(195f) Operation of  Dividing-Wall Distillation Columns: A Asymmetrical Temperature Control Scheme

Although double temperature difference control (DTDC) scheme appears to be more effective than temperature control and temperature difference control schemes in the operation of dividing-wall distillation columns (DWDCs), it never means that the DTDC scheme always represents the best option in the aspect of control system performance and complexity. Rather than a symmetrical structure with all control loops involving uniformly three temperature measurements, an asymmetrical structure with all control loops involving a different number of temperature measurements ranging from one to three can be derived and lead to not only improved system performance but also simplified control structure. In the current work, such possibility is highlighted with reference to a frequently studied DWDC system separating an equil-molar ternary mixture of benzene, toluene, and o-xylene. The three candidate controlled variables for each control loop, i.e., temperature, temperature difference, and double temperature difference, are assessed in face of all kinds of feed composition disturbances of frequently encountered magnitudes. Under the condition of strict maintenance of the four product compositions on their specifications, the one with the minimum average variation is selected as the most appropriate controlled variable. Because severe interaction occurs between the chosen controlled variables, three substitutes are then derived. Closed-loop evaluation of these resultant asymmetrical control schemes are carried out through in-depth comparison with the DTDC scheme. Despite the reduction of temperature measurements lowers somehow their abilities to reject feed composition disturbances, the formers all show comparable transient responses and reduced steady-state deviations in the three product purities as compared with the latter in face of feed composition disturbances as large as 15 mol %. Such outcomes demonstrate that simply adopting the DTDC scheme may not yield the expected control system performance; however, attempting an asymmetrical temperature control scheme through insightful process analysis is likely to be an effective alternative.