Ryskamp scheme

In addition to the energy balance or material balance options, there are various hybrid schemes - the most weU known of which is the simplified Ryskamp scheme (Reference 1). In this version of the scheme we add a reflux ratio controller to the energy balance scheme. 

A second advantage of the Ryskamp scheme is that, compared to the material balance scheme, it makes the bottoms composition more sensitive to changes in reboiler duty, as seen in Figure 12.66. This means that less adjustment of reboiler duty is required -reducing the interaction further. 

Of course, because the Ryskamp scheme offers benefits in one operating scenario does not mean it is universally the best scheme. Figure 12.67 shows how each of the schemes perform as feed composition changes. As we expect, the energy balance scheme maintains the distillate composition closer to its target than the material balance scheme. But, despite keeping the reflux ratio constant, the Ryskamp scheme performs poorly. 

Figure 12.66 Performance of Ryskamp scheme for bottoms composition...

Figure 12.69 shows the original version of the Ryskamp scheme. This maintains constant a different definition of reflux ratio, i.e. R/ R + D). This is chosen because it is the slope of the top operating line on the McCabe-Thiele diagram - see Equation (12.18). The output of the level controller isR + D. From this is subtracted R to generate the SP for the distillate flow controller. The reflux ratio target is multiplied by the level controller ouQjut to generate the SP for the reflux flow controller. 

Figure 12.70 Effect of feed composition on the original Ryskamp scheme...

Figure 12.88 Impact of lower temperature control (Ryskamp scheme)...

We have seen that the Ryskamp scheme largely breaks the interaction in one direction so that corrections made to the bottoms composition have little impact on the distillate. Although the converse is not true an adjustment to the reflux ratio will affect the bottoms composition, but when its controller takes corrective action it will not light the distillate composition controller. 

Figure 12.68 explains why Ryskamp does not handle feed composition changes as well as the energy balance scheme. As we know from Equation (12.25) distillate product rate (D) varies linearly with feed composition (LKf). Perhaps what is not immediately obvious is... 

The name material balance control was introduced by Shinkey (1984). The different control schemes that the author developed were based on the concept of relative gains (= power of control) of the different input-output combinations. Speed of control was only considered as a secondary factor. A simple explanation is given by Ryskamp (1980). Also Van der Grinten (1970) presented a nrrmber of common control schemes for distillation colnmns. The latter author used behavioral models in the eontrol scheme selection procedure. None of the mentioned references takes inverse responses into accoimt when X > 0.5. In the case of the more traditional approach, the energy balanee eontrol, the reflux ratio and/or vapor flow is used to eontrol the top product qrrality, while the distillate and bottom flow are nsed to maintain the mass balanee. In the ease of the material balance control, one of the prodnct flows is used to control product qrrahty, while the other product flow maintains the material balance. 

The basic distillation control configurations have been listed above. However, there are many other configurations which use linear or even nonlinear combinations of the basic manipulated variables. One common example, which is sometimes called Ryskamp s scheme [1], manipulates the reflux ratio L/D, via ratio control, and the reboiler duty V. Another relatively common scheme is the double ratio configuration, which manipulates the reflux ratio and the boil-up ratio. This scheme has been widely recommended as it results in relatively small interactions between the two control loops. This concept will be discussed in further detail at a later stage. 

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