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In ascientific sense, achemical process is a method or means of somehow changing one or morechemicals orchemical compounds.[1] Such a chemical process can occur by itself or be caused by an outside force, and involves achemical reaction of some sort. In an "engineering" sense, a chemical process is a method intended to be used inmanufacturing or on an industrial scale (seeIndustrial process) to change the composition of chemical(s) or material(s), usually using technology similar or related to that used inchemical plants or thechemical industry.
Neither of these definitions are exact in the sense that one can always tell definitively what is a chemical process and what is not; they are practical definitions. There is also significant overlap in these two definition variations. Because of the inexactness of the definition, chemists and other scientists use the term "chemical process" only in a general sense or in the engineering sense. However, in the "process (engineering)" sense, the term "chemical process" is used extensively. The rest of the article will cover the engineering type of chemical processes.
Although this type of chemical process may sometimes involve only one step, often multiple steps, referred to asunit operations, are involved. In aplant, each of the unit operations commonly occur in individual vessels or sections of the plant calledunits. Often, one or morechemical reactions are involved, but other ways of changing chemical (or material) composition may be used, such asmixing orseparation processes. The process steps may be sequential in time or sequential in space along a stream of flowing or moving material; seeChemical plant. For a given amount of a feed (input) material or product (output) material, an expected amount of material can be determined at key steps in the process from empirical data and material balance calculations. These amounts can be scaled up or down to suit the desired capacity or operation of a particular chemical plant built for such a process. More than one chemical plant may use the same chemical process, each plant perhaps at differently scaled capacities. Chemical processes likedistillation andcrystallization go back toalchemy inAlexandria,Egypt.
Such chemical processes can be illustrated generally asblock flow diagrams or in more detail asprocess flow diagrams. Block flow diagrams show the units as blocks and the streams flowing between them as connecting lines with arrowheads to show direction of flow.
In addition to chemical plants for producing chemicals, chemical processes with similar technology and equipment are also used inoil refining and otherrefineries,natural gas processing,polymer andpharmaceutical manufacturing,food processing, andwater andwastewater treatment.
Unit processing is the basic processing inchemical engineering. Together withunit operations it forms the main principle of the varied chemical industries. Each genre of unit processing follows the same chemical law much as each genre ofunit operations follows the same physical law.
Chemical engineering unit processing consists of the following important processes:
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