Volume 24, number 2, september 2010

Energy integration in processes: A powerful tool for cost-effective projects

In the industrial sector, huge quantities of energy are lost in several forms. Of course, heat recovery is an efficient method, but it can turn out to be complex and costly. Energy integration in processes is an approach that enables a company to improve its performance in terms of water, energy and/or raw material consumption while reducing GHG emissions. On average, the method can generate 25% in energy savings.

In an integration project, the process is analyzed as a whole and the interaction between its various components is examined. The integration can study areas as varied as steam production, refrigeration, and air compression. Depending on the industrial activity sector, the potential energy savings can vary between 10% and 40%. The payback period is usually between 6 months and 3 years.1

Generally, an integration study includes seven main steps:

  1. Simplified diagram showing the energy flows through the process
  2. Mass and energy balance
  3. Detailed energy balance of certain equipment
  4. Analysis of possibilities for modifying operating conditions
  5. Pinch analysis
      – Identify heat exchange points and extract data
      – Trace composite curves (CC) and large composite
      – Determine principal sources of energy inefficiency
      – Determine which exchanges are significant sources of inefficiency
      – Identify project options
  6. Conduct a technical-economic-feasibility study
  7. Select development projects of an implementation strategy

The first three steps involve identifying and diagramming energy flows, fluid flow, temperatures, and the quantity of energy exchanged at various stages and for the various equipment used in the process. Throughout the analysis, modifications to operating conditions (Step 4) need to be integrated into the mass and energy balances in order to determine the real potential for energy savings. Step 5, pinch analysis, is no doubt the most complex and it is central to process integration. Pinch analysis helps determine if the energy consumption of the process is optimal.

The analysis considers all thermal exchanges that may take place in a process in order to later verify if they are optimal. When analyzing heat exchanges, those made by conventional heat exchangers are considered, along with those made by evaporators, condensers, dryers, ovens, the injection of steam into a tank, etc.

Once these exchange points have been identified, their quality is checked and they are improved, if necessary. The goal of any improvement is not only to ensure the best possible exchange, but also to determine the minimal amount of energy the process needs.  In other words, once the minimal energy needs of an operation have been determined, any surplus will be the potential savings. The composite curves are based on these data.

The composite curves are a graphical representation of the total heating and cooling requirements of a process on a diagram showing temperature (T) as a function of heat flow (H). The curves are constructed by representing all the processes’ sources of, and demands for heating and cooling on a graph.

The famous pinch point, which shows where heat exchanges are most critical, can be seen in Graph 1. This value (Delta T minimum — ∆Tmin) is chosen during the course of the analysis, and it depends on the physical limits of the system as well as on the experience of the person doing the analysis. The graph also shows the minimal values of the heating (Q h min) and cooling (Q c min) sources for a given ∆Tmin and the maximal heat recovery.


Graph 1

The sources of energy inefficiency are identified and improvement projects proposed based on the pinch analysis. Originally, constructing this curve was a long and arduous task. Today, software designed by Natural Resources Canada (NRCan) does the job. The software is available to those who take the energy integration of industrial processes training (process integration) offered by Canmet. The last two steps in the integration are to conduct a technical-economic feasibility study based on the data from the pinch analysis and to select the most cost-effective projects.

To better understand the impact of this methodology, the following table shows the initial situation and the results of a project done recently.1

Customer Large distillery
Annual natural gas consumption ($) $5,000,000
Equipment Autoclave, distilling columns, evaporator, dryer, ≈ 35 heat exchangers 
Problem Several inconclusive energy audits
Conclusion with PI 9 projects proposed with potential of reducing energy consumption by 40%, or $2,000,000 per year.

Daniel Gendron, Eng.

1. Information and data taken from the training manual of the Energy Integration of Industrial Processes course offered by the Agence de l’efficacité énergétique in collaboration with CanmetENERGY (NRCan).