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Separation Process Engineering: Includes Mass Transfer Analysis, Third Edition by Phillip C. Wankat

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15.1 Molecular Movement Leads to Mass Transfer

On a molecular level, all molecules move and collide because of thermal energy. These molecular collisions result in mass transfer by diffusion. At every temperature above absolute zero, molecules are always moving. When they bump into another molecule, the kinetic energy of the two molecules is redistributed and the molecules move away at different angles. With a large number of molecules, the motion of each molecule is random and the molecules tend to distribute throughout the volume available. At equilibrium there is an equal number density of molecules throughout the container.

The number of molecules present in a volume (e.g., 1.0 ml) can easily be estimated by remembering that a gram mole ...

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