Mass transfer freeze drying

Since the thimble and the freeze-dried mass could hinder mass transfer, another set of experiments were conducted at 24 and 31 MPa In the cups of an aluminum sample holder with six cups (0.5 cm x 0.5 cm). It was reasoned that If the thimble was eliminated from the extraction, and If only the lipids were exposed to the SC CO2, there would be negligible mass transfer resistance to extraction. Solvent extracted lipids were placed In the cups, and extracted with SC CO2. 

In addition to the effects of formulation factors on freeze-drying behavior, it is important for the pharmaceutical scientist to understand basic principles of heat and mass transfer in freeze-drying [29,30]. Because of the high heat input required for sublimation (670 cal/g), transfer of heat from the heated shelf to the sublimation front is often the rate-limiting step in the coupled heat... 

Mass transfer in freeze-drying refers to the transfer of water vapor from the sublimation front through open channels in the partially dried layer, created by prior sublimation of ice, through the headspace of the vial, past the lyostopper, through the chamber, to the condenser. 

The reader is referred to basic studies of mass transfer in freeze-drying by Pikal and coworkers for in-depth treatment of the theoretical and practical aspects of mass transfer [29,32], Briefly, the rate-limiting step in mass transfer is transfer of water vapor through the partially dried matrix of solids. Resistance of the dried layer increases in a more or less continuous fashion as the depth of the dried layer increases, and the resistance also increases with the concentration of solids in the dried layer. Other factors can also affect the resistance of the dried layer, such as the method of freezing faster freezing tends to create a higher resistance in the dried layer. 

Heat and Mass Transfer in Low Pressure Gases Applications to Freeze Drying... 

In pharmaceutical systems, both heat and mass transfer are involved whenever a phase change occurs. Lyophilization (freeze-drying) depends on the solid-vapor phase transition of water induced by the addition of thermal energy to a frozen sample in a controlled manner. Lyophilization is described in detail in Chapter 16. Similarly, the adsorption of water vapor by pharmaceutical solids liberates the heat of condensation, as discussed in Chapter 17. 

The systematic description of heat and mass transfer applications in freeze drying is considered in this section. Mechanistic interpretations are discussed in the fol-... 

V. MECHANISMS OF HEAT AND MASS TRANSFER IN FREEZE DRYING... 

MJ Pikal. Use of laboratory data in freeze drying process design Heat and mass transfer coefficients and the computer simulation of freeze drying. J Parenter Sci Tech-nol 39 115-138, 1985. 

MJ Pikal, ML Roy, S Shah. Mass and heat transfer in vial freeze-drying of pharmaceutical Role of the vial. J Pharm Sci 73 1224-1237, 1984. 

Steinbach, G. Equations for the heat and mass transfer in freeze-drying of porous and non porous layers and bodiesm, p. 674 bis 683. International Institute of Refrigeration <1IR> (XIII, Washington 1971)... 

Sharon, Z., Berk, Z. Freeze-drying of tomato juice and concentrate, studies of heat and mass transfer, p. 115 bis 122. International Institute of Refrigeration (Comm. X, Lausanne, 1969)... 

Kochs, M., Korber, Ch., Nunner, B., Fleschel, I. The influence of the freezing process on vapor transport during sublimation in vacuum-freeze-drying. Journal of Heat and Mass Transfer 34, p. 2395-2408, 1991... 

With this information the maximum flow of energy can be estimated, which can be transported during freezing and main drying at a desired temperature difference between inlet and outlet temperature of the brine at the shelves, e. g. 2000 kj/h at a temperature difference of 3 °C. With this amount of energy approx. 0.7 kg ice could be sublimated per hour. (This estimate gives only the maximum possible sublimation-rate, whether it can be achieved or not depends from heat -and mass transfer conditions in the process (see Section 1.2.1 and Eq. (12)). 

Di Matteo, P, Donsi, G. and Ferrari, G., The role of heat and mass transfer phenomena in atmospheric freeze-drying of foods in a fluidised bed, /. Food. Eng., 59 (2003) 267-275. 

The most advanced technique of quick freezing is by pouring the material onto a freezing belt. Before drying, the material is granulated or sliced to improve heat and diffusional mass transfer. These operations are conducted in cold rooms at about —46°C. 

Tire rate of heat input to the frozen material is a function of the operating-vacuum method of heal transfer and the properties of ihe dried product. The operating vacuum determines the pressure difference and. in turn, the rate of mass transfer, which must be in balance with the rate of heat input. Otherwise, either melting will occur at the sublimation interface and the purpose of freeze-drying will be defeated or the sublimation temperature will decrease and the cost of processing will increase. 

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