Concepts development
This section first presents the necessary concepts used to perform
graphical targeting and design of the multi-contaminant material
network. Section 2.1 explains the proposed ’Polygon’ rule with graphical
visualisation. Section 2.2 describes the assignment of the sinks to
proper contaminant cascade. Section 2.3 compares the source allocation
pattern between the single contaminant and multiple contaminants.
Section 2.4 then describes the network design framework for each sink
using graphical method.
The typical Water Pinch Analysis (PA) could be performed to target
freshwater demand. There are various versions of graphical tools dealing
with material recycle/reuse problems. For fixed load operation, one of
the popular tools is the Water Surplus Diagram [6], which shows the
plot of impurity concentration vs flowrate. The plot is useful to
determine the fresh resource target only if no mixing between lower and
higher quality sources, relative to the sink is allowed. This added
benefit cannot be shown by the plot. If it is to be used, an initial
guess of a freshwater target is required. For fixed flowrate operation,
El-Halwagi et al. [9] proposed the load vs flowrate diagram as an
alternative to determining the freshwater target. The strength of this
plot is it accounts for the mixing of sources. This targeting tool is
also non-iterative and systematic in nature, providing a simpler
alternative for graphical targeting purpose. In this work, the load vs
flowrate used is used to solve the targeting problem with
multi-contaminants material recycle/reuse.
Polygon rule for sources
mixing
Yang et al. [31] presented a triangle rule in the graphical method
for hydrogen Pinch Analysis, accounting for the purification process. In
their work, they proposed the purification of the hydrogen feed stream
into purified products and tail gas. The mass of the contaminants is
conserved around the purification unit. Figure 1a shows the presented
’polygon’ (triangle) rule. The feed stream that has certain flowrate and
quality load is plotted on a load vs flowrate diagram. The horizontal
distance of line AC represents the flowrate of the feed stream, while
the vertical distance is the contaminant load of the stream. The
gradient of the line represents the contaminant concentration of the
feed stream. The feed stream is balanced by lines ’AB’ and ’BC’, which
represent the load and flowrate contributions of the purified product
and the tail gas. This concept can be applied to streams mixing as well,
but the process direction is reverse. A dirtier stream (line ’AB’) can
be mixed with cleaner stream (line ’BC’) to form a stream ’AC’. It
doesn’t matter which path to go from point ’A’ to point ’C’, as long as
the final point ’C’ is reached. The direction of the process has to be
denoted separately, as the diagram represents only the balance.
The ’triangle’ rule can be generalised into a ’Polygon’ rule, as
presented in Figure 1b. The feed stream AC can be balanced by more than
two streams such as AB, BD, DE and EC.