Energy Convers. Mgmt Vol. 27, No. 3, pp. 289-291, 1987 0196-8904/87 $3.00 + 0.00 Printed in" Great Britain. All rights reserved Copyright © 1987 Pergamon Journals Ltd

DRYING CHARACTERISTICS OF COARSE AGRICULTURAL GRAINS

RAM C H A N D R A and M. S. S O D H A Centre of Energy Studies, Indian Institute of Technology, Hauz Khas, New Delhi ! 10 016, India

(Received 7 May 1986)

Abstract--The drying characteristics of some common coarse agricultural grains, such as wheat, paddy, maize and peas, have been determined. Simple correlations for specific drying rates have been obtained. These correlations are functions of the initial moisture content of the grains, drying air temperature, air flow rate and bed mass. It is found that bed mass and air velocity affect the specific drying rate only marginally.

Drying Drying rate Moisture content Drying temperature Air flow rate Spouted bed dryer Coarse agricultural products

Bed mass

NOMENCLATURE

a x = Constants dp= Particle size (m) D = Diffusivity (m2/s) E = Error (%)

hog = Gas particle heat transfer coefficient (W/m 2 °C) M = Bed mass (kg) Q = Solid moisture content (kg/kg) Q0 = Initial Q of wet solids (kg/kg)

Q0*= Equilibrium moisture content (kg/kg) t = Time (s) T = Temperature (°C) T~ = Gas temperature T o = Initial air temperature (°C) pp = Particle density (kg/m 3) 2 = Latent heat of vaporization (J/kg)

1. INTRODUCTION

It is well known that through-circulation drying gives considerably superior drying performance compared to parallel-flow drying because the direct contact heat and mass transfer rates are much higher for the former case than that of the latter [1]. Amongst the through circulation dryers, the fluidized/spouted bed dryers give the best performance for granular free flowing materials [2-5]. Particularly for particle sizes of a few millimeters, such as wheat, paddy, maize and peas, the most suitable dryer is the spouted bed dryer. The advantage of such a dryer is the possibility of drying large amounts of material in a small size vessel. The thermal performance of these dryers can be improved by using higher drying temperatures without affecting the quality of the product because the particle stays in the spout for only a fraction of the total time. During this time, moisture evaporates only from the particle surface and a high intra- particle moisture gradient is developed. The high particle circulation and heat and mass transfer rates in a spouted bed ensure that the particle temperature never exceeds 50-80°C, even though the inlet air temperature may be as high as 160°C.

In the present paper, the dynamics of drying wheat, paddy, pegs and maize has been experimentally in- vestigated. The temperatures have been measured by using an on-line programmable data acquisition sys- tem. A simple expression has also been developed to predict the specific drying rates of these products. The predictions from this expression are in good agree- ment with experiment.

2. SPOUTED BED DRYING

The complete details of spouted bed drying are beyond the scope of this paper. There are various papers in the literature which give minimum spouting velocity, spouting pressure drop, maximum pressure drop, etc. [2, 6]. However, in what follows, the basic equations for modelling spouted bed drying have been presented.

2.1. Moisture content

The following equations were used by Becker and Sallans [7] for modelling wheat drying in a spouted bed:

dQ D d ( d Q ) dt r 2 dr r2 . . . . d r r (1)

with the boundary conditions

Q = Q 0 at t = 0 ; O<~r<~dp/2 (2)

and

k d t dQ [dt r=R = hps(Tg - Tr=R) - - Dpp2 ~r" (3)

The solution, using standard methods, is

Q - Q * 6 ~ 1 /-n2n2x2"~ Q 0 _ Q , - n 2 =,n-iexp~,- ~ ) . (4)

The above equation can also be simplified to (see Becker and Sallan [7])

289

290 CHANDRA and SODHA: DRYING CHARACTERISTICS OF AGRICULTURAL GRAINS

Velocity meter

g

DBI

OBT

I Dam logger

SBD

~Vo lve

Fig. 1. Schematic diagram of experimental set up. DBT--day bulb temperature; WBT--wet bulb temp0rature; SBD--spouted bed drier.

Q - Q *

Q0 -- Q * - - - e x p ( - 1.59x13), (5)

where

x = ~ p x/-~. (6)

2.2. Overall specific drying rate

The specific drying rate is defined as the rate of moisture evaporation per unit mass of the bed, dQ/dt. It can be obtained by differentiating equation (5) and is given by

dQd.__t_ ~ppD ~,. exp ( n292x2- ) - = + 2 4 ( a 0 - a * ) _ . ( 7 )

n=l

The average overall specific drying rate then becomes

dO D ~bm = - ~ - = 24 d~33 (Q0 - Q*) ,=

The above expression suggests a simple correlation for ~b m as follows:

Ckm=(Qo+alQo+a2To+a3g +a4m), (9)

where the as are constants to be determined from experimental data.

3. ANALYSIS OF EXPERIMENTS

The aim of the experimental study is to determine the drying characteristics of paddy, wheat, maize and peas. The experimental set-up is shown in Fig. 1. Air

from the blower passes through the spouted bed after being electrically heated. The temperature of the heated air is varied by changing the voltage in the electrical heaters, while the velocity is changed by a control valve. The spouted bed consists of a 0.1 m diameter cylinder. The height of the bed was 0.45 m. The bed was covered on top by fitting a thermocouple at its mouth so that sufficient air could pass over a wet wick. The wet and dry bulb temperatures were measured by an on-line programmable data acquisi- tion system. The air velocity was measured by a photoelectric air flow meter.

Experimental procedure

First, the product to be dried is spouted in the bed for some time to remove dust and other impurities. A known amount of water is then added to a known amount of product and is thoroughly shaken for about 25-30 min. The wet product is then left as such for a day so that the moisture concentration becomes uniform inside.

The blower and heaters are then switched on. The dry and wet bulb temperatures are continuously recorded by the data logger. When the air tem- perature becomes constant, a known amount of wet product is poured into the bed. Samples of bed material are taken after different time intervals. These samples are then dried in an oven for about 20 h for wheat, paddy and peas and for about 40 h for maize to determine the moisture content.

4. RESULTS AND DISCUSSION

The experimentally obtained moisture contents for wheat of different drying times are compared with the

C H A N D R A and SODHA: D R Y I N G C H A R A C T E R I S T I C S OF A G R I C U L T U R A L G R A I N S

Table 1. Drying characteristics of wheat

291

M V T O Qo Drying time Q©xp Qpr ~bm,exp ~bm, pr E S. No. (kg) (m/s) (°C) (kg/kg) (rain) (kg/kg) (kg/kg) (kg/kg) (kg/kg) (%)

1 1.7 0.55 45 0.281 28 0.1578 0.1577 6.8444 x 10 ~ 6.9681 x 10 -5 + 1.8 2 2.75 0.63 58 0.195 30 0.1150 0.130 4.444 4.501 a +1.28 3 1.82 0.35 36.5 0.195 28 0.1530 0.158 2.3333 2.408 +3.2 4 2.70 0.57 44.0 0.230 17 0.1741 0.182 3.1055 3.006 -3.2 5 2.67 0.52 58.0 0.23 30 0.1402 0.1455 4.9888 5.00 +0.224 6 2.67 0.43 40.5 0.201 30 0.1540 0.164 2.6111 2.6101 -0.038

Table 2. Drying characteristics of paddy, maize and peas

Drying M V T O Qo time Qexp ~bm,e~p

Material S. No. (kg) (m/s) (°C) (kg/kg) (min) (kg/kg) (kg/kg) Paddy 1 0.15 0.45 45 0.3729 30 0.1529 12.22 × 10 5

2 0.14 0.61 59.0 0.4023 30 0.1216 15.594 3 2.70 0.34 59.0 0.3648 30 0.182 10.1600 4 2.61 0.39 49.0 0.3638 30 0.143 12.2666 5 0.15 0.40 58.5 0.3381 30 0.1054 12.9277 6 0.149 0.38 58.5 0.3245 17 0.1021 12.3555

1 3.1 0.69 52.0 0.132 30 0 . 1 0 5 1.1111 ×10 5 2 3.04 0.75 60.5 0.15 30 0.1239 4.519 3 3.11 0.75 70.0 0.13 30 0.1001 2.9313 4 4.8 0.69 49.0 0.141 30 0.1145 1.4722 5 4.63 0.84 59.0 0.185 30 0.138 2.611

1 3.24 0.82 60 0.695 30 0.4231 2.665 × 10 4 2 3.55 0.75 74.5 0.595 30 0.239 3.49 3 3.15 0.8 60.5 0.735 30 0 . 3 0 1 4.284 4 3.40 0.79 70.5 0.6210 30 0.2948 3.198 5 3.27 0.77 67.5 0.6555 30 0.2963 3.5215

Maize

Peas

Table 3. Constants a0, a~ for different materials

as are Material a 0 a I a 2 a 3 a 4 multiplied by

Wheat - 11.8 48.5 0.115 0.005 0.00067 10 5 Paddy -8.84 54.0 0.054 0.0015 0.00042 10 -5 Maize -2.8 16.4 0.04 0.0018 0.00055 10 -5 Peas -42.96 38.0 0.32 0.0019 0.00035 10 -4

p r e d i c t e d va lue s o b t a i n e d f r o m e q u a t i o n (5) in T a b l e

1. It is seen t h a t t he p r e d i c t i o n s c o m p a r e e x t r e m e l y

well w i th t he m e a s u r e d da t a .

T h e e x p e r i m e n t a l overa l l d r y i n g ra te ~m, is ob-

t a i n e d f r o m (see V i s w a n a t h a n [4])

a o - Qt ~m - - , (10)

60/m

w h e r e

17 m i n for T o > 59°C (11)

tm = 30 m i n for To ~< 59°C.

T h e e x p e r i m e n t a l (])m v a l u e s fo r w h e a t a re g iven in

T a b l e 1. T h e p r ed i c t ed va l ue s o f thin a re a lso i nc l uded

in T a b l e 1 for c o m p a r i s o n . T h e p e r c e n t a g e d i f ference

E is c a l cu l a t ed f r o m

E (t]lm'pr- ¢~ . . . . P) X 100 (12) (~m,exp

a n d is a lso g iven in T a b l e 1. It is seen t h a t t he

d i f fe rence in t he e x p e r i m e n t a l a n d p red i c t ed v a l u e s o f

~b m is r e a s o n a b l y smal l . T h u s , it is seen t h a t e q u a t i o n s

(5) a n d (8) desc r ibe the d r y i n g cha rac t e r i s t i c s o f

c o a r s e a g r i c u l t u r a l g ra ins . T h e resu l t s for t he specific

d r y i n g ra tes for p a d d y , ma i ze a n d pea s a re g iven in

T a b l e 2.

T h e s e e x p e r i m e n t a l d a t a for specific d r y i n g ra tes

h a v e been f i t ted in e q u a t i o n (9). T h e c o n s t a n t s ax for

w h e a t , p a d d y , m a i z e a n d pea s a re g iven in Tab l e 3.

T h e s imp le c o r r e l a t i o n exp re s se s t he d r y i n g c h a r a c -

ter is t ics o f these g r a in s very well.

Acknowledgements--The financial support from DNES is gratefully acknowledged. The author is also grateful to Dr N. K. Bansal for helpful discussions.

REFERENCES

1. R. E. Treybal, Mass Transfer Operations. McGraw-Hil l , New York (1968).

2. K. B. Mathur and N. Epstein, Spouted Beds. Academic Press, New York (1974).

3. K. Viswanathan, D. S. Rao and B. C. Raychaudhuri , Indian chem. Engng 24, 12 (1982).

4. K. Viswanathan, Ph.D. Thesis, I.I.T. Delhi (1983). 5. P. Romankov, In Fluidization (Edited by J. F. Davidson

and D. Harnson). Academic Press, London (1971). 6. K. B. Mathur and P. E. Gischier AIChEJ. 1,157 (1953). 7. H. A. Becker and H. R. Sallans, Chem. Engng Sci. 13,

97 (1967).