Loop system

One of the most vexing aspects of pilot-plant work can be feed and product handling as a pilot plant is neither designed nor operated as a closed-loop system like a commercial plant. Indeed, the problems involved in handling and storing feed and product materials can sometimes seem to rival the pilot-plant process problems in difficulty. 

Flux is maximized when the upstream concentration is minimized. For any specific task, therefore, the most efficient (minimum membrane area) configuration is an open-loop system where retentate is returned to the feed tank (Fig. 8). When the objective is concentration (eg, enzyme), a batch system is employed. If the object is to produce a constant stream of uniform-quahty permeate, the system may be operated continuously (eg, electrocoating). 

Open-loop systems have inherently long residence times which may be detrimental if the retentate is susceptible to degradation by shear or microbiological contamination. A feed-bleed or closed-loop configuration is a one-stage continuous membrane system. At steady state, the upstream... 

Recycling and Reuse. (/) Install closed-loop systems. 2) Recycle on site for reuse. 3) Recycle off site for reuse. (4) Exchange wastes. 

Most controlled release dosage forms administer dmg according to thek design, whether conceptually simple, eg, a fixed release rate for a fixed amount of time, or complex, eg, several different rates for different amounts of time. Alternatively, closed-loop systems contain a sensor to monitor dmg concentration or to administer dmg according to a biological need (11). 

The oxygen sensor closed loop system automatically compensates for changes in fuel content or air density. For instance, the stoichiometric air/fuel mixture is maintained even when the vehicle climbs from sea level to high altitudes where the air density is lower. 

An open-loop system positions the manipulated variable either manually or on a programmed basis, without using any process measurements. This operation is acceptable for well-defined processes without disturbances. An automanual transfer switch is provided to allow manual adjustment of the manipulated variable in case the process or the control system is not performing satisfac torily. 

A closed-loop system uses the measurement of one or more process variables to move the manipulated variable to achieve control. Closed-loop systems may include reedfoi ward, feedback, or both. 

General Second-Order Element Figure 8-3 illustrates the fact that closed loop systems often exhibit oscillatory behavior. A general... 

Time-Delay Compensation Time delays are a common occurrence in the process industries because of the presence of recycle loops, fluid-flow distance lags, and dead time in composition measurements resulting from use of chromatographic analysis. The presence of a time delay in a process severely hmits the performance of a conventional PID control system, reducing the stability margin of the closed-loop control system. Consequently, the controller gain must be reduced below that which could be used for a process without delay. Thus, the response of the closed-loop system will be sluggish compared to that of the system with no time delay. 

In principle, ideal decouphng eliminates control loop interactions and allows the closed-loop system to behave as a set of independent control loops. But in practice, this ideal behavior is not attained for a variety of reasons, including imperfect process models and the presence of saturation constraints on controller outputs and manipulated variables. Furthermore, the ideal decoupler design equations in (8-52) and (8-53) may not be physically realizable andthus would have to be approximated. 

Loop Reactors For some gas-hquid-solid processes, a recirculating loop can be an effective reactor. These involve a relatively high horsepower pumping system and various kinds of nozzles, baffles, and turbulence generators in the loop system. These have power levels... 

The Mammoth-Paeifie plants are the world s first eommereial applieation ineorporating two elosed-loop systems (henee, binary proeess ) designed to proteet the environment and eonserve water. This is illustrated in Figure 4-43. 

Closed loop systems inelude either a feedbaek or feedforward, eontrol loop or both to eontrol the plant. In a feedbaek eontrol loop, the eontrolled variable is eompared to a set point. The differenee between the eontrolled variable to the set point is the deviation for the eontroller to aet on to minimize the deviation. A feedforward eontrol system uses the measured load or set point to position the manipulated variable in sueh a manner to minimize any resulting deviation. 

Many plants use off-line optimization. Off-line optimization is an open loop eontrol system. Instead of the elosed loop system, whieh eontrols the plant settings, data is provided to the operator so that he ean make the deeisions based on the findings of the operational data. Off-line systems are also used by engineers to design plants and by maintenanee personnel to plan plant maintenanee. Comparisons of the on-line systems to off-line systems ean be seen in Table 19-1. 

The bandwidth of the elosed-loop system should be equal to or less than 10 KHz. The amount of gain needed to be added by the error amplifier in order to aehieve this bandwidth is... 

The form of compensation is the 2-pole-2-zero method of compensation. This is to compensate for the effect of the double pole caused by the output filter inductor and capacitor. One starts by determining the control-to-output characteristic of the open-loop system. 

Before starting the design for the compensation of the error amplifier, it is desirable to know what constitutes a stable closed-loop system. The rule is elementary ... 

The closed loop phase shall never exceed -330 degrees of lag whenever the gain of the closed-loop system is greater than i (or OdB). 

Phase margin. This is the value of the phase of a closed-loop system at the gain cross-over frequency (G(s) = OdB). 

A control system may have several feedback control loops. For example, with a ship autopilot, the rudder-angle control loop is termed the minor loop, whereas the heading control loop is referred to as the major loop. When analysing multiple loop systems, the minor loops are considered first, until the system is reduced to a single overall closed-loop transfer function. 

Controllers for closed-loop systems 4.5.1 The generalized control problem... 

Thus, when r t) and f2(f) are unehanging, or have step ehanges, there are no steady-state errors as ean be seen in Figure 4.25. The seeond-order dynamies of the elosed-loop system depend upon the values of T[, T, K and K. Again, a high value of K will provide a fast transient response sinee it inereases the undamped natural frequeney, but with higher order plant transfer funetions ean give rise to instability. 

The Process Reaction Method assumes that the optimum response for the closed-loop system occurs when the ratio of successive peaks, as defined by equation (3.71), is 4 1. From equation (3.71) it can be seen that this occurs when the closed-loop damping ratio has a value of 0.21. The controller parameters, as a function of R and D, to produce this response, are given in Table 4.2. 

The physieal eonfiguration and bloek diagram representation of a CNC maehine-tool is shown in Figures 1.10 and 1.11. The fundamental eontrol problem here is that, by design, the lead-serew (by the use of re-eireulating ball-bearings) is frietion-free. This means that the positional eontrol system will have no damping, and will oseillate eontinuously at the undamped natural frequeney of the elosed-loop system. 

Fig. 4.39 Block diagrams for closed-loop systems. Amplifier...

Maximum value of the open-loop gain constant for the stability of a closed-loop system... 

The eharaeteristie equation for a elosed-loop system (5.24) may also be written as... 

Closed-loop poles (For K = 11.35) Since the closed-loop system is third-order, there are three closed-loop poles. Two of them are given in equation (5.81). The third lies on the real locus that extends from —5 to —oo. Its value is calculated using the magnitude criterion as shown in Figure 5.15. 

In practice, only the frequencies lu = 0 to+oo are of interest and since in the frequency domain. v = jtu, a simplified Nyquist stability criterion, as shown in Figure 6.18 is A closed-loop system is stable if, and only if, the locus of the G(iLu)H(iuj) function does not enclose the (—l,j0) point as lu is varied from zero to infinity. Enclosing the (—1, jO) point may be interpreted as passing to the left of the point . The G(iLu)H(iLu) locus is referred to as the Nyquist Diagram. 

The closer the open-loop frequency response locus G(ja )//(ja ) is to the (—l,j0) point, the nearer the closed-loop system is to instability. In practice, all control... 

Gain Margin (GM) The gain margin is the inerease in open-loop gain required when the open-loop phase is —180° to make the elosed-loop system just unstable. 

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