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Figure 3.6. Crude assay data, viscosities of vacuum gas oils.

Figure 3.7. Whole crude TBP temperature—yield curve, fuels—pitch operation.

2. The designer sets the amount of overflash depending upon the degree of purity required in the heavy vacuum gas oil. If color requirements and/or allowable metals level specifications are not severe, 1 to 2 volume percent of vacuum tower feed is normally taken as overflash. For tight specifications or for crudes having high metals content, as much as 4 percent may be taken. Two percent was used in Figure 3.7. In practice, operators never take any more overflash than necessary; thus, overflash capability may be considered as built-in extra capacity. Facilities should always be provided for measuring overflash and for yielding it as a product stream, even though it may be returned to the column sump or recycled to the furnace.

3. Item 2 above sets the sidestream yields of heavy and light vacuum gas oils.

4. Since a fuels-type tower operates more or less as a series of equilibrium condensations, the properties of the products are estimated by the following technique.

a. From the atmospheric EFV curve of the reduced crude, tabulate the EFV curves of the products and convert them to the ASTM and TBP curves.

b. Stream gravities are obtained from the crude assay, and molecular weights are calculated.

Flash Zone and Tower Base Calculations

The following discussion outlines procedures for calculating the overall heat balance around the flash zone and tower base. These procedures apply for both types of vacuum towers and are considered independently of the rest of the heat and materia! balance calculations. To this point, it is assumed that the following items have been completed.

1. An overall material balance for the system has been developed including overflash and the amount of hydrocarbon decomposition gases which are produced in the process by virtue of the high temperatures to

Pressure Drop Values As Recommended for Design Purposes

Sections

Drop Values, e.a.

Chimney trays

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