Page 98 - Chemical process engineering design and economics

Process Circuit Analysis

We can view any process as a circuit analagous an electrical circuit. Instead of
voltage differences between points in the circuit, there are pressure differences.
Instead of current flow, there is mass flow. Before a process can be completely
designed, all the mass flow rates, compositions, temperatures, pressures and en-
ergy requirements in all parts of the process must be known. Process engineers
usually specify pressure drops and temperatures from experience. They calculate
mass flow rates, which are traditionally treated in a course in mass and energy
balances. However, mass and energy balances are only a partial set of equations
that process engineers can write when analyzing a process circuit.
The objective of process circuit analysis is to determine specifications for
the process. These include temperatures, pressures, composition, and flow rates of
all streams. Also included is the energy transferred and the degree of separation or
reaction required of heat exchangers, reactors, and separators. After specifying
recoveries and conversions of components, the process engineer can calculate the
mass and energy requirements for a process. The process engineer will generate
specifications for all process units, which must be fulfilled by equipment design
experts. In a sense, process engineers are conductors, controlling the design of the
process. It is their responsibility to see that all the pieces fit.

STRATEGY OF PROBLEM SOLVING
Before proceeding, we will examine the structure of problem solving by consider-
ing the following procedure:

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