75 research outputs found

    Reducing Undesirable Powder Deposition

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    This paper describe how to reduce powder deposition

    Heat Integrated Milk Powder Production

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    Dairy processing is critical to New Zealand’s (NZ) economy producing NZ13billioninexportsfor2012whileconsuming32PJoffossilfuelsforprocessheat.ThreequartersofNZdairyexportsaremilkpowders.ThisthesispresentsmethodstoreduceprocessheatuseinMilkPowderPlants(MPP)throughimprovedheatintegrationandaddresseskeytechnicalchallengespreventingindustrialimplementation.Myoriginalcontributionstoliteratureinclude:(1)anoveldesignmethodcalledtheCostDerivateMethod(CDM)thatcostoptimallyallocatesareaindirectheatexchangenetworks,(2)anewdesignmethodologyforintegrationofsemicontinuousprocessclustersusingaHeatRecoveryLoop(HRL)withaVariableTemperatureStorage(VTS)systemforimprovedheatrecovery,(3)anexperimentallyvalidateddepositionmodelforpredictingcriticalairconditionsthatcausemilkpowderfouling,and(4)athermoeconomicassessmenttoolfortheoptimisationofindustrialspraydryerexhaustheatrecoveryprojectsviaaLiquidCoupledLoopHeatExchanger(LCHE)system.ByapplyingPinchAnalysistoanindustrialMMP,thisworkconfirmsthatheatmustberecoveredfromthemilkspraydryerexhaustair( 75°C)toachievemaximumheatintegrationinMPPs.ForstandaloneMPPsexhaustheatisbestusedtoindirectlypreheattheinletdryerairreducingsteamuseby12.7AkeybarrierpreventingexhaustheatrecoveryimplementationinNZMPPsisthepossibilityofmilkpowderfouling.Dryerexhaustaircontainsalowconcentrationofpowderthatwhenexposedtolowtemperaturesathighhumiditybecomessticky.Foraheatexchangerfaceairvelocityof4m/s,experimentaldatafrommilkpowderfoulingtestsofflatplates,tubesandfinsindicatesparticulatefoulingbecomesseverewhentheexhaustairtemperaturereaches55°C.Higherfacevelocitiesareshowntolowerthiscriticalexhausttemperatureforavoidingseverefouling,whichgivespotentialforincreasedheatrecoverybutforincreasedpressuredrop.Lowerfacevelocitiesshowtheoppositeeffect.Designingexhaustheatrecoverysystemsentailanacutetradeoffbetweenheattransfer,pressuredropandfouling.Twoimportantdesignparametersarethenumberoftuberowsintheexhaustheatexchangerandthefacevelocity.TheoutputsofathermoeconomicspreadsheettoolsuggestLCHEsystemsforadryerproducing23.5t/hiseconomic.Withafacevelocityof4m/sand14rowsoffinnedroundtube,theprojecthadanestimatedpaybackof1.6years,anetpresentvalueofNZ13 billion in exports for 2012 while consuming 32 PJ of fossil fuels for process heat. Three quarters of NZ dairy exports are milk powders. This thesis presents methods to reduce process heat use in Milk Powder Plants (MPP) through improved heat integration and addresses key technical challenges preventing industrial implementation. My original contributions to literature include: (1) a novel design method called the Cost Derivate Method (CDM) that cost optimally allocates area in direct heat exchange networks, (2) a new design methodology for integration of semi-continuous process clusters using a Heat Recovery Loop (HRL) with a Variable Temperature Storage (VTS) system for improved heat recovery, (3) an experimentally validated deposition model for predicting critical air conditions that cause milk powder fouling, and (4) a thermo-economic assessment tool for the optimisation of industrial spray dryer exhaust heat recovery projects via a Liquid Coupled Loop Heat Exchanger (LCHE) system. By applying Pinch Analysis to an industrial MMP, this work confirms that heat must be recovered from the milk spray dryer exhaust air (~75 °C) to achieve maximum heat integration in MPPs. For stand-alone MPPs exhaust heat is best used to indirectly preheat the inlet dryer air reducing steam use by 12.7 % for a 55 °C exhaust outlet. Additional economic heat recovery from condensate and vapour flows decreased steam use by a further 6.9 %. Application of the CDM to the liquid and vapour sections of new MMP maximum energy recovery networks reduced total cost by 5.8 %. For multi-plant dairy factories, a second industrial case study showed the exhaust heat may be integrated with neighbouring plants via a HRL with VTS to increase site heat recovery by 10.8 MW including 5.1 MW of exhaust heat recovery, compared to 7.9 MW using a conventional HRL design method with constant temperature storage. A key barrier preventing exhaust heat recovery implementation in NZ MPPs is the possibility of milk powder fouling. Dryer exhaust air contains a low concentration of powder that when exposed to low temperatures at high humidity becomes sticky. For a heat exchanger face air velocity of 4 m/s, experimental data from milk powder fouling tests of flat plates, tubes and fins indicates particulate fouling becomes severe when the exhaust air temperature reaches 55 °C. Higher face velocities are shown to lower this critical exhaust temperature for avoiding severe fouling, which gives potential for increased heat recovery but for increased pressure drop. Lower face velocities show the opposite effect. Designing exhaust heat recovery systems entail an acute trade-off between heat transfer, pressure drop and fouling. Two important design parameters are the number of tube rows in the exhaust heat exchanger and the face velocity. The outputs of a thermo-economic spreadsheet tool suggest LCHE systems for a dryer producing 23.5 t/h is economic. With a face velocity of 4 m/s and 14 rows of finned round tube, the project had an estimated payback of 1.6 years, a net present value of NZ3 million and internal rate of return of 71 %. This tool will empower industry with greater confidence to uptake exhaust heat recovery technology as a vital method for improving the heat integration of MPPs in NZ

    An investigation of milk powder deposition on parallel fins

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    One method to reduce the energy consumption of industrial milk spray dryers is to recover waste heat from the exhaust dryer air. A significant challenge associated with this opportunity is the air contains a small amount of powder that may deposit on the face and surfaces of a recuperator. This paper introduces a novel lab based test that simulates powder deposition on a bank of parallel plate fins at exhaust dryer air conditions. The fin bank acts like the face of a typical finned tube row in a recuperator. The aim of this study is to look at how deposition on the front of fins is affected by the air conditions. Results show similar characteristics to other milk powder deposition studies that exhibit a dramatic increase in deposition once critical stickiness levels are reached. As powder deposits on the face of the fins, the pressure drop across the bank increases until eventually an asymptote occurs, at which point the rates of deposition and removal are similar. For very sticky conditions, deposition on the face of the fins can cause a rise in the pressure drop by as much as 65%. The pressure drop has also been successfully related to the percentage of open frontal area of the fins with and without deposition. Deposition inside and at the rear of the fin bank was found to be minimal

    Integration of solar heating into heat recovery loops using constant and variable temperature storage

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    Solar is a renewable energy that can be used to provide process heat to industrial sites. Solar is extremely variable and to use it reliably thermal storage is necessary. Heat recovery loops (HRL) are an indirect method for transferring heat from one process to another using an intermediate fluid (e.g. water, oil). With HRL’s thermal storage is also necessary to effectively meet the stop/start time dependent nature of the multiple source and sink streams. Combining solar heating with HRL’s makes sense as a means of reducing costs by sharing common storage infrastructure and pipe transport systems and by lowering nonrenewable hot utility demand. To maximise the value of solar in a HRL, the means of controlling the HRL needs to be considered. In this paper, the HRL example and design method of Walmsley et al. (2013) is employed to demonstrate the potential benefits of applying solar heating using the HRL variable temperature storage (VTS) approach and the conventional HRL constant temperature storage (CTS) approach. Results show the VTS approach is superior to the CTS approach for both the non-solar and solar integration cases. When the pinch is around the hot storage temperature the CST approach is constrained and the addition of solar heating to the HRL decreases hot utility at the expenses of increased cold utility. For the VTS approach the hot storage pinch shifts to a cold storage pinch and increased heat recovery is possible for the same exchanger area without solar. With solar the VTS approach can maintain the same heat recovery while also reducing hot utility still further due to the presence of solar, but only with additional area. When the pinch is located around the cold storage temperature, solar heating can be treated as an additional heat source and the benefits of CTS and VTS are comparable

    Area targeting and storage temperature selection for heat recovery loops

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    Inter-plant heat integration across a large site can be achieved using a Heat Recovery Loop (HRL). In this paper the relationship between HRL storage temperatures, heating and cooling utility savings (heat recovery) and total HRL exchanger area is investigated. A methodology for designing a HRL based on a ΔTmin approach is compared to three global optimisation approaches where heat exchangers are constrained to have either the same Number of Heat Transfer Units (NTU), Log-Mean Temperature Difference (LMTD) or no constraints (actual global optimum). Analysis is performed using time averaged flow rate and temperature data. Attention is given to understanding the actual temperature driving force of the HRL heat exchangers compared to the apparent driving force as indicated by the composite curves. The cold storage temperature is also varied to minimise the total heat exchanger area. Results for the same heat recovery level show that the ΔTmin approach is effective at minimising total area to within 5 % of the unconstrained global optimisation approach. The study also demonstrates the efficiency of the ΔT min approach to HRL design compared to the other methods which require considerable computational resources

    Area targeting and storage temperature selection for heat recovery loops

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    Inter-plant heat integration across a large site can be achieved using a Heat Recovery Loop (HRL). In this paper the relationship between HRL storage temperatures, heating and cooling utility savings (heat recovery) and total HRL exchanger area is investigated. A methodology for designing a HRL based on a ΔTmin approach is compared to three global optimisation approaches where heat exchangers are constrained to have either the same Number of Heat Transfer Units (NTU), Log-Mean Temperature Difference (LMTD) or no constraints (actual global optimum). Analysis is performed using time averaged flow rate and temperature data. Attention is given to understanding the actual temperature driving force of the HRL heat exchangers compared to the apparent driving force as indicated by the composite curves. The cold storage temperature is also varied to minimise the total heat exchanger area. Results for the same heat recovery level show that the ΔTmin approach is effective at minimising total area to within 5 % of the unconstrained global optimisation approach. The study also demonstrates the efficiency of the ΔT min approach to HRL design compared to the other methods which require considerable computational resources

    Optimal waste stream discharge temperature selection for dryer operations using thermo-economic assessment

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    A typical drying process that has liquid and gas discharge streams has been analysed and the impact of selecting various combinations of soft temperatures on heat recovery, utility targets, area targets, capital cost and total cost is reported. The method is based on the plus-minus principle and traditional pinch analysis methods for utility, area and capital cost targeting with the modification of using a ΔT contribution. Results show that there is significant benefit from optimising discharge temperatures for total cost. To achieve minimum energy consumption and total cost, heat recovery from the dryer exhaust air is necessary. Heat recovery from liquid heat sources is shown to be preferable over gas streams due to a higher film coefficient resulting in less heat exchanger area and capital cost. There is also value in making process modifications, such as combining streams or removing small streams to be solely heated by utility, to reduce the number of network heat exchangers. For the best case, the discharge temperatures of the leaving streams are 18.0 °C for water condensate (liquid stream) and 52.4 °C for the exhaust air (gas stream)

    Optimal stream discharge temperatures for a dryer operation using a thermo-economic assessment

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    The application of traditional pinch analysis to processes involving waste streams require the discharge temperatures of the waste streams to be estimated prior to performing the pinch analysis
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