Variables to be controlled

Whenever possible, sense the process variable to be controlled. For example, for pressure control—sense pressure. 

Process variables requiring control in a system include, but are not limited to, flow, level, temperature, and pressure. Some systems do not require all of their process variables to be controlled. Think of a central heating system. A basic heating system operates on temperature and disregards the other atmospheric parameters of the house. The thermostat monitors the temperature of the house. When the temperature drops to the value selected by the occupants of the house, the system activates to raise the temperature of the house. When the temperature reaches the desired value, the system turns off. 

Three methods to derive a state space model for a crystallizer were discussed. The choice of a method in a specific situation depends on the crystallizer configuration, the growth rate kinetics and the variables to be controlled. 

The two prominent variables to be controlled in a continuous polymerization system are the reaction temperature and the monomer conversion achieved in the reaction system. Final polymer properties are directly influenced by changes in these process... 

Process control books and journal articles tend to emphasize problems with a single controlled variable. In contrast, many processes require multivariable control with many process variables to be controlled. In fact, for virtually any important industrial process, at least two variables must be controlled product quality and throughput. In this section, strategies for multivariable control are considered. 

In the first case, product purities are controlled indirectly by controlling front positions. In distillation columns the front positions are easily controlled with cheap, reliable and fast online temperature measurements on sensitive trays inside the column [27]. A similar procedure was recently proposed for moving-bed chromatographic processes with UV rather than temperature measurement [37]. However, the performance of such an approach is usually limited. Exact product specifications cannot be guaranteed because of this indirect approach. Furthermore, in combined reaction separation processes the relationship between the measured variable and the variable to be controlled is often non-unique, which may lead to severe operational problems as shown for reactive distillation processes [23], It was concluded that these problems could be overcome if in addition some direct or indirect measure of conversion is taken into account. 

Before proceeding to the designing methodology itself, it will be helpful to review and define some of the principles and concepts of partial control. We are concerned with the control of a system in which the number of process variables to be controlled is higher than the number of variables which are manipulated to realize this control. If all the process variables have to be controlled according to exact set points, the process has to abandoned or the design modified in order to provide the requisite number of manipulated variables. However, it is quite often the case that many of these variables need only be controlled within prescribed limits, hence the terminology of partial control. 

Two key variables to be controlled in this very simple process are production rate and product molecular weight. Polystyrene is commercially produced in... 

An elaboration of open-loop control which sometimes can be competitive with closed-loop control is feed-forward control, in which the controller is apprised of factors which affect the key process variable to be controlled, but is not directly apprised of the behavior of this variable. Thus for the kettle of Fig. 1, it would be possible to measure the flow rate and temperature of the feed and have a simple computer analyze the information and on the basis of this information set the steam pressure to be maintained by the controller. So long as the computer considered all pertinent factors, the control would be satisfactory, but an unaccounted factor, such as a change in agitator speed, might make the control ineffectual. 

This leaves four variables to be controlled by manipulating five streams. The fifth stream, often termed the "free stream, is usually flow controlled. If the free stream is the boilup rate, it is sometimes controlled by differential pressure. This technique is discussed in Chap. 19. At other times, the free stream is controlled by ratio control (often reflux to feed) or by a detuned composition controller (above). 

Applying first principles to measurements of the ESR signal and to all pertinent instrumental parameters, absolute numbers of spins can be determined. However, this is rarely done since the number of variables to be controlled is considerable and the labor involved is disproportionate. Relative concentrations of species with the same spectral shape and line widths can be determined simply by comparing peak heights of the normal first-derivative curve under identical conditions, i.e.. 

Various in vitro and in vivo methods have been used to predict drug absorption including Caco-2 cells, in situ intestinal permeability, whole-animal studies, and more recently chromatographic methods. Compared with in vivo absorption studies, evaluation of intestinal permeability in vitro requires less compound is relatively easy to study, often avoiding complicated surgery is rapid and can allow a wider variety of variables to be controlled. ... 

Redundancy and motor equivalence Bernstein [ 21 ] postulated the main problem of control of voluntary movement as the ehmination of redundant degrees of freedom. The method used by nature to resolve the problem of redundancy and increase the efficiency of movement comes fi-om the dependencies between components of the motor system. These dependencies could be understood as constraints and they ultimately decrease the number of variables to be controlled. 

The process variable to be controlled, PV that is, any stream- or operation-related variable in the flowsheet (e.g., pressure, temperature, liquid level, species mass or mole fraction, mass or molar flow rate). In addition, the minimum and maximum values of the PV are used to express the PV as a percentage of its full range ... 

Manipulated variables and final elements The manipulated variable, usually flow-rate, has a causal eflect on a key controlled variable, can be manipulated by an automated final element, provides fast feedback dynamics, has the capacity to compensate for expected disturbances and can be adjusted without unduly upsetting other parts of the plant. The final element, usually a valve, has a causal effect on the controlled variable. The number of final dements should be equal to or greater than the number of measured variables to be controlled and we must provide an independent means for controlling every variable. 

The next process variable to be controlled is film thickness (gauge). Either on-line or off-line measurements allow us to monitor film gauge. On-line devices usually employ a radiation source, such as a gamma backscatter system. These devices measure thickness by emitting radiation that reflects back to the sensor from both the near and far surfaces of the film. They can be mounted to measure a fixed location on the bubble or on a carriage to traverse around the bubble (Fig. 3.21). Also, a unit traversing back and forth across the flattened web can be used, but this measures two-layer thickness. Off-line devices typically operate on a capacitance principle, so they don t require the safety practices employed with radiation sources. 

The main variables to be controlled in a distillation column are the following ... 

The basic process control requirements can be established only after an operating philosophy has been developed. The process variables to be controlled must be established. The desired location of measuring elements, control valves, and controllers must be determined. The safety aspects and operational information must be evaluated. 

Moisture content is another key variable to be controlled during the production of glyceryl ether sulfonates. The minimum amount of water in the system should be enough to keep the sodium sulfite, with its limited solubility, completely dissolved in the system. This is another advantage for using the modified Streckerization reaction since bisulfite is more soluble than sulfite, and as less sulfite is needed, the amount of water required is also reduced. The sulfonate product is also highly viscous with poor heat transfer. Therefore, an overall moisture concentration of 50% is recommended to lower the viscosity of the solution and allow for better temperature control. ... 

The dynamic behavior of an extruder is significantly determined by the temperature control system on the extruder. It is, therefore, important to understand the basic characteristics of the various temperature control systems. Most control systems are closed-loop or feedback systems. The variable to be controlled is measured and this information is sent to a control unit. From the control unit a signal is sent to an actuator that adjusts the process such that the control variable is as close as possi-... 

Figure 5.11 shows a typical process, where the input is divided into two types manipulated variables and input disturbances. Input disturbances can be divided further, into measured and unmeasured disturbances. The output variables to be controlled are also divided into measured and unmeasured variables. 

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