The mass transfer by convection can take place under two conditions:

1. Forced Convection:
Forced convection occurs when the fluid motion is induced by an external force, such as a pump or a fan. In this mode of mass transfer, the fluid flows over a solid surface or through a channel, and the motion of the fluid enhances the mass transfer. The rate of mass transfer by forced convection is directly proportional to the velocity of the fluid and the concentration difference between the fluid and the solid surface. Examples of forced convection include the flow of air over a heated surface or the flow of water through a pipe.

2. Natural Convection:
Natural convection, also known as free convection, occurs when the fluid motion is driven solely by buoyancy forces resulting from temperature differences. This mode of mass transfer is typically observed in systems where there is no external force driving the fluid motion, such as in a still air environment or a quiescent liquid. The rate of mass transfer by natural convection is influenced by the temperature difference between the fluid and the solid surface, the surface area, and the properties of the fluid. Examples of natural convection include the rising of hot air or the sinking of cold water.

Both forced convection and natural convection are important in various engineering applications. Forced convection is commonly used in heat exchangers, cooling towers, and fluidized beds, where the fluid motion is controlled to enhance heat and mass transfer. Natural convection, on the other hand, is often encountered in passive cooling systems, such as natural draft cooling towers or solar chimneys, where the buoyancy forces drive the fluid flow without the need for external energy input.

In conclusion, the mass transfer by convection can take place under two conditions: forced convection and natural convection. These modes of mass transfer are influenced by different factors and are applicable in various engineering processes.