Qualitative Plantwide Control System Synthesis

The qualitative design procedure for plantwide control by Luyben et al. (1999) consists of the following steps. 

Step 1. Establish the control objectives. As mentioned above, these are related closely to the process objectives. For example, one may wish to impose a given production rate, while ensuring that the products satisfy the quality specified by the market, and guaranteeing that the process meets environmental and safety constraints. 

Step 6. Fix a flow rate in every recycle loop and control vapor and liquid inventories (vessel pressures and levels). Process unit inventories, such as liquid holdups and vessel pressures (measures of vapor holdups), are relatively easy to control. While vessel holdups are usually non-self-regulating (Guideline 1), the dynamic performance of their controllers is less important. In fact, level controllers are usually detuned to allow the vessel accumulations to dampen disturbances in the same way that shock absorbers cushion 

Step 7. Check component balances. In this step, control loops are installed to prevent the accumulation of individual chemical species in the process. Without control, chemical species often build up, especially in material recycle loops. 

Step 8. Control the individual process units. At this point, the remaining degrees of freedom are assigned to ensure that adequate local control is provided in each process unit. Note that this comes after the main plantwide control issues have been handled. 

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