f wall deposit would increase linearly with time. This type of
behaviour can be seen for skim milk in Fig. 2 more strongly
than for maltodextrin in Fig. 3, and it follows the same
behaviour as found by Ozmen and Langrish [4]. However, if
cohesion of particles to each other is more rapid than adhesion
of particles to the wall, the initial deposition rate (adhesion)
would be slow since there would not be many particles on the
spray dryer wall for cohesion to take place. This type of
behaviour is seen with maltodextrin in Fig. 3. The deposition
process is initially by adhesion on the clean surface of the dryer
walls and then, once the particles cover the walls, the cohesion
process of particles onto other particles is likely to control the
deposition process.
Adhikari et al. [5] noticed that there is a significant change in
the internal cohesiveness of maltodextrin droplets as they are
dried, with several regions of moisture contents showing
relatively easy adhesive failure, unlike the behaviour of
fructose. The relative lack of adhesive strength in maltodextrin
would also explain the lesser initial adhesion rate seen in the
experiments reported here, as well as the lower wall deposition
rate for maltodextrin than for skim milk. In this regard, the glass
transition temperature for pure dry lactose, the sugary
component of skim milk, is 101 °C, while that for pure dry
maltodextrin (DE 18) is around 155 °C. The higher glass
transition temperature for dry maltodextrin than for dry skim
milk is also consistent with the lower wall deposition rate with
maltodextrin.
In all cases, the greatest amount of wall deposition was seen
at the bottom of the cone, position 3, as also noted by Ozmen
and Langrish [4], which is consistent with the deposits being
dominated by semi-dried particles coming from the atomizer.
Fig. 4(a) shows a photograph of the interior (the cone) of the
spray dryer after two and a half hour of spraying skim milk,
while Fig. 4(b) shows the corresponding image for spraying
maltodextrin. The wall deposition for skim milk appeared to be
more evenly spread than with the maltodextrin, and the
maltodextrin powder appeared to be concentrated in the top
part of the conical section with all the three experiments using
maltodextrin. The greater unevenness in the appearance of the
deposit, with maltodextrin, may simply be connected with the
lower deposition rate for this material, since Figs. 2 and 3 show
as much if not more variation in the deposition rates for skim
milk, as a function of position, as for maltodextrin. Fig. 5 shows
that the average ratio of the amounts of skim milk to
maltodextrin deposition is approximately 2.8, and that this
average value is a representative ratio at all times and positions,
suggesting that this ratio is a function of the actual material
properties themselves.
Comparing these wall deposition fluxes with previous work,
the wall deposition flux for skim milk measured here on the top
plate was 15 g m−2 h−2
, compared with 13–16 g m−2 h−1
reported by Ozmen and Langrish [4], so these results are
comparable with previous work. The average deposition fluxes
f wall deposit would increase linearly with time. This type ofbehaviour can be seen for skim milk in Fig. 2 more stronglythan for maltodextrin in Fig. 3, and it follows the samebehaviour as found by Ozmen and Langrish [4]. However, ifcohesion of particles to each other is more rapid than adhesionof particles to the wall, the initial deposition rate (adhesion)would be slow since there would not be many particles on thespray dryer wall for cohesion to take place. This type ofbehaviour is seen with maltodextrin in Fig. 3. The depositionprocess is initially by adhesion on the clean surface of the dryerwalls and then, once the particles cover the walls, the cohesionprocess of particles onto other particles is likely to control thedeposition process.Adhikari et al. [5] noticed that there is a significant change inthe internal cohesiveness of maltodextrin droplets as they aredried, with several regions of moisture contents showingrelatively easy adhesive failure, unlike the behaviour offructose. The relative lack of adhesive strength in maltodextrinwould also explain the lesser initial adhesion rate seen in theexperiments reported here, as well as the lower wall depositionrate for maltodextrin than for skim milk. In this regard, the glasstransition temperature for pure dry lactose, the sugarycomponent of skim milk, is 101 °C, while that for pure drymaltodextrin (DE 18) is around 155 °C. The higher glasstransition temperature for dry maltodextrin than for dry skimmilk is also consistent with the lower wall deposition rate withmaltodextrin.In all cases, the greatest amount of wall deposition was seenat the bottom of the cone, position 3, as also noted by Ozmenand Langrish [4], which is consistent with the deposits beingdominated by semi-dried particles coming from the atomizer.Fig. 4(a) shows a photograph of the interior (the cone) of thespray dryer after two and a half hour of spraying skim milk,while Fig. 4(b) shows the corresponding image for sprayingmaltodextrin. The wall deposition for skim milk appeared to bemore evenly spread than with the maltodextrin, and themaltodextrin powder appeared to be concentrated in the toppart of the conical section with all the three experiments usingmaltodextrin. The greater unevenness in the appearance of thedeposit, with maltodextrin, may simply be connected with thelower deposition rate for this material, since Figs. 2 and 3 showas much if not more variation in the deposition rates for skimmilk, as a function of position, as for maltodextrin. Fig. 5 showsthat the average ratio of the amounts of skim milk tomaltodextrin deposition is approximately 2.8, and that thisaverage value is a representative ratio at all times and positions,suggesting that this ratio is a function of the actual materialproperties themselves.Comparing these wall deposition fluxes with previous work,ฟลักซ์สะสมผนังสำหรับนมพร่องมันเนยวัดนี่ด้านบนจานรับ h−2 m−2 15 กรัมเมื่อเทียบกับ h−1 m−2 g 13-16รายงาน โดย Ozmen และ Langrish [4], ผลลัพธ์เหล่านี้จึงเป็นเปรียบได้กับการทำงานก่อนหน้านี้ ตัวช่วยหลอมสะสมเฉลี่ย
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