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Introduction to Mass Transfer. Outline 1.Mass Transfer Mechanisms 1.Molecular Diffusion 2.Convective Mass Transfer 2. Fick’s Law for Molecular Diffusion.

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Presentation on theme: "Introduction to Mass Transfer. Outline 1.Mass Transfer Mechanisms 1.Molecular Diffusion 2.Convective Mass Transfer 2. Fick’s Law for Molecular Diffusion."— Presentation transcript:

1 Introduction to Mass Transfer

2 Outline 1.Mass Transfer Mechanisms 1.Molecular Diffusion 2.Convective Mass Transfer 2. Fick’s Law for Molecular Diffusion 3. Molecular Diffusion in Gases 1.Equimolar Counterdiffusion 2.Combined Diffusion and Convection 3.Uni-component Diffusion

3 Mass Transfer Mechanisms 1. Convective Mass Transfer2. Diffusion http://www.timedomaincvd.com/CVD_Fundamentals/xprt/xprt_conv_diff.html

4 Mass Transfer Mechanisms 3. Convective and Diffusion http://www.timedomaincvd.com/CVD_Fundamentals/xprt/xprt_conv_diff.html

5 Outline 1.Mass Transfer Mechanisms 1.Molecular Diffusion 2.Convective Mass Transfer 2. Fick’s Law for Molecular Diffusion 3. Molecular Diffusion in Gases 1.Equimolar Counterdiffusion 2.Combined Diffusion and Convection 3.Uni-component Diffusion

6 Fick’s Law for Molecular Diffusion For a binary mixture of A and B

7 Molecular Transport Equations RECALL: MOMENTUM HEAT MASS

8 Fick’s Law for Molecular Diffusion Example A mixture of He and N 2 gas is collected in a pipe at 298 K and 1 atm total pressure which is constant throughout. At one end of the pipe at point 1 the partial pressure p A1 of He is 0.60 atm and at the other end 0.2 m p A2 = 0.20 atm. Calculate the flux of He at steady state if D AB of the He-N 2 mixture is 0.687 x 10 -4 m 2 /s.

9 Convective Mass Transfer Coefficient

10 Outline 1.Mass Transfer Mechanisms 1.Molecular Diffusion 2.Convective Mass Transfer 2. Fick’s Law for Molecular Diffusion 3. Molecular Diffusion in Gases 1.Equimolar Counterdiffusion 2.Combined Diffusion and Convection 3.Uni-component Diffusion

11 Molecular Diffusion in Gases Equimolar Counterdiffusion Flux of one gaseous component is equal to but in the opposite direction of the second gaseous component A B BA

12 Molecular Diffusion in Gases Equimolar Counterdiffusion A B BA

13 Molecular Diffusion in Gases Equimolar Counterdiffusion A B BA Substitution of Fick’s law into the equation for equimolar counter diffusion,

14 Molecular Diffusion in Gases Equimolar Counterdiffusion A B BA

15 Molecular Diffusion in Gases Equimolar Counterdiffusion A B BA For gases,

16 Molecular Diffusion in Gases Equimolar Counterdiffusion A B BA In terms of mole fraction,

17 Molecular Diffusion in Gases Example A large tank filled with a mixture of methane and air is connected to a second tank filled with a different composition of methane and air. Both tanks are at 100 kN/m 2 and 0°C. The connection between the tanks is a tube of 2 mm inside diameter and 150 mm long. Calculate the steady state rate of transport of methane through the tube when the concentration of methane is 90 mole percent in one tank and 5 mole percent in the other. Assume that transport between the tanks is by molecular diffusion. The mass diffusivity of methane in air at 0°C and 100 kN/m 2 is 1.57 x 10 -5 m 2 /s.

18 Molecular Diffusion in Gases Diffusion plus Convection

19 Molecular Diffusion in Gases Diffusion plus Convection Total convective flux of A wrt stationary pt Diffusion flux wrt moving fluid Convective flux wrt to stationary point

20 Molecular Diffusion in Gases Diffusion plus Convection

21 Molecular Diffusion in Gases Diffusion plus Convection

22 Molecular Diffusion in Gases Uni-component Diffusion One component (A)diffuses, while the other (B) remains stagnant http://sst-web.tees.ac.uk/external/U0000504/Notes/ProcessPrinciples/Diffusion/Default.htm

23 Molecular Diffusion in Gases Uni-component Diffusion http://sst-web.tees.ac.uk/external/U0000504/Notes/ProcessPrinciples/Diffusion/Default.htm

24 Molecular Diffusion in Gases Uni-component Diffusion http://sst-web.tees.ac.uk/external/U0000504/Notes/ProcessPrinciples/Diffusion/Default.htm

25 Molecular Diffusion in Gases Uni-component Diffusion http://sst-web.tees.ac.uk/external/U0000504/Notes/ProcessPrinciples/Diffusion/Default.htm When P is constant,

26 Molecular Diffusion in Gases Example Water in the bottom of a narrow metal tune is held a t a constant temperature of 293 K. The total pressure of air (assumed dry) is 1.01325  10 5 Pa and the temperature is 293 K. Water evaporates and diffuses through the air in the tube, and the diffusion path z 2 -z 1 is 0.1524m long. Calculate the rate of evaporation of water vapor at 293 K and 1 atm pressure. The diffusivity of water in air is 0.250 x 10 -4 m 2 /s. Assume that the system is isothermal.

27 Long Exam Results LE 1LE 2 Mean33.3236.55 Median32.0030.75 Mode39.0025.50 Passing Rate0.009.09

28 QuizzesMachine ProblemsTarget Average Scores Student No.TotalQ 5/5Total*M 15/15LE3L 60/60FinalF 20/20 2011-18077 301.50240128230.48216.4 2011-57319 462.30240129027.89018.0 2010-04141 361.802401282318216.4 2010-01283 261.30240128929.18917.8 2010-31873 472.352401285298517.0 2011-07217 261.30240126733.36713.4 2011-03676 502.50240129326.99318.6 2010-36588 231.15240129228.79218.4 2011-18143 311.55240127331.97314.6 2011-18147 311.55240127731.37715.4 2011-09522 331.65240121022610220.4 2011-30507 231.15240128829.68817.6 2011-09270 190.95240128130.98116.2 2010-53270 361.80240127431.67414.8 2011-14930 613.052401251355110.2 2009-21119 80.40240129928.19919.8 2011-21884 482.40240127032.17014.0 2011-19280 90.45240129229.59218.4 2011-26790 1045.20240124534.1459.0 2010-21409 140.702401210027.310020.0 2011-01530 572.85240126532.56513.0 2011-30255 211.05240129128.99118.2


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