Air separation control technology

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Abstract

Achieving high performance process control (HPPC) requires that the control system operate the plant at optimal efficiency over the full range of steady state and dynamic conditions. Air separation processes present particular challenges because of their energy intensive nature and demanding production schedules. The HPPC challenges for both cryogenic and adsorption processes are presented, recent applicable research is summarized, and directions for future research are proposed. The value of the operability index to improved HPPC is also presented and discussed.

Section snippets

Process characteristics

There are two primary methods of separating air into its two main components. If a lower volume, gaseous oxygen or nitrogen product is required, then an adsorption process driven by the pressure difference between the adsorption step and the desorption/reactivation steps may be used. On the other hand, for liquid products, larger volume gaseous products, high purity products, or the recovery of argon, cryogenic processes will be used. Fig. 1 (Air Products, 1997) indicates the ranges of capacity

Operability index

The benefits and challenges of evaluating the impact of design complexity on the operability of a facility were the subject of work by the author (Vinson, 1997, Vinson, 2000). One of the results of that work was the definition of a measure called the operability index (OI). In order to calculate the OI, a number of operating spaces were defined, some of those spaces are identified here:

  • (1)

    Available input space (AIS)—represents the range over which the inputs of the process are able to change.

  • (2)

Proposed directions for future research

Air separation is often viewed as a mature technology, yet significant advances continue to be made in the design efficiency and operating optimization of these processes. The recent rise in energy prices will drive further improvements in the industry which may catalyze additional research aimed at further improvements in control and operability of air separation processes. Suggested directions for future research in HPPC are described below.

The air separation industry is characterized by the

Summary and conclusion

The air separation industry has demonstrated continued improvement in process efficiency and operating cost. A large contributor to the improvements has been the utilization of advanced control techniques. Recent increases in the cost of energy will continue to fuel the drive for optimized design and operation. The ability to achieve maximum improvements is dependent upon continued progress in the research of advanced process control techniques.

Acknowledgements

I would like to thank to my colleagues at Air Products and Chemicals Inc. for joining me in the continuing battle to achieve HPPC in our operating plants, especially those that have served with me over the years in Advanced Control.

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