Inengineering, themass transfer coefficient is adiffusion rate constant that relates themass transfer rate, mass transfer area, and concentration change as driving force:^[1]

${\displaystyle k_c=\frac{{\dot{n}}_A}{A\mathrm{\Delta }{c}_A}}$

Where:

• ${\displaystyle k_c}$ is the mass transfer coefficient [mol/(s·m²)/(mol/m³)], or m/s
• ${\displaystyle {\dot{n}}_A}$ is the mass transfer rate [mol/s]
• ${\displaystyle A}$ is the effective mass transfer area [m²]
• ${\displaystyle \mathrm{\Delta }{c}_A}$ is the driving force concentration difference [mol/m³].

This can be used to quantify the mass transfer betweenphases,immiscible and partiallymisciblefluid mixtures (or between a fluid and a porous solid^[2]). Quantifying mass transfer allows for design and manufacture ofseparation process equipment that can meet specified requirements, estimate what will happen in real life situations (chemical spill), etc.

Mass transfer coefficients can be estimated from many different theoreticalequations,correlations, andanalogies that are functions of material properties,intensive properties and flow regime (laminar orturbulent flow). Selection of the most applicable model is dependent on the materials and the system, or environment, being studied.

• (mol/s)/(m²·mol/m³) = m/s

Note, the units will vary based upon which units the driving force is expressed in. The driving force shown here as '${\displaystyle \mathrm{\Delta }{c}_A}$' is expressed in units of moles per unit of volume, but in some cases the driving force is represented by other measures of concentration with different units. For example, the driving force may be partial pressures when dealing with mass transfer in a gas phase and thus use units of pressure.

• Mass transfer
• Separation process
• Sieving coefficient

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