Automatic-control schemes

The detailed design and specification of the automatic control schemes for a large project is usually done by specialists. The basic theory underlying the design and specification of automatic control systems is covered in several texts Coughanowr (1991), Shinskey (1984) (1996) and Perry et al. (1997). The books by Murrill (1988) and Shinskey (1996) cover many of the more practical aspects of process control system design, and are recommended. 

In this chapter only the first step in the specification of the control systems for a process will be considered the preparation of a preliminary scheme of instrumentation and control, developed from the process flow-sheet. This can be drawn up by the process designer based on his experience with similar plant and his critical assessment of the process requirements. Many of the control loops will be conventional and a detailed analysis of the system behaviour will not be needed, nor justified. Judgement, based on experience, must be used to decide which systems are critical and need detailed analysis and design. 

Some examples of typical (conventional) control systems used for the control of specific process variables and unit operations are given in the next section, and can be used as a guide in preparing preliminary instrumentation and control schemes. 

The following procedure can be used when drawing up preliminary P and I diagrams  

Identify and draw in those control loops that are obviously needed for steady plant operation, such as  

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Experienced process control engineers are usually responsible for the design and specification of automatic control schemes on large chemical plants. The book by Shinskey (1979) provides details of the practical... 

Both modes usually are conducted with constant vaporization rate at an optimum value for the particular type of column construction. Figure 13.9 represents these modes on McCabe-Thiele diagrams. Small scale distillations often are controlled manually, but an automatic control scheme is shown in Figure 13.9(c). Constant overhead composition can be assured by control of temperature or directly of composition at the top of the column. Constant reflux is assured by flow control on that stream. Sometimes there is an advantage in operating at several different reflux rates at different times during the process, particularly with multicomponent mixtures as on Figure 13.10. 

Optical spectroscopies are appropriate for many process analysis applications because they can provide a rapid analysis of process stream composition in an industrially robust form requiring little regular maintenance. On-line implementation means that extracting manual samples is avoided and results obtained using computerized data handling methods can be hnked to automatic control schemes. Recently a review of spectroscopy in process analysis has been published which contains descriptions and describes applications of optical spectroscopies [2]. 

Manual control implies that the operator makes the changes in manipulated variable, which is used occasionally. Feedback control connotes that the manipulated variable is changed automatically in response to the error between the set point and controlled variable. The second approach serves as the basis for most automatic control schemes. Feedforward control is a technique where the manipulated variable is changed as a function of a disturbance variable. Both feedback and feedforward control are discussed in later sections of this chapter. 

As an initial (demonstration) application of the Icon/1000 control system, we automated two simultaneous acrylic lab polymerizations. In this application, heaters, agitators, and metering pumps are controlled. A batch proceeds automatically from state to state unless the operator intervenes through one of a series of color CRT touch screens allowing him to take complete manual control of the batch for as long as he desires. All important process variables are continually monitored and recorded. The entire control scheme was created, tested, and modified several times in the space of two months, without formal instruction, by a chemical engineer with little previous programming experience and no previous experience at all with this system. 

We have begun to include detailed automatic alarm-handling in our control schemes, in order to begin unattended, eventually overnight operation. At present, any polymerization requiring more than one work day must be stopped and restarted on a second day. This prevents us from accurately simulating plant processes extending over more than one shift. Safe, unattended automatic lab reactor automation should, then, improve scale-up efficiency for many of our polymers. 

The height of the liquid interface should be measured accurately when the liquid densities are close, when one component is present only in small quantities, or when the throughput is very small. A typical scheme for the automatic control of the interface, using a level instrument that can detect the position of the interface, is shown in Figure 10.40. Where one phase is present only in small amounts it is often recycled to the decanter feed to give more stable operation. 

Total unit heat dnty will typically be in the range of 500-1000 BTU per pound of feed to the unit. This set of process heat requirements establishes the amount of heat that must be supplied by combustion of coke. Because of the process control schemes that are normally employed in FCCUs, the unit operation will automatically adjust itself so that the energy produced via coke combustion equals the heat requirements of the process. If the balance is shifted by changes to the feed quality or operating conditions, shifts in catalyst circulation rate and regenerator temperature will occur until a new equilibrinm set of conditions is established. 

Hence, as a result of the unsuitability and instability of the automatic flow-control system, an alternative means of control is required This can be achieved by setting up a control scheme baaed on pressure drops, the third key characteristic of the liquid/liquid hydrocydone. 

X. Zhang, M.M. Polycarpou, and T. Parisini. A robust detection and isolation scheme for abrupt and incipient faults in nonlinear systems. IEEE Transactions on Automatic Control, 47 576-593, 2002. 

Continuous Control automatic and control loops, database storage, scan frequencies, complex control schemes (cascade, ratio, predictive)... 

We see from the above that with flow reversal a certain amount of the mixture of ions A and D is circulating in the apparatus. As a consequence the D ion can play the role of indicator. The presence of the transition zone, from the mixture of ions A and D to the A ions alone at the bottom of the columns, is characterized by appreciable change in the properties of the ion exchanger-solution system. This enables the establishment of automatic control for column operation. A similar scheme of flow reversal may be utilized in column I. 

A control system or scheme is characterized by an output variable (e.g., temperature, pressure, liquid level, etc.) that is automatically controlled through the manipulation of inputs (input variables). Suppressing the influence of external disturbances on a process is the most common objective of a controller in a chemical plant. Such disturbances, which denote the effect that the external world has on a process, are usually out of reach of the human operator. Consequently, a control mechanism must be introduced that will make the proper changes on the process to cancel the negative impact that such disturbances may have on the desired operation of the process. Control engineers usually refer to the combination of a sensing element and a control device with a set point as a control loop. ... 

In practice, SMB processes are controlled using similar manual schemes (Kiisters et al., 1995, Juza, 1999 and Miller et al., 2003). Antia (2003) suggested that these heuristic rules are included in a fuzzy controller to achieve full automatic control of SM B processes, but no applications have been described so far. Cox et al. (2003) recently reported a successful control and monitoring system for the separation of an enantiomer mixture based on the concentration profiles in the recycle loop. 

In this chapter we have advocated the use of online model-based optimization for the automatic control of SMB plants. This approach has the advantage that the process is automatically operated at its economic optimum while meeting all relevant constraints on purities and flow rates. Application to a pilot-plant-scale reactive SMB process for glucose isomerization showed that implementation at a real plant is feasible - the requirements for additional hardware are moderate (a high-level PC and online concentration measurements in the recycle line). The experiments confirmed the excellent properties of the proposed control scheme. The scheme is extremely versatile, and the cost function and constraints can easily be adapted to any specific separation task. 

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