Column 2

Ljx D'zxD.

V2 V2

96r 46xd2

Step 5 The solution for *Dj and xD{ must be obtained by trial and error.

Assume xDl = 0.99


The solution involves finding xDj and xDi such that column 1 and column 2 have an equal number of stages. The bottom section is the same, and has the same component balance line as in Example 2.1.

Figure 9.2 shows x-y diagrams for column 1 and column 2. Solution was obtained on first trial, giving 8 stages in each rectifying section. This compares to five theoretical stages for the case of no maldistribution (Example 2.1). The apparent (measured) efficiency in the rectifying section is V« = 62.5 percent of the efficiency achievable with a perfectly distributed column.

Effect ot changes In local L/V ratio on packing efficiency. Example 9.1 illustrates a 40 percent reduction in efficiency resulting from a liquid maldistribution of the order of as little as 5 percent between two halves of a column. A similar analysis of an actual troublesome maldistribution case history (140a) also showed that it does not take much maldistribution to cause a major efficiency loss. Example 9.1 also illustrates the following:

1. The reduction in efficiency depends on the relationships between the component balance lines and equilibrium curve of the x-y diagram. This makes it a function of factors such as reflux ratio, product purity, location of the feed point, thermal state of the feed, relative volatility, and shape of the equilibrium curve.

2. Maldistribution can cause localized pinching. The user is encouraged to repeat Example 9.1' for a ±10 percent liquid maldistribution (60/40 instead of 55/45). In this case, the required separation will never be achieved because "column 2" (Fig. 9.26) operates below minimum reflux, which drops the apparent rectifying section efficiency to zero. In practice, the operator will need to increase reflux ratio or accept a lower product purity. Lowering product purify or increasing reflux ratio eliminates the pinch, and the resulting apparent efficiency will return to a value greater than zero.

3. Adequate prediction of the effect of maldistribution on efficiency

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