Expert Advice Maximizes Optimizer Value

A combination closed-loop optimizer and G2 expert system allowed a largy chemical company to save more than a million dollars in reduced overall energy demand.

By James F. Sturnfield, Union Carbide Corp.

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The Seadrift plant has achieved savings in the operation of heat-recovery steam generators (HRSG). These generators create steam from the hot exhaust of a gas turbine for plant heating or to generate electricity via steam turbines. In manual-control mode, operators tend to run at a level that allows them to comfortably handle any fluctuations in steam demand. As a result, the electrical load on the steam turbines varies since the steam load changes inversely with changes in plant steam demand.

With the aid of the real-time expert system, the optimizer will automatically adjust the load to achieve the best savings. The optimizer quantitatively evaluates continually fluctuating electricity prices and steam demands. It can then determine the appropriate level at which to run the HRSG and best utilize the steam generators. The optimizer and expert system change operating setpoints as often as every five minutes, whereas operators typically made changes only once or twice per day.

G2 also has a built-in simulator that allows users to run cases offline and measure the results. These simulated data can run concurrently with real system data to improve or compare models in the test environment to the conditions in the actual plant. The simulator can be used for testing application logic throughout the development cycle, for monitoring actual-to-ideal performance in the finished application, and for what-if analyses to help identify optimal operating conditions and designs.

Dow engineers frequently use this tool to evaluate potential changes to the optimizer and to assist in operator training. The operators play a critical role in the use of a closed-loop system – they not only need to feel comfortable with its operation, but their invaluable knowledge helps improve it.

For example, when the closed-loop optimizer was first implemented, operators often turned it off. The optimizer would recommend running the turbines at partial load, but the operators were concerned that the hydrogen-rich fuel fed to the turbines might flash back and force a shutdown. When this problem was brought to their attention, Dow engineers used G2 to implement guidelines to the optimizer on the relationship between fuel richness and the limitation on partial loading of the gas turbine.

The optimizer provides additional savings by giving Dow a better understanding of plant operations. For example, the operators were controlling a header pressure by venting. Based on output from the optimizer, the header controls were changed to allow the operators to maintain a reliable steam supply to the units, which resulted in additional energy savings. Dow has validated the operation of the optimizer and expert system at Seadrift. The operators&rsquo confidence in the system is high enough so they rely on it in all but the most unusual of circumstances.

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Cost savings of $120 per hour have been attributed to the combined use of the closed-loop optimizer and the expert system. The success of Dow engineers in increasing the operators&rsquo comfort level is demonstrated by the fact that last year the Seadrift plant ran nearly continuously in closed-loop mode. The operators now view the optimizer as being an important part of their success.

Dow believes that substantially greater savings can be captured by extending the application to other areas of the plant&rsquos operation besides energy usage. Dow is also ready to take this concept to other plants. The use of closed-loop optimization in combination with a real-time expert system clearly provides Dow with important improvements to plant performance and the company intends to take full advantage of its capabilities.

James F. Sturnfield is a senior specialist for Union Carbide Corp., a subsidiary of The Dow Chemical Co., South Charleston, W.Va. Sturnfield has a Ph.D. in mathematics and specializes in simulation, optimization and expert systems. E-mail him at sturnfjf2@dow.com.

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