Distillation steam flow rate, dynamic

As discussed, modelled and simulated by Prenosil (1976), the dynamics of the process bring in the question of steam consumption, steam flow rate, starting time of the distillation, and shut-down time when the desired degree of separation has been reached. The modelling of steam distillation often involves the following assumptions. 

Figure 11.5a shows a typical implementation of feedforward controller. A distillation column provides the specific example. Steam flow to the reboiler is ratioed to the feed flow rate. The feedforward controller gain is set in the ratio device. The dynamic elements of the feedforward controller are provided by the lead-lag unit. 

A schematic of the stripping section of a distillation column with a ratio controller for the bottom products composition is shown in Figure 15.53. This application is similar to ratio control except that dynamic compensation is added to the measured column feed rate. If the steam flow to the reboiler were increased immediately for an increase in column feed rate, the corrective action would initially be an overcorrection. This results because, when a feed rate change occurs, it takes some time for the bottom product composition to respond. The purpose of the dynamic compensation (DC) element is to allow for the correct timing for the compensation for feed rate changes. 

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