Liquids vacuum freeze-drying

J Wolff, E., Gibert, H. Rudolf, F Vacuum freeze-drying kinetics and modeling of a liquid in a vial. Chem. Eng. Process., 25 (3), p. 153-158, 1989... 

Rene, F., WoKf, E., Rodolphe, F., 1993. Vacuum freeze-drying of a liquid in a vial determination of heat and mass-transfer coefficients and optimisation of operating pressure. Chem. Eng. Process. 32 245-251. 

This method involves atomizing of the solution into a freezing agent (for example, liquid N2) where tiny droplets are turned into solid particles and after necessary filtering, these solid particles are moved to a vacuum freeze-drying chamber in which they are heated and ice sublimes. The dried material is sintered to produce oxidized nanoparticles. Obviously freezedrying consists of two steps namely spray freezing... 

Process Intensification in Vacuum Freeze-Drying of Liquids 1139... 

Freeze-drying, like all drying processes, is a method to separate liquid water from a wet solid product or from a solution or dispersion of given concentration. However, the main difference is that the liquid water is separated by solidification (i.e., the formation of ice crystals) and subsequent vacuum sublimation instead of evaporation. This allows a drying at subzero temperatures which can be advantageous in case of heat-sensitive products. There are two general applications... 

Figure 5.6 Freeze-drying works by decreasing the pressure, and causing a phase change at higher pressure, the stable form of water is liquid, but the stable form at lower pressures is vapour. Consequently, water (as vapour) leaves a sample when placed in a vacuum or low-pressure chamber we say the sample is freeze-dried ...

Undercooling is the driving force in freeze drying. An aqueous salt solution is introduced dropwise into an immiscible liquid (hexane or a petroleum fraction such as kerosene) cooled below 243 K. The individual droplets are frozen instantaneously and the solid particles are decanted or filtered. The frozen particles are then sublimed in a vacuum to obtain a homogeneous powder of fairly uniform particle size. Important parameters in freeze drying are the final temperature of the salt solution and the cooling rate. These can be controlled to some extent, but only on a small scale. Hence the method is not very suited for large-scale manufacture of catalysts. 

Lyophilization (or freeze-drying) is a process utilized to convert a water-soluble material filled into a container to a solid state by removal of the liquid while frozen. The process requires the use of deep vacuums and careful control of temperatures. By conducting the process under reduced pressure, the water in the container converts from ice directly to vapor as heat is applied and is removed from the container by the vacuum. The dissolved solids in the formulation cannot undergo this phase change and remain in the container. At the completion of the cycle, the container will be returned to near atmospheric pressure stoppers are applied or fully seated and crimped as described above. Lyophilization is particularly common with biological materials whose stability in aqueous solution may be relatively poor. The time period in solution and the temperature of the solution are kept at a specified low temperature to prevent product degradation [35],... 

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