Paddy Dehydration by Adsorption

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Research Paper: PHPostharvest Technology

Paddy dehydration by adsorption: Thermo-physical

properties and diffusion model of agriculture residues

S. Tirawanichakula, Y. Tirawanichakulb,, E. Snisob

aDepartment of Chemical Engineering, Faculty of Engineering, Prince of Songkla University, HatYai, 90110 Songkhla, ThailandbDepartment of Physics, Faculty of Science, Prince of Songkla University, HatYai, 90110 Songkhla, Thailand

a r t i c l e i n f o

Article history:

Received 31 January 2007

Received in revised form

6 June 2007

Accepted 1 November 2007

Available online 18 December 2007

Dehydration of high-moisture grain kernels using adsorption is an interesting method,

especially when agricultural residues are used as adsorbents. This dehydration method is a

low-temperature drying process that is simple, requires no mechanical equipment, has low

energy consumption and maintains grain quality. The aim of this research was to

investigate the use of rice husk, coconut husk and sago palm rachis to reduce the moisture

content of fresh paddy. The thermo-physical properties and the drying kinetics of these

agricultural residues acting as moisture adsorbents were studied. The bulk density, specific

heat capacity, void fraction, equilibrium moisture content (EMC) and the effective diffusion

coefficient (Deff) of the adsorbents were determined. The bulk density, specific heat capacity

and void fraction were determined at initial moistures contents ranging from 4.1% to 72.4%

dry-basis (d.b.). The EMC andDeffvalues were measured at surrounding temperatures and

relative humidity varying from of 40 to 701C and from 10% to 90%, respectively. For

evaluating the Deffvalue using thin-layer drying (TLD), an experiment was carried out at

inlet air temperatures of 3070 1C. The bulk density and specific heat capacity of all three

adsorbents were linearly dependent on the moisture content and the void fraction was

inversely related to the moisture content. The four commonly cited EMC equations were

fitted to the experimental data. A modified Henderson equation provided the best fit for

describing the EMC isotherm of all the study cases. The modified Henderson and Pabis

model provided a good fit to the experimental TLD results and theDeffvalues, determined

by non-linear regression, were significantly affected by the inlet drying air temperature.

The Deff values were in the range 1.2 1086.2 108 m2 s1. Sago palm rachis had the

highest Deffvalue compared to coconut husk and rice husk. However, the most suitable

adsorbent material was rice husk because it was easy to prepare and it could be

regenerated by low-temperature heating.

&2007 IAgrE. Published by Elsevier Ltd. All rights reserved.

1. Introduction

Sorption processes are used to remove moisture from

materials (Rouquerol et al., 1999). The application of adsor-

bents for moisture reduction and the dehumidification of

paddy using silica gel were reported by Murata et al. (1993).

The results showed that the silica gel dehumidifier for cereal

grains at ambient air temperature was technically feasible.

Applications of moisture adsorption by agriculture residues

have previously been reported and studied (Inoueet al., 2002).

Thailand as an agro-industrial country has large amounts of

agricultural residues such as coconut shell and coconut husk,

ARTICLE IN PRESS

1537-5110/$ - see front matter & 2007 IAgrE. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.biosystemseng.2007.11.001

Corresponding author.

E-mail addresses:[email protected] (S. Tirawanichakul),[email protected] (Y. Tirawanichakul).

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rice husk, starch waste from cassava and sago palm rachis

that can be used in various applications (Malik, 2003).

It is generally agreed that moisture reduction from the grain

kernel, some fruits and food such as rough rice, corn, fish and

starch, is described by unsteady liquid diffusion based on

Ficks law (Henderson & Pabis, 1961). Hill (1949) andCrank

(1975)previously proposed isothermal liquid diffusion models

to describe the drying rate of a grain as a function of

diffusivity. Additionally, they found that the effective diffu-

sion coefficient (Deff) value depended on the inlet drying air

temperature and the initial moisture content. However, a few

previous investigations on application of moisture removal by

adsorption have been reported and studied (Murata et al.,1993; Inoue et al., 2002). The application of moisture deso-

rption for grain using agricultural residues is an interesting

and simple technique which removes the need for mechan-

ical equipment and results in lower energy inputs.

The objectives of this research were to determine the

physical parameters of agricultural residues (rice husk, coco-

nut husk and sago palm rachis) affecting dehydration for

fresh paddy kernel. Thermo-physical properties such as the

specific heat capacity, bulk density and void fraction values of

these agricultural residues need to be determined. In addi-

tion, the equilibrium moisture content (EMC) isotherm, the

effective diffusion coefficient (Deff) and the thin-layer drying

equation for all adsorbents required consideration.

2. Experimental procedures

2.1. Materials

The adsorbents rice husk, raw sago palm rachis and coconut

husk were uniformly rewetted by water spraying to initial

moisture contents of 5.621.5% (d.b.) to determine the EMC

and 14.9%, 20.1% and 27.1% (d.b.) to examine the thin-layer

drying equation, and to 4.172.4% (d.b.) to evaluate specific

heat capacity, bulk density and void fraction.

To determine the equilibrium moisture content, saturatedsalt solutions of potassium nitrate (KNO3), sodium chloride

(NaCl), magnesium nitrate decahydrate (Mg(NO3)2 6H2O),

magnesium chloride decahydrate (MgCl2 6H2O) and lithium

chloride (LiCl) were used.

2.2. Methods

2.2.1. Determination of physical properties

The physical properties of the adsorbents were determined in

terms of the specific heat capacity, bulk density and void

fraction. To determine the specific heat capacity, sago palm

rachis and coconut husk samples were cut into cubes

measuring 23.2mm 23.2mm 23.2mm and rice husk was

formed in a layer with dimensions of 7mm 15mm. Thespecific heat capacity of the agricultural residues was

determined using a calorimeter. Consequently, to calculate

the bulk density, the mass of the samples was weighed

by an electronic balance with an accuracy of 70.01g and

the volume measured using a volumetric flask. The bulk

density was calculated from the mass and volume of the

samples.

2.2.2. Determination of equilibrium moisture content (EMC)

To determine the EMC of the agricultural residues (adsor-

bents), the samples were weighed (2035 g) and kept in

stainless-steel wire nets, hanging in five bottles, whichcontained the five saturated salt solutions (in Section 2.2.1).

The glass bottles were then placed in an incubator at a

constant temperature and a relative humidity (HR) of 4070 1C

and 1090%, respectively. The time to achieve the equilibrium

state between the adsorbent and the saturated salt solution

was approximately 1220 days. The samples were then

removed and weighed to determine the final moisture

content (so-called equilibrium moisture content, Meq) follow-

ing the AOAC (1995) method. Four isotherm models for

predicting the equilibrium moisture content were chosen to

fit the experimental data of EMC, surrounding temperature ( T)

and the HR. They were the modified Brunauer et al. (1938)

(BET), the ChungPfost equation (Chung & Pfost, 1967), themodifiedHenderson (1952)and theHalsey (1948)equations.

ARTICLE IN PRESS

Nomenclature

A,B,C,g,h,k,m and n arbitrary coefficientscp specific heat capacity, kJ kg

11C1

Deff effective diffusion coefficient or effective diffu-sivity, m2 s1

Do Arrhenius factorEa activated energy, kJ kmol

1 K1

HR relative humidity, decimalL characteristic half-thickness, mM moisture content, % (d.b.)Meq equilibrium moisture content, % (d.b.)Mi initial moisture content, decimal (d.b.)Mt moisture content at drying time (t), decimal (d.b.)MR,exp,i experimental data of moisture ratio at the ith

drying time, decimal (d.b.)

MR,pre,i predicted value of moisture ratio at theith dryingtime, decimal (d.b.)

MR moisture ratio at the ith drying time which isdefined as (MtMeq)/(MiMeq), decimal

N number of samplesn number of termsR universal gas constant, 8.314 kJ kmol1 K1

R2 coefficient of determination, decimalT absolute temperature, KTin inlet drying air temperature, 1Ct drying time, sln root of the Bessel function of the first kind of

order zero

r bulk density, kg m3

e void fraction, %



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The equations used were

Modified BET equation (Brunaueret al., 1938)

HR1 HRMeq

1AB

HRB 1

AB , (1)

where HR is the relative humidity in decimals; A and B arearbitrary coefficients; and Meq is the equilibrium moisture

content in decimals (d.b.).

ChungPfost equation (Chung& Pfost, 1967)

lnHR A

RTeBMeq, (2)

whereRis the universal gas constant, 8.314kJ kmol1 K1;Tis

the absolute temperature in K.

Modified Hendersons equation (Henderson, 1952)

1 HR eATMB

eq

. (3)

Halseys equation (Halsey, 1948)

HR eA=RTMBeq. (4)

The constants in these models were determined by the

non-linear regression analysis from the experimental

data. The coefficient of determination (R2) value was used as

the criterion for determining the best fit of the experimental

data.

2.2.3. Development of a thin-layer drying equation and the

effective diffusion coefficient

The samples were dried in a thin layer (thickness of 15 mm)

to determine the effective diffusion coefficient. Drying

was performed at hot-air inlet temperatures of 3070 1C,

which were suitable for the dehydration of adsorbents.

Above drying temperatures of 701C the adsorbents

may burn and decompose. An air velocity for determining

the thin-layer drying kinetics of materials was fixed at

1.8ms1.

Samples were maintained at initial moisture contents of

14.9%, 20.1% and 27.1% (d.b.). The inlet, outlet and ambient air

(wet and dry bulb) temperatures were recorded using a

datalogger with an accuracy of 71 1C. Air velocity was

measured by a hot-wire anemometer with an accuracy of

70.1ms1.

(a)Empirical model:

Empirical models were developed using non-linear regres-

sion. The four conventional models were formulated as

follows:

Newton (Mujumdar, 1987)

MR ekt. (5)

Page (Page, 1949)

MR MtMeqMiMeq ektm. (6)

Two-term model (Sharaf-Eldeenet al., 1980)

MR Aektn Begt. (7)

Modified Henderson and Pabis (Karathanos, 1999)

MR Aekt Begt ceht, (8)

where MR is the moisture ratio at the ith drying time. It isdimensionless and is defined as (MtMeq)/(MiMeq) whereMt,

MiandMeqare the predicted, initial and equilibrium moisture

contents, respectively, decimal (d.b.); A, B, k, g,h, m and n are

arbitrary coefficients; and t is drying time in s.

The empirical equations were evaluated for best fit using

the coefficient of determination (R2). The experiments were

replicated.

(b)Theoretical model:

It is assumed that moisture is transferred by liquid

diffusion and that shrinkage of materials is negligible duringdrying. In the partial differential equations for moisture

diffusion to a single piece of adsorbent, the rice husk is

assumed to be an infinite slab and both the coconut husk and

the sago palm rachis are assumed to be cubic in shape. The

general solution for moisture ratio can be expressed by the

following equation:

For an infinite slab

MRMtMeq

Mi Meq

8p2

X1n1

1

2n 12ep

22n12=4LDefft; (9)

whereL is the characteristic half-thickness of the adsorbent

in m; t is the drying time in s.

For the cubic shape

MRMtMeq

Mi Meq

8p2

3 X1n0

1

2n12

3e2n1

23p2Defft=L2.

(10)

The effective diffusion coefficient (Deff) is conventionally

described by the Arrhenius-type equation as follows:

Deff DoeEa=RT, (11)

whereDois an Arrhenius factor and Eais an activated energy

in kJkmol1

K1

.Many studies have reported that Do depends on the

absolute drying air temperature (T) and the moisture content.

However, this Do value can be determined as a constant

(Muletet al., 1989;Rovedo et al., 1998). The experiments were

replicated three times.

3. Results and discussion

3.1. Determination of physical property

Fig. 1shows the variability of the specific heat capacity of rice

husk, coconut husk and sago palm rachis with the moisturecontent. The experimental data of each adsorbent were fitted

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to linear regression. The results show that the specific heat

capacity (cp) in kJkg1 K1 of all adsorbents has a linear

relation to the moisture content (M) in % (d.b.). These specific

heat capacities are determined corresponding to the pervious

works (Muletet al., 1989). The equations are as follows:

rice husk cp 0:0287M 1:6686, (12)

coconut husk cp 0:0232M 2:2325, (13)

sago palm rachis cp 0:0343M 2:2986 (14)

with values for R2 of 0.953, 0.948 and 0.935, respectively.

The results showed that rice husk had the lowest specific

heat capacity, while at lower moisture contents coconut husk

and sago palm rachis had similar specific heat capacities.

However, at higher moisture contents, the rice husk had a

relatively low heat adsorption compared to the others.

Fig. 2shows the relationship between the bulk density and

the moisture content of the three adsorbents. The bulk

density of the adsorbents was linearly dependent on the

moisture content. Moreover, the bulk density of rice husk hada relatively high value compared to the coconut husk and

sago palm rachis. The bulk densities (r) in kgm3 could be

expressed as follows:

rice husk r 1:27M 80:65, (15)

coconut husk r 0:26M 34:41, (16)

sago palm rachis r 0:26M 56:13 (17)

with values ofR2 of 0.973, 0.918 and 0.932, respectively.

The experimental and fitted void fraction data for all

adsorbents are shown in Fig. 3. The results indicated that

the percentage of void fraction (e) slightly decreases with anincrease in the moisture content. The least-squares regres-

sion analysis indicated that the fitted regression equations

were in close agreement with the experimental data. More-

over, the percentage of void fraction of rice husk was higher

than that for coconut husk and sago palm rachis, respectively

rice husk 0:0922M 98:75, (18)

coconut husk 0:0887M 81:19, (19)

sago palm rachis 0:0483M 69:75 (20)

with values ofR2 of 0.952, 0.974 and 0.973, respectively.

However, rice husk is a more suitable adsorbent than

coconut husk and sago palm rachis, especially in Thailand.

This is because rice husk is a direct by-product of dehusking

of paddy and it is not necessary to cut it into a in rectangular

shape like the others.

3.2. Determination of the equilibrium moisture content

The four EMC desorption models for rice husk, coconut husk

and sago palm rachis were evaluated as shown inTables 13,

respectively. The results indicated that the predicted values

using the modified Henderson model were in close agreementwith the experimental results for the temperature and

ARTICLE IN PRESS

0

1

2

3

4

5

0 10 20 30 40 50 60

Moisture content,% (d.b.)

Specificheatcapacity,

kJkg

1

K

1

Experiment Simulation

Fig. 1 Relationship between specific heat capacity and

moisture content of three Absorbents; , sago palm rachis;, coconut husk; , rice husk; , regression.

0

20

40

60

80

100

120

0 5 10 15 20 25 30

Moisture content, % (d.b.)

Bulkdensity

,kg

(m3)1

Fig. 2 Change in bulk density of rice husk, coconut husk

and sago palm rachis at various moisture contents; , sago

palm rachis; , coconut husk; , rice husk; , regression.

60

70

80

90

100

0 5 10 15 20 25 30

Moisture content, % (d.b.)

Voidfraction,

%

Fig. 3 Change in void fraction of rice husk, coconut husk

and sago palm rachis at various moisture contents; , sago

palm rachis; , coconut husk; , rice husk; , regression.



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relative humidity ranges. TheR2 values of the EMC absorption

isotherm of these three adsorbents simulated using the

modified Henderson equation were 0.942, 0.773 and 0.942

for rice husk, coconut husk and sago palm rachis, respec-

tively. Additionally, the modified EMC equations are as

follows:

rice husk 1HR exp3:03 104TM0:91eq , (21)

coconut husk 1 HR exp2:57 104TM0:92eq , (22)

ARTICLE IN PRESS

Table 1 Equilibrium moisture content constants andcoefficients of different desorption isotherm models forrice husk

Model Arbitrary constants inmodel

R2

A B

Modified BET 0.998 1.638 0.815

Chung and Pfost 7.426 103 0.199 0.996

Modified Henderson 2.750 104 1.130 0.988

Halsey 8.673 103 1.017 0.995

R2, coefficient of determination.

Table 2 Equilibrium moisture content constants andcoefficients of different desorption isotherm models forcoconut husk


R2

A B

Modified BET 0.613 1.557 0.902

Chung and Pfost 3.899 104 0.393 0.731


Halsey 1.010 106 3.103 0.842

R2,coefficient of determination.

Table 3 Equilibrium moisture content constants andcoefficients of different desorption isotherm models forsago palm rachis


R2

A B

Modified BET 0.725 1.415 0.944

Chung and Pfost 1.858 104 0.240 0.996


Halsey 5.071 105 2.528 0.999

R2, coefficient of determination.

0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60

Drying time, min

0 20 40 60

Drying time, min

Drying time, min

Moistureratio,

decimal

0.0

0.2

0.4

0.6

0.8

1.0

Moistureratio,

decimal

0.0

0.2

0.4

0.6

0.8

1.0

Moistureratio,

decimal

0 10 20 30 40 50 60

(a)

(b)

(c)

Fig. 4 Comparison between experimental data and

simulated data from the modified Henderson and Pabis

empirical model at various drying times, drying

temperatures of 3070 1C and a constant airflow rate of

1.8ms1; , drying temperature of 30 1C; , drying

temperature of 50 1C;K, drying temperature of 70 1C; ,

simulation. (a) Rice husk with initial moisture content of

14.9% (d.b.). (b) Coconut husk with initial moisture content

of 27.1% (d.b.). (c) Sago palm rachis with initial moisture

content of 14.9% (d.b.).



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sago palm rachis 1 HR exp1:02 104TM1:30eq (23)

with the values ofR2 of 0.942, 0.773 and 0.942, respectively.

The EMC values of all adsorbents will be used for evaluating

the empirical and semi-theoritical moisture ratio in the next

section.

3.3. Establishment of a thin-layer drying equation and an

effective diffusion coefficient

To validate the moisture ratio (MR) equation, including the

EMC value for adsorbents, the thin-layer drying of rice husk,

coconut husk and sago palm rachis was conducted at the

drying temperatures of 30, 50 and 70 1C. The inlet drying air

velocity was fixed at 1.8m s1. The experimental results were

used to formulate the thin-layer drying equation by curve

fitting [Eqs. (5)(8)]. The rate of measured and predicted

moisture transfer was significantly related to the inlet drying

air temperature. Additionally, drying at high temperatures

was more rapid.Fig. 4compares the measured and predicted

values for moisture content at various drying times. The

results also showed that the proposed modified Henderson

and Pabis model could predict the moisture content in close

agreement with the measurements. The R2 value is higher

than 0.99 as shown inTable 4.

Eqs. (9) and (10) were used to determine the effective

diffusion coefficient (Deff) and the diffusion coefficient at air

temperatures of 3070 1C was formulated using Eq. (11). The

equations were written as follows:

rice husk Deff 0:815e9:69103Tin, (24)

coconut husk Deff 0:914e2:01102Tin, (25)

sago palm rachis Deff 1:260e1:59102Tin , (26)

whereTinwas the inlet drying air temperature in 1C and with

values ofR2 of 0.997, 0.997 and 0.998, respectively.

Figs. 57 show the effective diffusion coefficient versus

drying air temperature. The results show that the Deffvalue

was exponentially related to the drying temperature. At the

drying air temperature range between 30 and 70 1C, the range

of values of theDeffvalue for rice husk [(110) 1010 m2 s1] is

lower than that obtained of coconut husk [(16) 108 m2 s1]

and sago palm rachis [(25) 108

m2

s1

]. This indicates thatcoconut husk and sago palm rachis more rapidly transfer

moisture compared to rice husk. TheseDeffvalues are useful

for predicting the drying kinetic of adsorbents. All of these

physical parameters are necessary to develop a mathematical

model for predicting paddy drying by adsorption. Moreover,

some physical parameters will be useful for studying thecombustion and adsorption of these agricultural residues.

ARTICLE IN PRESS

Table 4 Illustration of arbitrary constants of modified Henderson and Pabis drying model for three different absorbents

Type Absorbent Drying model coefficient of Modified Henderson and Pabis model R2

Mi, % d.b. Inlet drying temperature,1C

A B C g h k

Rice husk 20.1 50 0.4400 0.4393 0.1267 0.2258 0.0130 0.2258 0.999Coconut husk 27.1 50 0.5372 0.4217 0.0412 0.0017 1.7761 0.0526 0.999

Sago palm

rachis

14.9 50 0.1369 0.1776 0.1327 0.0226 0.7173 0.0226 0.991

Mi, initial moisture content;R2, coefficient of determination.

0.0

1.0

2.0

3.0

4.0

0 30 60 90 120

Temperature, C

Diffusioncoefficient,1

0

10,m

2s

1

Fig. 5 Relationship between the effective diffusion

coefficient and the temperature of rice husk; ,

experimental data of rice husk; , non-linear regression.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

0 30 60 90 120

Temperature, C

Diffusioncoefficient,

1

0

8m

2s

1


coefficient and the temperature of coconut husk; ,

experimental data of coconut husk; , non-linear

regression.



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4. Conclusions

The physical properties of three agriculture residues (rice

husk, coconut husk and sago palm rachis) using initial

moisture contents of 4.172.4% (d.b.) were studied. The

important parameters for drying kinetics such as equilibrium

moisture content, thin-layer drying equation and effective

diffusion coefficient were determined at drying temperatures

of 3070 1C and initial moisture contents of 14.9%, 20.1% and

27.1% (d.b.). In a future paper these physical properties will be

used for developing a mathematical model of paddy drying

using these agriculture residues. The following conclusions

are drawn:

1. The physical property of adsorbents in terms of the

specific heat capacity, bulk density and void fraction was

found to be linearly dependent on the moisture content.

The rice husk was found to have a high specific heat

capacity, bulk density and void fraction compared to

coconut husk and sago palm rachis.

2. The modified Hendersons equation developed to predict

the equilibrium moisture content for all adsorbents was

valid for ambient relative humidity in the range between

10% and 90% and temperature in the range of 4070 1C.

3. The empirical equation for thin-layer drying using themodified Henderson and Pabis model provided close

agreement between predicted and measured results for

all three adsorbents.

4. The drying kinetics of adsorbents was well explained by

the diffusion model while an effective diffusion coefficient

can be formed as an Arrhenius function of drying air

temperature.

Finally, it can be concluded that rice husk is the most

suitable agriculture residue for dehydrating fresh paddy. This

is because it is a direct by-product of paddy dehusking and

does not need to be prepared before being used in the drying

process. However, in the absence of rice husk, both sago palm

rachis and coconut husk can be used for moisture removal

from fresh paddy.

Acknowledgements

The authors wish to express their sincere thanks to Graduate

School, Department of Chemical Engineering, Faculty ofEngineering and Department of Physics, Faculty of Science,

Prince of Songkla University, Thailand for their financial

support and facilities. Finally, the authors would like to

gratefully acknowledge Dr. K. Inoue, Agricultural Engineering

Division, the National Agricultural Research Center for

Hokkaido Region (NARCH) Japan for his warmly advice.

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0.0

2.0

4.0

6.0

8.0

10.0

0 30 60 90 120

Temperature, C

Diffusioncoefficient,1

0

8m

2s

1


coefficient and the temperature of sago palm rachis; ,

experimental data of sago palm rachis; , non-linear

regression.


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Paddy Dehydration by Adsorption