INFLUENCE OF AIR VELOCITY IN DEHYDRATION ... - UIB Congres

European Drying Conference - EuroDrying'2011
Palma. Balearic Island, Spain, 26-28 October 2011
INFLUENCE OF AIR VELOCITY IN DEHYDRATION OF POTATO CUBES
G. Clemente 1 , A. Frías 2 , N. Sanjuán 1 , J. Benedito 1 , A. Mulet 1
1 Grupo de Análisis y Simulación de Procesos Agroalimentarios (ASPA)
Departamento de Tecnología de Alimentos
Universidad Politécnica de Valencia
C/ Camino de Vera s/n, 46022 Valencia, España
Tel.: +34 96 387 91 48 E-mail: gclemen@tal.upv.es
2 Departamento de Ingeniería Química. Facultad de Ingeniería Química.
Instituto Superior Politécnico “José Antonio Echeverría”, CUJAE
Ave.114, No. 11901, e/ 119 y 127, Habana, Cuba.
Tel.: +537 260 0641 E-mail: anabel@quimica.cujae.edu.cu
Abstract: Air velocity is an important operating variable operation on hot air drying. For
improving energy efficiency it is necessary to know its influence on the kinetics. The aim
of this work was to establish the effect of air velocity on the drying kinetics of potato
cubes. Potato cubes of 1 cm side were dehydrated at 60ºC and 0.5, 1.0, 1.5, 2.0, 3.0, 4.5,
7.0, 8.0 and 10.0 m/s. Experimental drying kinetics were modeled neglecting shrinkage
and external resistance. It was observed that there is no interest to increase air velocity
above 1.5 m/s, external resistance to mass transfer must be considered below this air
velocity value.
Keywords: potato, modeling, external resistance, air velocity
INTRODUCTION
Dehydrated potatoes are an important source of
carbohydrates for elaboration of food products such
as soups or salads. Manufacturing of dried potatoes
cubes should ensure a product of high quality (Iciek
and Krysiak, 2009) and also an economic
profitability when comparing drying with
refrigeration.
Modeling of drying is very useful for design an
optimization purposes. The most frequent models are
the diffusion ones because of their formulation is
relatively simple and the results obtained are usually
reasonably good. Nevertheless some simplifications
must be done and for that reason an effective
diffusivity is considered. Effective diffusivity
includes the effect of known hypothesis and
unknown phenomena not included in the model
(Mulet, 1994). Namely, the effect of air velocity will
be included if external resistance is not negligible.
In most of the drying models shrinkage is not taken
into account in order to simplify the model and
facilitate its solution.
Another point that must be considered is the
boundary conditions adopted on the material surface.
In order to facilitate the solution of the model, the
most common assumption is that the material surface
is in equilibrium with the drying air throughout the
process, thus external resistance to mass transfer is
neglected (Gou et al., 2004; Blasco et al., 2006).
Nevertheless, sometimes this means that the model
does not provide an accurate description of
experimental conditions although interesting
conclusions can be drawn.
The aim of this work was to establish the effect of air
velocity on the drying kinetics and the relative
importance of external resistance on hot air drying of
potato cubes.
MATHERIALS AND METHODS
Raw material
Monalisa potatoes (Solanum tuberosum) harvested in
Spain were used as raw material. Until sample
preparation potatoes were stored in a refrigerated
chamber at 2 + 0.1 ºC. For sample preparation,
potatoes were left to stabilize at room temperature
(around 24 ºC) for at least 15 hours. Then, they were
peeled and cut, into cubes of 12 ± 0.2 mm side. In
each experiment around 68.3 ± 2.1 g of the sample
were used.

Drying experiments
Drying kinetics were determined in triplicate at 60ºC
and 0.5, 1.0, 1.5, 2.0, 3.0, 4.5, 7.0, 8.0 and 10.0 m/s.
For that purpose a laboratory convective dryer,
described in Sanjuán et al. (2003) was used.
The final moisture content of the samples was
determined in triplicate by means of the AOAC
method (AOAC, 1997).
Mathematical modeling
Modeling of the drying process was done considering
that water transport from the centre of solid to the
surface took place mainly by diffusion (Fick’s law).
The model did not consider shrinkage and external
resistance to mass transfer. The model include the
governing equation (equation 1), the initial condition
(equation 2), the symmetry conditions (equations 3, 4
and 5) and the surface boundary conditions
(equations 6, 7 and 8).
∂Xl
∂t
⎛ 2 2 2 ⎞
⎜
∂ X ∂ ∂
= l X
+ l X
D
+ l ⎟
e⎜
2 2 2 ⎟
⎝ ∂x
∂y
∂z
⎠
l (x, y,z,0) X 0
(1)
X = (2)
∂X l (0, y,z, t) = 0
∂x
∂X l
(x,0,z, t) = 0
∂y
∂X l (x, y,0, t) = 0
∂z
l(L,
y,z, t) X e
(3)
(4)
(5)
X = (6)
X = (7)
l(x,
L, z, t) X e
X = (8)
l (x, y, L, t) X e
Equilibrium moisture content (X e ) was obtained by
modeling together different experimental potato
sorption isotherms from literature (Wang y Brennan,
1991; McLaughlin y Magee, 1998; Chou et al., 2000;
McMinn and Magee, 2003) by means of GAB model
with effect of temperature.
The model was solved by means of Separation of
Variables Method. The goodness of fit was evaluated
considering the explained variance (%var).
RESULTS AND DISCUSSION
According to Mulet (1994), when the plot of
(Ψ d(lnΨ)/dt) 2 versus Ψ is a straight line, external
resistance can be considered as non negligible on the
mass transfer process. If external resistance can be
neglected, the plot is a parabola. In Fig. 1, the plot of
(Ψ d(lnΨ)/dt) 2 versus Ψ is represented for all the air
drying velocities considered.
(Ψ d(lnΨ)/dt) 2
0,0000014
0,0000012
0,000001
0,0000008
0,0000006
0,0000004
0,0000002
0
0 0,2 0,4 0,6 0,8 1 1,2
Fig. 1. Plot of (Ψ d(lnΨ)/dt) 2 versus Ψ for the drying
conditions considered
As it can be observed in Fig. 1, for air velocities of
0.5, 1.0 and 1.5 m/s a straight line was obtained. It
seems that for air velocities higher than 1.5 m/s
external resistance to mass transfer can be neglected.
In Table 1 the values obtained for effective
diffusivity are shown. These results are the average
of the three replications. For all drying kinetics
considered, an explained variance higher that 90%
was obtained.
Table 1. Effective diffusivity values and standard
deviation (sd) for air velocity considered.
Ψ
Air velocity (m/s) D e + sd * 10 10 (m 2 /s)
0.5 2.98 + 0.11
1.0 4.35 + 0.09
1.5 6.02 + 0.17
2.0 8.21 + 0.20
3.0 8.28 + 0.32
4.5 8.74 + 0.17
7.0 8.75 + 0.11
8.0 8.72 + 0.14
10.0 8.70 + 0.11
For air velocity 0.5, 1.0 and 1.5 m/s the value of
effective diffusivity is influenced by air velocity.
Nevertheless, for air velocity of 2.0 m/s or higher, the
value of effective diffusivity does not depend on air
velocity. Effective diffusivity is a property which is
linked to a particular product and must not be
influenced by external conditions. If external
resistance is not considered when it is important,
effective diffusivity will be affected because it
includes the effect of this hypothesis (Blasco et al.
2006).
The values in Table 1 are represented in Figure 2.
0.5 m/s
1.0 m/s
1.5 m/s
2.0 m/s
3.0 m/s
4.5 m/s
7.0 m/s
8.0 m/s
10.0 m/s

De (m 2 /s)
1E‐09
9E‐10
8E‐10
7E‐10
6E‐10
5E‐10
4E‐10
3E‐10
2E‐10
1E‐10
0
0 2 4 6 8 10 12
air velocity (m/s)
Fig. 2. Influence of air velocity on effective
diffusivity.
The results shown in Figure 2 and Table 1 are in
agreement with those in Figure 1. For the
experimental conditions in this study, it seems that
external resistance does not influence mass transfer
process for air velocity 2.0 m/s or higher. These
results are in agreement with other authors in
literature (Blasco et al., 2006; Clemente et al., 2011).
These results show that the increase on air velocity
above 2 m/s do not bring an improvement on the
kinetics. As a consequence, 2 m/s seems to be a
threshold from an energy point of view. For air
velocity lower than 2 m/s an optimal air velocity
should be determined considering all the costs
involved.
CONCLUSIONS
During dehydration of potato cubes at 60ºC, external
resistance to mass transfer can be neglected if air
velocity is 2.0 m/s or higher. The results obtained
show that air velocity during drying should be
established to optimize the operation management
increasing also the energy efficiency.
NOMENCLATURE
D e effective diffusivity m 2 s -1
L half-lenght of the cube m
t time s
X e equilibrium moisture content db
X l local moisture content db
x length co-ordinate m
y length co-ordinate m
z length co-ordinate m
Ψ dimensionless moisture content
REFERENCES
AOAC (1997), Official Methods of Analysis.
Washington D.C.: Association of Official
Chemists.
Blasco, M., García-Pérez, J. V., Bon, J., Carreres, J.
E. & Mulet, A. (2006). Effect of blanching and
air flow rate on turmeric drying. Food Science
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323.
Chou, S.K., Chua, K.J., Mujumdar, A.S., Hawlader
M.N.A. and J.C. Ho (2000), On the intermittent
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Clemente, G., Bon, J., Sanjuán, N. and A. Mulet
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Mulet (2003) Dehydration kinetics of red pepper
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ACKNOWLEDGEMENTS
The authors acknowledge the financial support of the
Generalitat Valenciana from the project
PROMETEO/2010/062.