Page 165 - Chemical engineering design

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CHEMICAL ENGINEERING
the flows independently of the equipment design considerations; other stream flows and
compositions will be dependent on the equipment design and performance. To draw up
the flow-sheet the designer must use his judgement in deciding which flows can be calcu-
lated directly; which are only weakly dependent on the equipment design; and which are
determined by the equipment design.
By weakly dependent is meant those streams associated with equipment whose perfor-
mance can be assumed, or approximated, without introducing significant errors in the
flow-sheet. The detailed design of these items can be carried out later, to match the
performance then specified by the flow-sheet. These will be items which in the designer’s
estimation do not introduce any serious cost penalty if not designed for their optimum
performance. For example, in a phase separator, such as a decanter, if equilibrium between
the phases is assumed the outlet stream compositions can be often calculated directly,
independent of the separator design. The separator would be designed later, to give suffi-
cient residence time for the streams to approach the equilibrium condition assumed in the
flow-sheet calculation.
Strong interaction will occur where the stream flows and compositions are princi-
pally determined by the equipment design and performance. For example, the optimum
conversion in a reactor system with recycle of the unreacted reagents will be determined
by the performance of the separation stage, and reactor material balance cannot be made
without considering the design of the separation equipment. To determine the stream
flows and compositions it would be necessary to set up a mathematical model of the
reactor-separator system, including costing.
To handle the manual calculations arising from complex processes, with strong inter-
actions between the material balance calculations and the equipment design, and where
physical recycle streams are present, it will be necessary to sub-divide the process into
manageable sub-systems. With judgement, the designer can isolate those systems with
strong interactions, or recycle, and calculate the flows sequentially, from sub-system
to sub-system, making approximations as and where required. Each sub-system can be
considered separately, if necessary, and the calculations repeatedly revised till a satis-
factory flow-sheet for the complete process is obtained. To attempt to model a complex
process without subdivision and approximation would involve too many variables and
design equations to be handled manually. Computer flow-sheeting programs should be
used if available.
When sub-dividing the process and approximating equipment performance to produce a
flow-sheet, the designer must appreciate that the resulting design for the complete process,
as defined by the flow-sheet, will be an approximation to the optimum design. He must
continually be aware of, and check, the effect of his approximations on the performance
of the complete process.

4.3.1. Basis for the flow-sheet calculations
Time basis

No plant will operate continuously without shut-down. Planned shut-down periods will be
necessary for maintenance, inspection, equipment cleaning, and the renewal of catalysts