This article deals with the design of energy efficient water utilization systems allowing operation split. Practical features such as operating flexibility and capital cost have made the number of sub operations an important parameter of the problem. By treating the direct and indirect heat transfers separately, target freshwater and energy consumption as well as the operation split conditions are first obtained. Subsequently, a mixed integer non-linear programming (MINLP) model is established for the design of water network and the heat exchanger network (HEN). The proposed systematic approach is limited to a single contaminant. Example from literature is used to illustrate the applicability of the approach.
Energy and water are important resources to
process industries. Since the modem society faces
energy shortage and water scarcity problems, considerable
attention has been paid to reduce energy and
water consumption.
For water using operations such as absorption,
extraction, and washing, it is required to operate at
particular temperature levels. The water streams need
to be heated up or cooled down to satisfy the temperature
requirements of the operations. Under such
circumstances, energy consumption should be considered
in the design of water utilization systems.
During the past decade, considerable design
techniques have been developed to minimize the energy
and water consumption simultaneously. Conceptual
design tools such as two dimensional grid diagram
[ 1-31, separate system [ 1-31, source-demand energy
composite curve [4], and graphical thermodynamic
rule [5] have been introduced to design the water
network and the corresponding heat exchanger
network (HEN). Mathematical programming techniques
have also been used for the problem. Bagajewicz
et al. [6] proposed two sequential linear programming
(LP) problems to determine the minimum
water usage and energy consumption targets. Once
these targets are identified, a mixed integer linear programming
(MILP) model is generated to obtain the
detailed network. Zheng 171 considered the
multi-contaminant situation and obtained different
water networks under different economic targets.
Other researchers [8-11] treated the water network and
the HEN sequentially. Since the water network obtained
in the first step was not unique, the resulting
HEN may not be optimal. Du et al. 1121 introduced
genetic algorithm and simulated annealing algorithm
to solve the problem simultaneously. Leewongtanawit
et al. [13, 141 developed a mixed integer non-linear
programming (MINLP) model that relies on the combining
of water superstructure and HEN superstructure
to provide an overall network design. Recently, Liao
et af. [15] introduced a modified transshipment model
that treats direct and indirect heat transfers separately.
Kuo and Smith [16] first addressed that for particular
processes such as extracting and washing, the
operations are allowed to split, as seen in Fig. 1. They
applied these split options to further reduce the freshwater
usage. Later, Xu et al. [I71 developed a mathematical
programming model, which allows operation
split for the water network with regeneration reuse.
When the energy aspect is taken into account, the split
of operations may further reduce the energy consumption.
Consider now a two operation example of Table
1 to see how the operating cost can be decreased by
operation split. Given that the water and energy cost
are specified at 1.5 RMB Yuan.t-' and 38 YuamGJ-',
the approach of Liao et al. [15] is applied to ths problem.
The resulting minimum utility cost is 1114.6
RMB Yuan-h - ', with the corresponding network
shown in Fig. 2. If operation 1 is allowed to split at
the concentration of 83.67 pg.g-', then a network with
999.6 RMB Yuan-h-' utility cost can be obtained as
shown in Fig. 3. On the other hand, the split of operations
may cause additional equipment cost. Therefore,
the tradeoffs between capital cost increasing and operating
cost reducing should be evaluated.
+Q operation split
+-q-+... -++z+
Figure 1 Split of original operation
The designer now faces the question of whether a
better network may be possible using a different design
strategy. What is really the correct utility target?How many number of splits are required to achieve
this? It should be noted that when operation split is
allowed in the water network design, the design variables
will increase rapidly. For example, if the maximum
number of N is permitted to split in one operation,
the total number of streams increases from N&
to (NMW2w, here, NM is the number of operations. In
this article, an effective two step procedure is introduced
to deal with such a problem. In the first step, a
rigorous determination of the targets including utility
consumption and the number of the operation splits is
obtained. Once these targets are identified, the detailed
network design is carried out in the second step
by an MINLP formulation.water-stream interconnections among the processes
and to design a network of heat exchangers between
these streams. The objective is the simultaneous
minimization of the freshwater usage and the energy
consumption of the whole system. Several assumptions
are specified: 0 all operations operate isothermally;
0 the contaminant load is fixed and is independent
of the flow rate; 0 the operations are allowed
to split by concentration; 0 the water flow
rate does not change through an operation; 0 single
contaminant is permitted.water-stream interconnections among the processes
and to design a network of heat exchangers between
these streams. The objective is the simultaneous
minimization of the freshwater usage and the energy
consumption of the whole system. Several assumptions
are specified: 0 all operations operate isothermally;
0 the contaminant load is fixed and is independent
of the flow rate; 0 the operations are allowed
to split by concentration; 0 the water flow
rate does not change through an operation; 0 single
contaminant is permitted.
In the design of energy efficient water utilization
systems, the split of water using operations should be
considered to reduce the total utility cost. The synthesis
of networks allowing split is now carried out in
two stages. The utility and split targets are first obtained.
Then, the detailed network design is carried
out based on the MINLP model. Although this strategy
may lead to local optimum solution, the design procedure
of the example shows fairly low complexity,
which is favorable in the industry.