MONITORING OPERATIONS BETTER
Process operators are tasked with running their units safely while meeting production targets for throughput and quality. They also can address and impact energy efficiency. A necessary first step is to measure and report energy consumption in detail and in real time — however, in many cases, this alone doesn't suffice. When throughput rates, feed properties or product qualities highly vary, it's critical to provide energy consumption targets that account for current process conditions so operators can spot opportunities to improve. Figures 4 and 5 illustrate the importance of this point.
Figure 4 tracks steam consumption and charge rate for a refinery's fluid catalytic cracking unit. Ovals highlight several events of interest. In the first, steam consumption increased for a short time with no change in unit throughput. In the second, steam consumption rose with feed rate, as would be expected. Monitoring steam consumption alone doesn't provide guidance to operators about which changes are opportunities for improvement.
Adding intelligent targeting information for the unit (Figure 5) makes the picture clearer. The steam consumption targets, shown in blue, depend upon throughput, feed quality and reaction conversion levels. The excursion in steam consumption during the first event is worse than indicated when only considering throughput because the target energy actually decreased during the event due to process changes other than the throughput. In the second, although the steam consumption is expected to rise with feed rate, it increased more than it should have because the operation wasn't executed carefully. The situation was corrected shortly after the feed rate increase to bring the actual consumption again in line with the target rate.
Real-time monitoring and reporting of energy consumption will allow operators to make moves that result in energy conservation only if the following conditions are met:
• The energy consumption must be tracked in detail — for each energy/utility type, at least down to the process unit level.
• Tracking and reporting must take place in real time so responses can be made as soon as a change in performance occurs.
• The energy must be put into context by comparing actual consumption to a target that accounts for process conditions such as throughput, feed and product quality, conversion levels, etc.
USING AUTOMATION MORE EFFECTIVELY
One of the simplest ways to address energy efficiency through automation is to routinely review the performance of regulatory control loops that affect energy consumption. Examples are steam/oil ratio controls in oil-fired furnaces, steam pressure controls for sub-headers or individual heat exchangers, plant air header pressure controls, etc. Often, operations doesn't flag these loops for attention from process control staff as long as their performance doesn't impact production rate or final quality. Identifying such loops and periodically focusing on them frequently can improve energy efficiency without affecting the overall process. Routinely checking for poor controller tuning, valve stiction and transmitter problems can yield benefits for many plants.