Pressure-compensated temperature control

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  

TABLE 6.3 Design Results for Heat-Integrated Systems Acetone-Methanol. 

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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. 

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 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 ... 

During normal operation the flare valve is closed and the pressure in the gasifier is controlled through a by-pass valve at the booster compressor. Quite naturally is the gasifier a slow system concerning both pressure and temperature control. The output control of the gas turbine is completely different and it responds more or less instantaneously. Operation in the fully integrated mode made the pressure, temperature and gas quality in the system vary a bit when the gas turbine suddenly compensated for a small change in either parameter. 

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

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. 

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... 

Figure 6.28 Control structure with pressure-compensated temperature.

Since the pressure in the high-pressure column changes with operating conditions, a pressme-compensated temperature control structure is required to maintain product specification in this column. The detailed steps in implementing the various flowsheets and control structures in Aspen Plus and Aspen Dynamics have been discussed. 

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 ... 

Some operators have foimd it confusing to have both temperature and pressure-compensated temperature. We therefore have taken Figure 10.3 and plotted C versus T (both read at constant Pps) on a new plot. The operator s control station can then be calibrated directly in terms of composition. 

These modern computer controlled ignition systems use multiple sensors to determine optimum firing. This may include double pick-up sensors on the flywheel to determine rpms under acceleration and deceleration, intake and atmospheric pressure compensation, oxygen sensor levels to maximize combustion, temperature sensors and exhaust emission sensors. All this data is constantly fed into the on-board computer and processed using complex algorithms to determine optimum firing and fuel consumption levels. 

Most distillation columns are operated under constant pressure, because at constant pressure temperature measurement is an indirect indication of composition. When the column pressure is allowed to float, the composition must be measured by analyzers or by pressure-compensated thermometers. The primary advantage of floating pressure control is that one can operate at minimum pressure, and this reduces the required heat input needed at the reboiler. Other advantages of operating at lower temperatures include increased reboiler capacity and reduced reboiler fouling. 

Dirt and moisture are the worst enemies of the performance of all PD gas meters, so inlet filtering should be used when indicated. Pressure and temperature should either be controlled or compensated. The testing (or proving, as it is called in the gas utility industry) of gas meters is usually done by an accurately calibrated "bell" of cylindrical shape that is sealed in a tank by a suitable liquid. The lowering of the bell discharges a known volume of air through the meter under test. Other standards used to calibrate gas meters are calibrated orifices and critical flow nozzles. These devices compare rates of flow rather than fixed volumes. 

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