The purpose of this section is to provide a technical basis and rationale for examining and understanding a process flow diagram. In this section we show that all processes have a similar input/output structure whereby raw materials enter a process and are reacted to form products and byproducts. These products are separated from unreacted feed, which is usually recycled. The streams are then purified to yield products acceptable to the market place. All equipment in a process can be categorized into one of the six elements of the Generic Block Flow Process Diagram.

      In addition, methods for tracing chemical species through a process flow diagram are given. By following the paths of feed chemicals and reactants we obtain a much clearer picture of what is happening in the process and of the interaction between the equipment. Furthermore, the conditions at which different equipment operate are discussed and explained. The concept of conditions of special concern are explained and examples of such conditions are identified and explained in the context of the Toluene HDA process. Finally, the role that experienced-based rules-of-thumb play in the checking of design calculations is discussed. A comprehensive set of tables covering process engineering rules-of-thumb is presented.

      The basic input/output structure of a chemical process is presented, and it is shown that all chemical processes can be described in terms of a Generic Block Flow Process Diagram, which has six basic elements. These six elements are Reactor Feed Preparation, Reactor, Separator Feed Preparation, Separator, Recycle, and Environmental Control.

      In order to gain a better understanding of a PFD, it is often necessary to follow the flow of key chemical components through the diagram. This chapter presents two different methods to accomplish this. The tracing of chemicals through the process reinforces our understanding of the role that each piece of equipment plays. In most cases, the major chemical species can be followed throughout the flow diagram using simple logic without referring to the flow summary table.

      Once the connectivity or topology of the PFD has been understood it is necessary to understand why a piece of equipment is operated at a given pressure and temperature. The idea of conditions of special concern are introduced. These conditions are either expensive to implement (due to special materials of construction and/or the use of thick walled vessels) or use expensive utilities. The reasons for using these conditions are introduced and explained.

      When the design of a process is reviewed, it is prudent to compare the design with well established rules-of-thumb. Although the design will most often differ from the results of these short-cut methods, it does allow large differences and potential errors to be identified quickly. This, in turn, allows engineers to check their work as the design progresses.