Control of Separation Processes

A detailed look at the use of control in separation processes requires a much more corrqoehensive treatment than is possible here. The literature contains many well-written books and articles dealing with control in general and distillation column control in particular. Several references in each case are included at the end of this chapter.lJi 

It would be useful perhaps to provide some perspective as to how the nature of process control is evolving, particularly in the context of design and synthesis of separation processes- 

There is no doubt that, in many instances, relatively smtple approaches are still anfficient to accomplish particular control objectives. However, for example, in the case of separation sequences that involve complex flow arrangements and thermal onapling, more advanced process control strategies will be necessary. As process synthesis methodologies improve, control strategy evaluation and process design and 

In die perspective of process synthesis, process control should be viewed am as a separate element in process desiga and optimization bas rather as a component of a coordinated approach. Therefore, the design and sequencing of separation processes me si consider the relationship that process control will have to the final process structure. This can be done only through process modeling and simulation, 

The objective in the control or separation processes has evolved from the control of the components of a process that already has been designed and often constructed to an integrated plan involving simulation, desiga, and control. Chemical process synthesis approaches that jnclnde control structures have been suggested only recently and are not well developed. 

The first process simulators developed for general use were fbr steady-slate operation. Process dynamics usually were ignored to avoid excessive computation times and computational difficulties. While steady-state information is useful for control strategy evaluation, process control systems are des ned best wUh some knowledge of process dynamics. As nonlinearities and interactions become more pronounced, information from the time-dependent behavior of a process or processes becomes crucial. There is currently a great deal of effort focus on upgrading process simulators so that they can handle unsteady-state operation. Several of these programs are able to model process dynamics for specific situations. 

Because of the computational complexities associated with dynamic process simulation for multiunit processes, there is still much to be done before simulators of this type become available for general application. Another problem complicating their development is that process models for even individual separation units are usually for steady-state cases this is the result of both incomplete understanding of the chemical and physical nciples involved and computational difficulties. This is one of the rmun reasons why process control considerations are difficult to incorporate into chemical process simulation and thesis and why on-line plant optimization is still far away in most instances. 

Process modeling and simulation are nevertheless extremely important tools in the design and evaluation of process control strategies for separation processes. There is a strong need, however, for better process mc ls for a variety of separations as well as process data with which to confirm these models. Confidence in complex process models, especially those that can be used to study process dynamics, can come only from experimental verification of these models. This will require more sophisticated process sensors than those commonly available for temperature, pressure, pH, and differential pressure. Direct, reliable measurement of stream composition, viscosity, turbidity, conductivity, and so on is important not only for process model verification but also for actual process control applications. Other probes, which could be used to provide a better estimate of the state of the system, are needed to contribute to the understanding of the process in the same time frame as that of changes occurring in the process. 

A distributed control system may involve the use of microcomputers at the local level and the use of more powerful rruchines to coordinate overall plant control objectives. In this context, it has been suggested that process control might be described better as process management. Local control of the operation of individual separators is still important but, with the use of distributed control, reliability is maintained (microcomputers are dedicated to particular process units) while overall technical and economic objectives are pursued (mainframe computers can perform complex on-line/off-line optimization). The advantage to distributed control is that it makes effe ve use of current technology and provides a framework within which control and optimization developments can be implemented. These developments probably will include better simulation and optimization routines that will help to assess the current state of the process plant and to suggest improvements. 

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Different cell types use distinct Ca signals, as suits best their physiology. In particular, the possibility of local and global Ca signaling permits the control of separate processes in the same cell. 

Automatic control of separation process Possible with... 

Separability. One of the greatest advantages of a solid catalyst is that it can be separated easily from the products of reaction. To do this successfully requires careful control of the process conditions so that exposure of the catalyst to nonreactant liquids capable of affecting or dissolving either the catalytic material or the support is prevented or rninimi2ed. Solid catalysts typically are used in axial or radial flow beds and multitubular reactors. Many successful commercial processes maintain the reactants and products in the gas phase while in contact with the catalyst to avoid catalyst degradation problems. 

There are three separate requirements here. Control of further processing involves stopping the process and, as explained previously, should be carried out only by those responsible for the process. Controlling further delivery is somewhat different, as the authority to deliver may not be vested in the same person who performed the processing. 

Advancing the field of process engineering. Important generic goals for research include the development of separation processes for complex and fragile bioproducts the design of bioreactors for plant and mammalian tissue culture and the development of detailed, continuous control of process parameters by rapid, accurate, and noninvasive sensors and instruments. 

Chemical analysis finds important applications in the quality control of industrial processes. In an ideal situation a continuous analysis of the process stream is made and some aspects of this are discussed in Chapter 12. However, such continuous analysis is by no means always possible, and it is common to find a process being monitored by the analysis of separate samples taken at regular intervals. The analytical data thus obtained need to be capable of quick and simple interpretation, so that rapid warning is available if a process is going out of control and effective corrective action can be taken. 

In order to control the separation process it is necessary to control the dominant parameters of the process. Their dependence on each other is demonstrated in Figure 1. 

With micelles, microemulsions, or liposomes, a second phase is introduced into the separating system. As in chromatography, exchange of the analyte between the mobile and the stationary phases controls the separation process. Contrary to classical chromatography, both phases are mobile, moving with different velocities. As in all electrophoresis techniques, the net mobility of an analyte is the mean mobility of its fraction in the aqueous and the micellar phases ... 

Both bacteria and plants have separate enzymes that catalyze the individual steps in the biosynthetic sequence (Fig. 17-12). The fatty acyl group grows while attached to the small acyl carrier protein (ACP).54 58 Control of the process is provided, in part, by the existence of isoenzyme forms. For example, in E. coli there are three different P-oxoacyl-ACP synthases. They carry out the transfer of any acyl primer from ACP to the enzyme, decarboxylate malonyl-ACP, and carry out the Claisen condensation (steps b, e, and/in Eq. 17-12)58a e One of the isoenzymes is specialized for the initial elongation of acetyl-ACP and also provides feedback regulation.58c The other two function specifically in synthesis of unsaturated fatty acids. 

Description of the process. The simplified process flow diagram is shown in Figure 16.12. The shredder waste (ASR, plastic and electronic waste as well as MSW) is fed in an IRFB, which operates in a reducing atmosphere and at temperatures as low as 500-600°C, allowing easy control of the process. The IRFB reactor separates the combustible portion and the dust from the inert and metallic particles of the fed waste the obtained mixture of metallic and inert particles is sent to a mechanical metal separation while fuel gas and carbonaceous particles are burnt in a cyclonic combustion chamber for energy production and fine ash vitrification. Metals such as aluminium, copper and iron can be recycled as valuable products from the bottom off-stream of the IRFB as they are neither oxidized nor sintered with... 

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