For sensitivity analysis of the model, two reference cases were used (GF3_A_02 and GF3_C_03). Therefore, one parameter increased and decreased around the reference value while all other parameters stayed unchanged. An overview of the simulation parameters of the reference cases is given in
Table 3. Special focus lies on the underlying assumptions made in the model development and previous applications of those correlations.
The number of height classes and residence time classes represent the numerical grid of the simulations. The results are not affected by them, as long as the number of residence time classes is >100 and the number of height classes >50. The impact of the cut off residence time value
(see
Section 2.3) is more pronounced. The lower
, the more residence time classes are excluded from the calculations. As particles with the highest residence time will be closest to the equilibrium state, this leads to an increase in particle moisture content and a decrease in particle temperature. Detailed analysis shows that the model predictions lie within the experimental margin of error for
> 90%. It must mentioned that an increase in grid refinement as well as
leads to significant increase in computation time.
Parameters that influence the hydrodynamics of the fluidized bed are the Geldart group and
. The choice of the Geldart group defines the used correlations and parameters for calculation of the bubble volume fraction (
). The detailed sets of equations are described in [
6,
15]. A higher bubble volume fraction results in a smaller portion of gas in direct contact with the particles. Consequently, heat and mass transfer rates are reduced between them, which leads to higher particle moisture contents and lower particle temperatures. However, the overall effect of this is only marginal, i.e., results still lie within the experimental margin of error. Also, negligible impact on the overall results is observed for variation of
, which is used in the calculation of bubble volume fraction and bed height as well.
As observed and described in the literature [
25] as well as in our experiments, the vibration has negligible or no influence on the drying kinetics of the investigated particles. This is represented by the model, which consequently shows negligible sensitivity with respect to vibration frequencies and amplitudes in the investigated ranges. The vibration parameters were chosen similar to industrial applications [
6].
4.2.1. Heat Transfer between Particles and Dryer Wall
Another aspect of the model that has significant influence on the results, is the consideration of heat transfer from particles to the environment through the dryer walls. Therefore, the temperature of the inside of the dryer wall () needs to be estimated or assumed. For this sensitivity study, wall temperatures ranging from 10 –50 are assumed. The results are heavily affected by this. For below ambient conditions, particle moisture content increases and particle temperature decreases strongly. When is well above ambient conditions, the particles are much drier, and temperature increases significantly. These trends are to be expected due to high energy sink or source added to the process, respectively. Potential use of this feature are processes with heated dryer wall or internals as extra heat sources.
As mentioned above, deviations between measured and simulated particle temperature at high drying temperatures of Geldart B and D particles may be explained by not considering heat transfer between the dryer and the environment. The exact temperature of the inside of the dryer wall is unknown, thus it is assumed to have ambient temperature in a first step. This assumption includes the consideration of heat conduction through the dryer wall without thermal insulation. Furthermore, heat transfer resistance from the outside of the dryer wall to the ambient air is also neglected.
The influence of heat transfer between particles and dryer wall is shown exemplary for Cellets in the VFB dryer in
Figure 17 and compared to experiments. The wall temperature was set to ambient temperature of the respective experiment. The results show that predicted particle temperatures are closer to measured values, when heat transfer between particles and dryer wall is considered.
Assuming that the wall temperature inside the dryer equals ambient temperature is just an estimation of the currently investigated cases. In fact, heat transfer from the dryer wall to the ambient air is subject to resistance. Thus, the wall temperature at the inside of the dryer will be slightly higher than ambient temperature due to transfer resistances, i.e., heat conduction through the wall and heat transfer from the outside of the wall ambient air. Consequently, the temperature of the inside of the dryer wall must be slightly higher than ambient temperature which in turn would results in slightly higher particle temperatures than predicted. As mentioned earlier, the discussed deviations between model and experiment are not significant in the considered range of drying temperature for Geldart B and D particles. However, if higher drying temperatures are of interest, detailed knowledge of the wall temperature of the dryer is needed. A further increase in accuracy could be achieved by consideration of heat transfer from the drying air to the environment via convection or heat transfer via radiation.
For particles with smaller diameters (Geldart A), heat transfer between particles and dryer wall has significant influence and cannot be neglected (see
Section 4.1.4).
4.2.2. Number of Transfer Units
Another assumption, made in the model, is that the number of transfer units between bubbles and suspension phase (
) equals 1 at a bed height of 5 cm. Based on this, the heat transfer coefficient between bubbles and suspension (
) is calculated.
Theoretical limits of the number of transfer units are
(inactive bypass, i.e., no mass transfer from suspension to bubble phase) and
(no bypass, i.e., no resistance to mass transfer between suspension and bubble phase) [
37].
The assumption that
was used in preceding versions of this model ([
17,
18,
36,
38]). The assumption was introduced by Groenewold and Tsotsas [
16]. They stated that overall drying kinetics were dominated by the mass transfer between particles and suspension gas. Drying kinetics were less sensitive towards mass transfer between bubbles and suspension [
37]. This assumption seems to be valid, as its use in previous fluidized bed drying models delivered accurate results [
17,
18,
36,
38].
Most values for
, reported in the literature for fluidized bed applications, vary from 0.01–100 [
37].
depends on the bubble characteristics and thereby also on particle properties (i.e., Geldart group). Clear dependencies on Reynolds number or bed height have not been identified [
37]. The values for
, used in the validation cases of this work (
Section 4.1), varied from 1.36–2.6, under the assumption
at
. Considering that particles of different Geldart groups were used in this work and that
values two orders of magnitude higher or lower may still be valid for fluidized beds, the height, at which
may be viewed as a fitting parameter in the model.
Sensitivity analysis of the model regarding
for reference case GF3_A_02 are plotted in
Figure 18. It shows that the original assumption of
at
results in good agreement with the experiment for both particle moisture content and temperature. Further increase of
leads only to minor changes and approaches constant values of the predicted particle properties. These values agree slightly better with the experiments. Higher
negate the effect of bubbles acting as bypass. These observations support the conclusion of Chen et al. [
28] that the bubble phase has negligible effect on the drying kinetics in fluidized beds of Geldart group D particles and that the gas may be considered to be one phase. On the other hand, when
is reduced, the model predictions are sensitive towards the changes and deviate strongly from the experiments. However, lower values for
are unreasonable for Geldart group D particles, due to the general low bypass effect of the bubbles. As bubbles in fluidized beds of Geldart group D rise slower than the suspension gas, the suspension gas travels through those bubbles which results in (almost) ideal heat and mass transfer between the two phases [
28].
Bubbles of Geldart A and B particles travel faster than the suspension gas. Therefore, the bypass effect of the bubbles is more pronounced than for Geldart D particles [
1]. This observation is confirmed by the results of our sensitivity analysis of predicted particle temperature and moisture content of Cellet particles in the GF3 dryer. This is plotted in
Figure 19. The particle temperature is affected strongly by increase or decrease of
alike. The impact of
on particle moisture content is similar as observed in the example of Geldart D particles, i.e., it has less influence for higher
but strong influence for reduced
. Considering the agreement of particle temperature and moisture content with experimental data, the original assumption of
at
is confirmed to deliver accurate model predictions in the investigated cases.
All simulations for validation and sensitivity analysis were conducted on a regular PC (Windows 10 Pro x64, 16 GB RAM, Intel Core i7-6700). The duration of the simulations varied between 1 and 3 min.