So far we have only looked at single-unit processes. Here we might consider the system to be solely the process unit itself (we had no other choice).
A system is the region of a process on which we are currently performing a balance. You will typically think of a box surrounding this "system" and will perform the balance on the box.
In most real process, we have more than one process unit. In these cases, it will often be convenient (or even necessary) to define our system as something other than one single process unit. Look at the following diagram as an example:
Once we have chosen a sub-system, however, the problem becomes identical to a single-unit process (where our sub-system plays the role of the single unit!).
(New?) Method for solving multi-process units:
A multi-process system requires: that you be clever in choosing your system, that you solve equations in the "right" order, that you remember how to solve a system of algebraic equations.
Identify relevant sub-systems within a multi-unit process on which to perform a degree-of-freedom analysis (and subsequently do the required calculations)
Let's try one...