Significance of Chemical Engineering Thermodynamics: Process Plant Schema
Before we conclude the present chapter it would be appropriate to obtain a brief preview of the scope and utility of the principles of thermodynamics insofar as application to real world processes is concerned. Although based on relatively abstract principles, the laws of thermodynamics provide the fundamental constraints under which all real world process take place. The ultimate application of the knowledge of the core principles of chemical engineering is in the design of a chemical process plant. Engineering thermodynamics constitutes one of the principal elements of such knowledge. Typically such a plant converts a set of raw materials to a desired product through a variety of steps that are schematically represented by Fig. 1.6.
The raw materials most often are mixtures which need to be purified to obtain the right composition required for conversion to products A wide variety of separation processes are available for carrying out such purification; examples include distillation, liquid-liquid extraction, precipitation from solutions, crystallization, etc. Practically all such separation processes involve generation of two or more phases, in one of which the desired raw material components are preferentially concentrated, which is then used recover the substances in a relatively purer form. For a typical large scale chemical plant the separation process equipments may constitute more than half of the total capital investment.
The chemical reactor forms the “heart” of a chemical plant. It is here that once the feed materials are available in the right proportions (and compositions) they are reacted to yield the product. Obtaining the desired product requires an optimal choice of conditions under which the reactor may be operated. However, the product formed is very rarely obtained in a pure form. This is because typically the feed is never fully converted to product molecules and therefore the stream exiting the reactor is not a pure substance. In addition it is usually a common phenomenon that the intended chemical reaction is accompanied by often more than a single side reaction. The latter leads to the formation of side products, which results in “contamination” of the final product. Therefore, it is usually required to subject the reactor exit stream to another round of purification to obtain a product with the desired specifications of the product
With regards to all such processes of purification and reaction, the laws of thermodynamics play a very fundamental role: they allow the calculation of the principal entities that form the basis of design and operation of process plants:
1. The maximum degree of purification that is possible under a given set of processing conditions
2. The maximum degree of conversion possible under the reaction conditions
3. The optimal operating conditions for separation and reaction processes
4. The total energy required to achieve the intended degree of separation and reaction, and therefore the plant energy load.
The calculation of the above parameters tends to constitute 50-70% of the computational load encountered during the stage of basic process plant design. Thus, the principles of chemical engineering thermodynamics is one of the mainstays of knowledge needed to realize the goal of plant design and operation.