6 to 8 mm Hg
1 to 2 mm Hg
2 to 5 mm Hg
I to 2 mm Hg
1 mm Hg
which the reduced crude is subjected. The symbol, G, is used for this material in the calculations.
2. A value for inward air leakage is assumed which can be checked later by the methods outlined in Ludwig. For the heat balance, this leakage is assumed to occur totally at the flash zone. The symbol, A, is used for this material in the calculations.
3. The maximum allowable flash zone temperature has been established. As pointed out in Chapter 2, this has been the subject of much discussion throughout the industry. Normally, maximum flash zone temperatures range from 775 to 800 degrees F.
4. The tower top pressure has been established.
In order to determine the flash zone pressure, it is first necessary to allocate trays and/or other internals to various sections of the tower and then to assume pressure drops across these sections. The number of trays between lube tower draw trays is normally 3 to 5. Modern fuels towers use grid sections. Pressure drop values recommended for design purposes are given in Table 3,1. Having assumed the internal configuration of the tower, the flash zone pressure is then calculated arithmetically.
Knowing the flash zone temperature and the feed vaporization requirements, the hydrocarbon partial pressure, in the flash zone is found by inspection of the vacuum region EFV curves which were developed earlier in this chapter. The difference between the partial pressure and the total pressure in the flash zone must be made up by the air leakage and by steam. The required steam to the flash zone is calculated as
D = total hydrocarbon distillate products including the decomposition gas, moles per hour, and yHC= ratio of hydrocarbon partial pressure to total pressure in the flash zone, mole fraction. This analysis assumes essentially no stripping of the feed flash liquid in the base section of the tower, although, in a later step, a temperature drop across the bottoms stripping zone will be set. Nelsons' correlations show very little stripout at the steam rates usually employed, but experience shows that temperature drops as high as 30 degrees F between flash zone and bottoms do occur. This is of little real importance, however, since the heat input to the system can be calculated independently of the absolute thermal condition of the feed.
The error inherent in an incorrect estimate of the bottoms temperature carries two considerations.
1. From the viewpoint of the furnace, a low estimate of the exit bottoms temperature will cause the heat input to the system to be calculated as lower than actually required.
2. From the viewpoint of the tower heat balance, a low estimate of the exit bottoms temperature will cause the various tower heat removal quantities to be calculated as lower than actual. This applies especially to the cooling requirements for the vacuum residuum.
In practice, the value of these discrepancies is quite small in comparison to the total values of these heat quantities in question. The most conservative design approach would be to assume a zero temperature drop for the liquid from the flash zone to the base of the tower. This would maximize both feed heat input and heat removal duties. Conversely, assuming a high temperature drop, say 30 degrees F, would be a tighter, more competitive approach.
The calculations necessary to define the heat balance around the base section of the tower are shown on Figure 3.8. This illustration is self-explanatory as to the calculation of the total feed heat input and the furnace duty.
Note the role played by the condensed overflash liquid. If this stream is returned to the system, either as overflow from the draw tray to the flash zone or as recycle from the draw tray to the furnace, it must be taken into account in the heat balance. If it leaves the system as a product stream, it will not be seen as a heat input to the flash zone. This is the reason for the question mark in the equation for calculating the feed heat input.
The tabulation of the external heat quantities at the top of the flash zone is made to facilitate the heat balance
EXTERNAL HEAT QUANTITIES LEAVING FLASH ZONE Q'ifz= (Q f + Qa + Qsw), BTU/hr.
2D= TOTAL DISTILLATE PRODUCTS EXCLUSIVE OF OVERFLASH. Lo = OVERFLASH.
G = HYDROCARBON DECOMPOSITION GAS.
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