2.2. Kinetic modelMass transfer kinetic model was used to represent the experimentdata because of there is no reaction between the oil andthe solvents, hexane and petroleum ether (Liauw et al., 2008).Assumption is made that the main mechanism which controlsthe rate of extraction of oil is mass transfer of oil from the waste(solid) to the solvent (liquid). This assumption is in accordancewith other findings (Amin et al., 2010; Saxena et al., 2011). Masstransfer rate can be written as (Liauw et al., 2008):dWAdt ¼ k AðCAi CAÞ ð2Þwhere dWA/dt, mass transfer rate of the coconut waste oil (g/s); CAand CAi, concentration of coconut waste oil in liquid (solvents) attime t (g/m3) and at equilibrium (g/m3), respectively; k, mass transfercoefficient, ms1; A, surface area for mass transfer process, m2.Since the extraction was conducted in a batch process and thevolume was constant throughout the experiment, Eq. (2) can bewritten as:dWAdt ¼ k AV ½WAi WA ð3ÞdWAdt ¼ k a ½WAi WA ð4Þwhere, ka, is volumetric mass transfer coefficient. To solve Eq. (4)by integration, following condition was used where the mass ofcoconut waste oil is zero in liquid (WA) at the beginning of theextraction process. Considering this condition, integration of Eq.(4) resulted as:WA ¼ WAi½1 expðkatÞ ð5ÞRearranging Eq. (5) in terms of yield per mass of coconut waste,the kinetic model used in this study was:YA ¼ YAi½1 expðkatÞ
ð6Þ
where, YA and YAi is yield of coconut waste oil in liquid at time, t, s
and ka-volumetric mass transfer coefficient.
To determine the value of ka (s1
), YA and YAi a nonlinear least
square method was used to calculate numerically. ‘‘Origin 8.5’’ Program
was used to fit the data in order to obtain the mass transfer
value and the yield of oil.
2.3. Mass transfer within the particle
The study of mass transfer within the particle was conducted to
determine that extraction of oil using soxhlet extractor or batch
reactor was not controlled or limited by the internal diffusion. Thiele
modulus was used to investigate the mass transfer within the
particle. To determine the value of Thiele modulus, effective diffusivity
(Deff), m2 s1 was calculated. Fick’s second law was used to
determine effective diffusivity by assuming Deff is constant with
the Y, yield at time, t and initial yield of the oil. Pinelo et al.
(2006) calculated effective diffusivity using following equation
(Pinelo et al., 2006):
ln Y ¼ ln 6
p2
pDeff
r2 t ð7Þ
where, r is the radius of particles diameter (mm).
By using Eq. (4), a plot lnY versus time of experiment was plotted.
The value of the slope was used to determine the effective
diffusivity.
To determine the effect of mass transfer within the particle on
the extraction of the oil, Thiele Modulus, / was calculated based
on Giri and Sharma, 2000 equation (Giri and Sharma, 2000):
/ ¼ dp
6
Kqp
Deff 0:5
ð8Þ
The value dp represents the particle diameter, K is the extraction
rate, s1
, qp is the density of the coconut waste. The system is assumed
to have no internal mass transfer limitation if the Thiele
Modulus is <2, and the system suffers from the internal mass transfer
limitation if it is above 10 (Giri and Sharma, 2000).
2.4. Thermodynamic parameters
Thermodynamic parameters (DH, DS and DG) for the oil extraction
of coconut waste using hexane and petroleum ether were estimated
using following equation (Liauw et al., 2008):
ln K ¼ DG
R
1
T ¼ DH
R
1
T þ
DS
R ð9Þ
K ¼ YT
YU
¼ mL
mS
ð10Þ
where K is equilibrium constant, YT is the yield percent of oil at temperature
T, YU is the percent unextracted oil in coconut waste, mL is
the amount of coconut waste oil in liquid at equilibrium temperature,
mS is the amount of coconut waste oil in solid at T, C equilibS.
Sulaiman et al
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