Process control, automatic elements

Automatization of all stages of the analytical process is a trend that can be discerned in the development of modern analytical methods for chemical manufacture, to various extents depending on reliability and cost-benefit considerations. Among the elements of reliability one counts conformity of the accuracy and precision of the method to the specifications of the manufacturing process, stability of the analytical system and closeness to real-time analysis. The latter is a requirement for feedback into automatic process-control systems. Since the investment in equipment for automatic online analysis may be high, this is frequently replaced by monitoring a property that is easy and inexpensive to measure and correlating that property with the analyte of interest. Such compromise is usually accompanied by a collection of samples that are sent to the analytical laboratory for determination, possibly at a lower cost. 

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. ... 

Inside microfluidic channels the flow is laminar. Due to the absence of turbulences different media do not mix or mix only spontaneously and very slowly. Automatic elements can only operate correct in the presence of sufficient process medium. Otherwise, in the absence of stimulation, the control functionality can not be performed. Therefore, inside microfluidic channel networks the realisation of a recirculation is recommended which can provide an independency from the state (open or closed) of the active hydrogel components. 

Figure 9-17 is a schematic diagram of a typical closed control loop. If the error detection is done by an experienced human operator, then the process is controlled manually. Manual control is used only where the required instrumentation either is not available or is too expensive from either a first cost or maintenance standpoint. Mechanical and electrical error detecting elements or controllerB are almost always used in closed-loop control to give a fully automatic process control system. 

It would be advisable to look through this guide to become familiar with the concepts of process instrument elements. Their use in automatic process control systems will be explained next. 

As defined here, automatic process control always implies the use of a feedback. This means that the control instrument is continuously monitoring certain output variables of the controlled process, such as a temperature, a pressure, or a composition, and is also compeuing this output with some preestablished desired value, which is considered a reference, or a set point, of the controlled variable. An error that is indicated by the compeuison is used by the instrument to compute a correction to the setting of the process control valve or other final control element in order to adjust the value of the outpirt variable to its desired level and maintain it there. 

The process operators control strategy is based on anticipation, knowledge (know-how), and experience, and can be considered as a set of heuristic decision rules or rules of thumb. In order to recognize the human s basic elements and to include the operator s control actions in an automatic scheme, a good user-system interface is required. A useful way to combine this user-system interface with process control is to use intelligent control systems. 

Final control elements are typically automated valves however, motors or other electrical devices can be used. The final control element is the last link in the modern control loop and is the device that actually makes the change in the process. Automatic valves open or close to regulate the process. Control loops usually have (1) a sensing device, (2) a transmitter, (3) a controller, (4) a transducer, and (5) an automatic valve. Automatic valves can be controlled from remote locations, making them invaluable in modern processing. 

Iron sintering mix control and composition stabilization. For an efficient sintering process, a constant and optimized basicity of raw mix without short and long term fluctuations is a must. Achieving real-time automatic process control without human factor influence requires on-line elemental composition data. Figure 8.29a presents typical breakdown spectra of the sintering mix and the results of industrial LIBS unit test data, where laboratory CaO control data are compared with online analyzer readings. One hundred and forty samples have been taken from conveyer belt and send to laboratory for control analysis. It was found that the correlation of... 

Fluorescence spectrometers are widely used in the metal industry. Frequently, parallel spectrometers are employed. Such an instrument actually consists of a number of crystal spectrometers, each set for a particular emission line. The spectrometers are arranged around the sample, which is irradiated by an X-ray tube. One of the spectrometers is set for a standard sample that is contained in the sample holder. In this way the intensity of the X-ray tube can be monitored. Frequently, a measurement is terminated when a preset number of comits for the reference sample has been obtained. The corresponding number of counts from the other detectors can then be directly used for a pai allel assessment of the elemental composition of the sample. With a sequential spectrometer, a number of selected elements are measured sequentially by turning the crystal and the detector to preset positions. With computer steering the measurement process is automatic. This type of instrument is well suited for varying types of analysis, whereas parallel spectrometers are more suited to continuous control operation of, for example, a steel mill in near-real time. 

When expanded, the PFPF will be able to produce 15t MOX/y. Instead of using the existing fabrication method, which depends upon manual work in glove boxes, a fabrication process with automatic remote operation techniques has been applied in the PFPF to improve productivity and reduce external exposure. The main process consists of pellet fabrication, fuel element fabrication and assembling Each process involves remote handling controlled by a computer. 

The relay feedback experiment was made popular in the field of process control by Astrom and Hagglund (1984). This experiment was suggested as a means to automate the Ziegler-Nichols scheme for determining ultimate gain and frequency information about a process. Their approach followed directly from a describing function approximation (DFA) to the nonlinear relay element. The objective was to use the obtained process information for automatic tuning of PID controllers. 

The determining of sorting limits of steel parts after thermal processing in order to eliminate these, which indicate exceeded allowed content of residual austenite, requires elements of identical shape and dimensions, as the studied parts, and with known content of residual austenite. Such elements serve to define the sorting thresold, during manual control as well as automatic... 

BAL. In many cases, controller output signals of the automatic mode and manual mode may not be the same. If the controller were directly transferred from automatic to manual or manual to automatic, the controller output signal could suddenly change from one value to another. As a result, the final control element would experience a sudden change in position or "bump." This can cause large swings in the value of the process variable and possible damage to the final control element. 

Please note, that if the identified contributing types of latent conditions were resolved, i.e. sufficient information from the transformation process its history of deviations and no cost pressure from stockholders were present, the control element turns effective. However, this does not automatically imply that the corresponding precursor will be alleviated. It is still possible that a control element preceding the previous initial ineffective control element, is also ineffective. 

We will consider all the components of this temperature control loop in more detail later in this book. For now we need only appreciate the fact that the automatic control of some variable in a process requires the installation of a sensor, a transmitter, a controller, and a final control element (usually a control valve). Most of this book is aimed at learning how to decide what type of controller should be used and how it should be tuned, i.e., how should the adjustable tuning parameters in the controller be set so that we do a good job of controlling temperature. 

The interface with the process at the other end of the control loop is made by the final control element. In a vast majority of chemical engineering processes the final control element is an automatic control valve which throttles the flow of a manipulated variable. Most control valves consist of a plug on the end of a stem that opens or closes an orifice opening as the stem is raised or lowered. As sketched in Fig. 7.5, the stem is attached to a diaphragm that is driven by changing air pressure above the diaphragm. The force of the air pressure is opposed by a spring. 

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