Page 21 - Design and Operation of Heat Exchangers and their Networks

8 Design and operation of heat exchangers and their networks

imposing a series of design targets arranged by their decreasing impact on the
HEN total annual cost. Targeting synthesis algorithms based on mathemat-
ical programming models decompose the search for the optimal HEN into
three major tasks. The first one involves the solution of a linear program-
ming (LP) problem to target the process utility requirement even if some
heat matches have been forbidden (Papoulias and Grossmann, 1983a; Cerda
et al., 1983). This is the unique kind of structural constraint that is considered
in the determination of the utility usage target. In the next task, a mixed-
integer linear programming (MILP) problem is solved to find the minimum
number of matches needed to achieve maximum heat recovery. In this way,
the HEN at the level of matches can be found (Papoulias and Grossmann,
1983b; Cerda and Westerberg, 1983). The simultaneous HENS methods are
aimed at finding the optimal HEN in a single step. These methods are no
longer based on the assumption that the total annual cost is dominated by
the utility requirements. All of them belong to the mathematical program-
ming area and use a mixed-integer nonlinear programming (MINLP) prob-
lem formulation to seek the HEN featuring the least total annual cost in one
calculation. Yee and Grossmann (1990) propose a MINLP mathematical
formulation where all the design decisions can be optimized simultaneously.
The model is based on a superstructure resulting from a stage-wise problem
representation where a match between any pair of hot and cold streams may
take place at every stage. On the other hand, when attention is paid to heat
exchanger type, much work during the last three decades is focused on con-
ventional two-stream heat exchanger networks. A few studies have been
done on mathematical models and their solution techniques for HENS with
multistream heat exchangers.
A synthesis model of HEN with multistream heat exchangers was estab-
lished by Wei (2003) based on the concept of stage-wise heat exchanger net-
work superstructure. In this model, the assumption of isothermal mixing
used in literature was abnegated, and the model does not rely on any heu-
ristics such as pinch technology, and no constraint for process stream heat
transfer film coefficients was needed. Meanwhile, utility cost, area of heat
exchangers, number of units, and stream split fractions can be optimized
simultaneously.

1.5 Dynamic behavior of heat exchangers and their
networks

The earlier reviews deal with steady-state behaviors of heat exchangers
and their networks. A steady-state procedure can reveal the energy