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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
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Citation
Davis, J. L.; Knight, N. (2005). Integrating Process Unit Energy Metrics into Plant Energy Management Systems. Energy Systems Laboratory (http://esl.tamu.edu); Texas A&M University (http://www.tamu.edu). Available electronically from https : / /hdl .handle .net /1969 .1 /5587.