Most companies in the process industries are working hard to reduce their energy consumption, prompted, e.g., by concerns about carbon dioxide emissions as well as energy security and cost. Many sites have energy and emissions management programs in place or are preparing to implement them. Ad-hoc or limited-scope efforts can produce positive results — but the benefits usually disappear over time. Achieving lasting improvements requires a comprehensive program with clear goals and systematic actions.
To be truly effective, a program must address the four key areas of lifecycle energy management — design, operations, automation and maintenance (Figure 1). It must aim to:
• design the plant for efficiency;
• operate intelligently to keep efficiency high;
• control processes to reduce energy consumption; and
• maintain equipment at design performance.
These activities involve different people within the organization and different time frames. Design efforts generally occur only during the initial engineering phase or during subsequent capital projects. Operations and control activities are on-going; best-managed sites continually strive to improve efficiency. Maintenance takes place periodically, often during plant shutdowns but sometimes, depending on the equipment, while the plant is running. Table 1 lists some resources processors should provide in these four areas.
Decisions made during the design phase to a large extent determine the ultimate energy efficiency of a process unit. Choices, e.g., of technology to license, major equipment, heat recovery operations, etc., will have lasting effects throughout the life of the unit and typically constrain the maximum efficiency achievable once the unit is operating.
Evaluation of various options will consume time and budget; so among the most important steps during the design phase is to set aside resources so energy efficiency can be considered early in the process. Waiting until late in the design stage after most major decisions have been made often only results in a list of changes that would have been beneficial but can't be implemented without significant delays in the overall project schedule.
A common technique employed in process design for improving energy efficiency is pinch analysis. Typically a resulting solution will require additional heat exchange surface area, which normally will increase pressure drop. However, a creative and thorough analysis may yield improved hydraulics as well as better heat recovery. Consider Figures 2 and 3, which depict a crude unit preheat train. The original design (Figure 2) has a higher overall pressure drop for the crude feed path than the revised design (Figure 3), even though the latter includes new exchanger shells in the path. The changes allowed the unit to increase maximum crude throughput by 10% without changes to major equipment; the capital required was paid back in less than three years based solely on the energy savings.
Finally, even in relatively small projects such as turnarounds, it can be worthwhile to conduct an engineering review of the process during the planning stage to search for ways to save energy. An energy loss analysis is one assessment that process engineers not trained in pinch analysis can perform relatively quickly. It simply involves cataloging the energy lost to either cooling utilities or to surroundings in the unit, and heuristically identifying easy ways to capture some of that heat. Typical energy loss points are air or water coolers, furnace stacks and even insulation.