In order to show how the addition of feed-forward control improves the performance of the first compartment‟s temperature control (T1) and the control of the mass in 400-TK-20, a test was done which highlights the improvement. This test entails a sequence of changes in the set point (SP) of T1. Since the MV of the temperature controller is the flash recycle stream, these SP changes will result in flash recycle fluctuations, which in turn will introduce an important disturbance to 400-TK-20. Since the addition of feed-forward control aims that improving the response of the mass controller to this disturbance, the test done in this section showcases the success of the FF control. Starting with the steady state value as the SP, at 12 minutes into the run the SP is multiplied by 0.99, to return to its initial value after 1.2 minutes. After another 1.2 minutes the SP is multiplied by 0.995, and is kept there. While this is a small temperature change, the resulting change in flash recycle rate is large. These quick changes between reasonable values allow for a demonstration of the addition of feed- forward control.
Table 39 summarises the results from this evaluation:
Table 39: Summarised comparison between the performance of the controllers for the first temperature compartment and mass of 400-TK-20, for the base case and controller with feed-forward control.
Controller Comparative Measure Base Case FF-Feedback Controller Temperature
(400-TIC-2001)
Maximum Deviation 1.01% 1.01% Normalised IAE 2.997e-4 2.773e-4 SS Offset No No
Mass of 400-TK-20
Maximum Deviation 0.040% 0.046%
Normalised IAE 1.716e-5 2.764e-5
SS Offset Yes Yes
In this table it can be seen that the addition of feed-forward control improves the control of the first compartment‟s temperature. The normalised IAE is decreased by 7.5% from the base case, while the maximum deviation remains unchanged. The larger maximum deviation and IAE values for the FF- feedback control is caused by the fact that this mass controller is tuned to averaging control, whereas the base case is tuned using the variance of the tank‟s level in the data.
Note that, during the fine-tuning of the FF-feedback controller, it has been observed that tighter control on 400-TK-20 does not worsen the performance of the temperature controller, and therefore
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the above improvement in temperature control can be confirmed not be caused by a more loosely controlled 400-TK-20.
The analysis of the plots generated for this evaluation can be seen below.
First Compartment Temperature (oC) vs time (h)
Figure 26: Plot of the base case model’s SP (red) and measured value (blue) of the temperature in the first compartment (oC) vs time (hours). There is not offset at steady-state. Maximum deviation = 1.01%. nIAE =
2.997e-4.
In this figure the SP changes in the temperature can be clearly seen. While the temperatures move toward the SPs, it reaches it only again at 0.45 hours. The normalised IAE is less than 0.0003, which is a small value. The following plots show the flow rates of the flash recycle stream and of stream 7.
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Flash recycle mass flow (kg/h) vs time (h)
Figure 27: Plot of the base case model’s flash recycle flow rate (kg/h) vs time (h)
Mass flow of stream 7 (kg/h) vs time (h)
Figure 28: Plot of the base case model’s mass flow rate of stream 7 (kg/h) against time (h)
It can be seen from these figures how the outflow of 400-TK-20 resembles that of the flash recycle stream. This is due to the fact that the flow rate of the latter is limited by that of the former (to a ratio of 0.9 in this exercise). This problem has been identified and is the main motivation for the inclusion of feed-forward control. The mass in 400-TK-20 is given below:
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Mass in the 400-TK-20 (kg) vs time (h)
Figure 29: Plot of the base case model’s mass inside 400-TK-20 versus time (h). Steady-state offset. Maximum deviation = 0.04%. nIAE = 1.716e-5.
It can be seen how the maximum deviation is 0.04% of the SP. It can be noted that there is a steady- state offset, but this offset is negligibly small.
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First Compartment Temperature (oC) vs time (h)
Figure 30: Plot of the FF model’s SP (red) and measured value (blue) of the temperature in the first compartment (oC) vs time (hours). There is no offset at the new steady-state. Maximum deviation = 1.01%.
nIAE = 2.773e-4.
From Figure 30 it can be seen that the temperatures proceed closer to the SP values than it does in the base case. A zero steady-state offset is again reached.
Flash recycle mass flow (kg/h) vs time (h)
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Mass flow of stream 7 (kg/h) vs time (h)
Figure 32: Plot of the FF model’s mass flow rate of stream 7 (kg/h) against time (h). Offset at steady-state.
As in the case of the base case model, the two mass flow rates resemble one another. However, its shape differs from that of the base case plots in that there is a sharp initial change (compared to the slightly delayed one of the base case). The reason for this is the fact that the flash recycle flow rate is not inhibited by the flow rate of stream 7, but rather immediately changes its flow rate.
The mass in 400-TK-20 is plotted below.
Mass in the 400-TK-20 (kg) vs time (h)
Figure 33: Plot of the FF model’s mass inside 400-TK-20 versus time (h). There is a steady-state offset of - 0.0036%. Maximum deviation = 0.046%. nIAE = 2.764e-5.
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It can again be seen that 400-TK-20 has a steady-state offset. This is caused by the averaging control on this tank. The offset is less than 0.001% of the tank‟s mean, and is therefore negligible.
It can also be seen that the tank‟s mass does not follow the same trend as that of the flow rates entering and leaving the tank. These observations can be explained by the fact that an increase in the flash recycle stream‟s flow rate immediately causes the outflow to increase with the same amount. In the absence of controller lag (which is assumed to be zero in this project) the tank mass will not be influenced notably by the control of the first compartment‟s temperature.
From what has been shown, the following observations can be made:
The fact that the mass flow rate of stream 7 immediately changes in response to the change in the flash recycle rate (in the model with FF control) leads to a smaller disruption in the mass in 400-TK-20
In the absence of feed-forward control the response of the flash recycle rate is slow, due to the fact that it is limited by a mass controller which has one gain term by which it has to reject the disturbance constantly introduced by changes in the flash recycle rate. Introducing feed- forward control allows the mass control to be more effective, which in turn allows the temperature control to proceed without limitations on the MV.
Note that the temperature controller can be tuned much more aggressively for the model with feed- forward control without interfering with the mass controller. The reason for this is that the MV of the temperature controller is not limited by an externally enforced ratio. The same cannot be said for the model(s) without feed-forward control.
It is recommended that feed-forward control, as introduced and designed in this section, is used as supplementation to the feedback mass (or level) control of 400-TK-20.