massetransfer

Massetransfer refers to the movement of a component or species from regions of higher concentration to lower concentration due to gradients in chemical potential, concentration, temperature, or pressure. It encompasses diffusion, convection, and phase transfer and occurs in gases, liquids, and porous solids. In many systems diffusion dominates at small scales or in stagnant media, while convection enhances transport in flowing systems.

Diffusion is the molecular transport driven by concentration differences and is described, for simple one-dimensional systems, by Fick's first law N_A = -D_AB dC_A/dx, where N_A is the molar flux and D_AB is the diffusion coefficient. Fick's second law governs transient evolution, ∂C_A/∂t = D_AB ∂^2 C_A/∂x^2. In convective systems, a mass transfer coefficient k combines diffusion with fluid flow; the overall flux can be written as N_A = k (C* - C), where C* is the interfacial concentration in equilibrium with the bulk. Dimensionless groups, such as the Sherwood number Sh = kL/D and the Reynolds and Schmidt numbers, are used to correlate k in engineering applications.

Mass transfer across phase boundaries involves interfacial resistances and partitioning. At equilibrium, a distribution (partition) coefficient describes how a solute divides between phases; away from equilibrium, dynamic mass transfer is modeled by resistances in series (film theory) or by transport in porous media.

Common applications include distillation, absorption, extraction, drying, crystallization, membrane separations, and environmental engineering. In biology, mass transfer principles describe oxygen and nutrient transport across membranes. Accurate modeling relies on diffusivities, interfacial areas, and flow conditions, often validated by experiments and dimensionless correlations.