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Film Coefficients of Mass Transfer

As mentioned in Chapter 2, close analogies exist between the film coefficients of heat transfer and those of mass transfer. Indeed, the same type of dimensionless equations can often be used to correlate the film coefficients of heat and mass transfer. [Pg.84]


The film (individual) coefficients of mass transfer can be defined similarly to the film coefficient of heat transfer. A few different driving potentials are used today to define the film coefficients of mass transfer. Some investigators use the mole fraction or molar ratio, but often the concentration difference AC (kg or kmol m ) is used to define the liquid phase coefficient (m while the partial pressure difference A/i (atm) is used to define the gas film coefficient (kmolh m 2 atm ). However, using and A gp of different dimensions is not very convenient. In this book, except for Chapter 15, we shall use the gas phase coefficient (m h" ) and the liquid phase coefficient ki (m h ), both of which are based on the molar concentration difference AC (kmol m ). With such practice, the mass transfer coefficients for both phases have the same simple dimension (L T" ). Conversion between k and is easy, as can be seen from Example 2.4. [Pg.24]

The film model referred to in Chapters 2 and 5 provides, in fact, an oversimplified picture of what happens in the vicinity of interface. On the basis of the film model proposed by Nernst in 1904, Whitman [2] proposed in 1923 the two-film theory of gas absorption. Although this is a very useful concept, it is impossible to predict the individual (film) coefficient of mass transfer, unless the thickness of the laminar sublayer is known. According to this theory, the mass transfer rate should be proportional to the diffusivity, and inversely proportional to the thickness of the laminar film. However, as we usually do not know the thickness of the laminar film, a convenient concept of the effective film thickness has been assumed (as... [Pg.80]

Film coefficients of mass transfer inside or outside tubes are important in membrane processes using tube-type or the so-called hollow fiber membranes. In the case where flow inside the tubes is turbulent, the dimensionless Equation 6.25a, b (analogous to Equation 5.8a, b for heat transfer) provide the film coefficients of mass transfer k [7]... [Pg.84]

In the case where the fluid flow outside the tubes is parallel to the tubes and laminar (as in some membrane devices), the film coefficient of mass transfer on the outer tube surface can be estimated using Equation 6.26a and the equivalent diameter as calculated with Equation 5.10. [Pg.85]

In the case where the fluid flow outside tubes is normal or oblique to a tube bundle, approximate values of the film coefficient of mass transfer can be estimated by using Equation 6.27a [7], which is analogous to Equation 5.12a ... [Pg.85]

Consider the process of absorption. If [y] is the concentration in the bulk gas phase, the driving force toward the interfacial boundary is [y] - [yj and the rate of mass transfer is Ay([y] - [yJ), where ky is the gas film coefficient of mass transfer. For this rate of mass transfer to exist, it must be balanced by an equal rate of mass transfer at the liquid film. The liquid phase mass transfer rate is kJ Xj - [x]), where is the liquid film coefficient of mass transfer and [x] is the bulk concentration of the solute in the liquid phase. Thus,... [Pg.442]

Individual liquid film coefficient of mass transfer... [Pg.470]


See other pages where Film Coefficients of Mass Transfer is mentioned: [Pg.81]    [Pg.84]    [Pg.85]    [Pg.94]    [Pg.470]    [Pg.81]    [Pg.84]    [Pg.85]    [Pg.94]    [Pg.94]    [Pg.192]    [Pg.210]    [Pg.214]    [Pg.214]    [Pg.215]    [Pg.246]    [Pg.291]   


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