Materials Balance Models

This chapter presents an overview of the mass balance principle and its applications. It is an important tool for quantifying wastes which are produced by economic processes. These wastes are equal in mass to the difference between total raw material inputs to the process and useful material outputs. Products are becoming more complex which results in an increase of input mass and wastes. It is safe to say that nowadays process wastes far exceed the mass of materials that are finally embodied in useful products.
The application of the mass balance principle can take many shapes and forms, and this chapter illustrates a few. Using mass balance and chemical engineering knowledge of processes, we found that on a yearly basis, the inorganic chemical industry has a yield of 91% (9% of the inputs end up in waste), and the organic
chemical industry has a yield of 40%. A second example is the rare earth metal industry, where potential recovery of these scarce metals is quantified to motivate reuse and recycling. Presently less than 1% of rare earth metals are recovered from end-of-life products, but as the demand for these resources increases in the
near future for products such as electric motors and wind power turbines, recovery will become necessary.
An introduction to thermodynamics and exergy is included, since all wastes are thermodynamically degraded as compared to raw materials. The exergy of the inputs, products, and wastes is an important factor to consider for process efficiency and environmental evaluation.