Study a new atmospheric freeze drying system incorporating a vortex tube and multi mode heat input 6

Chapter-6 Simulation Results CHAPTER SIMUALTION RESULTS This chapter provides a comparison between the predicted and experimental results of a fix bed dryer on AFD process without using adsorbent. Model is described in chapter 4. Three heat input schemes were compared: case1-pure convection, case2-two-stage convection, case3-radiation-coupled convection. Drying kinetics phenomena under different ranges of operating conditions and effect of product thicknesses were compared. Numerical efforts were extended to predict the location and temperature of the sublimation front layer during the course of drying. Finally, moisture content and temperature distribution inside the dry layer were predicted. All results are discussed and presented. 6.1 Comparison Between Experimental and Simulation Results: Using Fix Bed Dryer and Multimode Heat Input 6.1.1 Drying kinetics Variation of moisture content with drying time for pure convection (case-1) at –11oC and –6oC for potato pieces of both disc and rectangle shaped are shown in Figure 6.1 and Figure 6.2, respectively. Higher drying rate was found at higher drying temperature as expected. Final dimensionless moisture content of the experimental and predicted results at –11oC and –6oC was 0.40 and 0.36, and 0.13 and 0.12 for discshape product, and 4.2 and 4.3, and 0.45 and 0.5 for cubical product, respectively, after eight hours of drying time. Good agreement between experiment and simulation results was found in both cases. Physical properties of the experimental dried products (Rahman et. al. 2007) for the AFD under this condition proved a sublimation process 122 Chapter-6 Simulation Results Predicted-Single stage: -6C 0.9 Dimensionless moisture content Predicted-Single stage: -11C 0.8 Measured-Single stage: -6C 0.7 Measured-Single stage: -11C 0.6 0.5 0.4 0.3 0.2 0.1 0 Time, hr Figure 6.1 Variation of measured and predicted dimensionless moisture content with time for disc shaped (16mm x 1mm) potato sample 8.0 Measured-Single stage: -11C Predicted-Single stage: -11C Measured-Single stage: -6C Predicted-Single stage: -6C Moisture content, kg/kg db 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 Time, hr Figure 6.2 Variation of measured and predicted dimensionless moisture content with time for rectangle shaped (10mm x 5mm x 1mm) potato sample 123 Chapter-6 Simulation Results from ice to vapor during drying. Therefore, porous structure and thereby nonshrinkable dried product due to absence of condensation inside the frozen product was observed which was a key assumption in the model, results in a fairly good match with experimental data. Figure 6.3 shows variations of the moisture content with time of disc shaped potato samples for case-2 and case-3. In first-stage, drying process was conducted upto four hours at –11oC for both cases through convection and then stepped up at –6oC for the next four hours through convention heat input for the case-2 and radiation coupled convection heat supply for case-3. In experiment, apparently a higher drying rate was found after four hours of drying time in which drying temperature was stepped up at –6oC from –11oC. High-intensity drying conditions play an important role in enhancing the sublimation rate, i.e. higher drying rate. In the case of multimode heat Predicted-Two stage: -11C & -6C 0.9 Dimensio nl ess mo isture cont ent Predicted-Two stage: -11C & -6C ( Rad) 0.8 Measured-Two stage: -11C & -6C 0.7 Measured-Two stage: -11C & -6C ( Rad ) 0.6 0.5 0.4 0.3 0.2 0.1 0 Time, hr Figure 6.3 Variation of measured and predicted dimensionless moisture content with time for disc shaped (16mm x 1mm) potato sample 124 Chapter-6 Simulation Results input (case-3) shows further improvement in drying rate due to the additional contribution of radiation heat input (Ratti and Mujumdar, 1995). These phenomena are also captured well in the predicted results. Final dimension less moisture content for both experimental and simulation results for case-2 and case-3 after eight hours of drying time were about 0.0775 and 0.049, and 0.058 and 0.0134, respectively. Slightly underprediction was observed in the predicted results just immediate after four hours of drying time for case-3. This is probably due to a minor condensation in sublimation layer during sublimation because of high intense drying condition at the beginning of second-stage drying. 0.9 Dimension less moistu re co nten t 0.8 0.7 0.6 0.5 0.4 Predicted-Thickness-1mm Predicted-Thickness -2 mm Predicted-Thickness -3mm Predicted-Thickness -4mm Measured-Thickness-1mm Measured-Thickness- 2mm Measured-Thickness-3mm Measured-Thicnkness-4mm 0.3 0.2 0.1 0 Time, hr Figure 6.4 Variation of measured and predicted dimensionless moisture content with time for disc shaped (16mm x 1mm) potato samples of different thickness Variation of product thickness of disc-shaped potato samples on the freeze-drying kinetics for case-3 is shown in Figure 6.4. Final dimensionless moisture content from 125 Chapter-6 Simulation Results experimental results was obtained about 0.07, 0.42, 0.61, and 0.68 for 1, 2, and mm product thickness, respectively, after hours of drying time while the corresponding predicted results were 0.047, 0.49, 0.68, 0.74, respectively. A good match was found between the experiment and simulation for all thicknesses in terms of the final moisture content; the curves also show similar behaviours. Figure 6.4 show that drying rate decreases with increase of product thickness. The increases of sample thickness implies an increase of dry layer thickness and increases in the water vapor diffusion path, which decreases the rate of migration of sublimated vapor from inside to the surface of the product. Therefore, it can be argued that product thickness is one of the key parameters in AFD process. This result agrees with the previous work of Matteo et al. (2003) and Wolf and Gibert (1991). 1.0 Measured-Single stage: -6C 0.9 Measured-Single stage: -11C Dimensionless moisture content 0.8 Predicted-Single stage: -6C 0.7 Predicted-Single: -11C 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Time, hr Figure 6.5 Variation of measured and predicted dimensionless moisture content with time for disc-shaped (16mm x 1mm) carrot 126 Chapter-6 Simulation Results Comparison between simulation and experimental results on freeze-drying kinetics was also carried out by using disc-shaped carrot samples for case-1, case-2 and case-3; this is shown in Figure 6.5 and Figure 6.6. After hours of drying time, the dimensionless moisture content from the experiment and simulation results were obtained 0.29 and 0.25, and 0.04 and 0.02 for case-1, respectively (Fig-6.5). The final moisture contents for case-2 and case-3 were about 0.43 and 0.45, and 0.25 and 0.24, 11 Predicted-Two stage: -11C & -6C 10 Predicted-Two stage: -11C & -6C (R) Moisture content, db/db kg Measured-Two stage: -11C & -6C Measured-Two stage: -11C & -6C ( R ) 0 Time, hr Figure 6.6 Variation of measured and predicted dimensionless moisture content with time for disc shaped (16mm x 1mm) carrot sample respectively (Fig-6.6). A similar trend of the drying kinetics curve between experiment and simulation results as well as a close match between the final dimensionless moisture content was found in both cases for carrot. However, at the end of the drying period, a slight discrepancy was observed between the predicted and simulation results. 127 Chapter-6 Simulation Results This is probably due to the melting of ice under such intense drying conditions during experiment, which causes damage of internal structure due to surface tension effects. Hence it can cause flexible cell walls to collapse as the liquid in the pores is emptied. 6.2 Predicted Parameters 6.2.1 Location and temperature of sublimation front Besides these slight discrepancies between experiment and simulation results, fairly good agreement was obtained to capture the drying phenomena of AFD system for Evaporation front temperature, o C -6 -8 Single stage: -6oC -10 Single stage: -11oC -12 -14 Single stage: -6C Single stage: -11C -16 -18 -20 10 Time, hr Figure 6.7 Variation of the predicted sublimation front temperature with time for potato for single stage drying process various samples of different geometry and under drying conditions. Therefore, the present model can be used as a good tool to predict other important phenomena in an AFD system, which usually not possible to measure experimentally. 128 Chapter-6 Simulation Results Figure 6.7 shows the variation of the predicted temperature with time at the interface for case-1. As seen from this figure initially the temperature of the interface increases rapidly from -17oC to –9oC within one hour of drying time at –6oC air and subsequently becomes stable with time. At the beginning of drying the interface ice layer receives sensible heat to raise the temperature of water inside the product matrix, which subsequently absorbs the latent heat for sublimation; this temperature is well below then the triple point temperature of pure water. At this point the sublimation process begins. After that the temperature of the evaporation front layer was reasonably constant at the freezing temperature due to continuous sublimation of the ice layer from the interface. This result also proofs the frozen integrity of the product during the entire AFD experiment. With air at –11oC, it takes two hours to approach a stable interface temperature of about –13oC. Due to the less intensity drying condition heat penetration rate decreases from the carrier gas through the product to the sublimation layer. This is because of less temperature gradient between the carrier gas and the product temperature and thereby takes more time to stabilize; consequently it decreases the drying rate. Figure 6.8 shows the predicted variation of the sublimation front temperature for case2 and case-3. In the first stage, for up to four hours of drying time, the two curves overlaps in terms of the temperature distribution as well as the numerical value of the final temperature, which is about -12oC. This result is consistent as in both cases; the drying condition was the same during this period. In the second stage (4 hrs to hrs), a slightly higher temperature (0.16oC) was observed for case-3 than case-2 because of the incorporation of the radiant heating, which provides uniform heating of the 129 Chapter-6 Simulation Results -6.00 Evaporation front temperature, C -8.00 -10.00 -12.00 -14.00 -16.00 Two stage: -11C & -6C -18.00 Two stage: -11C & -6C ( Rad ) -20.00 Time, hr Figure 6.8 Variation of predicted sublimation front temperature with time for potato for two stage drying process 0.0005 Stage: -11C Distance of evaporation front, mm Single stage: -6C 0.0004 Two stage: -11C & -6C Two stage: -11C & -6C ( Rad) 0.0003 0.0002 0.0001 0 Time, hr Figure 6.9 Predicted location of the sublimation front with time for potato 130 Chapter-6 Simulation Results product. The computed location of the evaporation front under different drying conditions is shown in Figure 6.9. Results show that at the beginning of drying, the location of the evaporating front was at the surface of the product. As drying progresses, the evaporation front recedes inside the product due to sublimation of the interface ice front, layer by layer. The final location of evaporation front from the surface of the product was about 0.000018 m, 0.000058 m, 0.0000028m, and 0.000007m, respectively, for case-1, case-2 and case-3. At the higher intensity drying conditions, particularly for case-3, a higher penetration rate of the evaporation front as the ice layer deepens was observed. 6.2.2 Temperature and moisture distributions inside the dry layer Figure 6.10 shows the predicted distribution of the moisture mass fraction inside the dry layer with distance for all four cases examined. It was observed that the moisture mass fraction increases from the surface towards the depth of the product as drying progresses. Water vapour formed by sublimation process at the interface layer results in a higher partial pressure of the vapor near the evaporation surface. Moisture then travels from interior to the surface of the product due to a partial pressure gradient i.e. from higher concentration region to a lower conductive region. During the flow of the sublimed vapor higher accumulation of moisture takes place near the evaporation front region due to the high concentration of water vapor and gradually decreases as moisture travels towards the surface of the product. Final moisture mass fractions for case-1, case-2 and case-3 were computed to be 0.000308, 0.000611, 0.0008 and 0.000810, respectively, after hours of drying. The depth of the dry layers from the 131 Chapter-6 Simulation Results 0.0009 Single stage: -11C 0.0008 Single stage: -6C 0.0007 Two stage: -11C & -6C 0.0006 Two stage: -11C & -6C ( Rad ) Y 0.0005 0.0004 0.0003 0.0002 0.0001 0.0000 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Distance, mm Figure 6.10 Predicted moisture distributions inside the dry layer with depth for potato -7.40 -7.45 -7.50 o T, C Single stage: -6C Two stage: -11C & -6C -7.55 Two stage: -11C & -6C ( Rad ) -7.60 -7.65 -7.70 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Distance, mm Figure 6.11 Predicted temperature distributions within the dry layer with depth for potato 132 Chapter-6 Simulation Results surface of the product for case-1, case-2 and case-3 were 0.182 mm, 0.058 mm, 0.028 mm, and 0.007 mm, respectively. From this finding, it can be argued that accumulation of moisture inside the dry layer is minimal. This also implies that the major portion of the sublimed water during the course of drying migrates to the carrier gas. For the twostage process, combination of convection and radiation heat input (case-3) results in a higher moisture distribution inside the dry layer in compared to other drying conditions. The temperature distribution within the dry layer for different drying conditions with distance is shown in Figure 6.11. After hours of drying, the dry layer temperatures for case-1, case-2 and case-3 were about -7.45oC, -7.64oC, and -7.51oC, respectively. The dry layer temperature increases with the increase of drying temperature, as expected. Higher temperature is observed near the surface of the product followed by a reduction as dry layer depth increases. Heat of sublimation penetrates through by conduction from the surface of the product and gradually reaches the evaporation front. As s results, the dry layer temperature near the surface and subsequently towards inside, come across more contact with inward heat flow and absorb more heat and hence increases temperature distribution. 6.3 Summary Atmospheric freeze drying system using a vortex tube and multi-mode heat input was studied experimentally and numerically. Results showed good agreement between simulation and experimental results which capture the drying phenomena of two different products of different size and carrier gas temperatures under single and multimode heat input. Therefore, numerical efforts were extended and predicted well 133 Chapter-6 Simulation Results the other important phenomena (sublimation front temperature, location of the sublimation front and moisture content as well as temperature distribution inside the dry layer) in AFD system, which usually not possible to measure experimentally. Results also illustrated that the process is recommended to work at the highest possible temperature, which should, of course, be compatible with a high quality of product conservation. This simple model can be used as a tool to optimize the process parameters. 134 [...]... surface of the product and gradually reaches the evaporation front As s results, the dry layer temperature near the surface and subsequently towards inside, come across more contact with inward heat flow and absorb more heat and hence increases temperature distribution 6. 3 Summary Atmospheric freeze drying system using a vortex tube and multi- mode heat input was studied experimentally and numerically... good agreement between simulation and experimental results which capture the drying phenomena of two different products of different size and carrier gas temperatures under single and multimode heat input Therefore, numerical efforts were extended and predicted well 133 Chapter -6 Simulation Results the other important phenomena (sublimation front temperature, location of the sublimation front and moisture... distance is shown in Figure 6. 11 After 8 hours of drying, the dry layer temperatures for case-1, case-2 and case-3 were about -7.45oC, -7 .64 oC, and -7.51oC, respectively The dry layer temperature increases with the increase of drying temperature, as expected Higher temperature is observed near the surface of the product followed by a reduction as dry layer depth increases Heat of sublimation penetrates... -11C & -6C ( Rad ) -7 .60 -7 .65 -7.70 0.0 0.1 0.2 0.3 0.4 0.5 0 .6 Distance, mm Figure 6. 11 Predicted temperature distributions within the dry layer with depth for potato 132 Chapter -6 Simulation Results surface of the product for case-1, case-2 and case-3 were 0.182 mm, 0.058 mm, 0.028 mm, and 0.007 mm, respectively From this finding, it can be argued that accumulation of moisture inside the dry layer... layer is minimal This also implies that the major portion of the sublimed water during the course of drying migrates to the carrier gas For the twostage process, combination of convection and radiation heat input (case-3) results in a higher moisture distribution inside the dry layer in compared to other drying conditions The temperature distribution within the dry layer for different drying conditions... moisture content as well as temperature distribution inside the dry layer) in AFD system, which usually not possible to measure experimentally Results also illustrated that the process is recommended to work at the highest possible temperature, which should, of course, be compatible with a high quality of product conservation This simple model can be used as a tool to optimize the process parameters 134...Chapter -6 Simulation Results 0.0009 Single stage: -11C 0.0008 Single stage: -6C 0.0007 Two stage: -11C & -6C 0.00 06 Two stage: -11C & -6C ( Rad ) Y 0.0005 0.0004 0.0003 0.0002 0.0001 0.0000 0.0 0.1 0.2 0.3 0.4 0.5 0 .6 Distance, mm Figure 6. 10 Predicted moisture distributions inside the dry layer with depth for potato -7.40 -7.45 -7.50 o T, C Single stage: -6C Two stage: -11C & -6C -7.55 Two stage: . Final dimensionless moisture content of the experimental and predicted results at –11 o C and 6 o C was 0.40 and 0. 36, and 0.13 and 0.12 for disc- shape product, and 4.2 and 4.3, and 0.45 and. temperature distribution. 6. 3 Summary Atmospheric freeze drying system using a vortex tube and multi- mode heat input was studied experimentally and numerically. Results showed good agreement. evaporation front. As s results, the dry layer temperature near the surface and subsequently towards inside, come across more contact with inward heat flow and absorb more heat and hence increases