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Process Circuit Analysis 109

Table 3.2.5: Stream Properties - Cooling-Tower Analysis

Stream Temperature Pressure Flow Rate Concentration, Mole Fraction
Number op psia lbmol/h a Water Air

1 94.0 14.7 164700 0.0302 0.9698
2 100.7 — 169800 0.0525 0.9475
3 109.0 - 291300 0 1.0
4 89.0 — 286100 0 1.0

"Multiply by 0.4536 to obtain kg mol/h.

Example 3.3 Flash Valves, Partial Condensers, and Partial Vaporizers____

Flashing, partial condensation, and partial vaporization are frequently occurring
process operations. Because partial separation occurs during these operations,
they are all separations. We will treat them together because the equations for cal-
culating downstream conditions are almost identical, differing only in the heat-
transfer term in the energy equation. The flash valve is essentially adiabatic, the
condenser removes heat, and the vaporizer adds heat to a process stream. The
pressure drops across these units also differ considerably. The pressure drop
across flash valves is about 1 Mpa (145 psi), for condensers, 10 kPa (1.45 psi), and
for the vaporizers, 1 kPa (0.145 psi). In all these units, we assume equilibrium
between the vapor and liquid streams leaving each process unit. This implies that
sufficient contact time will be allowed to reach equilibrium. The turbulence be-
tween the vapor and liquid streams in the flash valve and the vaporizer insures
good contact and hence a rapid approach to equilibrium. In the condenser, equilib-
rium may not be completely attained. Nevertheless, we will assume equilibrium.
Frequently, vapor-liquid phase separators follow and are combined with the
component separators, and equilibrium is assume between the exit streams of this
combination. Here, the phase separators are omitted as shown in Figure 3.3.1 to
keep the two kinds of separators divided according to their major function - one
where essentially component separation occurs and the other where essentially
phase separation occurs.

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