Optimize Energy Use in Distillation

The U.S. Dept. of Energy estimates that there are more than 40,000 distillation columns in North America, and that they consume about 40% of the total energy used to operate plants in the refining and bulk chemical industries (1). Improving the energy efficiency of this unit operation, therefore, is important to achieving overall plant energy savings. Reducing the energy consumption of distillation columns is not straightforward. First, columns come in many configurations with different operating objectives. These differences lead to distinct dynamic behaviors and different operational degrees of freedom, which necessitate specialized control configurations to optimize energy usage. In addition, many columns are subject to significant interaction among the control loops and have numerous constraints or limits on their operation, further complicating the dynamics and making it even more difficult to optimize control. The operation of distillation columns typically involves a tradeoff between energy usage and product recovery, and setting the proper target involves evaluating the relative economic value of these two factors. However, distillation is a non linear process, and normal product-valuation patterns add more nonlinearity to the economic objective function. Thus, calculating the correct operational targets can be complicated. Many books and papers have been published on advanced control of distillation columns and the design and analysis of these controls (2–4); the book by Blevins, et al. (2) provides a good introduction to the topic. This article discusses the nonlinear economic aspects of distillation control optimization and demonstrates a technique for calculating the correct energy-usage targets.