Crude preflash tower

A common example of foam formation in the bottom of a fractionator inducing flooding occurs in a crude preflash tower. In this case, stable foam accumulates in the bottom of the column as a consequence of the "flow improver" chemicals added to crude oil. These chemicals reduce pressure drop in the crude pipelines. Once the foam level rises to the feed inlet nozzle, the trays flood and black distillate is produced. Please see Chapter 18 (Preflash Towers). 

So far, all our examples have dealt with tworeflux rate is large compared to the overhead product rate, and many of our towers do run with a lot of reflux. But we can all think of distillation columns where the top reflux rate is small compared to the overhead product, and the overhead product itself consists of a dozen widely different chemical compounds. Figure 7.4 represents such a column. It is called a crude preflash tower. 

In existing refineries, nearly every possible combination of flow or of towers may be found. Figure 7-8 illustrates two of these (also see Fig. 7-7 again), and nearly all refiners employ a crude preflash tower to... 

Notice that there is no reboiler in this flash tower. All the heat input comes from the partially vaporized crude. Both the temperature and the percent vaporization of the crude are fixed. Hence, the external heat input to the preflash tower is constant. The pounds of vapor flowing to the bottom tray must also be constant. 

Fig. 18.15. Crude distillation—TOTALFINAELF andTechnip. Includes desalter and preheater (1), preflash towers (wet and dry) (3,2), gas plant and rectifier tower (4), main fractionation tower (5), and vacuum fractionation tower (6). (Source Hydrocarbon Processing, 2004 Refining Process Handbook. CD-ROM. September 2004 copyright 2004 by Gulf Publishing Co., all rights reserved.)...

Field tests have shown that the changes depicted in Figure 1-9 result in no measurable loss in separation efficiency between light and heavy naphtha. For a typical low-sulfur crude (35°AP1), 6-8 vol% of the crude is taken overhead in the preflash tower. The observed reduction in furnace duty was 5%-10%. 

Lately, a popular addition to a crude distillation system has been a preflash column ahead of the two stages shown in Fig. 15.10. The preflash tower strips out the lighter portions of a crude oil before the remainder enters the atmospheric column. It is the lighter portions that set the vapor loading in the atmospheric column, which, in turn, determines the diameter of the upper section of the column. 

Gas plants are integrated tower systems intended to recover LPG range material and separate it from naphtha products. This stabilizes the naphtha and reduces its vapor pressure. The LPG material may either be saturate gases going to LPG or unsaturates going to further processing. Gas plants on preflash and atmospheric crude processing units are saturate gas plants. Gas plants on FCC units are unsaturate gas plants. Coker and visbreaker gas plants are somewhere between the two. 

The preflash column is used when the crude oil reaches a temperature of about 400—450 °F. At this point, some of the crude is above its boiling point, and vaporization occurs. The lightest portion of the crude is flashed (vaporized and separated from the bulk liquid) in the preflash column. The overhead is condensed and added into the atmospheric tower as a reflux, and the noncondensible vapors are added to the atmospheric tower overhead vapors. This step serves to conserve energy and unit investment for preheating the bulk of the crude. 

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