A unit is a physical grouping of equipment and the unit control functions required to execute a batch. A process unit is a group of mechanical equipment, with each piece performing, somewhat independently, a portion of the chemical process. Examples of process units are filters, batch reactors, heat exchangers, and distillation columns. Control consists of the process states, known as phases, required to perform the unit operations. Examples are charging, heating, cooling, agitation, reacting, discharging, and washing.
Unit management interfaces with the user, batch management, sequential control, regulatory control, and discrete control in performing its function of (1) communicating with other unit equipment modules, loops, devices, and elements; (2) acquiring resources; (3) executing phases; and (4) handling phase exceptions. (In a batch program, the basic action is called a statement; several statements make up a step; a number of steps comprise a phase; phases can be combined into an operation; and a sequence of operations makes up a batch.) Figure 3.13.10 shows the unit management model defined by the ISA.
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