Controlling Two Products

Post-Oil Energy Technology After the Age of Fossil Fuels 

Configuration for controlling the composition of both products of a distillation column without much interaction (left) and with interaction (right). 

Consequently, the bottom composition (x) has to be controlled by manipulating the energy balance of the column. The control system computes V based on the equation, V = F(a + b[V/F]), where [V/F] equals the desired ratio of boil-up to feed. 

Interaction is unavoidable between the material and energy balances in a distillation column. The severity of this interaction is a function of feed composition, product specification, and the pairing of the selected manipulated and controlled variables. It has been found that the composition controller for the component with the shorter residence time should adjust vapor flow, and the composition controller for the component with the longer residence time should adjust the liquid-to-vapor ratio, because severe interaction is likely to occur when the composition controllers of both products are configured to manipulate the energy balance of the column and thereby fight each other. 

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Manipulated Inputs. Before delving into a detailed quantitative analysis, we need to identify the manipulated variables for these three different types of processes. As pointed out by Luyben, it is important to maintain stoichiometric balance for neat reactive distillation. Al-Arfaj and Luyben chose to use one of the feedrates. In this chapter the feed ratio is used as one manipulated variable. In addition to holding the stoichiometric balance, we need to control two product compositions using two manipulated variables. However, for reactions such asA + B C + D, if the conversion is properly maintained and the product flowrates are equally distributed, one-end composition control will do a fairly good job. 

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