Step-change disturbance Subject

The application of the SVD technique provides a measure of the controllability properties of a given d mamic system. More than a quantitative measure, SVD should provide a suitable basis for the comparison of the theoretical control properties among the thermally coupled sequences under consideration. To prepare the information needed for such test, each of the product streams of each of the thermally coupled systems was disturbed with a step change in product composition and the corresponding d3mamic responses were obtained. A transfer function matrix relating the product compositions to the intended manipulated variables was then constructed for each case. The transfer function matrix can be subjected to SVD ... 

Figures 21.18, 21.19, and 21.20 show the responses for the three configurations, subjected to the worst-case disturbance in which the feed flow rate and composition simultaneously undergo positive step changes to their design Unfits. As shown in Figure 21.18, the SC configuration is returned to its setpoints in approximately 100 min, with Tm most affected. Note that this response is quaUtatively similar to that of the linear approximation shown in Figure 21.3. The simulation can be reproduced using the METH SC.hsc file on the CD-ROM that accompanies this book.

The CSTR in the previous example is subjected to a step change in the system flowrate, while maintaining a constant volume for the reactor. Find the exit composition response, c (t), to such disturbances. 

One of the major disadvantages of PI controllers for liquid level is that they always cause the manipulated flow change temporarily to be greater than the disturbance flow change. For example, if the system we have been considering is subjected to a step change in inflow, we obtain the following ... 

How does this control scheme respond to disturbances The major control objective is to produce a side stream of essentially pure benzene, approximately 99 per cent. To test this control scheme, the column was subjected to two different types of disturbance. The first disturbance was that of an increase in the total volumetric flow rate of the feed introduced to the column. A strip chart of the feed flow rate, the three product flow rates and the side stream benzene purity is shown in Figure 8.12. A step change in the feed flow rate is introduced, increasing the flow rate from 11.5 m h to 12.075 m h This corresponds to an increase of 5 per cent. 

A processor does not have control of all the steps from basic material to finished product. Material processing capability is limited by what is received and by when inspection is made or required so selection tests are important and must be subject to change (see Chapter 10). In turn, the process line has many variables that must be coordinated. A practical procedure today, with all closed-loop process control systems, is to subdivide the controls into distinct subsystems (Fig. 3-33). They can then be controlled within single control loops or by simple intermeshed circuits. A single-loop feedback circuit has one input and one output signal disturbances that affect the process are registered by the controller directly, by an ad-... 

The size of the step must be large enough to have a noticeable effect on the process. If the PV is subject to noise, small disturbances will be difficult to analyse accurately. The change in PV needs to be at least five times larger than the noise amplimde. This may cause an unacceptable process disturbance. 

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