Problems in Process Control

Most controls, particularly the older ones, are the open loop type. They merely set mechanical or electrical devices to some operating temperature, time, and pressure. If this is all that is required, the control may remain in operation. However, this setup is subject to a variety of hard-to-observe disturbances that are not compensated by open loop controls. Thus process control must close the loop to eliminate the effect of process disturbances. 

There are two basic approaches to problem solving (1) find and correct the problem, applying only the controls needed or (2) overcome the problem with an appropriate process control strategy. The approach one selects depends on the nature of the processing problem, and whether enough time and money are available to correct it. Process controls may, in most cases, provide the most economical solution. To make the right decision, one must systematically measure the magnitude of the disturbances, relate them to product quality, and identify their cause so that proper control action can be taken. 

Before Investing in a more expensive system, the processor should me- 

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Flocculating agents differ from other materials used in the chemical process industries in that their effect not only depends on the amount added, but also on the concentration of the solution and the point at which it is added. The process streams to which flocculants are added often vary in composition over relatively short time periods. This presents special problems in process control. 

Church, D. F., "Cement and Projected Pulp and Paper Industry Problems in Process Control and Process Modeling," AIChE Symp. Ser., 1976, 72 (159), 19. 

Edgar, T. F., Current problems in process control, IEEE Control Systems Magazine, 13-15 (1989). 

The CO2 may be used directly in the process or may first be absorbed into a caustic solution to produce Na2C03 or K2CO3. In the latter case, absolute precision in stoichiometry is not necessary. While variations in composition can cause problems in process control, any deficiency in CO2 content of the solution will result in some hydroxide value accompanying the carbonate. This is compensated for by reducing the amount of caustic solution added to the brine. If the CO2 content is high, there will be a certain amount of bicarbonate present. This will consume OH when the caustic solution is added, and so the flow rate of that stream will have to be increased. This regenerates CO3 late in the process and is more undesirable than the opposite error. 

Cylinders may be single- or double-acting, and both unloaders and clearance pockets are used for capacity control (Section 9.1.6.3B). If the compressor takes less than the full cell output of hydrogen, the excess must be vented. This can be by way of the low-pressure seal pots or by controlled venting somewhere in the compression train. This can present problems in process control. Section 11.4.2.5A describes some of the systems that have been used. 

Chemical engineers working in the area of transport phenomena must frequently solve problems that involve linear second-order differential equatiOTis. These may occur as boundary value problems in diffusional systems or initial value problems in process control or reacting systems, but most frequently, they are the result of reduction of partial differential equations. 

Reconciliation of inaccurate process measurements is an important problem in process control that can be solved using optimization techniques. The flow rates of streams B and C have been measured three times during the current shift (shown in Fig. El9.7). Some errors in the measurement devices exist. Assuming steady-state operation = constant), And the optimal value of wa (flow rate in kg/h) that minimizes the sum of the squares of the errors for the material balance, + wc =... 

We end our series of specific examples by noting that even in healthy individuals there is a natural tendency for blood to concentrate in the lower regions of the lungs and air in the upper. Such misdis-tribution can cause serious problems. This is basically a problem in process control, but its diagnosis is achieved by measuring the response of the pulmonary system to a series of volatile tracers with different solubilities in blood [e.g., 45]. 

Many compounds can cause problems in pollutant-control equipment. Particulate matter, liquids, or solids in the waste stream can plug the adsorber beds, heat-recovery beds in regenerative thermal incinerator systems and biofilters. Conventional filtration systems are used to remove particulate matter before or after the process. 

Beishon, R. J. (1967). Problems of Task Description in Process Control. Ergonomics 10, 177-186. 

The extrusion process has fewer process control problems with TPs than does injection molding but has greater problems in dimensional control and shape. During... 

Linking techniques together might seem like a good idea in theory but in practise, there can be as many problems as potential advantages. HPLC-NMR does have undeniable use in the field of bio-fluid NMR and in process control in a production environment but we feel that it has little to offer the organic chemist looking to monitor a reaction. 

In this paper, three methods to transform the population balance into a set of ordinary differential equations will be discussed. Two of these methods were reported earlier in the crystallizer literature. However, these methods have limitations in their applicabilty to crystallizers with fines removal, product classification and size-dependent crystal growth, limitations in the choice of the elements of the process output vector y, t) that is used by the controller or result in high orders of the state space model which causes severe problems in the control system design. Therefore another approach is suggested. This approach is demonstrated and compared with the other methods in an example. 

We know that the temperature control of the shower in a bathroom is not so easy. To have a comfortable shower, a hot water valve needs to be manipulated carefully (control action) however, we usually rely on a trial and error action until the proper temperature of a shower is achieved. How can we achieve the most comfortable shower temperature more quickly This is a common problem in the control action of many processes that are controlled by a feedback loop. The difficulty comes from a delay in the response, which naturally exists in any process - in other words, the dynamic characteristics of a process. Therefore, the control action should be determined based on the dynamics of the process. In particular, some bioprocesses are known to have serious delays in response. 

In addition to device-level diagnostics, networked final control elements, process controllers, and transmitters can provide loop level diagnostics that can detect loops that are operating below expectations. Process variability, time in a limit (saturated) condition, and time in the wrong control mode are metrics used to detect problems in process loop operation. 

From a dynamic response standpoint, the electronic adjustable-speed pump has a dynamic characteristic that is more suitable in process control applications than those characteristics of control valves. The small amplitude response of an adjustable-speed pump does not contain the dead band or the dead time commonly found in the small amplitude response of the control valve. Nonlinearities associated with friction in the valve and discontinuities in the pneumatic portion of the control valve instrumentation are not present with electronic variable-speed drive technology. As a result, process control with the adjustable-speed pump does not exhibit limit cycles, problems related to low controller gain, and generally degraded process loop performance caused by control valve nonlinearities. 

The second example considered the absorption of soluble insulin from subcutus. The problem here was to establish a set of consistent hypotheses that could explain the observed volume and concentration effects. At the time when the model was formulated there was no notion of the possible role of polymerization in the absorption process for insulin. Most experiments were performed at normal pharmacological concentrations (40 IU/ml) and injection volumes (0.3 ml), and the work was oriented towards elucidating the importance of exercise and skin temperature at the absorption site. Such experiments are obviously important, since variations in skin temperature may pose a problem in the control of labile type I diabetes. Analyses of a single set of data, obtained partly at micro-dose levels, allowed us to identify processes in the skin that were not amenable to direct experimentation. 

A key problem in bioreactor control is the difficulty in obtaining reliable sensors and consequently of reliable on-line process information. Demands for product consistency and process productivity produce requirements for more process information.13 Especially in the case of fermentors, rapid, accurate on-line measurement of process variables is often a complex task. As a result, much research effort has focused on methods for quantitatively estimating compositions within reactors and on using model-based control techniques. 

Sometimes, as a series is sampled, the level of noise in each sample depends on diat of the previous sample. This is common in process control. For example, there may be problems in one aspect of the manufacturing procedure, an example being die proportion of an ingredient. If the proportion is in error by 0.5 % at 2 pm, does this provide an indication of die error at 2.30 pm ... 

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