Ammonia Henry coefficient

Whffle Ho = Henry coefficient for ammonia in pure water... 

Deviations from Henry s law are exhibited by most gases having absorption coefficients greater than 100. In some cases the discrepancies vanish at higher temperatures. Thus Roscoe and Dittmar (1860) found that ammonia did not follow the law of Henry at the ordinary temperature, but Sims (1862) showed that the deviations from the law became less as the temperature at which absorption occurred increased, until at 100° the amount of ammonia dissolved by water was directly proportional to the pressure. The deviations appear to be always greatest under small pressures, and to decrease with increasing pressure, and therefore with increasing concentration of the solution they are doubtless due to chemical interaction between the solvent and dissolved gas. 

Similar results are derived for other weak acids and for ammonia (Warneck, 1986). In each case one can define a factor /a, which expresses the dependence on proton concentration, and a modified Henry s law coefficient... 

So under atmospheric conditions practically all dissolved ammonia in clouds is in the form of ammonium ion. The aqueous-phase concentrations of [NH ] in equilibrium with 1 ppb of NH3(g) are shown in Figure 7.9. The partitioning of ammonia between the gas and aqueous phases inside a cloud can be calculated using (7.9) and the effective Henry s law coefficient for ammonia, // = Wnh, Ka [H+]/ATm. If the cloud pH is less than 5 practically all the available ammonia will be dissolved in cloudwater (Figure 7.10). 

The plots indicate that the non-ideality is significant at higher concentrations. Consequently, liquid activity coefficients should be used. Because NHj is subcritical, there is a choice between symmetric and asymmetric conventions. In the first case, Antoine equation is necessary for both components. In the second case, Henry constant for ammonia-water pair is required, but this may be obtained from the slope of the experimental curves plotted in Fig. 6.14. Vapour pressures and Henry constants are ... 

Time does not permit a detailed discussion but, in view of the importance of these solutions in chemical engineering, let me quickly show a few results. Figure 41, from Edwards et al. (23),shows how experimental data in the dilute region for aqueous ammonia can be reduced to yield Henry s constants (the intercept) and Guggenheim s coefficient 6 (the slope). Note that the abscissa gives the molecular molality of ammonia, not the total molality therefore. Figure 41 implicitly Includes the effect of ionization as determined by the independently-measured chemical dissociation constant. 

The activity coefficients and the fugacity coefficients consequently account for the deviation from Raoult s law for ammonia, and for the deviations from Henry s law for the dissolved gases. The validity of a published gas solubility for a binary system in a multicomponent system requires therefore that Henry s law... 

Example 8.5-4 Overall mass transfer coefficients in a packed tower We are studying gas absorption into water at 2.2 atm total pressure in a packed tower containing Berl saddles. From earlier experiments with ammonia and methane, we believe that for both gases the mass transfer coefficient times the packing area per tower volume is 18 Ibmol/hrft for the gas side and 530 Ibmol/hrft for the liquid side. The values for these two gases may be similar because methane and ammonia have similar molecular weights. However, their Henry s law constants are different 75 atm for ammonia and 41,000 atm for methane. What is the overall gas-side mass transfer coefficient for each gas ... 

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