Criteria For Selection of Best Temperature Control Tray

1 Criteria for selection of best temperature control tray 

Since we have two control degrees of freedom, our objectives in distillation are to control the amount of LK impurity in the bottoms product ( b.lk) and the amount of HK impurity in the distillate ( 5 Hk) Controlling these compositions directly requires that we have composition analyzers to measure them. Instead of doing this, it is often possible to achieve fairly good product quality control by controlling the temperature on some tray in the column and keeping one manipulated variable constant. Quite often the best variable to fix is the reflux flowrate, but other possibilities include holding heat input or reflux ratio constant. 

The typical procedure for selecting which tray to control is to look at the steady-state temperature profile in the column at the base-case conditions, as illustrated in Fig. 6.10. We look for a location in the column where there are large temperature changes from tray to tray. In Fig. 6.10, the slope of the temperature profile is the steepest in the 

But why is this a good way to select the best control tray It works because we want to find a tray where temperature is significantly affected by the compositions of the LK and HK components. Since temperature is also affected by other variables (pressure and other components), it is important that the effects of these variables are small compared with the effects of the compositions of the key components. 

A final method for selecting a temperature control tray location is to use singular value decomposition (SVD) techniques. This approach was first presented by Downs and Moore and is summarized on p. 458 in Luyben and Luyben (1997). A steady-state rating program is used to obtain the gains between the two manipulated variables and the temperatures on all trays. The gain matrix is decomposed by using SVD to find the most sensitive tray locations. This method requires more computation than the others. 

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