Experimental Study of Natural Composite Desiccant-Based Dehumidification System

Abstract

This study investigates the sorption and desorption characteristics of a natural com- posite desiccant based on dried cow dung (DCD). The first part of the study focuses on finding out an effective binding material for DCD. For this Polyvinyl Pyrrolidone (PVP) and clay are selected as binders. The moisture uptake capacity of composite desiccants is measured with an isotherm experiment under different DCD-to-binder ratios. Based on their isotherms, composite desiccants are chosen for the study under different humid conditions and compared with available literature data. Brunauer-Emmett-Teller and Barrett-Joyner-Halenda analyses are carried out to understand the physical properties of DCD, DCD+PVP (3:1 ratio) and DCD+Clay (3:1 ratio). Total heat load reduction, exergy efficiency and power required for these dehumidification systems are calculated for different inlet conditions. Desorption characteristics are tested at 328 K and 6% RH. Results show the maximum moisture uptake capacity of DCD and DCD+PVP as 9.87 and 9.01 g/100 g, respectively. The maximum exergy efficiency of the DCD+PVP dehumidification system is found to be 55%. The desorption time for DCD+PVP desic- cant is 17 minutes, which is 4 and 2 minutes higher compared to DCD, and DCD+Clay, respectively. In the second phase of the study, a natural composite desiccant, in which the unuti- lized portion of the spherical desiccant material is replaced with a metallic ball, is pro- posed. Stainless steel balls with a diameter of 4.75 and 6.35 mm are used to make different thickness ratios (TR = 1, 0.525, and 0.365) of metal-embedded natural com- posite desiccants (MENCDs). The natural composite desiccant is prepared from dried cow dung and polyvinyl pyrrolidone with a ratio of 3:1. Experiments are conducted to find theoptimumthicknessratioofMENCDs.Thetotalmoisturesorption,moisture sorption rate,totalheatloadreduction,andexergyefficiencyofthesedehumidification systems areinvestigatedunderdifferentrelativehumidities(RH=65%,75%and85%), and ataconstanttemperatureandvelocity.Desorptioncharacteristicsaretestedat328 K and5%RH.ThemaximummoistureupatakecapacityofMENCDswithaTRof 0.365 isfoundtobe11.84g/100g,whichis17%highercomparedtonaturalcomposite desiccants (i.e.,TR=1)at85%RH,whereas,thetotalmoisturesorptionrateis0.4 g/100 g·min, whichis20.57%higherforTRof0.365comparedtoTR=1.Themois- ture desorptionrateforTR=0.365is16.66%highercomparedtoTR=1.Thesystems exhibitanaverageexergyefficiencyof60%.However,whenemployingcompositedes- iccants withaTRof0.365,theiraverageexergyefficiencyimprovesby9.6%compared to thesystemsoperatingwithTR=1.Furthermore,theaveragereductionintotalheat load withTR=0.365is24%highercomparedtothoseutilizingTR=1. Further studiesarecarriedouttoreducethepressurelossacrossthedehumidifica- tion bedandincreasethemoisturesorptioncapacity.Toachievethisthedehumidifi- cation bedisdesignedwithstaggeredhexagonalaluminiumchannels(SHACs).The natural compositedesiccant(NCD)waspreparedbycoatingthemixtureontothechan- nels witha2:1ratioofDCD:PVPduetobettercoatability.Theresultsofthestudyshow that theNCD-coatedSHACsdehumidificationsystemhadahighmoisturesorptionca- pacity,withmaximummoisturesorptionvaluesrangingfrom8.34to14.31g/100gat differentRHandtemperatureconditions.Thesystemalsoshowsanaveragemoisture sorption rateof0.26g/100g·min andadesorptionrateof0.51g/100g·min. Further- more, themoistureflowdesignoftheNCD-coatedSHACsbedresultsinalow-pressure drop of0.13kPa, whichissignificantlylowerthantheNCD-packedbed.