Process improvements such as high-yield catalysts, equipment advances, better design and process control changes can result in substantial energy savings. However, it’s not always necessary to look for complex process changes to save energy. Simple modifications to operations also can complement energy optimization efforts, resulting in significant energy savings. Here are some examples.
Plants often use equipment designed to operate only during abnormal operations in normal situations. Operating a burner over 40–50% excess air level, instead of the optimum 10–20% level is one such example. Opportunities similar to this exist in plants that run multi-stage processes. In petroleum refineries and petrochemical plants, the product of one unit could be the raw material of another unit. Such plants store the intermediate product to improve the operational flexibility of the processing units. Product coolers generally facilitate storage. However, when the product could be supplied continuously as feed to the next unit, simply bypassing the intermediate product cooler can save unnecessary cooling and fuel.
For example, an air-fin cooler at a petrochemical plant cooled an intermediate product stored at lower temperatures in tanks. The product is stored for longer durations only when the next unit isn’t ready to take its feed. However, the air-fin cooler ran continuously, providing unnecessary cooling even after the intermediate product was sent to the next unit. Because the next unit required higher feed temperature, the intermediate product was again heated at its feed heater. When the unnecessary cooling was pointed out, engineers set the product to bypass the air-fin cooler, resulting in substantial fuel and electrical energy savings due to the stoppage of two air-fin-cooler fans. This concept is applicable to any plant where high temperature intermediate products could be fed to the next unit without much time delay.
In steel mills, we always recommend to hot-charge the slabs, blooms and billets to the reheating furnaces of the rolling mills. This case is a good example of operators tempted to utilize additional design provisions for convenience rather than necessity; over time that becomes the operating norm. Good operating procedures combined with training, and understanding the purpose and limitations of all the process equipment process can help optimize energy use.
Reviewing excessive safety margins in equipment operation also may help save energy. In an acid manufacturing plant in Europe, two steam turbines drove a large air compressor and an electric generator in a single shaft arrangement. The first turbine, a backpressure unit, let down the high-pressure (HP) steam from 930 psig to the medium-pressure (MP) header maintained at 290 psig. The second turbine, an extracting-condensing-type, used the MP, 290-psig steam extracted from the first turbine. Part of the supplied steam to the second turbine was extracted at 73-psig and supplied to the low-pressure steam header; the balance of the steam was expanded into a vacuum condenser. The original design called for the first turbine’s MP letdown steam to supply the second turbine. Because the backpressure steam from the first turbine was superheated and exceeded the 660°F allowed at the second turbine inlet, a desuperheater at the first turbine outlet reduced the temperature to 600°F. However, over the years, plant engineers identified additional MP steam available from process waste-heat boilers and routed that to the second steam turbine to increase steam system efficiency and electricity generation. Because this steam was at saturation temperature, the combined average temperature of the steam supplied to the second turbine inlet dropped to 550°F, much lower than the 660°F limit at the turbine inlet. Unfortunately, the steam turbine extracts maximum work when the steam supply temperature is higher. The desuperheater operation and mixing with saturated steam unnecessarily increased the safety margin at the expense of turbine performance. Because the saturated steam reduced steam temperature below the 660°F limit, we recommended bypassing the desuperheater. Eliminating the desuperheater resulted in 500kW additional electricity generation to the plant, obviating purchased electricity worth $360,000 annually. The capital investment required was paid back within a couple of weeks. While it’s always essential to operate within design limits, excessively large safety margins can lead to unnecessary energy efficiency losses.
Whenever changes are made, their impact in other areas of the plant may require appropriate corrective measures to minimize the adverse effects. With better understanding of the basic principles and limitations of the process equipment, plant and process engineers can identify simple and inexpensive means to save substantial energy.
VEN V. VENKATESAN is Chemical Processing's Energy Columnist. You can e-mail him at firstname.lastname@example.org