Process control, automatic controllers

An extraction plant should operate at steady state in accordance with the flow-sheet design for the process. However, fluctuation in feed streams can cause changes in product quaUty unless a sophisticated system of feed-forward control is used (103). Upsets of operation caused by flooding in the column always force shutdowns. Therefore, interface control could be of utmost importance. The plant design should be based on (/) process control (qv) decisions made by trained technical personnel, (2) off-line analysis or limited on-line automatic analysis, and (J) control panels equipped with manual and automatic control for motor speed, flow, interface level, pressure, temperature, etc. 

Specialized equipment for industrial measurements and automatic control have been developed (18) (see Process control). In general, the pH of an industrial process need not be controlled with great accuracy. Consequendy, frequent standardization of the cell assembly may be uimecessary. On the other hand, the ambient conditions, eg, temperature and humidity, under which the industrial control measurements are made, may be such that the pH meter must be much more robust than those intended for laboratory use. To avoid costiy downtime for repairs, pH instmments may be constmcted of modular units, permitting rapid removal and replacement of a defective subssembly. 

C. A. Smith and A. B. Corripio, Principles and Practice of Automatic Process Control, Am. Wiley Sons, Inc., New York, 1985. 

Adaptive Control Process control problems inevitably require on-hne tuning of the controller constants to achieve a satisfactory degree of control. If the process operating conditions or the environment changes significantly, the controller may have to be retuned. If these changes occur quite frequently, then adaptive control techniques should be considered. An adaptive control system is one in which the controller parameters are adjusted automatically to compensate for changing process conditions. 

In continuous processes where automatic feedback control has been implemented, the feedback mechanism theoretically ensures that product quality is at or near the set point regardless of process disturbances. This, of course, requires that an appropriate manipulated variable has been identified for adjusting tne product quality. However, even under feedback control, there may be daily variations of product quahty because of disturbances or equipment or instrument malfunctions. These occurrences can be analyzed using the concepts of statistical quahty control. 

System includes automatic actuation of most activities, with changing of feed lines to silos accomplished by diverter valves controlled automatically by process control computer. 

Bristol, E.H., 1966. On a new measure of interactions for multivariable process control, IEEE Transactions of Automatic Control, AC-11, 133. 

The alternative is hexane, which because of the explosion hazard requires a more expensive type of extractor construction. After the extraction the product is dull gray. The continuos sheet is slit to the final width according to customer requirements, searched by fully automatic detectors for any pinholes, wound into rolls of about 1 m diameter (corresponding to a length of 900-1000 m), and packed for shipping. Such a continuous production process is excellently suited for supervision by modern quality assurance systems, such as statistical process control (SPC). Figures 7-9 give a schematic picture of the production process for microporous polyethylene separators. 

The calender was developed over a century ago to produce natural rubber products. With the developments of TPs, these multimillion dollar extremely heavy calender lines started using TPs and more recently process principally much more TP materials. The calender consists essentially of a system of large diameter heated precision rolls whose function is to convert high viscosity plastic melt into film, sheet, or coating substrates. The equipment can be arranged in a number of ways with different combinations available to provide different specific advantages to meet different product requirements. Automatic web-thickness profile process control is used via computer, microprocessor control. 

Continuous in-line measurements and control of the mass material balance in the process, with automatic feedback to the reactants dosing devices (performed either by computerized system or by traditional flow control loops). 

A high pressure gel permeation chromatograph (GPC) has been used to monitor the performance of the reactor. A novel aspect of the GPC is that, it too, has been put on-line to the process control computer and both data collection and analysis have been made automatic while giving the operator full interactive facilities. 

J. W. Westwater, Boiling of Liquids Ernest E Johnson, Automatic Process Control... 

Automatic process control involves the maintenance of a desired value of a measured or estimated quantity (controlled variable) within prescribed limits (deviations, errors), without the direct action of an operator. Generally, this involves three steps ... 

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. 

Boiling of Liquids J. W. Westwater Automatic Process Control Ernest F. Johnson... 

Conductivity sensors are most commonly used for safety purposes in household appliances. Presence and absence of washing liquor, detergency, and water softener can be easily measured and proper operation ensured [71]. The various applications mainly differ by their design of electrode geometry and methods for electrical measurement. Due to the close relation between ionic conductivity and water hardness, the automatic water softener in an automatic dishwasher can be controlled by a conductivity sensor [72]. To isolate the transmission of the measured value from the process controller, the conductivity sensor could incorporate an opto-electronical coupling [73]. Thus, protective insulation of the electrodes in a washer-dryer could be ensured. 

The most utilized and reliable process control in the petrofeum and related industries is human observation and surveillance. Local pressure and level gages along with control room instrumentation are provided so that human observation and actions can occur to maintain the proper process conditions. First stage process alarms are provided to alert operators to conditions that they may not have already noticed. Typically when secondary alarm stages are reached, computer control systems employed to automatically implement remedial actions to the process. 

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