Porous drying

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

Blue coral, Heliopora coerulea, has a very different structure, though it is also an octocoral. It has a massive, aragonite skeleton, which takes the form of fingers, or of plates or baffles, according to the water currents in which it grows, forming a large, compact bush shape (Fig. 11.4). It has tiny holes all over the surface, and appears porous, dry... 

The porous dried polymer was opaque-white or translucent. 

Hanes J, Evora C, Ben-Jebria A, Edwards DA, Langer R. Porous Dry-Powder PLGA Microspheres Coated with Lung Surfactant for Systemic Insulin Delivery via the Lung. Proceed Inter Symp Control Rel Bioact Mater 1997, 24, 57-58. 

Basically, all products which contain water can be dried in this manner. After freezing, the ice crystals remain at first at their original location in the structure. If vacuum is applied, the ice sublimates directly without melting. This results in a porous, dry product which has retained its original form. The volume has hardly changed, the cell walls have been little affected, flavour and other main constituents have been treated gently. If the product is to be restored to its former condition - to be reconstituted -only water has to be added. Water sorption in freeze dried materials proceeds faster than in conventionally dried products. For detailed studies on the fundamentals of freeze drying see the literature [1-10]. 

Fig. 4.9 a Polymerizable bis-urea gelator G22 capable of producing highly porous dried gel b TEM micrographs of butyl acetate gel of G22 before polymerization (5 mg/mL, bar 1,000 nm) c SEM of a benzene gel of G22 after irradiation (5 mg/mL, bar 400 nm). Reprinted with permission from Ref. [59]. Copyright 1997 Ameriean Chemieal Society... 

King, C.J., Rates of moisture sorption-desorption in porous dried food. Food TechnoL, 22 50, 1968. 

The material balance equations for the water vapor and the inert gas are given by Equations 12.10 and 12.11, and the constitutive expressions for and N-a, are obtained from Equations 12.12 and 12.13. Equation 12.17 represents one possible form for the rate expression of the removal of bound water (see Section 12.2.2), and this equation (or its simpler form, as discussed above) could be used to describe the change in the concentration of sorbed water with time. The total mass flux Nf through the porous dried layer is obtained from Darcy s law, and is given by Equation 12.16. 

Fig. 10.2-5 Schematic model to elucidate the thermal conductivity of porous drying goods...

First, a hst of outstanding physical prop>erties of highly porous dry gels will be given, all of which are determined by sohd structure. In numerous research projects all over... 

In the next section, we will present experimental efforts to look for alternatives to the high-pressure process of supercritical drying. The goal is the production ofhighly porous dry gels by subcritical and ambient pressure routes or by freeze-drying. 

K permeability of the porous dried layer, m p. viseosity of the binary mixture of water vapor and inert gas in the porous dried layer, kg/m s Pie effeetive density of the dried layer, kg/m ... 

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