Unit Operations and Flowsheets

Every chemical process is a collection of units interconnected by streams. This is the concept of unit operations, the first, and most venerable, paradigm adopted by the fledgling discipline of chemical engineering near the beginning of the twentieth century. 

The five most common units are reactors, heat exchangers, pumps, mixers, and separators. The design and operation of these five units, which exist in hundreds of incarnations, are the focus of the traditional chemical engineering curriculum. Furthermore, 

As we saw in the ammonia process, the reactor is usually the key unit. The reactor will often dictate whether a chemical process is possible. Separators are secondary only to the reactor in most processes the performance of the separator(s) will often determine if a process is profitable. Examples of separators include dryers, filters, absorbers, adsorbers, and centrifuges. The method used to separate a mixture - distillation, condensation, or filtering - is usually indicated by a specific symbol on the flowsheet. We will introduce these specific units later, as needed. 

Additional units could be added to the ammonia process. Reactants should be mixed before entering a reactor, not just combined. Simply combining flour, eggs, water, and baking soda is not enough. The reactants must be mixed to bake a cake. The simplest mixer combines two or more streams into one stream  

Mixers are important for solids and liquids. Gases, such as N2 and H2, tend to mix on their own. 

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