Load Rejection Performance

In most chemical processes the principal control problem is load rejection. We want a control system that can keep the controlled variables at or near their setpoints in the face of load disturbances. Thus the closedloop regulator transfer function is the most important. 

The ideal closedloop relationship between the controlled variable and the load is zero. Of course this can never be achieved, but the smaller the magnitude 

The closedloop relationships for a multivariable process were derived in Chap. 15 [Eq. (15.64)] 

For a 3 X 3 system there are three elements in the x vector, so three curves are plotted of each of the three elements in the vector [[/ + Cm(, ) B(i )] for the specified load variable L. Table 17.3 gives a program that calculates the TLC curves for the 3 x 3 Oguimatke and Ray column 

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Reflux, on the other hand, can only be used to control a temperature in the upper section of the column. The hydraulic lags of liquid flowing down the column from tray-to-tray are typically 3-6 s per tray. Thus, attempting to control a temperature 40 trays down in the column will introduce a 2-A min lag in the loop, which will adversely affect load rejection performance. 

I Shunt reactors These are provided as shown in Figure 24.23 to compensate for the distributed lumped capacitances, C , on EHV networks and also to limit temporary overvoltages caused during a load rejection, followed by a ground fault or a phase fault within the prescribed steady-state voltage limits, as noted in Table 24.3. They ab.sorb reactive power to offset the charging power demand of EHV lines (Table 24.2, column 9). The selection of a reactor can be made on the basis of the duty it has to perform and the compensation required. Some of the different types of reactors and their characteristics are described in Chapter 27. 

Cascade control was discussed quahtatively in Sec, 8.2. There is a secondary (or "slave ) loop and a primary (or master ) loop. Both load rejection and performance can sometimes be improved by using cascade control. 

EXAMPLE 4.1. Consider the two blending systems shown in Fig. 4.8. The flow rate or composition of stream 1 is the disturbance. The flow rate of stream 2 is the manipulated variable. In Fig. 4.8a the sensor is located after the tank, and therefore the dynamic lag of the tank is included in the feedback control loop. In Fig. 4.8h the sensor is located at the inlet of the tank. The process lag is now very small since the tank is not inside the loop. The control performance in part h, in terms of speed of response and load rejection, would be better than the performance in part a. In addition, the tank now acts as a filter to average out any fluctuations in composition. ... 

Cascade control was discussed qualitatively in Section 4.2. It employs two control loops the secondary (or slave ) loop receives its setpoint from the primary (or master ) loop. Cascade control is used to improve load rejection and performance by decreasing closedloop time constants. 

Cellulose acetate was the first high-performance reverse osmosis membrane material discovered. The flux and rejection of cellulose acetate membranes have now been surpassed by interfacial composite membranes. However, cellulose acetate membranes still maintain a small fraction of the market because they are easy to make, mechanically tough, and resistant to degradation by chlorine and other oxidants, a problem with interfacial composite membranes. Cellulose acetate membranes can tolerate up to 1 ppm chlorine, so chlorination can be used to sterilize the feed water, a major advantage with feed streams having significant bacterial loading. 

The next step was to develop technology to rapidly quantify each such that the load could be accepted/rejected in an acceptable time frame. Kupina (1984) developed a high-performance liquid chromatographic (HPLC) method that simultaneously quantified each of the msgor metabolites in 11 min. Briefly, the procedure involves chromatographing a composite filtered juice sample on a HPLC column which separates the three components from each other and other matrix compounds. Quantification is accomplished by comparing the component peak areas to those of a standard solution chromatographed in the same way. 

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