Gas film coefficient

any turbulence velocity and length scale are sufficient to use in these dimensionless parameters, as long as they are used in all parameters. Tamburrino and Gulliver used the bottom shear velocity and channel depth. Equation (8.69) provides a measure of surface renewal rate  

this fundamental approach to determining gas transfer coefficient is still sorting out which flows have all scales contribute to gas transfer coefficient, such that /Srms, and which flows have only the larger scales of p contribution, such that Ki It will be some time before the approach is taken to the field. 

The gas film coefficient is dependent on turbulence in the boundary layer over the water body. Table 4.1 provides Schmidt and Prandtl numbers for air and water. In water, Schmidt and Prandtl numbers on the order of 1,000 and 10, respectively, results in the entire concentration boundary layer being inside of the laminar sublayer of the momentum boundary layer. In air, both the Schmidt and Prandtl numbers are on the order of 1. This means that the analogy between momentum, heat, and mass transport is more precise for air than for water, and the techniques apphed to determine momentum transport away from an interface may be more applicable to heat and mass transport in air than they are to the liquid side of the interface. 

Saturation vapor pressure at any air temperature may be computed by the Magnus-Tetons formula  

Because values of and Va can be found from tables such as those in the Handbook of Chemistry and Physics (CRC, 2005), all that remains is to determine Pt before we can estimate Kq from free convection. The equation 

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Log arithmic-Mean Driving Force. As noted eadier, linear operating lines occur if all concentrations involved stay low. Where it is possible to assume that the equiUbrium line is linear, it can be shown that use of the logarithmic mean of the terminal driving forces is theoretically correct. When the overall gas-film coefficient is used to express the rate of absorption, the calculation reduces to solution of the equation... 

Experimental K g< and Ki a data are available for most absorption and stripping operations of commercial interest (see Sec. 15). The solute concentrations employed in these experiments normally are very low, so that Ki a — Ki/i and K g< Pt where pf is the total pressure employed in the actual experimental-test system. Unlike the individual gas-film coefficient /cg, the overall coefficient will... 

Gas-Film Coefficient Since the gas film is not affected by the liquid-phase reaction, one of the many available correlations for physic absorption may be apphcable. The coefficient also may be found directly after elimination of the hquid-film coefficient by employing a solution that reacts instantaneously and irreversibly with the dissolved gas, thus cancehng out any backpressure. Examples of such systems are SO2 in NaOH and NH3 in H2SO4. 

Hog = height of transfer units, based on overall gas film, coefficients, ft... 

Calculate the shell-side dry-gas film coefficient, hg or h, for outside tube conditions. Assume a baffle spacing or about equal to one shell diameter. Use the shell-side method described in Equation 10-48 and Figure 10-54. This is necessary for inlet conditions and then must be checked and recalculated if sufficient change occurs in the mass flow rate, G, to yield a change in hg. 

Gas excitation, as cause of color, 7 326t, 328 Gas feeders, for swimming pools, 26 178 Gas-film coefficient, 15 695 Gas flammkohle coal grade (Germany), 6 713t... 

These correlations are applicable to all the systems employed, provided that the initial maximum values of the transfer coefficients are used. This suggests that the extrapolation gives the true gas-film coefficient. This is borne out by the fact that the coefficient remained unchanged for a considerable period when the pores were large, though it fell off extremely rapidly with solids with a fine pore structure. It was not possible, to relate the behaviour of the system quantitatively to the pore size distribution however. 

The results for the heat transfer coefficient were satisfactorily correlated by equation 6.68 as shown in Figure 6.30. Since the resistance to heat transfer in the solid could be neglected compared with that in the gas, the coefficients which were calculated were gas-film coefficients, correlated by ... 

The simplest method of representing data for gas-film coefficients is to relate the Sherwood number [(hod/Dv)(PBm/Z3)] to the Reynolds number (Re) and the Schmidt number (p,/pDv). The indices used vary between investigators though van Krevelen and Hoftijzer(28) have given the following expression, which is claimed to be valid over a wide range of Reynolds numbers ... 

Rathbun RE, Tai DY. 1986. Gas-film coefficients for the volatilization of ethylene dibromide from water. Environ Sci Technol 20 949-952. 

The gas film coefficient due io free convection (Figure 8.15a) is described by the relation of Shulyakovskyi (1969) ... 

EXAMPLE 8.6 Computation of gas film coefficient over a water body... 

Figure E8.6.1. Computations of gas film coefficient for the unstable boundary layer of Example 8.6 and for a neutral boundary layer that is otherwise similar.

T/F) The gas film coefficient is linearly dependent on diffusion coefficient of the compound. 

The evaporation of water is generally used to determine the gas film coefficient. A loss of heat in the water body can also be related to the gas film coefficient because the process of evaporation requires a significant amount of heat, and heat transfer across the water surface is analogous to evaporation if other sources and sinks of heat are taken into account. Although the techniques of Section 8.D can be used to determine the gas film coefficient over water bodies, they are still iterative, location specific, and dependent on fetch or wind duration. For that reason, investigators have developed empirical relationships to characterize gas film coefficient from field measurements of evaporation or temperature. Then, the air-water transfer of a nonvolatile compound is given as... 

The relationships developed from field measurements have been made dimensionless with the assumptions that v = 1.33 x 10 m /s and AijO = 2.6 x 10 m /s to facilitate comparisons between relations and avoid dimensional problems. They are given in Table 9.2. The early measurements were to investigate the loss of water from the reservoirs of the Colorado River in the United States, and the later measurements were designed to investigate heat loss from heated water bodies. A revelation occurred in 1969, when Shulyakovskyi brought in buoyancy forces as related to natural convection to explain the heat loss from heated water at low wind velocities. This was picked up by Ryan and Harleman (1973), who realized that natural convection could explain the need for a constant term in front of the relationship for gas film coefficient, as had been found by Brady et al. (1969), Kohler (1954), Rymsha and Dochenko (1958), and Shulyakovskyi (1969). Finally, Adams et al. (1990) rectified... 

EXAMPLE 9.3 Application of characteristic relations for gas film coefficient... 

Table 9.2 Relationships developed to characterize gas film coefficient over water surfaces...

Kg, gas film coefficient A, surface area of water body 7), diffusion coefficient of compound in air W, wind velocity at 2 m above the mean water surface v, kinematic viscosity of air a, thermal diffusion coefficient of air g, acceleration of gravity thermal expansion coefficient of moist air AP, temperature difference between water surface and 2 m height APv virtual temperature difference between water surface and 2 m height. 

A 10-km-long lake is exposed to various wind speeds. On a cold day in the fall, the water temperature at 10°C has not cooled yet, but the air temperature is at -5 °C. The relative humidity is 100%. Compute the gas film coefficient for water vapor at various wind fetch lengths for wind speeds at 2 m height and at 2,6,10, and 16 m/s. Compare your results with those of Example 8.6. 

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. 

A gas component A in air is absorbed into water at latm and 20 °C. The Henry s law constant of A for this system is 1.67 X 10 Pa m kmol h The liquid film mass transfer coefficient and gas film coefficient I(q are 2.50x10 and 3.00 X10" ms respectively, (i) Determine the overall coefficient of gas-liquid mass transfer (ms ). (ii) When the bulk concentrations of A in the gas phase and liquid phase are 1.013 X 10 Pa and 2.00 kmol m , respectively, calculate the molar flux of A. 

This equation can be written Nco,s k[CCor where k[ = VA, Deo, and may be regarded as a liquid-film mass transfer coefficient enhanced oy the fast chemical reaction. This is very convenient because it allows us to use the expression in Volume 2, Chapter 12 for combining liquid-film and gas-film coefficients to give an overall gas-film coefficient ... 

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