Maximized Recovery or Constant Separation

In unit operations control, the individual column variables are treated only as constraints and so long as the values of these constraints are within acceptable limits, the column is controlled (optimized) to maximize production rate, profitability, etc. Economics of individual fractionators may continually change throughout the life of the plant, because energy savings can be important at one particular time, whereas product recovery can be more important at other times. 

A distillation column operating under constant separation conditions has one fewer degree of freedom, because its energy-to-feed ratio is constant. This means that, for each concentration of the key component in the distillate, a corresponding concentration exists in the bottoms. Therefore, if the concentration of a component in one product stream is held constant, that 

The controls required to keep the internal reflux flow of the column constant (a) and the controls needed to eliminate the effect of accumulator lag in controlling the external reflux to a column (b). 

If one product, such as the distillate (D), is worth much more than the other, the control system can be designed to maximize the production of the more 

Post-Oil Energy Technology After the Age of Fossil Fuels 

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