4.Diffusion

DAY 2: CHEMICAL ENGINEERING – Diffusion

DIFFUSION Review Questions 1. The unit of volumetric diffusivity is a.) cm2/ec b.) cm/sec c.) cm3/sec d.) cm3/sec3 2. The diffusivity (D) in a binary gas mixture is related to the temperature (T) as a.) D  T b.) D  T0.5 1.5 c.) D  T d.) D  T3 3. The diffusivity (D) in a binary gas mixture is related to the pressure (P) as a.) D  T1.5 b.) D  1 P0.5 c.) D  1 d.) D  1 P P1.5 4. Molecular diffusion is caused by a.) transfer of molecules from low concentration to high concentration region b.) thermal energy of the molecules c.) activation energy of the molecules d.) potential energy of the molecules 5. Pick out the correct statement a.) Diffusivity decreases with increase in temperature b.) Diffusivity increases with increase in molecular weight c.) Diffusivity increases with the size of the individual molecule d.) None of these 6. Mass transfer co-efficient is defined as a.) Flux = (Co-efficient) / (concentration difference) b.) Co-efficient = Flux / concentration difference c.) Flux = concentration difference / co-efficient d.) None of these 7. Mass transfer co-efficient (K) and diffusivity (D) are related according to film theory as a.) K  D b.) K  D c.) K  D1.5 d.) K  D2 8. Penetration theory relates average mass transfer co-efficient (K) with diffusivity (D) as a.) K  D b.) K  D 1.5 c.) K  D d.) K  D2 9. Corresponding to Prandtl number in heat transfer, the dimensionless group in mass transfer is a.) Schmidt number b.) Sherwood number c.) Peclet number d.) Stanton number 10. Schmidt number is a.)  DAB b.) Sh x Pe

b.) Re.Pe d.) Re / Pe

DAY 2: CHEMICAL ENGINEERING – Diffusion

11. In physical terms, Schmidt number means a.) thermal diffusivity / mass diffusivity b.) thermal diffusivity / momentum diffuisivity c.) momentum diffusivity / mass diffusivity d.) mass diffusivity / thermal diffusivity 12. Corresponding to Nusselt number in heat transfer, the dimensionless group in mass transfer, is a.) Scherwood number b.) Schmidt number c.) Peclet number d.) Stanton number 13. Mass transfer coefficient of liquid is a.) affected more by temperature than that for gases b.) affected much by less temperature than that for gases c.) not affected by temperature d.) none of these 14. Lewis number (Le) is a.) Sc x Pr c.) Sc x Pr

b.) Pr x Sc d.) St x Sh

15. Which of the following is not a unit of mass transfer coeffiecient? a.) moles transferred (time)(area)(mole fraction) b.) moles transferred ( time)(area)(pressure) c.) mass transferred (time)(area)(mass A / mass B) d.) none of these 16. In case liquid-liquid binary diffusion , diffusivity of one constituent into another is not dependent on a.) temperature and pressure b.) concentration c.) nature of the constituents d.) none of these 17. Steady state equimolal counter diffusion is encountered in a.) separation of a binary mixture by distillation b.) absorption of NH4 from air by water c.) all liquid-liquid diffusion systems d.) all liquid-solid diffusion systems 18. Which of the following has the same dimension as mass diffusivity? a.) momentum flux b.) kinematic viscosity c.) thermal diffusivity d.) both b and c 19. Diffusivity in concentrated solutions differs from that in dilute solutions because of changes in a.) viscocity with concentration b.) degree of ideality of the solution c.) both a and b d.) neither a nor b 20. The value of Lewis number Le = Sc Pr a.) 1 c.) 3.97

for air-water vapour system is around b.) 0.24 d.) 600.0