Gas-phase coefficient

B. Absorption counter-current, gas-phase coefficient Hq, for random packing... 

The expression for the effective gas phase coefficient that would account for axial dispersion and hence give a proper mass transfer zone length is ... 

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. 

The bubble column is shown in Figure 6.2c. In this type of equipment, gas is sparged from the bottom into a liquid contained in a large cylindrical vessel. A large number of gas bubbles provide a very large surface area for gas-liquid contact. Turbulence in the liquid phase creates a large liquid-phase mass transfer coefficient, while the gas-phase coefficient is relatively small because of the very... 

For the gas phase, coefficient units are lb-mole/(hr)(ft2) (atm of partial pressure difference). For Kl, units are lb-mole/(hr)(ft2) (concentration difference /ft3). 

However, the sorption coefficient in Equation (2.67) is a liquid-phase coefficient, whereas the sorption coefficient in Equation (2.72) is a gas-phase coefficient. The interconversion of these two coefficients can be handled by considering a hypothetical vapor in equilibrium with a feed solution. This vapor-liquid equilibrium can then be written... 

Equation (2.79) expresses the driving force in pervaporation in terms of the vapor pressure. The driving force could equally well have been expressed in terms of concentration differences, as in Equation (2.83). However, in practice, the vapor pressure expression provides much more useful results and clearly shows the connection between pervaporation and gas separation, Equation (2.60). Also, the gas phase coefficient, is much less dependent on temperature than P L. The reliability of Equation (2.79) has been amply demonstrated experimentally [17,18], Figure 2.13, for example, shows data for the pervaporation of water as a function of permeate pressure. As the permeate pressure (p,e) increases, the water flux falls, reaching zero flux when the permeate pressure is equal to the feed-liquid vapor pressure (pIsal) at the temperature of the experiment. The straight lines in Figure 2.13 indicate that the permeability coefficient d f ) of water in silicone rubber is constant, as expected in this and similar systems in which the membrane material is a rubbery polymer and the permeant swells the polymer only moderately. 

A local overall gas-phase coefficient for mass irairsfor with chemical reaction may he defined by the... 

In practically all the gas-bubble liquid systems, the liquid-phase mass-trans-fer resistance is strongly controlling, and gas-phase coefficients are not needed. The liquid-phase coefficients are correlated by (Hughmark, 1967)... 

A local overaU gas-phase coefficient for mass transfer with chemical reaction may be defined by the equation... 

Another approach to obtaining pure gas-phase coefficients is to make measurements when a pure liquid evaporates into a gas. Here there is no liquid resistance since there is no concentration gradient within the liquid. The resulting volumetric coefficients kyO and F a, however, do not agree with those obtained in the manner first described above. The reason is the different effective interfacial areas, as explained below. 

For Raschig rings and Berl saddles, the gas-phase coefficient is given byf... 

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