Feedforward Control, Blending

Ratio and Multiplicative Feedforward Control. In many physical and chemical processes and portions thereof, it is important to maintain a desired ratio between certain input (independent) variables in order to control certain output (dependent) variables (1,3,6). For example, it is important to maintain the ratio of reactants in certain chemical reactors to control conversion and selectivity the ratio of energy input to material input in a distillation column to control separation the ratio of energy input to material flow in a process heater to control the outlet temperature the fuel—air ratio to ensure proper combustion in a furnace and the ratio of blending components in a blending process. Indeed, the value of maintaining the ratio of independent variables in order more easily to control an output variable occurs in virtually every class of unit operation. 

However, we can describe the basic structure of several feedforward control systems. Figure 8.7 shows a blending system with one stream which acts as a disturbance both its flow rate and its composition can change. In Fig. 8.7a the conventional feedback controller senses the controlled composition of the total blended stream and changes the flow rate of a manipulated flow. In Fig. %.lb the manipulated flow is simply ratjoed to the wild flow. This provides feedforward control for flow rate changes. Note that the disturbance must be measured to implement feedforward control. 

Although we have indicated two methods of breaking the (r) transmission, thereby simplifying composition control system design arid improving the control of yris), it should be noted that the first procedure—feedforward control—is more desirable than the use of a large condensate receiver. Actually the control of top-tray vapor composition is not as important as the control of composition of condensate to the next step. It can be shown that blending is much more effective if done outside the composition control loop. 

Two physical examples, a continuous blending system and a distillation column, have been used to introduce basic control concepts, notably, feedback and feedforward control. We also motivated the need for a systematic approach for the design of control systems for complex processes. Control system development consists of a number of separate activities that are shown in Fig. 1.10. In this book we advocate the design philosophy that for complex processes, a dynamic model of the process should be developed so that the control system can be properly designed. 

Feedforward Controller Design Based on Steady-State Models 15.3.1 Blending System... 

To further illustrate the design method, consider the blending system and feedforward controller shown in... 

Figure 15.9 Feedforward control of exit composition in the blending system.

An alternative feedforward control scheme for the blending system is shown in Fig. 15.10. Here the feedforward controller output signal serves as a set point to a feedback controller for flow rate W2. The advantage of... 

Feedforward control is a powerful strategy for control problems wherein important disturbance variable(s) can be measured on-line. By measuring disturbances and taking corrective action before the controlled variable is upset, feedforward control can provide dramatic improvements for regulatory control. Its chief disadvantage is that the disturbance variable(s) must be measured (or estimated) on-line, which is not always possible. Ratio control is a special type of feedforward control that is useful for apphcations such as blending operations where the ratio of two process variables is to be controlled. 

Design a feedforward-feedback control system for the blending system in Example 15.5, for a situation in which an improved sensor is available that has a smaller time delay of 0.1 min. Repeat parts (b), (c), and (d) of Example 15.5. For part (c), approximate GyGpGm with a first-order plus time-delay transfer function, and then use a PI controller with ITAE controller tuning for disturbances (see Table 12.3). For the feedforward controller in (15-34), use a = 0.1. 

It is desired to design a feedforward control scheme in order to control the exit composition X4 of the two-tank blending system shown in Fig. E15.13. Flow rate 2 can be manipulated, while disturbance variables, and X5, can be measured. Assume that controlled variable X4 cannot be measured and that each process stream has the same density. Also, assume that the volume of liquid in each tank is kept constant by using an overflow line. The transmitters and control valve have negligible dynamics. 

Ratio control is a simple form of feedforward control that is commonly employed in controlling reactor feed compositions and in blending operations. It is also used to control the fuel-to-air ratio in heaters and boilers and to control the reflux ratio in distillation columns. The flow rate of one stream is used to provide the set point for another stream so that that ratio of the two streams is kept constant even if the flow of the first stream varies. Alternatively, the actual ratio between two flows can be used as the input to a controller. 

There are many advanced strategies in classical control systems. Only a limited selection of examples is presented in this chapter. We start with cascade control, which is a simple introduction to a multiloop, but essentially SISO, system. We continue with feedforward and ratio control. The idea behind ratio control is simple, and it applies quite well to the furnace problem that we use as an illustration. Finally, we address a multiple-input multiple-output system using a simple blending problem as illustration, and use the problem to look into issues of interaction and decoupling. These techniques build on what we have learned in classical control theories. 

Flow controllers set the rates of both streams, one being under flow-ratio control. In principle, either caustic soda or dilution water can be the master stream, with the other following it to maintain the ratio. Blending is controlled by a feedforward system, ultimately reset by the product concentration or density. Feedback from caustic concentration measurement (usually by density) could be used for final adjustment, but the concentration of the hypochlorite solution is the more important variable. The simple flow-ratio controller mentioned here can be replaced by a multi-stream version that allows use of other streams in addition to the principal 50% NaOH and dilution water. A cooler downstream of the mixing point removes the heat of dilution. The standard design is a titanium plate exchanger, which can also provide turbulence to complete the mixing process. Chlorine joins the diluted caustic in the reactor. Its rate of addition is controlled by an oxidation-reduction potential (ORP) instrument. The reaction mass recirculates from a collection tank around the system to reduce the increase of temperature across the reactor and to promote turbulence. The net production is removed from the tank, normally under level control. 

Consider the blending system of Section 15.3, but now assume that a pneumatic control valve and an I/P transducer are used. A feedforward-feedback control system is to be designed to reduce the effect of disturbances in feed composition xi on the controlled variable, product composition X. Inlet flow rate 1V2 can be manipulated. Using the information given below, design the following control systems and compare the closed-loop responses for a +0.2 step change in xi. 

Figure 15.12 Block diagram for feedforward-feedback control of the blending system.

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