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FLOW-SHEETING
To illustrate the use of a commercial computer aided design program, Figure 4.2 has
been redrawn using the program FLOSHEET and is shown as Figure 4.2a. FLOSHEET is
a versatile flow-sheet drafting program. It is used by many chemical engineering depart-
ments in the UK; see Preece (1986) and Preece and Stephens (1989).
FLOSHEET is part of a suite of programs called PROCEDE which has been developed
for the efficient handling of all the information needed in process design. It aims to
cover the complete process environment, using graphical user interfaces to facilitate the
transfer of information, Preece et al. (1991). The equipment specification sheets given in
Appendix G are from the PROCEDE package.

4.3. MANUAL FLOW-SHEET CALCULATIONS

This section is a general discussion of the techniques used for the preparation of flow-
sheets from manual calculations. The stream flows and compositions are calculated from
material balances; combined with the design equations that arise from the process and
equipment design constraints.
As discussed in Chapter 1, there will be two kinds of design constraints:
External constraints: not directly under the control of the designer, and which cannot
normally be relaxed. Examples of this kind of constraint are:

(i) Product specifications, possibly set by customer requirements.
(ii) Major safety considerations, such as flammability limits.
(iii) Effluent specifications, set by government agencies.

Internal constraints: determined by the nature of the process and the equipment
functions. These would include:
(i) The process stoichiometry, reactor conversions and yields.
(ii) Chemical equilibria.
(iii) Physical equilibria, involved in liquid-liquid and gas/vapour-liquid separations.
(iv) Azeotropes and other fixed compositions.
(v) Energy-balance constraints. Where the energy and material balance interact, as for
example in flash distillation.
(vi) Any general limitations on equipment design.

The flow-sheet is usually drawn up at an early stage in the development of the project. A
preliminary flow-sheet will help clarify the designer’s concept of the process; and serve
as basis for discussions with other members of the design team.
The extent to which the flow-sheet can be drawn up before any work is done on
the detailed design of the equipment will depend on the complexity of the process and
the information available. If the design is largely a duplication of an existing process,
though possibly for a different capacity, the equipment performance will be known and
the stream flows and compositions can be readily calculated. For new processes, and for
major modifications of existing processes, it will only be possible to calculate some of