If you want sustainable energy savings, install advanced control systems to maximize energy efficiency. After all, the most problematic portion in all sustainable programs involves personnel. Many of the most efficient energy programs can suddenly become energy wasters because the person who championed the program was promoted, retired, or given a different responsibility.
In my first plant job, there was an old supervisor, named Mitch, who knew everything about the plant. To ensure his knowledge wasn’t lost, we hired an advanced controls company to meet with him to capture his knowledge and incorporate it into control systems so they would make smarter moves.
Energy systems aren’t as complicated as most plant controls. Most of the time, there’s no worry about catalyst, reactions, or non-linear responses. However, the key variables must be known for the systems to do the right thing. Here are four systems that are sure winners.
Minimum pressure control. Most distillation towers fractionate certain components at designed temperatures and pressures. However, there’s some leeway and built-in capacity. On most units, operating your tower at the same temperature and pressure provides the best results. Nonetheless, a few large distillation towers making very specific cuts can save a lot of energy by taking natural swings from summer to winter and day to night. If overhead gas is moved by a compressor with a motor, these swings also may include the cost of horespower.
Lower pressure requires less reboiler duty for the same fractionation. This only applies if the overhead condenser’s cooling medium can remove the necessary heat at lower pressure. If that medium has more ability at night or in the winter, there’s a possibility of saving some energy by reducing pressure. Instead of an operator constantly checking temperatures, use advanced controls to monitor key variables, including cost of compression at the current electrical rate, to minimize total energy cost at the same separation.
Product quality control. Adding analyzers on product streams can allow advanced control systems to reduce product fluctuation and hold is set point near actual specification. Before advanced controls, most plants used a buffer between actual specification and the plant product. Lab samples sometimes take up to 12 hours for results, meanwhile many changes can occur. If a product from a tower overhead was to have no more than 5% unwanted material, plants required samples to fall between 3% to 4% unwanted material. That way when the sample contained 6% unwanted material, the storage tank could dilute the unwanted. Even with this buffer, some tanks became “off-spec” and the tank had to be reduced in price or become rerun material.
Advanced controls with analyzers reduce that requirement and allow for tighter controls. So, plants can target between 4% and 5% or even 4.5% to 5%. In general, allowing more unwanted material on a constant basis, even with occasional excursions above specification, requires less energy than allowing less unwanted material. Additionally, not needing to rerun off-spec material avoids a hidden energy cost and can save a plant hard-earned money. But if you also attach a minimum pressure control to product quality control and optimize the entire system, you’ll have a tower running with the industries’ best practices.
Furnace air ratio control. It’s difficult to be specific about this because the range of furnaces is so vast. But control of air at the burner is probably a plant’s least adjusted control. In most systems, a mechanical flow device or variable speed motor controls air. To optimize air intake, you need to know the amount of oxygen available at the burner after combustion, draft pressure in the box, and the amount of fuel required. As a safety precaution and a better way to control multi-burner systems, monitor carbon monoxide in the stack, because it can only be produced at the burner. Furnace air controls should be slow to remove and quick to add air. A fuel feed-forward control increases air before increasing fuel to prevent fuel starvation, which can create a false response signal.
Energy management systems. There’s nothing more important for your sustainable system. A 24-hour monitor on plant demand, fuel and electric costs, and supply options saves money and quantifies savings. The system should help operators, not engineers, to make decisions to minimize total energy costs. In some cases, certain decisions can be automated. Although some find this unacceptable, because these systems are new and not entirely vetted, automated energy management should be a long-term goal for maximum sustainability.
These four advanced control systems will allow you to capture more savings on an ongoing basis and make your savings easier to sustain over time.
Gary Faagau is Chemical Processing's energy columnist. You can e-mail him at GFaagau@putman.net.