Difference Between Mass Transfer and Diffusion

Main Difference – Mass Transfer vs Diffusion

Mass transfer and diffusion and are two important terms used to explain the spread or aggregation of solutes in a fluid. Mass transfer is a general term, and diffusion is a form of mass transfer. Mass transfer is the transport of mass from one place to another. Diffusion is the even distribution of solutes throughout the system. The main difference between mass transfer and diffusion is that mass transfer may or may not occur across a concentration gradient whereas diffusion occurs across a concentration gradient.

Key Areas Covered

1. What is Mass Transfer
     – Definition, Different Forms, Applications
2. What is Diffusion
     – Definition, Explanation, Determination of the Rate of Flow
3. What is the Difference Between Mass Transfer and Diffusion
     – Comparison of Key Differences

Key Terms: Absorption, Concentration, Diffusion, Distillation, Evaporation, Mass Transfer, Precipitation, Sublimation

What is Mass Transfer

The term mass transfer refers to the transport of a mass from one place to another. There are many different chemical processes in which mass transfers occur. Some of these processes are listed below.

• Absorption
• Evaporation
• Sublimation
• Drying
• Precipitation
• Membrane filtration
• Distillation, etc.

In most of these cases, mass transfer occurs along with a chemical reaction. Some of the processes involve phase transitions i.e., evaporation of water is the conversion of liquid water into steam.  Here the water in the container is transferred to the air.

Figure 1: Evaporation is a Type of Mass Transfer

There are some important applications of mass transfer. For example, dispersion of contaminants, drying, humidifying, segregations and doping in materials, vaporization and condensation of mixtures, cooling towers, respiration, sweating, etc.

What is Diffusion

Movement of a solute or a gas from an area of high concentration to an area of low concentration through air or water is known as diffusion. In other words, the molecules move through a concentration gradient. Therefore, the factors that affect the concentration gradient will affect the diffusion.

Figure 2: Diffusion is the Spread of Solutes throughout the System Evenly

The diffusion terminates when the concentration of the whole system (gas or liquid) is equal. This means diffusion occurs until all the solutes distribute evenly throughout the system. But the diffusion can be monitored using different membranes. For example, impermeable membranes do not allow diffusion to occur. Semi-permeable membranes allow some particular solute particles to pass through.

The rate of flow in diffusion is given by the following equation. According to this equation, diffusion of a solute is proportional to the concentration gradient.

J = -D (dc/dx)

J is the amount of a substance passing through a reference surface of unit area per unit time, D is the diffusion constant, c is the concentration of solute and x is the direction (or location).

Difference Between Mass Transfer and Diffusion

Definition

Mass Transfer: Mass transfer refers to the transport of a mass from one place to another.

Diffusion: Diffusion is the movement of a solute or a gas from an area of high concentration to an area of low concentration through air or water.

Principle

Mass Transfer: Mass transfer can occur as a phase transition, separation or as a result of a chemical reaction.

Diffusion: Diffusion causes the transfer of mass from a high concentration area to a low concentration area.  

Concentration Gradient

Mass Transfer: Mass transfer may or may not occur across a concentration gradient.

Diffusion: Diffusion occurs across a concentration gradient.

Movement of Solutes

Mass Transfer: Mass transfer may involve the movement of solutes (ex: in evaporation) or stop the movement of solutes (ex: precipitation).

Diffusion: Diffusion always involves the movement of solutes.

Conclusion

Mass transfer is the movement of mass from one place to another. Diffusion is a form of mass transfer. The main difference between mass transfer and diffusion is that mass transfer may or may not occur across a concentration gradient whereas diffusion occurs across a concentration gradient.

Reference:

1. “Multiphysics Cyclopedia.” COMSOL, Available here.
2. “Diffusion.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 24 Feb. 2016, Available here.
3. “Mass transfer.” Wikipedia, Wikimedia Foundation, 24 Jan. 2018, Available here.

Image Courtesy:

1. “Blausen 0315 Diffusion” by Tristan Schmurr (CC BY 2.0) via Flickr
2. “Vaporization” By BruceBlaus – Own work (CC BY 3.0) via Commons Wikimedia