thesis

Heat Integrated Milk Powder Production

Abstract

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

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