Diffusion principle and interphase mass transfer process

Extraction separation is the separation of the mixture by the distribution of the components in the equilibrium between the two phases. The extraction equilibrium reflects the possible transfer direction and degree of the solute. The extraction mass transfer process reflects the mechanical properties of extraction, that is, the rate of solute transfer or the difficulty of the process, which is related to the mechanism of the mass transfer. The transfer of solute from one phase to another can be divided into three steps:

The solute diffuses from the main body of one phase to the interface of the two phases;

② The solute changes from one phase to another at the interface;

③ The solute diffuses from the other side of the interface to its main body.

In order to understand the mass transfer between two phases, we must first discuss the diffusion of solute from the fluid body to the interface and from the interface to the fluid body, that is, diffusion in the single phase, and then discuss the transfer between two phases.

The measured movement of certain components through the dry mixture resulting from the presence of a concentration gradient is called diffusion. Because the diffusion motion causes the components to move in the direction that the visibility gradient decreases, the diffusion motion will proceed continuously and steadily if the concentration gradient remains constant.

Diffusion can also be caused by pressure gradients, temperature gradients, or external force fields such as centrifugal force, but these phenomena are rarely used in liquid-liquid extraction processes, so the diffusion caused by concentration gradients is considered. The diffusion phenomena caused by the concentration gradient can be divided into two types: molecular diffusion and eddy diffusion. The former is caused by the thermal motion of molecules while the latter is caused by electrical dry eddies and turbulence in turbulent fluids. Eddy diffusion is faster than molecular diffusion.

(2) The role of diffusion in the extraction process

Since the diffusion process causes the transfer of substances, the unit operation using the diffusion process in chemical engineering is usually called the mass transfer operation. Common are rectification, absorption, rate extraction, leaching, finishing, drying, etc., they are widely used in the chemical industry. Dry liquid-liquid extraction involves mass transfer separation between two liquid phases. The density between the liquid phase is small, the degree is large, the interface phenomenon is complex, and the molecular diffusion coefficient of the liquid phase is small, so it is necessary to analyze the diffusion phenomenon in the liquid-liquid extraction process.