Flash zone

As shown in Fig. 13-92, methods of providing column reflux include (a) conventional top-tray reflux, (b) pump-back reflux from side-cut strippers, and (c) pump-around reflux. The latter two methods essentially function as intercondenser schemes that reduce the top-tray-refliix requirement. As shown in Fig. 13-93 for the example being considered, the internal-reflux flow rate decreases rapidly from the top tray to the feed-flash zone for case a. The other two cases, particularly case c, result in better balancing of the column-refliix traffic. Because of this and the opportunity provided to recover energy at a moderate- to high-temperature level, pump-around reflirx is the most commonly used technique. However, not indicated in Fig. 13-93 is the fact that in cases h and c the smaller quantity of reflux present in the upper portion of the column increases the tray requirements. Furthermore, the pump-around circuits, which extend over three trays each, are believed to be equivalent for mass-transfer purposes to only one tray each. Bepresentative tray requirements for the three cases are included in Fig. 13-92. In case c heat-transfer rates associated with the two pump-around circuits account for approximately 40 percent of the total heat removed in the overhead condenser and from the two pump-around circuits combined. 

Cohimn pressure at the reflux drum is established so as to condense totally the overhead vapor or some fraction thereof. Flash-zone pressure is approximately 69 kPa (10 psia) higher. Crude-oil feed temper-... 

Try the following problem to sharpen your skills in working with material and energy balances. Crude oil is heated to 525° K and then charged at a rate of 0.06 m /hr to the flash zone of a pilot-scale distillation tower. The flash zone is maintained at an absolute temperature of 115 kPa. Calculate the percent vaporized and the amounts of the overhead and bottoms streams. Assume that the vapor and liquid are in equilibrium. 

In single-stage units which do not produce kerosene or other critical stocks, flash zone temperatures may be as high as 750 - 775 F. The principal limitation is the point at which cracking of distillates to less valuable gas or the rate of coke formation in the furnace tubes becomes excessive. 

In two stage units, it is often economical to distill more gas oil in the vacuum stage and less in the atmospheric stage than the maximum attainable. Gas formed in the atmospheric tower bottoms piping at high temperatures tends to overload the vacuum system and thereby to reduce the capacity of the vacuum tower. The volume of crude vaporized at the flash zone is approximately equal to the total volume of distillate products. Of course, the vapor at this point contains some undesirable heavy material and the liquid still contains some valuable distillate products. The concentration of heavy ends in the vapor is reduced by contact with liquid on the trays as the vapor passes up the tower. This liquid reflux is induced by removing heat farther up in the tower. 

The heated oil is flashed into the VPS flash zone where vapor and liquid separate. Split between distillate and bottoms depends on flash zone temperature and pressure. Separation of vapor and liquid in the flash zone is incomplete, since some lower boiliug sidestream components are present in the liquid while bottoms components are entrained in the vapor. The liquid from the flash zone is steam stripped in the bottom section of the VPS to remove the lower boiling components. 

Before desalters came into common use, crude pipe stills were frequently equipped with flash drums to minimize salt deposition on hot surfaces. In the flash drum system, the crude is heated to about 300°F. under enough pressure to suppress vaporization. The pressure is released as the crude enters the flash drum and all of the water (along with a small amount of crude) is flashed off, leaving the salt as a suspension in the oil. The flashed vapor is recombined with the crude near the furnace outlet or in the flash zone of the fractionating tower. 

To obtain a low flash zone pressure, the number of plates in the upper section of the vacuum pipe still is reduced to the minimum necessary to provide adequate heat transfer for condensing the distillate with the pumparound streams. A section of plates is included just above the flash zone. Here the vapors rising from the flash zone are contacted with reflux from the product drawoff plate. This part of the tower, called the wash section, serves to remove droplets of pitch entrained in the flash zone and also provides a moderate amount of fractionation. The flash zone operates at an absolute pressure of 60-90 mm Hg. 

Since cracking stocks generally do not have to meet the color specifications that lube distillates do, higher flash zone temperatures (up to 8(X)°F) can be tolerated. Fuel units are normally designed to distill material boiling up to 1100°F (at atmospheric pressure) from the feed, and some units have distilled beyond 12(X)°F at low feed rates. 

Xf = mol fraction of a component in feed Xf = mol fraction of any component in the feed. Ft where Xf = F Xf/F( for all components in F for the non-condensable gases, Xf = V /Ft F = mols of feed entering flash zone per unit time contains all components except non-condensable gases Ft=F-tV,... 

Vt = V + Vs mols of vapor at a specific temperature and pressure, leaving flash zone per unit time Vs = mols of non-condensable gases entering with the feed, F, and leaving with the vapor, V, per unit time... 

Feed rate to tower, lb mols/hr or, mols of feed, (batch distillation) entering flash zone/time all components except non-condensable gases Factor for contribution of other feed flow to minimum reflux Mols of liquid feed Mols of vapor feed... 

V = Vt = Total vapor leaving flash zone/tmit time at... 

One method of maximizing the LCO end point is to control the main fractionator bottoms temperature independent of the bottoms pumparound. Bottoms quench ( pool quench ) involves taking a slipstream from the slurry pumparound directly back to the bottom of the tower, thereby bypassing the wash section (see Figure 9-9). This controls the bottoms temperature independent of the pumparound system. Slurry is kept below coking temperature, usually about 690°F, while increasing the main column flash zone temperature. This will maximize the LCO endpoint and still protect the tower. 

Concentration changes observed between mother liquor in the flash zone and liquid product in the melt zone of an experimental triple-point crystallizer have been dramatic. A qualitative concentration profile typical of those observed in the experimental unit is shown in Figure 8. The mother liquor concentration is relatively uniform above the packed bed, but a sharp drop in contaminant concentration occurs within the top several inches of the loosely packed crystal bed. Concentration changes of the order 500 to 5000 have been observed for representative sulfurous compounds and trace contaminants, including hydrogen sulfide, carbonyl sulfide, methyl mercaptan, ethane, and ethylene. Concentration profiles calculated for the packed bed of solid carbon dioxide using a conventional packed bed axial dispersion model agree very well with the observed experimental profiles. 

Figure 2.1 presents the simplistic basis upon which all separations are commonly made in our industry. Even membrane separations depend to a large degree upon the vapor pressure and temperature effects shown. A typical temperature dashed line shows how the temperature variance effects a vapor-liquid separation. Notice also the variance for pressure and enthalpy. Inside the phase envelope, the temperature and pressure remain constant while the enthalpy varies. This constant T and P occur in what is called the flash zone. 

Example The dew point is defined an all-vapor system except for one very small increment of liquid. Now take the feed again, and consider the fact that this is a flash-off crude still sidestream vapor at 20 psig (34.7 psia). The dew point is to be determined to set the proper sidestream stripper overhead temperature for this desired product. This problem is worked similarly to the bubble point. Simply hold the pressure (34.7 psia) constant, and vary the temperature. Note that when you find the temperature at which just a small amount of liquid is formed, the sign SYSTEM IS ALL VAPOR goes off and the flash component summary appears. Note also that the previous flash summary will remain on the screen (if you had a previous run) until you input a flash zone temperature and click on Run Prog. 

The first text box immediately under Run Prog, is named txtPrt. See Fig. 9.12. This is a very unique answer box, as it displays a special message if you don t choose a temperature and pressure within the flash zone. If the temperature and pressure are not in the flash zone, the message System is all liquid or System is all vapor will appear in this text box. Otherwise, no message will appear in this box. 

DPTS represents the sum value of component mole fractions divided by their respective K values. The K value is the component s equilibrium value, y/x. BPTS represents the sum value of the component mole fraction times its respective K value. If the sum DPTS is less than 1.0, it means that the respective K values and temperature are far too great for the system to be within the flash zone where both vapor and liquid exist. BPTS is the same, except the K values are not large enough for any vapor to exist. The code lines that make this calculation are as follows ... 

That is, for a given temperature and pressure and inflows of species, the ECES model calculated the flow rates of the species in the liquid phase leaving the flash zone. This information must be in a form suitable for use in the fractionator program and the fractionator program must be able to utilize this information correctly. 

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