Preflash tower

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. 

When we raise the top reflux rate to our preflash tower, the tower-top temperature goes down. This is a sign that we are washing out from the upflowing vapors, more of the heavier or higher-molecular-weight, components in the overhead product. Of course, that is why we raised the reflux rate. So the reduction in tower-top temperature is good. 

As the reflux rate is raised, the weight flow of vapor through the top tray, and to a lesser extent through all the trays below (except for the bottom tray), increases. This increase in the weight flow of vapor occurs even though the external heat input to the preflash tower is constant. The weight flow of vapor to the bottom tray is presumed to be solely a function of the pounds of vapor in the feed. 

The tray temperatures in our preflash tower, shown in Fig. 4.4, drop as the gas flows up the tower. Most of the reduced sensible-heat content of the flowing gas is converted to latent heat of evaporation of the downflowing reflux. This means that the liquid flow, or internal reflux rate, decreases as the liquid flows down the column. The greater the temperature drop per tray, the greater the evaporation of internal reflux. It is not unusual for 80 to 90 percent of the reflux to evaporate between the top and bottom trays in the absorption section of many towers. We say that the lower trays, in the absorption section of such a tower, are drying out. The separation efficiency of trays operating with extremely low liquid flows over their weirs will be very low. This problem is commonly encountered for towers with low reflux ratios, and a multicomponent overhead product composition. 

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.)...

The answer to these problems is to install a prellash tower. If the unit has a flash drum upstream of its furnace, this drum can be retrofitted as a preflash tower. Figure 1-9 illustrates a preflash lower arrangement used with success at several refineries. Significant features of this arrangement are ... 

The preflash tower is located midway in the preheat exchanger train, and its pressure "floats" on the fractionator pressure. 

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%. 

A method to calculate energy savings resulting from retrofitting a flash drum as a preflash tower is ... 

SH = Specific heat of the hydrocarbon vapor in the preflash tower (about 0.5-0.6)... 

M = Total pounds per hour of the preflash tower net overhead (liquid plus vapor) product... 

Tp2 = Temperature of the fractionator flash zone Tpf = Preflash tower feed temperature... 

Hrp = Preflash tower reflux heat duty (i.e., Ib/hr of reflux multiplied by about 170 BTU/lb)... 

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). 

Preflash tower, 27-31 energy saving, 27-30 foaming, 30 Preheat exchanger train, 28 Preheater problems (distillation), 388... 

Conversion units may employ a full-fledged fractionation train, with a preflash tower to remove light ends an atmospheric fractionator to separate light naphtha, heavy naphtha, middle distillates, and unconverted oil and a vacuum tower to maximize the recovery of diesel. Some hydrocrackers use the atmospheric tower to produce full-range naphtha, which is then separated into light and heavy fractions in a naphtha splitter. 

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. 

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. 

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... 

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