Free moisture content

Other Measurements. Other tests include free moisture content, rate of dissolution and undissolved residue in acids and alkaH, resin and plasticizer absorption, suspension viscosity, and specific surface area. Test procedures for these properties are developed to satisfy appHcation-related specifications. 

Free moisture content is the Hquid content that is removable at a specific temperature and humidity. Eree moisture may include bound and unbound moisture, and is equal to the total average moisture content minus the equiHbrium moisture content for the specific drying conditions. 

Free-moisture content is that liquid which is removable at a given temperature and humidity. It may include bound and unbound moisture. 

Strips of a material 10 mm thick are dried under constant drying conditions from 28 per cent to 13 per cent moisture in 25 ks. If the equilibrium moisture content is 7 per cent, what is the time taken to dry 60 mm planks from 22 to 10 per cent moisture under the same conditions, assuming no loss from the edges All moisture contents are expressed on the wet basis. The relation between E, the ratio of the average free moisture content at time t to the initial free moisture content, and the parameter / is given by ... 

Free market, for silver, 22 648 Free moisture content, 9 97 Free on board (FOB), 25 329 Free-radical addition polymerization,... 

During this period the rate of drying is, approximately, directly proportional to the free moisture content (w — we), or ... 

The relation between E, the ratio of the average free moisture content at time t to the initial free moisture content, and the parameter J is given by ... 

Oven dry Moisture content (%) SA Free moisture content (%)... 

The authors concluded that water which cannot be removed at 100 °C is bound in such a way that it cannot jeopardize the pharmaceutical product. Only the free water can diffuse from the stopper to the product. The moisture content is measured by the Karl Fischer method with different temperatures in the oven, 100 °C to determine the free water content and up to 300 °C to measure the free and bound water. The authors suggested developing a similar program for other stoppers, since the time for such measurements is relatively short (1 week) instead of observing the RM in a product over long times. Table 1.15.2 summarizes the results with the stoppers described above. Table 1.15.3 lists the limits of the free moisture content in 2 types of stoppers and for different cake weights under the assumption that a maximum RM increase of 0.5% in the product is acceptable. 

The moisture content at which the drying rate begins to decline is called critical moisture content. Some of the variables on which the transition point depends are indicated in Figures 9.3(c) and (g)—for example, the nature of the material, the average free moisture content, and so on. The shape of the falling rate curve sometimes may be approximated by a straight line, with equation... 

The free moisture content of a substance is the amount of moisture that can be removed from the material by drying at a specified temperature and humidity. The amount of moisture that remains associated with the material under the drying conditions specified is called the EMC. One should note that the EMC can be altered in a transfer step from a dryer to subsequent processing. This exposure to the ambient conditions of... 

Consider the drying of a wet solid under fixed drying conditions. In the most general cases, after an initial period of adjustment, the dry-basis moisture content X decreases linearly with time t following the start of the evaporation. This is followed by a nonlinear decrease in X with t nntil, after a very long time, the solid reaches its equilibrium moisture content X and drying stops. In terms of free moisture content, defined as... 

The model assumes one-dimensional hquid diffusion with constant effective diffusivity and no heat effects. X2 is the average free moisture content att= /obtained by integrating the analytical... 

X, average free moisture content a, half-thickness of slab... 

For solids with continuous pores, a surface tension driven flow (capillary flow) may occur as a result of capillary forces caused by the interfacial tension between the water and the solid particles. In the simplest model, a modified form of the Poiseuille flow can be used in conjunction with the capillary forces equation to estimate the rate of drying. Geankoplis (1993) has shown that such a model predicts the drying rate in the falling rate period to be proportional to the free moisture content in the solid. At low solid moisture contents, however, the diffusion model may be more appropriate. 

Figures 24.5 and 24.8 show the drying rate per unit area R plotted against the free moisture content X - — X, or X. Figure 24.5 is the curve for a nonporous clay slab Fig. 24.8 is that for a porous ceramic plate. The difference in the shapes of these curves reflects the differences in the mechanism of internal moisture flow in the two materials.

Xj- = average total moisture content at time t-p h X = average free-moisture content at time tp h X = equilibrium-moisture content... 

Xji = initial moisture content at start of drying when t = 0 Xi = initial free-moisture content D = diffusivity of moisture through solid s = one-half slab thickness... 

Equation (24.18) shows that when diffusion controls, the rate of drying is directly proportional to the free-moisture content and inversely proportional to the square of the thickness. Equation (24.17) shows that if time is plotted against the logarithm of the free-moisture content, a straight line should be obtained, from the slope of... 

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