Determination Of Diffusivity And Overall Mass Transfer Coefficient

So far, it has been assumed that the transfer coefficients are known constants. In reality transfer coefficients in food systems are unknown and not constant. Constant transfer coefficients can be used only in special situations and under restricted conditions. A number of experimental methods (7) and several empirical equations (1) have been developed to determine and predict the mass transfer coefficients. In the following subsections the theory that underlines some of the simpler experimental methods used is presented.

Diffusivity in Gases

One of the simplest methods for determining the diffusivity of a vapor or gas into another gas is to evaporate pure liquid A using a graduated cylinder with small diameter, as shown in Figure 4. The vapor A diffuses through an inert gas B, which is gently passed over the opening of the cylinder. The gas B moves slowly so that the layer at the top of liquid A (in the cylinder) can be assumed stagnant. The whole apparatus should be maintained at constant

Gas B

Liquid A'

Ay al

Figure 4. Evaporation of a liquid.

temperature and pressure. The drop of the liquid's level is measured over time and used to determine the diffusivity. The amount of liquid A evaporated in a small time interval is

Agitator cb

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