Since the pressure drop directly impacts the operating cost for the blower to deliver the feed gas, the design engineer should choose the tower diameter to meet the pressure drop specification. Figure 5.20.3 shows the value for a widely used correlation for the pressure drop calculation. The correlation includes a parameter, packing factor FP, which is a constant for a given packing size and shape (Fair 1987). The value of Fp can be obtained from Table 5.20.2. With known physical properties and flowrates for the liquid and gas streams, the engineer first determines the specific gas rate G, kg/m2 • sec in the column and then calculates the column cross-sectional area by dividing the total gas flowrate (GT,kg/s) by G.
In cases where the pressure drop is not a prime consideration (e.g., treatment of a pressurized process stream), the column can be designed to operate at 70-80% of flooding rate. For packings below 2.5 cm size, the design engineer can use the flooding line shown in Figure 5.20.3 to calculate the flooding rate. If the packing is larger than 3.7 cm, the following empirical equation determines the pressure drop at flooding (McCabe 1993):
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