PROCESS AND ENERGY OPTIMIZATION IN DRYING OF FOAMED MATERIALS

??? 66.047

PROCESS AND ENERGY OPTIMIZATION
IN DRYING OF FOAMED MATERIALS

T. Kudra1, C. Ratti2

Canmet Energy Technology Centre – Varennes, PQ, Canada J3X 1S6 (1);
Department of Soils and Agri-Food Engineering, Laval University,
Quebec, QC, Canada G1K 7P4 (2)

Key words and phrases: apple juice; drying efficiency; drying kinetics; drying
time; energy consumption; foam; foam-mat drying.

Abstract: The paper deals with optimization of drying time and energy
consumption in foam-mat drying of liquid bio-materials. A clarified apple juice mixed
with methylcellulose in mass concentrations 0,5; 1,0; 2,0 and 3,0 % was whipped in a
blender to generate foams of different density varying from 0,21 to 0,45 g/cm3. The
foam samples in layers of 6, 12 and 26 mm thick were then dried in a drying tunnel
using air at 55 °C flowing past the sample surface at a superficial velocity of 0,7 m/s.
Experimental data were interpreted in terms of drying kinetics and energy performance.
It was found that the drying rate of foamed juice depends on foam density and its
characteristics, as well as on the layer thickness. Shorter drying times were obtained for
thinner layers of foams of the same density. A distinct minimum of a drying time was
obtained for foam density of 0,21 g/cm3 induced with 1 % methylcellulose. This foam
exhibited the highest drying efficiency and the lowest energy consumption.


Nomenclature
b – thickness, m;
Y – absolute air humidity, kg/kg;
cH – humid heat, kJ/(kg?K);
?D – instantaneous drying efficiency;
C – concentration, % w/w;
? – density, g/cm3.
E

D – cumulative drying efficiency;
Subscripts
G – mass flow rate, kg/s;

?H – latent heat of evaporation, kJ/kg;
ev – evaporation;
g – gas;
m – mass, kg;
m – material.
Q – heat rate, W;

t – time, min;
Acronyms

T – temperature, °C;
d. m. – dry matter;
u – superficial air velocity, m/s;
MC – methylcellulose;
X – moisture content dry basis, kg/kg;
w/w – weight-by-weight.



Introduction

Foaming of liquid and semi-liquid materials has long been recognized as one of
the methods to shorten drying time. Over the past decade, this relatively old technology
known as foam-mat drying, received renewed attention because of its added ability to
process hard-to-dry materials, obtain products of desired properties (e.g., favorable
rehydration, controlled density), and retain volatiles that otherwise would be lost during
the drying of non-foamed materials. Thus, current research is directed not only to
convective drying of purposely foamed materials in spray dryers, plate dryers, and band
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dryers but also to conventional freeze-drying, as well as microwave drying of frozen
foams with and without dielectric inserts as complementary heat sources (Ratti and
Kudra, 2006).
In general, drying of foamed materials is faster than that of non-foamed ones,
although certain foams such the one from the soymilk (Akintoye and Oguntunde, 1991)
or starfruit (Karim and Wai, 1999) exhibit higher drying rates in the beginning of foam-
mat drying whereas for other materials such as tomato paste (Lewicki, 1975), bananas
(Sankat and Castaigne, 2004), and mango (Cooke et al., 1976) drying rates are greatly
accelerated at the end of drying.
Besides accelerated transport of liquid water to the evaporation front, drying
experts have repeatedly pointed to the increased interfacial area of foamed materials as
the factor responsible for reduced drying time. Because density of foamed materials is
lower than that of non-foamed ones and extends from 0.3 to 0.6 g/cm3, the mass load of
the foam-mat dryer is also lower. However, shorter drying time can not only offset the
reduced dryer load but also increase the dryer throughput. For example, the dryer
throughput can be higher by 32 % when drying foamed apple juice with pulp (own
unpublished data) and by 22 % when drying foamed banana puree (Rajkumar et al.,
2007). A higher throughput can result in a smaller dryer, which would translate into cost
savings. In case of foamed mango, the capital costs of the belt conveyer dryer and the
drum dryer could be lower by about 11 % and 10 %, respectively (Kudra and Ratti,
2006). Our comparative study on drying of non-foamed apple juice and foamed apple
juice of density equal to 0,21 g/cm3 had indicated that foamed juice dries faster, and the
energy consumption is only 0,2 of the one for drying of non-foamed juice which results
from shorter drying time and higher drying efficiency (Kudra and Ratti, 2006).
This study presents the experimental results on drying kinetics and energy
consumption during convective drying of apple juice foams of different density and
layer thickness as these two parameters have the prevailing effect on drying
characteristics over temperature and air velocity.

Materials and methods

Fig. 1 summarized the most important steps of the experimental methodology.
Foam preparation
Clarified apple juice (Rougemont, Québec, Canada) in 250 g batches was whipped
with a pre-selected amount of the foaming agent in a domestic blender (Sunbeam,
Mixmaster) operated at 800 rpm for 5 min. Following our own studies on foam
characteristics (Raharitsifa et al., 2007), the methylcellulose (Methocel, 65HG, Fluka,
Switzerland) at 0,5; 1,0; 2,0 and 3,0 % w/w, were used to generate foams having the
respective density of 0,21; 0,21; 0,30 and 0,45 g/cm3. Characteristically, the lowering of
Methocel concentration from 1,0 to 0,5 % w/w did not result in lower foam density, and
the similar trend was reported by Karim and Wai (1999) and Raharitsifa et al. (2006).
Foam density was determined using the method described by LaBelle (1966). Thus,
foam from the blender was transferred to a brand-crystallizing dish (80 mm in diameter
and 40 mm height) and weighed with accuracy of 0,01g. Transferring of the foam was
carried out gently to not destroy its structure or trap air voids while filling the dish up to
the rim. Foams were deemed mechanically and thermally stable (Bates, 1964) as no
drainage and collapse was noted for foams at density below 0,5 g/cm3. An exception is
only the foam generated with 0,5 % w/w of Methocel, which appeared to be
mechanically stable but thermally instable as noticeable drainage was observed during
drying.
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Fig. 1. Experimental protocol

Drying trials
Drying experiments were carried out with a commercial hot-air dryer (Armfield
UOP80, UK), which resembles a single-pass wind tunnel having a 0,55 m long drying
chamber with a 0,277 × 0,277 m cross-sectional area. In the centre of this chamber a
removable rack was placed to support two identical Petri dishes located one above the
other in an adjustable distance allowing air flow past both samples at the same velocity.
In these experiments, the superficial air velocity was kept constant at 0,69 ± 0,03 m/s,
and air temperature entering the drying chamber was set at 55 °C and monitored
throughout the experiment. To generate data for energy calculations, the air temperature
at the dryer outlet was also measured, and the outlet air humidity was calculated from
the mass of water evaporated and ambient air humidity (measured with the wet and dry
bulb aspirated psychrometer). To trace the variation of material temperature with time, a
K-type 0,5 mm open junction thermocouple (Omega Engineering, Inc., Stamford, CT,
USA) was inserted into the geometrical centre of each sample.
Prior to each experiment, the dryer was thermally stabilized by passing hot air at
pre-set temperature and velocity for 60 min. The temperature difference at the dryer
inlet and outlet, resulting from heat losses, was then used to adjust the measured
temperature difference for energy calculations.
Two separate experiments were performed with identical samples. In drying
kinetics experiments, the foamed apple juice was poured flash to the rim of a Petri dish
(14,7 cm in diameter) and two dishes were placed on the rack in a drying chamber.
After a given time interval (5 min at the beginning and then 10 and 15 min) the dishes
were withdrawn from the chamber for mass determination (Mettler Toledo balance,
accuracy m 0,01 g). At the same time, the samples were visually inspected for cracks,
shrinkage, color, etc. The bone-dry mass of the samples was determined by drying at
60 °C for 48 hours in a vacuum oven at 12 kPa, using P2O5 as desiccant. Measurements
were done in duplicate, and the arithmetic averages were taken for data interpretation.
In the second type of experiments, twin samples each having embedded
thermocouple for material temperature measurements, were not removed for weighing
but dried continuously over the same time as in drying kinetics experiments. Separate
experiments for drying kinetics and determination of energy consumption as well as the
use of two identical samples in each experiment minimized errors due to the position of
the sample, contact of the thermocouple with the drying material, and the variation of
thermal conditions when withdrawing the sample for mass loss determination.
Data interpretation
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Despite a number of studies on drying of foamed materials, only a few papers
report on the effect of foam characteristics and operating parameters on drying kinetics.
An analysis of the published data indicates that the drying characteristics, quantified by
the drying time and final moisture content, are affected by foam density, layer
thickness, air temperature, and air velocity. It is generally accepted that drying time
shortens with increasing temperature. However, higher air temperatures and prolonged
drying times result in such a significant deterioration of quality attributes that drying at
lower temperatures of about 60 to 70 °C is recommended (Lewicki, 1975; Cooke et al.,
1976; Karim and Wai, 1999; Sankat and Castaigne, 2004) unless higher foam
temperature is compensated by shorter processing time as it is the case of microwave
drying (Brygidyr et al., 1977).
The effect of air velocity was studied by Sankat and Castaigne (2004) and Lewicki
(1975), who found the positive but not profound effect of air velocity on the drying rate.
Thus, in this study, both air temperature and air velocity were not considered as process
variables.
As expected for most of the food products, where the drying rate is controlled by
internal mass transfer, the drying time extends with the layer thickness. Such an effect
of the layer thickness on drying kinetics for foamed apple juice is shown in Fig. 2. A
similar trend was observed for drying of foamed bananas (Sankat and Castaigne, 2004)
and foamed tomato paste (Lewicki, 1975).
Aside from the foam microstructure, defined by the bubble size and size
distribution (LaBelle, 1966; Brygidyr et al., 1977), the foam density regarded as a
lumped parameter appears to have a crucial effect on drying characteristics. This effect
is explicitly revealed only in papers by Lewicki (1975) and Brygidyr et al. (1977) who
studied foam-mat drying of tomato paste. The paper by Karim and Wai (1999) presents
this effect indirectly as drying characteristics of foamed starfruit were related to the
Methocel concentration. As a result of extensive studies Lewicki (1975) found that
drying time shortens with lowering foam density. Interestingly, he spotted the critical
foam density of 0,32 g/cm3 below which the drying either increases when 3-mm layer is
dried at 60,2 °C, remains constant at 75,2 °C, or reduces progressively at 88,5 °C. These
trends become more pronounced when drying progresses and the final moisture content


X/X 0
1,0

b = 26 mm
0,9

b = 12 mm
0,8

b = 6 mm
0,7

0,6

0,5

0,4

Apple juice
0,3

C = 1 % w/w MC
0,2
? = 0,21 g/cm3

0,1

0
0 50 100 150 200 250 300 350 400 450 t, min

Fig. 2. Normalized drying curve for foamed apple juice – effect of layer thickness
(Tg = 55 °C; u = 0,7 m/s)
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approaches 0,05 kg/kg. This critical density did not exist when the tomato paste was
dried in a 1 mm layer. These findings are supported by the results of Brygidyr et al.
(1977) who also found shorter drying time for lower density of tomato paste. No critical
value for foam density in these experiments is credible as drying of 3,2 mm foam with
density equal to 0,34 g/cm3 was performed at 76,7 °C that is for values where no
minimum was found by Lewicki (1975). Such a critical density noted for certain
operating parameters indicates that foam-mat drying can be optimized with respect to
drying time and thus to energy consumption.
A similar optimization problem was identified in the present study. As shown in
Fig. 3, the fastest drying and thus the shortest drying time was obtained for apple juice
foamed with 1 % Methocel (? = 0,21 g/cm3) dried in a 12 mm layer at 55 °C. Further
reduction of Methocel concentration did not affect foam density but resulted in longer
drying time. This effect can be ascribed to the minimum concentration of the foaming
agent that allows generation of the mechanically and thermally stable foam down to
1 % w/w. Although foam of the same density (0,21 g/cm3) obtained with 0,5 % w/w
Methocel was mechanically stable prior to drying, it became thermally-instable, as
progressive drainage was observed in the course of drying. Such a phenomenon
deteriorated the foam structure and altered the heat and mass transfer characteristics.
In our previous study we found that the drying efficiency varies with moisture
content, and that the foamed materials exhibit higher values of drying efficiency and
lower energy consumption than the not foamed ones (Kudra and Ratti, 2006). To
identify the effect of foam properties, the cumulative drying efficiency was calculated
from the following relationships (Kudra, 1998)

energy used for
n
evaporatio at time t

?D =
(1)
input
(
energy ? output energy) at time t

and

t

E
1
D
=

?
? D (t)dt
. (2)
t 0

The energy used for water evaporation during an incremental drying time was
calculated as
X/X

0
1,0

C = 0,5 % w/w MC; ? = 0,21 g/cm3
0,9


C = 1,0 % w/w MC; ? = 0,21 g/cm3
0,8

C = 2,0 % w/w MC; ? = 0,30 g/cm3


0,7
C = 3,0 % w/w MC; ? = 0,45 g/cm3

0,6

0,5

0,4

0,3

Apple juice
0,2
b = 12 mm

0,1

0 0 50 100 150 200 250 300 350 t, min


Fig. 3. Normalized drying curve for foamed apple juice – effect of foam density
(Tg = 55 °C; u = 0,7 m/s)
ED

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0,70
0,60
1,0 % w/w MC (0,21 g/cm3)
0,50
2,0 % w/w MC (0,30 g/cm3)
0,40
0,30
0,5 % w/w MC (0,21 g/cm3)
0,20
Apple juice
0,10
b = 12 mm
3,0 % w/w MC (0,45 g/cm3)
0

0 1 2 3 4 5 6 7 8 X, kg H

2O/kg d.m
Fig. 4. Variation of cumulative drying efficiency with moisture content f
or foamed apple juice of different density
(Tg = 55 °C, u = 0,7 m/s)

?m
?m

ev
Q =
H
? =
(
3
,
2502 ? 376
,
2
m
T ),
(3)
?t
t
?

where Tm, °C is the average material temperature over the incremental time ?t.
The input and output energy with drying air was calculated from the following
relationship with respective parameters determined for inlet and outlet conditions


Q = GgcHTg = Gg ( ,
1 0059 + 861
,
1
Y )T .
g
(4)

The outlet air humidity was calculated from the inlet humidity and mass of evaporated
water.
Fig. 4 presents the variation of the cumulative drying efficiency with moisture
content for different foam density. All curves exhibit very similar runs. Namely, after
reaching a weak depression at the beginning of drying the cumulative drying efficiency
increases to its maximum and the gradually decreases as water evaporates.
Characteristically, the maximum drying (energy) efficiency was obtained for foam with
density of 0,21 g/cm3 induced with 1 % w/w of Methocel. Much lower efficiency was
obtained for the same density foam but induced with 0,5 % w/w of Methocel. This
maximum coincides with the maximum of drying rate and minimum drying time, and
can be attributed to thermal instability of such foam. Characteristically, the maximum of
energy efficiency and minimum of energy consumption were obtained also for foam
prepared with 1 % w/w of Methocel. For example, taking the foam induced with 1 % of
Methocel as the reference one, the specific energy consumption for foams induced with
0,5 % and 2 % w/w of Methocel were respectively higher by 3,2 % and 13, %, which
documents the optimum conditions for both the process and energy use.

Conclusions

Drying kinetics of foamed apple juice depend on foam density and layer thickness.
The highest drying rate thus the shortest drying time was obtained for the thinnest layers
and the foams of the lowest density. However, a notable difference in drying rates is
noted for the foams of the same density but generated with different concentrations of
Methocel as the foaming agent. A distinct maximum of the drying rate with respect to
Methocel concentration can be attributed to the combined effect of foam density and its
structural stability which dramatically reduces at the lowest Methocel concentration.
The maximum of drying efficiency thus minimum of energy consumption appear for the
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same foam density induced with the same Methocel concentration. This indicates that
drying of foamed materials needs optimization of the process in order to minimize
energy consumption.

The authors would like to acknowledge FQRNT (Fonds québécois de la recherche
sur la nature et les technologies, Québec) for their financial support as well as the help
of Monica Araya-Farias and Valérie-Anne Brouillard during the experiments.

References

1. Akintoye, O.A. and Oguntunde, A.O. 1991. Preliminary investigation on the
effect of foam stabilizers on the physical characteristics and reconstitution properties of
foam-mat dried soymilk. Drying Technology 9( ): 245–262.
2. Bates, R.P. 1964. Factors affecting foam production and stabilization of tropical
fruit products. Food Technology 18: 93–96.
3. Brygidyr, A.M., Rzepecka, M.A. and McConnell, M.B. 1977. Characterization
and drying of tomato paste foam by hot air and microwave energy. J. Inst. Can. Sci.
Technol. Aliment. 9(4): 313–319.
4. Cooke, R.D., Breag, G.R., Ferber, C.E. M., Best, P.R. and J. Jones, J. 1976.
Studies of mango processing; 1. The foam-mat drying of mango (Alphonso cultivar)
puree. J. Food Technology 11: 463–473.
5. Karim, A.A. and Wai, C.C. 1999. Foam-mat drying of starfruit (Averrhoa
carambola L.) puree. Stability and air drying characteristics. Food Chemistry 64:
337–343.
6. Kudra, T. 1998. Instantaneous dryer indices for energy performance analysis.
Inzynieria Chemiczna i Procesowa 19: 163–172.
7. Kudra, T. and Ratti, C. 2006. Foam-mat drying: energy and cost analyses.
Canadian Biosystems Engineering 48: 3.37-3.32.
8. LaBelle, R.L. 1966. Characterization of foams for foam-mat drying. Food
Technology 19: 89–94.
9. Lewicki, P.P. 1975. Mechanisms concerned in foam-mat drying of tomato paste.
Trans. Agricultural Academy in Warsaw 55: 1–67 (in Polish).
10. Raharitsifa, N., Genovese, D.B. and C. Ratti, C. 2006. Characterization of
apple juice foams for foam-mat drying prepared with egg white protein and
methylcellulose. J. Food Science 71(3): E142–E151.
11. Rajkumar, P., Kailappan, R., Viswanathan, R., Raghavan, G.S.V. and Ratti, C.
2007. Studies on foam-mat drying of alphonso mango pulp. Drying Technology 25(2):
357–365.
12. Ratti, C. and T. Kudra, T. 2006. Drying of foamed biological materials:
opportunities and challenges. Drying Technology 24(9): 1101–1108.
13. Sankat, C.K. and Castaigne, F.F. 2004. Foaming and drying behaviour of ripe
bananas, Lebensm.-Wiss. U.-Technol. 37: 517–525.


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ISSN 0136-5835. ??????? ????. 2008. ??? 14. ? 4. Transactions TSTU.

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????????????????? ? ???? ????? ?????????????? ???? ?????? ?????????????.
????????? ???????? ???, ???????????? ? ??????????????? ? ????????
???????????? 0,5; 1,0; 2,0; ? 3,0 %, ?????????? ? ???????? ?? ????????? ????
????????? ????????? ?? 0, 21 ?? 0,45 ?/??3. ??????? ???? ?????? ???????? 6, 12
? 26 ?? ???????????? ? ????????? ??????? ??? ???????????? ??????? ???
55 °C, ???????????? ?? ??????????? ??????? ?? ????????? 0,7 ?/?. ??????
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????. ??????????? ????? ????? ???? ???????? ??? ????????? ???? 0,21 ?/??3,
????????? ? 1 %-? ???????????????. ???????????? ????????????? ????? ?
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Optimisierung der Prozesse und der Energieaufwände
bei dem Trocknen der Schaumelstoffe

Zusammenfassung: Im Artikel wird der Prozess der Optimisierung der Zeit des
Trocknens und des Energieverbrauches bei dem Trocknen der Kunstschaumstoffmatte
der flüssigen Biomaterialien betrachtet. Der mit der Metilzellulose vermischte
gereinigte Apfelsaft in der Massenkonzentration 0,5; 1,0; 2,0 und 3,0 % wird im
Blender bis zur Schumerhaltung der verschiedenen Dichte von 0,21 bis 0,45 g/cm3
geschlagen. Die Schaummuster 6, 12 und 24 mm dick warden im Trocknentunnel unter
der Einwirkung der auf die Oberfläche des Musters mit Geschwindigkeit 0,7 m/s
gerichteten Luft bei 55 °C getrocknet. Die Experimentalangeben haben die Interpre-
tation von den Stellungen der Kinetik des trocknens und des Energieverbrauches
bekommen. Es wurde festgestellt, dass die Geschwindigkeit des Trocknens des
Schaumelsaftes von der Schaumdichte, ihren Charakteristiken und auch von der
Schichtdicke abhängt. Für das Trocknen der dünnen Schaumschichten braucht man
weniger Zeit bei der enlichen Schichtendichte. Minimale Zeit des Trocknens wurde bei
der Dichte des mit der 1 % Metilzellulose vermischten Schaumes von 0,21 g/cm3
erhalten. Maximale Effektivität des Trocknens und minimaler Energieverbrauch sind
für diese Schaumart kennzeichend.


Optimisation du procédé de séchage de matériaux moussés

Résumé: Ce travail vise à l’optimisation du temps de séchage et de la
consommation d’énergie durant le séchage en tapis de mousse des biomatériaux
liquides. Du jus de pomme clarifié mélangé avec de la methylcellulose à des
concentrations massiques de 0,5; 1,0; 2,0; et 3,0 % a été fouetté avec un mixeur pour
générer des mousses avec des masses volumiques entre 0,21 à 0,45 g/cm3. Les
échantillons de mousse en couches de 6, 12 et 26 mm d’épaisseur ont été séchés dans un
séchoir à tunnel avec de l’air à 55 °C passant en flux parallèle à la surface avec une
vitesse de 0,7 m/s. Les données expérimentales ont été interprétées en fonction de la
cinétique de séchage et de la performance énergétique. Il a été observé que la vitesse de
séchage du jus moussé dépend de la masse volumique de la mousse et de ses
caractéristiques, ainsi que de l’épaisseur de l’échantillon. Des temps de séchages plus
petits ont été trouvés à des épaisseurs d’échantillon plus minces pour des mousses avec
des masses volumique identiques. Un temps de séchage minimal a été obtenu pour la
mousse de masse volumique de 0,21 g/cm3 fait avec 1 % de methylcellulose. Le séchage
de cette mousse a montré également une efficacité de séchage maximale et une
consommation d’énergie minimale.
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