of process-elements or "microprocesses,,;18)
and
Cc) products, which
are grouped in accordance with the "Standard Industrial Classification" or SIC code. The basic line of thought in the original version of the MPP model is that all possible production processes could be described as sequences of basic process elements taking into consideration all constraints upon the sequences of microprocesses imposed by the states and properties of the materials used.
In a later version of the model, 19 however, a substantially different view has been adopted. ) In the first version, the "space of technological possibilities" was regarded .as a continuum and the observed process differentiation wa.s the resul t of more or less random explorations of this "space of possibilities" rather than the result of some evolutionary process.
This view implied
the assumption that macroprocess changes occurring as a result of
chan~s
in economic conditions would basically consist of rearranging sequences of unit operations and more efficient use of energy.
The later version
of the model, in contrast, takes the view that processes tend to evolve as distinguishable "species" and that each industrial process represents an optimal (or sub-optimal) choice from among a very large number of competing possibilities.
The way in which most macroprocess innova-
tions achieve their economic advantage is through the utilization of cheaper inputs.
Such input substitution is made possible, for example,
through the utilization of new microprocesses.
Once a new microprocess
(e.g. a new chemicalreaction path) has been specified, the process of optimization (or sub-optimization) will lead to the development of a relatively efficient combination of components and physical flow pat20) terns. b.
Environmental models
The production see tor models described above can, as pointed out, be 18) In chemical engineering terminology, microprocesses involving physical changes are called unit operations whereas microprocesses involving chemica1 changes are generally called unit processes. 19) See Ayres, R.U. & Kneese, A.V., Models of the Activity Analysis of Productian, Consumption, and Externalities. Unpublished mimeograph. Resources for the Future, Inc., Washington, D.C. 1973. 20) An application of this model can be found in: Ayres, R., Saxton,J. & Stern, M., M책terials-Process-Product Model. A Feasibility Demonstration Based on the Bottle Manufacturing Industry. International Research Corporation. Arlington, Va. 1974. 41