Objectives of process control

Ensuring safe plant operation is one of the most important tasks of a control system. This is achieved by monitoring process conditions and maintaining variables within safe operating limits. Potentially dangerous situations are signalled by alarms and a plant may even be shut down automatically. 

Safe operation is consistent with the economic objective of process control which is to operate the plant in such a way to minimize total costs. This involves maintaining product quality, meeting production targets and making efficient use of utilities such as steam, electricity, cooling water and compressed air. 

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The broader objectives of process control engineering include ... 

The primary objective of process control is to maintain a process at the desired operating conditions, safely and efficiently, while satisfying environmental and product quality requirements. The subject of process control is concerned with how to achieve these goals. Luyben3 gives the following process control laws ... 

The main economic objective of process control is to achieve maximum productivity or efficiency while maintaining a satisfactory level of product quality. Manufacturing facilities in the production of chemicals, paper, metals, power, food, and pharmaceuticals require accurate and precise control systems. Although the methods of production vary from industry to industry, the principles of automatic control are generic in nature and can be universally applied, regardless of the size of the plant. 

The primary objective of process control is to cause some controlled variable to remain at or near to some desired specific value. When variables are dynamic, a corrective action must be continually provided to keep the variables in agreement. This is the case in the control of various parameters in textile dyeing, including pH, temperature, fluid flow/direction, multiple injections of chemical anxiliaries and dyes, and, of course, the concentration of dye in the dye liqnor. 

For regulatory control, repeatability is of major interest. The basic-objective of regulatory control is to maintain uniform process operation. Suppose that on two different occasions, it is desired that the temperature in a vessel be 80°C. The regulatoiy control system takes appropriate actions to bring the measured variable to 80°C. The difference between the process conditions at these two times is determined by the repeatability of the measurement device. 

Static charge generation causes an ignition hazard only if the accumulated charges create an electric field sufficient to produce an electrical discharge in a flammable atmosphere. In most processes, this means that the electric field intensity at some location must reach the breakdown strength of air (nominally 3 X lO " V/m). The objective of static-control measures is to ensure that electric field intensities cannot reach this value. 

A lot of natural as well as technological objects of analytical control are colloidal systems, i.e. human blood, biological liquids, sol and suspension forming in different technological processes (ore-dressing, electrochemical deposition, catalysis and other), food, paint-and-lacquer materials, sewage water and other. 

Despite the efforts being made by some research groups to develop a molecular method for identification of parasites in food in developing countries, the main objective of a control program of oral transmission of T. cruzi remains to be prevention, because the processing of many food that may pose some risk is the main source of income for the population. 

The application of this concept to liquid samples is what we already refer to electronic tongue . It entails the use of multidimensional information coming from an array of chemical sensors, mimicking the animal sense of taste. As several possibilities exist on the side of which sensors form the array, the general response shown by the different sensors used is of paramount importance that is, cross-selectivity features are needed in order to profit from the multidimensional aspects of the information [7]. The performance of electronic tongues can be suited not only to qualitative purposes like identification of species and classification of sample varieties, but also to quantitative uses, normally the multidetermination of a set of chemical species, an interesting objective for process control. A more bioinspired trend is the artificial taste [8] in order to perform automated taste perception, especially in the industrial field. 

A risk analysis is not an objective by itself, but is one of the elements of the design of a technically and economically efficient chemical process [1]. In fact, risk analysis reveals the process inherent weaknesses and provides means to correct them. Thus, risk analysis should not be considered as a police action, in the sense that, at the last minute, one wants to ensure that the process will work as intended. Risk analysis rather plays an important role during process design. Therefore, it is a key element in process development, especially in the definition of process control strategies to be implemented. A well-driven risk analysis not only leads to a safe process, but also to an economic process, since the process will be more reliable and give rise to less productivity loss. 

Throughout the design of a chemical plant, issues relating to safety, economics and environmental impact must be considered. By doing so, the risks associated with the plant can be minimised before actual construction. The same principle applies whatever the scale of the process. The field of process control (Chapter 8) considers all these issues and is, indeed, informed by the type of hazard analyses described in Chapter 10. The objectives of an effective control system are the safe and economic operation of a process plant within the constraints of environmental regulations, stakeholder requirements and what is physically possible. Processes require control in the first place because they are dynamic systems, so the concepts covered in the earlier chapters of this book are central to process control (i.e. control models are based on mass, energy and momentum balances derived with respect to time). Chapter 8 focuses on the key aspects of control systems. 

Selection of Process Controlled and Manipulated Variables. At the calculated optimum X (x, m, d ) of the above problem (Pi), some of the inequality constraints will be active. The regulatory control objectives and the active design constraints (i.e. g ) at the current optimum will constitute the class of primary controlled variables denoted by c, i.e. 

Steps 1 and 2 establish the objectives of the control system and the available degrees of freedom. Step 3 ensures that any production of heat (entropy) within the process is properly dissipated and that the propagation of thermal disturbances is prevented. In Steps 4 and 5 we... 

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. ... 

PI encompasses not only the development of novel, more compact equipment but also the development of intensified methods of processing, such as the use of alternative energy sources [4]. In addition, one of the objectives of process intensification is to move away from batch processing to small continuous reactors. The better control of the reaction conditions often allows us to improve not only the... 

The most important lesson to remember is that our focus as engineers must be on the process. We must understand its operation, objectives, constraints, and uncertainties. No amount of detailed modeling, mathematical manipulation, or supercomputer exercise will overcome our ignorance if we ignore the true subject of our work. We need to think of Process control with a capital P and a small c. 

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