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Opportunities for heat exchanger applications in environmental systems R K Shaha B Thononb D M Benforadoc aDelphi Harrison Thermal Systems GM Lockport NY 14094 1896 USA bCEA Grenoble DTP GRETh 38054 Grenoble Cedex 9 France c3M Retired Environmental Engineering 7100 Glenross Road Woodbury MN 55125 USA Received 24 October 1998 accepted 9 May 1999 Abstract There is a worldwide interest in using pollution prevention methods to eliminate or lessen air water land and thermal pollution problems Pollution prevention is designing processes that do not create pollution in the fi rst place Heat exchangers play an essential role in pollution prevention and in the reduction of environmental impact of industrial processes by reducing energy consumption or recovering energy from processes in which they are used They are used 1 in pollution prevention or control systems that decrease volatile organic compounds VOCs and other air pollutant emissions 2 in systems that decrease pollutants in wastewater discharges the amount of the discharge and thermal pollution and 3 used to recover 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 of many other pollution prevention or control related processes In this paper fi rst presented is background information on the role of heat exchangers their types and a discussion of environment pollution problems Next the role of heat exchangers is outlined in the prevention and mitigation of the following pollution problems air pollution from VOCs sulphur oxides SOx nitrogen oxides NOx water pollution from industrial processes thermal pollution and land pollution resulting from municipal solid wastes or industrial hazardous wastes Specifi c Research and Development needs for environmental heat exchangers are then summarized in the paper It is hoped that this paper will challenge the heat transfer engineering community to further enhance the role of heat exchangers for pollution prevention and global sustainable development 2000 Elsevier Science Ltd All rights reserved Keywords Heat exchanger Environment Applied Thermal Engineering 20 2000 631 650 1359 4311 00 see front matter 2000 Elsevier Science Ltd All rights reserved PII S1359 4311 99 00045 9 Corresponding author Tel 33 476 883 079 fax 33 476 885 172 E mail address thonon dtp cea fr B Thonon 1 Background Heat exchangers have found wide applications in pollution control facilities designed to mitigate various types of air water land or thermal pollution problems In the past heat exchangers have been used to help justify the use of traditional end of the pipe pollution control equipment because they help minimize operating costs Intherecentyears heatexchangershavebecomeimportantinimplementingthe concept of Pollution Prevention The prevention approach is preferred because it is a long term solution to pollution problems For Pollution Prevention the preferred hierarchy for waste management consists of four steps 1 generation prevention 2 recovery and recycling 3 treatment to reduce volume and toxicity and 4 proper disposal of any residual waste which cannot be prevented recycled or treated In Pollution Prevention engineers are challenged to innovatively develop new processes which do not generate pollution in the fi rst place or which make better use of available resources 32 Heat exchangers play an important role in helping to make these new processes economically feasible 33 Heat exchangers will continue to play an important role in the foreseeable future in environmentalmanagement Todaygovernmentandbusinessleadersworldwideare committed to a relatively new concept called Sustainable Development which is defi ned as meeting the needs of the present without compromising the ability of future generations to meet their own needs 1 The concept of Sustainable Development recognizes that economic growth and environmental protection are inextricably linked goods and services must be provided in an eco e cient manner while progressively reducing environmental impact and resource intensity throughout their life cycle Heat exchangers can help reduce the energy intensity of goods and services Heat exchangers allow for the utilization of waste heat processes to be integrated to reduce energy requirements and energy to be exchanged between processes 2 2 Heat exchanger types A heat exchanger is a device that is used for transfer of thermal energy enthalpy between two or more fl uids at di erent temperatures and in thermal contact Typical applications involve heating or cooling of a fl uid stream of concern evaporation or condensation of a single or multicomponent fl uid stream and heat recovery or heat rejection from a system In some heat exchangers the fl uids exchanging heat are in direct contact In other heat exchangers heat transfer between fl uids takes place through a separating wall or into and out of a wall in a transient manner In most heat exchangers the fl uids are separated by a heat transfer surface and ideally they do not mix Such exchangers are referred to as direct transfer type or simply recuperators In contrast exchangers in which there is an intermittent heat exchange between the hot and cold fl uids via thermal energy storage and rejection through the exchanger surface 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 are used such as in boilers fi red heaters and fl uidized bed exchangers Mechanical devices may be R K Shah et al Applied Thermal Engineering 20 2000 631 650632 used in some exchangers such as in the scraped surface exchangers agitated vessels and stirred tank reactors 3 Heat exchangers could be classifi ed in many di erent ways such as according to transfer processes number of fl uids surface compactness fl ow arrangements heat transfer mechanisms type of fl uids gas gas gas liquid liquid liquid gas two phase liquid two phase etc and industry Heat exchangers can also be classifi ed according to the construction type and process function 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 as follows shell and tube for liquids vaporization and condensation applications and also for gases at high pressures and temperatures plate and frame and spiral plate for liquids at relatively low pressures and temperatures and regenerators rotary and fi xed matrix type and recuperators prime surface plate fi n tube fi n type for fl ue gas and other polluted gas applicationsthatdonothaveheavyfouling Whentheaforementionedconventional exchangers are used for pollution prevention mitigation applications they do not require any additional special construction design features in general However heat exchangers for high temperature applications for corrosive environment and having dual function of catalytic action and heat transfer need to be developed for pollution prevention applications For further details on the construction features design methodology and operating problems for heat exchangers see Ref 3 Fig 1 Classifi cation of heat exchangers according to construction and process function R K Shah et al Applied Thermal Engineering 20 2000 631 650633 3 Environmental pollution Environmental Pollution is the release of any substance into air water or land that is detrimental to the quality of life Air Pollution is the release of any potentially harmful substances into the atmosphere which endanger human health or the environment Air pollutants can be gases liquid droplets particles or fi bers The role of heat exchangers to prevent or minimize air pollution from industrial processes will be discussed in this paper in more detail Water Pollution is the release of potentially harmful chemical physical or biological substances into surface water lakes streams and estuaries groundwater and oceans Land Pollution is the release of potentially harmful solid or liquid substances into the soil Thermal Pollution of water is the discharge of heated water from industrial processes that can kill or injure aquatic organisms Most thermal pollution come from the hot cooling water discharges from electric power plants followed by that from cooling operations of industrial facilitiessuchasmetalsmelters processingmills petroleum refi neriesandchemical manufacturing plants Heated wastewater can harm the environment in two ways fi rst by raising the temperature of the receiving stream or body of water above the range that can support the aquatic habitat and second by causing a reduction in the dissolved oxygen content of the receiving stream a ecting the kinds of aquatic organisms that can live there Steps that can be taken to reduce the impact of thermal pollution on the receiving body of water include the following 1 Use of a holding pond large enough to dissipate the energy of the heated water into the atmosphere 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 prior to discharge 4 Find industrial uses for the waste heated water such as using it with heat exchangers to provide heat for buildings or other processes 5 Thermal pollution of air from industrial stack emissions is not a practical problem since it does not harm natural habitat However if the exhaust air gas temperature from the industrial stack is su ciently high it is quite common to employ some waste heat recovery exchangers to recover thermal energy and save the fuel cost 6 In some cases in colder climates solid waste incinerators need to reheat the stack gases in the winter to prevent the creation of fog by condensation of water vapor to avoid people s misconception that the emissions are polluting the surrounding environment In many applications both material and thermal pollution of air and water is founded such as hot dirty air or water streams 4 Air pollution prevention from VOCs Volatile organic compounds VOCs are precursors when combined with NOx to the ozone problem Some of the VOCs or e uent contaminants and odor are organic solvents R K Shah et al Applied Thermal Engineering 20 2000 631 650634 phenols aldehydes oil mists phthalic anhydride sulphides mercaptans odors and sewage gases SomeoftheapplicationsinwhichVOCsgeneratedareasfollows general manufacturing wire enameling facilities paint bake ovens glass fi ber curing phenols and solvent adhesive tape and label curing ovens sandpaper curing ovens rubber processing asphalt blowing petroleum processing paper and pulp mills paint spraying printing phenolic coating laminating converting fl exible packaging oil refi ning 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 process must be performed This includes characterization of the total gas fl ow rate VOCs concentration composition temperature pressure solid suspension humidity etc All these parameters will enter in the fi nal selection of the technical solution There is no universal process for the VOCs treatment but rather a combination of two or more individual techniques Five main methodologies can be identifi ed see Fig 2 for the VOCs prevention or treatment biofi ltration high temperature oxidation adsorption absorption and condensation The last three technologies allow recovery and possible recycling of the VOCs For a more comprehensive description of these techniques refer to Corbitt 7 Ruddy and Caroll 8 or Le Cloirec 9 Heat exchangers are directly involved in condensation processes and are used for energy recovery as services for cooling heating the streams in all other methods used for the VOCs removal In the following subsections we will describe the function of these heat exchangers in the processes and the main parameters that a ect their selection From an economic viewpoint the annual operating cost of the process electricity gas nitrogen etc ranges from 5 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 enhanced Fig 2 Range of operating conditions for various VOCs treatments R K Shah et al Applied Thermal Engineering 20 2000 631 650635 technologies is cost e ective as they have lower energy consumption than conventional heat exchangers 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 Biofi ltration Biofi ltration is a relatively new technology which uses the microorganisms in compost to destroy objectionable pollutants in exhaust air from a facility In applying the technology the exhaust air is passed through a biofi lter system fi lled with a medium that is essentially the same as a gardener s compost pile The microorganisms in the medium decompose the pollutants in the exhaust similar to the action that breaks down composted leaves and grass clippings into a soil conditioner The biofi lter s action requires no energy other than electricity to operate the fan which circulates the exhaust air through the system If successful this can be a considerable advantage over 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 by one company to use a biofi lter to control odorous solvent vapors from a manufacturing operation was not successful Biofi lters are sensible to temperature on one hand the rate of reaction increases with the temperature and on the other the solubility of the VOC decreases Furthermore if the temperature of the biofi lter remains too longer the active microorganism can be a ected and loose 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 cooling the air stream and of increasing the humidity which is favorable for biodegradation In winter time it may be necessary to heat the air stream Heat exchangers can be used in the conditioning of the air stream depending upon the economics of the system The technology of these heat exchangers should be similar to those used for controlling humidity and temperature in greenhouses where low cost and corrosion resistance heat exchangers are required Plastic heat exchangers are suitable for such heat exchangers 4 2 High temperature oxidation High temperature oxidation is an air pollution control process in which organic waste gases and organic particulates are converted to odorless carbon dioxide and water vapor with an e ciency of 99 or greater The contaminants are destroyed by exposure of the waste gases to the proper conditions of temperature time and turbulence in a combustion chamber Oxidation temperatures range mainly from 315 to 9808C 600 18008F Some inorganic compounds such as hydrogen sulphide ammonia and cyanides can also be destroyed by high temperature oxidation but there is a limit on the concentrations This is because these compounds are converted to their oxides by oxidation which can be objectionable in themselves 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 631 650636 are waste gases containing halogens or phosphates When halogens are oxidized the reaction products include free halogens fl uorine 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 heat recovery equipment recuperator regenerator is essential to reduce fuel costs and make the pollution control process economically feasible There are two high temperature oxidation methods of eliminating the VOCs in the exhaust air fumes thermal oxidation and catalytic oxidation Under certain special conditions the oxidation process can be designed to be self sustaining Thermal oxidation is the predominant technology used today In this process the VOCs are oxidized to carbon dioxide and water vapor at temperatures of 760 9808C 1400 18008F In catalytic oxidation the polluted air is passed through a catalytic bed at potentially lower temperatures of 315 5408C 600 10008F to oxidize the VOCs Several types of catalytic materials are used and their selection depends on the VOC composition Catalytic oxidation o ers the potential advantage of operation at lower temperatures 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 various contaminants and inorganic particulates that may be present in the waste gases Catalyst systems are generally not recommended for variable processes where the contaminants can change radically depending on the products being manufactured Thermal oxidizers which utilize regenerative heat exchanger a fi xed bed regenerator can be designed to be self sustaining depending on the conditions of the VOC process to be controlled The oxidation process is exothermic Hence once the VOC waste gases are heated to 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 the exhaust air containing VOCs High temperature oxidation can also e ectively eliminate some of th