Tower pressure selecting optimum

The first two factors help make fractionation better, the last factor makes fractionation worse. How can an operator select the optimum tower pressure, to maximize the benefits of enhanced relative volatility, and reduced tray deck dumping, without unduly promoting jet flooding due to entrainment ... 

Figure 3.5 illustrates this relationship. Point A is the incipient flood point. In this case, the incipient flood point is defined as that operating pressure that maximizes the temperature difference across the tower at a particular reflux rate. How, then, do we select the optimum tower pressure, to obtain the best efficiency point for the trays Answer—look at the temperature profile across the column. 

Packing characteristics. We have shown that the optimum steam rate that leads to minimum tower volume and minimum power is that of the ideal cascade (13.14). The optimum typ>e of packing, optimum pressure, and optimum vapor velocity is that which makes the expression in braces (1327) a minimum. We shall not attempt to evaluate a number of types of packing, but shall use Spraypak no. 37 packing as an example of the selection of optimum vapor velocity and pressure. This is the type of packing recommended by McWilliams and co-workers [M4] for a water distillation plant. 

Air-stripping tower diameter is selected as a function of the liquid loading rates necessitated by the required design flow capability. The optimum tower diameter may be determined with the use of pressure-drop curves developed by Eckert (11) as shown in Fig. 3. The volumetric air-to-water ratio, calculated by Eq. (9), is converted to a weight-to-weight ratio and plotted on the abscissa in the form ... 

Thns, for a valne of G/L of 4.0, the valnes of Z/HTU for XJX = 0.1 are 1.49 for 85°F and 1.95 for 75°F. The height of the tower at 85°F is 1.48 x 23.5 = 34.8 ft, whereas at 75°F it becomes 1.95 x 23.5 = 45.8 ft. This shows that setting the GIL ratio to 4.0 instead of 2.0 wonld resnlt in a shorter tower. The amount of air requirecf, however, is not doubled 34.8 times 2 divided by 49.5 = 1.40. Thus, at this higher air loading rate, only 40% more air is reqnired. To find the optimum GJL ratio, the entire design mnst be priced and the minimnm cost tower selected. This requires repetitive calculations using a computer and incorporating reasonably accurate cost data as well as mass transfer, enthalpy transfer, and pressure drop characteristics on the detailed analysis. 

In the design of any vacuum tower, the first question to be settled is the selection of the optimum operating pressure of the system. In order to simplify this discussion, let us consider certain facts, assuming that a maximum allowable flash zone temperature has been set. 

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