Henry’s law coefficient

The diffusion coefficient, sometimes called the diffusivity, is the kinetic term that describes the speed of movement. The solubiHty coefficient, which should not be called the solubiHty, is the thermodynamic term that describes the amount of permeant that will dissolve ia the polymer. The solubiHty coefficient is a reciprocal Henry s Law coefficient as shown ia equation 3. 

Henry s law is useful for handling equiUbria associated with gas absorption (qv) and stripping problems. Henry s law coefficients are useful for estimating terminal activity coefficients and have been tabulated for many compounds in dilute aqueous solutions (27). 

Solution. For TGE in water, the Henry s law coefficient may he taken as 417 atm/mf at 20°G. In this low-concentration region, the coefficient is constant and equal to the slope of the eqnihhrinm hne m. The solnhility of TGE in water, based on H = 417, is 2390 ppm. Because of this low solnhility, the entire resistance to mass transfer resides in the liquid phase. Thus, Eq. (14-25) may he used to obtain Nql, the nnmher of overall hqnid phase transfer units. 

The solubility of gases varies widely. Gases with a low solubility (e.g. N2, O2) have large values of the Henry s Law coefficient. This means that the liquid-film resistance in Equation 7.6 is large relative to the gas-film resistance. On the other hand, if the gas is highly soluble (e.g. CO2, NH3), the Henry s Law coefficient is small. This leads to the gas-film resistance being large relative to the liquid-film resistance in Equation 7.6. Thus,... 

Dissociation of the neutral acid in water necessitates modifications for air-sea exchange in the model, which is based on Henry s law. Other possible pathways, e.g. sea spray, are neglected. Henry s law is restricted to concentrations of physically solved, non dissociated substances. Since only the non-dissociated acid is volatile, it is important to correct the air-water partition coefficient as to reflect the relative proportions of volatile and non-volatile components. The corrected parameter is the effective Henry s law coefficient, which is related to the Henry s law coefficient as a function of pH (modified Henderson-Hasselbalch equation) ... 

The concentrations of dissolved atmospheric gases in the oceans, whether of water or other liquids, can be calculated using the Henry s Law coefficients listed in... 

Table 8.3 Henry s Law coefficients for gases dissolved in water and benzene...

Tse, G., Orbey, H., Sandler, S. I. (1992) Infinite dilution activity coefficients and Henry s law coefficients for some priority water pollutants determined by a relative gas chromatographic method. Environ. Sci. Technol. 26, 2017-2022. 

Henry s law coefficient 3 = PA/CA for hydrogen dissolved in feed liquid = 2240 bar/(kmol/m3). 

The proposed catalyst loading, that is the ratio by volume of catalyst to aniline, is to be 0.03. Under the conditions of agitation to be used, it is estimated that the gas volume fraction in the three-phase system will be 0.15 and that the volumetric gas-liquid mass transfer coefficient (also with respect to unit volume of the whole three-phase system) kLa, 0.20 s-1. The liquid-solid mass transfer coefficient is estimated to be 2.2 x 10-3 m/s and the Henry s law coefficient M = PA/CA for hydrogen in aniline at 403 K (130°C) = 2240 barm3/kmol where PA is the partial pressure in the gas phase and CA is the equilibrium concentration in the liquid. 

Table 5.3 Solute and solvent solubility isotope effects for (benzene-water) solutions at 306.2 K obtained from IE s on Henry s Law coefficients, Ki and Kn- [Isotope effects on free energies of transfer, ideal gas to solution in the limit of infinite dilution] (Dutta-Choudhury, M., Miljevic, N.

Adsorption constant, Henry s Law coefficient, slope of the chord of the isotherm, dimensionless... 

Benkelberg, H.J., Hamm. S.. and Warneck, P. Henry s law coefficients for aqueous solutions of acetone, acetaldehyde and acetonitrile, and equilibrium constants for the addition compounds of acetone and acetaldehyde with bisulfite. /. Atmos. CAem.,20(l) 17-34,1995. 

Tse, G., Orbey, H., and Sandler, S.I. Infinite dilution activity coefficients and Henry s law coefficients of some priority water pollutants determined by a relative gas chromatographic method, Environ. Sci Tecbnol, 25(10) 2017-2022, 1992. Tsierkezos, N.G., Kelarakis, A.E., and Palaiologou, M.M. Densities, viscosities, refractive indices, and surface tensions of dimethyl sulfoxide + butyl acetate mixtures at (293.15, 303.15, and 313.15) K, /. Chem. Eng. Data, 45(2) 395-398, 2000. Tsierkezos, N.G. and Molinou, I.E. Densities and viscosities of ethylene glycol mixtures at 293.15 K, /. Chem. Eng. Data, 44(5) 955-958, 1999. 

Cost of an air stripping system is site specific and contaminant specific. In 1991, the cost of air stripping contaminants with a Henry s law coefficient from 0.1 to 10 was estimated to range from 0.07 per 1000 gal of water treated to 0.70 per 1000 gal of water treated (packed-tower system). As the Henry s law coefficient was decreased to 0.005, costs rapidly rose to 7.00 per 1000 gal of water treated (D164241, p. 7). 

TABLE 5.6 Henry s Law Coefficients (H) of Some Atmospheric Gases Dissolving in Liquid Water at 25°C... 

Zielinska et al. (1996) and Kelly and Holdren (1995) have summarized the stability in canisters of organics, some of which are U.S. EPA designated HAPs (hazardous air pollutants). Kelly and Holdren propose that for compounds whose stability in canisters is not known, estimates can be made based on species of similar physical and chemical characteristics. These characteristics include their vapor pressure, polarizability, water solubility, Henry s law coefficient in water, and estimated lifetimes with respect to reactions in air and in the aqueous phase. 

Another method to determine infinite dilution activity coefficients (or the equivalent Henry s law coefficients) is gas chromatography [11, 12]. In this method, the chromatographic column is coated with the liquid solvent (e.g., the IL). The solute (the gas) is introduced with a carrier gas and the retention time of the solute is a measure of the strength of interaction (i.e., the infinite dilution activity coefficient, y ) of the solute in the liquid. For the steady-state method, y is given by [11, 12] ... 

Transfer velocity across gaseous boundary layer typically between 0.1 and 1 cm s"1 (up to 5 cm s 1, see Fig. 20.2). Km is the nondimensional liquid/gas distribution coefficient (for air-water interface inverse nondimensional Henry s law coefficient, i.e., Jfr w) with typical values between 10-3 and 103. DA is the molecular gaseous diffusivity, typical size 0.1 cm2s . 

The air-seawater Henry s law coefficients, A, seawater, and the usual Henry s law constants are given in the margin. 

Figure 20.1 Schematic view of the overall air-water exchange velo-city, via/w, as a function of the air-water partition coefficient, Ku/w, calculated from Eq. 20-3 with typical single-phase transfer velocities v,a = 1 cm s"1, vM = 10 3 cm s1. The broken line shows the exchange velocity v a/w (air chosen as the reference system). The upper scale gives the Henry s Law coefficient at 25°C, Km = 24.7 (Lbar mol"1) x Ku/W.

Figure 20.7 Overall air-water transfer velocity vla/w as a function of Henry s Law coefficient for two very different wind conditions, 10 = 1 m s l (calm overland condition) and Kl0 = 20 m s 1 (rough ocean conditions). The solid lines are calculated for average compound properties Diz = 0.1 cm2 s 1 and Sc,w = 600. The dashed line indicates the boundary between air-phase- and water-phase-controlled transfer velocities. See Table 20.5 for definitions of parameters and substances.

Before we discuss these models, we note that, in contrast to v,w, the air-phase exchange velocity, via, is not strongly affected by the flow. Thus, the following considerations are not relevant for compounds with very small Henry s law coefficients. This is no longer true when the air-water interface is broken up by bubbles and droplets. Some models attempt to incorporate the effect of air bubbles into the exchange velocity v,w (see Eq. 20-38 below), yet air bubbles also lead to a modification of Eq. 20-3 describing the overall exchange velocity, via/w. In the context of river flow, this situation will be treated in Section 24.4. 

Air-water exchange of PCE is liquid-film controlled (Table 20.56). For BC, the size of die Henry s law coefficient (K = 0.015) suggests a slight influence of the air-film which you may disregard. To relate v vw of PCE and BC, combine Eqs. 18-55 and 20-29 into the simple expression ... 

Air-water exchange deserves closer inspection. On one hand, regarding the depth and size of the river the small-eddy model (Eq. 20-32) may be appropriate to calculate v of chloroform. Since the Mississippi River is wide and its waters flow rather slowly, we can, on the other hand estimate air-water exchange from wind speed while noting that the Henry s law coefficient of chloroform indicates a water side-controlled process. The reader is invited to compare the two approaches. 

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