Manipulated streams

If there are any differences it then adjusts the steam valve. If the downstream pressure changes, a correction in the control valve is made immediately, instead of waiting for a product temperature change. Should the output temperature of the process stream rise, this would cause a set point change of the steam-pressure controller, which would cause a decrease in the steam pressure in the heat exchanger. Cascade control is very useful when the variation in the quality of a utility or other manipulable stream can cause deviations from the desired output. 

As the name implies, ratio control involves keeping constant the ratio of two or more flow rates. The flow rate of the "wild or uncontrolled stream is measured and the flow rate of the manipulated stream is changed to keep the two streams at a constant ratio with each other. Common examples include (1) holding a constant reflux ratio on a distillation column, (2) keeping stoichiometric amounts... 

Exchange of Sensible Heat When there is no change in phase, the rate of neat transfer is no longer linear with the flow of the manipulated stream, but is a function of the mean temperature difference A Tm ... 

The flow rate F of a manipulated stream through a control valve with equal-percentage trim is given by the following equation ... 

Hgure 16.1 Controlled variables and manipulated streams in a typical colunm. 

Manipulated streams. A stream is manipulated by varying the opening of its control valve. The stream flow rate is thereby varied to control a desired variable. Figure 16.1 shows positions of control valves in a typical distillation system. There are five manipulated streams top and bottom product flow rates, condensation rate, boilup rate, and reflux flow rate. 

Some ground rules for control system synthesis. The essence of control system s3mthesis is suitable pairing of the five control variables with the five manipulated streams. Theoretically, 5 (= 120) pairing combinations are possible, but only a handful of these are commonly used. The following ground rules can be applied for initial screening out of undesirable combinations ... 

In some systems, the reflux drum is omitted or run flooded (e.g.. Fig. 17.56). This eliminates one variable (accumulator level). This also eliminates a manipulated stream by making the condensation rate a slave to the top product rate. Similarly, when an internal condenser is employed, the accumulator level is eliminated, together with one manipulated stream (reflux becomes a slave to the condensation rate). In either situation, the rest of the variables are paired in the normal manner. The discussions below therefore apply to that situation too. 

Pressure is often considered the prime distillation control variable, Pressure affects condensation, vaporization, temperatures, compositions, volatilities, and almost any process that takes place in the column. An unsatisfactory pressure control often implies poor column control. Pressure is therefore paired with a manipulated stream that is most effective for providing tight pressure control. When the top product is liquid, this stream is almost always the condensation rate when the top product is vapor, this stream is almost always the top product rate (see Sec. 17.2.). 

Figure 16.2 Initial pairing of variables and manipulated streams, (a) Liquid top product (6) vapor top product (c) liquid and vapor top products.

Dynamic response. The closer the control temperature to the manipulated stream, the faster the response of the temperature to a change in the manipulated stream. This is particularly true when the manipulated stream affects the liquid flow rate. Liquid moves down the column in a tray-by-tray manner, first increasing tray liquid holdup, and only then flowing over the weir to the tray below. Because of this lag, a controller located far below the reflux inlet will keep calling for more reflux for some time after a corrective action has been taken. 

Figure 19.1 A nonlinear level controller designed to reduce manipulated stream flow fluctuations.

Side products complicate column control. Each side product adds two control variables (side product lights and heavies concentrations), but only one manipulated stream (side product flow) to the column (Sec. 16.2). Only one of these two compositions can be controlled by the additional manipulated stream the other is either allowed to vary, or "rides on the nearest end-product composition. Alternatively, both lights and heavies content of the side product can be controlled, at the expense of letting the composition of one of the end products vary. Side drawoffs also escalate the potential for interaction among control loops. 

The total product flow is set into the controller manipulating stream, 4. The incoming concentration x is multiplied by the flow of the plant effluent F1, whih when added to stream Fj, gives a measure of total product in the blend. This same signal serves as the measurement for flow controller 1, and as a set point for flow controller 2. Blend concentration y is set into the ratio station at the right, manipulating the flow of solvent B. If the flow of the blend is F,... 

Manipulated stream u Figure 15.5 Ratio control, Method I. 

Ratio control can be implemented in two basic schemes. For Method I in Fig. 15.5, the flow rates for both the disturbance stream and the manipulated stream are measured, and the measured ratio, = Um/d, is calculated. The output of the divider element is sent to a ratio controller (RC) that compares the calculated ratio Rfn to the desired ratio R and adjusts the manipulated flow rate u accordingly. The ratio controller is typically a PI controller with the desired ratio as its set point. 

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