(67b) Advances in Process Synthesis - Energy & Environment

The origins of much of the research into process synthesis in the late 1970's and early 1980's were rooted in energy conservation, and in particular the design of heat exchanger networks. Today, the activity in process synthesis covers a much wider area than energy conservation. However, significantly increasing energy prices and new pressures to deal with environmental problems, such as the emission of greenhouse gases, has led to a resurgence of interest in the application of process syntheses for energy and environment.

During the 1980's, tools for process synthesis based on thermodynamic and graphical methods were developed. This process synthesis technology for energy efficiency has seen widespread application throughout the world. There are now many thousands of successful applications in a wide range of industries from petroleum, petrochemical, chemical, fine chemical, through to steel manufacture, food and drink, pulp and paper. In parallel with the thermodynamic and graphical approaches that were developed, alternative approaches based on the optimisation of superstructures were also developed. In this approach a superstructure is first created that has embedded within it all feasible operations and all feasible interconnections that are candidates for an optimal design. The problem is formulated mathematically and the superstructure optimised in a structural and parameter optimisation. The optimisation then removes redundant features, reducing the complexity of the design and setting operating conditions. Thus, the decision making aspect of design are carried out by automated procedures in software. Commercial software is now available that follow this approach.

Process synthesis techniques for heat exchanger network retrofit have also improved significantly. This problem was difficult to address using the thermodynamic methods developed in the 1980's, since when they were applied to retrofit, they effectively considered the problem as a pseudo new design. The more recent methods of heat exchanger network retrofit start from the existing network and gradually evolve towards a new design in steps that allow the process designer to maintain control over the complexity of the project. Advances in heat transfer enhancement in heat exchanger technology also allow simpler retrofit designs to be developed by minimising the number of structural modifications necessary to the retrofit of heat exchanger network.

To complement these network design tools, new methods for utility system design and optimisation have been developed. These allow a more complete understanding of the interactions between the processes and the utility system on a site, but also allow a better understanding of the interactions between different processes through the utility system.

Even though these tools are well established, with commercial software available, the understanding of such tools is still patchy around the process engineering community as a whole. Whilst simulation has moved out of the realm of specialists in the last 20 years, process synthesis is still largely carried out by specialists.

The reduction of flue gas emissions as a result of combustion of fuel to produce heat and power can be brought about by:

? Changing fuels

? Changes to utility systems

? Improved heat recovery

? Process changes

? Waste heat export

? CO2 capture and sequestration

? Carbon trading

Process synthesis techniques have significant role to play in developing an effective strategy for minimisation of emissions that might call for an integrated combination of these actions to achieve environmental targets at minimum cost. Finally, process synthesis has also had a significant role to play through the minimisation of water use. Techniques for minimisation of water use have been developed, but are as yet not widely practised, due to the low cost of water, despite water shortages being problematic in certain parts of the world.