Revista Brasileira de Produtos Agroindustriais, Campina Grande, v.12, n.1, p.45-54, 2010
ISSN 1517-8595
45
INFLUENCE OF AIR PARAMETERS ON SPRAY DRYING ENERGY
CONSUMPTION
Tatumi Kajiyama 1, Kil Jin Park 2
ABSTRACT
The objective of this present research was to simulate and evaluate the performance of
spray dryers in function of air characteristics using Matlab software. Simulations were
conducted to identify the effect of the influence of inlet air temperature (from 100ºC to
140ºC) and of relative humidity of ambient air (from 20% to 80%) on energy
consumption and thermal efficiency. It was found that energy consumption increased as
drying air temperature and as relative humidity of ambient air increased. The thermal
efficiency of the process diminished as drying air temperature and as relative humidity
of ambient air increased.
Keywords: thermal efficiency, temperature, relative humidity, matlab
INFLUÊNCIA DOS PARÂMETROS DO AR NO CONSUMO ENERGÉTICO DO
SECADOR ATOMIZADOR
RESUMO
Este trabalho foi realizado com o objetivo de simular e avaliar o desempenho do
secador atomizador em função das características do ar utilizando o software Matlab.
Simulações foram conduzidas para identificar as influências dos efeitos da temperatura
do ar na entrada (de 100ºC a 140ºC) e da umidade relativa do ar ambiente (de 20% a
80%) no consumo energético e na eficiência térmica. Verificou-se que o consumo
energético cresce com o aumento da temperatura do ar de secagem e da umidade
relativa do ar ambiente. A eficiência térmica diminui com o aumento da temperatura do
ar de secagem e da umidade relativa do ar ambiente.
Palavras-chave: eficiência térmica, temperatura, umidade relativa, matlab
Protocolo 103.012-10 de 28/04/2010
1
PhD - Technology Department of State University of Feira de Santana, Feira de Santana, BA, Brazil, E-mail: [email protected].
2
Professor - School of Agricultural Engineering, University of Campinas, P.O. Box 6011. CEP: 13084-971. Campinas - SP, Brazil. E-mail:
[email protected]. Address to correspondence should be sent.
46
Influence of air parameters on spray drying energy consumption
INTRODUCTION
Drying is one of the oldest and most
common operations employed in industrial
processes to preserve food. Drying is a process
in which water activity in food is reduced by
removing water in order to minimize
deterioration caused by enzymatic and
microbiological reactions and, therefore,
prevent physical and chemical modification of
products. Spray drying is a commonly used
method of drying a liquid feed through hot gas
in the production of powders. This technique is
widely used in food and pharmaceutical
manufacturing and presents not only a low
operating cost, but also a short contact time.
The development of new products has used an
encapsulating agent in microencapsulation by
spray drying. This wide application of spray
drying in research of new products increases the
need for engineers to better understand energy
calculation with mass and heat balance
concerning the spray drying process. With the
scarcity of energy and its rising cost, it is
important to evaluate energy consumption on
the drying process. The cost of heating the
drying air is the most important economic
factor for the drying process. Therefore, the
influence of air characteristics on energy
balance is important when attempting to
understand the drying process.
Spray drying is the process that transforms
fluids to a solid state in order to obtain powder
products. The aim is to dry biological material
as quickly as possible. Although the inlet air
temperature is high, the thermal damages are
few, due to very fast drying (short drying time).
In the development of new products, an
encapsulating agent, with good emulsifying
capacity and low viscosity in aqueous solution,
has been used in microencapsulation by spray
drying. The encapsulation process, initiated
with essential oil to prevent oxidation, volatile
substance loss, and control liberation of flavor,
extended to the incorporation of natural
additives and ingredients to alter texture,
improve nutritional quality, prolong shelf life,
and control properties of processed foods (RÉ,
1998 and 2000). Spray drying is one of the
most employed drying methods in the
encapsulation process due to the wide disposal
of equipment, low process cost, possibilities of
using various types of encapsulation agents,
good retention of volatile compounds, and good
stability of the product (Reineccius, 1989;
Desai & Park, 2005). The process of spray
Kajiyama & Park
drying is realized by dispersion of suspension
drop into the drying chamber, where it contacts
the hot air, in the form of a spray. The drying of
1 m3 of liquid generates 2x1012 uniform
particles measuring 100µ m in diameter,
equivalent to a surface area of 60.000m2
(Masters, 1972). The increase of surface area is
a function of necessity of the drying rate
increase subjected to drying air conditions.
Duffie & Marshall, 1953, presented the
variation
of
feed
concentration,
physicochemical
feed
properties,
feed
temperature, air temperature, and method, as
well as atomization conditions as variables that
affect the properties of powder. The important
advantages of spray drying are: efficient control
of product properties and qualities; drying
thermal sensitive food and pharmaceutical
products by short exposition time of drying;
great quantities to process in continuous
operation; relatively simple equipment;
relatively uniform dimension of particles; and,
low process costs (Filková & Mujumdar, 1995).
The object being dried is always related to the
prediction of the drying process time. The
drying process time is fundamental in order to
design and optimize the industrial drying plant.
The drying rate must be related to a specific
product with a determined process and drying
equipment. The first rate of drying is
characterized by constant rate period, in which
water evaporates as a free liquid without
bounding energy. In this period, the heat and
mass transfer is equivalent. The second rate of
drying is characterized by a falling rate period.
The quantities of water present in the surface of
the drying product are less than the quantities to
evaporate. In this second period, heat transfer is
not compensated by mass transfer, and the
migration of water from inside the material to
the surface limits the drying rate (Park et al.,
2007). With spray drying, these two drying rate
periods occur and the inflection point of these
periods is referred to as a critical point, which
relates to critical moisture content. SINGH &
Heldmann, 1998, and Pereda et al., 2005, cited
that the constant rate period is predominant for
spray drying where the mass transfer occurs at
drop surface. Once the critical moisture content
is reached, the particle structure causes a
reduction in the drying rate and, therefore, the
interior diffusion of the drying material limits
the drying process. Nowadays, with the scarcity
and rising cost of energy, it is important to
evaluate the energy consumption of the drying
process. Masters, 1981, present some
Revista Brasileira de Produtos Agroindustriais, Campina Grande, v.12, n.1, p.45-54, 2010
Influence of air parameters on spray drying energy consumption
disadvantages of spray drying. The outlet air
has residual heat and to reach saturation
condition to have maximum use of energy is
difficult. The possible methods of saving
energy are increasing the inlet air temperature
and diminishing the outlet air temperature
(Kessler, 1981). Brennan, 1992, presents some
changes that would reduce energy consumption:
(1) isolation of equipment; (2) recirculation of
exhaust air; (3) heat recovery of exhaust air; (4)
use of direct heater; (5) drying in two stages;
(6) concentration of feed; (7) automation of the
outlet air temperature. More details regarding
energy consumption reduction can be found in
Zargorzycki, 1983, Masters, 1972, Grikitz,
1986, Heldman & Hartel, 1977, and Driscoll,
1955. The cost of heating the drying air is the
most important economic factor in the drying
process. In addition, the improvements of
computational methods are some of the most
important contributions of technological
progress that help in understanding the drying
process. Advances in computer science and the
facilities provided by the personal computer
enable experimental data to be adjusted to
mathematical models and simulations to be
tested using specific software. Park et al., 2007,
obtained the numerical solution of the Fick’s
Law and Kajiyama & Park, 2008, studied the
influence of feed parameters on spray drying
energy consumption using Matlab, 2001. The
objective of this work was to evaluate the
influence of air properties on spray drying
Kajiyama & Park
47
energy consumption and thermal efficiency
using Matlab, 2001.
MATERIAL AND METHODS
Material
Properties of dairy products according to
Singh & Heldmann, 1998:
•
•
•
Feed flow rate, F=1kg/s;
Critical moisture content, wet basis,
Xc=0,45kgwater/kgproduct;
Final moisture content to calculate
drying time, Xf =0,05kgwater/kgproduct.
The concurrent flow dryer configuration was
used, according to Brennan et al., 1998. The
ambient air considered for simulation was
measured at a temperature of 30º C, 60%
relative humidity, and 16 gwater/kgdryair of
absolute humidity at Feira de Santana, BA,
Brazil. The ambient air was heated to the
desired simulation temperature before entering
the dryer. After drying, the outlet air relative
humidity was fixed at 70%. The characteristics
of the heated air are presented in Table 1. The
characteristics of ambient air with different
relative humidity used in simulation are
presented in Table 2. (Singh & Heldmann,
1998, Himmelblau, 1998 and Smith et al.,
2000).
Table 1- Characteristics of heated air according to temperature
Ta
(ºC)
Twb
(ºC)
λwb
(kJ/kg)
Yf
(gwater/kgdry air)
100
110
120
130
140
37
39
40
41
42
2414.1
2409.3
2406.9
2404.5
2402.1
42.0
45.5
49.0
52.5
56.0
Table 2 - Characteristics of ambient air at various levels of relative humidity
RH
(%)
20
30
40
50
60
70
Twb
(ºC)
36
37
38
39
40
41
λwb
(kJ/kg)
2416.4
2414.1
2411.7
2409.3
2406.9
2404.5
Y0
(gwater/kgdry air)
6
8
10
13
16
19
Yf
(gwater/kgdry air)
38
40
42
44
47
50
Revista Brasileira de Produtos Agroindustriais, Campina Grande, v.12, n.1, p.45-54, 2010
48
Influence of air parameters on spray drying energy consumption
80
42
2402.1
23
Where: Ta is heated air temperature; Twb is wet
bulb temperature; λwb is latent heat at Twb; RH
is the relative humidity of ambient air; Y0 is the
absolute humidity of inlet drying air; and Yf is
absolute humidity of outlet drying air.
Mathematical considerations
The humid heat of moist air is given by:
C s = 1.005 + 1.88Y0
 X − Xf
W = F 0
 1− Xf



(2)
W = M (Yf − Y0 )
(3)
The energy needed to raise
temperature is calculated by Equation 4:
PG aq = M(H aq − H atm )
53
TE = (energy required to evaporate moisture at
T temperature/energy supplied to dryer)
This leads to the following equation used
to calculate TE (Strumillo & Kudra, 1986 and
Kaminski et al., 1989):
TE =
(1)
Where, Cs is humid specific heat of moist air.
According to mass balance (Barbosa-Cánovas
& Veja-Mercado, 2000):
air
(4)
The energy consumption is simulated
using Matlab, 2001 using Equations (1), (2),
(3), and (4).
The energy efficiency (or thermal
efficiency: TE) of the dryer is defined as:
Kajiyama & Park
W∆H
MC pa (Tain − Taatm )
(5)
The energy needed to raise air
temperature is calculated using Equation 6:
PG aq = M(H aq − H atm )
(6)
So, the thermal efficiency was simulated
with Equation 7 using Matlab:
TE =
Wλ wb
PG ev
=
M(H aq − H atm ) PG aq
(7)
The algorithms used in this work with
software Matlab are presented in Figure 1. The
calculations of energy consumption and thermal
efficiency in function of air temperature are
indicated.
The calculations of energy
consumption and thermal efficiency in function
of relative humidity are indicated in
parentheses.
Revista Brasileira de Produtos Agroindustriais, Campina Grande, v.12, n.1, p.45-54, 2010
Influence of air parameters on spray drying energy consumption
Kajiyama & Park
49
START
Data values:
air mass flow (Idem), ambient air temperature
(Idem) and absolute humidity (or drying air temperature), initial
Calculate humid specific heat of
moist air, equation (1).
Calculate Evaporated water flow
Yes
Energy consumption
thermal efficiency
for temperature (or
relative
humidity)
range?
no and
END
No
Data dependent of temperature:
Drying air
temperature, Wet bulb temperature (Idem), Latent
heat (Idem), Absolute humidity of outlet drying air
(Idem), (Absolute humidity of inlet drying air)
Calculate humid specific heat of
moist air, equation (1).
Calculate air mass flow (Idem),
equation (3).
Calculate energy consumption
(Idem), equation (4).
Calculate
thermal
(Idem), equation (7).
efficiency
Figure 1 - Algorithms used to calculate energy consumption and thermal efficiency
RESULTS AND DISCUSSION
Influence of drying air temperature
According to Equations (2) and (3), air
flow decreases as drying air temperature
increases. This diminishing trend is shown in
Figure 2 in function of drying air temperature.
The necessity of air flow is less when a high
temperature is employed, which increases the
drying potential, i.e. the absolute humidity of
inlet drying air diminishes. To visualize the
effect of temperature on relative humidity see
Figure 3, which shows the relative humidity
(RH) in function of drying air temperature at 16
gwater/kgdry air of absolute humidity.
Figure 4 demonstrates the simulation
result of the amount of energy consumption
(PGaq) used to heat the drying air to desired
temperature (Ta).
Revista Brasileira de Produtos Agroindustriais, Campina Grande, v.12, n.1, p.45-54, 2010
50
Influence of air parameters on spray drying energy consumption
Kajiyama & Park
Figure 2 - Air flow (M) in function of drying air temperature (Ta)
Figure 3 - Relative humidity (RH) in function of drying air temperature (Ta)
Figure 4 - Energy consumption (PGaq) in function of drying air temperature (Ta)
Revista Brasileira de Produtos Agroindustriais, Campina Grande, v.12, n.1, p.45-54, 2010
Influence of air parameters on spray drying energy consumption
As expected, more energy is needed to
obtain high temperatures of drying air
(Equation 4). The function is non-linear
because of the variation of the air’s heat
capacity (Equation (1)) and because of air flow
variation (M). The M diminishes as temperature
increases to evaporate the water (W), but this
Kajiyama & Park
51
decreasing is less than heating energy (air
enthalpy).
Figure 5 shows the thermal efficiency
(TE) in function of drying air temperature (Ta).
The thermal efficiency diminishes as drying air
temperature increases, resulting in an increase
of energy consumption in function of increasing
drying air temperature, as shown in Figure 4
.
Figure 5 - Thermal efficiency (TE) in function of drying air temperature (Ta)
Influence of relative humidity
Figure 6 shows the simulation result of
energy consumption to heating air (PGaq) in
function of relative humidity of ambient air
(RH).
Energy consumption increases as
ambient air relative humidity increases. This
fact results in increasing air mass flow (M) with
increasing relative humidity to evaporate the
same water (W).
The energy needed to evaporate water in
food diminishes slightly as relative humidity
increases, as shown in Figure 6. The increasing
wet bulb temperature of heated air diminishes
latent heat as shown in Figure 7.
Figure 6 - Heating (PGaq) and evaporation energy (PGev) in function of relative humidity (RH)
Revista Brasileira de Produtos Agroindustriais, Campina Grande, v.12, n.1, p.45-54, 2010
52
Influence of air parameters on spray drying energy consumption
Kajiyama & Park
Figure 7 - Latent heat (λwb) in function of relative humidity (RH)
The thermal efficiency (TE) diminishes
as relative humidity (RH) increases, as seen in
Figure 8. This result is due to the increase in air
heating energy and the decrease of energy used
for evaporation, which can be seen in Figure 6.
Figure 8 - Thermal efficiency (TE) in function of relative humidity (RH)
CONCLUSIONS
When the drying air temperature rises,
the relative humidity diminishes, leading to an
increase of the water absorption capacity. This
increase of absorption capacity diminishes air
mass flow to the same evaporation rate in
drying. The magnitude of decreasing drying air
flow is less than the magnitude of air heating
energy that caused an increase in energy
consumption as drying air temperature
increased. This heating energy behavior
exhibits decreasing thermal efficiency as drying
air temperature increases. The water absorption
capacity decreases as the air relative humidity
increases, thereby, needing a major quantity of
air mass flow for the evaporation rate in drying.
Therefore, the energy consumption to heat the
air increased as relative humidity increased.
The evaporation energy decreased as relative
humidity increased. These energy behaviors
exhibit decreasing thermal efficiency as air
relative humidity increases.
AKNOWLEDGEMENTS
We are grateful to the State University of
Campinas (UNICAMP), the State University of
Revista Brasileira de Produtos Agroindustriais, Campina Grande, v.12, n.1, p.45-54, 2010
Influence of air parameters on spray drying energy consumption
Feira de Santana, and the National Council for
Scientific and Technological Development
(CNPq).
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Filková, I.; Mujumdar, A. S. 1995. Industrial
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Himmelblau, D.M. Engenharia química: princípios e cálculos, Livros Técnicos e
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C.
1989.
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optimization of drying process. Drying
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Kessler, H. G. 1981. Food engineering and
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Masters, K. 1981. Spray drying handbook,
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Park, K. J. B. 2007. Conceitos de processo e
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<http://www.feagri.unicamp.
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Revista Brasileira de Produtos Agroindustriais, Campina Grande, v.12, n.1, p.45-54, 2010
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Influence of air parameters on spray drying energy consumption
Kajiyama & Park
NOMENCLATURE
Cpa
Cs
F
Hatm
Haq
∆H
M
PGaq
PGev
RH
Ta
Tain
Taatm
Twb
TE
W
Xf
Heat capacity at constant pressure
humid specific heat of moist air
Feed flow rate
Ambient air enthalpy
Hot air enthalpy
Heat of vaporization
Air mass flow
Thermal energy per time
Energy for evaporation per time
Relative humidity
Heated air temperature
Inlet air temperature
Ambient air temperature
Wet-bulb temperature of drying air
thermal efficiency
Evaporated water flow
Moisture content at transition drying rate point, wet
basis
Final moisture content, wet basis
kgwater/kgproduct
X0
Initial moisture content, wet basis
kgwater/kgproduct
Yf
Y0
wb
Absolute humidity of outlet drying air
Absolute humidity of inlet drying air
Latent heat at Twb
Xc
kJ/kg ºC
kJ/kgdry air ºC
kg/s
kJ/ kgdry air
kJ/ kgdry air
kJ/kg
kg/s
kJ/s
kJ/s
%
ºC
ºC
ºC
ºC
kg/s
kgwater/kgproduct
gwater/kgdry air
gwater/kgdry air
kJ/kgwater
Revista Brasileira de Produtos Agroindustriais, Campina Grande, v.12, n.1, p.45-54, 2010