a pumparound heat removal improves separation between cuts above the pumparound, but increases furnace energy consumption.

Effect of Changing Stripping Steam Open steam is used in the strippers to remove the light material that is in the liquid withdrawn from the main column. Changing stripping steam flowrate affects the initial part of the boiling point curve, but has less of an impact on 5% point and essentially no impact on the 95% point and product flowrates. Of course, using more steam increases steam consumption and increases the load on water purification facilities required to handle the water decanted off the reflux drum.

For example, assume that the stripping steam to the top (kerosene) stripper is increased from 3300 to 5000 lb/h. All other degrees of freedom remain unchanged. The initial TBP boiling point of the kerosene changes from 311 to 321 °F. The initial ASTM boiling point of the kerosene changes from 366 to 374°F. The ASTM 5% boiling point changes only from 395 to 399°F. The ASTM 95% point changes only from 502 to 503°F.

Principle 11.3 The flowrate of stripping steam affects the initial boiling point or the flashpoint of the cut.

The steady-state design is now complete. We are ready to investigate dynamics and control of this complex system.

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