Batch operators face extra factors that can complicate any energy efficiency improvement. Unlike continuous operations, energy project economics is spread over fewer hours per year and may not help during ramping up and cooling down. Also, these operations tend to be more specialized, so industry standards or comparable plants are hard to find. Still, general energy efficiency principles do apply, but flexibility is needed during design.
For batch operations, make the unit energy sound from the start. A furnace that’s only 50% efficient when running must be revamped or replaced to be more efficient. A steam system with several leaks or faulty traps still needs to be repaired. Insulation on hot lines and hot vessels has to be maintained.
The nature of batch operations poses unique problems. One is energy recovery after the operation is over. One operator I know heats liquid to 450ºF and then must cool it to store it. All that energy is wasted.
[pullquote] The first possible solution is hot oil storage consisting of a hot oil tank, a cold oil tank, and a series of heat exchangers. After the operations are complete, the product is run through a series of heat exchangers where it’s exchanged against cold oil. The heated oil is stored in a hot tank until the hot oil is exchanged with the feed for the next batch. Sometimes, when there’s a series of batch operations, the heat from one unit can be used to heat up another unit. Other solutions involve using the oil to heat water.
Another problem is the need to reach reaction temperature and then remove heat from an exothermic reaction. In most cases, cooling water or fin-fans remove heat while a furnace produces the reaction temperature. Depending on reaction temperature, you can use the reaction to create hot oil, similar to the previously described processor , you can produce steam to run a steam turbine or supplement your steam system.
Most solutions for batch operating efficiency can be solved by frequency and management. One plant I audited had three batch operations, each with different energy requirements and hot oil and steam systems to produce the energy needed. Each operation had several reactors, so some required multiple heaters. I checked frequency and patterns and concluded that everything could be done with two furnaces, instead of five, if the plant spent more time planning batch timing and added systems for hot oil temperature control.
By setting the hot oil system to the highest temperature needed, some operations could be satisfied by blending return oil from higher temperature reactions to supply oil of lower temperature operations. This required running certain batches before others. Managing this can result in as much as a 50% energy savings. Most savings came from shutdown of furnaces that sat on idle or worked at reduced rates because not all batches were operating simultaneously.
One problem with the recent recession for batch operators is production cost increase per batch because of less frequency. Some have gone to one-shift operations, which means equipment must be cooled every night and then reheated each morning. Checking the economics may show that keeping the most energy dependent operations going around the clock and running less energy dependent batches during the day would save energy. This eliminates cooling off and start-up periods that substantially increase costs.
Idle energy is the time between batches when equipment must be maintained. Some equipment must be completely cooled while others are kept at a temperature to reduce the need to reheat from ambient. Running fewer reactors more often is a great way to reduce idle energy. One plant I knew used to run one batch every two to three days because an undersized heater required a day to get the reactor to the right temperature and undersized cooling equipment prevented products from being sent to storage tanks.
The plant replaced its heater with a system that could ramp the reactor temperature up in two hours and added heat recovery equipment for faster product cooling. The results were amazing with batch operations done twice a day — a 600% production increase, and a 40% cut in energy consumption per batch.
Being a batch operator doesn’t mean you have to have inefficient operations. Maintaining systems at high efficiency, storing heat in a hot oil medium, reducing downtime by running shifts on energy-intense batches, managing batches to take advantage of available heat, and improving batch frequency can help reduce per batch energy costs.
Gary Faagau is Chemical Processing's Energy Columnist. You can e-mail him at [email protected].