systems—both engineering processes and products—for minimum pollution. This represents one of the greatest challenges currently facing the engineering profession. The process has barely begun, and only faint outlines of how and where we are going are visible. As a starting point, we ought to identify those engineering design procedures or paradigms that can best be adapted to the design of clean technologies. In some cases, existing procedures suitably modified will be directly applicable. An example taken from chemical engineering is given below. In other cases, it will be necessary to invent new procedures. This may well be true of important classes of consumer product design.

In what follows, extensive use is made of chemical engineering design paradigms to follow both the conversion of raw materials to chemical products and the simultaneous generation of pollutants. The chemical engineering approach makes use of material balances, chemical kinetics, thermodynamics, and transport processes to track the conversion of a set of reactants (raw materials) into a set of products. The power of the approach makes it applicable not only to chemical plants and refineries but also to power plants, microelectronics processing, aerospace factories, and other industries that make extensive use of chemical processes. It can be applied at several different but interacting scales (Figure 1). An example at the largest scale is the work of Ayres (1989) on "industrial metabolism," which makes extensive use of material balances in tracking the large-scale flow of metallic elements through various industry sectors and into the environment.

Macroscale analyses (such as "industrial metabolism" or the closely related "industrial ecology") are useful in developing national and international strategies for reducing material losses to the environment and planning future technological development. On a mesoscale—the individual chemical plant or petroleum refinery—the approach is used to design plants for the conversion of raw materials, such as crude petroleum, or a particular chemical feedstock, such as propylene, to desired products. Finally, at the microscale this approach is employed in the design of individual chemical reactors, employing differential balances on elemental reactor volumes. The three scales of organization are closely linked as shown in Figure 1.

While instructive, rates of flow of chemical species through industry and environment tell only part of the story. Chemical compound form and the physical and chemical properties of the mixtures may strongly affect public health and ecology. In addition, concentrations change by orders of magnitude during processing and environmental transport and transformation. The concentration has a determining influence on the economics of recovery and reuse (see Allen and Behmanesh, in this volume).

The extension of the chemical engineering paradigm to the formation and control of undesired chemical species—pollutants—is in some cases straightforward at the meso (manufacturing plant) and micro (chemical processes) scales. However, as discussed later in this paper, the design skills required for predicting

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