Control Structure

Although the control objectives have been established, the actual controlled variables and manipulated variables are still to be decided. Many choices are available but the most likely candidates for the controlled variables are the various flows, temperatures, pressures and compositions in the system. However, the final choice is not restricted to these parameters and the use of composite variables such as the column flooding factor or the energy consumption could be advantageous for some economic scenarios. The manipulated variables are effectively selected in choosing the control structure that will be used.

A control structure which adequately supports all three control modes is required. Although the specific controlled variables have not yet been selected for each mode, some general properties of the various control configurations allow a satisfactory selection to be made forthwith.

The energy-balance control configurations (e.g. LV) manipulate the internal liquid and vapour flows, which affccl the feed-split and fractionation immediately and directly, while the material-balance configurations (e.g. DV, LB, etc.) manipulate external variables (i.e. the product flow rates) and rely on these changes being transmitted to the column via the level control loops. These level control loops should be stable and in near-perfect control in order to maintain good dynamic responsiveness with a material-balance control structure. Conditions for the stability of the inventory controllers (with linear control algorithms) were derived in Section 10.2.2 and it was concluded that these conditions do not hold for the material-balance control configurations where output multiplicity exists. Although output multiplicity was not detected in this particular column, the presence of multiple steady states in other similar columns (both MTBE and ETBE) suggests that the selection of an energy-balance structure would be prudent.

This configuration is very widely used in industrial practice for non-reactive distillation. The considerations described above suggest that it is likely to be widely used for reactive distillation too and this was confirmed for open-loop, one-point and two-point control of the 10 stage ETBE column in Chapter 9 and Section 10.1.

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