Balances on Multiple-Unit Processes

In the preceding sections, we referred rather loosely to the system, as in the statement At steady state the rate at which benzene enters the system equals the rate at which it leaves. Not much was said about what the system was. Up to now, however, not much needed to be said, since we have considered only processes involving a single unit—a mixer, or a distillation column, or a reactor—and that unit necessarily constituted the system. 

Industrial chemical processes rarely involve just one process unit. One or more chemical reactors are often present, as are units for mixing reactants, blending products, heating and cooling process streams, separating products from each other and from unconsumed reactants, and removing potentially hazardous pollutants from streams prior to discharging the streams to the plant environment. Before we analyze such processes, we must take a closer look at what we mean by a system. 

In general terms, a system is any portion of a process that can be enclosed within a hypothetical box (boundary). It may be the entire process, an interconnected combination of some of the process units, a single unit, or a point at which two or more process streams come together or one stream splits into branches. The inputs and outputs to a system are the process streams that intersect the system boundary. 

Boundary encloses the entire process the system defined by this boundary has as inputs Feed Streams 1,2, and 3 and Product Streams 1,2, and 3. (Convince yourself.) Balances on this system are referred to as overall balances. The stream that connects Units 1 and 2 is internal to this system and so would not enter into overall system balances. 

Boundary encloses a feed stream mixing point. Feed Streams 1 and 2 are inputs to this system and the stream flowing to Unit 1 is an output. Boundary encloses Unit 1 (one input stream and two output streams). Boundary encloses a stream splitting point (one input stream and two output streams), and Boundary encloses Unit 2 (two input streams and one output stream). 

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