A Beginner's Guide of Mass Transfer - Chemo Concept

The journey of chemical engineers revolves around the term called unit operation. Unit operation is a single unit in a whole chemical process.

This single unit can be differentiated from other operations based on changes happening on the chemical. Based on these changes, we can classify unit operations into four major groups:

1. Fluid Flow Operations: Pressure differences act as a driving force for this kind of operation.
2. Heat Transfer Operations: Temperature differences act as a driving force for this kind of operation.
3. Mass Transfer Operations: Concentration differences act as a driving force for this kind of operation.
4. Mechanical Operations: Operations related to solids come under this kind of operation.

In this post, We are going to learn about the basics of Mass Transfer and Mass Transfer Operations. These both terms look similar but there is a difference between these terms which I will clear in this article.

Topics Covered in This Article

• What is Mass Transfer?
• Examples of Mass Transfer
• Types of Mass Transfer
• Calculations of Mass Transfer
• Fick's First Law for Molecular Diffusion
• Convective Mass Transfer Flux

What is Mass Transfer?

Mass Transfer is a phenomenon in which molecules transfer from higher concentration region to lower concentration region due to concentration difference driving force.  This transfer of molecules can happen can be due to the thermal energy of the molecules or due to any external energy source.

Examples of Mass Transfer

• Let's understand mass transfer with an example: When we put a drop of ink in the tub full of water, Ink will disperse in the water as time passes. After some time, the whole water becomes blue due to the homogenous concentration of ink in it.
• Another example of mass transfer is that when a person having scent applied on his/her body enters the room, the Smell of that scent will reach the people nearby.

Mass transfer alone can happen without any external force. But the rate of mass transfer will be low if no external mean is not involved. If we involved any external source, that will increase the rate of mass transfer greatly. We will discuss most of the mass transfer operations will have an external source to increase mass transfer further in this article.

So let's classify mass transfer.

Types of Mass Transfer

Mass transfer operations can be classified into two parts:

1. Molecular Diffusion: In this type of mass transfer, Molecules transfer without any external force involved. Molecules transfers based on their own thermal energy. This type of mass transfer is slower.
2. Convective Mass Transfer: In this type of mass transfer, Molecules transfer with the help of an external medium such as a pump or fan which makes the mass transfer faster.

There are also two different types of diffusion based on the species involved:

1. Binary diffusion: This type of diffusion involves only two species. In these two species, one or both species might diffuse into another. Equimolar counter diffusion and diffusion of one species into another stagnant medium are two important examples of binary diffusion.
2. Multicomponent diffusion: This type of diffusion involves more than two species. In these more than two species diffuse into another species.

Calculations of Mass Transfer

In mass transfer, Molecular flux is calculated as the number of molecules transferred. This molecular flux can be defined as moles of species transferred per unit time and per unit area perpendicular to transfer.

According to the type of mass transfer involved, Molecular flux is calculated with the help of a different set of equations. For molecular diffusion, the mass transfer can be calculated with Fick's first law. This law makes it easy to calculate molecular flux for steady molecular diffusion. Fick's second law is also available for unsteady molecular diffusion.

Fick's First Law for Molecular Diffusion

Fick's first law states that molecular flux is proportional to the concentration gradient. Concentration gradient is a change in concentration per unit length. Mathematically it is written as

Diffusional Flux = (Mass Diffusivity)*(Concentration Gradient)

J_AB = D_AB * (dC_A / dx)

Where,

J_AB =  Molecular diffusional flux of species A in species B

D_AB  = Diffusivity of species A in species B

(dC_A / dx) = Concentration gradient

Convective Mass Transfer Flux

For convective mass transfer, the equation for molecular flux can be obtained by analogy with the fluid flow or heat transfer. This can be mathematically written as:

Mass Flux = (Mass Transfer Coefficient)*(Concentration Difference)

N_AB = K_C * (C_A1 - C_A2)

Where,

N_AB = Convective diffusional flux of species A in species B

K_C  = Mass Transfer Coefficient

(C_A1 - C_A2) = Concentration difference

The above equation can be written in terms of the difference in mole fraction or pressure according to the type of system. Concentration difference calculations are convenient for liquid-liquid contact mass transfer.

Read also: Mass Transfer Operations