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毕业设计（论文）任务书课题名称 大型空分氮主换热器的优化设计 学院(部) 电子与控制工程学院 专 业 自 动 化 班 级 自动化三班 学生姓名 黄 俊 心 学 号 201132010318 3 月 9 日至 6 月 12 日共 14 周指导教师(签字) 教学院长(签字) 年 月 日一、设计内容（论文阐述的问题） 铝制板翅式换热器具有体积小、重量轻、传热效率高的特点，所以在大型空分成套装置中大量使用。氮主换热器作为最重要的一种换热器，位于冷箱中，对冷箱的传热介质进行冷量回收，使空气冷却后进入下塔，膨胀空气冷却后进入膨胀机，氮气加热后作为工业产品在各个行业中大量使用。大型空分氮主换热器的成功设计使得空分设备具有更高的效率、更低的成本及更快的生产速度。2、 设计原始资料（实验、研究方案） 本课是基于的基本原理，主要利用Aspen-HYSYS及Aspen-muse两套设计软件和公司的f、j两种数据包，在雷诺数Re于50010000范围内的6500级别的大型空分主换热器设计。设计要求：温度T（K）进/出压强P（bar）进/出流量（N/h）空气290/99.95.8/5.6517795.46膨胀空气290/1708.13/7.982107.7氮气97/2871.29/1.1419699三、设计完成后提交的文件和图表（论文完成后提交的文件）1. 计算说明书部分： (1) 热力计算书 (2) 压力容器强度计算书2、 图纸部分： 产品整体及零件图：KB1173全套图纸。 四、毕业设计（论文）进程安排序号 设计（论文）各阶段名称 日期（教学周）1 了解板翅式换热器的基本工作原理及生产工艺，在车间实习， 了解生产、焊接及组装过程； 1周2 对空分设备进行学习，掌握空分设备的工作原理； 2周3 结合资料和任务书，完成开题报告； 3周4 学习Aspen-HYSYS、Aspen-muse等专业软件； 4周5 学习板翅式换热器中的热力计算，及设计中的热力分析； 5周6 根据原始资料及设计要求计算设计压力，换热面积等参数； 6周7 对产品进行初步设计，包括选材、结构、通道位置设计等； 7-8周 8 对设计进行优化，完善设计细节，提高效率，节约成本； 9周 9 完成全套图纸的绘制，强度计算书的制作，强度校验； 11-12周 10 完成论文，准备答辩。 12-14周5、 主要参考资料1蒋旭等当前国产空分设备发展状况J低温与特气，2013(B1):89-942陈永东，陈学东我国大型换热器的技术进展N 机械工程学报，2013-03-26(A2)3嵇训达我国板翅式换热器技术进展J 低温与特气，1998 (1):22-274SUESSMANN W，MANSOUR APassage arrangementin plate-fin heat exchangersCProceedings of XVInternational Congress of Refrigeration，1979 (1): 421-4295 PRASAD B S V Fin efficiency and the mechanisms ofheat exchange in multi-stream plate-fin heat exchangers:formulation JInternationalJournal of Heat and MassTransfer，1996，39 (2): 419-4286刘景成等板翅换热器导流结构非线性映射与性能多目标优化N化工学报， 2015-02-12 (C1)7孙石桥大型空分设备工程设计探讨N杭氧科技，2013-12-31(C2)8王婵等板翅式换热器的散热性能研究N广东化工，2014-01-15(B1)4Opportunities for heat exchanger applications inenvironmental systemsR.K. Shaha, B. Thononb,*, D.M. BenforadocaDelphi Harrison Thermal Systems, GM, Lockport, NY 14094-1896, USAbCEA-Grenoble, DTP/GRETh, 38054, Grenoble Cedex 9, Francec3M, Retired, Environmental Engineering, 7100 Glenross Road, Woodbury, MN 55125, USAReceived 24 October 1998; accepted 9 May 1999AbstractThere is a worldwide interest in using pollution prevention methods to eliminate or lessen air, water, landand thermal pollution problems. Pollution prevention is designing processes that do not create pollution inthe first place. Heat exchangers play an essential role in pollution prevention and in the reduction ofenvironmental impact of industrial processes, by reducing energy consumption or recovering energy fromprocesses in which they are used. They are used: (1) in pollution prevention or control systems that decreasevolatile organic compounds (VOCs) and other air pollutant emissions; (2) in systems that decreasepollutants in wastewater discharges, the amount of the discharge and thermal pollution; and (3) used torecover energy in facilities that incinerate municipal solid waste and selected industrial hazardous wastes.Heat exchangers are also used in the heating, cooling and concentration of process streams that are part ofmany other pollution prevention or control related processes. In this paper, first presented is backgroundinformation on the role of heat exchangers, their types, and a discussion of environment pollutionproblems. Next, the role of heat exchangers is outlined in the prevention and mitigation of the followingpollution problems: air pollution from VOCs, sulphur oxides (SOx), nitrogen oxides (NOx); waterpollution from industrial processes, thermal pollution, and land pollution resulting from municipal solidwastes or industrial hazardous wastes. Specific Research and Development needs for environmental heatexchangers are then summarized in the paper. It is hoped that this paper will challenge the heat transferengineering community to further enhance the role of heat exchangers for pollution prevention andglobal sustainable development. # 2000 Elsevier Science Ltd. All rights reserved.Keywords: Heat exchanger; EnvironmentApplied Thermal Engineering 20 (2000) 6316501359-4311/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.PII: S1359-4311(99)00045-9/locate/apthermeng* Corresponding author. Tel. +33-476-883-079; fax. +33-476-885-172.E-mail address: thonondtp.cea.fr (B. Thonon).1. BackgroundHeat exchangers have found wide applications in pollution control facilities designed tomitigate various types of air, water, land or thermal pollution problems. In the past, heatexchangers have been used to help justify the use of traditional end-of-the-pipe pollutioncontrol equipment because they help minimize operating costs.Intherecentyears,heatexchangershavebecomeimportantinimplementingtheconcept of Pollution Prevention. The prevention approach is preferred because it is along-term solution to pollution problems. For Pollution Prevention, the preferred hierarchyfor waste management consists of four steps: (1) generation prevention, (2) recovery andrecycling, (3) treatment to reduce volume and toxicity, and (4) proper disposal of anyresidual waste (which cannot be prevented, recycled, or treated). In Pollution Prevention,engineers are challenged to innovatively develop new processes which do not generatepollution in the first place, or which make better use of available resources 32. Heatexchangers play an important role in helping to make these new processes economicallyfeasible 33.Heat exchangers will continue to play an important role in the foreseeable future inenvironmentalmanagement.Todaygovernmentandbusinessleadersworldwidearecommitted to a relatively new concept called Sustainable Development, which is defined asmeeting the needs of the present without compromising the ability of future generations tomeet their own needs 1. The concept of Sustainable Development recognizes that economicgrowth and environmental protection are inextricably linked goods and services must beprovided in an eco-e?cient manner while progressively reducing environmental impactand resource intensity throughout their life cycle. Heat exchangers can help reduce theenergy intensity of goods and services. Heat exchangers allow for the utilization of wasteheat, processes to be integrated to reduce energy requirements, and energy to be exchangedbetween processes 2.2. Heat exchanger typesA heat exchanger is a device that is used for transfer of thermal energy (enthalpy) betweentwo or more fluids at di?erent temperatures and in thermal contact. Typical applicationsinvolve heating or cooling of a fluid stream of concern, evaporation or condensation of a singleor multicomponent fluid stream, and heat recovery or heat rejection from a system. In someheat exchangers, the fluids exchanging heat are in direct contact. In other heat exchangers, heattransfer between fluids takes place through a separating wall or into and out of a wall in atransient manner. In most heat exchangers, the fluids are separated by a heat transfer surface,and ideally they do not mix. Such exchangers are referred to as direct transfer type, or simplyrecuperators. In contrast, exchangers in which there is an intermittent heat exchange betweenthe hot and cold fluids via thermal energy storage and rejection through the exchangersurface or matrix are referred to as indirect transfer type, or simply regenerators.Combustion and chemical reaction may take place in the process in which heat exchangers areused such as in boilers, fired heaters, and fluidized bed exchangers. Mechanical devices may beR.K. Shah et al./ Applied Thermal Engineering 20 (2000) 631650632used in some exchangers such as in the scraped surface exchangers, agitated vessels and stirredtank reactors 3.Heat exchangers could be classified in many di?erent ways such as according to transferprocesses, number of fluids, surface compactness, flow arrangements, heat transfer mechanisms,type of fluids (gasgas, gasliquid, liquidliquid, gastwo-phase, liquidtwo-phase, etc.) andindustry. Heat exchangers can also be classified according to the construction type and processfunction as outlined in Fig. 1. Refer to Shah and Mueller 4 for further details.The most commonly used heat exchangers for pollution prevention and mitigation are asfollows: shell-and-tube for liquids, vaporization and condensation applications, and also forgases at high pressures and temperatures; plate-and-frame and spiral plate for liquids atrelatively low pressures and temperatures; and regenerators (rotary and fixed-matrix type) andrecuperators (prime surface, plate-fin, tube-fin type) for flue gas and other polluted gasapplicationsthatdonothaveheavyfouling.Whentheaforementionedconventionalexchangers are used for pollution prevention/mitigation applications, they do not require anyadditional special construction/design features in general. However, heat exchangers for hightemperature applications, for corrosive environment, and having dual function of catalyticaction and heat transfer need to be developed for pollution prevention applications. Forfurther details on the construction features, design methodology and operating problems forheat exchangers, see Ref. 3.Fig. 1. Classification of heat exchangers according to construction and process function.R.K. Shah et al./ Applied Thermal Engineering 20 (2000) 6316506333. Environmental pollutionEnvironmental Pollution is the release of any substance into air, water or land that isdetrimental to the quality of life. Air Pollution is the release of any potentially harmfulsubstances into the atmosphere, which endanger human health or the environment. Airpollutants can be gases, liquid droplets, particles or fibers. The role of heat exchangers toprevent or minimize air pollution from industrial processes will be discussed in this paper inmore detail. Water Pollution is the release of potentially harmful chemical, physical, orbiological substances into surface water (lakes, streams and estuaries), groundwater andoceans. Land Pollution is the release of potentially harmful solid or liquid substances into thesoil.Thermal Pollution of water is the discharge of heated water from industrial processes thatcan kill or injure aquatic organisms. Most thermal pollution come from the hot cooling waterdischarges from electric power plants, followed by that from cooling operations of industrialfacilitiessuchasmetalsmelters,processingmills,petroleumrefineriesandchemicalmanufacturing plants. Heated wastewater can harm the environment in two ways: first byraising the temperature of the receiving stream or body-of-water above the range that cansupport the aquatic habitat; and second by causing a reduction in the dissolved oxygen contentof the receiving stream, a?ecting the kinds of aquatic organisms that can live there. Steps thatcan be taken to reduce the impact of thermal pollution on the receiving body-of-water includethe following:1. Use of a holding pond large enough to dissipate the energy of the heated water into theatmosphere, prior to discharge.2. Use of an evaporative cooling tower prior to discharge.3. Use of air-cooled heat exchangers to reduce the temperature of the waste heated water priorto discharge.4. Find industrial uses for the waste heated water such as using it with heat exchangers toprovide heat for buildings or other processes 5.Thermal pollution of air from industrial stack emissions is not a practical problem since it doesnot harm natural habitat. However, if the exhaust air (gas) temperature from the industrialstack is su?ciently high, it is quite common to employ some waste heat recovery exchangers torecover thermal energy and save the fuel cost 6. In some cases, in colder climates, solid wasteincinerators need to reheat the stack gases in the winter to prevent the creation of fog bycondensation of water vapor to avoid peoples misconception that the emissions are pollutingthe surrounding environment.In many applications, both material and thermal pollution of air and water is founded suchas hot dirty air or water streams.4. Air pollution prevention from VOCsVolatile organic compounds (VOCs) are precursors (when combined with NOx) to the ozoneproblem. Some of the VOCs (or e?uent contaminants and odor) are: organic solvents,R.K. Shah et al./ Applied Thermal Engineering 20 (2000) 631650634phenols, aldehydes, oil mists, phthalic anhydride, sulphides, mercaptans, odors, and sewagegases.SomeoftheapplicationsinwhichVOCsgeneratedareasfollows:generalmanufacturing (wire enameling facilities, paint bake ovens, glass fiber curing), phenols andsolvent adhesive tape and label curing ovens, sandpaper curing ovens, rubber processing,asphalt blowing, petroleum processing, paper and pulp mills, paint spraying, printing, phenoliccoating, laminating, converting, flexible packaging, oil refining, sewage plants, scrap salvaging,metal decorating, food processing and chemical processing.In order to prevent the generation of VOCs, or reduce them, an evaluation of the processmust be performed. This includes characterization of the total gas flow rate, VOCsconcentration, composition, temperature, pressure, solid suspension, humidity, etc. All theseparameters will enter in the final selection of the technical solution. There is no universalprocess for the VOCs treatment, but rather a combination of two or more individualtechniques. Five main methodologies can be identified (see Fig. 2) for the VOCs prevention ortreatment: biofiltration, high temperature oxidation, adsorption, absorption and condensation.The last three technologies allow recovery and possible recycling of the VOCs. For a morecomprehensive description of these techniques, refer to Corbitt 7, Ruddy and Caroll 8 or LeCloirec 9.Heat exchangers are directly involved in condensation processes and are used for energyrecovery as services for cooling/heating the streams in all other methods used for the VOCsremoval. In the following subsections, we will describe the function of these heat exchangers inthe processes, and the main parameters that a?ect their selection. From an economicviewpoint, the annual operating cost of the process (electricity, gas, nitrogen, etc.) ranges from5 to 30% of the total investment cost of the process equipment (that include heat exchangers,fans, controls, burners, etc.). Therefore, adopting compact heat exchangers and enhancedFig. 2. Range of operating conditions for various VOCs treatments.R.K. Shah et al./ Applied Thermal Engineering 20 (2000) 631650635technologies is cost e?ective, as they have lower energy consumption than conventional heatexchangers. The payback time can be less than one year for such enhanced equipment.Furthermore, the reduced volume of the equipment will allow developing skidmounted units.4.1. BiofiltrationBiofiltration is a relatively new technology, which uses the microorganisms in compost todestroy objectionable pollutants in exhaust air from a facility. In applying the technology, theexhaust air is passed through a biofilter system filled with a medium that is essentially the sameas a gardeners compost pile. The microorganisms in the medium decompose the pollutants inthe exhaust, similar to the action that breaks down composted leaves and grass clippings into asoil conditioner.The biofilters action requires no energy other than electricity to operate the fan, whichcirculates the exhaust air through the system. If successful this can be a considerable advantageover a conventional system such as a thermal oxidizer, which requires non-renewable resources.While several hundreds of these compost technology systems are already in use in Europe,experience has been limited and variable in the United States. An attempt, a few years ago, byone company to use a biofilter to control odorous solvent vapors from a manufacturingoperation was not successful.Biofilters are sensible to temperature, on one hand, the rate of reaction increases with thetemperature and on the other the solubility of the VOC decreases. Furthermore, if thetemperature of the biofilter remains too longer, the active microorganism can be a?ected andloose its e?ciency. In most of the bio-reactors the temperature is kept between 108C and 408C.In case that the reaction is exothermic it may be required to cool down the air before blowing.The most simple technique used is to spray water, which has the double advantage of coolingthe air stream and of increasing the humidity (which is favorable for biodegradation). In wintertime, it may be necessary to heat the air stream. Heat exchangers can be used in theconditioning of the air stream depending upon the economics of the system. The technology ofthese heat exchangers should be similar to those used for controlling humidity and temperaturein greenhouses, where low cost and corrosion resistance heat exchangers are required. Plasticheat exchangers are suitable for such heat exchangers.4.2. High temperature oxidationHigh temperature oxidation is an air pollution control process in which organic waste gasesand organic particulates are converted to odorless carbon dioxide and water vapor, with ane?ciency of 99% or greater. The contaminants are destroyed by exposure of the waste gases tothe proper conditions of temperature, time and turbulence in a combustion chamber.Oxidation temperatures range mainly from 315 to 9808C (60018008F). Some inorganiccompounds such as hydrogen sulphide, ammonia, and cyanides can also be destroyed by hightemperature oxidation, but there is a limit on the concentrations. This is because, thesecompounds are converted to their oxides by oxidation, which can be objectionable inthemselves, depending on the concentration.Compounds, which cannot be satisfactorily controlled by high temperature oxidation alone,R.K. Shah et al./ Applied Thermal Engineering 20 (2000) 631650636are waste gases containing halogens or phosphates. When halogens are oxidized, the reactionproducts include free halogens (fluorine, chlorine, bromine or iodine), halogen acids, phosgene,etc., all of which are toxic, and must be removed by chemical scrubbing/absorption 10.Because of the excessive energy required for proper high temperature oxidation, heatrecovery equipment (recuperator/regenerator) is essential to reduce fuel costs and make thepollution control process economically feasible.There are two high temperature oxidation methods of eliminating the VOCs in the exhaustair/fumes: thermal oxidation and catalytic oxidation. Under certain special conditions, theoxidation process can be designed to be self-sustaining. Thermal oxidation is the predominanttechnology used today. In this process, the VOCs are oxidized to carbon dioxide and watervapor at temperatures of 7609808C (140018008F). In catalytic oxidation, the polluted air ispassed through a catalytic bed at potentially lower temperatures of 3155408C (60010008F) tooxidize the VOCs. Several types of catalytic materials are used and their selection depends onthe VOC composition. Catalytic oxidation o?ers the potential advantage of operation at lowertemperatures and lower costs. However, a very careful evaluation of the process must be made,before using catalyst systems. This is because catalyst beds are easily poisoned by variouscontaminants and inorganic particulates that may be present in the waste gases. Catalystsystems are generally not recommended for variable processes where the contaminants canchange radically depending on the products being manufactured.Thermal oxidizers, which utilize regenerative heat exchanger (a fixed-bed regenerator), can bedesigned to be self-sustaining, depending on the conditions of the VOC process to becontrolled. The oxidation process is exothermic. Hence, once the VOC waste gases are heatedto a temperature, at which the oxidation reaction becomes self-sustaining in the regenerator,there is no additional heat needed to maintain the temperature required for oxidation of theexhaust air containing VOCs. High temperature oxidation can also e?ectively eliminate someof the odorous inorganic combustibles, such as ammonia or hydrogen sulphide, depending onthe concentrations.Commercial technology is available today for self-sustaining thermal oxidizers using a fixed-bed regenerative heat exchanger, to destroy more than 99% of VOCs in exhaust air. Theresidual organic content can be reduced to less than 20 mg/m3(1.25 ? 10?6lbm/ft3) undernormal operating conditions, meeting the most stringent air pollution control regulations.However, the actual amount of residual VOCs depends upon the kinds of VOCs beingdestroyed, their concentrations, and the operating temperature of the oxidizer, 7008C (13008F)and above. When applicable, this self-sustaining method reduces an additional source of airpollution by minimizing the amount of fossil fuels required, compared with a conventionaloxidizer.The resultant clean exhaust gases from thermal or catalytic oxidizers are at very hightemperature, and contain a significant amount of useful thermal energy. In order to reduce thefuel cost, it is essential to use heat exchangers to recover heat from the high temperatureexhaust clean air coming out of the oxidizer/regenerator. Primary heat exchangers are used torecover this heat by preheating the incoming polluted air using the high temperature exhaustclean air with heat recovery e?ciency up to 6075% depending upon the application.Sometimes a secondary heat exchanger is also used to recover additional heat from the exhaustair which can be used to heat water, generate steam or heat makeup air for other processes. InR.K. Shah et al./ Applied Thermal Engineering 20 (2000) 631650637some applications, waste heat recovery boiler is used as primary exchanger to recover heat bygenerating steam. In the case of a catalytic oxidizer, waste heat recovery may be justified, if theoperating temperature is higher than 3758C (7008F).Heat exchangers, used with any of the VOC oxidation processes, operate at hightemperatures, and hence they must be designed properly: (1) by using appropriate materials forheat transfer surfaces so that they last the design life, (2) to reduce di?erential thermalexpansion between tubes and tubesheet, and (3) to allow for the thermal displacement of thecore to the casing by using appropriate expansion devices. The most common heat exchangersused with the VOCs removal process are recuperative tubular exchangers with natural gas orpropane thermal oxidizers and regenerative fixed-bed regenerators using a gravel bed (whichalso acts as an oxidizer). While currently plain tube shell-and-tube exchangers made fromdi?erent grades of stainless steel (304, 316, refractory steel, Corten) are used for the VOCsremoval, enhanced corrugated tubes are being considered for the application 11. For coste?ective treatments, Straitz 12 suggests to use plate-and-frame heat exchangers to preheat thegas before combustion or to install a boiler before the stack if steam is required on site.In the fixed-bed regenerative heat exchanger, the heat transfer surface is made of gravel,sand or rocks. Gas-fired burners or resistance heating through an embedded electric wireinitially raise the bed temperature to the self-sustaining oxidation temperature of the VOCs(typically 9808C or 18008F). Once the temperature of the bed is raised to the right temperaturelevel, the auxiliary heat supply is terminated unless the temperature falls below 8708C (16008F).The temperature of the incoming polluted air is increased as it passes through the regenerator,reaches the self-sustaining temperature and oxidizes all VOCs, and then cools down in the restof the regenerator. The net e?ect is its temperature rise by about 258C (458F) from the inlet tothe exit of the regenerator. In order to maintain the proper temperature distribution in theregenerator bed, the direction of air is changed in the regenerator at a regular time interval,typically every 4 min 13. The next generation regenerators replace gravel, sand or rocks withstructured monolithic extruded triangular or rectangular flow passage ceramic matrix, similarto that used in rotary regenerator applications. While the cost of such a regenerator is high,the pressure drop is substantially reduced. Such regenerative heat exchangers are already usedin the glass industry 14.4.3. Adsorption using activated carbonAny gas or vapor will adhere to some degree to any solid surface. Adsorption is useful as ameans of concentrating gaseous contaminants, such as VOCs from an airstream, facilitatingtheir recovery. In activated carbon adsorption processes, heat exchangers are not directlyinvolved in the treatment, but are essentially used as external devices for heating or cooling thefluids before or after the column. Furthermore, in these treatments, there is a need toregenerate the activated carbon material, which adsorbs the VOCs, and often a cyclic processor an external regeneration system is adopted. In the case, where the solvent is immiscible inwater, the most common desorption technique is using superheated steam or hot water. Thesteam is then condensed and the VOC is separated in a phase separator by gravity. To reducethe steam consumption and increase the process e?ciency, heat exchangers are used ascondensers,reboilersandliquidliquidexchangers.Insomecases,heatexchangersR.K. Shah et al./ Applied Thermal Engineering 20 (2000) 631650638incorporated in such processes need to handle solvent or corrosive fluids and are either in highquality metallic materials or non-metallic materials (plastic or graphite) 10.4.4. AbsorptionContaminant removal by absorption scrubbing depends on the greater solubility of thecontaminant in a selected scrubbing solution than in the contaminant carrying airstream.Chemical scrubbers have been used e?ectively to control low concentrations of odorousorganic emissions including VOCs. Chemical scrubbing solutions used include potassiumpermanganate, sodium hydroxide or other oxidizing agents. The gas scrubber most widely usedby chemical engineers is the countercurrent packed tower, in which the scrubbing liquid isintroduced at the top, and trickles down through a packed bed designed to expose an extendedliquid surface in thin films to the contaminants in the rising gas stream 10. The spent solutionis then sent to waste water treatment facility.Similar to adsorption processes, there is a need to regenerate the chemical scrubbing solutionwhich is liquid, and a continuous regeneration can be envisioned (for example, see Silverberg15 ). After the absorption column, the liquid is fed in a desorption column which operates ata lower pressure than the absorption column. To reduce the energy consumption, heatexchangers are installed at the inlet and outlet of the columns to increase the absorption anddesorption processes 34.4.5. CondensationThis process consists in cooling the polluted gas to such an extent that the partial pressure ofthe VOCs is equal to their saturation pressure. As a result, the VOCs are subject to partialcondensation and are recovered in a liquid phase. However, all VOCs present in the gas cannotbe eliminated by this technique. This is because as the vapor concentration decreases, thesaturation temperature decreases and is limited by the temperature of the cold source.Two main processes are considered: in the first case, only the condensation process issu?cient to insure an e?cient VOC treatment. For instance, a temperature below ?808C(?1108F) is required for the solvents (see Table 1) and requires a cryogenic cold source. Forless volatile compounds, more conventional refrigeration cycles can be used. In the secondcase, firstly the condensation process insures a pre-treatment of the e?uent and then a secondtechnique is applied (thermal oxidation, absorption or adsorption). In this case, the level oftemperature of the condensation process is optimized by taking into account the second stagetreatment.The required condensation temperature is calculated from the vapor equilibrium of di?erentcomponents, and requires the knowledge of the VOC composition. For binary or ternarymixtures, the equilibrium diagram can be easily established applying semi-empirical methods.For a more complex mixture or a more precise evaluation, advanced methods must be applied16.Basically to condense a fluid, either its temperature is decreased or its pressure is increased.For example, consider a saturated e?uent of air and toluene. The concentration of toluene at atotal pressure of 1 bar and 208C is 109 g/m3, at 1 bar and ?208C is 8.3 g/m3(see Table 1), andR.K. Shah et al./ Applied Thermal Engineering 20 (2000) 631650639at 10 bars and 208C is 10.9 g/m3. Thus, to obtain the final required concentration, severalcombinations of pressure and temperature are available. From a more practical point of view,several stages of condensation process are adopted. In the first stage, the cooling medium forthe e?uent is ambient air or industrial water supply. The second stage uses a cooling mediumbetween 08C and ?308C (328F and ?228F) produced by a refrigeration unit. In the final stage,a cryogenic cooling medium (liquid nitrogen) must be used. At each stage of the process,condensate is recovered and this cascade of condensers acts as partial distillation process.Three main problems may arise in condensation processes1. Formation of frost at the wall, which leads to partial or complete blockage of the channels.This situation implies a regular cleaning procedure.2. Formation of the crystal of a VOC when the condensate temperature is such that themelting point of one compound is reached. This situation also implies a cleaning procedure.3. Formation of fog rather than a liquid film. The transport of small droplets of liquid withinthe air requires an e?cient separation device between di?erent condensation stages.Furthermore, severe variations of the flow rate or VOCs concentration may increase one ofthe aforementioned problems 17.5. Removal of SOxIn the thermal power plants that use fossil fuels as the heat source, the flue gas coming outof the steam generator usually goes through the economizer and air preheater, and thenTable 1VOC concentration in g/m3for a saturated e?uent at 1 bar total pressureFluidTemperature (8C)40200?20?40?60?80?100?120?140?160?180Chlorinated compoundsTrichlorethylene98038512536Chloroform23391026438122305.5Chlorethylene76723855705020MethyleneChlorine28241264530162438.51.2HydrocarbonPropylene3050.15O-Xylene88287.31.4Toluene295109338.31.530.190.01Alcohol1-Propanol171491120.260.02Isopropanol340103254.70.640.06Methanol4631705213.52.660.380.03Ethanol314110326.6R.K. Shah et al./ Applied Thermal Engineering 20 (2000) 631650640exhausted to the ambient. The thermal power plant boiler e?ciency is limited by theconventional economizers and air preheaters operating above the acid (SO2and SO3) dewpoint to avoid cold-end corrosion. In the past, recovering waste heat from the flue gas waseconomically viable above the acid dew point of the gas (1161638C or 2403258F) dependingupon the sulphur content of the fuel. To clean the hot gas in conventional processes, a quenchis installed on the hot fumes (3008C or 5758F) that cools the fumes below 708C (1608F) beforepassing through stripping columns. If a heat exchanger can be designed to condense out theacid (SO3and water), a significant thermal energy can be recovered from the flue gas (with theexhaust temperatures of 38498C or 1001208F to the stack) with a reduction of up to about7% fuel consumption. At the same time, releasing the exhaust gas at a much lowertemperature reduces thermal pollution of the atmosphere. In addition, a continuous rain ofcondensate in the exhaust gas (due to gas side heat exchanger surfaces well below the waterdew point) cleans the particulates from the flue gas, just the way rain cleans the atmosphere.Thus, by using a condensing heat exchanger after the air preheater, sulphur and particulateemissions are reduced, thermal pollution is reduced, and the thermal power plant e?ciency isimproved 18.A condensing heat exchanger is a shell-and-tube unit with Teflon coated (0.38 mm or 0.015in. thickness) tubes, and Teflon-to-Teflon seals between the shell and tubes. Thus, it is inert toacid corrosion, and hence the outlet temperature of the exhaust gas can be reduced to the limitimposed by the sink temperature. In this exchanger, the feed water flows through the tubes andthe exhaust gas over the tube array on the shell side. The condensate flows downward throughthe tube arrays and leaves from the bottom drain carrying with it particulates and acidsscrubbed from the gas, and washed from the tubes. Thus, this exchanger recovers both thesensible and latent heat (from the condensation of water vapor in the flue gas) and removessubmicron particulates and reduces other emissions. Full teflon heat exchangers could also bemanufactured, but in most of the cases, due to cost and mechanical constraints, coating isadopted.An alternative to Teflon coating or plastic heat exchangers (that are limited in temperature)is to use graphite heat exchangers. Such heat exchangers can operate at higher temperatures(3008C or 5758F) and avoid the use of quench technologies to reduce the gas temperature.Graphite heat exchangers are either gas-to-gas or liquid-to-gas, and have a high thermale?ciency due to the good thermal conductivity of the material compared to plastics.Condensing heat exchangers are currently used in some thermal power plant and have proventheir e?ciency.6. Removal of NOxElimination of nitrogen oxides is now required for air quality control, and specific systemsare added to the more conventional heat recovery and SOxequipment. Two main NOxremoval systems are: (1) Selective Non-Catalytic Reduction (SNCR), and (2) Selective CatalyticReduction (SCR). SNCR is a method in which ammonia or urea is injected into the furnace inthe temperature window of 87012608C (160023008F) while in the SCR method, ammonia isinjected at lower temperatures of 2005508C (40010258F) in the presence of catalysts. In bothR.K. Shah et al./ Applied Thermal Engineering 20 (2000) 631650641methods, ammonia reacts selectively with NOxto reduce it to nitrogen. SCR is more e?ectivethan SNCR in the NOxreduction with minimum ammonia slip, while the capital cost is high.SNCR has lower NOxremoval e?ciency (3050%), high ammonia consumption and ammoniaslip, low capital cost, but high operating cost. Both are commonly used depending on theammonia slip requirement. Ammonia slip is the term used for the quantity of ammoniaunreacted or residual/leftover ammonia after SCR/SNCR. Under sulphur environment (such ascoal or heavy-oil fired furnace), it reacts with SO3to form ammonium sulphates/bisulphateswhich can block/plug airheater openings. Therefore, it is usually limited below 5 ppm in coal-fired applications.The SCR reactor or a container is made up of baskets of ceramic honeycomb catalyst cells(like a rotary regenerator matrix) or the plate type. Typical cells are square with the side lengthvarying from 3.7 to 7.5 mm (0.150.30 in.). Ammonia is injected through a grid in theupstream duct of an SCR system. A variety of catalysts are available for various applications.The major components of the catalysts are titanium dioxide (TiO2), tungsten trioxide (WO3),vanadium pentoxide (V2O5) and molybdenum trioxide (MoO3) with the actual compositionbeing of the proprietary nature. The NOxremoval e?ciency depends upon the applicationvarying from 42% to 90%. Some of the applications where SCR has been used are chemicalprocess industry and refinery heaters and boilers, gas turbines, and coal fired cogenerationplants 19.7. Water pollution prevention from wastewater7.1. Municipal wastewater treatmentModern wastewater treatment plants utilize anaerobic digestion techniques for biologicalstabilization of municipal sewage sludge, and consume a large amount of energy. The digestersludge temperature must be maintained within tight tolerances for proper sludge stabilization,approximately358C(958F),formesophilicdigestion.Thedigestersludgecontainsapproximately 4 wt% solids and fibers, which would plug conventional shell-and-tube andother exchangers. Spiral plate heat exchangers having a single spiral passage (the passageheight ranges from 6 to 32 mm or from 0.25 to 1.25 in., and the width ranges from 0.23 to 2.4m or from 9 to 96 in.) are ideal for this application since fouling and nonuniformity in theflows are not problems in such a heat exchanger. Spiral plate heat exchangers are also used forraw sludge pre-heating by recovering waste heat from digested sludge. This raw sludgepreheating greatly reduces the size and operating cost of the digester sludge heater. The spiralplate heat exchanger provides 60% heat recovery and can handle such solid and fiber ladenfluids.7.2. Industrial wastewater (liquid hazardous wastes)The major sources of liquid hazardous wastewater are from chemical industries, pulp andpaper industries, primary metals industries, cement industries, agro-food industries, etc. Thewastes are generated continuously (washing procedures) or incidentally (leakage, overflows,R.K. Shah et al./ Applied Thermal Engineering 20 (2000) 631650642pipe breakdown, etc.). Generally, multiple treatments are used in combination for pre-treatment of the final treatment. The most common ones are adsorption, absorption,biological, chemical, filtration, gravity separation or neutralization. These processes are eitheron-site (continuous or not) or o?-site (which implies the transportation). For safety and costreasons, it is highly desirable to reduce the amount of the liquid waste before transportation,therefore, concentration processes must be used. Heat exchangers are directly used only in theconcentration process by evaporation. However, similar to the VOCs treatments, they are usedas services for heating or cooling the fluids. Alternatively to discharging the liquid e?uent, itcan be recycled through process heat exchangers where the water will be partially evaporatedbefore reinjection in the process. This saves water a growing problem, but would needexamination of fouling prevention techniques.Concentration of liquid wastes by evaporation is widely used in industry and a large varietyof techniques are adopted. Two cases are considered: (1) the e?uent is dissolved in water (saltsolution for example); (2) the e?uent and the water act as a mixture (wateracid solutions forexample).In the first case, the concentration process is very e?cient and high concentration can beobtained. In specific cases, crystallisation of the e?uent can be achieved. Most of thetechnological developments have been obtained on the projects dealing with desalination of seawater or sugar industries.For mixtures, the liquid e?uent to be concentrated is partially evaporated in a heatexchanger, at the outlet, the vapor phase is richer with more volatile compound and the liquidphase is richer with less volatile compound. The evaporator acts as a first stage of distillation.The e?ectiveness of the evaporation process depends essentially on the mixture phaseequilibrium.Forwideboilingtemperaturerangemixtures(wateracidsolutions),theconcentration process is very e?cient as the dew and bubble curves as functions of molarcomposition are almost parallel. For fluids having low boiling temperature range, theconcentration process is less e?cient as all components are evaporated simultaneously.Consequently, the fluid temperature has to be controlled more closely, which implies a goodknowledge of the heat transfer coe?cients.Di?erent types of evaporators are used in concentration processes: (1) flash evaporationthrough a discharge valve, (2) horizontal tubular or plate reboilers (submerged or falling film),(3) vertical tubular or plate evaporators (climbing or falling film), (4) specific evaporators(direct contact, scraped surface, etc.).Flash type evaporators are mainly used for large capacities as the cost is very low, but theprocess requires superheating the fluid before the discharge valve (which is not alwaysacceptable) and also requires the hot fluid at a temperature higher than that for otherprocesses. Submerged reboilers are often used in the process industries; and if enhanced tubesare used, the temperature approach (ThotTsat) of the process can be reduced to 5108C.However, the liquid hold up on the process side is generally too high, and as a result, fallingfilm evaporators are adopted. These evaporators have very high e?ciency, but the distributionof the fluid needs to be controlled carefully and fouling fluids cannot be used. Verticalevaporators are often use in concentration processes as the e?ciency is good and the liquidholdup is low. Plate or tubular type evaporators have been used in the sugar industry and haveR.K. Shah et al./ Applied Thermal Engineering 20 (2000) 631650643proved their technical and economical feasibility 2022, and their generalization to liquidwaste concentration is under consideration.In most cases, the heat source in the evaporator is steam, which is often available on site.When steam is not available, mechanical recompression of the vapor can be used. Thistechnique is energy e?cient and adopting high performances heat exchangers and compressorcan lead to very compact units. An innovative technology using direct heating technique byJoule e?ect has been recently developed for concentration of sulphuric acid 23.8. Land pollution prevention and site remediationIn recent years, disposal problems have changed the way the nation thinks about waste.Improperly handled hazardous and solid wastes can contaminate drinking water supplies,release hazardous vapors into the air, or cause explosions. In 1991, the President of the USAissued an Executive Order on waste reduction and recycling, and the United States Congressadopted laws to accomplish, among other things, the following objectives: (1) preventpollution, (2) manage hazardous wastes, (3) address the growing volume of municipal solidwaste being generated (roughly 5 lb or 2.3 kg per person per day or 180 million tons in 1988)24.Pollution prevention e?orts are substantially reducing the volumes of solid and hazardouswaste being generated. However, all of the waste cannot be recycled or prevented from beinggenerated. Properly designed and operated solid and hazardous waste incinerators, utilizingheat recovery equipment, are helping to prevent land pollution by reducing the remainingamount of solid and hazardous waste which must be sent to landfills.Municipal Waste-to-Energy facilities, utilizing boilers to generate steam and electricity, playa very important part in reducing the impact of the solid waste problem in a beneficial,environmentally acceptable manner. There are hundreds of resource recovery waste-to-energyfacilities, which use municipal residential and commercial solid waste as a fuel for boilers tomake steam which is used to generate electricity for power companies. Combusting 1000 tonsof solid waste saves the burning of 1600 barrels of fuel oil or 500 tons of coal. The majorproblems with such an application are fouling and corrosion as well as slagging of boiler tubes,and waste heat boiler companies provide specific solutions for individual applications.Hazardous Waste Incineration is an important part of the US EPA Waste ManagementPriority Hierarchy. The hierarchy was developed in response to the Waste Minimizationrequirements by Congress in the 1984 Hazardous and Solid Waste Amendments (HSWA) tothe Resource Conservation and Recovery Act of 1976 (RCRA) 25.One of the first rotary kiln hazardous waste incinerators in the United States was built by3M at their Cottage Grove, Minnesota facility in 1971. It was initially designed to burn 90 ?106Btu/h (26.4 MW) of hazardous waste without heat recovery, and in 1983 an in-line wasteheat boiler was added. Two concerns with adding the waste heat recovery boiler were tubecorrosion and slagging. After three years of addressing these problems, an on-line percentageof 70% was reached. However, problems with slagging and high maintenance continue toplague the facility. The company plans to build a new incinerator in 1999, but do not plan toadd a waste heat boiler this time, because of its unreasonably high cost, and the potential forR.K. Shah et al./ Applied Thermal Engineering 20 (2000) 631650644continuing problems due to the variability of the waste to be burned. There is a greatopportunity for heat exchanger engineers to find better ways to build waste heat recoveryboilers for incinerators handling industrial waste.During the last 20 years, the land pollution policy has progressively shifted from determiningthe nature and extent of the contamination of each site to a remedial action, and finallypollution prevention by on-site continuous cleaning processes. The cleanup goal for each siteand contaminants depends strongly on the impact on potential receivers. For instance, themost stringent limits must be applied at sites near drinking water supply for a city. Similar tothe VOCs treatment processes, before the selection of the cleanup technology, several criteriamust be analyzed:1. What is the type of contaminants?2. What is the geologic configuration of the site?3. What are the legislative requirements?4. How much is the delay allowed before the action is taken?Once these basic questions answered, the most appropriate cleaning technology can be selected.Another important aspect to take into account is the nature of the residues and how they aregoing to be treated. Four main categories of residue treatments are physical treatments (airventing and air stripping; water, vapor or solvent washing; supercritical extraction), biologicaltreatments (bioventing; bioreactors), thermal treatments (thermal desorption; incineration), andchemical treatments (in situ oxidation; ultra-violet oxidation). Refer to Hyman and Bagaasen26 for a more complete description of the processes and how to evaluate cost e?ectiveness ofdi?erent technologies.Most of the treatments generate residues which contain the contaminants that are either invapor phase for venting and thermal desorption processes or dissolved in liquid for washingand solvent extraction. The contaminants in the vapor phase are assimilated to VOCs and theprocesses have been described in Section 5. For liquid residues, if they are immiscible, gravityphase separation devices are adopted (oilwater separation); in other cases, the contaminantsmust be either extracted or eliminated. As most of the land treatments are on-site processes,the available space is a constraint as well as the weight if the system has to be transportable. Inthe case, when the treatment cannot be carried out on site, a concentration process(evaporation or filtration) is adopted to extract the water from the solution. Afterwards, theconcentrated liquid e?uent or the solid residues are transported for final destruction inspecialized plants.9. R&D needs for environmental heat exchangers9.1. Compact and high performance heat exchangersIncreasing thermal performance of a heat exchanger intrinsically contributes to the reductionof its environmental impact by reducing the energy consumption. Furthermore, reducing theheat transfer surface requirement also contributes to reduce the overall environmental impactconsidering the life cycle of the product and the energy consumption for manufacture of theR.K. Shah et al./ Applied Thermal Engineering 20 (2000) 631650645raw material. This is especially true for high quality materials for corrosive or hightemperature applications. These two aspects emphasize the potential important role of compactheat exchangers and enhancement technologies with environmental processes. However, designand optimization of environmental exchangers are often neglected since they are not the keycomponents of the processes. But, as indicated in the previous sections, increasing heatrecovery (thus reducing the energy consumption) and recovering some of the exhausts (VOCsor acids) as liquids are the possibilities to reduce the payback time for exchangers. In order toadopt compact and high performance heat exchangers in environmental processes, there is aneed to prove that such heat exchangers are technically suitable and are cost e?ective. TheR&D actions should focus on pilot scale demonstration projects in order to validate sizingmethods and to obtain experimental results under industrial operating conditions.9.2. Phase change heat transferHeat exchangers employed in single-phase applications are not posed with any specialproblems when used in environmental processes. However, for phase-change (vaporization orcondensation) heat exchangers, the general predictive methods developed in the process andrefrigeration industry cannot be applied directly. Two critical aspects must be studied: (1) thephysical properties of the e?uent and more precisely the dew and bubble point curves(temperature versus concentration for mixtures), and (2) the heat transfer correlations forvaporization and condensation. For example, most of the correlations available for enhancedheat exchangers have been obtained with pure refrigerants and refrigerants mixtures whichhave generally a low boiling temperature range (less than 108C or 208F). The extension ofthese correlations to other operating conditions and fluids is doubtful and there is a need toestablish and validate new design methods. For boiling and evaporation, viscous fluids andfluids with wide boiling temperature ranges should be studied, and industrial e?uent should beused rather than simulation fluids. During condensation, heat and mass transfer aspects are ofequal importance.9.3. High temperature applicationsThe developments of thermal oxidation techniques and high e?ciency burners require heatexchangers that can operate at high temperatures. For temperatures higher than 10008C(18008F), ceramic material is the only technological solution for durable heat exchangers. Analternative is the development of heat exchanger core/matrix made up of composite materialsof metal with ceramics brazed/coated for high performance (high e?ectiveness) heat exchangers27,28. Currently, most of ceramic heat exchangers for high temperature applications are notenhanced and have a rather low thermal e?ciency compared to heat exchangers that operate atlower temperatures. The reason is that the R&D e?orts have been focused on the mechanicaldesign rather than the thermal-hydraulic design. Now, there is a need to develop more e?cienthigh temperature heat exchangers having enhanced heat transfer surfaces.R.K. Shah et al./ Applied Thermal Engineering 20 (2000) 6316506469.4. Fouling of heat exchangersFoulingishighlydetrimentaltothermal-hydraulicperformance,andmustbewellunderstood if the heat exchange capability of practical equipment needs to be accuratelypredicted. In water-cooling or liquid-waste treatments, particulate and precipitation foulingmechanisms are frequently responsible of the decrease of heat transfer performance. Enhancedheat transfer surfaces provide higher heat transfer coe?cients than conventional plain tubes,but could be more sensitive to fouling. Furthermore, the fouling margin implies an extrasurface, which generally costs more than that for plain stainless steel or copper tubes. As aresult, some specific recommendations need to be given for both fouling resistance values andoperating conditions. Mitigation of fouling of enhanced heat exchangers must be taken intoaccount at the design stage, and should include several factors such as the operatingconditions, transient and start-up procedures, maintenance capabilities, etc. Thus, factorsa?ecting the fouling rate must be studied for di?erent enhanced geometries (low-finned tubes,structured surfaces, plate heat exchangers, etc.) under controlled flow and fouling conditions.The present knowledge on fouling and complexity of the mechanisms encountered do not allowthe development of predictive tools which could be incorporated in the process control systems,hence long-term studies should be devoted to this topic. Equally, the emphasis should beplaced on improving the foulant technology such as better soot blower or sonic technologies,etc.9.5. Heat exchangers for corrosive fluidsIn many cases, the e?uents to be treated are corrosive and the equipment installed needs toresist corrosion from aggressive chemicals. Four corrosion resistance materials can beidentified: (1) special high quality metals, (2) ceramics, (3) plastics, and (4) graphite. Theselection of the material depends on the operating pressure and temperature. For metallic heatexchangers, the main limitations come from the cost of the raw material and the cost ofenhancementtechniquethatdonotallowimplementationofhighperformancemoresophisticated surface enhancement. Ceramic-based heat exchangers are not very common andare generally used only for high temperature applications. However, the development of newmanufacturing techniques may reduce the cost of these heat exchangers resulting in increasedapplications at lower temperatures. Graphite is not a new material for heat exchangers and isalready used in many industrial processes. Most of graphite heat exchangers are of the shell-and-tube type design, but recently manufacturers of plate heat exchangers have proposed platetype heat exchangers. As graphite is quite expensive, enhancement of surfaces and plate typeheat exchangers should be developed. Teflon coated heat exchangers are plain tubular type; thetechnology should be developed for Teflon coated finned tube and compact heat exchangerssince Teflon is also quite expensive. Nevertheless, coating of heat exchangers frequently leadsto corrosion via small holes, and finned surface will be even more likely to create problems.Alternative coating techniques (metal deposition or surface treatment) should also bedeveloped. Plate heat exchangers made of aluminium and coated with stainless steel are alreadyused in the agro-food industry, and the development of Titanium coating should be allowed touse these heat exchangers for corrosive applications. The major limitations of plastic heatR.K. Shah et al./ Applied Thermal Engineering 20 (2000) 631650647exchangers are their low resistance to higher temperatures and pressures, and low thermalconductivity that yields low overall heat transfer coe?cients due to their high wall thermalresistance. Most of the recent developments on plastic heat exchangers have employed newermaterials such as liquid crystal polymers 29 or Poly-Ether-Ether-Keton 30, and are all primesurface (plate type) heat exchangers. Nearly all of the studies have been carried out undersingle-phase flow (gas or liquid) but applications with phase-change heat transfer are arisingand should provide encouraging results.9.6. Combined heat and mass transferWhile dealing with mixtures, heat and mass transfer occur simultaneously, and the processcould be either mass transfer limited or heat transfer limited. Heat and mass transfer has beenextensively studied in plain geometries, and predictive models are available. But for enhancedgeometries or compact heat exchangers, there is a lack of basic information on heat and masstransfer under actual flow conditions. Studies have been carried out using refrigerant mixturesbutthephysicalpropertiesandtheoperatingconditionsarenotrepresentativeofenvironmental processes.10. Concluding remarksIn this paper, starting with the types of pollution and heat exchangers, the role of heatexchangers is discussed to prevent or mitigate air, water,
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