Alternative Control Strategies

There are several different strategies for air pollution control. The strategy just discussed and shown in Fig. 5-1 is called the air quality management strategy. It is distinguished from other strategies by its primary reliance on the development and promulgation of ambient air quality standards. 

Ultimate disposal taxes Land use taxes Tax remission 

Property taxes Fines, effluent charges, and fees Fines 

Domestic production restraints Quotas, land and offshore use restraints 

On fuel, fuel additives, ingredients in fuel, pollution-producing equipment 

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An alternative control strategy fixes the reactor-inlet toluene flow rate [16]. Fresh toluene is fed into the condenser drum of the last distillation column, on level control. Production-rate changes can be achieved by changing the setpoint of the toluene reactor-inlet flow, or the setpoint of the reactor-inlet temperature controller. When this control structure is used, the whole range of conversion becomes stable. Drawing of this control structure is left as an exercise to the reader. 

M. Deacon, R.W. Horrocks, C.J. Brace, N.D. Vaughan, C.R. Burrows "Impact of alternative controller strategies on emissions from a diesel CVT powertrain - preliminary results." IMechE Seminar on "Application of Powertrain and Fuel Technologies to meet Emission Standards." 24.-26. June 1996, London. 

The motivations for a catalytic control strategy are simple. Most alternative control strategies transfer the pollutant into another form, producing contamination of another valuable resource. Soil ventilation or ground-water stripping, for example, changes a problem of contaminated soil or water into an air pollution problem. If the pollutants are trapped by activated carbon from the contaminated air stream thereby produced, the activated carbon itself becomes contaminated and must then be disposed of. If a furnace flue gas is to be scrubbed, the resulting contaminated scrubber liquid must be dealt with. 

Thc.se decisions naturally eliminate certain alternative control strategies. 

Bozer, Y. A., Quiroz, M., and Sharp, G. P. (1988), An Evaluation of Alternative Control Strategies and Design Issues for Automated Order Accumulation and Sortation, Material Flow, Vol. 4, No. 4, pp. 265-282. 

To test alternative control strategies, Prett and Morari (1986) provide a linearized model, referred to as the Shell process, of a distillation tower to separate crude oil into fractions in a refinery. Part of the model describes the dynamics of the two top compositions as a function of the manipulated variables (the two top draw rates) and two key disturbances (the heat removal loads in pump-around streams used to remove heat and create intermediate reflux). For this example, it is sufficient to examine die matrices specific to the nominal model ... 

The principal points arising from this evaluation of costs and possible direct economic benefits of alternative control strategies are ... 

Another strategy to determine decision variables is to consider how the process is controlled. Any variable that must be controlled is a decision variable. There are alternative control strategies for equipment and for processes, but a well-designed control system reduces the degrees of freedom to zero without overconstraining the process. The controlled variable is a secondary decision variable the variable manipulated by the final control element is the primary decision variable. 

Model-Based Approach. A dynamic model of the process is first developed that can be helpful in at least three ways (i) it can be used as the basis for model-based controller design methods (Chapters 12 and 14), (ii) the dynamic model can be incorporated directly in the control law (for example, model predictive control), and (iii) the model can be used in a computer simulation to evaluate alternative control strategies and to determine preliminary values of the controller settings. 

Develop a control strategy for a new process. A dynamic model of the process allows alternative control strategies to be evaluated. For example, a dynamic model can help identify the process variables that should be controlled and those that should be manipulated. For model-based control strategies (Chapters 16 and 20), the process model is part of the control law. 

Methods 1 and 2 are based on simple transfer function models and will be considered in Section 12.2. The controller tuning relations of Method 3 are analytical expressions and correlations for the PID controller settings. They are considered in Section 12.3. Design techniques based on frequency response analysis (Method 4) are the subject of Chapter 14. Computer simulation of the controlled process (Method 5) can provide considerable insight into dynamic behavior and control system performance. In particular, software such as MATLAB and Lab VIEW facilitates the comparison of alternative control strategies and different controller settings. (See Appendices C and E of Doyle (2000).) Method 6, on-line tuning, is considered in Section 12.5. 

Table G.4 A Comparison of Alternative Control Strategies for the Reactor for the Reactor/Distillation Column Plant...

Depew, C.A., Hashemi, M.H., Davis, J. 1988. Evaluation of alternative control strategies for delayed coker. Proc. Am. Control Conf. 88(l) 240-246. 

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