Heat Input To Process

. /-N irN j-i^ w \ Qf-Q1 — O2, BTU/hr Qfzi=Q2*(QV + QI)FZI f '

HYDROCARBON PARTIAL PRESSURE IN FLASH ZONE P'hC«={[VhCFZ=(V'+VSo)]/(VhcFZ +Sw)}PFZ, PSIA WHERE ALL QUANTITIES ARE MOLES.

Figure 2.11. Flash zone heat and material balance.

order of 3 psi. However, as the exchangers become fouled, the extra drop allowed will be available.

Most atmospheric towers have 25 to 35 trays between the flash zone and the tower top. Recommended ranges for the number of trays in various sections of the tower are given in Table 2.1. Using these numbers of trays and reasonable levels of heat recovery will provide adequate fractionation between products. The allowable pressure drop for trays will be in the range of 0.1 to 0.2 psi per tray. In the design example, a pressure drop of 5 psi was taken between the flash zone and the tower top.

At this point, the flash zone pressure is set as the sum of the reflux drum pressure and the combined pressure drops across the condensers and trays above the flash zone. A pressure drop of 5 psi between the flash zone and the furnace outlet is recommended.

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