Abstract

An extended Pinch-based targeting and synthesis procedure in the domain of multiple constraints (e.g. multi-contaminants) for material recycle/reuse networks has been developed in this contribution. The main steps are the identification of the ranking order of material sinks/demands or sources/supplies. This is dependent on the limiting contaminants of sinks. Each contaminant/constraint is assigned a Pinch Diagram (Load vs Flowrate), and the Source and Sink Composite Curves (CC) are plotted for each diagram. The Source CC should be shifted until its lines could form a ’polygon’ with the Sink CC, for which the points represent the vertices of the polygons. The sequential approach used is to first identify a preliminary resource target and allocated sources for each sink, then to follow certain heuristics for further reduction of the freshwater. The proposed approach provides a minimum resource target and a network design that achieves the targeted fresh resource along with graphical representation.
Keywords : Process Integration; Multi-contaminant Water Pinch Analysis; Material Recycle/Reuse; Water Network Design
Introduction
To progress toward Circular Economy and to reduce the reliance on natural resources, the process industries have pursued material conservation as a key approach, which is mainly based on the concept of Process Integration [1]. El-Halwagi and Manousioutakhis [2] used the analogy with Heat Integration and introduced the problem of mass exchange network (MEN) for mass transfer operations in the process. Mass transfer processes include absorption, stripping, extraction, leaching, adsorption or ion exchange. They used the analogy to develop the related Composite Curves (CC) and Composition Interval Table (CIT) to determine the minimum water target, and the final network again is using the analogy from the Heat Grid Diagram [3] for design heuristics aided with Water Grid Diagram. Wang and Smith [4] again extended the Heat Pinch Approach to target the minimum freshwater consumption and wastewater discharged by the transfer of contaminants from process streams to water streams. A simplified design procedure was later introduced by Olesen and Polley [5], which results in better constructions for regeneration reuse and recycles designs. Hallale [6] introduced an extended targeting method for water minimisation. Gomes et al. [7] presented a heuristic algorithmic procedure, water source diagram (WSD), to synthesise water mass exchange networks. El-Halwagi and Manousiouthakis [8] showed that it is possible to target the minimum usage of external lean streams using again systematic representations passed on the physical understanding such as Composite Curves. El-Halwagi et al. [9] provided a single-stage targeting method to identify minimum resources for a single contaminant water network, with the solution strategy which identified through rigorous analysis. Both methods apply to mass exchange processes, and they rely on the basic principle of concentration driving force. For a review of the historical development of Water Pinch Analysis see, e.g. Foo [10]. Klemeš and Kravanja [11] provided an overview of the development of Process Integration prior to 2013, and Klemeš et al. [12] had more recently conducted a comprehensive overview of various extensions of PA in Mass Integration, including water and hydrogen integration.
However, a complicated issue is a solution in the case of multi-contaminant water flows. Various other strategies have been proposed to tackle this problem. Alva-Argaez et al. [13] developed Mixed-Integer Linear Programming (MILP) based multi-contaminant transhipment model used for targeting, particular for mass exchange networks and wastewater minimisation problems. Their model could tackle the general problem of mass exchange networks, but it could not evaluate the mixing effect of multi-contaminant problems. Gomes et al. [7] developed an algorithmic and graphical procedure, which is called Water Source Diagram (WSD) for a fixed load single contaminant system. This method is flexible and simple to use. It aims at minimising water consumption and provides the corresponding system network simultaneously directly. A series of works related to the use of WSD with multiple contaminants have been developed. An extension of the WSD for multiple contaminant systems was presented by Ulson de Souza et al. [14] to reduce freshwater consumption in an oil refinery. As the reference contaminant, they chose the one which was critical in the process, that is, the one which needs more water in operation. Karthick et al. [15] used the WSD with a mathematical method and generated a hybrid procedure to deal with the minimisation of freshwater for wastewater treatment. The WSD was used to provide starting points for the mathematical model. Gomes et al. [16] extended this method for maximum reuse and FL operations. This approach requires the choice of a reference contaminant and a reference operation to carry out the adjustments of concentrations in the required operations. Calixto et al. [17] developed a decomposition approach to be used with WSD, where the objective is to prevent the need of calculations to avoid violations in flowrate and/or concentration from the proposed network. This method also was applied to FL systems. Francisco et al. [18] extended this tool to fixed flowrate problem with multiple contaminants. The main steps involve the determination of the correct reference contaminants and operations, and later to adjust the inlet or outlet concentrations accordingly. Calixto et al. [19] provided an overview of the WSD and its application. Other attempts include the work of Castaño and Higuita [20], who used the property of turbidity (which sums a number of contaminants) in the design of water networks. The authors regarded turbidity as the key measured parameter and linear correlations of it were made with the concentration of the suspended solids. Mabitla and Majozi [21] presented a hybrid of graphical and mathematical approaches in solving multi-contaminant water and regeneration networks. The graphical approach involves the pre-processing steps to identify minimum water target and optimal regenerator removal ratios.
Another concept proposed by Liu et al. [22] is the concentration potential for a multi-contaminant problem. The concepts are presented based on the overall allocating possibility of source streams to demand streams. The concept is analogous to the single contaminant water network as it identifies the concentration order of the streams. Fan et al. [23] extended the concepts of concentration potential to the fixed flowrate operations. Li et al. [24] provide a review on this approach with their extension and applicability, and Zhao et al. [25] utilised the concept in designing heat-integrated water networks.
The Water Source Diagram (WSD) is a powerful tool and is widely used for industrial implementations due to its graphical visualisation platform [19]. Their algorithm to determine the resource target and network design require the adjustment on the concentration of the contaminant. In most of their works, they assumed the linear mass load ratio from Wang and Smith [26]. This might not be practical in realistic mass transfer operations. Calixto et al. [17] also explained that additional algorithm is needed to predict contaminant violation for the design framework. Francisco et al. [18] implemented this to the WSD algorithm, but the overall algorithm can be complicated. The potential concentration concept from Liu et al. [22] is another successful implementation, but it relies on the prioritisation of the sources. Chin et al. [27] mentioned that the prioritisation of the sources might not be the optimal allocation strategy, but to satisfy all the contaminant limits of the sinks as much as possible. This is applicable for fixed flowrate operations.
Using Mathematical Programming in MEN synthesis, Short et al.[28] formulated a comprehensive mixed-integer nonlinear program for MEN design. Oladosu et al. [29] presented an algebraic approach for simultaneous targeting and design of MENS with streams splitting. The proposed segregated Composition Interval Table (SECIT) was used to determine MEN Pinch Point and simultaneously target and design MENS. Yanwarizal et al. [30] adopted the Pinch based graphical tools and plotted the Composite Curves separating the streams instead of compositing the streams with similar concentration level. This strategy allows the simultaneous targeting and design of the MEN with a minimum fresh resource. However, their study was limited for single contaminant only.
In spite of the benefits of using MP in synthesising MEN network dealing with multi-contaminant problems, the approach is less favoured by design engineers due to the full automation of the solution. The superstructure formulation is demanding when the reasonable confidence in the proposed solutions and to produce solutions away from the optima. The solution strategy also provides restricted physical insights and analysis of the problem, making the internal bottlenecks unknown. This is true; especially when an authentic problem involves a lot of contaminants or streams properties constraints. The development of PA is an essential step for industrial application because it reveals the inherent system limitations – targets of the resource supply and the internal bottlenecks. These targets are then used in the detailed design model setting the optimisation strategy and providing bounds on the key variables. This is the logic which has led to the Process Integration strategy to identify the target for minimum usage of fresh resources ahead of detailed design, e.g. Heat Integration [3].
Based on the analysis, the objective of this paper is to develop a Pinch-Based simultaneous targeting and design of minimum fresh resource using interactive graphical representations. The specific problem of water using systems with multiple contaminants is studied in this paper. The proposed method involves the ranking of different resources to be reuse/ recycling according to its composition of various contaminants as well as assigning sinks to the proper contaminant cascades. The graphical plots using the load vs flowrate diagram can show the Pinch Point for different contaminants, maximum mass recovery, source allocation and minimum external resource targets for individual sinks simultaneously. The plot result can then be translated into source allocation network with the optimal design without the need to perform calculations to check for mass transfer feasibility.