Pressure Compensated Temperature

We have dealt with many of the problems that can arise with tray temperature control by selecting the optimum trays. There remain some other issues that we need to address with 

The technique for dealing with this problem is known as pressure compensated temperature PCT). This was covered briefly as an example of signal conditioning in Chapter 5. In its simplest form a linear correction is applied to the measured tray 

Taking our example of a reduction in pressure, the value of PCTwill be higher than T. If we make PCT the PV of the tray temperature controller then it will compensate for the increase by reducing T. Provided we have quantified dT/dP correctly for each temperature controller (as AT/AP) then the product compositions will remain constant. 

It is possible, although unusual, to apply the pressure correction to the SP of the tray temperature controller (Psp). Some operators prefer this since it emulates what they would do when the pressure changes. Further the controller still displays the real tray temperature. 

Theoretically the pressure transmitter should be at the same location in the column as the temperature transmitter. Fortunately on most columns the pressure difference, between the 

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The partial derivatives are usually assumed to be constants that are evaluated at the steadystate operating level from the vapor-liquid equilibrium data. Thus, pressure and temperature on a tray can be measured, as shown in Fig. 8.3c, and a composition signal or pressure-compensated temperature signal generated and controlled. 

Pressure variations can be compensated for by measuring both temperature and pressure at a tray location and computing a pseudocomposition signal. This computed composition signal (pressure-compensated temperature) can then be controlled ... 

Computed variable control, (a) Mass flow rate, (b) Heat input, (c) Composition (pressure-compensated temperature). 

A differential temperature control (e.g., Fig. 18.8o) is in essence a pressure-compensated temperature control. The control tray temperature is measured in the usual manner. A second temperature is measured at a point where temperature is relatively insensitive to composition, such as near the bottom or the top of the column. The second temperature is subtracted from the first, giving a differential temperature measurement. This differential temperatiire is used for control. Since the second temperature varies little with composition, the differential temperature will reflect the composition variations measured by the first temperature. When column pressiu-e changes, both temperatures change equally, but the temperature difference remains constant. 

PRESSURE-COMPENSATED TEMPERATURE CONTROL IN DISTILLATION COLUMNS... 

Tray temperature control is used in most distillation columns to infer product composition, but changes in pressure on the control tray can adversely affect the estimation of composition. Pressure is typically controlled in the condenser, not on the control tray, so changes in vapor flow rates will change tray pressure due to changes in tray pressure drops. Pressure-compensated temperature control was proposed over four decades ago to solve this problem. Measurements of both temperature and pressure on the control tray are used to estimate composition. The method has been qualitatively described in many practical distillation control books, but the author is not aware of any quantitative evaluation of its effectiveness that has appeared in the open literature. 

In this chapter, we present a numerical example to illustrate quantitatively the performance of pressure-compensated temperature control. In addition, a simple but accurate method for finding temperature/pressure/composition relationships is described, and the techniques for implementing pressure compensation in Aspen Dynamics are presented. 

A more flexible approach was documented in 1973 and used measurements of both pressure and temperature to calculate a pressure-compensated temperature signal to be controlled. Equation (16.1) shows the relationships among the measured variables (temperature and pressure) and the calculated composition on the tray. 

Figure 16.7 Flowsheet equations for pressure-compensated temperatures.

Figure 16.8 Aspen Dynamics process control diagram with pressure-compensated temperature control.

Two types of disturbances are imposed on the process, and the performance of conventional temperature control is compared with that of pressure-compensated temperature control. 

The dashed lines (PTC) in Figure 16.9 are for the pressure-compensated temperature (composition) control. It should be remembered that we are estimating the nC4 composition on Stage 55 and controlling this calculated C4 composition by manipulating the QR/F ratio. The set point of the composition controller is 19mol% nCA. The increasing... 

To set up a pressure-compensated temperature scheme, VLB data are calculated at two different pressures for the same liquid composition. For a liquid mole fraction of 0.90 THF, the temperature is 139.499°C at 7.91 bar and 142.858°C at 8.5 bar. Therefore, the temperature signal is adjusted as given below ... 

Figure 6.27 Flowsheet equations with pressure-compensated temperature.

The dotted lines in Figure 6.26 (labeled +20 Tpc ) give results for a +20% change in feed flowrate using the pressure-compensated temperature scheme. The temperature on Stage 9 in the high-pressure column is not held at 144°C, but increases to about 149°C at the higher pressure. The water impurity XB2(water) in the THF product stream is held very close to the desired 10 ppm level. 

The energy requirement in the low-pressure column is 14.79 MW, so the auxiliary reboiler must provide 14.79-5.86 = 8.93 MW (32GJ/h, as shown in the TCI faceplate in Fig. 6.30). Flowsheet equations similar to those described in the THF-water system are needed in this system. The pressure-compensated temperature measurement uses the temperature (411.5 K) and pressure (10.354 atm) on Stage 53. 

Figure 11.12 Aspen Dynamics flowsheet equations for heat-integrated extractive process and pressure-compensated temperature.

The third equation is used to provide a pressure-compensated temperature measurement in the methanol column. This is needed because, in the heat-integrated system, the pressure in the methanol column is not controlled. It floats with operating conditions. If more heat transfer is required in the reboiler/condenser, a larger temperature difference is required, and this is achieved by the pressure in the methanol column increasing, which raises the bubblepoint temperature in the reflux drum. 

If a distillation tray temperature controller keeps the temperature constant as the pressure changes, the composition wiU move away from target. We can resolve this by using the pressure to condition the temperature measurement. The subject of pressure compensated temperatures is covered in full in Chapter 12. 

The output of the compensator, is a measure of the pressure-compensated temperature. The process operator must know what this relationship is in order to adjust the set point for the controller properly. It is common practice to adjust the compensator bias so that it reads mid ale when T = Tps and P = Pp, or more generally when ... 

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