Dried layer

FIG. 16-42 loD-exchaDger regcDeratioD. a ) CoDveDtioDal. Acid is passed down-flowthrough the cation-exchange resin bed. (h) Coiinterflow. Regenerant solution is introduced iipflow with the resin bed held in place hy a dry layer of resin. 

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. 

Fig. 14.19 (A) SEM image of cracks in a dry layer of paint with inclusion of halloysite nanotubules containing antifouling agent ...

The vapor is only transported from the ice front through the porous dried layer. 

The heat transport in the already dried layer is neglected. 

The advantage of the use of linear horizontal development is the reduced developing time. In this case the gravitation does not decrease the mobility of the mobile phase more than in the traditional linear ascending development. Plates are placed horizontally in the chamber and the transport of the eluent is assured by a glass frit strip, a capillary split or any other method. Circular development techniques employ circular TLC plates, the mobile phase enters the centre of the plate and the development occurs out of the centre of the plate. The sample can be applied either onto the dry layer or onto the layer under the flow of the mobile phase. 

Giilzow, E., Schulze, M., Wagner, N., Kaz, T., Reissner, R., Steinhilber, G., and Schneider, A. Dry layer preparation and characterization of polymer electrolyte fuel cell components. Journal of Power Sources 2000 86 352-362. 

By its chemical nature, the very thin dry layer of glass permits ion exchange without any possibility of electrochemical redox reactions. If the membrane is a sufficiently thin layer, one can achieve millisecond response times to changes in pH, and these properties can be put to full advantage with rapid mixing devices to study chemical reactions with half-lives in the 5-10 msec range. 

As the sublimation interface recedes in (he material (Fig. 2). (he dry layer presents a resistance to the flow of water vapor and a pressure difference must exist between the ice interface and the surface of the dry... 

The overall heat-transfer coefficient U depends upon the properties of the dry product and the method of heal transfer. The heat-transfer rate A is influenced by the mechanical design of the heating elements and the conditioning of the frozen mass. The temperature gradient AT is limited by the maximum allowable temperatures al the sublimation interface and dry-layer surface. In the constant-rate period, the lirst one-half to two-thirds of the drying cycle, about 8fl + of the water is removed. 

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