Practical Process Control Part 14: Pressure Compensated Temperatures

Myke King shows how to pressure compensate distillation column tray temperature controllers

THE TEMPERATURE, measured on a tray in a distillation column, can be a good indication of product composition. Separation by distillation relies on the components having different boiling points. So, there will be a correlation between composition and boiling point. Tray temperature measures boiling point (at the operating pressure) and so provides a simple inferential property. Figure 1 shows a typical control scheme. Our example is a column separating a mixed propane/butane stream. The product specifications permit 5% propane in the bottom butane product and vice versa.

Figure 2 shows the correlation between bottoms composition and tray temperature. If we were to control the tray temperature at 84.4°C, this should keep the butane on grade. However, the relationship between composition and tray temperature is not fixed. Firstly, the tray will be a few trays up from the bottom of the column, so we are effectively controlling the composition on the tray. Any change in the vapour and liquid flows below this tray will change the product composition. However, such changes are small when compared to the effect of column pressure. Secondly, boiling point is a function of pressure. While pressure is likely to be controlled (in this example at 12 barg), it can be attractive to manipulate its setpoint. Reducing pressure saves energy or debottlenecks a limiting reboiler. Increasing it helps relax any condenser constraint, potentially allowing feed rate to be increased. Adjusting pressure can be economically attractive and is often included in a multivariable predictive controller (MPC).

Linear compensation

Figure 2 shows the effect of reducing the column pressure from 12 to 11 barg. Keeping the tray temperature fixed will reduce the propane content of the butane to 2.4%. We can see, that to stay on grade, the tray temperature should be reduced to 80.5°C. This is the key to pressure compensation; we require a change of 3.9°C per bar of pressure.

Figure 3 shows the same effect at the top of the column. If we were to have a temperature controller in this section, then dT/dP would be about 3.1°C/bar.

The simplest way to apply this technique is to use signal conditioning. The measurement (PV) of the temperature controller becomes the pressure compensated temperature (PCT):

The reference pressure is any sensible constant value; it might be design pressure or the average operating pressure. The use of the minus sign might appear incorrect; we want the tray temperature to increase with pressure. But remember that this is the PV of a controller. By “tricking” the controller into believing the temperature has dropped, it will take corrective action to increase it.

It would be unlikely that the pressure measurement is on the same tray as the temperature. Indeed, as is shown in Figure 1, it is common for the pressure to be measured (and controlled) at the top of the column. However, although far from the tray, this measurement is normally suitable. The pressure drop across the column will be small compared to the operating pressure and, in any case, we are concerned with only changes in pressure. The exception to this can be vacuum distillation columns. Distillation under vacuum is used to reduce the boiling point of heavy components, for example, to avoid thermal cracking. As we have seen, the heavier the hydrocarbon, the higher the value of dT/dP. Under these circumstances, this can be as high as 800°C/bar – increasing the impact of using the wrong pressure. Secondly, the pressure drop across a vacuum column will be comparable to the operating pressure. Any change in pressure drop would cause a significant error. Under these circumstances the pressure measurement should be close to the tray used for temperature control.

Use the pressure controller setpoint

On most columns, the pressure measurement we use for compensation will also be the measurement used by the pressure controller. This offers the option of using the pressure controller setpoint, rather than its PV, for compensation. The most compelling reason to do this is when the pressure controller output is saturated. Caused by reaching the condenser duty limit, the controller output has reached 100% but the PV is still higher than the setpoint. If we were to use the PV for compensation then the temperature controller would increase the tray temperature by increasing the reboiler duty. This would overload the condenser further and cause a further increase in pressure. Further, using the setpoint also means that the pressure compensation will not unnecessarily respond to PV disturbances that will quickly be corrected by the pressure controller. Another advantage is that the setpoint will be noise-free. If an operating issue requires the pressure controller to be switched to manual, PV tracking means the setpoint will follow the measured pressure and so can still be used in the PCT.

However, as with our example of vacuum distillation, it may only be feasible to use a measurement different from the one used for control. Then, it may be necessary to filter noise and include logic to ensure that the temperature controller will not aggravate any violation of the condenser limit.