Chemical Process Control

This was mainly in the hands of electrical/instrument engineers until the mid-1960s, and most of the published literature reflected their particular expertise. Since that time process control has developed as a significant area of chemical engineering expertise and publication. Many academics have adopted and published in this field (see Luyben, 1992) and there are also several handbooks and practical texts dealing with plant operations (e.g. Shinskey, 1984). Distillation was one of the primary chemical engineering unit operations to be researched and developed in depth for process control applications. This was because of the scope offered by distillation as a control problem and the range of options and alternatives available. The focus of distillation control is usually the product specification, but the critical operations of reboiler and condenser performance, the uncertainty of feed-point location, the reflux rate, and the need for correct internal functioning (e.g. fluid flow on and between the trays) means that several complex relationships and problems need to be considered together. The number of publications dealing specifically with distillation control attests to the complex nature of the problem and the number of approaches and applications that are possible within any single situation. An additional consideration for the designer is the possible number of column arrangements, utilizing several columns, and their possible alternative specifications (hence the advantages offered by simulation packages), and therefore the integration of individual column control within the overall control of a set of distillation operations.

Developments in distillation control have focused on several areas, generally searching for optimal solutions to the following:

1. Individual column control (including reflux streams, and reboiler and condenser operation and performance).

2. Control of a column required to perform a specific separation (e.g. heat-sensitive mixtures) or a particular type of application (e.g. separation of azeo-tropes or close-boiling mixtures; reactive distillation).

3. Control of sequences and arrangements of several columns.

Distillation control has developed into a multi-faceted problem incorporating mass transfer and separations, process modelling and optimization techniques, instrumentation and control functions, and the application of simulation packages in order to evaluate a range of possible problem solutions. Control theory has now developed to include a range of advanced techniques such as adaptive control, modelbased control, and the use of neural networks which supplement the basic approaches offered by combina tions of proportional, integral and derivative (P-I-D) control functions.

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