Chapter-8 Conclusions and Recommendations CHAPTER CONCLUSIONS AND RECOMMENDATIONS 8.1 CONCLUSIONS A new atmospheric freeze drying (AFD) system was designed, fabricated and tested for drying of heat sensitive materials. A series of atmospheric freeze-drying experiments with different food and biotechnological products were carried out under different drying conditions. Experimental and numerical studies were carried out to compare the traditional vacuum freeze drying (VFD), existing atmospheric freeze drying (AFD) and heat pump drying (HPD) methods with the AFD process. The experimental results are compared with simulation results. Finally, a three-dimensional CFD simulation of the vortex tube used to generate sub-zero temperature was carried out. Based on extensive experimental and analytical results, the main conclusions drawn are as follows: • A vortex tube can be used as a suitable means to achieve and maintain desirable low temperature for an AFD drying process on a laboratory scale. • The multi-mode AFD process examined in this research project, using conduction and radiation coupled with convection yields faster drying kinetics for pieces of potato and carrot without compromising on dried product quality, which compared favorably with vacuum freeze drying. Experimental results also showed that use of a vibro-fluidized bed with an adsorbent presents significant improvement in terms of freeze drying kinetics when compared with existing commercial AFD systems which use heat pump assisted fluidized beds with convection heat input. 168 Chapter-8 • Conclusions and Recommendations Experimental results revealed that osmotic treatment is not a suitable option in case of AFD and VFD processes as it reduces the quality of the dried product. • On the basis of extensive parametric studies, it is concluded that the proposed AFD system for drying pieces of heat-sensitive materials (Vibro-fluidized bed dryer with a vortex tube, multimode heat input and mixed adsorbent) is an attractive alternative to overcome some of the drawbacks of existing AFD as well as VFD systems. A technoeconomic feasibility study needs to be carried out, however, after scale-up to a pilot scale. • A simplified one-dimensional simulation model for atmospheric freeze-drying of foods in a fixed bed dryer was developed. Simulation results compared well with the experimental results. The model gave a good prediction of the drying kinetics. This simple model can be used as a tool to optimize the process parameters. • Three-dimensional CFD simulation of the vortex tube used in this study suggests that the standard RNG k-є turbulence model leads to a better agreement between the numerical and experimental data than the standard k-є, k-omega and the swirl RNG k-є turbulence models. This model captured well the aerodynamic and thermal characteristics of the vortex tube. Predicted results showed that energy separation between the two opposite counterrotating vortices in the vortex tube occurs mainly due to transfer of loss of angular momentum in the form of heat from the inner vortex to the outer vortex. Results also revealed that the magnitude of energy separation increases as the inlet pressure increases. They further confirm that the location of strip inside the commercial vortex tube is the optimal position for maximum energy 169 Chapter-8 Conclusions and Recommendations separation. It is expected that such a model will be useful in future for design and scale-up of vortex tubes. In summary, the key objectives of this research project were met successfully. It is noted that the AFD process can compete with the traditional VFD process in terms of both capital and operating costs as it does not need vacuum operation. 8.2 RECOMMENDATIONS FOR FUTURE WORK Recommendations for future work are summarized as follows: • Further work on the experimental study using vortex tube. Our results showed the potential of using vortex tube to supply cryogenic air for atmospheric freeze drying. For better energy efficiency experiments with an AFD system where heating and cooling outputs of vortex tubes are concurrently used are recommended • Optimize drying condition in AFD using vortex tube. An extensive series of experiments is recommended to identify the optimal drying conditions for twostage conditions: below and above sub-zero temperatures. A large varity of products, including those biological origins, should be tested to determine viability of AFD. If successful the capital and operating cost will be reduced significantly. • Scale-up of vortex tubes in AFD systems for industrial application is suggested through a careful CFD numerical simulation. Finally, a technoeconomic feasibility study should be carried out before commercialization can be recommended. 170 . experimental and analytical results, the main conclusions drawn are as follows: • A vortex tube can be used as a suitable means to achieve and maintain desirable low temperature for an AFD drying. Chapter -8 Conclusions and Recommendations 1 68 CHAPTER 8 CONCLUSIONS AND RECOMMENDATIONS 8. 1 CONCLUSIONS A new atmospheric freeze drying (AFD) system was designed, fabricated and. vortex tube used in this study suggests that the standard RNG k-є turbulence model leads to a better agreement between the numerical and experimental data than the standard k-є, k-omega and the