THE USE OF NONLINEAR PROGRAMMING TO OPTIMAL WATER ALLOCATION

Environmental protection, shortage of fresh-water and rising costs for wastewater treatment are all convincing motives for reducing fresh-water consumption and wastewater discharge of the chemical, petrochemical, petroleum refining and other process industries. Maximizing water reuse, regeneration re-use, and regeneration recycling within the chemical plant, as well as optimal distribution of waste streams for end-of-pipe treatment can reduce fresh-water usage and wastewater discharge, while they are also significant in shrinking capital investment in wastewater treatment systems.

Optimal assignment and design of water consuming, regenerating, and treatment systems is a complicated task that can be mathematically formulated as mixed integer non-linear programming (MINLP). In the present article the superstructure based 'Cover and Eliminate' approach with NLP is applied with the tools of the GAMS/M1NOS/CONOPT package and compared to previous results. After introducing the problem in the context of chemical process synthesis, a mathematical model is described and the use of the methodology is explained. Experience with the use of GAMS is discussed. Several case studies are solved including basic examples from the literature and their variants.

The main conclusion is that the application of the mathematical programming for the optimal water allocation problem is essential owing to the broad variety of the specification opportunities. The complex nature of re-use, regeneration re-use, and recycling with multiple pollutants and multiple treatment processes cannot be simultaneously taken into account by conceptual approaches. It is also shown that the assumption on the independency of contamination rates, generally applied in earlier works, are not necessarily valid; and the NLP approach can deal with the more reliable specifications.     

Optimisation of petroleum refinery water network systems retrofit incorporating reuse,regeneration and recycle strategies

Scarcities in freshwater supply and increasingly stringent rules on wastewater discharges have emerged as major environmental concerns for petroleum refineries. Hence, this work attempts to develop an optimisation framework for refinery water network systems design and retrofit that integrates the complementary advantageous features of water pinch analysis (WPA). The framework explicitly incorporates water minimisation strategies by first postulating a superstructure representation that embeds all feasible flowsheet alternatives for implementing water reuse, regeneration and recycle (W3R) opportunities. Subsequently, a nonlinear programming (NLP) model is formulated based on the superstructure and computational experiments on a real‐world case study are conducted using the GAMS/CONOPT3 modelling language platform. Post‐optimality analysis on the numerical results are performed to achieve the desired water reuse quality, hence presenting a viable framework to aid decision‐making in water network systems synthesis. © 2011 Canadian Society for Chemical Engineering     

含再生再利用的用水网络的优化设计

对单组分含再生再利用的用水系统,提出了一种基于序贯操作模型的整体优化设计方法。该方法分别针对单组分含再生再利用的用水网络设计可能存在的三种情况,以新鲜水和再生水用量最小为目标,给出了相应的设计策略,数学的表达是一个非线性规划。首先依据规则将操作对贫流的要求进行分段排序,并通过引入操作的质量交换分配系数αi,将各操作依据规则虚拟地分配为再生前过程和再生后过程,然后按操作序列逐级进行操作和操作水源之间的的优化匹配,建立含再生再利用的用水网络的序贯操作模型,最后将此综合问题归结为非线性规划问题来求解,从而完成用水过程的设计。给出了一个实例,计算结果表明本文方法是有效和简便易行的。     

基于粒子群优化算法的水分配网络系统综合的最优化研究

The problem of optimal synthesis of an integrated water system is addressed in this study, where water using processes and water treatment operations are combined into a single network such that the total cost of fresh water and wastewater treatment is globally minimized. A superstructure that incorporates all feasible design alterna- tives for wastewater treatment, reuse and recycle, is synthesized with a non-linear programming model. An evolutionary approach--an improved particle swarm optimization is proposed for optimizing such systems. Two simple examples are .Presented.to illustrate the global op.timization of inte.grated water networks using the proposed algorithm.     

考虑非等温混合的能量集成多杂质水网络优化设计

提出了一种分步综合多杂质体系水网络和换热网络的新方法。对于水网络,考虑进入操作单元的新鲜水与回用水之间的非等温混合,确保流股间的直接热回收;对于换热网络,可以只考虑水网络中的新鲜水和回用水流股之间的换热匹配。采用无进化次数的改进粒子群算法对本文建立的多杂质体系水网络非线性模型和换热网络混合整数非线性模型进行求解。实例表明,与不考虑非等温混合时比较,考虑非等温混合时的最优网络结构更简单,且年度总费用要节省2.4%。因此,本文提出的方法在实际生产中有较好的可行性和有效性。     

多杂质用水网络最优化设计——水质分析及数学规划法

在多杂质用水系统中,各操作出水水质会以不同杂质为主要因素,难以直接比较水质优劣,本文提出了水质分析与数学规划法相结合的用水网络最优化设计方法,对进出口水的极限数据进行分析得出水质状况并对它们按优劣进行排序,优先将排在前面的、节水效果好的操作作为供水源,在水量足够的情况下,对后面的操作不予考虑,以此来简化用水网络超结构模型,然后利用GAMS软件对经过简化后的超结构进行非线性规划并求解,通过两个例子的分析,证明了本方法的有效性。     

逐步线性规划法求解废水最小化问题

对用水过程的优化设计即废水最小化问题进行了研究 ,提出了操作水源和操作序列的概念 .分别对单组分再利用、多组分再利用及单组分再生再利用问题 ,提出了逐步线性规划的解算方法 .新方法首先依据规则将操作对贫流的要求进行分段排序 ,然后按操作序列逐级进行富流和贫流水源的优化匹配 ,最终完成用水过程的设计 .给出了 4个实例 ,计算结果表明本文方法是有效和简便易行的     

Multilevel strategies for the retrofit of large‐scale industrial water system: A brewery case study

This article presents an approach to designing a large‐scale water system, which integrates water‐using operations and wastewater treatment units in different production sections within the same network. This approach uses a mixed‐integer nonlinear programming (MINLP) model for water reuse and regeneration reuse in batch and semicontinuous processes. The application of this mathematical formulation to large‐scale industrial problems with changing daily production schedule leads to huge and complex mathematical models. Two alternative multilevel strategies are proposed to solve such problems by means of temporal decomposition. The approach is illustrated with a brewery case study that integrates water consumers in two production sections. The results obtained show that, despite the high piping cost, integration of both sections yields better result than the separate water network design in each section. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

多杂质水网络设计和零排放

The paper presents a procedure to design water network. First of all, water reuse system, water regeneration reuse system (including regeneration recycle) and wastewater treatment system are designed separately.But the interaction between different parts demands that each part is designed iteratively to optimize the whole water network. Therefore, on the basis of the separated design a water network superstructure including reuse,regeneration and wastewater treatment is established from the system engineering point of view. And a multiobjective adaptive simulated annealing genetic algorithm is adopted to simultaneously integrate the overall water network to balance the economic and environmental effects. The algorithm overcomes the defect of local optimum of simulated annealing (SA), avoids the pre-maturation of genetic algorithm (GA) and finds a set of solutions (pareto front) in acceptable computer time. From the pareto front, a point with minimum fresh water consumption will be extended to zero discharge as our ultimate goal.     

Simulation of mass exchange networks using modified genetic algorithms

The optimum water usage network leads to both a minimum of freshwater consumption and a minimum of generated wastewater. This work is to develop a mass-exchange networks (MENs) module for a minimum freshwater usage target. This module works as an interface to retrieve supplemental data of chemical processes from a process simulator and to communicate this to the genetic algorithm optimizer. A reuse system and a regeneration/recyclingsystem with a single contaminant are considered as approaches for freshwater minimization. In the formulated model, as mixed integer nonlinear programming (MINLP), all of the variables are divided into independent and dependent variables. The values of independent variables come from randomization, whereas the values of dependent variables come from simultaneous solutions of a set of equality constraints after assigning the values of independent variables. This method is applied to the steps of initialization, crossover and mutation. The MENs module is validated with a tricresylphosphate process consisting of five unit operations. Water is used to remove a fixed content of cresol. From the result, the module gives a reliable solution for freshwater minimization, which can satisfy mass balance and constraints. The results show that reuse and regeneration/recycling strategies can reduce freshwater consumption, including wastewater generated. Reuse cannot decrease the mass load of the contaminant, while regeneration/recycling can. In addition, regeneration requires less freshwater than the reuse process.     

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     

Application of water system optimization in a titanium pigment plant ——Mathematical programming

钛白粉行业是化工行业中的耗水大户,具有较大的节水潜力。本文在对某钛白粉厂水系统现状进行调研的基础上,采用数学规划法,分别从废水直接回用和再生循环两方面对其水系统进行优化设计,结果表明,改造后的两种水系统均比改造前节约新鲜水用量40.0%,并且废水直接回用更加适合钛白粉行业开展节水工作。     

多杂质体系间歇过程用水网络优化

对多杂质体系完全间歇过程用水网络优化进行了研究,建立了用水网络超结构和非线性规划数学模型,利用中间储罐跨越时间约束,设置用水过程流量上下限以加速收敛。首先确定关键杂质,并将关键杂质浓度最大的水源直接排往废水,获得简化的用水网络超结构;其次,运用GAMS求解,得到最小新鲜水量、废水量、中间储罐数目及优化的用水网络结构。实例研究证明本文提出的方法可行,并能以较少周期获得优化的网络结构,同时可以减少中间储罐数目。     

AquoMin: A software tool for Mass-Exchange Networks targeting and design

This study addresses the work developed at Process Integration Group of CPQ to build a software tool in the area of Pollution Prevention, regarding the Mass-Exchange Networks (MEN) targeting and design.Given a set of mass transfer operations and their limiting data, AquoMin can analyse different options to reduce fresh water consumption and minimise wastewater production. The available strategies to optimise these targets are (i) process without waste water reutilisation, (ii) process with waste reutilisation, and (iii) process with waste regeneration reuse. These options are analysed in two stages: first the targeting and then the design. The former stage uses algorithms based on pinch analysis to obtain the minimum consumption of external water and also the minimum wastewater production. These targets are then used in the second stage, the MEN design.The regeneration reuse strategy needs an extra step in the MEN design stage, which uses pinch analysis concepts to generate a mass balance model to obtain a set of optimal design parameters, the Split Operation Parameters (SOP). Furthermore, work on regeneration with recycle and reuse has been also developed. It was created a general algorithm to obtain the targets for this strategy: the external water source flowrate and the regeneration and recycle flowrate.A small AquoMin tutorial is presented using an Example Problem. The regeneration reuse strategy is focused and the methodology employed in both targeting and design stages were improved. Different scenarios evaluating the MEN structure's complexity and equipment cost were also developed and compared in the regeneration reuse strategy. In the regeneration recycle and reuse strategy, the Example Problem was used to emphasise a new option for the regeneration concentration value and the results of all the strategies are compared.Finally, a five operations’ Case Study was solved to obtain the targets and designs to accomplish the three Process Integration Strategies with a final reduction of 60% in the fresh water consumption.     

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.     

A global optimal formulation for the water integration in eco-industrial parks considering multiple pollutants

A mathematical programming formulation for the water integration in eco-industrial parks considering streams with several pollutants is presented. The formulation is based on a superstructure that allows the wastewater reuse in the same plant, the water exchange with different plants, and a shared set of interceptors that must be selected to determine the network configuration that satisfies process equipments and environmental constraints. The model formulation considers wastewater with several pollutants, and optimizes the network according to the minimum total annual cost, which includes the costs of fresh water, piping and regeneration. A new discretization approach is also proposed to handle the large set of bilinear terms that appear in the model in order to yield a near global optimal solution. The results obtained in several examples show considerable savings with respect to the solutions of the individual plant integration policy commonly employed for these types of problems.     

超滤预处理系统的优化设计

描述了超滤预处理系统的流程。依据微孔理论建立了超滤膜单元及系统模型,将系统的优化设计表达为一个非线性规划(NLP)问题,以年费用最小为目标函数,采用GAMS软件进行求解。结果表明,当进料液质量浓度高、产水水质要求低时,宜选用高通量膜;当进料液质量浓度低、产水水质要求高时,宜选用低通量膜。     

Aeration control of a wastewater treatment plant using hybrid NMPC

In the operation of wastewater treatment plants a key variable is dissolved oxygen (DO) content in the bioreactors. As oxygen is consumed by the microorganisms, more oxygen has to be added to the water in order to comply with the required minimum dissolved oxygen concentration. This is done using a set of aerators working on/off that represents most of the plant energy consumption. In this paper a hybrid nonlinear predictive control algorithm is proposed, based on economic and control aims. Specifically, the controller minimizes the energy use while satisfying the time-varying oxygen demand of the plant and considering several operation constraints. A parameterization of the binary control signals in terms of occurrence time of events allows the optimization problem to be re-formulated as an nonlinear programming (NLP) problem at every sampling time. Realistic simulation results considering real perturbations data sets for the inlet variables are presented.     

并联连续再生/再循环的多杂质间歇用水系统优化方法

为了有效地减少多杂质间歇用水系统的新鲜水消耗量和再生处理量,针对多杂质间歇过程用水网络提出连续操作并联再生处理单元水网络结构模型及其优化设计方法。通过在用水网络中设置中间储罐和再生单元以实现对不同水质废水并联分质处理,建立了减少用水系统的新鲜水用量和再生水量以及废水排放量的数学规划模型,采用GAMS软件对一个实例进行求解。计算和分析表明：提出的水网络结构与优化设计方法可有效地解决按水质对废水进行并联处理的间歇用水系统,使系统的新鲜水用量和再生水流率同时达到最小。     

Progress of water network integration with multiple contaminants

介绍了近年来多杂质水网络集成研究进展,讨论了当前设计多杂质水网络的几种方法,包括水夹点及图解方法、数学规划法、中间水道技术及基于经验的设计方法。探讨了各种方法的特点以及各方法涉及废水的直接回用、再生回用和再生循环。最后对今后的发展方向做了展望。