Plant operability and controllability

The topic of synthesis will be be covered in the following paper by Douglas [ 1 ]. I myself have recently written two papers, each in part a review of process synthesis [2, 3]. Several articles have reviewed synthesis in the last decade, one over 40 journal pages long. The brief summary in this paper will be limited to a few ideas that are not usually discussed but that are thought to be important in process synthesis when heat effects do not dominate the decision making. A more extensive section on analysis completes this work. The paper by Morari [4] will present ideas on plant operation and control. 

Before proceeding any further with the development of a process design and its associated economics, it will be desirable to consider an overall view of the various functions involved in a complete plant design. Particular emphasis in this discussion will be placed on important health, safety, loss prevention, and environmental considerations. Other items that will be noted briefly include plant location, plant layout, plant operation and control, utilities, structural design, storage, materials handling, patents, and legal restrictions. 

In the design of an industrial plant, the methods which will be used for plant operation and control help determine many of the design variables. For example, the extent of instrumentation can be a factor in choosing the type of process... 

Plant operation and control are easy. Thermal efficiencies of spray-drying systems are inherently better than those of most other dryers. Extra equipment to recover low-grade heat in exhaust is not necessary. 

PRISM offers multiple layers of defense against events that challenge plant safety. The PCS provides the normal plant operation and control function and also prevents challenges to the RPS by taking action before RPS setpoints are reached. However, if RPS setpoints... 

For plant operation and control The designer should be aware of Instrumentation Maintenance Utilities... 

A key consideration for plant operability and controllability is variable interaction. We have learned that dead time is one of our enemies, as it always makes tight control more difficult to achieve. Variable interaction places similar restrictions on the way we can control a process and can significantly reduce the overall control system performance. Three common sources of variable interaction are the nature of the process (i.e. distillation), the combination of multiple unit operations and heat integration. Each of these points can be highly advantageous in the steady state, but they can also create operability and controllability problems that may not be evident without considering the process dynamics at the design phase. 

This section develops an operational strategy for the Direct Gas Brayton space nuclear power plant. General methods of plant operation and control that will allow it to most effectively meet its mission requirements within the constraints of the space application are identified. The most significant conclusions regarding operational strategy include ... 

The development of computer capabiUties in hardware and software, related instmmentation and control, and telecommunication technology represent an opportunity for improvement in safety (see COMPUTER TECHNOLOGY). Plant operators can be provided with a variety of user-friendly diagnostic aids to assist in plant operations and incipient failure detection. Communications can be more rapid and dependable. The safety control systems can be made even more rehable and maintenance-free. Moreover, passive safety features to provide emergency cooling for both the reactor system and the containment building are being developed. 

Most sulfonation plants monitor and control operations by computer. Sulfur-buming catalytic SO -generating equipment may require a 1—2 h stabilization period on startup. The unit can be kept in a standby position by maintaining heat to the unit when it is off-line. Liquid SO -based sulfonation plants do not require such a stabilization period and hence are more flexible to operate than sulfur-buming sulfonation plants. 

Operation and Maintenance. Engineering and constmction firms can be contracted to take charge of treatment-plant operation and maintenance and manage other aspects necessary to meet performance goals, such as hiring and cost controls (7). Costs to the municipalities are reported to be competitive with the more traditional approach of management by municipal employees (8). 

Companies usually include in the charge for overhead the following items operating supphes, supervision, indirect payroll expenses, plant protection, plant office, general plant overhead, and control laboratory. This overhead charge is frequently taken as an equivalent percentage of the direc t labor cost. 

Analysts The above is a formidable barrier. Analysts must use limited and uncertain measurements to operate and control the plant and understand the internal process. Multiple interpretations can result from analyzing hmited, sparse, suboptimal data. Both intuitive and complex algorithmic analysis methods add bias. Expert and artificial iutefligence systems may ultimately be developed to recognize and handle all of these hmitations during the model development. However, the current state-of-the-art requires the intervention of skilled analysts to draw accurate conclusions about plant operation. 

Another common approach is to use an information-processing model to classify human errors. The classification models the information processing which occurs when a person operates and controls complex systems such as processing plants. One such classification (Rouse and Rouse, 1983) identifies six steps in information processing. Exhibit 6.1 lists the six steps, and provides some examples of errors that can occur at each of these steps. 

The monitoring of pollutant concentration or mass flow of pollutants is of interest to both plant owners and control agencies. Industry uses such measurements to keep a record of process operations and emissions for its own use and to meet regulatory requirements. Control officials use the... 

The volume represents the initial fulfillment of a series, and is aimed at assisting process engineers, plant managers, environmental consultants, water treatment plant operators, and students. Subsequent volumes are intended to cover air pollution controls, and solid waste management and minimization. 

Despite the need for good control in many process systems, most engineers do not allow the proper pressure drop for the control valves into their calculations. Many literature sources ignore the problem, and many plant operators and engineers wonder why the actual plant has control problems. 

While it produces very pure water, de-ionization is an expensive process to operate. It uses acid and caustic for regeneration and produces an effluent which may need neutralization before it can be discharged. On the other hand, all sizes of de-ionization plants are available with fully automatic operation and control so that the plant... 

For smaller boiler plants all relevant information is recorded in logbooks that provide a permanent operating and control record. The full and correct completion of logbook entries is mandatory. 

In designing a continuous plant, therefore, it is essential to establish the measurement and control strategy based on an understanding of which critical aspects are indicative of good or poor plant operation, and how deviation of these measurements can be exploited to perform a corrective process control action. Estabhshing suitable on-hne real-time measurement techniques may be a blocker to the implementation of continuous processing. In contrast, implementation of a successful measurement and control strategy may be the enabler for improved product yields and product quahty. 

In order to estimate emissions from pickling facilities, U.S. EPA developed 17 model plants to represent five types of pickling operations and one acid regeneration process.12 The model plants include one or more size variation for each process model. The model plants were developed from information obtained from a survey of steel pickling operations and control technologies. U.S. EPA estimated emission rates for model facilities. Using these emission rates and the production and hours of operation for the model pickling plants, emission factors were calculated. These appear in Table 28.12. 

Mutalib MIA, Zeglam AO and Smith R (1998) Operation and Control of Dividing Wall Distillation Columns Part II Simulation and Pilot Plant Studies Using Temperature Control, Trans IChemE, 76A 319. 

Ghezell-Ayagh et. (2001) Operation and control of direct reforming fuel cell power plant, Proc. IEEE Power Eng. Soc. Winter Meeting. 

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