Overview of Steady-State Design

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 Ib/h. The bottoms stream from the pipestill ( reduced crude ) goes to a downstream vacuum pipestill in which more gas oil is recovered. The low pressure in the vacuum furnace produces more vapor for the same furnace temperature. 

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 375 °F, 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 Ib/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 At Stage 6, some liquid is withdrawn and fed to a 4-stage stripper. Open steam (3300 Ib/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 502 F, respectively. 

A 2-stage strippCT at Stage 18, using 800 Ib/h of open steam, produces 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. 

---