2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
- 167 -
Abstract--
The purpose of this research is to verify model
for describing equilibrium moisture content of fresh rubber
sheet and to study effect drying conditions on kinetics and
quality of unsmoked sheet (USS) rubber. The five different
drying strategies were carried on by infrared drying, hot
air drying, open sun drying, green house drying and green
house and ventilation. Initial moisture content of fresh
rubber sheet was varied between 30 and 50% dry-basis and
dried samples had final moisture content of 1±0.5% dry-
basis. The experimental results were mathematical
simulated by non-linear regression analysis. Equilibrium
moisture contents (EMC) of natural rubber (NR) were
evaluated using gravimetric-static method among
surrounding temperature of 40-60 °C correlated to
surrounding relative humidity ranging of 10-90%. The
results showed that equilibrium moisture content decreased
with increasing temperature at constant relative humidity.
The Modified Halsey model was found to be the most
suitable for describing the relationship among equilibrium
moisture content, relative humidity and temperature and
mathematical models of Approximation of diffusion model
can predict the behavior of unsmoked sheet rubber was
good. For evaluating the effective diffusion coefficient by
Fick’s law of diffusion was found that the effective diffusion
coefficient increased with temperature. Additionally, drying
at a low temperature has a low specific energy consumption
compared to those drying of high temperature. Moreover,
determine qualities following the highest standard Thai
rubber (STR5L) grade, the results showed that quality of
samples was acceptable in market level.
Index Terms
--
EMC/ unsmoked sheet rubber/ Green house
drying/ Hot air drying/ Quality
I.
N
OMENCLATURE
MR
moisture ratio (dimensionless
)
M
t
average moisture content at drying time (% dry-
basis)
M
i
initial moisture content (% dry-basis)
M
eq
equilibrium moisture content (% dry-basis)
t
drying time (h)
D
effective diffusion coefficient (m
2
/h)
t
drying time (h)
l
thickness of the slab sheet (m)
D
’
Arrhenius factor of the heterogeneous solid (m
2
/h)
E
a
activation energy (kJ/mol-K)
R universal gas constant (8.314 kJ/mol-K)
T
abs
absolute temperature (K)
Data
predicted
moisture content predicted by mathematical
models
Data
experiment
measured moisture content
N number of data points
H
initial
geometric mean thickness of the rubber sheet at the
beginning of the drying experiment
H
final
geometric mean thickness of the rubber sheet at the end
of the drying experiment
M
f
final moisture content (% dry basis)
W
d
mass of dry sample
(
kg
)
P
the amount of energy used (kilowatt - hours)
II.
I
NTRODUCTION
Rubber is the most important economic agricultural
crops in Thailand because of its largest natural rubber
export product. Natural rubber (NR) was roughly
harvested about of 11.37 million rais and raw production
of rubber sheet was about of 3.17 million tons [1]. NR
products are mostly delivered in form of cream
concentrated latex, air dried sheet (ADS) rubber, ribbed
smoked sheets (RSS) rubber, unsmoked sheet (USS)
rubber and block rubber etc. Like most agricultural
products including all natural rubber export is
hygroscopic and the storage environment could
adversely affect its quality [12]. Understanding Moisture
and heat transfer between products and air surrounding
are essential, especially in tropical climate.
Consequently, the study of moisture sorption
characteristics of rubber sheet under various
environmental conditions is imperative. Knowledge of
the moisture sorption characteristics is needed for shelf-
life prediction and determination of critical moisture
content for acceptability and storage of products. They
also
provide
valuable
information
on
the
thermodynamics of moisture sorption [7] and serves as
useful tools for determining the interaction of water and
the substance. It is realized that moisture content is held
in hygroscopic material by physical and chemical forces
[12] and the mechanism of moisture binding is affected
by sorption characteristics of the material. The heat of
sorption therefore provides useful information on the
heat and free energy changes during moisture sorption
processes in foods [11]. Determination of equilibrium
moisture contents was evaluated by gravimetric-static
method [6]. To understanding the drying kinetic, the
experimental data should be carried on and the suitable
effective diffusion coefficient of moisture content should
be presented. This is because diffusion phenomena let us
to know how mechanisms of moisture transfer during
dehumidification of samples.
Thus the objectives of this research are to obtain
experimental equilibrium moisture content of NR and to
determine simulated mathematical model for prediction
evolution of moisture transfer during drying.
Additionally, the quality
of dried natural rubber sheet
Sorption Isotherm and Liquid Diffusion Model
for Unsmoked Sheet Rubber Drying
Y. Tirawanichakul*, W. Suchonpanit** and S. Tirawanichakul**
*Plasma and Energy Technology Research Laboratory, Department of Physics,
Faculty of Sciences, Prince of Songkla University,
(Thailand)
**Energy Technology Research Center and
3
Department of Chemical Engineering
Faculty of Engineering, Prince of Songkla University,
(Thailand)
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