Reducing Distillation Column Energy Usage

I saw some startling facts in a presentation by Emerson’s Pete Sharpe, whom you may recall from many process optimization-related posts. This week he visited The Automation Group (TAG) office in Houston, to discuss distillation and fired heater optimization. I was speaking with Aaron Crews who was able to catch this presentation. Aaron’s expertise has also has been highlighted in several automation modernization and social media related posts.

Did you know that there are over 40,000 distillation columns in the U.S. alone? And that they consume 40-60% of the total energy in the chemical and refining industry. And that these distillation columns consume 19% of the total energy consumed by U.S. manufacturers. And finally, that these consume 6% of the total U.S. energy usage. If you drive along the Texas Gulf Coast and see all the refineries, petrochemical, and chemical complexes, it’s not hard to accept these statistics.

I think it’s safe to say that anything that can be done to improve the energy efficiency of these distillation columns will reduce operating costs, improve emissions, and lower the overall domestic need for energy. Pete noted that reducing energy consumption is one of the best ways that U.S. manufacturers can meet the Environmental Protection Agency (EPA) greenhouse gas reporting rules.

Pete and his team of refining and chemical consultants, work with process manufacturers to optimize the performance of their distillation processes. SmartProcess Distillation packages the team’s subject matter expertise, couples it with Emerson products such as the DeltaV system with its embedded advanced process control, and provides planning, engineering, commissioning, and training support on these optimization projects.

For those not as close to the process of distillation, a distillation column separates components based on different boiling points. The column is made up of trays, and the temperatures of these trays usually reflect composition on that tray. These temperatures need be compensated for pressure.

From a control strategy perspective, in any column, you are trying to control a composition of the top product and the composition at the bottom of the column – simultaneously. The mental framework one follows is, “What comes in must go out.” This translates into a material balance and energy balance. The material balance is represented by the fraction of the feed that goes overhead, or the overhead (OH) rate divided by the feed rate. The energy balance is represented by a reflux ratio, or the internal reflux flow rate divided by the feed rate. Dual composition control uses both the material and energy balance handles to simultaneously control both overhead and bottoms compositions. Since these are highly interactive, it typically requires a model predictive controller like PredictPro to accomplish closed loop control.

Aaron relayed a key point that Pete emphasized. As you approach 100% purity on either end of the column, the change in energy per change in purity increases dramatically, making high-purity columns prime targets for energy savings. Pete stressed that Advanced Process Control will almost always reduce the column variability, push to minimum reflux limits and allow operators to run closer to the specifications. This in turn results in improved product quality controls, less quality giveaway, lower specific energy consumption and less emissions. Good for profit, good for the environment.

Pete shared some typical results achieved in installing the SmartProcess Distillation application. For 1-2 weeks of engineering to design, install, and commission, plants have been able to achieve a 40-80% variability reduction, 5-10% throughput increase, 5-10% energy cost reduction, less off-spec / rework and improvements in safety and environmental metrics.

If you see opportunities to improve control on your distillation columns, it seems like the energy reductions and improved operations can fund it.

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