Power of control

Another very great step forward has been made during the past two years by the discovery of substances which can control the development of iral infections. Penicillin, streptomycin, and the sulfa drugs aie effectn e against bacteria but not against viruses. It has recently been found, however, that chloramphenicol (Chloromycetin) and au-reumycin, both of which are substances manufactured by molds (the molds Streptomyces venezuele and Streptomyces aureofaciens respectively), have the power of controlling certain viral infections. 

Even when teacher has to move from his desk his power of control over the class should be such that students continue their work satisfactorily. 

The sulfa drugs and many other substances, such as penicillin, are effective against bacteria but not against viruses. Some substances with the power of controlling certain viral infections have been discovered in recent years. One of these substances is chlortetracycline, C22H23O8N2CI. 

The power of controlled radical polymerisation is demonstrated convincingly and the limitations of the synthetic approaches clearly indicated. 

The environmental model is the product of a problem analysis. It forms the basis for the development of behavioral models of the process. This holds for physical models, in which the internal dynamic mechanism is described by pltysical laws, as well as empirical (black box) models, in which only overall dynamic relationships between process inputs and outputs are formulated. The environmental model can also be used for the development of a process control scheme. By evaluating the static (power of control) and dynamic relationships (speed of control) between process inputs and outputs, the process control scheme can be selected. In general no dynamic model of the process is required yet at this stage. This is the starting point that is used in the chapters on process control. 

In this chapter, the design of a control scheme for an entire plant will be discussed. On the basis of the relationship between process outputs and inputs, the control scheme will be developed. The first part of the procedure is similar to the procedure for the development of an environmental model, which is identifying the inputs and outputs of the process. Measurement problems and costs of the correcting devices, however, should now also be taken into consideration. The result of this procedure is a table with interactions, in which the relationships between the manipulated and controlled variables is shown. The static relationship determines the power of control the dynamic relationship determines the speed of control. The design procedure is illustrated by an example. Subsequently, methods for optimization and extension of the control scheme are discussed. 

Sometimes a weighting between power of control and speed of control is necessary. An example of such a situation is shown in Fig. 33.4. 

When the temperature sensor is located in the top of the column, the speed of control for the reflux is high. However, owing to small temperature fluctuations, often within 0.1 °C, the power of control is limited. If the temperature sensor is located in the middle of the column, the power of control is at its maximum and the speed of control is lower. 

B and D should not be used for pressure control, owing to a lack of power of control. R cannot be used either, unless the degree of under-cooling is extremely large. C and H have a large power of control, since they affect the incoming and leaving vapor flow irrunediately. 

D is not suitable for bottom level control, since the power of control is nil. B is less suitable when the vaporization ratio Vh IB) is large. H, C and R possess usually a large power of control. The response of the liquid flow from the first tray (Li) to changes in R consists of a cascade of first-order responses ... 

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. 

As pointed out earher, the power of control of D is limited when R/D >5. If the reflux flow would stay constant during a change in vapor flow, a relatively large change has to be made in the top draw-off to maintain a constant reflux drum level. This might lead to a violation in a minimum or maximum draw-off constraint violation. Thus, at high reflux ratios (>5), manipulating the reflux flow would result in better controllability and sensitivity of Hhe... 

If energy balance control is used in the case of R/D > 5, the small power of control can be increased by maintaining a constant R/D ratio. The product flow D is still used as correcting variable to maintain a constant reflux dmm level. A higher level will then also result in an increased reflux flow, consequently, the level will respond faster. 

In the case of material balance control, the correcting actions of the quality controller on the product flow have no effect on the distillation column, i.e. the top product quality, until the level controller adjusts the reflux ratio. The controller should therefore be carefully tuned, such that the dynamics of the level control loop are reduced to a minimum. If material balance control is applied and the reflux drum is large, the power of control can be increased by keeping the ratio R/D constant with a flow ratio controller, which is adjusted by the level controller as a master controller. In that case the reflux is still the adjustable variable. 

In principle the setpoints of the controllers that are now in place, eould be used for master-slave control. These are Tj, Fair,sp, and Pr,sp- It may be expected that the setpoint of the reactor pressure controller Pr,sp will not have much impact on the proeess variables that still have to be controlled, i.e. it is expected that the power of control of Pr sp will be very small This leaves us with the remaining three setpoints that can be used for eontrol. Tte situation is given in Table 35.5, in which... 

To determine which setpoint should be used to control which variable, step changes in the three setpoints were given STin sp = + 10 °F, SFair = +0.1 mol/s, SAPrr,sp = -0.3 psia. The results are shown in Figs. 35.10, 35.11 and 35.12. It can easily be seen from Fig. 35.10 that the stack gas oxygen concentration can best be controlled by APrr,sp The response to the other two setpoints do not only give a major inverse response, the power of control is also very small compared with the power of control that is achieved for a change in M rr,sp-... 

---