About six years ago, Gary Faagau wrote in his Energy Saver column that being a batch operator doesn’t mean you must have inefficient operations (See, “Improve Batch Processing”). I’m going to expand on the lessons learned from Faagau’s column and discuss some of my experiences with batch operations and ways to analyze and improve their energy efficiency.
Generally, chemical plant operations are continuous and most analyzing and quantifying methodologies are very robust when we have “steady state, steady flow” conditions. We, as energy engineers, also neglect startup and shutdown conditions during an energy assessment because usually they occur for a negligible fraction of the time compared to the total operating hours in a year. However, this fundamental assumption breaks down when working with batch operations. Hence, all analyses must pay close attention to cycle time and understand ramp up and ramp down of the utility and feed streams. An engineer should understand every step of the process, its utility requirements and the corresponding time associated with that step. Drawing a temporal cycle plot is an excellent tool to visualize the process. You can use temperature versus time or load (heat duty) versus time profiles to understand the batch process (See “Bridge Batch and Continuous Steps Better”).
On the electrical side, pumps, fans and other mechanical motor-driven operations are relatively simple. Apart from a spike in electrical demand when these operations are started, the analysis methodology is straightforward when you take into account the time of operation. However, analysis for compressed air and batch operations can get complex. For batch operations, using storage effectively is the simplest solution to meet peak air demand. It also ensures extra compressors aren’t running continuously and eliminates short-cycling of air compressors. Using a master controller on the compressed air generation can be vital to maintaining the highest level of system efficiency.
Most processes require heat for the feed reactants. Reaction temperature is controlled for a certain amount of time and the product removed appropriately. Heat can come from several different sources but the most common are direct-firing (heaters), steam and oil (or a heat transfer fluid).
A fired heater cycles on and off on demand. If multiple heaters are used for multiple batch processes, opportunity exists for substantial energy gains. Turning a heater on and off leads to significant energy loss while purging and short-cycling. Instead, staging different batch trains, if possible, may provide sustained firing on one common heater, allowing tight excess air controls, almost steady-state operations, high energy efficiency and increased system redundancy because the other non-operating heaters can serve as 100% backup.
Steam is a distributed commodity in a plant; a boiler goes to low-fire or hot-standby as steam demand decreases. With the sophisticated controls and turndown capabilities now available on the market, managing steam demand isn’t an issue. Nevertheless, you can optimize steam generation based on load cycles. In some plants, I have seen steam accumulators used effectively to overcome peak steam demands and maintain steady steam generation.
Heating with oil loops (or a heat transfer fluid) may require more innovative strategies such as cascading heat exchangers and thermal storage. The oil heater may still cycle on and off depending on batch process demand. But the circulating oil loop has a lot of thermal energy and will cool down, eventually losing all thermal energy. Thermal storage may be a good option if the holding tanks are conveniently located, rated for the high temperatures and well insulated. It may also save pumping energy because the loop can be turned off. In multiple-batch operation, the excess heat from one batch could be used to pre-heat another batch. However, this would require a creative network of oil piping and heat exchangers and might not be feasible.
In summary, batch processes have been neglected from an energy efficiency perspective and they offer significant opportunities. I have tried to scratch the surface in this column, and hope to provide some targeted case studies in the future of such opportunities.
Riyaz Papar, PE, CEM, is director, Global Energy Services, at Hudson Technologies Company, Pearl River, N.Y. He has more than 20 years of experience in industrial energy systems and with best practices. He also is a U.S. Department of Energy (DOE) Steam Best Practices senior instructor and a DOE steam energy expert. He has provided energy consulting services in 100+ industrial plants in the U.S. and internationally. You can email him at email@example.com.