Drying of Solid

8/3/2019 Drying of Solid

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3068 I n d . E n g . Chem. Res. 1995,34 , 3060-3077

SEPARATIONSDrying of Solids in Fluidized Beds

C. Srinivasa Kannan, P. P. Thomas, and Y. B. G. Varma"Department of Chemical Engineering, Indian Znstitute of Technology, Madras, Madras 600 036, I nd ia

Solids are dried in batch and in continuous fluidized beds corresponding to cross-flow andcountercurrent flow of phases covering a wide range in drying conditions. Materials thatessentially dry with constan t drying rat e an d then givea falling drying rat e approximately linearwith respect t o solids moisture content (sand ) as well as those with an extensive falling rateperiod with the subsequent falling rate being a curve with respect t o the moisture content(mustard, ragi, poppy seeds) are chosen for the study. The performance of the continuousfluidized bed driers is compared with that of batch fluidized bed driers; the performance ispredicted using batch kinetics, the residence time distribution of solids, and the contact efficiencybetween the phases.

IntroductionFluidized bed drying is advantageously adopted inindustrial practice for drying of granular solids such asgrains, fertilizers, chemicals, and minerals either forlong shelf life or to facilitate further processing orhandling. The drying rate in the fluidized bed isstrongly influenced by the material characterist ics andthe fluidization conditions. Materials with no internalporosity dry essentially at a constant r ate while otherswith internal porous structure give both constant andfalling drying rates.A knowledge of drying kinetics is essential for theestimation of the drying time needed t o reduce themoisture content to the desired level and for suggestingthe optimal drying conditions. Toward this , it is re-quired t o know the constant drying rate, the criticalmoisture content at which the drying rate beginsto fall,the falling drying rate a t the different levelsof moisture,and the equilibrium moisture content. The informationis however not readily available in literature to facilitatean a priori estimationof the drying rate in fluidized beddrying of solids (Kunii and Levenspiel, 1991). This ispartly due to lack of sufficient experimental data,arising out of the specificity of the drying rate to thematerial and t o the drying conditions.It is attempted in the present study to experimentallyinvestigate the fluidized bed drylng of solids (a) ha t giveessentially a constant drying rate period and a shortfalling rate period, where the falling drying rate may

be approximated to a linear relationship with respectt o the moisture content of the solid; and (b) that giveessentially a falling drying rate period and a shortconstant drying rate period, where the falling dryingrate is a curve with respect t o the moisture content ofthe solid. The variables covered in the s tudy includethe temperature and flow rate of the heating medium,the initial moisture content of solid, the particle size,and the solids holdup. The objective is to identify thevariables and their extent of influence on the drying ra tein a fluidized bed a t different levelsof moisture contentof solids.Fluidized bed drying may be carried out either as* To whom correspondence should be addressed.

0888-5885/95/2634-3068$09.QQIQ

9 -- 10l a )

12 -3W l 4

I C )Figure 1. Schematic diagram of experimental setup. (a)Batchfluidized bed. (b)Continuous single-stage spiral fluidized bed. (c)Multistage fluidized bed. (1)Air compressor; (2 )control valve; (3)orifice meter; (4 ) air heater; (5 ) air chamber; (6) temperaturecontroller; ( 7 ) air distributor plate; (8) calming section; (9) ther-mocouple;(10) luidization column; (11) piral baffle; (12)vibratoryfeeder; (13) calibrated orifice; (14) olids hopper; ( 15 ) solidsdischarge pipe; (16) olids downcomer.batch or as continuous operation. Batch driers aremainly used fo r small scale operation while continuousdriers are preferred for large capacity (Reay and Baker,1985). The second objective of the present study is tocompare the performance of batch and different typesof continuous fluidized bed driers.

0 995 American Chemical Society


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Ind. Eng. Chem. Res., Vol. 34,No. 9, 1995 3069Table 1. Materials and Range of Variables Investigated in the Study

(a) Materialsragi mustard poppy seeds(Elesine coracana Linn) (Brassica napus Linn) (Pappaver sominiferum) sand

density, kg/m3 1207 1100 800 2650particle size, m ( x 103) 1.48 1.7-2.15 0.363-0.6 0.363-0.6(b) Range of Variables

multistagebatch single stage single-stage spiral two-stage spiral N = 1 N = 2 N = 3

ai r flow rat e, m3/s 0.03-0.098 0.06-0.098 0.074-0.098 0.074 0.03 0.03 0.03solids flow rate, kg/(m2s ) 0.03-1.5 0.03-0.12inlet air temp, K 313-373 313-353 333-352mean holding time, s 8000 220-1160 300-1300solids holdup, kg 0.15-2.6 1.3-3.2 1.3-2.6Experimental Section

Batch Drying. Drying experiments were conductedusing fluidization columns of 148 mm i.d. and of 245mm i.d. The gas distributor was 2 mm thick with 2 mmperforations and 13% free area. A fine wire mesh of0.2 mm openings was spot welded over the distributorplate to arres t the flow of solids from the fluidized bedinto the air chamber. Air from the blower was heatedand fed into the air chamber and into the fluidizationcolumn (Figure la). The electrical heater consisted ofmultiple heating elements each of 2 kW rating. Atemperature controller, provided t o the air chamber,facilitated control of air temperature to f0 .5 "C, for theoperating range of 30-110 "C. Air flow as measuredusing a calibrated orifice meter.A known quantity (at 2 kg) of known initial moisturecontent of solids was taken in the batch fluidized bed,and air at the desired rate was introduced into thecolumn. As fluidization continued, solid samples ofapproximately 5 g each were scooped out of the bed fo rmoisture determination. In general 10-20 sampleswere collected in each experiment.Continuous Drying. Experiments on continuousfluidized bed drying of solids were conducted under (i)under cross flow and (ii) under countercurrent flow ofphases. In the former, a copper foilof 1mm thickness,would in the form of a spiral, was placed within thecircular cross section of the fluidization column. Theheight of the copper foil was 200 mm; the channel widthand length of the spiral were 25 and 1600 mm, respec-tively. Solids, fed at the center, discharged from thefluidized bed at its periphery through a 12 mm i.d.downcomer tube (Figure lb). These are termed "spiralfluidised beds'' (Chandran et al ., 1990).In the second arrangement, the fluidization columnwas sectioned into a number of stages using horizontalperforated plates. The plates were 2 mm thick, with 2mm perforations and 13% free area, with a fine wiremesh at the top t o arrest solids downflow through theperforations. Downcomers of 12 mm i.d. were providedt o the perforated plates a t diametrically opposite loca-tions to serve for flow of solids from stage to stage.These are termed "multistage fluidised beds" (SrinivasaKannan e t al., 1994.)Solids feed t o the fluidized bed was controlled usinga vibratory feeder and a calibrated orifice at thedischarge end. With spira l fluidized beds, fluidizedsolids moved along the spiral from the center to theperiphery of the bed in cross flow t o the upflowing gas.Solids mixing in spiral fluidized beds may be describedusing the axial dispersion model (Pydisetty et al., 1989).With multistage fluidized beds, solids fed at the top of

.- . ._0.03-0.09 0.058-0.306 0.058-0.306 0.058-0.306333-352 333-373 333-373 333-373500-1300 40-100 70-190 100-3102-2.6 0.09-0.22 0.18-0.38 0.03-0.68the column moved in the fluidized state from stage t ostage countercurrent t o the upflowing gas and dis-charged from the bottom stage. Solids mixing in eachstage may be assumed t o correspond to ideal mixing(Krishnaiah et al., 1982). Single-, two-, and three-stagebeds were studied in this arrangement (Figure IC).

With the continuous feeding of solids and air a t thedesired rates and choice of experimental conditions, asample of solids was collected at steady state from thesolids discharge tube for moisture determination. Steadystate was assumed when the solids discharge rate andthe moisture content remained constant. The holdupof solids was determined by the weighing method afterstopping the flow of solids and air into the column. Themean holding time was estimated from the solids holdupand solids discharge rate. The solids moisture contentwas determined by drying the sample till constantweight in an air oven at 105 "C. The moisture contentis expressed on dry basis as kilograms of moisture perkilogram of dry solid. The experimental da ta werechecked fo r reproducibility, especially when the sampletimes were small, and were found t o deviate less than4% from the reported value.Table 1 gives the details of the materials and therange of variables covered in the study.

Results and DiscussionBatch Fluidized Bed Drier. Typical experimentaldata showing the drying rate -dC/dt versus moisturecontent of solids are shown in Figures 2 and 3 coveringthe effectsof temperature and flow rate of air, the initialmoisture content of solids, the particle size, and thesolids holdup. The following observations are madebased on drying of ragi, mustard, poppy seeds, and sandin batch fluidized beds.The materials exhibit constant and falling rate peri-

ods; the extent and the value of each depend upon thematerial characteristics and the drying conditions. Forexample, sand dries essentially at constant ra te whileragi shows a considerable falling ra te period, comparedto the constant r ate period.An increase in air temperature increases significantlythe drying rate in the constant and falling rate periodsfor all the materials. This increase in the constantdrying rate is attributed t o the increase in surfacetemperature of the particle resulting in higher surfacehumidity and an increased evaporation from the sur-face. The increase during the falling drying rate periodis due t o the solid particle attaining a higher temper-ature; this increases intraparticle moisture diffusion tothe particle surface.


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3070 Ind. Eng. Chem. Res.,Vol. 34,No . 9, 1995

34F2c 2X-0.IU0

00 0. 1 0.2 0.3 0 . 40. 1 0.2 0.3 0 . 4 i.561 I I I I 115 0 35 3 0.060 1 . 4 8 1. 3 R a g iXc-z 3IU0I-

2

1

0 0 0.1 0. 2 0.3 0 .4 0.5M o i s t u r e c o n t e n t , C

Figure 2. Effect of (a) temperature and flow rate of air and (b)particle size an d holdup of solids on drying rat e for mustard andragi i n batch fluidized bed.I I I I

3 . 5 Ma t l . :S a n d AcU 2 4 - /7IUDI 1I

1

0

. 5c

, 5 ~0 0.1 0 .2 0 . 3 0 . 4 0 .5Moisture content, C XIO

Figure 3. Effect of temperature, flow rate of air, and holdup ofsolids on drying rat e for sand in batch fluidized bed.An increase in air rate increases the drying rate inthe constant rate period due t o a decrease in gas filmresistance surrounding the particle. The influence ofair rate on the drying rate in the falling rate period ishowever small as this resistance plays a minor roleduring the falling rate period.The solids initial moisture content influences thedrying rate, especially during the falling rate period.

C ldrv bas is )

0.02 0.10 0.18 0.26 0.34C ( w e t b a s i s )

Figure 4. Effect of solids initial moisture content on drying rat ebased on wet and dry basis: Ci = 0.4 (0,) ;Ci = 0.21 (a, ).

When the drying rate is expressed on a dry basis, theconstant drying rate appears not influenced by theinit ial moisture content. On the other hand, whenexpressed on a wet basis (Figure 41, he drying rateduring constant and falling rate periods is influencedby the solids initial moisture content. Solids with highinitial moisture content will have less dry solids i.e.,fewer number of particles, and have reduced drying rateper unit weight of initial charge. As drying progresses,close to equilibrium, solids with low initial moisturecontent due to higher holdup may show a marginaldecrease in drying rate than solids with high initialmoisture content.An increase in particle size decreases the drying ratein the constant and falling rate periods. This reductionis due to reduction in surface area per unit weight ofsolids. An increase in solids holdup decreases thedrying rate in the constant and in the falling rateperiods. This is due to a reduction in air t o solids ratiofor the given air flow rate.The experimental data presented in Figures 2 and 3show that the critical moisture content is influenced bythe drying conditions. It increases with increase in airrate and temperature but decreases with increase in

particle size and solids holdup. It also increases withincrease in solids init ial moisture content. The equi-librium moisture content, however, is found t o dependprimarily on the temperature of the drying process. Thedrying rate during constant rate period is of importanceas i t forms the maximum drying rate for the material,and i t constitutes in certain materia ls (e.g., sand) themajor portion of the drying process. On the basis of theexperimental data, the constant drying rate is empiri-cally related t o the system variables as follows:


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3072 Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995

*,A,. - B a t c hO,A,o - S i n g l e s t a g e s p i r a l

C l o c k t i m e , t I s )

E q n s ( l 1 4 ) T i dpx1o3 C i V f ws M a t l . 'o- 13 0.565 0.47 0.092 .089 R e s i nA -- - 2 3 0.650 0.48 0.092 .748 doo - - . - e 3 4 3 0.650 0 . 47 0.092 1.748 do0- 03 0.900 0.0030.1217 6.000SandA --- 03 0.900 0.0030.1517 3.000 do

C l o c k t i me , t Is 1- 0 . 5 1 1 I I I I I 1 I_ _E Materiol: Zerolit resin0

:0 . 1 1000 2000 300 0 coo0C l o c k t ime , t I s )

'0 1000 2000 3000 4000 5000 6000C l o c k t i m e , t ( S I

Figure 6. (A , top left) Comparison of experimental data of Chandran e t al. (1990) with predicted data using eqs 1and 4 . (B,ottom left)Comparison of experimental data of Mckenzie and Babu (1991) with predicted data using eqs 1and 4. (C, right) Comparison of experimentaldata of (a ) Uckan and Ulku (1986) and (b) Thomas and Varma (1992) with perdicted data using eqs (1 and 2) .

where K = 2.6 for ragi, mustard, and poppy seeds and1.4 fo r sand. The critical moisture contents, as readfrom the experimental drying rate curves, are howeversubjective to some extent. This is more so fo r materialsthat exhibit very short constant rate periods (e.g., ragi).However, the large amount of data used in the develop-ment of eq 5 permits prediction of C, o a fai r degree ofaccuracy.The equilibrium moisture content is empiricallyre-lated to air temperature, as (Srinivasa Kannan et al.,1994)

C*= K exp[2000/Til ( 6 )whereK= 1.3 x fo r mustard, ragi, and poppy seedsand 4 x for sand.The predictions using the aforementioned equationsare satisfactorily compared in Figure 6 with the experi-mental data of earlier investigators for drying of resinand sand (Chandran, 19901, zerolit resin (Mckenzie andBahu 19861, corn (Uckan and Ulku, 19861, and mustard(Thomas and Varma, 1992) in batch fluidized beds.

For the chosen material and drying conditions, thecritical and equilibrium moisture contents are predictedusing eq 5 and 6. Knowing the initial moisture content,the flow rate and temperature of air, and the solidsholdup in the batch fluidized bed, the drying rate in theconstant rate period is estimated using eq 1. The solidsmoisture content for a given drying time during thefalling rate period is predicted using eq 2 for materials

I I I I I

having internal moisture and using eq 4 for materialswith no internal porosity.Continuous Fluidized Bed Driers. (a) SpiralFluidized Bed Drier. Figure 7 shows typical variationof the relative moisture content of solids with airtemperature and solids mean holding time in the spiralfluidized bed drier. Mean holding time is the ra tio ofsolids holdup t o the solids flow rate. Solids holdup inthe continuous fluidized bed is influencedby he air flowrate, the solids rate, and the downcomer height pro-jected into the fluidized bed. An increase in air r atedecreases solids holdup. Further, the increase in airrate provides a larger heat input t o the smaller holdup


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Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995 30733 6 3r I I I , 11.0

3 1 30 0 .6 0. 8 1 . 2 1 . 6Length of sp i ra l ( f rom cen tre 1 ( m

8

6 .-U.4'u

2

Figure 8. Variation of temperature and solids moisture contentalong the length of the single-stage spiral fluidized bed drier.

!I1 3 dp =1.48 x 10Vf :0.06;WS: 2 . 6Matl . : R a g iC i - 0 . 2 7 4

IIB a t c h1IJrlll0.2 0.4 0 . 6 0.8 1.0030 -c ICiFigure 9. Variation of bed temperature and relative moisturecontent in batch and continuous single-stage spiral fluidized beddriers.of solids; the effect is to decrease the moisture contentof solids leaving the fluidized bed drier.An increase in solids rate increases solids holdup;

however it decreases the mean holding time of solids,since the increase in solids holdup is not proportionalto the solids flow rate. A decrease in solids holding timecoupled with an increase in solids holdup increases themoisture content of solids leaving the fluidized bed drier.An increase in solids holdup by increasing the down-comer height, keeping the flow rates of air and solidsconstant, decreases the moisture content of solids leav-ing the fluidized bed drier due to decrease in heat inputper unit solids holdup.Figure 7 also compares the performance of the spiraland batch fluidized bed driers. Solids moisture content,determined experimentally by taking solids samplesfrom different locations along the length of the spiralfluidized bed drier, and the air temperature notedlikewise along the spiral length using thermocouples,are shown in Figure 8 and compared with the dataobtained in the batch fluidized bed drier in Figure 9.The similarity and close agreement of the temperatureand concentration profiles in the spiral and batchfluidized bed driers explain the identical performanceof the units.The performance of the spiral fluidized bed drier ispredicted from batch kinetics and the solids residencetime distribution (RTD). RTD of solids in spiral fluid-ized beds has been investigated by Pydisetty et al.(1989) for different configurations of bed geometry usingdifferent materials . The axial dispersion number wasrelated t o the system variables as (Pydisetty et al., 1989)

The average moisture content of solids leaving the spiralfluidized bed drier is given by

where E(8) he exit age distribution function is givenby (Pydisetty et al., 1989)E(@ = ,&e- exp[-(l - I2 "1 d e (9)4n 3312 4e

Substituting eq 9 in eq 8, the integral is numericallyevaluated for the batch kinetics (eq 2) given fo r thematerials with internal moisture. Figure 10acomparesthe experimental data with the predictions using eq 8.Likewise on substitution of eqs 1 and 4a fo r batchkinetics and eq 9 fo r RTD of solids, the experimentaldat a are satisfactorily matched with t he predictions ofthe model for drying of sand in the spiral fluidized beddrier (Figure lob). The analysis of the experimentaldat a on spiral fluidized bed driers covering comparisonwith the performance of batch fluidized bed drier as wellas with the model predictions shows that the cross-current driers are comparable to their performance t othe batch driers for materials with interna l moistureand for materials possessing only the external moisture.(b)Single-Stage and Multistage Fluidized BedDriers. Typical experimental data showing the effectof air temperature and solids holding time on therelative moisture content of solids in a single-stagecontinuous fluidized bed drier are shown in Figure 11.The effects of air rate , solids rate, and downcomer heighton solids holdup and on solids holding time in single-stage and multistage fluidized bed driers are qualita-tively similar to the effects of these variables reportedearlier for the spiral fluidized bed driers.The performanceof the single-stage continuous fluid-ized bed drier is modeled using eq 8 on the assumptionof ideal mixing fo r solids, viz., E(8)= exp(-8), andcompared satisfactorily with experimental data in Fig-ure 11.Figure 12 compares the performance of single-stage,two-stage, and three-stage continuous countercurrentflow fluidized bed driers with the performance of thebatch fluidized bed drier. It is seen that the batchfluidized bed drier gives better performance than thesingle-stage continuous fluidized bed drier; the perfor-mance of two-stage and three-stage fluidized bed driersis superior to the performance of the batch fluidized beddrier (and therefore the spiral fluidized bed drier). Theimproved performance may be at tributed to the follow-ing:

(1) Countercurrent operation gives higher effectiveconcentration driving force than the cross-current op-eration; (ii) an increase in the number of stages in-creases the solids holdup in the multistage fluidized bed,thereby increasing the mean holding time of solids inthe drier; (iii) staging of the fluidized bed using hori-zontal perforated plates offers the flow close t o pistonflow for both phases (solids phase alone approachespiston flow in spiral beds); (iv) horizontal perforatedplates in the multistage fluidized bed facilitate crossflow between bubble phase and emulsion phase for the


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3074 Ind. Eng. Chem. Res., Vol.34,No. 9, 995

1 1M a t e r i a l : Mustardo E x p t l- qn 18)-- 0 6 - d p = 1 7X10-3 -

D = 0 . 2 4 5h = 6 O X I O - ~IU

0 6 - V I - 0 . 0 7 4 -TI = 3 3 3

Ma te r i a l : Ragi0 E x p t l .- W. 81 -

.-U- 0 . 6 -tu - 3d = 1.48 X 10o,41' m O . 2 4 5h : .8 ~ 1 0 - i1 V = 0 .074 II T i 3 3 3 , I200 400 600 80020

1,r i d : S a n d

o E x p t l .-Eqn. I 8 1

0 = 0.2LS -2h = S ~ 1 0Ti ~ 3 0 3Vf = 0.1517

' 50 1 0 0 150 200 250 300Mean holding time, 7 ( s

Figure 10. Comparison of experimental data of single-stage spiralfluidized bed drier with predicted data using eq 8: (a, top) ragi;and (b, middle) mustard; a nd (c , bottom) sand.gas phase (Raghuraman and Varma, 1973). This im-proves the concentration driving force from the particlet o its neighborhood.

Murphree Stage Efficiency. It is attempted t oanalyze the performance of the single-stage and multi-stage fluidized bed driers using stage efficiency concept.Murphree stage efficiency,M P E , nd Murphree overallefficiency, M O E , re defined as (Figure 13)

C,* is the equilibrium solids moisture content corre-sponding t o temperature T, in stage n. CN" is theequilibrium value corresponding to temperature TN nthe bottom stage of the multistage fluidized bed drier.Figure 14 shows the variation in solids relativemoisture content and in air temperature, measuredexperimentally, in the three stages of the three-stage

I I I IMa t e r i a l : M u s t a r d

O . S o 200 400 600 800 1000 12001 . 0 * I I 1 I I I

.-U- 0 . 6I U

-Mean holding t ime, t I s )Figure 11 . Effect of a ir temperature and solids holding time onsolids moisture content in continuous single-stage fluidized beddrier for (a)mustard and (b) sand. Comparison of experimentaldata (0 ,A ) with prediction using eq 8.

I I I 1 I0

.U. .Iu

0.

0.I I I 1 1 I

-U.lu

0 2 -

1111111' 100 200 300 4 0 0 500 600Mean holding t ime, 7 I s 1

Figure 12. Effect of number of stages on outlet moisture con tentof solids in the multistage fluidized bed drier for (a) ragi and (b)poppy seeds.fluidized bed drier. Figure 14 ls o gives Murphree stageefficiency estimated for each stage of the three-stagedrier.Murphree stage efficiency is the highest for the topstage t o which solids with the highest moisture contentare fed into the fluidized bed drier. Air temperature ishowever the lowest in the top stage of the fluidized beddrier. The highest Murphree efficiency in the top stagecorresponds t o constant drying rate and the initial stageof the falling rate period.Figure 15 is the plot of Murphree stage efficiencyagainst A (A = P G d G , ) for the three stages of the three-stage fluidized bed drier. i s the ratio of slopes of


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Ind. Eng. Chem. Res., Vol.34, No. 9, 1995 3075

r TI I cn-l t II I ' Y II c n tYn+lItc - cw f fI I cN-l t II t 1 " I

I 1 I " II i I iFigure 13. Schematic representation of solids moisture contentand air humidity in a countercurrent multistage fluidized beddrier.

operating and equilibrium lines. An increase in Rrepresents an increase in air t o solids ratio which isfavorable for drying of solids.Figure 16 shows the variation of Murphree overallefficiency with R for single-, two-, and three-stagefluidized bed driers. It is seen tha t an increase in airtemperature or an increase in R increases Murphreeoverall efficiency. An increase in air inlet temperaturegives higher average temperature for the entire fluid-

ized bed. Likewise, an increase in air-to-solids ratioprovides higher heat input per unit quantity of solidsin the drier. This gives a smaller temperature drop forthe heating medium and a higher Murphree overallefficiency. Figure 17 shows Murphree overall efficiencyfor single-, two-, and three-stage fluidized bed driers.The variation in the band shown in the figure representsthe variation in M O Ewith the experimental conditionscovered in the study. M O Evaries with the air inlettemperature, the air-to-solids ratio, and the air inlethumidity. Air inlet humidity has not been varied in thestudy. As seen from the figure, Murphree overallefficiency increases rapidly with increase in the numberof stages from the single-stage t o three-stage fluidizedbed drier.

1.01 I I I 350

0. 4

0.20.06

3302-t

3 2 0

310

Wa. 0 . 4 .z

0.06 0.10 0.1 4G, (k g m-.' s")Figure 14. Variation of (a )bed temperature and solids moisturecontent, and (b) Murphree stage efficiency with solids flow ra tein a three-stage countercurrent fluidized bed drier.

aS

0.3Material :Ragi

0 0 0.1 0.2 0.3 0. 4 0.5A

Figure 15. Variation of Murphree stage efficiency with in eachof three stages of the multistage fluidized bed drier.(c) Two-Stage Spiral Fluidized Bed Drier. A fewexperiments were conducted using a two-stage spiralfluidized bed drier. Figure 18 compares the variationof relative moisture content of solids leaving single-stageand two-stage spiral fluidized bed driers with a varia-tion in the solids holding time. The performance of

single- and two-stage spiral fluidized bed driers isqualitatively similar t o the performance of single- andtwo-stage countercurrent multistage driers.The superior performance of the two-stage drier overthat of the single-stage drier is due t o near-piston flowfor solids in each stage and due t o the countercurrentoperation in the two-stage bed (see Figure 17). Thevariation in solids relative moisture content and in a irtemperature along the spiral length are typically shownin Figure 19. It is noticed that the significant variationin air temperature and in solids moisture content occursin the top stage of the two-stage drier. In the two-stagedrier, solids with the highest moisture content are fedt o the top stage and air is fed to the bottom stage.


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3076 Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995

0.8-

0" 0 . 6 -r0 . 4 -

0. 2

0.91 I I I I 1

0 M u l t i s t a g e f l u id i s edb e d d r i e r

-1

o . 2t0 10.7 0 .2 0 . 3 0. 4 0 . 5 0.6A

Figure 16. Variation of Murphree overall efficiency with 1 andwith air temperature for single-stage, two-stage, and three-stagefluidized bed driers.

1 . 0 I I I I I 14

Number of s t a g e sFigure 17. Effect of number of stages on Murphree overallefficiency for spiral and mult istage fluidized bed driers.

A S i n g l e s t a g eo Two s t a g e

I U 0 .40.2u

4 0 0 80 0 1200 1600-M e a n h o ld i ng t i m e , t ( sFigure 18. Comparison of performance of single- and two-stagespiral fluidized bed dr iers.Summary and Conclusion

Experiments are conducted in batch, continuoussingle-stage (cocurrent), spiral (cross-current), and mul-tistage (countercurrent) fluidized bed driers, usingmaterials that essentially dry at a constant rate andthose that essentially dry during a falling rate periodunder intraparticle moisture diffusion control. "he inletair temperature, air rate, solids rate, solids holdup,initial moisture content of solids, particle size, andnumber of stages are the variables investigated in thestudy.

3 5 3 I I 1 I I I I I

A , A 3 4 3 0.09083 4 31 I., 0 35 3 jO.0889 1

M a t l . : Ragidp = 1 .4 8 Y

3 3 3t 0 = 0 . 2 4 5It-I 3 2

3 1

$ 00.80.6 .-0 .4 ' "

U.I . 23031 1 1 I 1 I I 1 I 100 0 .8 1 . 6 2 . 4 3 : 2I- I _Top stage 6ot tom s t a g eL e n g t h o f s p i r a l { r o m f e e d l o c a t i o n ( m )-

Figure 19. Variation of solids moisture content and ai r temper-ature along the spiral length in a two-stage spiral fluidized beddrier.

Fluidized bed drying of solids exhibits constant andfalling rate periods. The constant drying rate is influ-enced by the air rate, its temperature, the solids holdup,and the particle size. The critical moisture content isinfluenced in addition by the initial moisture contentof solids. Air temperature is the principal variableinfluencing the falling drying rate and the equilibriummoisture content. The empirical equations givenfo r thecritical and equilibrium moisture content and the ef-fective diffusivity are developed on the basis of experi-mental data of the present study. Though they arespecific to the materials, they are able to show adiscernible trend fo r materials with internal moisturefrom materials without internal moisture. The predic-tions of drying rate using the empirical correlationsagree well with the data of the present study as well asthat reported in the literature for widely differentmaterials.

Air inlet temperature and solids holding time are thetwo principal variables influencing the drying rate incontinuous fluidized bed dryers. Solids holding timedepends on air rate, solids rate, and the geometry ofthe solids discharge tube. The performance of spiralfluidized bed driers fo r materials with o r withoutinternal moisture closely corresponds t o the perfor-mance of batch fluidized bed driers, and it is satisfac-torily predicted using batch drying kinetics and theresidence time distribution of solids in the spiral fluid-ized beds.

The performanceof the continuous single-stage fluid-ized bed drier is inferior to the performanceof the batchfluidized bed drier; the performance of the continuousdrier is satisfactorily predicted using batch kinetics andassuming of ideal mixing for solids.The performance of two- and three-stage countercur-rent fluidized bed driers is superior t o that of the batchfluidized bed drier. This is attributed t o a highereffective driving force resulting from countercurrentoperation, to a higher holding time for solids due t omultistaging of the fluidized bed, and t o cross flow ofair between the emulsion and bubble phases to give


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Ind. Eng. Chem. Res., Vol.34,No. 9, 1995 30771= K * G ~ G ,a = R/(C,- C*)S u b s c r i p t sc = criticalg = gasi = initial, inlets = solidsSuper scr ip t s

= average* = equilibriudsaturationLiterature CitedChandran, A. N.; Subba, Rao, S. ; Varma, Y. B. G. Fluidised beddrying of solids. ACIhE J. 990, 36, ( l) , 29-38.Chu, S.T.; Hustrul id, A. Numerical solution of diffusion equations.Trans . ASA E. 196Sa, 11, 705-708.Krishnaiah, Y.; Pydisetty, Y.; Varma, Y. B. G. Residence timedistribution of solids in multistage fluidisation. Chem. Eng. Sci.1982 ,37 (9), 1371-1377.Kunii, D.; Levenspiel, 0.Fluidisation Engineering; Butterworth-Heinemann: London, 1991.McKenzie, K. A.; Bahu, R. E. Material model for fluidised beddrying. In Drying '91; Mujumdar, A. S., Filkova, I., Eds.;Elsevier: New York, 1991; pp 130-141. In Mujumdar, A. S.,Ed. Drying of Solids: Recent developments; John Wiley: NewYork, 1986.Pydisetty, Y.; Krishnaiah, K.; Varma, Y. B. G. Axial dispersion ofsolids in spiral fluidised beds. Powder Technol. 1989,59, 1-9.Raghuraman, J. ; Varma, Y. B. G. A model for residence timedistribution in Multistage system with cross flow between activeand dead regions. Chem. Eng. Sci . 1973,28,585-591.Reay, D.; Baker, C. G. J. Drying. In Fluidisation 1985; Davidson,J. F., Clift, R., Harrison, D., Eds.; Academic Press: London,1985; pp 529-562.Srinivasa Kannan, C.; Subbarao, S.;Varma, Y. B. G. A study ofstable range of operation of Multistage fluidised beds. PowderTechnol. 1994a, 78 (31, 203-211.Srinivasa Kannan, C.; Subbarao, S.; Varma, Y. B. G. A kineticmodel for drying of solids in batch fluidised beds. Ind. Eng.Chem. Res. 1994b,33,363-370.Thomas, P. P.; Varma, Y. B. G. Fluidised bed drying of granularfood materials. Powder Technol. 1992, 69, 213-222.Treybal, R. E. Mass Transfer Operations; McGraw Hill BookCompany: New York, 1981.Uckan,G.; Ulku, S. Drying of corn grains in a batch fluidised beddrier. In Drying of solids; Recent developments; Mujumdar, A.S. , Ed.; Wiley: New York, 1986; pp 91-96.

Received for review December 12 , 1994Revised manuscript received May 3, 1995Accepted May 10 , 1995@IE940733Q

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improved contact time distribution. Murphree stageefficiency is the highest forthe stage to which the solidsare fed into the drier, and Murphree overall efficiencyincreases with increase in air-to-solids ratio and thenumber of stages in multistage fluidized bed driers. TheMurphree overall efficiency for the two-stage spiralfluidized bed drier is higher than the efficiency for thetwo-stage countercurrent fluidized bed drier. The ex-perimental data and its analysis clearly indicate theadvantage of the spiral fluidized bed drier over thesingle-stage continuous drier and that of multistagedriers over the batch fluidized bed drier.NomenclatureA = total surface area of dry solids, m2Ar = Archimedes number, gdp3qg(g, &Ipg2Bi = mass Biot number, K,R$D,nC = moisture content of solids, kg of moisturekg of drydCldt = drying rate, kg of waterl(kg of dry solid-s)D = fluidization column diameter, mD e = axial dispersion coefficient, m2/sDeff = effective diffusivity of moisture, m21sd , = particle size, mE(@) exit age distribution function for solidsFr = Froude number, U,2lgd,g = gravitational constant, m/s2G = mass flow rate of air, kg/(m2s)h = height of downcomer, mK = constant in eq 3 , 5 , and 6K, = external mass transfer coefficient,m /sK* = equilibrium coefficientL = length of the spiral, mM P E= Murphree stage efficiencyMOE= Murphree overall efficiencyN = number of stagesn = nth stagePe = Peclet number, U J J D ,R = constant drying rate, kg of moisture/(kgof dry so1ids.s)Re = Reynolds number, d p e g U g / p gR, = particle radius, mT = temperature of air, Kt = drying time, sU = superficial velocity, d sVf = volumetric gas flow rate, m3/sw d = Holdup of dry solids, kgW, = holdup of wet solids, ( = w d ( l f CJ), kgY = humidity of gas, kg of moisturekg of dry airGreek Let terse = density, kg/m3p = viscosity, kg4m.s)8 = dimensionless time (=t/i)

solid

@ Abstract published in Advance ACS Abs t rac t s , June 15,1995.

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