New Metrics for Gas Separation

To measure progress towards sustainability, many efforts are being made to define indicators of the industrial process and, in particular their impact on three specific areas.Thus the following indicator were defined  

Most of these indicators are calculated in the form of appropriate ratios which can provide a measure of impact independent of the scale of the operation, or to weigh costs against benefits and, in some cases, they can allow the comparison between different operations. As indicated by Sikdar, to guarantee the sustainability of a process these three areas (environmental, economic and societal) should be inter-related and the metrics or indicators can be used for capturing the three aspects of sustainability. 

Two classes of metrics are under development to indicate the state and performance of a system. These metrics are more popularly known as indexes. Those indicating a system state are known as content indexes and those measuring the system behavior, performance indexes. The following discussion is focused on the performance ones only, as we are mostly focused on the means of improving the sustainability eharacteristics of a membrane system. 

Membrane operations are well known for their modularity, compactness and flexibility, therefore they can be eonsidered as new operations developed in the logie of proeess intensifieation. Reeently, new metrics for comparing membrane performanees with those of eonventional units have been introduced. These new metries take into aeeount the size, weight, flexibility, yield and modularity 

In order to compare different systems, energy and mass intensity were calculated as reported below, taking into account the power required by the system (eqn (19.1)), the H2 produced and its fraction recovered in the permeate (eqn (19.2)) as valuable product with respect to the total mass fed and the steam and cooling water necessary (eqn (19.3)). Moreover, another index was defined as productivity to footprint ratio (eqn (19.4)). Lower values of these indicators are related to an intensified process. In the ideal situation the mass intensity indicators would approach 1, whereas the energy intensity and the productivity to footprint ratio should be as low as possible  

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