1. Chapter 8. Mass Transfer ( Mixing the Acid and Base )
Thermophysical Properties Laboratory
Chapter 8. Mass Transfer ( Mixing the Acid and Base )
Department of Chemical and Biological Engineering,
Seoul National University

2. Contents 8.1 MOLECULAR DIFFUSION 8.2 MASS CONVECTION
8.3 MASS CONVECTION WITH TRANSFER ACROSS PHASE BOUNDARIES
8.4 MULTI-STEP MASS TRANSFER
8.4.1 Membrane Separation
8.4.2 Liquid-Liquid Extraction
8.4.3 Gas-Liquid Absorption
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3. 8.1 MOLECULAR DIFFUSION
Molecular diffusion refers to the movement of individual molecules through a group of molecules without the aid of bulk fluid flow
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4. Fick’s law
The rate of transfer of species A through species B by molecular diffusion in the x direction is
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5. Transfer of electricity (current or I) Driving force (Voltage or V)
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<< Compare >>
Driving force (ΔCA)
Resistance (ΔDABA)
Transfer of electricity (current or I)
Driving force (Voltage or V)
Resistance (R )

6. Binary diffusivity ( DAB )
: describes the ease with which a molecule of species A moves through molecules of species B
DAB is large  transfer occurs rapidly, the resistance is small
DAB is small  transfer occurs slowly, the resistance is large
DAB depends on the properties of the molecules of A and B
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DAB varies with the physical conditions of the system ( T, P )

7. 8.2 MASS CONVECTION
Mass convection is the method by which velocities and flow aid in the mixing of molecules of different types
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8. Even in the presence of flow, molecules also continue to carry on molecular diffusion. However, when flow is present, the convection usually dominates as a mechanism of transfer
Mass convection distributes molecules much more quickly than does molecular diffusion alone
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9. 8.3 MASS CONVECTION WITH TRANSFER ACROSS PHASE BOUNDARIES
A frequent Application of mass transfer in chemical engineering processes is the transfer of mass across “phase boundaries”( interfaces ) where two different phases meet
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10. In designing processes where mass transfer is to occur across phase boundaries, chemical engineers usually include mass convection to increase that transfer
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11. Driving force : Concentration difference Resistance : 1/hmA
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Driving force : Concentration difference
Resistance : 1/hmA

12. 8.4 MULTI-STEP MASS TRANSFER
8.4.1 Membrane Separation
Diffusion and convection occur in a process to separate chemicals using selective membranes, where the transferring substance passes through a membrane while other substances do not
Selectivity on the basis of molecular size
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Selectivity on the basis of solubility
Hydrophilic membranes & Hydrophobic membranes

13. The process involves at least three steps
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14. The fraction of the total membrane area consisting of open pores
Porocity ( εpore )
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The fraction of the total membrane area consisting of open pores
The area for membrane transfer ( Pore Area )

15. In steady-state State, NA is the same for all three steps of the process
( 1 )
( 2 )
( 3 )
( 1 ) + ( 2 ) + ( 3 )
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16.  Total resistance for the process is the sum of resistances for the individual steps
Limiting resistance
If one of the resistances is much larger than the other resistances, that larger resistance will be dominant and will limit the total transfer rate  Limiting resistance
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When an improvement is needed in a process, the chemical engineer must find where the limiting resistance(s) is(are)

17. 8.4.2 Liquid-Liquid Extraction
Immiscible liquids
When two liquids are mixed together, some pairs of liquids will blend to form a single phase while others will remain separate as two phases
Liquids that will not mix with each other are said to be immiscible
Liquid-Liquid Extraction
Given two immiscible liquids in contact with each other, a third chemical compound may be more soluble in one of the phases than in the other
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The difference in solubility can be used to create a driving force for mass transfer

18. Liquid-liquid extraction is promoted when two immiscible liquids are brought into contact so that a particular compound will transfer from one phase to another
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In that process, the transferring compound will travel through the original phase by diffusion or possibly convection, then through the liquid-liquid interface between them, and then into the new phase by diffusion or convection

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20. 8.4.2 Liquid-Liquid Extraction
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Gas-liquid absorption is similar to liquid-liquid extraction where the two phases are a gas and a liquid instead of two liquids