(525d) Assessment of Sustainability-Potential: Hierarchical Approach

Our capability to conceive or identify alternative synthetic routes, i.e., reaction paths, to manufacture a single or set of chemical compounds from various precursors is being enhanced acceleratedly. This has been brought about by the availability of ever expanding chemical databases and the advent of efficient modern experimental techniques and computational methods. In the current intensely competitive economic environment, we need to avail ourselves of the mechanisms or tools to speedily weed out those alternative synthetic routes that are estimated to generate unequivocally unprofitable processes prior to initiating costly developmental efforts. The gross-profit or profit-potential estimation is one of such tools; it evaluates the potential profit from each synthetic route as the difference between the prices of the final reaction products and starting reactants for manufacturing a unit quantity of the desired product. Nevertheless, it is totally untenable to scale?up a process even if the profit-potential is estimated to be exceedingly large when its operation is unsustainable.

The sustainability of a chemical process has been variously defined. In general, however, its assessment entails the consideration of such factors as energy and material requirements; safety and health effects; ecological and environmental impacts; and societal and regulatory constraints. The energy and material requirements will be reflected on the thermodynamic performance, particularly in terms of the dissipation of available energy, i.e., exergies, of reacting species and the cost of or profit from the process both of which are quantifiable. Substantial progress has been made to quantify the safety and health effects in terms of the toxicity indices of the materials to be involved in the process, even though the values of these indices are more nebulous than those of exergies and costs. This is also the case for the ecological and environmental impacts; fortunately, the process' thermodynamic performance in terms of the exergy dissipation due to the reaction is one of the effective indicators of the thermal and material dissipations due to the reaction involved. The societal and regulatory constraints are least amenable to quantification; nevertheless, much effort is being spent to quantify them by social and political scientists and economists.

A hierarchical approach is proposed here to assess the sustainability of a chemical process based on any of the alternative synthetic routes. This novel notion of sustainability-potential is a generalization of the notion of profit-potential. Analogous to the profit-potential, the sustainability-potential is estimated from the reaction products and starting reactants of the synthetic route. Alternative synthetic routes yielding a reaction product or a set of reaction products, however, tend to form a complex reaction network: These synthetic routes often share some common starting reactants and intermediates. It is indeed daunting to completely recover all the individual feasible synthetic routes from the network. This difficulty can be overcome by identifying them by resorting to a highly efficient algorithmic method for network synthesis based on process graphs (P-graphs) at the outset of assessment.

The individual feasible synthetic routes yielding the desired product are hierarchically or sequentially assessed in descending order of quantifiability. In the first step, the synthetic routes are assessed in terms of the exergy dissipation, subject to a criterion specified on the basis of the extent of dissipation. In the second step, those synthetic routes surviving the scrutiny of the first step are assessed in terms of the profit-potential, subject to a criterion specified on the basis of the size of profit. In the third step, those synthetic routes surviving the scrutiny of the first two steps are assessed in terms of the toxicity indices, subject to a criterion specified on the basis of the magnitude of toxicity indices. Naturally, the assessment can proceed further as the ecological and environmental impacts and/or the societal and regulatory constraints become sufficiently quantifiable. It is worth noting that the first step of assessment in light of exergy dissipation should be of the most fundamental and practical significance; the second step, the next most; and so on. The efficacy of the proposed hierarchical approach is demonstrated with an example. An effort is being made to render it possible to execute hierarchical assessment of sustainability-potential online.