Ratio control controlled stream

Mass flow measurement has been shown to be efficient as a mole ratio control, even where S03 mass flow was not measurable with sufficient accuracy in the diluted gas stream. 

Cascade control, along with ratio control, is used to control the temperature. The cold-water line is to have an air-to-close control valve. In case of failure in the air supply, the valve would open fully and a runaway reaction would be prevented. The hot-water line will have an air-to-open valve for similar reasons. After the two streams are mixed, the temperature will be measured. If it is above the desired temperature, the amount of air supplied to the valves will be reduced. This will increase the cold-water flow rate, and decrease the hot-water throughput. The result will be a reduction in the inlet water temperature. The desired temperature will be determined from a measurement of the reactor temperature. A deviation from the desired temperature will cause the set point of the second controller to be changed. This will result in a change of the inlet water temperature. 

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... 

In Fig. 8.7c the ratio of the two flows is changed by the output of a composition controller. This system is a combination of feedforward and feedback control. Finally in Fig. %.ld a feedforward system is shown that measures both the flow rate and the composition of the disturbance stream and changes the flow rate of the manipulated variable appropriately. The feedback controller can also change the ratio. Note that two composition measurements are required, one measuring the disturbance and one measuring the controlled stream. 

Control. The control scheme for a hydrocyclone installation in which wc have identified flow rale as a key parameter would logically be based on total flow rate with a ratio device adjusting a flow-control valve in the reject line to maintain the desired reject ratio. However, problems can be expected with an installation based on flow/ratio control because of gas breakout, which exists preferentially with the reject stream and creates two-phase flow. Normal metering devices will not tolerate two-phase flow and give sufficiently accurate and reproducible results for control purposes. 

Flow ratio control is essential in processes such as fuel-air mixing, blending, and reactor feed systems. In a two-stream process, for example, each stream will have its own controller, but the signal from the primary controller will go to a ratio control device which adjusts the set point of the other controller. Figure 3.17(a) is an example. Construction of the ratioing device may be an adjustable mechanical linkage or may be entirely pneumatic or electronic. In other two-stream operations, the flow rate of the secondary stream may be controlled by some property of the combined stream, temperature in the case of fuel-air systems or composition or some physical property indicative of the proportions of the two streams. 

An alternative type of ratio control system is shown in Fig. 7.72 where both flowrates are measured and the ratio between them determined. This measured ratio is compared to the desired ratio (acting as the set point) and the difference is used as the error signal for the controller which adjusts the flowrate of the controlled stream accordingly. 

The process control scheme for the absorption column is presented in Figure 9.6. It was designed from the recommendations presented in the HAZOP analysis, the results of which are reported in Section 9.8. It features ratio control on the make-up water stream. The signals from flow transmitters on this line and on the gas input line are fed to the ratio controller, whereby the make-up water stream is adjusted. 

MORE OF More Flow 4. Increased feed Possible reduction in absorption efficiency. May cause flooding. f) Ratio control on the liquid feed streams should be sufficient. g) Install HIGH LEVEL ALARM on the... 

The disturbance considered is a step increase in the flowrate of the ethylene feed. We want to increase the benzene feed whenever the ethylene feed is increased, so a ratio control structure is installed. Figure 3.89 shows a Mulitply block selected from the list of ControlModels, dropped on the flowsheet, and renamed ratio. A control signal is attached to the FE stream and connected to ratio.Inputl, as shown in the upper window of Figure 3.90. Another control signal is attached from the output of the multiply block to the setpoint of the benzene flow controller. Clicking the ratio icon, clicking the right... 

Controlled stream is manipulated to maintain a constant ratio (R) of controlled flow (B) to wild flow (A). 

Use a ratio controller to adjust the quench flow rate by maintaining a constant ratio of the quench to the total flash liquid stream (ratio controller, RC2). 

Ratio control is a special type of feedforward control where two disturbances (loads) are measured and held in a constant ratio to each other. It is mostly used to control the ratio of flow rates of two streams. Both flow rates are measured but only one can be controlled. The stream whose flow rate is not under control is usually referred to as wild stream. 

Figure 21.9a and b show two different ratio control configurations for two streams. Stream A is the wild stream. 

V.26 (a) Develop a ratio control system to regulate the temperature T of the resulting mixture of Figure PV.5. The following specifications are provided (1) Stream 1 is the wild stream. Its temperature remains constant but its flow rate changes in an unpredictable manner. (2) The temperature of stream 2 remains constant. 

For oxidation to formaldehyde, 1 lb of pure, methanol theoretically requires 26.7 cu ft of dry air measured under standard conditions (2.18 lb). Exact control of the ratio is made possible by the use of separate, controlled streams of air and liquid methanol introduced to a vaporizing chamber, which is supplied with heat to vaporize the liquid. Cleaned, dry air is supplied through a storage tank by a compressor, and methanol enters the mixing chamber as a liquid. Only small amounts of mixed mr and vapor... 

Ratio control is achieved by two alternative schemes, shown in Fig. 4.1. In the scheme shown in Fig. 4. la, the two flow rates are measured and their ratio is computed (by the divider). This computed ratio signal is fed into a conventional PI controller as the process variable (PV) signal. The setpoint of the ratio controller is the desired ratio. The output of the controller goes to the valve on the manipulated variable stream, which changes its flow rate in the correct direction to hold the ratio of the two flows constant. This computed ratio signal can also be used to trigger an alarm or an interlock. 

This leaves four variables to be controlled by manipulating five streams. The fifth stream, often termed the "free stream, is usually flow controlled. If the free stream is the boilup rate, it is sometimes controlled by differential pressure. This technique is discussed in Chap. 19. At other times, the free stream is controlled by ratio control (often reflux to feed) or by a detuned composition controller (above). 

Scheme 16.6c is a bottom product demand scheme analogous to scheme 16.4a. Similarly, bottom product demand schemes analogous to schemes 16.46 and d, and distillate demand schemes analogous to schemes 16.4a,6, and e can be devised. Often, a ratio control is used on the free stream (e.g., reflux-to-feed ratio in Fig. 16.6c) to minimize the impact of feed fluctuations on the column. Several product demand schemes with more sophisticated ratio controls are described by Buckley et al. (68). 

In all these situations, the two prominent products are MB-controlled using one of the normal schemes (Fig. 16.4), as if the small stream does not exist. There is little incentive to tightly control the composition of the small stream, and it is often assigned a non-MB control. The small stream may be withdrawn on flow control, flow-to-feed-ratio control, or flow-to-main-product-ratio control (Fig. 19.6a-c). A generous flow or ratio setting is usually fixed as a means of positively preventing impurity accumulation. If this leads to excessive product losses, the small flow can be manipulated by a temperature (or composition) controller in the pasteurizing section. For simplicity. Fig. 19.6 shows the small stream to be on flow control, but the discussion below also applies when the small steam is temperature- or ratio-controlled as described above. 

In some side-drawoff columns, three or more products are prominent. Typical examples are an ethanol-water still (Fig. 19.9) and some alkylation depropanizers and deisobutanizers. The control philosophy is to pick the two most important product purities and have these compositions controlled. Other product purities are allowed to vary. Any additional nonprominent streams are drawn on flow or ratio control. 

Figure 24.2. Reagent addition system. The ratio controller compares reagent flow to sample flow and maintains a preset ratio by operating control valve V on the reagent stream. The streams are then mixed in or before) the measurement chamber where the desired analytical measure ment is performed. Courtesy of the Foxboro Company.

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