The increased toxicity associated with waste solvents can hardly be overstated. Conventional waste disposal techniques such as incineration, onsite and offsite disposal, tend to increase the overall carbon and ecological footprints. Solvent recovery methods present a better alternative to improve the circularity of industrial processes.

Emergy is one way to quantify the ‘energy sustainability’ of a process. It is defined as the amount of available energy of one kind that is directly and indirectly used in transformations to make a product. Different forms of energy flows need to be converted into solar energy equivalent, termed as solar emjoules (seJ) for Emergy calculations. Another metric that presents a good basis for ‘environmental sustainability’ is the Sustainable Process Index (SPI), which estimates the total area needed by a process to produce a unit amount of product. This is based on the idea that solar energy is the real long-term input with its utilization bound to the limited surface area of earth. Thus, SPI represents the area needed to provide one unit of product, while Emergy accounts for the work done by nature and human economy in the production process. Therefore, it is essential to integrate both SPI and Emergy metrics to offer a holistic view and predict the overall circularity of a given process.

To this end, we have developed a superstructure-based solvent recovery framework that considers multiple separations and purification technologies simultaneously. We integrated mathematical models that consider SPI, Emergy, and Economics of the recovery pathways. We then formulated a multi-objective optimization problem that seeks to minimize all three metrics simultaneously. Our results indicate that solvent recovery presents a good alternative to conventional waste solvent disposal techniques. Additionally, the integrated SPI-Emergy analysis offers an approach to quantify circularity in terms of the overall pollution absorbing, processing, and regeneration capacity of the Earth as well as its limitations.

C. Stewart Slater

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