full2011_inter.pdf - page 139

2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
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ൌ ටͳ σ ሺ ‡š’ െ ƒŽ ሻʹ ‹ൌͳ (2)
ൌ ටσሺ ‡š’ െ ƒŽ ሻʹ
†ˆ (3)
where Yexp is the measured in the experiment, Ycal the
calculated using the models, Y exp the mean of the
measured in the experiment data and N the number of
data points.
C. Drying Experiment
The drying experiment was performed in a laboratory
fixed-bed dryer. The experiment were carried out at
various drying air temperature and at constant specific
mass flow rate of 0.07 kg of dry air/s-kg of dry rubber
with the block rubber depths bed of 0.30 m. Drying was
continued till the final moisture content of about 0.5%
dry-basis was reached. The drying air temperature was
controlled by a Proportional, Integral, and Derivative
(PID) controller with an accuracy of ±1°C. The inlet and
outlet drying air temperature, ambient air temperature and
the grain temperature in each rubber bed depth were
measured by K-type thermocouple connected to a data
logger with an accuracy of +1qC. The moisture contents
of rubber were determined by ASABE standard method
(ASABE Standards, 1988).
After drying, the dried skim rubber was qualitatively
analyzed for compliance with the STR 20 standard. All of
qualities were measured in duplicate.
D. Mathematical Model
Mathematical model for deep layer drying can be
classified those into three types as follows: non-
equilibrium model (Ababneh, Srivastava, & Lu, 2000;
A.W Aregba & Nadeau, 2007; Brooker, Bakker-Arkema,
& Hall, 1981), near-equilibrium model (Soponronnarit,
1988; Yuttana Tirawanichakul, Prachayawarakorn,
Tungtrakul, Chaiwatpongskorn, & Soponronnarit, 2003;
Yutthana Tirawanichakul, Prachayawarakorn,
Varanyanond, & Soponronnarit, 2004) and logarithmic
model (A.W. Aregba, Sebastian, & Nadeau, 2006). In this
work, development of the mathematical deep layer for
skim rubber drying model based on a near-equilibrium
model was modified from Soponronnarit (Soponronnarit,
1988). A mathematical drying model is composed of
drying model and performance model. The following
assumptions were used in the development of this
mathematical model.
1. The granular of skim block rubbers were of uniform
size and a sphere shape.
2. The thermal equilibrium existed between drying air
and skim block rubbers.
3. The temperatures gradients within the individual
kernel of granular are negligible.
4. The volume shrinkage of a bed of granular skim
block rubbers is negligible during the drying process.
5. During any short time interval, the heat capacities of
the moist air and skim rubber are constant.
6. The air flow is a plug-flow type.
7. The drying chamber walls are adiabatic, with
negligible heat capacity.
Fig.3. Schematic diagram of a fixed-bed dryer.
The major assumption was that there exists
thermal equilibrium between skim rubber and drying air
and the outlet drying air condition from the first thin layer
is used as the input drying air condition for the next layer
and the same calculation procedure is made for all thin
layers with advancing time step. Considering the energy
analysis of fixed-bed dryer, a schematic diagram of a
fixed-bed dryer was illustrated in Fig. 3. The details of
model are as follows:
Drying model
The moist air properties at outlet drying chamber can
be calculated by using the principle of mass and energy
conservation to control volume 1 (CV1) in Fig. 3. From
mass conservation, the assumption is that water vapor
evaporated from the thin layer is equal to the changes of
water vapor in flowing stream. The equation of mass
transfer between hot air inlet and skim rubber can thus be
written as (Soponronnarit, 1988).
‹š ο–ሺ ˆ െ ‹š ሻ ൌ ሺ ‹ െ ˆሻ’ (4)
when, Wmix and Wf is the humidity of dry air before and
after drying (kg of H2 O/kg of dry air), Min and Mf is the
initial and final moisture content (% dry-basis),
respective, mp is the dry mass of product (kg of dry
rubber), mw is the mass of water evaporated from rubber
(kg of water).
From energy conservation, it is assumed that moist
air and product are in thermal equilibrium. The equation
can be derived based on the first law of thermodynamic.
It can thus be written as (Soponronnarit, 1988).
ˆ ൌ ܿ ܽ ‹š ൅ ‹š ൫Š ˆ‰ ൅ܿݒ ‹š ൯൅ ܿݎ ” െ ˆ Š ˆ‰
ܿܽ ൅ ˆ ܿ ݒ൅ ܿݎ (5)
when, c a, c v, cr is the specific heat capacity of dry air,
water vapor and wet rubber (kJ/kg-°C), Tf, Tmix, T r is the