Introduction to Process Control and Instrumentation

In this chapter the goal for controlling the process will be discussed first, followed by a review of different elements of the control loop. Subsequently, different types of measuring device and their associated dynamics, as well as the characteristics of the correcting device, will be briefly explained. The discrete and continuous controller equation and a simulation of a control loop will be discussed. 

A standard feedback control loop consists of several elements, as shown in Fig. 31.1. 

The process output can be a flow, level, concentration or another type of measurement. The sensor or measuring device to measure the value of the process output variable can therefore be veiy different. Whether the dynamics of the measuring device should be taken into account when analyzing the control loop depends on major time constant of the sensor compared with the major time constant of the process. In a subsequent section, different types of measuring devices will be briefly reviewed. 

Process Dynamics and Control Modeling for Control and Prediction. Brian Roffel and Ben Betlem. 2006 John Wiley Sons Ltd. 

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The book is divided into five parts. Part I provides an introduction to process control and an in-depth discussion of process modeling. Control system design and analysis increasingly rely on the availabihty of a process model. Consequently, the third edition includes additional material on process modeling based on first principles, such as conservation equations and thermodynamics. Exercises have been added to several chapters based on MATLAB simulations of two physical models, a distillation column and a furnace. These simulations are based on the book. Process Control Modules, by Frank Doyle, Ed Gatzke, and Bob Parker. Both the book and the MATLAB simulations are available on the book Web site (www.wiley.com/college/seborg). National Instruments has provided multimedia modules for a number of examples in the book based on their Lab VIEW software. 

Introduction to Process Technology Process Technology 1—Equipment Process Technology 2—Systems Process Instrumentation Safety, Health, and Environment Process Technology 3—Operations Quality Control Troubleshooting Chemistry and Physics Math... 

You should review the contract and the detail specifications to identify whether your existing controls will regulate quality within the limits required. You may need to change the limits, the standards, the techniques, the methods, the environment, and the instruments used to measure quality characteristics. One technique may be to introduce Just-in-time as a means of overcoming storage problems and eliminating receipt inspection. Another technique may be Statistical Process Control as a means of increasing the process yield. The introduction of these techniques needs to be planned and carefully implemented. 

Introduction The chemical processing industry relies on many types of instrumented systems, e.g., the basic process control systems (BPCSs) and safety instrumented system (SIS). The BPCS controls the process on a continuous basis to maintain it within prescribed control limits. Operators supervise the process and, when necessary, take action on the process through the BPCS or other independent operator interface. The SIS detects the existence of unacceptable process conditions and takes action on the process to bring it to a safe state. In the past, these systems have also been called emergency shutdown systems, safety interlock systems, and safety critical systems. 

Automation has been applied for a number of years in process control instrumentation, but the major impetus to introduce automatic devices into laboratories stems from three sources (1) the introduction of the continuous-flow principles as outlined by Skeggs [1] (2) the general demand for clinical chemical measurements, which represents a ready and sizeable market for instrument companies, and, more importantly, (3) the abihty to handle large volumes of data and package them in a form suitable for presentation to analysts and customers, through the use of mini- and micro computer systems hnked to a control computer. 

In analogy to other modern analytical instruments, a computer-based data processing and evaluation system is inserted in all mass spectrometers constructed today. All processes, from sample introduction using an autosampler, e.g., in an ICP-MS, to optimization of experimental parameters in the ion source, ion extraction, separation of ion beams and their registration, the vacuum system and the whole measurement procedure are supported and controlled by a fast and powerful... 

Identifying pharmaceuticals, whether APIs or excipients used to manufacture products, and the end products themselves is among the routine tests needed to control pharmaceutical manufacturing processes. Pharmacopoeias have compiled a wide range of analytical methods for the identification of pharmaceutical APIs and usually several tests for a product are recommended. The process can be labor-intensive and time-consuming with these conventional methods. This has raised the need for alternative, faster methods also ensuring reliable identification. Of the four spectroscopic techniques reviewed in this book, IR and Raman spectroscopy are suitable for the unequivocal identification of pharmaceuticals as their spectra are compound-specific no two compounds other than pairs of enantiomers or oligomers possess the same IR spectrum. However, IR spectrometry is confronted with some practical constraints such as the need to pretreat the sample. The introduction of substantial instrumental improvements and the spread of attenuated total reflectance (ATR) and IR microscopy techniques have considerably expanded the scope of IR spectroscopy in the pharmaceutical field. Raman spectroscopy,... 

In-line moisture of a granulation was discussed by Rantanan et al. in 2000. A binder was added in a solution and the granulation evaluated for residual moisture. Derivatives were used to minimize the particle size effects. Although the measurements were performed (initially) with a full scanning instrument, the authors were able to use fixed wavelength instruments for routine usage. This is an important introduction to simple process control. 

The key to progress in this field was the introduction of the so-called instrumented tablet press by Higuchi and others in the mid 1950s, in which force transducers were fitted to the press to measure the applied load. This revolutionized research into the tabletting process and in addition has lead to the development of presses with automatic tablet weight control, since the mass of particles in the die governs the force detected by the transducer (see the article on Automation of Tablet Presses in this encyclopedia). 

These explanations serve as an introduction to the principles of process control. For further study, see the Additional Selected References. The Process Instruments and Controls Handbook cited previously is particularly recommended. 

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