外文Centrifugal Fluidized Bed dryer.pdf
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Chemical Engineering Journal 78 (2000) 107113Experimental investigation of the heat and masstransfer in a centrifugal fluidized bed dryerM.H. Shi, H. Wang, Y.L. HaoDepartment of Power Engineering, Southeast University, Nanjing 210096, ChinaReceived 9 November 1998; received in revised form 25 June 1999; accepted 29 June 1999AbstractAn experimental study of the heat and mass transfer characteristics of wet material in a drying process in a centrifugal fluidized bed(CFB) dryer was carried out. The rotating speed ranged from 300 to 500rpm. Wet sand, glass beads and sliced food products were usedas the testing materials. The gas temperature and the wet bulb temperature at the inlet and outlet, as well as the bed temperature, weremeasured.Themoisturecontentsweredeterminedinstantaneouslybythemassbalancemethodinthegasphase.Influencesofthesuperficialgas velocity, particle diameter and shape, bed thickness, rotating speed of the bed and initial moisture on the drying characteristics wereexamined. One empirical correlation which can be used to calculate the heat transfer coefficients of the gas particles in the centrifugalfluidized dryer were obtained. 2000 Elsevier Science S.A. All rights reserved.Keywords: Drying; Heat and mass transfer; Centrifugal fluidized bed1. IntroductionCentrifugal fluidized bed (CFB) drying is a new technol-ogy in which the wet material undergoes a highly enhancedheat and mass transfer process in a centrifugal force field byrotating the bed. The bed essentially is a cylindrical basketrotating around its symmetric axis with a porous cylindricalwall. The drying material is introduced into the basket andforced to form an annular layer at the circumference of thebasket due to the large centrifugal forces produced by rota-tion. The gas is injected inward through the porous cylin-drical wall and the bed begins to fluidize when the forcesexerted on the material by the fluidizing medium balancethe centrifugal forces. Instead of having a fixed gravitationalfield as in a vertical bed, the body force in a centrifugal bedbecomes an adjustable parameter that can be determined bythe rotation speed and the basket radius. Minimum fluidiza-tion can, in principle, be achieved at any gas flow rate bychanging the rotating speed of the bed. By use of a strongcentrifugal field much greater than gravity, the bed is ableto withstand a large gas flow rate without the formation oflarge bubbles. Thus, the gassolid contact at a high gas flowrate is improved and heat and mass transfer can be achievedduring the drying process. For this reason, the CFB dryerhas received much attention in the drying industry.Corresponding author.Only a few research works dealing with drying in the CFBcould be found in the literature. Lazar and Farkas 1,2 andBrown 3 have conducted the drying process in a CFB forsliced fruits and vegetables, while Carlson 4 investigatedthe drying of rice in the CFB. These research works are veryinstructive, but they are mainly focused on the possibility ofan industrial application for CFB. The flow behaviour anddrying characteristics in the CFB are very complicated andstill unclear. A knowledge of heat transfer from the gas to thematerial is desirable in order to estimate the material surfacetemperature from the measured temperature of the gas. Aquantitative knowledge of the heat transfer characteristics ofCFBs is therefore necessary for design purposes 5.In this paper, an experimental study of the flow behaviourandgassolidheatandmasstransfercharacteristicsinaCFBdryer was performed and the main factors which influencethe drying process were examined and discussed.2. Experimental apparatusA schematic diagram of the experimental apparatus isshown in Fig. 1. A cylindrical basket rotated about a hor-izontal axis is mounted in a sealed cylindrical casing. Thebasket is 200mm in diameter and 80mm in width. The sidesurface of the basket contains 3mm diameter holes whichserve as a gas distributor, with an open area of 22.7%. A1385-8947/00/$ see front matter 2000 Elsevier Science S.A. All rights reserved.PII: S1385-8947(00)00148-0108M.H. Shi et al./Chemical Engineering Journal 78 (2000) 107113Fig. 1. Experimental apparatus.200 mesh stainless steel screen is coated on the inside sur-face to prevent the bed material from leaching out. There isone hole 80mm in diameter located at the centre of the endwall of the basket to exit the gas. A variable speed motoris used to rotate this basket by means of a shaft connectedto the other end wall of the basket. Rotational speeds of themotor are measured using an LZ-45 revolution counter.Air is blown in from a blower. The mass flow rates of airare measured using an orifice meter. Air is heated using anelectric heater. A tee valve is used to control the flow direc-tion. After the air temperature is steady at the desired value(about 100C), the drying experiments begin by turning thetee valve on; the hot air flows through the distributor to thebed and then is exhausted into the atmosphere. The pressuredrop is measured by a U-shaped pressure gauge. A pres-sure probe is stretched into the basket along the centreline10mm away from the end wall of the basket. Experimentsare also conducted without bed material to obtain the pres-sure differentials across the distributor under the same oper-ating conditions. The pressure drop through the bed is thencalculated by1pBed= 1pTotal 1pDistributorThe inlet gas temperature, the outlet gas temperature andthe bed temperatures at various positions versus time aremeasured using the bare thermocouple probes, and the dataare recorded by a 3497A data acquisition/control unit. Mois-ture contents of the test material during the drying processare measured by the moisture balance method in the gasphase, i.e. by measuring the inlet and outlet wettabilities inthe gas phase with wet and dry bulb thermometers.Fig. 2. A differential section in a centrifugal fluidized bed.The water balance in the time interval from tjto tj+1isGZtj+1tj(Hout Hin)dt = MsZxj+1xjdx(1)and thus, the moisture content of the test material at timetj+1isxj+1= xjGMsZtj+1tM.H. Shi et al./Chemical Engineering Journal 78 (2000) 107113109Fig. 3. The fluidized curve of sand in the CFB (dp=0.245mm, n=400rpm).Material (up/down): (m/h) sand; (d/s) glass beads.speeds during the drying tests. In the initial fluidizing stage,the pressure drop increases linearly with increasing gas ve-locity.Afterreachingthecriticalpoint,thepressuredropwillbe almost constant. However, different results are observedfor sliced and block materials. The pressure drop curve hasa maximum value that corresponds with the critical fluidiza-tion point as shown in Fig. 4. In the initial fluidizing stage,the pressure drop increases slowly with increasing gas ve-locity. After reaching the critical point, the pressure dropwill decrease with increasing gas velocity. This is becausethe self-lock phenomenon of the sliced material under a cen-trifugal force field will be weakened and because the bedbecomes uniform. This causes a decrease in the flow resis-tance. Decreasing the bed rotating speed would decrease thebed pressure drop and the critical gas velocity remarkably,as also shown in Fig. 4. This is because decrease in the bedrotating speed would weaken the centrifugal force field andcause the flow resistance to decrease. It can be seen fromFig.4thatthecriticalfluidizedvelocityforpiecesofpotatoissomewhat smaller than that of blocks of potato owing to theFig. 5. Intermittent drying curve in the CFB (sand, dp=0.411mm, M=2.48kg, =41.9rads1, U0=1.71ms1, Hin=0.016kgkg1): (1) Tg,in; (2) Tg,out;(3) Tb; (4) R; (5) x.Fig. 4. The fluidized curves for materials with different shapes: (4) piecesof potato 10mm10mm1.5mm, n=300rpm; (h) blocks of potato5mm5mm5mm, n=300rpm; (s) block of potato 5mm5mm5mm, n=250rpm.larger upwind surface area for pieces of material. Further-more, pressure drop of the piece material bed is also smallerthan that of the block material bed because the pieces ofmaterial show better fluidization character in the CFB. Theinitial fluidizing relationships obtained from the theoreticalmodel for granular material 6 do not fit the sliced mate-rial. The initial fluidizing conditions for the sliced materialwith different shapes should be determined experimentallyand individually.3.2. Drying curvesTypical gas temperature and bed temperature curves aswell as the drying curve of wet sand in the intermittent dry-ing process are shown in Fig. 5. This shows that the drying110M.H. Shi et al./Chemical Engineering Journal 78 (2000) 107113Fig. 6. Variations in the moisture content (Curve 6) and drying rate (Curve7) for sliced potato.characteristics of materials like sand in the CFB, in whichthe moisture content is mainly surface water, are the sameas in an ordinary dryer, i.e. the whole drying process can bedivided into three stages. At a short initial stage, the materialis preheated and the drying rate increases rapidly; the bedtemperature is increased to a stable value. The second stageis a constant drying rate stage in which the heat transferredfrom gas to material is expended totally for evaporation ofthe surface water of the material. The material temperatureremains constant and the drying rate is also constant. Thelast stage is called the falling rate stage in which the ma-terial temperature increases gradually and the drying ratedecreases until the end of drying.The drying behaviour for sliced food products in the CFBis somewhat different from sand as shown in Fig. 6. It isobvious that sliced potato has a drying character in the CFBthat is basically similar to that in the conventional dryingprocess. In the beginning, there is a short initial period. Inthis period, the bed material is preheated; the bed temper-ature approaches a stable value quickly and the drying rateincreases very rapidly. This initial period is followed by aperiod of a constant rate of drying. In the constant rate pe-riod, the surface of the test material is covered with a thinwater film. The heat transferred from the gas flow to the ma-terial is used completely to evaporate the moisture, so thatthe temperature of the sliced material remains at an equi-librium temperature and the drying rate is at the maximumvalue. As the main moisture content in potato is cell water,the constant rate period is very short. The most importantdrying process is completed in the falling rate period. Inthe falling rate period, the dry layer appears and graduallybecomes thicker near the surface owing to the larger trans-port resistance of the inner moisture outward. This causesthe heat transfer resistance to increase and the drying rateto decrease rapidly in the first stage. After the dried layerstemperature has increased to a certain value, a slow decreasein the drying rate occurs. This indicates that the falling rateperiod for the sliced potato in the CFB dryer can be dividedinto two different stages. This is significant for engineeringdesign and operation.The experimental results show that the pieces of potatoin the drying process have a larger drying rate and a shorterdrying time than blocks of potato in the CFB. This is be-cause the transport distance of moisture from the innercell to the outer evaporating surface in the pieces of ma-terial is much shorter than in the blocks of material; inparticular, the second stage of the falling rate period isshorter for the pieces of material during the drying process.In general, because the sliced material could be fluidizedand mixed very well in the CFB, the drying time is ex-tremely short. For example, the drying time is 15 timesshorter in the CFB for sliced potato than in the tunnel dryerand five times shorter than in the conventional fluidizeddryer.3.3. Influences of the operational parameters3.3.1. Superficial gas velocityIt is obvious that an increase in the superficial velocitywould increase the degree of fluidization, and thus, the heatand mass transfer between the gas and the solid phase wouldbe greatly enhanced. This causes the drying rate to be largerand the drying time to be shorter, as shown in Fig. 7. Thecritical moisture content would be increased with increas-ing gas velocity, indicated by the broken line in Fig. 7. Forfood material, the experimental results show that the dry-ing rate in the constant rate period and the first stage ofthe falling rate period would increase with increasing gasvelocity in the low gas velocity range. Thus, the total dry-ing time would be decreased. However, when the gas veloc-ity is increased to a certain value, the constant rate periodwould disappear, the first stage of the falling rate periodFig. 7. The influence of superficial velocity on the moisture con-tent (dp=0.411mm, M=2.50kg, =41.9rads1, Hin=0.016kgkg1): (1)U0=1.66ms1; (2) U0=2.17ms1.M.H. Shi et al./Chemical Engineering Journal 78 (2000) 107113111would decrease and the second stage would increase. Thetotal drying time would remain unchanged; this is becausethe main water content in potato is the inner cell water andthe main drying process is in the second stage of the fallingrate period. With an increase in the inlet gas temperature,the drying rates in all drying periods increase and the totaldrying time will decrease. However, the increase in gas tem-perature would be limited by the quality of the dried foodproducts. In our test, the best inlet gas temperature is about100110C.The experimental results also show that pieces of radishwith given dimensions show a larger drying rate than piecesof potato under the same operating conditions. This is be-cause the microstructures of the test examples indicate thatradish has a larger cell structure with a more regular ar-rangement than potato, and furthermore, the liquid in theradish cell is less viscous; these structural characteristicsmake radish easy to dry.3.3.2. Rotating speedAt the same gas velocity, a decrease in the bed rotatingspeed will reduce the centrifugal force acting on the materialand increase the fluidized degree of the material; this causesthe heat and mass transfer between the gas and the solidphase to increase. Thus, when decreasing the bed rotatingspeed, the drying rate will be larger, as shown in Fig. 8,and the drying process will be much more uniform overthe whole bed. This means that, for a given material dryingin the CFB, the bed rotating speed should be as low aspossible until the fluidization state cannot be maintained.When it is desired that the drying process be enhanced byincreasing the gas velocity in the CFB dryer, the bed rotatingspeed must be increased simultaneously to avoid the dryingmaterial from blowing out of the bed. Theoretically, thebed can be operated in the optimum fluidized condition atany gas velocity by regulating the bed rotating speed in theCFB.Fig. 8. The influence of rotating speed (dp=0.411mm, M=2.41kg, U0=1.43ms1, Hin=0.0123kgkg1): (1) =52.4rads1; (2) =41.9rads1.Fig. 9. The influence of particle diameter (M=2.4kg, =41.9rads1, U0=1.43ms1, Hin=0.0123kgkg1): (1) dp=0.245mm; (2) dp=0.411mm.3.3.3. Partial diameterFig. 9 shows the influence of particle diameter on thedrying behaviour in the CFB. It is clear that, owing to thelarger slip velocity between gas and solid particles for parti-cles with larger diameters, the heat and mass transfer in thedrying process would be enhanced; thus, the drying rate inthe CFB would increase with increasing particle diameter asshown in Fig. 9. However, with increasing material dimen-sions, the internal heat and mass transfer resistance wouldbe increased; thus, for a given material to be dried, it is im-portant to determine the optimum material dimensions in thedrying process under certain given operating conditions.3.3.4. Bed thicknessFig. 10 shows the effect of initial bed thickness on thedrying process. It can be seen that, with increasing bed thick-ness, the drying rate would be decreased; this is because theheat and mass transfer driving force between the gas and thesolid phase is larger in the shallow bed situation.Fig. 10. The influence of bed thickness (dp=0.411mm, =41.9rads1,U0=1.43ms1, Hin=0.0123kgkg1): (1) L0=30mm; (2) L0=20mm.112M.H. Shi et al./Chemical Engineering Journal 78 (2000) 107113Fig. 11. The initial moisture content (dp=0.411mm, M=2.48kg, =41.9rads1, U0=1.71ms1, Hin=0.016kgkg1): (1) x0=0.221kgkg1; (2)x0=0.0574kgkg. The effect of initial moisture contentIt is obvious that a material with a large initial moisturecontent has a much longer drying time (Fig. 11), but thedrying characteristics are the same. The only difference isin the duration of the constant rate stage.3.4. The heat transfer correlationSixty-five experimental runs of wet sand and glass beadswere carried out under the conditions of a static bed thick-ness range from 10 to 30mm, Reynolds number from 5.47to 35.3 and centrifugal force from 10.08 to 28 multiples ofgravity. The heat transfer coefficients were converted intoNusselt numbers using the mean diameter and the thermalconductivity of air at the average temperature.The dimensionless correlation of heat transfer betweengas and particles in the CFB during drying is obtained by useof a regression procedure. The exponent of the diffusivityratio (Prandtl number) has been assumed to be 1/3; thus,Fig. 12. Comparison of experimental and calculated results.Nu =5.33 105Pr1/3Re1.59F0.48c?L0dp?0.21?sg?0.79(7)The suitable parameter ranges for the above two correla-tions are Re=5.042.0, Fc=10.028.0. In Eq. (7), the Nus-selt number is defined as Nu=hdp/; the Reynolds number isRe=gU0dp/; the Prandtl number is Pr=cpg/; and then,dimensionless centrifugal force is defined as Fc=r02/g.Comparison of the experimental heat transfer data withthe values calculated by Eq. (7) is shown in Fig. 12. Thedeviation for all test data obtained in this work is within25%.4. Conclusions1. The CFB may be operated in the packed bed, incipientfluidization or fluidized bed states at a given gas velocity.Steady fluidized states can be maintained at large gasflow rates by using a strong centrifugal force field.2. There is no evident active region near the distributorof the CFB. The gassolid heat transfer comes under theinfluence of the superficial gas velocity, particle diameter,particle shape factor, particle density, bed thickness androtational speed of the bed.3. The drying process can be divided into three stages inthe CFB dryer and the drying rate increases with increas-ing superficial gas velocity and particle diameter and de-creasing bed rotating speed and initial bed thickness.4. Sliced food products can be fluidized and mixed v
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