15.7 Summary

This chapter has provided a brief overview of perhaps the most interesting and challenging aspects of process control. The intuitive placement of controllers on a process with recycle can often lead to extreme sensitivity to disturbances known as the snowball effect. This effect can be minimized by fixing the flow rate of one stream in the recycle loop.

Recycle can also significantly increase the dynamic timescale of the process. The specification of a plantwide control structure is based on years of operational experience with similar plants and will involve input from a large number of engineers and technicians. If you are interested in a much more detailed presentation of plantwide control, including many "rules of thumb" and other suggestions, the book by Luyben et al. (1999) is highly recommended.

Corporations have a large number of levels where decisions need to be made (Section 15.4). The various levels range from the long-term corporate purchasing and scheduling decisions at the top, to fundamental control instrumentation at the bottom, as shown in Figures 15-15 and 15-16.