Pairing for MB Control

Application of MB control, as well as the constraints discussed in the previous section, considerably narrows the options for pairing the remaining variables and streams. For the system shown in Fig. 16.2a, the free stream can only be either the reflux flow or the boilup rate. The other remaining variables are two levels and a composition. 

Additional pairs can be eliminated by recognizing that accumulator level is insensitive to bottom flow rate and bottom level is insensitive to distillate flow rate. This leaves six possible control schemes (Table 16.1). The first three, and to a lesser degree, also the fourth, are the most common schemes and are discussed in detail below. The others are uncommon. An additional uncommon scheme, which defies one of the previously stated ground rules (it does not use pressure to control condensation), but is reported to have worked well in some installations (234), is also included in Table 16.1. 

The author recommends caution if the control system used does not conform to one of the schemes in Table 16.1. Unless different peiram-eters are used or unique circumstances exist, such a method is likely to violate one of the ground rules mentioned esu-lier, and may be troublesome. The first four schemes are generally the least troublesome and should be used whenever possible. 

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