Problems and Opportunities in Classification

Forging various relationships into orderly schemes of classification is an important part of science. The theoretical study of classification is itself a science and is termed taxonomy [2]. Its fruits pervade many fields. In chemistry, the classic example is the periodic table. While discovered empirically, it is based on the grouping of elements with a common number of outer-shell electrons. This electron count is so fundamental that many 

Clearly, a complete classification of separations would require an accounting based on similarities and differences in many properties. In taxonomy, this would be termed a polythetic classification, as opposed to monothetic classifications based on only one property. Monothetic classifications are not necessarily erroneous—they are simply incomplete. In a complex field such as separations, many monothetic classifications exist, each one different, each with an element of validity, but each incomplete. 

The focus on general taxonomy is made here to explain to the reader the reasons for the many different logical systems for classifying separations in the literature. The approaches range from the simple and traditional division into equilibrium and rate processes by Karger et al. [3] to the historically important classification of driving forces and resistive forces by Strain et al. [4]. We note also a contribution to classification by Rony [5] and an important study by Lightfoot and his co-workers [6]. Many other authors have discussed the matter [7-12]. 

It is not our object here to review the details of existing classificatory systems. Instead, we follow an alternate route to classification suggested by our emphasis on transport. 

We found earlier that a theoretical consideration of displacement and transport exerts a unifying influence on separation science, bringing diverse methods under a common descriptive umbrella. The theory leads in a natural way to the formation of categories of separations which can be considered the beginning of a fundamental classificatory system. Here we generalize the results of transport theory to develop a fundamental basis for classification. While the resulting scheme will not be a complete polythetic classification, it will be based upon some of the most fundamental features of the separation process. These basic features, incorporated in the classification, should correlate well with other properties of separations in the same way that the number of outer-shell electrons is directly related to the diverse properties of the elements of the periodic table. This transport-oriented 

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