Mass Transfer

Mass Transfer
Mass transfer: Mass is transferred in drying as a liquid or vapor within the solid and as vapor from the exposed surfaces. Movement of moisture within the solid results from a concentration gradient, which is dependent on the characteristics of the solid.

Mass transfer is involved—

i.                    in the diffusion of water through the material to the evaporating surface

ii.                  in the subsequent evaporation of the water from the surface and

iii.                in diffusion of the resultant vapor into the passing air stream.

Factors affecting the mechanism of drying: Mechanism of drying is influenced by a number of factors, which can be divided into two categories—

1. Internal mechanism &
2. External mechanism.

Internal mechanism: Internal liquid flow may occur by several mechanisms depending on the structure of the solid, some of the possible mechanisms are as follows—

1. Diffusion: Drying is greatly influenced by diffusion. So, if diffusion increases, the drying rate also increases.

I.e. drying rate µ diffusion rate

2. Capillary flow: It is also responsible for drying of solids. Capillary flow is seen when particles are in granular forms.
3. Shrinkage and pressure gradients: Drying rate is directly proportional to the pressure gradients acting on the surface i.e.

Drying rate µ pressure gradients.

4. Gravity
5. Vaporization-condensation sequence: Repetition of vaporization and condensation facilitates the drying process.

External mechanism: The principle external factors involved in drying mechanism are as follows—

1. Agitation: Drying rat is directly proportional to the exposed surface area and thus to the agitation. Agitation during drying reduces drying time.
2. Temperature: Temperature is an important factor for drying process. The more is the temperature, the more is the evaporation of moisture. So, the drying rate is directly proportional to the temperature.
3. Humidity: The rate of drying is inversely proportional to the humidity of surrounding area.
4. Airflow:
5. Surface area: The evaporation of moisture is directly proportional to the surface area of the particles. The greater the surface area, the increasing the drying rate and decreasing the drying time.
6. Method of supporting the solids

Contact of solid with heating source.