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Fig. 10.25 Sensitivity of lab-scale methyl formate column. Concentrations of key components in both product streams (upper left). Nominal steady state (upper right) and disturbed steady states (bottom row)

product as a function of the heat duty. Obviously, there is virtually only a single heat duty, which at the same time results in a high methyl formate and a high water concentration in both products. The concentration profiles at this favorable operating point are shown in the upper right diagram of Fig. 10.25. The diagrams below show a significant shift of the steady state concentration profiles if the heat duty is just varied by 1 % from the nominal value. Control is essential to stabilize the desired operating point.

Nonlinear waves are very useful for a qualitative understanding of the concentration and temperature dynamics in an RD column. So far, only an incomplete understanding of the relation between the physicochemical complexity of the mixture, the design and operation of the column, and the observed spatiotemporal patterns is available. Much research is required to resolve the open issues. In addition, the phenomenon of a propagating wave can also be exploited to derive a simplified quantitative description of the column dynamics in the non-reactive [27, 67] as well as in the reactive [4, 51, 52] case. These reduced nonlinear models are most suitable to design and implement advanced model based control systems as discussed in the next section.

10.8 Control

In general, two different modes of operation are possible for distillation columns, no matter whether reactive or non-reactive. The first is the continuous and the second is the batch mode of operation. Focus in this section is on control of continuously operated RD columns. For optimal operation and control of batch columns we refer to the literature [4, 20, 75, 101, 110].

So far, only little work on RD column control is available in the open literature. Most of this work is concerned with case studies for esterification and etherifica-tion processes. Furthermore, the ethylene glycol process has received some attention because of its interesting dynamic behavior, which was summarized and illustrated above. A recent review on control case studies is available in [2]. Only very few general guidelines are available for RD column control including the work of Sneesby et al. [97] and Alarfaj and Luyben [2].

Like in non-RD, a hierarchy of different control tasks exists. On the lowest level, column pressure and the levels in the reboiler at the bottom and the accumulator at the top of the column have to be controlled. On a higher level, product purities have to be controlled. For single product columns only one product composition has to be controlled (one-point control), whereas for two product columns we usually want to control the specifications of both product streams (two-point control). Pressure and level control are similar to non-RD and can be achieved with standard methods. However, composition control of an RD column may introduce some additional complexities as will be illustrated subsequently. In the remainder focus is on two-point composition control.

Control system design consists of two steps: control structure selection includes the choice of suitable manipulated and measured variables as well as their pairing; design and parameterization ofsome control algorithm defines the computation of the required values ofthe manipulated variables from the measurements and given set-points. Let us first focus on the control structure selection problem.

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