Practical Ways to Save Energy in Your Plant

A recent Wall Street Journal article, U.S. Expects Steady Climb in Energy Prices opens:

Energy prices are undergoing a long, slow march higher as major economies shake off the effects of last year’s recession, the U.S. Energy Information Administration said Tuesday in its monthly outlook.

This outlook has many process manufacturers in energy-intensive industries again looking for opportunities to increase their energy efficiency. Emerson’s Dr. Douglas C. White has written a great article, Save Energy through Automation, in the January 2010 edition of AIChE’s Chemical Engineering Progress magazine.

Doug takes a reference Olefin plant with a naphtha feed that produces 500K tons/year of ethylene, total energy usage of 30MMBtu/ton ethylene, and total steam usage of 11MMBtu/ton ethylene. You can see the other process parameters on page 2 of the article.

He provides very specific energy saving examples and business result quantification based on the reference plant. I’ll highlight only a few specific examples but you’ll want to read the full 8-page article if energy efficiency improvements are part of your 2010 plans. Doug notes:

Advanced control and optimization systems can have a large impact on energy usage. However, their general functionality and energy impacts have been covered previously [Nasi, M. et al Experience with ethylene plant computer control, Hydrocarbon Processing, V62, N6, June, 1983, p. 74] and will not be discussed further here.

It’s important to make sure the key plant variables are being measured accurately, “in terms of location, frequency, accuracy, and number.” Deadtime can be introduced if the measurement device is physically located some distance away from a more appropriate location. Doug shares that this deadtime reduction can have a greater impact on loop performance than any tuning or algorithm change.

Many petrochemical plants measure fuel gas by volume instead of mass and density. If a plant has variations in fuel gas composition, flowmeters which measure mass and gas density will reduce the variability in cracking furnace combustion control. He cites an example naphtha plant:

…this improvement might be 0.2% in furnace efficiency, which is worth approximately $200,000/yr…

Market conditions, feedstock availability, furnace decoking, and process equipment availability impacts the load/throughput at which a plant operates. Effective anti-surge control on the cracked gas and refrigeration compressors is important since these units are major energy consumers within the plant. Doug writes:

The recycle valve needs to open very quickly and accurately to recirculate gas from the stage discharge to the suction. The ability to stably and safely operate the compressor closer to the surge limit saves energy, and the required operating constraint margin depends on the response characteristics of the anti-surge valve.

These anti-surge recycle valves can go from open to close and close to open in under two seconds. Doug quantifies:

If the plant is in low-throughput conditions 25% of the time, the value of the reduced recycle would be $125,000/yr for the reference plant.

Other energy saving examples Doug describes include stack gas CO measurements for improved combustion efficiency, key control loop dynamic analysis and tuning, and energy management and information systems (EMIS).

Energy Study Structured Approach-Doug-White
He closes the article with the importance of having a, “…structured program to assess, implement, and sustain energy savings achieved through automation…” This includes and energy study block diagram showing this structured approach to identify the top priorities.

There is a wealth of wisdom of practical ways that you can reduce the energy consumption of your process operations. Give the article a read if this imperative is part of your 2010 plans.

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