We talk a lot in these columns about ways to improve energy efficiency in existing chemical processes. However, we haven’t spoken much about how to identify energy-saving opportunities for specific plants. Each process is to some degree unique, so it is not always possible simply to replicate ideas. Instead, you must evaluate and understand each plant’s needs in order to identify inefficiencies and develop improvement options.
One of the most effective methods for identifying improvement opportunities is a process flow diagram (PFD) review. This is essentially a “structured brainstorming” activity. The procedure resembles that customarily used for HAZOP studies. It starts with a marked-up PFD for the process unit (e.g., hydrotreater, crude unit, etc.), showing the major equipment items and their interconnections, together with the heat and material balance. The inlets and outlets of each major piece of equipment should include temperatures, flow rates and pressures, together with energy flows, such as the thermal duty for each fired heater, heat exchanger or cooler, as well as the power requirements for pumps and compressors. In addition, the PFD should indicate any place where steam is used for heating or stripping, or where steam is generated, with flow rates labeled.
With the marked-up PFD, operations and technical support personnel from the operating site, with assistance from visiting energy efficiency specialists, review each of the main streams, equipment items and systems, to identify inefficiencies and areas of opportunity. The plant operations and technical staff bring their knowledge of day-to-day plant issues to the table. The visiting specialists bring their experience of similar processes at different locations, and the types of energy efficiency opportunities that have worked elsewhere. Together, they brainstorm ideas for the process unit under consideration.
Typically, a PFD review will generate a large number of ideas. These can range from adjusting set points and operating targets, through new control schemes, minor piping changes and equipment modifications, to completely new processes and novel technologies. During the PFD review, document these opportunities, and then later evaluate them more thoroughly to quantify the potential savings, estimate the implementation costs, and identify technical risks. I’ll discuss this evaluation process in a future column.
PFD reviews can serve as a stand-alone technique for identifying and organizing opportunities for improving energy efficiency on virtually any type of process plant. However, most often they form part of a larger energy efficiency initiative, such as an overall site energy assessment. They also often are used in conjunction with a pinch analysis to explore a wide range of energy efficiency options for a process or production site.
PFD reviews also provide an opportunity for site personnel to showcase their ideas. An example from a chemical plant illustrates this. The plant’s control engineer explained a new control algorithm he had written to optimize the operation of a large compressor. The new application had been ready for several months, but it had not been turned on because the operations department had concerns about how it might impact the stability of the plant.
The operations supervisor was in the meeting, and initially he expressed very strong objections to any changes to the existing control scheme. However, the control engineer demonstrated that the energy savings with the new operating mode were far greater than the operations supervisor had realized. Further, the visiting energy management specialist endorsed the new control scheme based on experience at other facilities. An animated discussion followed on strategies for testing the new algorithm and steps needed to safeguard plant operations. By the end of the meeting the operations supervisor was a man with a mission. He was committed not only to testing the new control scheme, but also to making it work.
This incident is by no means an isolated case, and highlights a key fact: Successful energy management is not only about good technological solutions. It is also about human behavior, engaging people in the pursuit of energy efficiency, and motivating them to succeed at it.
For further details and examples, see: Alan P. Rossiter & Beth P. Jones, Energy Management and Efficiency for the Process Industries, Wiley-AIChE, 2015, pp. 313-325.
