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.