Overview of Steady State Design

The flowsheet of the pipestill is shown in Figure 11.32. The column is very large in diameter (20.3 ft), operates with a top pressure of 15.7 psia, and has a total of 25 stages. The bottoms stream from the preflash column (see Fig. 11.25) is pumped to a furnace that heats the stream to 684°F. The higher the temperature of the furnace exit [coil outlet temperature (COT)], the more of the stream is vaporized and the more gasoil that can be recovered. However, there is a limit to the furnace temperature due to excessive thermal decomposition ("cracking") of the crude in the furnace. If the furnace tube wall temperatures are too high, coke will be formed. This interferes with heat transfer and eventually requires a shutdown of the unit to remove the coke. The heat duty in the furnace is 201 x 106 Btu/h.

The feed is partially vaporized: 2278 lb.mol/h of vapor with a feed of 3644 lb. mol/h. It is introduced into the flash zone on stage 22. There are three stages below the flash zone that are used to strip out any light material that is in the liquid leaving the flash zone. Open steam is fed to the bottom of the column at a rate of 12,000 lb/h. The bottoms stream from the pipestill ("reduced crude") goes to a downstream vacuum pipestill in which more gasoil is recovered. The low pressure in the vacuum furnace produces more vapor for the same furnace temperature.

Figure 11.32 Pipestill flowsheet: (a) engineering units; (b) metric units.
Figure 11.32 Continued.

The vapor from the pipestill flash zone flows up the column. At stage 14 a pumparound removes 15 x 106 Btu/h, which reduces the vapor flowing up the column and increases the liquid flowing down the column. This high-temperature heat (558OF) is used for feed preheating. At stage 8 a second pumparound removes 40 x 106 Btu/h, which further reduces the vapor flowrate and increases the liquid flowrate. This high-temperature heat (441 OF) is also used for feed preheating.

The vapor leaving the top of the column is condensed in a water or air-cooled condenser. The liquid distillate is a heavy-naphtha stream, which is used for the production of gasoline. It has ASTM 5% and 95% boiling points of 195 and 375OF, respectively. In some refineries it is sent to a reforming unit to produce aromatics (benzene, toluene, and xylenes) and hydrogen. The condensed water is decanted off the reflux drum. Note that this water stream is quite large (17,180 lb/h) because of all the open stripping steam that is used in the column base and sidestream strippers.

The reflux ratio is 3.71. At stage 6 some liquid is withdrawn and fed to a four-stage stripper. Open steam (3300 lb/h) is used to strip light material from the liquid leaving the main column. A kerosene product is produced from the bottom of the stripper. It has ASTM 5% and 95% boiling points of 396 and 502OF, respectively.

In the distillation of distinct chemical components, we talk about separation in terms of the compositions of the impurities in the product streams. In the distillation of petroleum fraction, separation is expressed in terms of "gaps" and "overlaps." These terms refer to the difference between the 95% boiling point of a lighter product and the 5% boiling point of the adjacent next-heavier product. If there were perfect separation of the petroleum cuts, the final boiling point of a lighter product would be equal to the initial boiling point of the next-heavier product. But separation is not perfect. The 95%-5% difference is used as a measure of fractionation. It can be improved by using more trays or by increasing the liquid to vapor ratio in the section of the column in which the separation between the two cuts is occurring. For example, there is a gap of 21°F between the heavy naphtha (95% point of 375°F) and the kerosene (5% point of 396°F). This fairly good separation is achieved because of the 3.71 reflux ratio and the five trays between these two products. As we will see, the separations between the other products have overlaps instead of gaps because of the smaller liquid to vapor ratios in the lower sections of the column.

Liquid from stage 13 is withdrawn and fed to a three-stage stripper. Open steam (1000 lb/h) is fed to the bottom of the stripper. A diesel product is produced from the bottom of the stripper. It has ASTM 5% and 95% boiling points of 489 and 640°F, respectively. Note that there is a 13°F overlap between the 95% point of the kerosene (502°F) and the 5% point of the diesel (489°F).

A two-stage stripper at stage 18, using 800 lb/h of open steam, produces atmospheric gasoil (AGO) with ASTM 5% and 95% boiling points of 589 and 782°F, respectively. There is a 51°F overlap between the 95% point of the diesel (640°F) and the 5% point of the AGO (589°F). This sloppy separation between petroleum cuts is typical of petroleum separation. The values of the different products are usually not drastically different, and improved fractionation can seldom be justified.

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