Control ratio

Ratio control is a special type of feedforward control that has had widespread application in the process industries. Its objective is to 

As shown in Ref 33, the feedforward control equations were obtained by writing component material balances around the recycle addition point. For example, for monomer A this balance is given by Eq. (101). 

This equation is then solved in Eq. (102) for the fresh feed of monomer A since it is desired to keep the goal flow of monomer A to the reactor (q ia) constant. 

The recycle composition (yaz) is typically measured by online gas chromatographs, which may have significant time delays. If a faster response time of the analyzer is required, an infrared or Raman spectroscopy probe may be used. As discussed in Section 12.2.8 flow q 2 is typically measured and controlled by manipulating the 

As shown in Refs. 22 and 33, the performance of the feedforward control allows perfect compensation of disturbances that can arise, for example, from a step-change in the purge ratio, so that the reactor polymer characteristics (composition, molecular weight) are unaffected. Without the presence of the feedforward controllers the reactor dynamics and hence its control can be affected directly by the presence of three lags in series (reactor, separator, hold tank) and thus become unnecessarily more complex. 

Ratio control is a form of feedforward control which is widely used in the chemical industry and has proven very useful in polymerization reactor control. As is evident from its name, its purpose is to keep the ratio of two process variables at a 

In this first scheme, both flows are measured and divided to obtain the actual ratio. This is then compared with the set point and the flow of y is adjusted based on the difference. The set point to the ratio controller is the desired ratio. 

A common example of ratio control is the case of an adsorption column, where a fixed ratio of V/L is desired. The wild flow rate is the vapour feed V to the column, and the controlled flow rate is the liquid flow rate L. The ratio control seeks to maintain constant absorption factors in the column by keeping a constant V/L profile. 

However, over the years a number of slightly more complex structures have been developed that can, in some cases, significantly improve the performance of a control system. These structures include ratio control, cascade control, override control, etc. We will devote much of this chapter to these subjects. 

Also covered in this chapter will be some guidelines for developing an appropriate control system structure for a single unit and for a group of units that form a plant. Several realistic examples will be presented. 

Finally, a brief discussion is given of a new type of control algorithm called dynamic matrix control. This is a time-domain method that uses a model of the process to calculate future changes in the manipulated variable such that an objective function is minimized. It is basically a least-squares solution. 

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 

Ratio control is achieved by two alternative schemes, shown in Fig. 8.1. In the scheme shown at the top of the figure, 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 measurement signal. The setpoint of the ratio controller is the desired ratio. The output of the controller goes to the valve 

There are many situations when the flowrate of one stream needs to be changed when the flowrate of another stream changes. The blending of two or more streams is a common example. In distillation column control, it is often desirable to control a reflux ratio or to control a reflux-to-feed ratio. We may also want to control a reboiler heat input-to-feed ratio. 

The distillation column used as a numerical example is shown in Figme 4.47. A binary mixture of methanol and water are separated in a 16-stage column operating a 14.7 psia in the reflux drum. The feed is 40 mol% methanol. Distillate purity is 99 mol% methanol. Bottoms purity is 99.5 mol% water. A reflux ratio of 1.27 is required to achieve these specifications. Reboiler heat input is 152 x 10 Btu/h. Column diameter is 0.474 ft. The reflux drum and column base are sized for 5 min holdup when half full. Valve pressure drops are 30 psi. 

The Aspen Plus file is pressure checked and exported into Aspen Dynamics. Controllers are installed to achieve the following control structure (see Fig. 4.48). 

Reflux drum level is controlled by manipulating distillate flowrate. 

Measured value of flowrate of concentrated sodium hydroxide 

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. 

Stephanopoulos(6) lists common uses of ratio control as follows  

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Several parameters come into the relation between density and equivalence ratio. Generally, the variations act in the following sense a too-dense motor fuel results in too lean a mixture causing a potential unstable operation a motor fuel that is too light causes a rich mixture that generates greater pollution from unburned material. These problems are usually minimized by the widespread use of closed loop fuel-air ratio control systems installed on new vehicles with catalytic converters. 

Polyvinylpyrrohdinone/vinyl acetate copolymer (PVP/VA) was developed as an improved, less hygroscopic version of PVP. The monomer ratios control the stiffness and the resistance to humidity however, too high a vinyl acetate monomer content requires another solvent in addition to water to completely solubilize it. 

Fig. 16. Methods of ratio control implementation, where -I- and x indicate the division and multiplication of signals, respectively (a) use of a divider and ratio controller and (b) use of a multiplier. Calculated ratio = actual value of control variable both setpoint and gain are equivalent to the desired ratio.

Ratio control and multiphcative feedforward control, in general, are subject to the same considerations. Ratio control can be of a steady-state or a dynamic form. It is often implemented using a setpoint as the load variable when the load variable has a controller associated with it and the controller is in auto mode. 

A. J. Beumont, A. D. Noble, and A. Scariobrick, "Adaptive Transient Ah Euel Ratio Control to Minimise Gasoline Engine Emissions," Fisita Congress, London, 1992. 

The multiloop controller contains a variety of func tion blocks (for example, PID, totalizer, lead/lag compensator, ratio control, alarm, sequencer, and Boolean) that can be soft-wired together to form complex control strategies. The multiloop controller, as part of a DCS, communicates with other controllers and man/machine interface (MMI) devices also on the DCS network. 

SBR is produced by addition copolymerization of styrene and butadiene monomers in either emulsion or solution process. The styrene/butadiene ratio controls the glass transition temperature (To) of the copolymer and thus its stiffness. T ... 

Recommended nominal steam rates at 60 m/s exit velocity for a typical flare tip are shown in Figure 2. At lower velocities, higher steam ratios are required. Typical steam control consists of a flow ratio controller with adjustable ratio set point, related to flare gas flow. The ratio adjustment, located in the control house, provides for the higher steam ratios necessary at low flaring rates. 

The topic of flare gas measurement is treated below. If necessary, continuously vented surplus low pressure steam can be used for smoke control at low flaring rates, with high pressure steam cutting in through a flare ratio controller designed to handle large releases. 

The operator thought valve B was open, so he shut valve A. This stopped the flow of gas to No. 1 reaetor. The oxygen flow was controlled by a ratio controller, but it had a zero error, and a small flow of oxygen continued. 

The discussion below will focus briefly on the design of the graphic displays in order to illustrate the methodology used. The aim of the furnace operation (see Figure 7.15) is to achieve a specified output temperature of the crude oil. This is done by means of a master temperature controller which regulates the pressures of the fuels used. An air/fuel ratio controller regulates the flow of the combustion air, receiving as input the flow rates of the fuels... 

It may seem surprising that isocitrate dehydrogenase is strongly regulated, because it is not an apparent branch point within the TCA cycle. However, the citrate/isocitrate ratio controls the rate of production of cytosolic acetyl-CoA, because acetyl-CoA in the cytosol is derived from citrate exported from the mitochondrion. (Breakdown of cytosolic citrate produces oxaloacetate and acetyl-CoA, which can be used in a variety of biosynthetic processes.) Thus, isocitrate dehydrogenase activity in the mitochondrion favors catabolic TCA cycle activity over anabolic utilization of acetyl-CoA in the cytosol. 

Most oxygen trim systems interpose an additional link in the air/gas ratio controller. Others use an additional valve. Most types are based on the zirconia cell installed in the flue, while others use paramagnetic or electrolytic cell methods. The zirconia type has the advantage that there is no time lag in sampling, nor is there a risk of contamination of the sample. 

Types of air/gas ratio control There are various types of air/gas ratio device commonly used, including ... 

Electronic ratio controller. In this type of controller, a proportion of both gas and air is diverted through a bypass in which a thermistor sensor measures the flow. The air and gas flows can be compared and the ratio calculated and displayed. A ratio control valve in the air or gas supply, depending on whether the mode of operation is gas- or air-led, will automatically restore a deviation from the pre-set ratio. The electronic controller maintains ratio over a 19 1 turndown. The principle of operation is based on mass flow, so that it can be used with preheated air in recuperative systems. 

The development of reliable zirconia cells which can measure the gas analysis in situ without recourse to gas-sampling techniques has led to systems which provide feedback to the air/fuel ratio control system. 

The feed pump will be re-rated for the new conditions. With higher viscosity and higher gravity, the pump driver may need work. If the system is not adequate, heavier feed can be piped through a separate circuit in parallel with the existing circuit, preferably on flow ratio control. 

Excess of S03 (poor mole ratio control), i.e., high conversion, low free oil... 

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. 

Fig. 8-7 Three principal ratios control the style of runoff generation prevalent in a landscape (1) ratio of rainfall intensity to the infiltration capacity of the soil (2) ratio of bedrock conductivity to soil conductivity and (3) the topographic index defined by the ratio of the upslope drainage area to the ground slope. HOF = Horton overland flow SOF = saturation overland flow SSS = subsurface stormflow GWR = groundwater flow.

It was left to Kent Wilson (1968), in unpublished observations, to discover that in glasses based on Si02-Al203-CaF2 compositions the Al/Si ratio controlled the rate at which the cement paste set. These observations laid the foundation for the development of the glass-ionomer cement, during which most of the work on fluoride glasses was done. This topic is covered in detail in Section 5.9.2. 

F, as the second letter indicates ratio eg. FFC indicates a flow ratio controller. 

Ratio control can be used where it is desired to maintain two flows at a constant ratio for example, reactor feeds and distillation column reflux. A typical scheme for ratio control is shown in Figure 5.21 (see p. 233). 

More Flow FR2/ratio control Danger of high ammonia... 

NH3 cone. Failure of ratio control Temperatures fall TA1 alarms... 

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. 

Apply classical controller analysis to cascade control, feedforward control, feedforward-feedback control, ratio control, and the Smith predictor for time delay compensation. 

To regulate the air flow rate with respect to the fuel gas flow rate, we can use ratio control. Fig. 10.5 illustrates one of the simplest implementations of this strategy. Let s say the air to fuel gas flow rates must be kept at some constant ratio... 

Figure 10.5. Simple ratio control of air flow rate.

A more sophisticated implementation is full metering control (Fig. 10.6). In this case, we send the signals from the fuel gas controller (FC in the fuel gas loop) and the air flow transmitter (FT) to the ratio controller (RC), which takes the desired flow ratio (R) as the set point. This controller calculates the proper air flow rate, which in turn becomes the set point to the air flow controller (FC in the air flow loop). If we take away the secondary flow control loops on both the fuel gas and air flow rates, what we have is called parallel positioning control. In this simpler case, of course, the performance of the furnace is subject to fluctuations in fuel and air supply lines. 

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