FIG. 13-92 Methods of providing reflux to crude units. (a) Top reflux. (b) Pump-back reflux. (c) Pump-around reflux.

ature at flash-zone pressure must be sufficient to vaporize the total distillates plus the overflash, which is necessary to provide reflux between the lowest sidestream-product draw-off tray and the flash zone. Calculations are made by using the crude-oil EFV curve corrected for pressure. For the example being considered, percent vaporized at the flash zone must be 53.1 percent of the feed.

Tray requirements depend on internal-reflux ratios and ASTM 5-95 gaps or overlaps, and may be estimated by the correlation of Packie (op. cit.) for crude units and the correlation of Houghland, Lemieux, and Schreiner (op. cit.) for main fractionators.

Example 9: Simulation Calculation of an Atmospheric Tower

The ability of a rigorous calculation procedure to simulate operation of an atmospheric tower with its accompanying side-cut strippers may be illustrated by comparing commercial-test data from an actual operation with results computed with the REFINE program of ChemShare Corporation, Houston, Texas. The tower configuration and plant-operating conditions are shown in Fig. 13-94. Light-component analysis and the TBP and API gravity for the feed are given in Table 13-27. Representation of this feed by pseudocomponents is given in Table 13-28 based on 16.7°C (30°F) cuts from 82 to 366°C (180°F to 690°F), followed by 41.7°C (75°F) and then 55.6°C (100°F) cuts. Actual tray numbers are shown in Fig. 13-94. Corresponding theoretical-stage numbers, which were determined by trial and error to obtain a reasonable match of computed- and measured-product TBP distillation curves, are shown in parentheses. Overall tray efficiency appears to be approximately 70 percent for the tower and 25 to 50 percent for the side-cut strippers.

Results of rigorous calculations and comparison to plant data, when possible, are shown in Figs. 13-95, 13-96, and 13-97. Plant temperatures are in good

FIG. 13-93 Comparison of internal-reflux rates for three methods of providing reflux.

stage q

Relative internal molar liquid rate

FIG. 13-93 Comparison of internal-reflux rates for three methods of providing reflux.

TABLE 13-27 Light-Component Analysis and TBP Distillation of Feed for the Atmospheric Crude Tower of Fig. 13-94

Light-component analysis


Volume percent


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