Mass Transfer Processes and Their Efficiency

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Questions and Answers

What primarily drives mass transfer by diffusion?

Pressure fluctuations

Mechanical agitation

Temperature differences

Concentration differences (correct)

Which statement best differentiates molecular diffusion from eddy diffusion?

Molecular diffusion occurs in turbulent fluids only.

Eddy diffusion has a higher transfer rate than molecular diffusion. (correct)

Eddy diffusion is due to concentration differences.

Molecular diffusion involves external forces.

What characterizes forced convective mass transfer?

Induced by external means such as fans or pumps (correct)

Driven by natural buoyancy forces

Operates only in laminar flow conditions

Results from concentration gradients alone

Which analogy can be drawn to describe the rate of diffusion mass transfer?

Comparable to a crawling turtle Signup and view all the answers

What example demonstrates forced convective mass transfer?

Wind carrying water particles off a lake Signup and view all the answers

Which process is generally faster in achieving mass transfer?

Convection mass transfer Signup and view all the answers

How does convection mass transfer relate to diffusion mass transfer?

Convection mass transfer includes a diffusion component. Signup and view all the answers

What condition is necessary for free convective mass transfer to occur?

Density differences due to buoyancy forces Signup and view all the answers

What primarily influences interphase mass transfer?

Differences in physico-chemical properties of molecules Signup and view all the answers

Which of the following gases can be absorbed using liquid water according to the examples provided?

Ammonia from the ammonia-air mixture Signup and view all the answers

What is the primary concept behind the two-film theory?

Gas and liquid layers exist through which gas is transferred by molecular diffusion Signup and view all the answers

What is assumed about the interface during steady-state conditions?

Equilibrium is achieved at the interface Signup and view all the answers

Which solvent is used for the absorption of CO2 in the provided examples?

Amine solvents Signup and view all the answers

In the context of interphase mass transfer, what is a characteristic of the two layers formed at the gas-liquid interface?

They allow for molecular diffusion of gas Signup and view all the answers

What kind of resistance is illustrated in the communication analogy of the two-film theory?

Resistance due to different languages Signup and view all the answers

Which of the following processes is an example of absorption at an interface?

Absorption of CO2 by amine solvents Signup and view all the answers

Study Notes

Modes of Mass Transfer

Mass transfer occurs through diffusion, which can be molecular or eddy-based, driven by concentration differences.
Molecular diffusion involves the transfer of substances through a stagnant layer of fluid, whereas eddy diffusion happens in a turbulent flow.
Eddy diffusion is generally more efficient than molecular diffusion, as seen in example scenarios like the dispersion of purple dye in water or smoke from a stack.

Mass Transfer by Convection

Convection refers to large-scale mass transfer resulting from the bulk movement of fluids, influenced by transport properties and flow dynamics.
Free convective mass transfer occurs due to buoyancy, such as ether evaporation.
Forced convective mass transfer is enhanced by external forces (e.g., fans or pumps), exemplified by wind rapidly transporting water particles on a lake's surface.

Comparisons of Mass Transfer Processes

Diffusion mass transfer operates slowly, limiting its effectiveness for large-scale processes.
In contrast, convection mass transfer is much faster and suitable for significant transfers, akin to a train's speed compared to a turtle's.
Both convection and diffusion processes coexist, with convection overshadowing diffusion.

Interphase Mass Transfer

Interphase transfer relies on molecules migrating between distinct phases, influenced by disparities in physical and chemical properties like vapor pressure or solubility.
A concentration gradient exists between the bulk fluid and the interface, with equilibrium often assumed at steady states.
Examples include the absorption of ammonia by water, CO2 from flue gas by amine solvents, and H2S from natural gas.

Two-Film Theory

The Lewis-Whitman two-film model describes mass transfer with gas-liquid interfaces, highlighting double layers through which diffusion occurs.
This model illustrates the complexities of communication between two languages, where barriers slow down interaction, analogous to gas transferring through molecular layers at an interface.

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Description

Explore the various modes of mass transfer, including molecular and eddy diffusion, as well as convection. Understand how these processes differ and their respective efficiencies in different scenarios. This quiz will challenge your knowledge and application of mass transfer concepts in fluid dynamics.