The quality of chemical operations

In the preceeding chapters the emphasis was on the rates of chemical processes, and particularly on the influence of various physical factors on process rates. These rates are not only important in relation to the dimensions of the equipment, but they also determine the selectivity of the chemical process. Aiming for higher selectivities is desirable for three reasons to minimize raw materials consumption, to improve product quality and to reduce chances of pollution of the environment. There are therefore important similarities between requirements for the quality of products of the chemical industry and for the quality of waste streams. In addition to the quantitative methods that were demonstrated above, a qualitative engineering approach is also valuable. This is not always reconcilable with quantitative economic calculations. It is necessary to consider the quality of chemical operations in general, and of chemical reactors in particular. This is also a part of chemical reactor developement. 

In the preceeding chapters various aspects of chemical reactor development were presented. It was silently assumed that we know what product we wish to make in a chemical reactor. When the product is an essentially single chemical compound, it can be well defined. However, many products are not completely pure, or are deliberately manufactured as mixtures. In general, a product from a chemical process is characterized by several parameters, regarding purity and performance characteristics. When the product is a solid, its performance is also related to its shape and structure. TTie total of these characteristics define the quality of the product. There is a general tendency in the chemical and related industries to raise the quality requirements of manufactured products. In almost every chemical process byproducts are formed. These may end up as waste products, that have to be disposed of, or they may be present as traces in waste waste water or in off-gases, that are released into the environment. The composition of these have to meet requirements that have become more stringent. 

At first sight, the problems of product quality and environmental protection appear to be quite unrelated. However, there are remarkable similarities. In the first place, the objectives are similar all materials that leave a chemical plant have to meet specific quality requirements, be it valuable products or waste streams. Secondly, the methods for meeting quality requirements are similar, regardless of the naure of the products. These methods comprise  

The quality of the products of the chemical industry, and the quality of the waste streams leaving a chemical plant, are both relat to the quality of chemical operations. 

Traditionally, the attention of chemical engineers has been focused mostly on the optimization of chemical operations with respect to production costs. The product quality could often be expressed in terms of purity. A higher quality simply required further purification, so a cost-benefit relation was a matter of straightforward engineering calculations. In fact, the same applied to the required purity of waste streams. Environmental regulations were seen primarily as boundary conditions for plant design. 

---