Exergy analysis of a combined RO, NF, andEDR desalination plant

A brackish water desalination plant in California that incorporates RO, NF, and EDR units was analyzedthermodynamically using actual plant operation data. Exergy flow rates were evaluated throughout the plant, and the exergy flow diagrams were prepared. The rates of exergy destruction and their percentage are indicated on the diagram so that the locations of highest exergy destruction can easily be identified. The analysis shows that most exergy destruction occurs in the pump/motor and the separation units. The fraction of exergy destruction in the pump/motor units is 39.7% for the RO unit, 23.6% for the NF unit, and 54.1 % for the EDR unit. Therefore, using high-efficiency pumps and motors equipped with VFD drives can reduce the cost of desalination significantly. The plant was determined to have a Second Law efficiency of 8.0% for the RO unit, 9.7% for the NF unit, and 6.3% for the EDR unit, which are very low. This indicates that there are major opportunities in the plant to improve thermodynamic: performance by reducing exergy destruction and thus the amount of electrical energy supplied, making the operation of the plant more cost effective.     

Exergy analysis of a reverse osmosis desalination plant in California

The exergy analysis of a 7250 m³/d reverse osmosis (RO) desalination plant in California was conducted by using actual plant operation data, and an alternative design was investigated to improve its performance. The RO plant is described in detail, and the exergies across the major components of the plant are calculated and illustrated using exergy flow diagrams in an attempt to assess the exergy destruction distribution. The primary locations of exergy destruction were the membrane modules in which the saline water is separated into the brine and the permeate, and the throttling valves where the pressure of liquid is reduced, pressure drops through various process components, and the mixing chamber where the permeate and blend are mixed. The largest exergy destruction occurred in the membrane modules, and this amounted to 74.07% of the total exergy input. The smallest exergy destruction occurred in the mixing chamber. The mixing accounted for 0.67% of the total exergy input and presents a relatively small fraction. The second law of efficiency of the plant was calculated to be 4.3%, which seems to be low. The analysis of the alternative design was based on the exergy analysis. It is shown that the second law of efficiency can be increased to 4.9% by introducing a pressure exchanger with two throttling valves on the brine stream, and this saved 19.8 kW electricity by reducing the pumping power of the incoming saline water.     

Exergy as a tool for measuring process intensification in chemical engineering

Exergy is defined as the maximum shaft work that can be done in a process to bring the system into equilibrium with the environment. Thus, exergy analyses are the first step to understand where the weak points of processes are. It considers intrinsically the quality of energy: when energy loses its quality, exergy is destroyed. In addition, optimization of processes aiming at the minimization of exergy destruction can be done as a function of the topology and physical characteristics of the system, such as finite dimensions, shapes, materials, finite speeds, and finite‐time intervals of operation, establishing a direct relationship between exergy and process intensification. However, the emphasis on exergy in chemical engineering is still very poor compared with other fields, in spite of being one of the areas in which more exergy is destroyed due to reaction and separation . This paper gives an overview of the current application of exergy analyses in chemical engineering, showing the main fields in which exergy studies are performed and focusing the attention on two critical points of action: separation technologies (distillation and membrane technology) and CO₂ capture. New research trends in chemical engineering using exergy as a tool for process intensification are highlighted. © 2013 Society of Chemical Industry     

基于热力学的石油蒸馏过程积累(火用)消耗的分配的改进计算

One of the most common problems arising from the application of exergy analysis is the allocation of cumulative exergy consumption （CExC） in the petroleum distillation process yielding several useful products. Based on the concept of exergy, an improved calculation of the minimum separation power of product （MSPP） in the petroleum distillation process is provided in this article. The calculation of MSPP can be derived from the concept of exergy. The related mathematical models are established. Finally, application of this method to a case study is given, and the results are compared with the ones using mass as an allocation parameter.     

循环水流量对ORC余热发电系统性能影响的试验分析

基于实验室3 kW有机朗肯循环（ORC）低温余热发电试验装置,参考石化行业能耗设计标准将循环水作为耗能工质,采用总能系统方法进行能耗分析,对比了不同热源温度下不同分析边界的系统及主要设备的热力学性能。结果显示：发电机输出功、膨胀机输出功、ORC子系统净输出功、ORC子系统热效率和?效率均随着热源温度和循环水流量的增加而增加;不同热源温度下,最大系统净输出功与最大系统?效率出现的工况一致。本试验在热源温度为120℃时取得最大系统净输出功0.731 kW和最大系统?效率11.81%,此时对应循环水流量为1.629 t·h^-1。该研究为ORC余热发电系统性能与能耗分析提供了参考。     

集成NGL回收的新型天然气液化系统AP-XTM的概念设计与模拟分析

为提高液化天然气能量集成与设备共用水平,提出了一种基于大型AP-X^TM液化流程，综合气体过冷技术（GSP）的集成NGL（天然气凝液）回收工艺的天然气液化系统的概念设计。基于化工流程模拟软件Aspen HYSYS进行模拟和分析，将集成工艺多流股换热器性能、全流程的单位功耗和乙烷回收率作为衡量系统性能的三项指标。模拟和分析的结果表明，集成NGL回收的AP-X^TM液化工艺单位功耗降低至0.45 kW·h·(kg LNG)^-1，较单产系统能耗降低了6%，同时乙烷回收率达到93%，实现了NGL的高效分离。通过热力学分析、?分析和经济性分析得出本设计流程具有较高的性能和经济价值，可为天然气液化工艺的集成设计和技术改造提供指导借鉴。     

Application of an adsorption non-flow exergy function to an exergy analysis of a pressure swing adsorption cycle

Pressure swing adsorption (PSA) is a popular gas separation technology for the process industries and is commonly used for air separation, hydrogen purification, and isomer separation. In this study, we apply a second law analysis to this technology to identify sources of irreversibility in the process and, in particular, identify which steps in the PSA cycle are responsible for the major losses. Unlike previous exergy analyses, we derive and use expressions for the exergy of the adsorbed phase using adsorption thermodynamics. In this way, exergy loss (or entropy generation) within the adsorption cycle in each step is clearly identified. We illustrate the use of these exergy functions with the application of binary linear isotherm (BLI) theory to a four-step Skarstrom cycle. Major losses in the process are shown to be the exergy loss across the valve in the blowdown step, and feed compressor aftercooler losses. Feed repressurisation is shown to be more efficient than product repressurisation for the separation factor examined in this study since part of the feed gas is introduced at a low pressure. During the cycle, bed exergy loss during the feed step is significant, while there is no exergy loss in the adsorbent bed during the blowdown or purge steps. The exergy functions derived in this study can readily be applied to more complex PSA cycles and provide a basis for cycle design.     

Energy advantages of reverse osmosis in seawater desalination

The very rapid increase in energy costs during the past three years is causing a change in the preferred process technology for seawater desalination. The phase changes, evaporation, and condensation, required in the distillation processes make them more energy intensive than the ambient temperature liquid separation that occurs in the reverse osmosis (RO) process. This paper describes the RO process and how to calculate its energy consultation.The RO process requires only 5–7 KWh/m³ of product water compared to 15–16 KWh/m³ required by the most efficient distillation process. The productivity of a large dual purpose electricity/RO water plant is compared to the productivity of a commercially purchased state-of-the-art dual purpose electric/distillation water plant that is currently under construction. The RO potable water productivity is about 2X the distillate flow at the same fuel rate     

A new exergy method for process analysis and optimization

A two level idealization concept involving the definition of intrinsic and extrinsic exergy destruction is incorporated into exergy analysis, exergoeconomic analysis and exergoeconomic optimization. The intrinsic exergy destruction is caused by the configuration constraints, whereas the extrinsic exergy destruction is caused by the transport rate limitations. For exergy analysis, intrinsic and extrinsic exergy destructions can be quantified for each process operation. For exergoeconomic analysis, the monetary costs associated with these exergy destructions can be determined. For exergoeconomic optimization, the variables associated with process configuration and transport rate limitations can be optimized independently. Methods for analysis as well as optimization are described and demonstrated by two case studies, an ethylene product recovery and separation plant and a benzene-toluene distillation column. Improvements demonstrated from these case studies are significant.     

Exergy analysis of a seawater reverse osmosis plant

Exergy analysis is a powerful tool to determine how inefficiencies of the processes influence system performance. The exergy analysis of a seawater reverse osmosis desalination plant with 21,000 m³/d of nominal capacity located in Tenerife (Canary Islands, Spain) was studied. Once defined, the flow chart of the process, the exergy rate and exergy cost of flows were determined as well as the exergy destruction rate in equipment. The main results indicate that 80% of the exergy destruction is placed on core processes (high pressure pumping and valve regulation, reverse osmosis separation and energy recovery), 29% extra exergy is necessary to obtain the unit of feed exergy from previous stages (seawater pumping and pretreatment) and extra exergy of 1.06 kJ is needed to generate 1 kJ of final product exergy (exergy performance about 50%). In addition, the moderate fluctuations of seawater environmental conditions in the Santa Cruz de Tenerife metropolitan area (and Canary Islands as a whole) establish that environmental parameters present a less important influence on exergy analysis.     

酯交换法制备碳酸二甲酯过程模拟与系统火用分析

针对酯交换制备过程中甲醇?碳酸二甲酯共沸体系难分离的问题,分别选择变压精馏、碳酸乙烯酯(EC)萃取精馏与乙二醇(EG)萃取精馏3种分离过程进行模拟与能量集成,对比了3种工艺流程的分离能耗,采用有效能(?)分析方法分析了能耗最低的变压分离过程的有效能(?)损失. 结果表明,3种工艺流程的能耗EG萃取精馏>EC萃取精馏>变压精馏,碳酸二甲酯生产过程中内部循环物流能量是输入总能量的1.55倍,变压共沸分离过程的?损失为7.9%。     

复合膜反渗透装置微生物的污染及防治

反渗透膜分离以其优异的特点已成为最受欢迎的分离技术之一。在反渗透膜分离技术的实际应用过程中,膜污染问题是影响反渗透膜分离技术可靠性的决定性因素。本文概述了复合膜反渗透装置中微生物污染的机理、预防措施及其清洗方法。     

液化天然气冷能构成及其利用方式探讨

液化天然气(LNG)在汽化过程中会释放大量冷能,如果这部分冷能被成功回收利用,其节能效果和对系统效率的提高都十分显著。文中对LNG冷能从冷量和冷量的角度进行分析,把LNG冷能回收方式分为冷量回收与冷量回收,揭示了目前各种LNG冷能回收利用形式的能量利用实质:发电、空分中主要是利用LNG的冷量;冷藏、空调和制干冰利用了LNG的冷量。最后对不同的冷能回收系统提出指导性建议:动力回收系统中,应充分利用其在低温下的高品质能量;冷量回收系统中应减少跑冷。     

有机朗肯循环膨胀机入口过热度实验

在给定热源条件下,探讨有机朗肯循环(ORC)膨胀机入口过热度对膨胀机性能和ORC系统性能的影响。建立了带前置泵的ORC实验系统,采用涡旋式膨胀机,R123为工质,在140℃热源下进行实验。通过改变膨胀机转矩调节系统蒸发压力,从而实现对膨胀机入口过热度的调节。实验获得最大膨胀机轴功和膨胀机实际运行效率分别为2.35kW和59.7%;ORC系统净输出功、热效率和(火用)效率分别为1.75kW、5.3%和21.8%。分析表明,随着膨胀机入口过热度递减,膨胀机机械效率递增,膨胀机等熵效率递减,膨胀机轴功和实际运行效率呈先增后减的变化趋势。膨胀机入口过热度为20℃左右时,有最大膨胀机轴功、最大系统净输出功、最高系统热效率和最高系统(火用)效率。此外,过热度影响系统的损失分布,随着膨胀机入口过热度减小,膨胀机(火用)损呈先增后减变化。     

Identification,Modelling, and Control of Continuous Reverse Osmosis Desalination System: A Review

Desalination is a separation process used to reduce the amount of dissolved salts in seawater or brackish water to a usable or potable level by distillation, multiple effect vapor compression, evaporation, or by membrane processes such as electro-dialysis reversal, nano-filtration, and reverse osmosis (RO). RO is the most widely used desalination process. Recent advances in RO technology have led to more efficient separation and now is the most cost-effective process to operate. The performance of the Reverse Osmosis process is dependent on the concentration of dissolved solids in the feed-water, feed-water pressure, and the membrane strength to withstand system pressure, membrane solute rejection, membrane fouling characteristics, and the required permeate solute concentration. RO is a promising tool that uses cellulose acetate (or) polyamide membrane and is widely chosen as the cost of production is reduced by the use of energy-efficient and process-control techniques. This article presents a review of literature survey of identification of parameters, dynamic modelling, and control of desalination system in the past twenty years by collecting more than 65 literatures.     

反渗透处理煤层气产出水中试示范及其控制模型研究(英文)

This study presents the first demonstration project in China for treatment of coal-bed methane(CBM) co-produced water and recycling.The work aims to provide a research and innovation base for solving the pollution problem of CBM extraction water.The reverse osmosis(RO) unit is applied to the treatment of CBM co-produced water.The results indicate that system operation is stable,the removal efficiency of the total dissolved solids(TDS) is as high as 97.98%,and Fe,Mn,and F-are almost completely removed.There is no suspended solids(SS) detected in the treated water.Furthermore,a model for the RO membrane separation process is developed to describe the quantitative relationship between key physical quantities-membrane length,flow velocity,salt concentration,driving pressure and water recovery rate,and the water recovery restriction equation based on mass balance is developed.This model provides a theoretical support for the RO system design and optimization.The TDS in the CBM co-produced water are removed to meet the "drinking water standards" and "groundwater quality standards" of China and can be used as drinking water,irrigation water,and livestock watering.In addition,the cost for treatment of CBM co-produced water is assessed,and the RO technology is an efficient and cost-effective treatment method to remove pollutants.     

煤间接液化中费托合成单元装置的分析

在基于PRO/Ⅱ对低温费托合成系统进行模拟及优化的基础上,采用热力学分析方法,对低温费托合成系统进行了量衡算,分析了系统中各主要能耗单元的效率和损失状况。计算结果表明,系统中损失最大的过程是费托合成反应过程。费托合成反应器是损失最大的设备,效率为86.80%,损失占了总损失的85.15%;冷凝液回流泵效率最低,只有6.71%;效率最高的为石蜡收集槽、石蜡泵、石蜡中间槽等,几乎没有损失。采用热力学分析方法可以更准确地揭示系统中各环节和设备的最大损失,为改进设备、节约能源提供目标和对策。     

Simulation and thermodynamic analysis of an integrated process with H2 membrane CPO reactor for pure H2 production

A one-dimensional steady-state heterogeneous model has been used to simulate the H₂ membrane reactor. The simulation work is the basis for the thermodynamic analysis of the integrated pure H₂ production process. The simulation and analysis also provide a quantitative tool for insight into and understanding the process.The simulation and thermodynamic analysis results indicate that increasing the inlet ratio H₂O/CH₄ cannot enhance the pure H₂ production rate. With increasing the inlet ratio H₂O/CH₄, the overall exergy efficiency of the process decreases, because a large amount of energy is required to obtain the steam.When the geometric parameters of membrane reactor and inlet temperature are given, there is a maximum feeding rate of methane for the integrated process. The pure hydrogen production rate increases with the inlet methane rate increasing, while the overall exergy efficiency decreases as inlet methane rate increases.For the same inlet rate of methane, operating the process at higher inlet temperature increases hydrogen production rate. Whereas, the overall exergy efficiency is lowered.Three suggestions are discussed to improve the overall exergy efficiency. All of them require more equipment investment. There will be an optimal point to balance equipment investment, pure hydrogen production rate and overall exergy efficiency. To find the optimum, thermo-economic analysis will be helpful.     

基于热流逸效应的燃煤电厂烟气二氧化碳分离系统

分离捕集CO₂是实现“双碳”目标的重要途径之一。常规的CO₂分离方法普遍能耗较高,若能以余(废)热为动力来分离CO₂则可综合利用能源、降低能耗。本文针对高碳排放但却拥有丰富余(废)热资源的燃煤电厂,提出了一种基于热流逸效应的烟气CO₂分离系统,并建立了相应的分离过程数学模型和系统性能评价指标。分析表明,CO₂的浓度和回收率均随热流逸式气体分离器串联级数的增加而升高,但浓度和回收率达到某一阈值后效果不再明显;典型的1000MW燃煤电厂烟气经该系统中串联的24级分离器处理后,CO₂的物质的量分数最高可达98.89%,回收率达72.53%。此外,该系统可梯级利用烟气的余热,?效率为64.8%,单位能耗为0.047GJ/tCO₂,与传统CO₂分离方法相比具有一定节能潜力。利用热流逸效应分离CO₂符合当下净零碳排放的政策导向,为CO₂的分离捕集提供了新思路。     

Thermodynamic Analysis‐Based Improvement for the Boil‐off Gas Reliquefaction Process of Liquefied Ethylene Vessels

A new optimization design of the boil‐off gas (BOG) reliquefaction process for liquefied ethylene (LEG) vessels is proposed in order to reduce the reliquefaction process energy cost and improve its cold exergy efficiency. The exergy loss of each component is calculated and the efficiency of the available energy utilization is evaluated on the basis of a detailed thermodynamic analysis. The exergy analysis results indicate that the exergy efficiency of the improved BOG reliquefaction process is about 19.0 % higher than that of the existing process, and the amount of refrigerant used in the improved process is reduced by about 44.9 % per hour. The power consumption could be decreased by 16 %. The circulation volumes of the refrigerant and BOG are both significantly reduced, thus lowering the equipment and operation costs of the BOG reliquefaction process.