Figure 2.12. EFV curves for whole crude at 14.7 psia, flash zone inlet pressure (Pp^j) and furnace outlet pressure (PpQ)-
Maximum allowable crude-oil temperature should be set by the owner. It is general knowledge that, at temperatures of 650 to 675 degrees F, most paraffinic and naphthe-nic base crudes begin to undergo thermal decomposition. This contributes to poor color and poor smoke properties in the light and heavy distillate fractions. Although oil temperatures as high as 735 degrees F for kerosene operation and 800 degrees F for general operations have been reported (6), the author recommends that temperatures of the oil should not exceed 650 degrees F for kerosene production and 700 degrees F for other designs.
Overflash is an extra amount of .vaporization above that required by material balance considerations which is taken to ensure that adequate reflux will be available in the trays between the flash zone and the lowest sidestream product draw tray. Overflash is measured as volume percent of crude oil feed to the tower.
The value of overflash to be used in design has been the subject of much discussion through the petroleum industry. An overflash of 2 volume percent of feed is recommended for competitive design although some refiners will specify values as high as 5 volume percent. On the other end of the scale, the author has observed a successful although very sensitive tower operation at overflash values as low as!0.2 volume percent. Since the gross hydrocarbon vapor from the flash zone is potentially all product, an actual operation will allow as much of the specified over-flash as possible to come up into the lowest sidestream, rejecting only that amount necessary to control color in the distillates'by refluxing the trays above the flash zone.
A conservative approach to design and one providing maximum operating flexibility would be to design the atmospheric section on the basis of maximum oil temperature and zero overflash. This means that the lowest sidestream product could contain all the overilash. Accordingly, this would require that the heat exchangers in the lowest side-stream circuit be sized to accommodate this high potential rate. On the other hand, the vacuum section would be designed on the basis that the atmospheric tower would return the design value of overflash to the reduced crude. This would require additional vacuum furnace capacity and sizing the heat exchangers in the lightest vacuum gas oil circuit for the highest potential rate. Because of the significantly higher cost, this approach to crude unit design would seem indicated only where a high degree of operating flexibility is required and where the penalties for failure to meet product quality standards are severe. For competitive design work, this approach would lead to financial disaster.
The calculations outlined in this subsection are based on having predetermined the maximum temperature of the oil leaving the furnace. Occasionally, one may define the thermal basis as the flash zone temperature. If this is the basis, the procedure described in the chapter covering the vacuum tower—specifically Figure 3.8—will apply for flash zone calculations.
Figure 2.11 illustrates the flow rates around the flash zone and gives the equations used in defining the appropriate heat quantities. Figure 2.12 shows how the basic heat input to the process is calculated by using the EFV curves. The sequential calculation procedure is as follows.
1. Convert the 14.7 psia EFV curves to the pressures which have been set for the furnace outlet and the entrance to the flash zone. This latter value is, for all practical purposes, equal to the flash zone pressure. The vapor pressure charts and nomographs in Section 4 of Maxwell are useful for these conversions.
2. At the furnace outlet conditions of temperature and pressure, find the volume percent vaporized as Point 1 on Figure 2.12. From crude assay data, calculate the weight of the vaporized crude. Using the weight of the vapor and liquid phases, calculate the heat content of the mixture leaving the furnace as qi=qfo
3. Next, determine the vaporization which exists at the flash zone entrance as Point 2 on Figure 2.12. This is a trial-and-error procedure which requires assuming a new temperature at the flash zone entrance, determin ing the new percentage vaporization and then calculating the heat content until satisfying the criterion
The temperature drop between the furnace outlet and the flash zone inlet will usually be on the order of 4 to 6 degrees F. The amount of heat which the feed introduces into the process has now been defined as
4. Set the value for overflash. This liquid returns to the flash zone from the tray immediately above it and mixes with the feed flash liquid. The sum of these two liquids is the feed to the top tray of the bottoms stripping section.
5. Set a stripping steam rate-10 pounds per barrel of net bottoms is recommended. From Figure 2.13, find the volume percentage of the feed to the stripping section which will be vaporized. From this difference, calculate the volume of net bottoms. From crude assay data, calculate the weight of the bottoms product.
6. The total vapor leaving the flash zone is the sum of the feed flash vapor and the stripout. From this is calculated the volume, weight and molal quantities around the flash zone.
7. Calculate the hydrocarbon partial pressure in the total vapor leaving the flash zone as P^- Find the 14.7 psia EFV temperature^for the indi«ted vo 1 umtTj>ercerUige vaporization and^convert this to the pressure~leveFof P'^q. TTiis-is the flash-zone temperature.
8. Assume that thVlemperHurc^oTlbt jiet bottoms liquid, W, is 30 degrees F lower than the flash zone temperature. Calculate the heat quantity leaving the system in this stream.
9. Calculate the heat input to the system from the stripping steam.
10. Calculate and tabulate the external heat quantities at the flash zone.
11. Tabulate the quantities and properties of the hydrocarbon flows around the flash zone.
12. Total distillate yield is the total hydrocarbon vapor leaving the flash zone minus the overflash.
A typical calculation of total distillate yield is given in the appended example calculations.
General Properties of Petroleum Fractions
Most petroleum distillates, especially those from atmospheric distillation, are usually defined in terms of an
Was this article helpful?