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UNIT I

1. Define diffusion.

Diffusion is the movement, under the influence of a physical stimulus, of an individual component through a mixture. The most common cause of diffusion is a concentration gradient of the diffusing component.

2. What is thermal diffusion?

Diffusion generally caused by activity gradient sometimes it occurs by temperature is thermal diffusion.

3. What is forced diffusion?

When the molecule as diffusion is induced by an external field, it is said to be forced diffusion. Generally thermal and forced diffusion are uncommon in chemical engineering only diffusion under concentration gradient is considered

4. Define eddy diffusion.

Diffusion also takes place when fluids of different compositions are mixed. The first step in mixing is often mass transfer caused by the eddy motion characteristic of turbulent flow. This is called eddy diffusion.

5. What is the role of diffusion in mass transfer?

In all the mass-transfer operations and often in both phase and often in both phases. In distillation the low boiler diffuses through the liquid phase to the interface and away from the interface into the vapour. In leatering diffusion of solute through the solid phase is followed by diffusion into the liquid. In liquid extinction the solute diffuses through the raffinate phase to the interface. In crystallization, solute diffuses through the mother liquorto the crystals. In humidification there is no diffusion through the liquid phase.

6. Explain theory of diffusion?.

Here the attention is focused on diffusion in a direction perpendicular to the interface between the phases and at a definite location in the equipment. Generally, steady state is assumed, and concentrations at any point do not change with time.

7. What is the difference between heat transfer and mass transfer?

It arises from the fact that heat is not a substance but energy in transit, whereas diffusion is the physical flow of material.

Heat transfer is based on one temperature gradient but in ass transfer there are different concentration gradients.

8. What are the diffusion quantities?

They are,

Velocity (u)

Flux across the plane (N)

Flux relature to a plane of zero velocity (J)

Concentration (c)

Concentration gradiert (dc/db)

9. What is a diffusion co efficient?

The diffusivity or diffusion coefficient, DAB of a constituent A in solution in B, which is a measure of its diffusive mobility, is then defined as the ratio of its fluex JA to its concentration gradient.

Here DAB is the diffusivity

10. What is Ficks first law?

JA =

Which is Ficks first law written for the Z direction. The negative sign emphasizes that diffusion occurs in the direction of a drop in concentration.

11. What is interstitial mechanism in the diffusion of solids

Interstitial sites are places between the atoms of a crystal lattice small diffusing solute a forms may pass from an intestinal site to the next when the matrix atoms of the crystal lattice move aspect temporarily to provide the necessary space.

12. What is vacancy mechanism?

If lattice sites are unoccupied an a form in an adjacent site may jump into such a vacancy.

13. What is interstitially mechanism?

In this care a large atom occupying an interstitial site pushes one of its lattice neighbours into as interstitial position and moves into the vacancy thus produced.

14. What is crowd -ion mechanism?

An extra a form is a chain of close packed a forms can displace several a forms in the line from their equilibrium position, thus producing a diffusion flux.

15. Define diffusion along group boundaries.

The diffusion in a single crystal metal is always substantially smaller than that for a multi-crystalline sample because the latter has diffusion along the grain boudaces which is nothing but the crystal interfaces.

16. What is an unsteady state operation?

In an unsteady state operation the concentrations at any point in the apparatus change with time.

17. What is a steady - state operation?

In a steady state operation concentrations at any position in the apparatus remain constant with the passage of time

18. What is stage wise operation?

In an insoluble phase if the phase are separated by mechanical means the entire. Operation and the equipment repaired to carry it out are said to constitute one state.

19. Write continuity equation.

For a fluid of constant density the result is,

where = k/lcp and Q is the rate of heat generation within the fluid per unit volume firm a chemical reduction.

20. Give equation for steady state molecular diffusion in fluids.

21. Give the equation for steady state diffusion of A through non- diffusing B.

substitute A diffuses by the virtue of its concentration gradiest, - dpA / de substance B is also diffusing relative to the average molar velocity at a flux J, which depends upon but like a fish which swims upstream at the same velocity as the water flows downstream NB = 0 relative to a fixed place.

22. What is steady-state equimolal counter diffusion?

This frequently per fairs in distillation operations NA = - N3 = conil we have,

23. Define steady state diffusion in multi component mixtures.

It can be effectively handled by using an effective diffusivity where it can be synthesized firm its binary diffusivities.

24. Give the expression, when the diffusion is in stageart conditions.

25. What is an effusion?

The gaseous diffusion is also known as effusion. Here, the membrane micro-porous. If a gas mixture whose components are pf different mokalae weight is brought into contact with such a diaphrapn the various components of the gas pair through the pores of rates dependent upon the mollule weights. This leads to different compositions on opposite sides of the membrane.

26. What is permeation?

In permeation, the member is not porous and the gas transmitted through the membrare first deisolves in it then diffures through. Separation in this case is brought by difference in solubility of the components

27. What are the types of solid diffusion?

The various types of solid diffusion are

Diffusion through polymers.

Diffusion through crystalline solids

Diffusion in porous solids

28. When will the hydrodynamic flow of gores occur?

If there is a difference is absolute pressure across a power solid, a hydrodynamic flow of gas through the solid will occur.

29. Define krudren diffusion.

Knudres diffusion is not known for liquids, but important reductions in diffusion rates occur when the molecular size of the diffusing solute becomes significant relative to the pare size of the solid.

30. What is surface diffusion?

Surface diffusion is a phenomenon accompanying adsorption of solutes onto the surface of the pores of the solid. It is an activated diffusion involving the jumping of adsorbed modules firm one adsorption site to another.

31. Discuss the velocities in diffusion.

Several velocities are needed to describe the movements of individual substances and of the total phase. Since absolute motion has no meaning any velocity must be based on as arbitrary state of rest. In this, velocity without qualification refers to the velocity relative to the interface between the phases and is that apparent. To as observes at rest with respect to the interface.

32. What is turbulent diffusion?

In a turbulent stream, the moving eddies transport matter firm one location to another, just as they transport momentum and heat energy. For momentum transfer and heat transfer in turbulent streams, the equation for mass transfer is,

JAt = -

33. Discuses on prediction of diffusivities.

Diffusivities are best estimated by experimental measurements, and where such information is available where such information is available for the system of interest, it should be used directly. Often the derived values are not available. However they must be estimated from published correlations. Sometimes a value is available for one set of conditions of temperature and pressure.

34. Briefly explain equimolal diffusion.

For equimolal diffusion in gases the net volumetric and molar flours are zero and the equation is

35. What is Schmidt number?

The ratio of the kinematic viscosity to the molecular diffusivity is known as the Schmidt number and it is designated by Sc

36. What is prandtl number?

Prandtl number is defined as the ratio of the kinematic viscosity to the thermal diffusivity

37. Give the expression for diffusivity of dilute solution for of non- electrolytes.

For dilute solutions of nonelectrolytes the empirical correlation of wilke and chang,

DAB =

Where, DAB = Diffusity of A.

MB= Moloculare weigh of solvent

T = Temperature

= Viscosity

Q = Arrociation factor.

38. Give the expression for the diffusivity of liquids in concentrated solution.

The diffusivity in concentrated solutions differ from that in dilute solutions because of change in viscosity with concentration and also because of changes in the degree of non ideality of the solution.

39. Write a note on rate coefficient.

The rate coefficients for the various components in a given phase will differ from each other to the greatest extent under conditions where molecular diffusion prevails, but even then the difference is not really large.

40. Write a short note on diffusion throu

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