Simultaneous integration of water and energy in heat‐integrated water allocation networks

This article proposes a new methodology for simultaneous integration of water and energy in heat‐integrated water allocation networks (WAHEN). A novel disjunctive model is first developed to determine an optimal water allocation network (WAN) where water and energy are integrated in one step. Based on the optimal WAN, a detailed heat exchanger network (HEN) to satisfy the utility target is then synthesized. Although the final network structure is obtained through two steps, the targets of freshwater and utility are optimized simultaneously. The proposed method has specific advantages. First of all, it can capture a tradeoff among freshwater usage, utility consumption, and direct heat transfer by nonisothermal mixing. Second, it can greatly reduce the complexity of subsequent HEN design. Finally, it is effective for simultaneous water and energy integration in large‐scale WAHEN systems. The advantages and applicability of this new method are illustrated by three examples from literature. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2202–2214, 2015     

Targeting and design of energy allocation networks for carbon emission reduction

In this work, the unified conceptual approach developed earlier for water and hydrogen networks is extended to energy allocation networks for reduced carbon emissions. The approach involves two distinct stages, namely, targeting and network design. The composite table algorithm (CTA), based on the limiting composite curve, is first used to target the minimum clean energy resources (zero-carbon and/or low-carbon) required in energy sector planning problems with carbon emission constraints aimed at reducing climate change effects. The nearest neighbors algorithm (NNA), where each demand is satisfied by sources that are the nearest available neighbors in terms of emission factor, is then used to synthesize energy allocation networks to meet the already-established targets. Importantly, the NNA allows several network designs, all achieving the minimum resource target, to be systematically generated depending on the order in which the demands are satisfied. Two case studies are presented to illustrate the algorithms.     

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.     

Heat exchanger network area targeting considering stream allocation to shell or tubes

An optimal heat recovery network requires optimum values of area and energy targets. Current heat exchanger network targeting methods do not consider the optimal allocation of each stream to shell or tube side of the exchanger during the network cost estimation. Some researchers pre-set the allocation of the streams ahead of targeting [Polley, G. T., & Panjeh Shahi, M. H. (1991). Interfacing heat exchanger network synthesis and detailed heat exchanger design. Transactions of the Institute of Chemical Engineers, 69(Part A), 445–457]. In real design case however, some practical considerations such as fouling and corrosion constrain the allocation of streams. Apart from those, other streams are allowed to be allocated to either tubes or shell. Appropriate allocation of these streams can considerably affect the network cost estimation. This paper introduces a new area-targeting procedure which utilizes the optimal allocation of streams in all enthalpy intervals. The procedure evaluates two possible options for each stream split passing through each exchanger in the spaghetti network. Thus, two different exchanger area requirements can be estimated and the one with less area requirement will be selected. During this evaluation process, the optimal distribution of each stream pressure drop within enthalpy intervals is fully utilized. The proposed targeting procedure is applied on a case study and comparison of the results with previous method (Polley & Panjeh Shahi, 1991) shows reduction of around 18% in the network area. In another case study, the area–energy trade offs using the new procedure shows a reduction of 12.4% in minimum network area requirement and 14.5% in total annual cost. Therefore, the new procedure can considerably alter the area–energy trade offs.     

A new technique for simultaneous water and energy minimisation in process plant

This paper presents a new technique for simultaneous minimisation of water and energy in process plants through a combination of numerical and graphical tools. The technique consists of three steps, namely, setting the minimum water and wastewater targets; design of minimum water utilisation network, and finally, heat recovery network design. This technique offers two key advantages over current state-of-the art techniques. Firstly, it is applicable to mass transfer based and non-mass transfer based water-using operations. Secondly, it introduces a new graphical visualisation tool through a plot of temperature versus stream flowrate, termed as heat surplus diagram to guide water and energy reduction simultaneously. The heat surplus diagram provide insights on the energy demand as well as on stream matching scenarios during design of a maximum water and energy recovery network. A case study on a paper mill plant demonstrates that significant reductions in water and energy consumption can be achieved using this approach.     

通过流股的合理合并改进用水网络的能量效率

Water-using operations in the process industry have demands for water usually both on water quality and temperature, and the existing mathematical models of heat exchange networks cannot guarantee the energy performance of a water network optimal. In this paper, the effects of non-isothermal merging on energy performance of water allocation networks are analyzed, which include utility consumption, total heat exchange load, and number of heat exchange matches. Three principles are proposed to express the effects of non-isothermal merging on energy performance of water allocation networks. A rule of non-isothermal merging without increasing utility consumption is deduced. And an approach to improve energy performance of water allocation network is presented. A case study is given to demonstrate the method.     

Optimal operation of heat exchanger networks through energy flow redistribution

This article presents a novel energy flow redistribution methodology to achieve optimal operation of heat exchanger networks. The proposed method aims to manipulate the propagation path of a disturbance through the network to reduce its impact on utility consumption. Specifically, an optimization problem is formulated to generate new duty targets for heat exchangers of the network when a disturbance is encountered. Subsequently, a feedback control system is designed to track these targets by manipulating bypasses around the process heat exchangers. The effectiveness of the proposed framework is illustrated with the help of three benchmark examples. The proposed approach can handle disturbances in inlet as well as target temperature, inlet flow and heat transfer coefficient of individual heat exchangers.     

数学规划与图形方法相结合设计热集成用水网络

将数学规划法与图形方法相结合探究单杂质用水网络与换热网络的集成问题。首先构建混合整数非线性规划模型（MINLP）,在最小公用工程消耗下优化流股参数未知情况下的分离系统组合曲线面积,得到了最为节能、换热面积最小的用水网络结构。在此基础上,提出了新的分离系统组合曲线演化步骤和规则,可以得到换热单元数目较小的换热网络结构。算例表明,与现有的基于分离系统的热集成用水网络设计方法相比,在最小化公用工程用量的同时可以进一步降低换热器数目与总换热面积。     

Targeting and design of water networks for fixed flowrate and fixed contaminant load operations

Water-using processes are typically modeled as either fixed flowrate operations or fixed contaminant load operations. A new method for targeting the minimum freshwater and pinch in a single-contaminant water network is proposed, which can be applied to both kinds of operations. The method consists of plotting separate source and demand composites with flowrate as the horizontal axis and contaminant load unusually as the vertical axis. It is elegant, non-iterative, and can handle hybrid problems where both kinds of operations coexist.To design minimum freshwater networks for fixed flowrate problems, an algorithm is presented based on a newly developed principle of nearest neighbors. The principle simply states that the source streams to be chosen to satisfy a particular water demand must be the nearest available neighbors in terms of contaminant concentration.To design minimum freshwater networks for fixed contaminant load problems, the nearest neighbors algorithm is applied to process units that lie across the pinch. Units that lie entirely on one side of the pinch are satisfied by the cleanest source available on that side of the pinch. In other words, below-pinch units are satisfied by freshwater and above-pinch units are satisfied by the cleanest available stream above the pinch. Designs based on this methodology, apart from meeting the minimum freshwater target, also minimize the water flowing through the process units resulting in reduced network capital cost.     

KIRCHHOFF'S LAW AND LOOP-BREAKING FOR THE DESIGN OF HEAT EXCHANGER NETWORKS

Traditional evolutionary methods for the synthesis of heat exchanger networks normally produce an initial network containing excess units when compared to the minimum units target. Subsequently, this network is simplified by energy relaxation. However, current methods fail to guarantee that the evolution follows a pathway ensuring that the minimum energy penalty is incurred. A new method to guide this evolution is presented. It is based on the application of Kirchhoff's Law to the network. This enables easy prediction of the actual energy penalties incurred by instituting different loop-breaking strategies. The minimum energy penalty may be discovered and the units to be removed to constitute an optimal solution readily identified. The precedence order for loop breaking in networks containing multiple loops can also be determined according to their respective energy penalties. The proposed algorithm enables the designer to undertake the loop-breaking phase of network design with confidence. The minimum energy path may be followed until a final network is discovered.     

Energy optimization in heat integrated water allocation networks

In this paper, linear programming formulations, complemented by concept based pinch analysis results, are developed to target the minimum energy requirements in a heat integrated fixed flow rate water allocation networks. These formulations can be applied for the cases of heat integration through isothermal and non-isothermal mixing in water allocation networks involving single as well as multiple contaminants. The earlier reported approaches are based on linear programming formulation for the case of isothermal mixing and either mixed integer non-linear programming or discontinuous non-linear programming formulations for the case of non-isothermal mixing. It has been observed that the earlier reported methodologies produce sub-optimal results for the case of non-isothermal mixing. However, the proposed methodologies produce the optimum results because of the rigorously proved linear formulation. Utility requirements for isothermal as well as for non-isothermal mixing cases are compared over a range of minimum approach temperatures, to evaluate the energy performance using illustrative examples.     

A rigorous targeting algorithm for resource allocation networks

Techniques of process integration can be applied to conserve resources such as energy, freshwater, cooling water, hydrogen, solvent, etc. Process integration methodologies are broadly classified into two categories: methodologies based on the mathematical optimization techniques and methodologies based on the conceptual approaches of pinch analysis. In this paper, a mathematically rigorous methodology is proposed to minimize the requirement of a natural resource in a chemical process industry. The proposed methodology combines the simplicity of the pinch analysis with the mathematical rigor of mathematical optimization techniques. Conservation of resource in a chemical process industry is posed as a network flow optimization problem and a simple algebraic methodology is proposed to solve the optimization problem. The proposed algebraic methodology is mathematically proved in this paper. The proposed algorithm is numerically faster than the general mathematical optimization methods used for solving optimal resource allocation problems.     

Synthesis of multipass heat exchanger networks using genetic algorithms

In this work, a methodology based on genetic algorithms (GAs) is developed for the optimal synthesis of multipass heat exchanger networks (HENs). The network model is based on a stagewise superstructure, and the problem of finding the optimum number of 1–2 shells in series of multipass heat exchangers is aided by an efficient optimization model that uses the standard F_T design method. The proposed methodology allows for proper handling of the trade-offs involving energy consumption, number of units, number of 1–2 shells and network area to provide a network with the minimum total annual cost. The results of the examples show that the new approach is able to find more economical networks than those generated by other methods.     

A heuristic design procedure for water‐using networks with multiple contaminants

On the analogy of the water‐using networks with single contaminant, we will introduce new methodology concepts: the concentration potentials of the demand streams and those of the source streams in the water‐using systems with multiple contaminants, based on the overall allocation possibility of the source streams to the demand streams. In the design procedure, the performing order of the processes is determined by the inlet concentration potentials of the processes. The processes with the lowest inlet concentration potential will be performed first. When satisfying the inlet stream of the process being performed, the source with the largest quasi‐allocation amount, which is defined in this article, will be used first. A few literature examples are investigated to show the method proposed. The results show that the method proposed in this work is very simple and the freshwater consumptions of the designs obtained are very close to the minimum freshwater targets. © 2008 American Institute of Chemical Engineers AIChE J, 2009     

SePTA—A new numerical tool for simultaneous targeting and design of heat exchanger networks

Pinch Analysis is an established insight-based methodology for design of energy-efficient processes. The Composite Curves (CCs) is a popular Pinch Analysis tool to target the minimum energy requirements. An alternative to the CCs is a numerical technique known as the Problem Table Algorithm (PTA). The PTA however, does not show individual hot and cold streams heat cascades and cannot be used for design of heat exchanger networks (HEN). This paper introduces the Segregated Problem Table Algorithm (SePTA) as a new numerical tool for simultaneous targeting and design of a HEN. SePTA shows profiles of heat cascade across temperature intervals for individual hot and cold streams, and can be used to simultaneously locate pinch points, calculate utility targets and perform SePTA Heat Allocation (SHA). The SHA can be represented on a new SePTA Network Diagram (SND) that graphically shows a heat exchanger network together with the amount of heat exchange on a temperature interval scale. This paper also shows that SePTA and SND can be a vital combination of numerical and graphical visualisation tools for targeting and design of complex HENs involving stream splitting, threshold problems and multiple pinches.     

基于热损失的换热网络夹点设计法

现行的换热网络目标方法,在进行换热网络价格估算时,未考虑热物流的热损失。在真实的设计中,换热器壳体保温后仍与环境温度相差较大,则其热损失不可忽略。文章提出了一个新的基于热损失的换热网络夹点设计法,该方法首先以综合费用最小为目标确定出最小温差,然后建立问题表格确定出夹点位置及最小公用工程消耗,最后再进行换热网络设计。文中采用某石油常减压换热网络系统为典型算例对该方法的前两步进行了分析研究,论证了该方法的必要性及可行性。结果表明:该方法与基本Linnhoff夹点技术法估算的投资费用有较大的差距之外,在一定的最小温差下,其与基本Linnhoff夹点技术法确定的夹点位置不同,公用工程消耗也有较大的差距。     

SYNTHESIS OF HEAT EXCHANGER NETWORKS WITH DESIGNER IMPOSED CONSTRAINTS

A new sequential, evolutionary design method is proposed for the synthesis of heat exchanger networks with designer-imposed constraints. The design parameters for generating the network are obtained with conventional algorithms by solving a modified trans-shipment formulation of the transportation linear program. Candidate network topologies can then be easily configured by hand computation and the “best” designs are quickly identified even for large problems. The procedure also identifies problems which may have multiple feasible near optimum topolgoies, characterized by minimum energy consumption as well as minimum number of units.     

Improved area—energy targeting for fired heater integrated heat exchanger networks

Effective integration of various subsystems into the overall process, results in an energy efficient and economic plant design. In this paper, issues related to the area-energy targeting for fired heater integrated heat exchanger networks are studied. Performance of a fired heater is affected by the variables such as fuel fired and air-preheat temperature. These variables along with the minimum approach temperature difference for the heat recovery of the background process, affect the performance of the overall system. A methodology is proposed for the area-energy targeting for fired heater integrated processes. In the proposed methodology, the fired heater heat duty split between the radiation and the convection section is determined using the one gas zone furnace model.     

考虑非等温混合的能量集成用水网络综合

提出了考虑非等温混合的新的水网络模型,并结合线性规划（LP）转运模型同时优化水网络的水耗及公用工程目标。新的水网络模型引入非等温混合以改善用水网络的能耗特性及减少模型中参与集成换热网络的流股数,从而降低设计换热网络的复杂程度。在确定水网络的水耗及公用工程目标后,采用夹点法设计详细的换热网络结构。两个算例结果表明,新的水网络模型不仅能确定用水网络的最优水耗及公用工程目标,而且还能得到一个更加简单的换热网络。这对节省设备投资及减少操作费用具有重要意义。     

Unified methodology for thermal energy efficiency improvement: Application to Kraft process

A unified methodology that can be used to identify the interactions between the utilities systems and the process, as well as their impacts on the implementation of energy efficiency measures is presented. It takes into account steam and water systems to analyze the process and formulate energy enhancement measures. It has been applied to an operating Kraft mill in Eastern Canada. The methodology consists of five stages: base-case process definition and characterization, pre-benchmarking, systems interactions analysis, implementation strategy and post-benchmarking. A simulation focused on the energy and water systems is first developed and used as basis of the analysis. The pre-benchmarking characterizes the current energy efficiency of the process by three techniques: energy and exergy content indicators, comparison to the current industrial practice and establishing targets for minimum energy and water requirements determined by the Thermal Pinch and Water Pinch methods. The systems interactions are analyzed to develop complementary energy efficiency measures by applying several energy enhancing techniques. A three-phase strategy is proposed to implement the identified measures. The application of the unified methodology results in an eco-friendly process that does not require fossil fuel for steam production and generates revenues by producing green electricity from biomass. In the case study presented, very significant energy gains have been proposed (26.6% steam requirement reduction and 33.6% fresh water intake reduction).