The whole idea behind mass transfer is Chemical potential equilibrium or for simplification, you can call it Concentration equilibrium -but it would be not very accurate term-. Anyhow, do you think that it is possible to find a place on the surface of earth that isn't occupied with air?
Of course not! All of earth's face is covered with air. OK, why do you think that air can't be depleted even if we used a vacum pressure to suck it? This is because of "Mass transfer" Air molecules -and any other molecules in the universe- tend to move from the high chemical potential point to the less one -can be expressed with concentration too-. Mass transfer process can take place in a gas or vapour or in a liquid, and it can result from the random velocities of the molecules (molecular diffusion) or from the circulating or eddy currents present in a turbulent fluid (eddy diffusion). Mass transfer rate is governed by Fick's law which expresses the mass transfer rate as a linear function of the molar concentration gradient. Na=-Dab*(dCa/dy)
where NA is the molar flux of A (moles per unit area per unit time),
Ca is the concentration of A (moles of A per unit volume),
Dab is known as the diffusivity or diffusion coefficient for A in B, and
y is distance in the direction of transfer.
It is also worth mentioning that concentration can be expressed in partial pressure in case of gases.
Diffusivity is extremely important factor in our course, There is two kinds of diffusivities, molecular diffusivity (Which is D is a physical property of the system and a function only of its composition, pressure and temperature) and Eddy diffusivity (Which is dependent on the flow pattern and varies with position). the higher the velocity of the molecules, the greater is the distance they travel before colliding with other molecules, and the higher is the diffusivity D. The higher pressure applied, the less distance a molecule travels before it colloids with another and hence, less mean free path and consequently, less diffusivity.
When the mass transfer rates of the two components are equal and opposite the process is said to be one of equimolecular counter diffusion. Such a process occurs in the case of the box with a movable partition, where each partition contains different gas. It occurs also in a distillation column when the molar latent heats of the two components are the same. At any point in the column a falling stream of liquid is brought into contact with a rising stream of vapour with which it is not in equilibrium. The less volatile component is transferred from the vapour to the liquid and the more volatile component is transferred in the opposite direction. If the molar latent heats of the components are equal, the condensation of a given amount of less volatile component releases exactly the amount of latent heat required to volatilise the same molar quantity of the more volatile component. Thus at the interface, and consequently throughout the liquid and vapour phases, equimolecular counterdiffusion is taking place.
Of course not! All of earth's face is covered with air. OK, why do you think that air can't be depleted even if we used a vacum pressure to suck it? This is because of "Mass transfer" Air molecules -and any other molecules in the universe- tend to move from the high chemical potential point to the less one -can be expressed with concentration too-. Mass transfer process can take place in a gas or vapour or in a liquid, and it can result from the random velocities of the molecules (molecular diffusion) or from the circulating or eddy currents present in a turbulent fluid (eddy diffusion). Mass transfer rate is governed by Fick's law which expresses the mass transfer rate as a linear function of the molar concentration gradient. Na=-Dab*(dCa/dy)
where NA is the molar flux of A (moles per unit area per unit time),
Ca is the concentration of A (moles of A per unit volume),
Dab is known as the diffusivity or diffusion coefficient for A in B, and
y is distance in the direction of transfer.
It is also worth mentioning that concentration can be expressed in partial pressure in case of gases.
Diffusivity is extremely important factor in our course, There is two kinds of diffusivities, molecular diffusivity (Which is D is a physical property of the system and a function only of its composition, pressure and temperature) and Eddy diffusivity (Which is dependent on the flow pattern and varies with position). the higher the velocity of the molecules, the greater is the distance they travel before colliding with other molecules, and the higher is the diffusivity D. The higher pressure applied, the less distance a molecule travels before it colloids with another and hence, less mean free path and consequently, less diffusivity.
When the mass transfer rates of the two components are equal and opposite the process is said to be one of equimolecular counter diffusion. Such a process occurs in the case of the box with a movable partition, where each partition contains different gas. It occurs also in a distillation column when the molar latent heats of the two components are the same. At any point in the column a falling stream of liquid is brought into contact with a rising stream of vapour with which it is not in equilibrium. The less volatile component is transferred from the vapour to the liquid and the more volatile component is transferred in the opposite direction. If the molar latent heats of the components are equal, the condensation of a given amount of less volatile component releases exactly the amount of latent heat required to volatilise the same molar quantity of the more volatile component. Thus at the interface, and consequently throughout the liquid and vapour phases, equimolecular counterdiffusion is taking place.
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