Page 154 - Chemical engineering design

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CHEMICAL ENGINEERING
4.2.1. Block diagrams
A block diagram is the simplest form of presentation. Each block can represent a single
piece of equipment or a complete stage in the process. Block diagrams were used to
illustrate the examples in Chapters 2 and 3. They are useful for showing simple processes.
With complex processes, their use is limited to showing the overall process, broken
down into its principal stages; as in Example 2.13 (Vinyl Chloride). In that example each
block represented the equipment for a complete reaction stage: the reactor, separators and
distillation columns.
Block diagrams are useful for representing a process in a simplified form in reports
and textbooks, but have only a limited use as engineering documents.
The stream flow-rates and compositions can be shown on the diagram adjacent to the
stream lines, when only a small amount of information is to be shown, or tabulated
separately.
The blocks can be of any shape, but it is usually convenient to use a mixture of squares
and circles, drawn with a template.

4.2.2. Pictorial representation

On the detailed flow-sheets used for design and operation, the equipment is normally
drawn in a stylised pictorial form. For tender documents or company brochures, actual
scale drawings of the equipment are sometimes used, but it is more usual to use
a simplified representation. The symbols given in British Standard, BS 1553 (1977)
“Graphical Symbols for General Engineering” Part 1, “Piping Systems and Plant” are
recommended; though most design offices use their own standard symbols. A selection of
symbols from BS 1553 is given in Appendix A. The American National Standards Institute
(ANSI) has also published a set of symbols for use on flow-sheets. Austin (1979) has
compared the British Standard, ANSI, and some proprietary flow-sheet symbols.
In Europe, the German standards organisation has published a set of guide rules and
symbols for flow-sheet presentation, DIN 28004 (1988). This is available in an English
translation from the British Standards Institution.

4.2.3. Presentation of stream flow-rates
The data on the flow-rate of each individual component, on the total stream flow-rate,
and the percentage composition, can be shown on the flow-sheet in various ways. The
simplest method, suitable for simple processes with few equipment pieces, is to tabulate
the data in blocks alongside the process stream lines, as shown in Figure 4.1. Only a
limited amount of information can be shown in this way, and it is difficult to make neat
alterations or to add additional data.
A better method for the presentation of data on flow-sheets is shown in Figure 4.2.
In this method each stream line is numbered and the data tabulated at the bottom of the
sheet. Alterations and additions can be easily made. This is the method generally used by
professional design offices. A typical commercial flow-sheet is shown in Figure 4.3. Guide
rules for the layout of this type of flow-sheet presentation are given in Section 4.2.5.