LP-based solution strategies for the optimal design of industrial water networks with multiple contaminants

This paper presents a new approach for the optimal design of water-using networks. Starting with a superstructure that accounts for all possible connections between water sources and water-using operations, different substructures are generated that consider all possible sequences of operations. Then, each operation is tackled one at a time, from the first to the last element in the sequence. This procedure has the advantage of replacing a nonlinear program (NLP) by a succession of linear programs (LP) that are solved for all operation sequences. Although part of the feasible region is lost in the process, the results have shown that the optimal solution is often obtained. The new procedure is at the same time a good approach of generating structurally different solutions that can be used as starting points for the full NLP. By doing this, local solutions can be avoided and the probability of finding the global optimal solution to the problem is increased. When compared to the standard initialization procedure that features a single starting point, the new approach is more efficient but substantially more demanding computationally. A trade-off can be reached by using the standard technique with multiple starting points originated from the possible operation sequences.     

Graphical approach to minimum flowrate targeting for partitioning water pretreatment units

Pretreatment is often necessary when the fresh water available to industrial plants is impure, and when some processes are particularly sensitive to contaminants. Partitioning processes such as membrane separation units are often used for such applications. However, the use of pretreatment units adds to capital and operating costs for a water system. Hence, it is of interest to develop design procedures to minimize the cost for such system. This work presents a graphical pinch analysis approach for targeting minimum flowrate of partitioning water pretreatment systems in single component problem. The approach determines how product and reject streams from the treatment unit can be allocated, along with bypassed freshwater, to satisfy multiple process sinks with their respective flowrate and purity requirements. Hypothetical case studies are presented to illustrate the approach, and generalized design principles based on pinch analysis heuristics are drawn from the examples.     

间歇过程单杂质用水网络设计

提出了一种利用时间-纯度两维用水图进行用水网络设计的方法,分别提出了无中间储罐和有中间储罐的设计规则。该方法把水的纯度作为一维,把时间作为另一维,从而更好地同时描述间歇用水过程的时间特性和杂质特性。它首先通过水级联分析技术确定间歇过程用水网络的夹点及最小公用工程,然后利用时间-纯度图和设计规则来合成用水网络,对有储罐的情况,确定了储罐的数量与容量,最后利用时间-水网络图描述了所合成的最优间歇过程用水网络,并探讨了用水过程流量对网络的影响。通过文献实例计算,表明该方法简单、直观、有效。     

MILP-based initialization strategies for the optimal design of water-using networks

The optimal design of water-using systems involves necessarily the exploitation of all possible water reuse and recycling alternatives. The general problem can be formulated as a non-convex nonlinear program (NLP), but due to the presence of bilinear terms, it may be difficult for local optimization solvers to attain global optimal solutions. To overcome this difficulty, this paper presents two mixed integer linear programming (MILP)-based procedures to generate a few structurally different starting points for the NLP. In both, the problem is decomposed into calculation stages by assuming that the water streams progress in series through the water-using units, with the binary variables selecting which unit belongs to a certain stage. Their main difference concerns the way fixed flowrate units are handled, either separately or in conjunction with a fixed load operation, since the former comprise a linear subsystem. The two algorithms are compared to a closely related LP-based method taken from the literature and to the one employed by the global optimization solver BARON. The results from a large set of example problems confirm their effectiveness in avoiding local solutions despite the small number of starting points. In contrast to the previous method they are easily scalable and, for some of the larger problems, could find better solutions than BARON with significantly fewer computational resources. The results have also shown that the option of tackling one unit at a time is the most favorable.     

Integrated water resource management through water reuse network design for clean production technology: State of the art

This article considers new and existing technologies for water reuse networks for water and wastewater minimization. For the systematic design of water reuse networks, the theory of the water pinch methodology and the mathematical optimization are described, which are proved to be effective in identifying water reuse opportunities. As alternative solutions, evolutionary solutions and stochastic design approaches to water system design are also illustrated. And the project work flow and an example in a real plant are examined. Finally, as development is in the forefront in process industries, this paper will also explore some research challenges encountered in this field such as simultaneous water and energy minimization, energy-pinch design, and eco-industrial parks (EIP).     

Sensor network design for contaminant detection and identification in water distribution networks

Water distribution networks (WDN) are vulnerable to either intentional or accidental contamination. In order to protect against such intrusions, effective and efficient online monitoring systems are needed. Due to cost and maintenance reasons, it is not possible to locate sensors at each and every potential intrusion point. In this work, we design minimal sensor networks which satisfy the two important properties of observability (ability to detect an intrusion) and identifiability (ability to identify the point of intrusion). Based on the hydraulic analysis of the network, a bipartite graph is constructed between intrusion points and the corresponding nodes that can potentially be affected by the contaminant. The problem of sensor network design is converted to a minimum set cover problem on the bipartite graph, and is solved using a greedy heuristic algorithm. The proposed method is illustrated using a medium scale urban WDN.     

Unified targeting algorithm for diverse process integration problems of resource conservation networks

A single algorithm is developed to establish minimum resource targets for diverse process integration problems including those of heat/mass exchange, water, hydrogen, carbon emission and material reuse networks. Previous algorithms such as the problem table algorithm for heat exchange networks and the composite table algorithm for resource allocation networks are special cases of the newly proposed unified targeting algorithm (UTA). The conversion of streams to equivalent inlet-outlet (demand-source) pairs is shown to be a key basis for the unified approach. The tabular data from the UTA may be plotted to obtain the grand composite curve (GCC) or the limiting composite curve (LCC). These provide graphical representations of the net load deficit/surplus at various levels for resource targeting and pinch identification. For allocation networks with system loss/gain, the UTA with increasing level sort order yields the Deficit LCC to target the minimum resource, whereas the UTA with decreasing level sort order provides the Surplus LCC to target the minimum waste/excess. A single UTA calculation along with the use of fundamental overall system balance equations is sufficient to establish complete targets for a problem. Six practical case studies from diverse domains are presented to illustrate the detailed steps of the UTA.     

Heuristic evolution strategies for simplifying water-using networks with multiple contaminants

In designing a realistic water-using network with more than one contaminant, it is often desirable to obtain a simple structure so as to achieve high levels of controllability, operability, and safety. Based on any given preliminary design, the proposed heuristic evolution procedure can be systematically applied to produce an improved network with fewer interconnections, less total throughput and near minimum freshwater usage. Specifically, the minimum interconnection number is first targeted on the basis of graph theory and, then, three basic evolution strategies, i.e., loop breakage, two-source shift and path relaxation, can be utilized to strive for the number target. The required calculations in this procedure can be easily realized with Microsoft Office Excel or a hand calculator. Two examples are presented in this paper to illustrate the implementation steps and to demonstrate the effectiveness of the proposed approach.     

Optimal design of single‐contaminant regeneration reuse water networks with process decomposition

Water network with regeneration schemes (e.g., regeneration reuse, regeneration recycling) can reduce freshwater consumption further than water network merely with direct reuse. Regeneration reuse, compared with regeneration recycling, can additionally avoid unexpected accumulation of contaminants. Owing to these features, process decomposition can help to reduce freshwater usage and wastewater discharge of regeneration reuse water systems and achieve the results, which graphical method delivers. In this article, the effect of decomposition on water‐using process and further on regeneration reuse water system is briefly analyzed on the concentration‐mass load diagram. Then a superstructure and three sequential mathematical models, which take process decomposition into account, are in turn developed to optimize single contaminant regeneration reuse water systems. By several examples, the reliability of the models is verified. Moreover, several decomposition strategies are summarized to realize the regeneration reuse water network, which attains the targets from graphical method. The results indicate that postregeneration concentration has a major impact on the scheme of process decomposition. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

涉及固定流量用水单元的多杂质水网络直观设计方法

水网络集成是过程工业一项重要的节水技术,如今受到越来越多的关注。固定流量过程与固定杂质负荷过程是工业水网络中典型的两类操作过程。不同于固定杂质负荷过程,固定流量过程是一类不涉及传质的用水操作单元。改进了针对固定杂质负荷过程的多杂质用水系统提出的浓度势概念,并将其应用于固定流量过程的多杂质用水系统设计。设计中各过程的执行顺序由需求水流的浓度势(CPD)决定：按照CPD值由小到大的顺序依次执行各过程。满足需求水流时,按照虚拟分配量由大到小的顺序来选择内部水源,按照价格由低到高的顺序选择外部水源。实例结果表明,该方法只需简单计算即可获得多杂质固定流量水系统的设计网络。     

Coupling pinch analysis and rigorous process simulation for hydrogen networks with light hydrocarbon recovery

In refineries, some hydrogen-rich streams contain considerable light hydrocarbons that are important raw materials for the chemical industry. Integrating hydrogen networks with light hydrocarbon recovery can enhance the reuse of both hydrogen and light hydrocarbons. This work proposes an automated method for targeting hydrogen networks with light hydrocarbon recovery. A pinch-based algebraic method is improved to determine the minimum fresh hydrogen consumption and hydrogen sources fed into the light hydrocarbon recovery unit automatically. Rigorous process simulation is conducted to determine the mass and energy balances of the light hydrocarbon recovery process. The targeting procedures are developed through combination of the improved pinch method and rigorous process simulation. This hybrid method is realized by coupling the Matlab and Aspen HYSYS platforms. A refinery hydrogen network is analyzed to illustrate application of the proposed method. The integration of hydrogen network with light hydrocarbon recovery further reduces fresh hydrogen requirement by 463.0 m³·h^-1 and recovers liquefied petroleum gas and gasoline of 1711.5 kg·h^-1 and 643 kg·h^-1, respectively. A payback period of 9.2 months indicates that investment in light hydrocarbon recovery is economically attractive.     

Capital and total cost targets for mass exchange networks: Part 1: Simple capital cost models

This paper presents a method for targeting the capital and total cost of a mass exchange network. This part of the paper considers stagewise systems and uses a simplified capital cost correlation in order to convey the fundamentals. The method starts by determining the number of stages required, using the y−x composite curve plot presented previously, and then converts this to a capital cost. This target is combined with utility targets to give a target for the total annual cost. Simple design guidelines are presented, which allow these targets to be approached to within a few percent. The method is applied to a coke-oven gas sweetening example, which is a case involving two mass separating agents (MSAs) which do not overlap. It is also applied to an example involving phenol removal, in which there are overlapping MSAs. A final example deals with multicomponent transfer by extending the coke-oven gas problem. The paper introduces a new graphical tool, the y−y* composite curve plot, for handling systems with overlapping MSAs. The paper demonstrates that, contrary to previous belief, using the minimum number of units does not necessarily lead to a minimum cost design.     

Design and evolution of water networks by source shifts

This paper proposes the concept of source shifts to design many different water networks, all of which satisfy the minimum freshwater target. The concept is also used to evolve these networks to simpler designs with fewer matches for implementation in practice. A matching matrix is introduced as a compact network representation to perform such shifts.     

Automated targeting for inter-plant water integration

Apart from in-plant water recovery, inter-plant water integration (IPWI) offers another promising mean for the reduction of fresh water and wastewater flowrates for process plants. This paper extends the automated targeting technique that was developed for single water network into IPWI. This optimisation-based technique is based on the concept of pinch analysis, which enables the setting of various network targets prior to detailed design. The automated targeting technique is formulated as a linear programming model for which global optimum is guaranteed. The proposed technique is demonstrated using several industrial and literature examples.     

Targeting water utilities for the threshold problem without waste discharge

Increasing water scarcity and stringent environmental regulation have necessitated effective water conservation policies. Pinch analysis has been proved as one of the powerful tools to locate targets of waste water minimization. Two earlier water pinch targeting methods known as Water Cascade Analysis and Material Recovery Pinch Diagram have focused on the “threshold problems”. However, these methodologies have not systematically analyzed the introduction of external utility. In this work, three scenarios are proposed for this reason. The “Infeasible Threshold Problem” is addressed prior to employing external utility through the proposed scenarios. By systematically analyzing this specific problem, it is revealed that existing Water Cascade Analysis method cannot locate correct infeasible targets. Some adjustments are proposed to deal with this drawback. Moreover, to illustrate the applicability of proposed scenarios, Water Cascade Analysis and Material Recovery Pinch Diagram approaches are utilized for addressing a literature problem as a case study. It is shown that harvesting the impure fresh water source with a higher quality, in the “threshold problem with zero discharge”, leads to more pure fresh water saving.     

Thermodynamic analysis of non-isothermal mixing's influence on the energy target of water-using networks

In this paper, a thermodynamic analysis of non-isothermal mixing's influence on the energy target of water-using network is presented. Firstly, water streams in the network are divided into two categories, and then based on the classification all the non-isothermal mixing patterns between two streams are defined. Through thermodynamic analysis from energy composite curve of hot and cold water streams, the influences of non-isothermal mixing on the energy target of water-using network are explored and some mixing rules are obtained, which can be used to simplify the heat exchanger network of a given water-using system and improve the system's energy performance through identifying the beneficial non-isothermal mixings. The applicability of these rules is illustrated by an example in the paper and the result is very encouraging.     

Synthesis of water networks considering the sustainability of the surrounding watershed

This paper presents a new mathematical programming model for the optimal synthesis of recycle and reuse networks considering simultaneously the integration of the water network system and the surrounding watershed to satisfy process and environmental constraints. The model considers the optimal location of the new industrial facility to integrate its wastewater discharge to the environment with the surrounding watershed through a disjunctive formulation. The pollutants discharged for the new plant are tracked simultaneously with the other discharges to the watershed (i.e., residential, sanitary, industrial and extractions), and the natural phenomena that affect the composition of the watershed (i.e., evaporation, filtration, etc.), in addition to the chemical reactions that are carried out in the rivers. The objective function consists in minimizing the total annual cost that is constituted by the installation of the new plant cost (including the transportation for raw materials, products and services, as well as the land cost), the wastewater treatment costs (including the piping cost) and the fresh sources cost. Two example problems were used to show the applicability of the proposed methodology.     

Optimal design of chemical processes with chance constraints

Generally, chemical processes (CP) are designed with the use of inaccurate mathematical models. Therefore, it is important to create a chemical process that guarantees satisfaction of all design specifications either exactly or with some probability. The paper considers the issue of chemical process optimization when at the operation stage the design specification should be met with some probability and the control variables can be changed. We have developed a common approach for solving the broad class of optimization problems with normally distributed uncertain parameters. This class includes the one-stage and two-stage optimization problems with chance constraints. This approach is based on approximate transformation of chance constraints into deterministic ones.     

Decentralized fault-tolerant control system design for unstable processes

Fault-tolerant control is an important issue in control of mission critical processes. In this paper, a new approach to fault-tolerant control of unstable processes is proposed based on the Passivity Theorem. The control system is designed in two sequential steps: A multi-loop proportional controller is used to stabilize the unstable process; a passivity-based decentralized unconditionally stabilizing (DUS) controller is then applied to the stabilized process. While the multi-loop stabilizing controllers need to be built with redundancy, the DUS controller is inherently fault tolerant and can maintain closed-loop stability when any of its loops fail. By using a stabilizing proportional controller with the fewest loops, control redundancy can be reduced to the minimum level.     

Optimal design of integrated agricultural water networks

This paper presents a mathematical programming model for the optimal design of water networks in the agriculture. The proposed model is based on a new superstructure that includes all configurations in terms of use, reuse and regeneration of water in a field constituted by a number of croplands. The model also includes the allocation of pipelines, pumps and storage tanks in different irrigation periods. The objective function consists in maximizing the annual profit that is formed by the economic incomes owing to the crop sell minus the costs for fresh water, fertilizer, storage tanks, treatment units, piping and pumping. The proposed multi-period optimization problem is formulated as a mixed integer non-linear programming formulation, which was applied to a case study to demonstrate the economic, environmental and social benefits that can be obtained.