Control continuous

Fig. 6. Schematic for a computer-controlled, continuous RO unit pressurized by a pump. A/D = analog to digital.

Modem plants generally produce carbon disulfide of about 99.99% purity. High product quaUty is ensured by closely controlled continuous fractional distillation. Reagent and U.S. Federal specifications, and typical commercial-grade quaUty are Hsted in Table 5. 

Eunk, J. C., and E. McAllister. 1989. Controlling Continuous Processes with DCS. Chemical Engineering, 96(5), 91-96. 

Use continuous reactors if possible. It is usually easier to control continuous reactors than batch reactors. If a batch reaction system is required, minimize the amount of unreacted hazardous materials in the reactor. Figures 12-40 and 12-41 show typical examples. 

The use of computers and microprocessors (also known as programmable electronic systems [PES]) in process control continues to grow. They have brought about many improvements but have also been responsible for some failures. If we can learn from these failures, we may be able to prevent them from happening again. A number of them are therefore described below. Although PES is the most precise descnption of the equipment used, I refer to it as a computer, as this is the term usually used by the nonexpert. 

Beishon, R. J. (1969). An Analysis and Simulation of an Operator s Behavior in Controlling Continuous Baking Ovens. In A. de Brisson (Ed.), The Simulation of Human Behavior. Dunuod Paris. 

Water-level controls continuously monitor the level of water in a steam boiler in order to control the flow of feedwater into the boiler and to protect against a low water condition which may expose the heating surfaces with consequent damage. The controls may be either float operated or conductivity probes. 

All these methods will require careful monitoring initially to set up and determine the correct rate of blowdown once the plant is operating. In order to take the necessary sample from the boiler the boiler(s) should be fitted with a sample cooler. To automate the continuous blowdown a conductivity-controlled system may be installed. Here a controller continuously compares the boiler water electrical conductivity with a value set in the controller. Depending on whether this is above or below the set rate, it will automatically adjust the blowdown flow rate. 

This is used when (1) the room needs heating instead of cooling or (2) for reheat as described above. It is vital in close control systems that its capacity is sufficient to maintain room temperature under these conditions, otherwise the system may fall into a loop, with the controls continuing to see high rh due to temperature. Using only part of the cooling coil for dehumidification will alleviate this situation. A heater capacity of the sensible heat extracted during dehumidification plus half the peak winter fabric loss is recommended where the room load could be nil in winter such as a start-up situation. 

Containment buildings are sometimes used to moderate the impact of a spill of an especially toxic material. When containment is used, special precautions are included to ensure worker protection, such as remote controls, continuous monitoring, and restricted access. 

Critical Alarms, Operator Supervision, and Manual Intervention of Process Control. Continuously during operations and emergencies. None ... 

Continuous operation provides high rates of production with more constant product quality. There are no downtimes during normal operation. Reactant preparation and product treatment also have to run continuously. This requires careful flow control. Continuous operation can involve a single stirred tank, a series of stirred tanks or a tubular-type of reactor. The latter two instances give concentration profiles similar to those of batch operation, whereas in a single stirred tank, the reaction conditions are at the lowest reactant concentration, corresponding to effluent conditions. 

When experience or testing indicates the above mitigation methods will not control continuing corrosion to an acceptable level, the segment shall be reconditioned or replaced and suitably protected. 

In dynamic thermogravimetric analysis a sample is subjected to conditions of predetermined, carefully controlled continuous increase in temperature that is invariably found to be linear with time. 

Adaptive control has been an active area of research for many years. The fullblown ideal adaptive controller continuously identifies (on-line) the parameters of the process as they change, and retunes the controller appropriately. Unfortunately, this on-line adaptation is fairly complex and has some pitfalls that can lead to poor performance. Also, it takes considerable time for the on-line identification to be achieved, which means that the plant may have already changed to a different condition. These are some of the reasons why on-line adaptive controllers are mot widely used in the chemical industry. 

Absorbed vanadium is primarily excreted in the urine, and it was detectable in 12 of the workers for periods of up to 2 weeks. Urinary vanadium concentrations were elevated in workers exposed to mean air concentrations of 0.1-0.28mg/m but there was no correlation between the air and urinary concentrations. Although most absorbed vanadium was excreted within 1 day after cessation of exposure, increased excretion relative to unexposed controls continued for more than 2 weeks among chronically exposed workers. ... 

Gas hydrates were first reported at the beginning of the 19th century, and until the 1930s they remained a scientific curiosity. At that time it was realized that hydrates were more likely to be the causative agent in blocking pipelines than ice. Today, gas hydrate control continues to be a problem in the oil and gas industry. In the 1960 s it was realized that natural gas hydrates are present in the geo-sphere with worldwide reserves estimated at 10,000 to 40,000 trillion cubic meters (TCM). Considerable efforts are underway to refine global estimates and to develop technology and exploit this resource. On the other hand these hydrates may... 

In this chapter we consider the fundamentals of reaction in continuous isothermal reactors. Most industrial reactors are operated in a continuous mode instead of batch because continuous reactors produce more product with smaller equipment, require less labor and maintenance, and frequently produce better quahty control. Continuous processes are more difficult to start and stop than batch reactors, but they make product without stopping to change batches and they require rninimum labor. 

Several trials have been performed with antidepressants in the prevention of recurrence of major depression. In an exemplary trial of this kind, Reimherr et al. (1998) explored the optimal length of therapy in a long-term, placebo-controlled continuation study of patients who had responded to 12 14 weeks of open-label fluoxetine treatment (20mg/day) for major depression. Different maintenance schedules were represented by four treatment arms ... 

M. Nishioka, T. Takeno, and C.K. Law. A Flame-Controlling Continuation Method for Generating S-Curve Responses with Detailed Chemistry. Combust. Flame, 104 328-342,1996. 

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