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1、1SECTION IV Mass Transfer and Its Applications 2Content 1. Mass transfer operations2. Unit operations included in mass transfer operations3. Terminology and symbols4. Diffusional processes and equilibrium stages31. Mass-transfer operations A group of operations for separating the components of mixtu

2、res is based on the transfer of material from one homogeneous phase to another.4Separation processes of homogeneous phasehomogeneous phaseProcess Two phases systemutilize differences in vapor pressure, solubility, or diffusivitySeparate the two phasesMass transfer from one phase to another Separatio

3、n of homogeneous mixture52. The unit operations include in mass transfer operationsa. Distillation(蒸馏)b. Gas absorption（吸收）c. liquid extraction（萃取） d. Drying of solid（干燥） Dehumidification（减湿）6e. Crystallization（结晶）f. Leaching（浸取） g. Adsorption（吸附）h. Membrane separations（膜分离）7a. Distillation（蒸馏）Separ

4、ating a liquid mixture of miscible and volatile substances into individual components or, in some cases, into groups of components , by vaporization.Liquid mixturevolatile(A)+nonvolatile(B)HeatingPartial vaporizingVapor phase(more A)Liquid phase(more B)8Examples of distillation1. The separation of a

5、 mixture of alcohol and water into its components; 2. The separation of liquid air into nitrogen, oxygen, and argon; 3. The separation of crude petroleum into gasoline, kerosene, fuel oil, and lubricating stock.9b. Gas absorption（气体吸收）The solute gas in a gas mixture is absorbed from an inert gas int

6、o a liquid in which the solute is more or less soluble. Gas mixturesolute(A)+inert gas(B)Solvent(S)Gas phase(B)Liquid phase(S+A)10Example of gas absorptionThe washing of ammonia from a mixture of ammonia and air by means of liquid water. Desorption or stripping（解吸）airammoniawaterairAmmonia water11c.

7、 Liquid extraction (Solvent extraction)（萃取）A mixture of two components is treated by a solvent that preferentially dissolves one or more of the components in the mixture. The solvent in the extract leaving the extractor is usually recovered and reused. Mixture(solute+diluent)Solvent（溶剂）Extract（萃取相）R

8、affinate(萃余相)12d. Drying of solid（固体干燥）In the drying of solids, a liquid, usually water, is separated by the use of hot, dry gas (usually air) and so is coupled with the humidification of the gas phase.Wet solidHot dry gasDry solidHumid gas13e. Crystallization（结晶）Crystallization is used to obtain ma

9、terials in attractive and uniform crystals of good purity, separating a solute from a melt or a solution and leaving impurities behind. 14f. Leaching(extraction of solids)（浸取）Soluble material is dissolved from its mixture with an inert solid by means of a liquid solvent. The dissolved material, or s

10、olute, can then be recovered by crystallization or evaporation. Solid mixture(solute A+inert solid B)Solvent（S）Liquid phase（A+S）Solid phase (B)15g. Adsorption（吸附）Solute is removed from either a liquid or a gas through contact with a solid adsorbent（吸附剂）, the surface of which has a special affinity f

11、or the solute.16h. Membrane separations（膜分离）They include gas separations, reverse osmosis(反渗透), and ultrafiltration（超滤）. 17In this semester, we will discuss the operation of distillation, gas absorption, liquid extraction and drying of solid. Other operations will be introduced briefly.183. TERMINOL

12、OGY AND SYMBOLS TABLE A Terminology for streams in mass-transfer operations (p507)OperationV phaseL phaseDistillationVaporLiquidGas absorption, dehumidificationGasLiquidMembrane separationsGas or liquidGas or liquidAdsorptionGas or liquidSolidLiquid extractionExtractRaffinateLeachingLiquidSolidCryst

13、allizationMother liquorCrystalsDryingGas (usually air)Wet solid19Note on concentrations = mole or mass fraction of component i= molar or mass density of mixture, kg/m3 or mol/m3 = corresponding concentration of component, mol/m3= pressure fraction of component i, Pa.20The relation of different kind

14、of concentration(17.8)214. DIFFUSIONAL PROCESSES AND EQUILIBRIUM STAGES Mass-transfer processes can be modeled by two distinctly different methods:a. based on diffusion rate processes -packed towers(填料塔), b. using the concept of equilibrium stages - plate towers（板式塔）. 22Phase Equilibria A limit to m

15、ass transfer is reached if the two phases come to equilibrium and the net transfer of material ceases. Equilibrium data can be shown in tables, equations, or graphs. 23phase rule where F = number of degrees of freedom c = number of components p= number of phases 24Applications of the phase rule to t

16、hree typical operations follow: Gas absorption: F= 3-2+2=3 Distillation: F=2-2+2=2 Liquid extraction :F=3-2+2=325Chapter 17 Principles of diffusion and mass transfer between phases26Content 17.1 CONCEPT OF DIFFUSION17.2 THEORY OF DIFFUSION17.3 PREDICTION OF DIFFUSIVITIES17.4 MASS-TRANSFER THEORIES17

17、.5 MASS-TRANSFER COEFFICIENTS2717.1 CONCEPT OF DIFFUSION1. Concept of diffusion2. Diffusion manner in fluid3. Diffusion process in different unit operations281. Concept of diffusionDiffusion is the movement, under the influence of a physical stimulus, of an individual component through a mixture.29D

18、iffusion can be caused by:Temperature gradient: thermal diffusion热扩散An external force field: force diffusion强制扩散Concentration gradient: mass diffusion302. Diffusion manner in fluida. Molecular diffusion (分子扩散) Caused by molecular movement transport from high concentration to low concentrationJust as

19、 conduction in heat transferb. Eddy diffusion（涡流扩散）Caused by fluid physical mixing fluid eddies of turbulent flowJust as convection in heat transfer31Bulk flow(主体流动)Drift of the entire phase in the direction parallel to the direction of diffusion.Sometimes the diffusion process is panied by bulk flo

20、w. 323. Diffusion process in different unit operationsIn all of the mass transfer operations(unit operations),diffusion occurs in at least one phase and often in both phases.33In Gas adsorptionCarrier gas + solute gas phase solvent liquid phase(interface)34In distillation, A is high volatile compone

21、ntB is low volatile component(interface)A+B vapor phase A+B liquid phase35In leaching, diffusion of solute through the solid phase is followed by diffusion into the liquid. A: soluteSolid phaseALiquid phaseinterface36In liquid extraction, the solute diffuses through the raffinate phase to the interf

22、ace and then into the extract phase. extract phase A+B(interface)Sraffinate phase3717.2 Theory of diffusion1. Comparison of diffusion and heat transfer 2. Diffusion quantities 3. Velocities in diffusion4. Molal flow rate, velocity, and flux5. Ficks first law of diffusion*6. Interpretation of diffusi

23、on equations 7. Equimolal diffusion *8. ponent mass transfer (one-way diffusion)* 38The discussion is restricted to:(1). Diffusion in the direction perpendicular to the interface between the phases. (2). Steady state molecular diffusion(3). Binary mixture391. Comparison of diffusion and heat transfe

24、rSimilarities: Driving force: heat transfer by conduction: temperature gradient mass transfer by diffusion: concentration gradient40Rate of transfer (flux): heat transfer by conduction: proportional to temperature gradient mass transfer by diffusion: proportional to concentration gradient41Differenc

25、es:Heat transferMass transferTransfer substanceTransfer energy422. Diffusion quantities 1). Velocity u, m/s.2). Flux across a plane N, mol/m2 s.3). Flux relative to a plane of zero velocity J, mol /m2 s.434). Concentration cM : molar density, mol/m3 x, y : mole fraction .5). Concentration gradient d

26、c/db, where b is the length of the path perpendicular to the area across which diffusion is occurring.443. Velocities in diffusionAbsolute velocityVelocity of bulk flowVelocity of diffusion454. Molal flow rate, velocity, and fluxFlux = (concentration) X (velocity)Molar densityMolar fractionAbsolute

27、velocityBulk flow velocityDiffusion velocity464. Molal flow rate, velocity, and flux(17.2)(17.3)(17.1)(17.4)(17.5)475. Ficks first law of diffusion The diffusion flux is assumed to be proportional to the concentration gradient , and the diffusivity扩散系数of component A, in its mixture with component B, is denoted by