Diffusion and Reactions in the Liquid Phase

the diffusion process is considered after a molecule X is taken into the liquid phase. Assuming the liquid phase is the aqueous solution, and the diffusion of dissolved molecules occurs in one-dimension, the process is expressed by the one-dimensional diffusion equation. 

As expected, the diffusion rate is dependent on the difference of the concentrations near the gas-liquid interface and in the bulk liquid. Also, the diffusion rate decreases in inverse proportion to the square root of time. This is because the number of re-evaporating molecules from gas-liquid surface to the gas phase increases with time. 

putting N q buik = 0 at t = 0, the above equation becomes 

By normalizing J ot (0 with the flux Jcoi, the gaseous molecules collide at the interface in a unit surface area and unit time (Eq. 2.79), the diffusion conductance Fsoi in the liquid phase in Eq. 2.84 is given by 

As shown in the above equation, r oi decreases with time, reflecting the re-evaporation process from the liquid to gas phase. Therefore, after enough time has elapsed (t oo), uptake rate and re-evaporation rate gets equal to reach the gas-liquid equilibrium and F oi O. Meanwhile, when the liquid particles are very small, and the interface layer forms bulk layer, gas-liquid equilibrium is completed instantaneously, and Eq. (2.98) does not apply. 

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There are several examples where it is not practical to separate chemical and physical kinetics. There may be very rapid gasAiquid reactions where phase equilibria, diffusion coefficients and similar physical data are not available, but where the combined effect of diffusion and reaction in the liquid phase can be measured accurately as a function of concentration and temperature. Often the combined kinetics can be used successfully on a larger scale. 

Chemical reactions in the liquid phase are either reversible or irreversible. Typical reversible reactions are involved in the absorption of H2S into ethanolamines, or the absorption of CO2 into alkali carbonate solutions. These reversible reactions permit the resultant solution to be regenerated so that the solute can be recovered in a concentrated form. Some irreversible reactions are the absorption of NH3 into dilute acids and the absorption of CO2 into alkaline hydroxides. The solute in such absorptions is so tightly bound in the reaction product that there is no appreciable vapor pressure of solute above the liquid phase. Under these conditions, regeneration of the solute is not possible, and the reacting component in the liquid is consumed. The purpose of such a reactant is to increase the solubility of the solute in the liquid phase and/or reduce the liquid-film resistance to mass transfer. Much theoretical work has been conducted since the 1950s to study diffusion and reaction in the liquid phase [19]. To calculate the effect of the rate of chemical reaction on the mass transfer requires the prediction of physical/chemical constants of salt solutions, such as equilibrium constants, reaction velocities, solubilities, and diffusion coefficients. Often, these constants must be available at elevated temperatures and/or pressures. 

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