theoretical bases for gas/liquid exchanges

The following constitute the main laws governing gas/liquid exchanges :

• din the liquid phase and at equilibrium, the Henry law that links, for a given temperature, the partial pressure of a gas p to its molar fraction x in liquid phase p = Hx, H being the Henry constant. Please refer to section characteristic constants of gases, for the Henry constant applicable to the main gases ;
• in the gas phase: Dalton’s law and the ideal gas law.

Thus, for a gas mixture occupying a volume V at a temperature T and a pressure P and constituted by m₁, m₂… m_n quantities by mass of gases having gramme-molecular weight of M_l, M₂ M_n respectively, applying partial pressures p₁, p₂ p_n, we can write :

For the transfer : the Whitman & Lewis theory used to quantify the global stream N of the compound transferred through the exchange surface S in the absence of any accumulations at the interface (figure 108) :

C_L and C_g are the gas concentrations in the liquid and gas phases that are only accessible to measurement.

C_iL, and C_ig. are the concentrations at the interface, k_L and k_g being the transfer coefficients in the liquid and gas phases that are dependant on the interface and on turbulence conditions.

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These laws highlight the parameters that are essential to an efficient transfer :

• maintaining a high concentration gradient between the liquid and the gas phase, this gradient acting as a driving force;
• creating a gas/liquid interface that is as large as possible;
• using high levels of turbulence for each of these 2 phases.