Category: Mass Transfer

We can use concepts of mass transfer resistance to make the analysis of mass transfer with combined mechanisms much easier. Definition of resistance A resistance is the driving potential (or driving force) divided by the transfer rate: resistance=driving potentialtransfer rate We are probably familiar with Ohm’s law, in which the driving potential is the voltage and the…

What do we do in cases like this hypothetical mass transfer device? Given the concentrations of species A at points 1 and 2 (��1, ��2), how do we compute the rate of mass transfer between them? We need to consider convective and diffusive mass transfer in multiple media. Concentration profiles The concentration profile owing purely to…

As we saw earlier, mass transfer refers to mass in transit due to a species concentration gradient in a mixture, and mass convection is one of the mechanisms for this transit. Mass transfer by convection involves the transport of material between a boundary surface (such as solid or liquid surface) and a moving fluid or…

One of the big unknowns in Fick’s equation above is the diffusion coefficient, ���. The diffusion coefficient (or diffusivity) is a proportionality constant between the molar flux due to molecular diffusion and the gradient in the concentration of the species (or the driving force for diffusion). On a theoretical basis, the diffusion coefficient is proportional to…

Describing diffusion quantitatively Exercise: Diffusion Suppose we have molecule � diffusing in a domain between boundary 1 and boundary 2, where ��,1 and ��,2 are held fixed. What happens to �� between boundary 1 and boundary 2 and what is the rate of diffusional mass transfer (moles/time) across some area �? Note that � represents an area � by � into the figure. Here is a particle simulation. Fick’s law of diffusion…

A concentration profile is a sketch that indicates the magnitude of the concentration as a function of position. This sketch is often superimposed on a process to indicate where these changes occur. This is analogous to a velocity profile for fluid flow. Exercise: Concentration profile Based on the molecular distribution below, draw the concentration profile of species � as…

Mass transfer Mass transfer is mass in transit due to a species concentration gradient in a mixture. By concentration gradient, we mean a spatial difference in the abundance of the chemical species. Modes of mass transfer diffusion convection Example: The story of engineering the contact lens The cornea of the eye needs oxygen from the air Mindful of…