Diagrams for Understanding Chemical Processes
The chemical process industry (CPI) is involved in the production of a wide variety of products, including polymers, that improve the quality of our lives and generate income for the companies and their stockholders.
In general, chemical processes are complex, and chemical engineers in industry encounter a variety of chemical process flow diagrams. These processes often involve substances of high chemical reactivity, high toxicity, and high corrosivity operating at high pressures and temperatures. These characteristics can lead to a variety of potentially serious consequences, including explosions, environmental damage, and threats to people’s health. It is essential that errors or omissions resulting from missed communication between persons and/or groups involved in the design and operation do not occur when dealing with chemical processes. Visual information is the clearest way to present material and is least likely to be misinterpreted. For these reasons, it is essential that chemical engineers be able to formulate appropriate process diagrams and be skilled in analyzing and interpreting diagrams prepared by others.
The most effective way of communicating information about a process is through the use of flow diagrams.
We concentrate on three diagrams that are important to chemical engineers: block flow, process flow, and piping and instrumentation diagrams. Of these three diagrams, we will find that the most useful to chemical engineers is the PFD.
BLOCK FLOW DIAGRAMS (BFDs)
This diagram is a series of blocks connected with input and output flow streams. It includes operating conditions (temperature and pressure) and other important information such as conversion and recovery, given in the problem statement. It does not provide details regarding what was involved within the blocks, but concentrated on the main flow of streams through the process.
The block flow diagram can take one of two forms. First, a block flow diagram may be drawn for a single process. Alternatively, a block flow diagram may be drawn for a complete chemical complex involving many different chemical processes.
An example of a block flow process diagram is shown in Figure, and the process illustrated is described below.
Reaction:C 7H8 + H2 -> C6H6 + C H4
Toluene and hydrogen are converted in a reactor to produce benzene and methane. The reaction does not go to completion, and excess toluene is required. The noncondensable gases are separated and discharged. The benzene product and the unreacted toluene are then separated by distillation. The toluene is then recycled back to the reactor and the benzene removed in the product stream.
Such a diagram is very useful for “getting a feel” for the process. Block flow process diagrams often form the starting point for developing a PFD. They are also very helpful in conceptualizing new processes and explaining the main features of the process without getting bogged down in the details.
