Effect of Nonkeys on Temperature Control

When nonkey components enter the column, the light nonkeys rise to the top, while heavy nonkeys descend to the bottom. The concentrations of nonkeys undergo little stage-to-stage variation, except for the regions near the very top, near the very bottom, or near the feed (Fig. 18.2a). As long as the control temperature is kept out of these regions and the concentration of nonkeys in the column feed remains constant, temperature control will not be adversely affected by nonkeys. Consider an increase in the content of heavy nonkeys in the feed. 

In brief, an increase in heavy nonkeys concentration will fool a bottom section temperature controller into letting more light keys out of the bottom, but will have little impact on a top section temperature controller. Conversely, an increase in light nonkeys in the feed will fool a top section temperature controller into letting more heavy non-keys out of the top, but will barely affect a bottom section temperature controller. In two different troublesome cases (239, 378), the top section temperature controller of an isobutane-normal butane splitter was frequently fooled into letting normal butane into the top product each time propane concentration in the column feed suddenly rose. In one of these (378), the problem was cured by using an analyzer/temperature control (see Sec. 18.3). The author is familiar with several similar experiences of temperature controller fooling. 

Intermediate key components can also fool a temperature controller. These tend to build up in close to the feed, often in the neighborhood of the preferred temperature control location, and their accumulation rate and location are seldom predictable. Techniques for preventing intermediate component accumulation are discussed in Sec. 13.7. 

Severe interference of nonkeys or intermediate keys may entirely preclude satisfactory temperature control without composition compensation. An excellent example has been described by Anderson and McMillan (13). Their column (Fig. 18.7) sepeirated water and heavy ether from a mixed alcohol stream, the top product consisting of a volatile water-ether azeotrope and a volatile ether-alcohol azeotrope. Wa- 

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