Page 122 - Chemical engineering design

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S h5
S h3
S h1
S h4
E2
E6
E5
E4
E1 S h2 CHEMICAL ENGINEERING S h6 S
E3
C1
E6 E7
E8
S u1 S u2
S h1 D residue (360 ° C)
S h2 D reflux stream (260 ° C)
S h3 D heavy gas oil (340 ° C)
S h4 D light gas oil (260 ° C)
S h5 D reflux steam (180 ° C)
S h6 D reflux stream (165 ° C)
S c1 D crude oil (15 ° C)
S u1 and S u2 D cooling water (50 ° C)
Figure 3.11. Typical heat-exchanger network

particularly in respect of optimising heat recovery in crude petroleum distillation. An
example of crude preheat train is shown in Figure 3.11. The general problem of the
synthesis and optimisation of a network of heat exchangers has been defined by Masso
and Rudd (1969).
Consider that there are M hot streams, S hi i D 1, 2, 3,... ,M to be cooled and N cold
streams S cj j D 1, 2, 3,..., N to be heated; each stream having an inlet temperature t f ,
or an outlet temperature t 0 , and a stream heat capacity W i .There mayalsobe S uk k D
1, 2, 3,.. . , L auxiliary steam heated or water-cooled exchangers.
The problem is to create a minimum cost network of exchangers, that will also meet the
design specifications on the required outlet temperature t 0 of each stream. If the strictly
mathematical approach is taken of setting up all possible arrangements and searching for
the optimum, the problem, even for a small number of exchangers, would require an
inordinate amount of computer time. Boland and Linnhoff (1979) point out that for a
process with four cold and three hot streams, 2.4 ð 10 18 arrangements are possible. Most
workers have taken a more pragmatic, “heuristic”, approach to the problem, using “rules
of thumb” to generate a limited number of feasible networks, which are then evaluated.
Porton and Donaldson (1974) suggest a simple procedure that involves the repeated
matching of the hottest stream (highest t f ) against the cold stream with the highest
required outlet temperature (highest t 0 ).
A general survey of computer and manual methods for optimising exchanger networks
is given by Nishida et al. (1977); see also Siirola (1974).
The design of heat exchanger networks is covered in more detail is Section 3.17.

3.16.3. Waste-heat boilers
If the process streams are at a sufficiently high temperature the heat recovered can be
used to generate steam.