Sherwood number

The Sherwood number, Sh (also called the mass transfer Nusselt number) is a dimensionless number used in mass-transfer operation. It represents the ratio of convective to diffusive mass transport, and is named in honor of Thomas Kilgore Sherwood.

It is defined as follows

$\mathit{Sh} = \frac{K \cdot L}{D} = \frac{\mbox{Convective mass transfer coefficient}}{\mbox{Diffusive mass transfer coefficient}}$

where

• L is a characteristic length (m)
• D is mass diffusivity (m².s⁻¹)
• K is the mass transfer coefficient (m.s⁻¹)

According to Froessling equation it can also be further defined as a function of the Reynolds and Schmidt numbers; for example, for a sphere it can be expressed as:

$\mathit{Sh} = 2+0.6Re^{\frac{1}{2}}Sc^{\frac{1}{3}}$

This form is particularly valuable to chemical engineers in situations where the Reynolds number and Schmidt number are readily available. Since Re and Sc are both dimensionless numbers, the Sherwood number is also dimensionless. These correlations are the mass-transfer version of an analogous technique in heat transfer of writing the Nusselt number in terms of the Reynolds number and Prandtl number. For a correlation for a given geometry (e.g. spheres, plates, cylinders, etc.), a heat transfer correlation (often more readily available from literature and experimental work, and easier to determine) for Nusselt number Nu in terms of the Reynolds number (Re) and the Prandtl number (Pr) can be used as a mass transfer correlation by replacing the Prandtl number with the analogous dimensionless number for mass transfer, the Schmidt number, and replacing the Nusselt number with the analogous dimensionless number for mass transfer, the Sherwood number. As an example, a heat transfer correlation for spheres is given by:

${Nu} = 2 + 0.6Re^{\frac{1}{2}}Pr^{\frac{1}{3}}$

This correlation can be made into a mass transfer correlation using the above procedure, which yields:

${Sh} = 2 + 0.6Re^{\frac{1}{2}}Sc^{\frac{1}{3}}$

This is a very concrete way of demonstrating the analogies between different forms of transport pheonomena.

As to Sherwood number,several different expressions are as follows:

Ranz-Marshall Correlation:

$Sh=2+0.6\times R{{e}^{0.5}}\times PrP{{r}^{0.3}},~0\le ~Re<200,~0\le Pr<250$

See also

• Churchill-Bernstein Equation