POWDER AGGLOMERATION DURING SPRAY DRYING PROCESS by aez97357

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									  POWDER AGGLOMERATION DURING SPRAY DRYING PROCESS
                     A.GIANFRANCESCO1), C.TURCHIULI1), E.DUMOULIN1)
                                1)
                                     ENSIA – UMR GénIAL 1145, Massy Cedex (France)


                                                       ABSTRACT

Instant food powders are usually produced by spray drying of formulated liquid and agglomeration, either in the
drying chamber or in an external fluid bed. During drying, according to the liquid composition, the viscosity of the
initial liquid drops increases more or less rapidly, until the surface reaches a rubbery state considered as sticky,
before further drying. This sticky property is used to realize agglomeration inside the chamber by adding small
solid particles, as fines.
The present study deals with the identification of regions where the drops could be sticky in relation with drying
operating conditions, using a pilot spray dryer. Trials were performed with water and a maltodextrin solution
(40%w/w, DE12) using a rotary atomizer for pulverization. Drying parameters included air temperature (160-180-
220°C) and atomized liquid flow rate (15 to 75ml/min), drying air flow rate was kept constant. The first
experimental step was to get a map of temperatures and relative humidity for drying air. Later these properties will
be related to the sticky state of drops. Conditions where maltodextrin powders were sticky will be used for
agglomeration trials with introduction of solid particles at different places in the drying chamber. Results will be
confirmed with scale-up trials and used in modelling.



1 INTRODUCTION                                                   The first objective of our study was to identify sticky
                                                                 regions inside the spray dryer chamber, that means
The spray drying process is widely used in industry to           zones in which specific particles can become sticky on
produce food powders like milk, coffee, fruit juices or          surface. Then these sticky regions could be exploited
soups. In order to improve instant properties (wettability,      to perform controlled agglomeration trials by
dispersability, solubility) of the final dry powder a further    introduction of other solid particles. Varying the
agglomeration step often follows the simple spray drying.        operating parameters of drying (liquid flow rate and
Agglomeration enlarges particle size (typically from 50-         concentration, air temperature and flow rate) might
80μm to 250-500 μm) and it could be performed either             vary the position of sticky regions and/or the efficiency
outside the drying chamber in an external fluid bed or           of such possible agglomeration.
integrated in the spray drying process. In the first case the
already dried particles are rewetted on surface by sprayed       This study dealt with the characterization of air
water (or other binder solution) while in the second case dry    properties evolution inside the chamber, which is
fines (particles with d<100 μm) are reintroduced into the        assumed to be representative of droplet drying. The
drying chamber. The study of this second kind of                 drying behavior of pure water droplets was compared
agglomeration process is the objective of our work.              with the behavior of “real” solution droplets in which
During spray drying, the initial liquid drop is dried by         water is more or less bound with other constituents.
the surface and its viscosity increases gradually [1],           That could lead to identify regions and conditions for
until reaching a possible sticky state, before further           which controlled agglomeration could be obtained.
drying. Surface stickiness depends on surface
temperature, water content and presence of some
components (i.e. carbohydrates), in relation with their
glass transition temperature Tg [2]. The sticky                  2 MATERIALS AND METHODS
temperature Ts is assumed to be some 10°C higher
than Tg, and it is represented as a function of water
content (or water activity) of the considered substance          The experimental work consisted in obtaining cartographies
in the so-called sticky curve. When the particle surface         of air temperature (T) and relative humidity (RH) inside the
reaches this state, collisions with other particles (sticky      chamber at steady state within a co-current pilot spray dryer
or dry) could lead to agglomeration, also depending on           (NIRO Minor Lab) equipped with a rotary atomizer.
velocity, force, angle and time of contact between
particles [3], [4].                                              Internal air temperature was measured with 12
                                                                 thermocouples (K-type), while air relative humidity was
In industrial practice dry fines are usually introduced at the   determined with a capacitive hygrometer (ROTRONIC)
top of the spray drying tower, but a better comprehension of     and a dew point hygrometer (GENERAL EASTERN)
the involved mechanism could lead to other operational           with sampling of air. Psychrometry was also exploited
choices with consequences on the agglomeration process.          to obtain inlet and outlet air moisture content. Figure 1




                                                                                                                 Page 16
shows the position of temperature and humidity probes           3.1 Pure water drying trials
inside the chamber
                                                                Table 1 summarizes the values of exit air temperature
                                   Water                        and moisture content for all tested conditions of pure
                                                                water drying
  -10
                  +                        PR A                 Inlet air                              144                  159                     174
   0                                                             T (°C)
              +       +
                                                                 Water                         18      36       54     18         54        18      36         54
                  +                        PR B                   flow
                                                                (ml/min)
   22         +        +
                                           PR C                 Exit air                       96      75       51     103        63        114     94         73

                                                                 T (°C)
                                                                  ∆T                           49      70       93     56         96        60      80     101
   44         +        +                                        IN/OUT


                                                                 HR air                       3,6      12       53     2,9        31        2,3     5,6        22
                       +                                        exit (%)
                                             Air
   80                     +R
                                                                Table 1: Air characteristics for the different pure water
                               +                                                     drying trials

height (cm)                                                     The decrease in air temperature from inlet to exit was
                                                                from 50 to 100 depending on the liquid flow rate. For
                                                                any tested condition, the decrease in temperature was
                                                                almost instantaneous at the entrance of the chamber
Figure 1: Positions of probes inside the dryer chamber          while air water content increased rapidly. Figure 2 and
    (+: thermocouples, R: ROTRONIC capacitive                   3 show results for inlet air temperature = 144°C and
       hygrometer, PR: dew point hygrometer)                    different flow rates. From a water mass balance we
                                                                can see that 65 to 89% of the water was quickly
                                                                evaporated in the upper part of the chamber (~15cm).
                                                                Below this zone, there was a small decrease of air
Two sets of atomization trials were performed: one by           temperature (ΔT ~ 5°C) and humidity along the
atomizing pure water and one by atomizing aqueous               chamber height (~1m) with no radial variation. For any
solutions of maltodextrin DE12 as a model product               tested condition, drying at the exit of the chamber was
able to stick. Drying parameters were hot air inlet             completed, with almost all the water evaporated.
temperature (144-159-174°C), atomized liquid flow
                                                                                            0,0500
                                                                    Y (kgH2O/kg dry air)




rate (water to evaporate: 18 – 36 - 54 ml/min) and                                                                                                 98 %
                                                                                                                                             water evaporated
aqueous solutions of maltodextrin (30%, 40%, 50%                                            0,0400     89 %
w/w) while air flow rate (80 kg/h) and rotary atomizer                                           water evaporated                                  94 %
speed (30000 rpm) were kept constant.                                                       0,0300                                           water evaporated
                                                                                                             88 %
                                                                                            0,0200water evaporated                               100 %
Final powder properties were determined, as size by laser
                                                                                                                                            water evaporated
granulometry, the moisture content and the instant properties                               0,0100        65 %
(wettability, flowability, density).                                                                water evaporated
                                                                                             0,0000
                                                                                           -20      0           20       40            60         80      100
                                                                                                                                                   water 18 ml/min
                                                                                                                     height (cm)                   water 36 ml/min
                                                                                                    Air inlet
                                                                                                                                                   water 54 ml/min
                                                                Figure 2: Air water content evolution inside the drying
                                                                chamber for different sprayed water flow rates
                                                                (T inlet hot air = 144°C)
3 RESULTS AND DISCUSSION




                                                                                                                                                  Page 17
                                                                                                                                                                                         Solution 40% MD12
                        180                                                                                                             Water 54 ml/min                                  75 ml/min ( 54 ml/min eau)
                                                                                   water 18 ml/min                                      T water: 14°C                                    T solution: 20°C
                        160                                                        water 36 ml/min                   +                                                     +
                        140                                                        water 54 ml/min                   42             144       +                            50            144
                                                                                                                                                                                                +
                                                                                   Sé i 4                                                                                                       61 HR : 23,5%
         T (°C )




                                                                                                                +         +                                           +         +
                        120                                                                                                                   53 – HR : 44%
                                                                                                                53 +      54                                          63        62
                                                                                                                                                                           +
                        100                                                                                     +
                                                                                                                     54
                                                                                                                          +                    +                      +
                                                                                                                                                                           63
                                                                                                                                                                                +               +
                                                                                                                                               54 – HR : 45%                                    64 HR : 23,7%
                         80                                                                                     55        56                                          63        64

                         60
                                                                                                                +         +                                           +         +
                         40                                                                                     53        54                                          62        62
                  -20 -10 0        10    20      30   40       50        60        70   80   90                           +                         Air 80 kg/h                 +                     Air 80 kg/h
                                                                                                                          54                        RH : 0,2%                   62                    RH : 0,2%
                                    height (cm)                                                                               +                                                     +
                                                                                                                              52 RH : 51%                                           58 – HR : 33%
                                                                                                                                    +                                                   +
Figure 3: Air temperature variation inside the drying                                                                              51                                                   59
chamber for different sprayed water flow rates                                                                                                                    b
                                                                                                            a
(T inlet hot air = 144°C)

                                                                                                            Figure 5: Air temperature and relative humidity inside
3.2 Maltodextrin solution drying trials                                                                     the drying chamber during drying of pure water (a) and
                                                                                                            40% maltodextrin solution (b)
Results of pure water trials were compared with                                                             (with the same amount of water to evaporate, 54 ml/min; air inlet
                                                                                                            temperature = 144°C)
maltodextrin solutions drying trials with the same amount of
water to evaporate.
                                                                                                         Maltodextrin solution drying behavior was different
For maltodextrin aqueous solution (40% w/w) and low                                                      from pure water droplets drying maybe because of
liquid flow rates (25 ml/min corresponding to 18 ml/min                                                  higher viscosity leading to larger size with lower
of water to evaporate), there was a good agreement                                                       exchange surfaces. The maltodextrin solution drying
with air temperature and humidity cartographies                                                          trials also allowed us to identify difficult drying
obtained with pure water, showing that water and                                                         conditions (low air inlet T (144°C) and high liquid flow
maltodextrin droplets followed a similar drying behavior                                                 rate (75ml/min)) in which final powder still had high
(Fig.4).                                                                                                 moisture content (>8%), with important powder
                                                                               Solution 40% MD12         adhesion on the walls of the chamber and loss of
                     Water 18 ml/min                                           25 ml/min ( 18 ml/min eau)product. The corresponding relative humidity of air at
                     T water : 20°C                                            T solution: 20°C
                            +                                                          +                 the exit is high (~33%). Final powder properties are
                                                           +
     +
     102
                            96 – HR : 2,4%
                                                           101
                                                                                       98,5 HR : 1,7%    showed in figure 6.
+
                    144                                +        +              144      +
          +
101       100               +                          102      101                     100 HR : 1,9%
     +                                                      +
                            99,8 – HR : 2,0%
     100                                                    101                         +                                               Property                           value
+          +                                          +             +
99         99                 +                       100           100                 99 HR : 1,8%
                              99,0 – HR : 2,1%                                                                                     d Sauter (μm)                                29
                                                                                                                              Water content (%)                                 8,7
+          +                                           +            +
98         98                                          100          99                                                             Flowability (s)                              NO
           +                        Air 80 kg/h                     +                         Air 80 kg/h                         Wettability (min)                            ~ 10
           98                       RH : 0,2%                       98                        RH : 0,2%
              +                                                     +                                       Figure 6: Final powder properties for maltodextrin trials
              95 RH : 3,2%                                          97 – HR : 3,0%                          (air inlet T = 144°C, liquid flow rate = 75 ml/min , 40%w/w)
                    +                                                         +
                   95                                                         96
                                                                                                            Droplet surface temperature will vary from initial
a                                                     b
                                                                                                            temperature (ambient, aw = 1) to air wet bulb
Figure 4: Air temperature and relative humidity inside                                                      temperature and to exit air temperature (upper limit) in
the drying chamber during drying of pure water (a) and                                                      a limited time (particle residence time). Obtained
40% maltodextrin solution (b) (with the same amount of water                                                experimental data permit to estimate product
to evaporate, 18ml/min; air inlet temperature = 144°C)                                                      temperature and water content during drying (water
                                                                                                            mass balances allow to calculate the amount of water
On the contrary, for high liquid flow rates of                                                              evaporated at different positions inside the chamber
maltodextrin solution (75 ml/min corresponding to 54                                                        and so to estimate water content of drying droplets)
ml/min of water to evaporate) air temperature inside                                                        and so to determine possible sticky regions. These
the chamber was some 10 degrees higher than for                                                             conditions are interesting to consider for improving
pure water drying trials and only less than 70% of                                                          drying and/or performing agglomeration by fines
water is evaporated in the upper part of the dryer                                                          introduction inside the chamber.
(fig.5).




                                                                                                                                                                                    Page 18
4 CONCLUSIONS
An experimental approach to obtain air temperature
and relative humidity cartographies inside the dryer
chamber has been tested and validated during drying
of pure water and of aqueous solution of maltodextrin
for different drying parameters. For any tested
condition, the biggest part of drying occurred in the
upper part of the chamber. The drying behaviour of
maltodextrin solution appeared to be different from that
of pure water especially with increasing liquid flow
rates. Possible sticky regions inside the dryer chamber
and operating conditions, suitable for agglomeration
have been identified.
The further steps of the work will consist in introducing
various fine particles in defined locations inside the
chamber, varying the drying parameters, to study the
feasibility of the agglomeration and the effect of
parameters on final products properties at different
scales (pilot, semi-industrial and industrial). These
experimental data and Computational Fluid Dynamics
(CFD) technology will also be used to simulate the
drying and agglomeration process.


REFERENCES:
[1] Roos, Y.H., Karel, M., 1991. Applying State
Diagrams to Food Processing and Developement,
Food Technology, 66-107.
[2] Roos, Y.H., Karel, M., 1991. Water and Molecular
Weight Effects on Glass Transition in Amorphous
carbohydrates and Carbohydrate Solutions, Journal of
Food Science 56 (6), 1676-1681.
[3] Huntington, D.H., 2004. The Influence of the Spray
Drying Process on Product Properties, Drying Technology
22 (6), 1261-1287.
[4] Palzer, S., 2005. The effect of glass transition on the
desired and undesired agglomeration of amorphous food
powders. Chemical Engineering Science 60, 3959-3968.


ACKNOWLEDGMENTS:

This work was financially supported by the EU 6th
Framework programme through the Marie Curie
Research Training Network Biopowders (MRTN-CT-
2004-512247).




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