Selective control systems

Exhaust controls, 26 717-719 Exhaust control system, selection of, 10 74t Exhaust control technology, 10 78-79, 96-104... 

Unfortunately, most set points are near the neutral or equivalence points on the titration curve, which coincide with location of the maximum slope. After an upset or start-up, the pH accelerates as the operating point approaches the steepest point on the curve. To the pH controller that knows only what it sees, it appears to be a fast runaway or positive feedback response. Most of the beneficial value of the investment in a well mixed vessel in terms of slowing down a disturbance has been lost, The decrease in the effective process time constant seen by the pH controller will be quantified in Chapter 10 on pH control system selection. Operators will ask whether you can do anything to slow down the response because it tends to zip right past the desired set point. In Chapter 8 on pH control systems, we will see how signal characterization can slow down the response by the translation of the controlled variable from pH to percent reagent demand (X axis of the titration curve) to soothe the operator s nerves and restore the process time constant. 

For control system selection, the main questions pertain to the number of stages, type and size of equipment, location of the set points, and the most appropriate advanced control technology. The question as to the number of stages and type of equipment mostly applies to neutralization systems. For pH control of unit operations for production such as fermentation, reaction, and crystallization, the process equipment is predetermined. 

A new dimension to acid-base systems has been developed with the use of zeolites. As illustrated in Fig. XVIII-21, the alumino-silicate faujasite has an open structure of interconnected cavities. By exchanging for alkali metal (or NH4 and then driving off ammonia), acid zeolites can be obtained whose acidity is comparable to that of sulfuric acid and having excellent catalytic properties (see Section XVIII-9D). Using spectral shifts, zeolites can be put on a relative acidity scale [195]. An important added feature is that the size of the channels and cavities, which can be controlled, gives selectivity in that only... 

Defining the requirements for a pilot-plant control system is often difficult because process plant experience for comparison and evaluation is commonly lacking and the design is frequentiy performed by personnel inexperienced in either instmmentation systems or pilot-plant operations. The isolated and often intermittent nature of pilot-plant operations also inhibits evolution and promotes individual unique installations. This compHcates the selection process. 

The support resources available to service, repak, and caUbrate the control system must also be considered when selecting a system they are considerable and can be estimated from the Hterature (48—50). 

Extensive efforts have been made to develop catalyst systems to control the stereochemistry, addition site, and other properties of the final polymers. Among the most prominant ones are transition metal-based catalysts including Ziegler or Ziegler-Natta type catalysts. The metals most frequentiy studied are Ti (203,204), Mo (205), Co (206-208), Cr (206-208), Ni (209,210), V (205), Nd (211-215), and other lanthanides (216). Of these, Ti, Co, and Ni complexes have been used commercially. It has long been recognized that by varying the catalyst compositions, the trans/cis ratio for 1,4-additions can be controlled quite selectively (204). Catalysts have also been developed to control the ratio of 1,4- to 1,2-additions within the polymers (203). 

Fig. 1. Process flow diagram for the selection of an exhaust control system.

Selective and Override Control When there are more controlled variables than manipulated variables, a common solution to this problem is to use a selector to choose the appropriate process variable from among a number of available measurements. Selec tors can be based on either multiple measurement points, multiple final control elements, or multiple controllers, as discussed below. Selectors are used to improve the control system performance as well as to protect equipment from unsafe operating conditions. 

The selection of controlled and manipulated variables is of crucial importance in designing a control system. In particular, a judicious choice may significantly reduce control loop interactions. For the blending process in Fig. 8-40(d ), a straightforward control strategy would be to control x by adjusting w, and w by adjusting Wg. But... 

Neveril, R. B., Capital and Operating Costs of Selected AirFollution Control Systems, EPA Report 450/5-80-002, Gard, Inc., Niles, IL, December 1978. ... 

The fact that batch processes are not carried out at steady state conditions imposes broad demands on the control system. The instrumentation and control system have to be selected to provide adequate control for a wide variety of operating conditions and a wide variety of processes. In addition, basic process control and shutdown systems have to deal with sequencing issues. This chapter presents issues and concerns related to safety of instrumentation and control in batch reaction systems, and provides potential solutions. 

Load sharing or selective load shedding is of interest to many users of hot gas expanders. A particularly successful European FCC application is illustrated in Figure 6-43. The addition of an expander-generator set to the FCC unit at a major refinery presented a challenge because a trip of the expander could upset the process. The company that is the subject of this application case study, GHH Borsig, solved this problem with the installation of a computerized control system and through computer simulation of trips. 

One of the methods of controlling air pollution mentioned in the previous chapter was pollution removal. For pollution removal to be accomplished, the polluted carrier gas must pass through a control device or system, which collects or destroys the pollutant and releases the cleaned carrier gas to the atmosphere. The control device or system selected must be specific for the pollutant of concern. If the pollutant is an aerosol, the device used will, in most cases, be different from the one used for a gaseous pollutant. If the aerosol is a dry solid, a different device must be used than for liquid droplets. 

Not only the pollutant itself but also the carrier gas, the emitting process, and the operational variables of the process affect the selection of the control system. Table 29-1 illustrates the large number of variables which must be considered in controlling pollution from a source (1-4). 

Thermal energy, power generation, and incineration have several factors in common. All rely on combustion, which causes the release of air pollutants all exhaust their emissions at elevated temperatures and all produce large quantities of ash when they consume solid or residual fuels. The ratio of the energy used to control pollution to the gross energy produced can be a deciding factor in the selection of the control system. These processes have important differences which influence the selection of specific systems and devices for individual facilities. 

Selection and insteillation of an integrated air pollution control system do not end the concern of the utility industry. Maintenance and operational problems of the system are considered by many engineers to be the weak link in the chain of power generation equipment (6). The reliability of the... 

Fluoride Removal Efficiencies Selected Aluminum Industry Primary and Secondary Control Systems... 

The selection of the PID controller parameters K, T[ and can be obtained using the classical control system design techniques described in Chapters 5 and 6. In the 1940s, when such tools were just being developed, Ziegler and Nichols (1942) devised two empirical methods for obtaining the controller parameters. These methods are still in use. 

The elements of a PM plan include periodic inspection, cleaning, and service as warranted, adjustment and calibration of control system components, maintenance equipment and replacement parts that are of good quality and properly selected for the intended function. Critical HVAC system components that require PM in order to maintain comfort and deliver adequate ventilation air include a outdoor air intake opening, damper controls, air filters, drip pans, cooling and heating coils, fan belts, humidification equipment and controls, distribution systems, exhaust fans. 

The type of control system and instrumentation to be selected will depend on its location (e.g., the danger of fire, explosion, pollution, moisture, the conditions of temperature and its variations). The control equipment can be classified as... 

Depending on the control system and components selected, the process variable from the sensors and signals to the actuator control unit is connected either directly or through transducers. Normally, the input and output are adjusted from pneumatic sensors and control units. The signals produced then are transformed into standard electronic values before connecting to the controller s input and output modules,... 

Exhauster and Motor Air movers for LVHV systems are not conventional fans. The low static pressures needed to operate LVHV systems can be generated by multiple-stage centrifugal (turbine-type) exhausters. These utilize high-precision rotating blades that can be damaged by dust. Consequently, it is always necessary to have an air cleaner in an LVHV dust control system to protect the fan. The low static pressures also require air volume flow rates to be corrected to standard conditions for exhauster selection. 

Detailed designs for fume control systems and selection [Pg.1268]

The first essential step in the design of a fume control system and selection of gas-cleaning equipment is the characterization of the fume emission source. Design procedures which can be used for new and existing industrial plants follow. The characterization of fume emission sources includes parameters such as plume flow rates (mVs), plume geometry (m), source heat flux (J/s), physical and chemical characteristics of particulates, fume loadings (mg/m ), etc. 

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