Integrating Process Unit Energy Metrics into Plant Energy Management Systems

Abstract

As energy costs continue to rise across the process industry, many plants have responded by developing improved energy monitoring and reporting programs. At the center of such programs are typically spreadsheet or database applications that pull information, such as fired heater excess oxygen and steam vent rates, from the plant data historian and generate summary reports that compare and trend actual performance relative to targets. On average, plants can expect to reduce overall energy costs by up to 10% through improved management of plant variables, or metrics, that influence energy consumption. Energy metrics can generally be classified into three categories: Equipment, Utility System, and Process. Examples of each type of metric will be given in the paper. As a percentage of the overall savings sited above, the energy savings through stewardship and optimization of Equipment, Utility System and Process metrics are generally 50%, 40% and 10%, respectively. Plants have generally done a good job of stewarding the 90% of savings available through Utility and Equipment related energy metrics, primarily because target setting is fairly straightforward. However, the 10% of savings available from Process metrics, such as tower reflux ratios, pumparound rates, and steam stripping ratios, are typically missing from energy management systems due to the difficulty in first identifying them, and second in determining the optimum targets. Target setting is most difficult because with process metrics, yield considerations must be included in the target setting process. It is quite easy, for example, to save energy by cutting tower reflux rates. But too much reduction will sacrifice yield performance. Even at elevated energy prices, even the smallest reduction in yield will typically offset any energy savings that might have been captured. Therefore, in order to effectively incorporate Process energy metrics into the plant energy management system, knowledge of both energy and yield parameters is required. This paper will explore an effective methodology for determining what process unit energy metrics are important, how to effectively set their targets, and how to incorporate them into an effective energy management system. In terms of how to identify process energy metrics, the paper will discuss which energy intensive processes should be examined first, such as crude distillation and cat cracking. In terms of target setting, the paper will describe how process engineering experience is combined with simulation to develop meaningful targets that characterize the point where yield and energy are simultaneously optimized. Finally, the work process required