Temperature measurement single tray

We started this section on inferential composition control by using a single tray temperature. Then we used two temperatures to control a temperature difference. Then we used multiple temperatures for average temperature control. The logical extension of this approach is to use whatever measurements are available to estimate product compositions. There are several types of composition estimators. Steady-state... 

As with many drying calculations, the most reliable design method is to perform experimental tests and to scale up. By measuring performance on a single tray with similar layer depth, air velocity, and temperature, the SDR (specific drying rate) concept can be applied to give the total area and number of trays required for the full-scale dryer. 

This chapter discusses a number of indirect composition-estimation Kchniques. They vary from the measurement of a single temperature on a tray somewhere in the column to the measurement of a number of variables that are fed into an on-line computer that performs rigorous tray-to-tray calculations. 

Probably the most commonly used composition-estimation technique is the measurement of a temperature on a single tray in die column. This tray is normally located in the rectifying section if cfistillate composition is more important, or in die stripping section if bottom composition is more important. On some columns two temperatures are controlled, one in each section. 

The sharp temperature profile in C2 presents control difficulties. The process gain between a single tray temperature, for example, stage 7, and reboiler heat input is very large. This means that the controller gain must be small, which results in poor load rejection. A simple solution is to measure several temperatures on trays in the column and control the average temperature. 

Nonisothermal Gas Absorption. The computation of nonisothermal gas absorption processes is difficult because of all the interactions involved as described for packed columns. A computer is normally required for the enormous number of plate calculations necessary to estabUsh the correct concentration and temperature profiles through the tower. Suitable algorithms have been developed (46,105) and nonisothermal gas absorption in plate columns has been studied experimentally and the measured profiles compared to the calculated results (47,106). Figure 27 shows a typical Hquid temperature profile observed in an adiabatic bubble plate absorber (107). The close agreement between the calculated and observed profiles was obtained without adjusting parameters. The plate efficiencies required for the calculations were measured independendy on a single exact copy of the bubble cap plates installed in the five-tray absorber. 

If the feed composition and the column pressure are constant, temperature can be used as an indirect measure of composition. When the bottom product composition is being controlled, the temperature sensor is located in the lower half of the column and when overhead composition is controlled, in the upper half of the column. The temperature sensor should be located on a tray that strongly reflects changes in composition (Figure 2.84). When two compounds of relatively close vapor pressures are to be separated, two temperatures or a temperature difference can be used instead of a single sensor. This configuration can also be used to eliminate the effects of column pressure variations. 

We want to compare tray temperature control with two types of composition control. In both, the composition of the distillate propane product is measured directly and controlled at 2 mol% isobutane impurity. The first type is direct composition control in which a single PI controller is used with reboiler heat input manipulated. The second type uses a cascade composition-to-temperature control structure. 

Two different control structures are explored for the single reactive column process. The first is the two-temperature structure considered in Chapter 10. This system uses only temperatures and does not require a direct composition measurement. The second control structure uses a composition analyzer to measure a composition of one of the reactants on a selected tray in the column to adjust one of the fresh feeds. 

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