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METHODS FOR EVALUATING THE SUSTAINABILITY OF GREEN PROCESSES
Citation:
Smith*, R L. AND M A. Gonzalez*. METHODS FOR EVALUATING THE SUSTAINABILITY OF GREEN PROCESSES. Presented at European Symposium on Computer Aided Process Engineering, Lisbon, PORTUGAL, May 16 - 19, 2003.
Impact/Purpose:
To inform the public
Description:
Methods for Evaluating the Sustainability of Green Processes
By Raymond L. Smith and Michael A. Gonzalez
U.S. Environmental Protection Agency
Office of Research and Development
26 W. Martin Luther King Dr.
Cincinnati, OH 45268 USA
Theme: New Challenges in CAPE (Sustainability and Environment)
Keywords: Sustainability, Green Chemistry, Green Engineering, Metrics, Pollution Prevention, GREENSCOPE
A considerable amount of research is being performed under the banners of "sustainable" and/or "green." The development of chemistries and technologies under these banners needs to be analyzed to support these claims and to guide future research. A methodology, called GREENSCOPE (Gauging Reaction Effectiveness for the ENvironmental Sustainability of Chemistries with a multi-Objective Process Evaluator), has been developed in the U.S. EPA's Office of Research and Development to directly compare the sustainability of processes that employ various chemistries or technologies. Evaluations using the method answer two questions: is an alternative green (i.e., lower environmental and energy burdens) and is it sustainable? For evaluating sustainability, methods are being developed in four areas, called the four E's: Efficiency, Environment, Energy and Economics. A process that is better in these four areas will most likely be sustainable, although one can expect that most process evaluations will result in tradeoffs.
The calculation of efficiencies for chemical reactions provides chemists with a measure of how green their reactions are (Constable et al., 2002). This is reflected in values such as conversion and selectivity, which define yields, reactor effluent product distributions, and recycle flows to make a desired amount of product. Another measure of how green a reaction is can be obtained from the atom economy (i.e., how many carbons from the feed end up in the product). These measures, which are well known in green chemistry, are related to environmental impacts, as the product distribution defines what chemicals and amounts may leave a process. These efficiencies can be considered an initial evaluation of a process, and they represent a bridge between the lab-scale experiments of a chemist and further engineering calculations.
To evaluate the environmental aspects of alternative chemistries or technologies GREENSCOPE employs the Waste Reduction (WAR) algorithm (Young and Cabezas, 1999). The WAR algorithm is used to determine the potential environmental impacts of releases from a process in eight impact categories: human toxicity by ingestion and dermal/inhalation routes, aquatic toxicity, terrestrial toxicity, acidification, photochemical oxidation, global warming and ozone depletion. While these potential impacts are defined as mid-point indicators (as opposed to end-point indicators which could specify the physical effects of acidification, for example), the measures for the various categories are well defined, which is a substantial improvement over arbitrary environmental or mass-based scores.
Energy is a basic component of chemical processes. Its use depletes resources and creates potential environmental impacts. Likewise, a less efficient process can be expected to use more energy, and so one can see that these measures of sustainability are interconnected. By evaluating energy balances for alternative processes another measure of sustainability is obtained.
Finally, the economics of alternative processes are measured according to their costs. For economists this is an oversimplified view of markets, but for our engineering calculations the annualized costs are significant measures. Of course, these costs are tied into the process through efficiencies, energy and environmental impacts. Without a positive economic performance no process is sustainable.
Example calculations have been performed on an alternative reactor system for the oxidation of cyclohexane (catalysis system described in Becker and Gonzalez, 2003). Since experiments on this reactor system have not been performed yet, this analysis represents a "what-if" evaluation. The results provide an indication of how beneficial this new reactor technology could be, i.e., what are the incentives for further research in this (or in a general sense, any) area. From the results, one can gain an increased understanding of potential increases in greenness and sustainability that a technology could possess. Once one knows what success would mean, the decision on pursuing research can be made on a much firmer basis.
Becker, T.M. and Gonzalez, M.A. (2003) "Homogeneous Catalytic Air Oxidation of Cyclohexane; Catalyst Design and An Overall Evaluation of Reaction Parameters", Ind. Eng. Chem. Res., submitted.
Constable, D.J.C., Curzons, A.D. and Cunningham, V.L. (2002) "Metrics to 'Green' Chemistry - Which are the Best?" Green Chemistry 4:521-527.
Young, D.M. and Cabezas, H. (1999) "Designing Sustainable Processes with Simulation: The Waste Reduction (WAR) Algorithm" Comput. Chem. Engng. 23:1477-1491.