Journal of Scientific & Industrial Research Vol. 6 1 , April 2002, pp 275-279

Hydrodynamics of Annular Circulating Fluidised Bed Drier with Millet and Ragi Particle Mixture

P Sivashanmugam*, Sree Lakshmi and S Sundaram

Department of Chemical Engineering, Regional Engineering College, Tiruchirapalli 620 0 1 5, India.

Received: 1 7 July 200 I ; accepted: 04 January 2002

. Millet and ragi particle mixture with the millet particles having average 2250 Ilm diam and density 1 1 63 .9 kglm3and ragi particles average 1 500 Ilm diam and density 1 3 1 0.4 kglm3, with the mass fraction 0.25 of millet particles and 0.75 of ragi particles were used to study the hydrodynamics of an annular circulating fluidised bed drier. Gap openings were varied from 1 2mm to 72mm and it was found that mass flux of solids remained constant for all air velocities greater than 4.05m/s and for gap openings greater than 56 mm. Dilute phase flow and dense slugging flow regime models were verified. A correction factor, K was incorporated for the dense slugging flow regime. The error in the use of the model for pressure drop calculation was 20 per cent.

Introduction

Fluidised bed have wide applications in the area of drying, combustion and other bioprocess downstream operations because of higher rates of heat and mass transfer, better sol id-gas contact and uniform mixing. However, food grains, when dried in such beds are l ikely to get charred or roasted. Thus, interest in annular circulation fluidised bed drier (ACFBD), for drying food materials has attracted the attention of researchers. In a ACFBD, two aspects, first the hydrodynamics and secondly the drying characteristics of the material handled are of importance.

In earlier works 1 .2 the hydrodynamics of the ACFBD with ragi and sand particle has been reported. The characterisation of various regimes such as, dilute phase and dense phase flow phase have also been discussed. For millet and ragi particles mixture slugging flow regime was observed for an air velocity up to 2 . 1 1 6 mls and di lute phase flow regime in the range of 2.469 mls to 4.405 mls. Mathematical models for pressure drop for the two regimes have been successfully tested 1 .2 and are indicated below:

� p = Ps I ( 1 - E ) g ,

� P = � P..£ + � P k+ � Pf ' � P f = � P f g + � Pf s.

* Author for correspondence

. . . ( 1 )

. . . (2)

. . . (3)

Equation ( 1 and 2) are for dilute phase and slugging flow regimes, respectively. The same models are adopted in the present work.

Experimental Procedure

Figure 1 gives the details of the experimental set up. It consists of a central tube (fluidisation chamber) of 0.0 1 5 m diam, where the solids are fluidised and an outer annular chamber of 0.046 m diam, for re­circulation of solids, which are deflected into it by the hemispherical dish. Placed below the central tube is an air distributor and the opening between the

Rotometer k L .. .

k Porticle movement o b�"rv01ion se ction of l e ngth · L" A __ � L Feed in let

Figure ) -Experimental set up

276 J SCI IND RES VOL 6 1 APRI L 2002

fluidisation chamber and the distributor (X) can be varied. This opening fixes the mass flux of solids into the fluidisation chamber. The position of the movable distributor and hence the opening is measured, using a cathetometer. The overall height of the annular chamber including the hemispherical dish is 1 .90 m.

Uniformly sieved and cleaned mi llet particles with the average 2250 11m diam and density 1 1 63 .9 kg/m3and ragi particles with the average 1 500 11m diam and density 1 3 1 0.4 kg/m3 and with the mass fraction 0.25 of millet particles and 0.75 of ragi particles were used to study the hydrodynamics. For gap openings ranging from 1 2 to 72 mm and air rates from 1 .4 1 1 to 4.409 mls the pressure drop was measured with a Rosemount differential pressure transducer. The mass rate of transport of food grain particles mixtures was determined by noting the time for their downward flow over a predetermined length in the annular chamber. The mass flux of solids, GJ was calculated using the relation :

. . . (4)

Results and Discussion

Figure 2 shows the variation of mass flux of solids with gap opening for the two flow regimes.

The data was fitted in Eq. (5) by regression analysis,

Gs=(JSOOU,,-3SJ6)X-J JS, . . . (5)

with average absolute deviation of 1 2 per cent (Figure 3). It is seen from Figure 2 that for both regimes the mass flux of solids increases with gap opening up to 56 mm and air velocity up to 4.05 mls and thereafter remains constant; for a specific gap opening, the mass flux increases with air velocity. Thus, gap openings of 56 mm and an air velocity of 4.05 mis, l imits the capacity of this set-up for handling solids.

The experimental and calculated values [Eqs ( 1 -3)] of pressure drop for dilute phase and slugging dense flow are shown in Figures 4 and 5 . The maximum error was 2 5 and 5 0 per cent for dilute and slugging flow regimes, respectively. The large errors in the slugging flow regime were reduced by introduction of a suitable correction factor K, called erratic factor, to account for the erratic flow pattern in this regime. Thus Eq.( 1 ) was modified as:

f1 P = K P s i ( 1 -E ) g , . . . (6)

where K was estimated by regression analysis as 0.9 for the particles mixture studied. Figure 5 and 6

4S0.-------------------------------------�----------------�

400

'Wi' 350 E c:r -& 300

� .. ::! 250 "0 II) ... � 200 :I C .. ; ISO �

SO

Millet 25 per cent and Ragi 75 per cent -x'------x

o 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76

X, Gap Opening (mm) Figure 2-Mass flux of solids vs gap opening

� 1 .4 1 1 m/s

- 1 .587 m1s

-tr- 1 .764 mls.

- 2. 1 16 m/s

� 2.469 m1s

� 2822 m/s

�3. 1 7S m/s

-x- 3.527 m/s

-+- 3.704 m/s

-0- 4.056 m/s

- 4.233 m1s

-6- 4.409 m/s

SlY ASHANMUGAM el al. : HYDRODYNAMICS OF ANNULAR CIRCULATING FLUDISED BED DRIER 277

700

600

.-�500 N 5 b1J

�oo � � .... � =300 (oJ -� (oJ �200 Co-'

100

0

.-� Q.. '-' c. 0 ... "CI � ... = til til � ... C.

"CI � .... � '3 (oJ -� U

,---------'--_. __ ._---------

0

scm

4(0)

3CID

2(0)

1(0)

0

0

Millet 25 per cent and

Ragi 75 per cent

100 200 300 400 500

Gs, experimental (kgIrn2.s)

600

Figure 3-Gs experimental vs Gs calculated

MIlet 25 per cent and

Ragi 75 per cent

1(0)

mute Jlha'ie flow

Experirrmtal pm;sure drop (Pa)

700

L ffiX)

Figure 4- Pressure drop calculated using model equation vs experimental values

0 L41 1 rnIs

11 1 587 rnIs

x L764 rnIs

x 2. 1 16 rnIs

0 2.469 rnIs

+ 2.822 rnIs

- 3. 175 rnIs

¢ 3.527 rnIs

• 3.704 rnIs

£ 4.056 rnIs

D 4.233 rnIs

A 4.409 rnIs

x 2.469 rrls I 0 2.822 rr1s

x 3. 175 rrls

¢ 3.527 rrls

0 3.704 rrls

+ 4.056 rr1s

D 4.233 His

11 4.400 nis

278

� 1 .41 1 IDS

o 1 .587 m's

11 1 .764 m's

X 2. 1 16 IDS

1 sel lND RES VOL 6 1 APRIL 2002

MIlet 25 per cent and Ragi 75 per cent

Slugging deme flow

O�----------,-----------,-----------.-----------.---------�

o HXXl

ExperirrEntaI p� drop (Pa) Figure 5 -Pressure drop calculated using model equation vs experimental values without correction factor

� 1 .41 1 IDS

o 1 .587 IDS

I:l 1 .764 IDS

X 2. 1 16 m's

Slugging deffie flow

I I o�----------�-----------.-----------,------------,----------�

o HID 2ero 3ero 5(XX)

Experimntal � drop (Pa)

Figure 6-Pressure drop calculated using model equation vs experimental values with correction factor

SlY ASHANMUGAM et at. : HYDRODYNAMICS OF ANNULAR CIRCULATING FLUDISED BED DRIER 279

indicate the improvement in calculated values of pressure drop in the slugging flow regime with maximum error being 20 per cent instead of 50 per cent.

Conclusions

The position of the air distributor tube (gap opening) has a significant effect on the mass flux of solids handled by the unit. Maximum mass flux of solids for opening of 56 mm has been established. The hydrodynamic model, proposed for pressure drop, can be utilised in scale up studies. A new factor K has been introduced to account for the erratic flow of solids in the slugging flow regime.

References

Sivashanmugam P & Sundaram S, Hydrodynamics of annular circulating fluidized bed drier, Powder Technol. 103 ( 1 999) 1 65- 1 68.

.

2 Sivashanmugam P & Sundaram S, Hydrodynamics of a annular circulating fluidized bed drier with sand particles, J Bioprocess Eng, 23 (2000) 539-54 1 .

Notations Used

A c-Cross-sectional area of f1uidisation chamber (m 2) G s-Mass flux of solids (kg/ m 2 s)

g -Acceleration due to gravity (m/s 2) K -Correction factor to account for erratic flow

behaviour in the slugging flow regime (-)

(-Length of the fluidising section (m)

L - Length of the down flow particle observation section in the annular portion (m)

U 0 -Superficial gas velocity (m/s)

V p-Velocity of particle in the annular portion (m/s)

W -Mass of solid particle in the pre-calibrated annular portion (kg)

X - Gap opening, distance between the distributor and fluidisation chamber (m)

L1 P - Total pressure drop (pa)

L1 Pc - Pressure drop calculated (pa)

L1 P E - Pressure drop experimental (pa)

L1 P f - Pressure drop due to frictional resistance of gas solid mixture (pa)

L1 P fK -Pressure drop for empty tube using fanning friction factor (pa)

L1 P fs -Pressure drop due to frictional resistance of solid (pa)

L1 P k - Pressure drop due to kinetic energy change of solids (pa)

L1 P s - Pressure drop due to static head of solid (pa)

Greek Letters

E -Porosity of bed under fluidising condition (-)

p s -Density of solid (kg/m ')