Process control, automatic time constant

Adaptive Control. An adaptive control strategy is one in which the controller characteristics, ie, the algorithm or the control parameters within it, are automatically adjusted for changes in the dynamic characteristics of the process itself (34). The incentives for an adaptive control strategy generally arise from two factors common in many process plants (/) the process and portions thereof are really nonlinear and (2) the process state, environment, and equipment s performance all vary over time. Because of these factors, the process gain and process time constants vary with process conditions, eg, flow rates and temperatures, and over time. Often such variations do not cause an unacceptable problem. In some instances, however, these variations do cause deterioration in control performance, and the controllers need to be retuned for the different conditions. 

Traditional schedules result in a drying rate that decreases with time, only partly countered by increases in the dry-bulb temperature and the wet-bulb depression as the schedule proceeds. Modem automatic process control means that the kiln schedule can be adjusted continuously so ensuring a more constant rate of heat transfer and evaporation. Also this avoids any shock that an abrapt change in the schedule imposes on the timber. 

The services of the analytical chemist are constantly increasing as more and better analytical tests are developed, particularly in the environmental and clinical laboratories. The analyst often must handle a large number of samples and/or process vast amounts of data. Instruments are available that will automatically perform many or all of the steps of an analysis, greatly increasing the load capacity of the laboratory. The data generated can often be processed best by computer techniques computers may even be interfaced to the analytical instruments. An important type of automation is in process control whereby the progress of an industrial plant process is monitored in real time (i.e., online), and continuous analytical information is fed to control systems that maintain the process at preset conditions. 

Some typical important industrial applications of coulometry include the continuous monitoring of mercaptan concentration in the materials used in rubber manufacture. The sample continuously reacts with bromine, which is reduced to bromide. A third electrode measures the potential of B12 vs. Br and, based on the measurement, automatically regulates the coulometric generation of the bromine. Coulometry is used in commercial instruments for the continuous analysis and process control of the production of chlorinated hydrocarbons. The chlorinated hydrocarbons are passed through a hot furnace, which converts the organic chloride to HCl. The latter is dissolved in water and the Cl titrated with Ag" ". The Ag" " is generated by coulometry from a sUver electrode, Ag°. It is necessary for the sample flow rate to be constant at all times. Integration of the coulometric current needed to oxidize the silver to silver ion results in a measurement of the Cl concentration. 

Since the columns must be interchangeable, the full set of connections must be provided on them all, and this involves a fairly extensive set of control valves. Since the start of one feed corresponds precisely with the cessation of another, it is convenient to employ multi-port valves, and various mechanical designs are available. These may be manually operated or may be driven by motors controlled by time switches if operating on a constant time cycle and with constant flow rates. A poppet valve assembly of this type is shown in Fig. 3.5. If each cycle must be unique to cope with variations of feed concentrations, etc., it is possible in some processes to control the motorized valve assemblies by means of automatic analysers on the effluent, eluent and wash streams. Instrumentation of this type can only be installed after very thorough development trials with process solutions under a wide variety of anticipated operating conditions, if reliable operating results are to be obtained. 

Steady state operations play a very important role in chemical engineering due to the easiness of material and energy recycling and the ability of set point control. Nevertheless it is very unlikely that steady state operations provide the best in conversion and selectivity. Since progress in automatic process control brings nowadays essentially ev forcing function within reach, there is no need to keep the process steady state from that point of view. Also the recyclability of mass and energy is still possible for non-steady operations if the time constant of the cycles is within reasonable bounds. 

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... 

The titration process has been automated so that batches of samples can be titrated non-manually and the data processed and reported via printouts and screens. One such instrument is the Metrohm 670 titroprocessor. This incorporates a built-in control unit and sample changer so that up to nine samples can be automatically titrated. The 670 titroprocessor offers incremental titrations with variable or constant-volume steps (dynamic or monotonic titration). The measured value transfer in these titrations is either drift controlled (equilibrium titration) or effected after a fixed waiting time pK determinations and fixed end points (e.g. for specified standard procedures) are naturally included. End-point titrations can also be carried out. 

Fig. 5.2a shows examples of the results obtained on the dissolution of 8-AI203. In batch experiments where pH is kept constant with an automatic titrator, the concentration of AI(III)(aq) (resulting from the dissolution) is plotted as a function of time. The linear dissolution kinetics observed for every pH is compatible with a process whose rate is controlled by a surface reaction. The rate of dissolution is obtained from the slope of the plots. 

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