Masstransfer

Mass transfer is the net movement of mass from regions of higher concentration to lower concentration, driven by concentration gradients, and occurring across phase interfaces and within phases. It is a core transport phenomenon in chemical engineering, environmental science, and related fields.

The mechanisms include molecular diffusion, convection with bulk fluid flow, and phase-change-driven transfer such as dissolution or evaporation. Diffusion is described by Fick's laws; convection involves transport by fluid motion; the combined behavior is captured by the convection-diffusion equation, often written in words: the rate of change of concentration equals diffusion flux divergence plus convective transport.

In engineering, mass transfer is analyzed using characteristic coefficients such as the mass transfer coefficient k or the overall coefficient in a boundary layer; the interfacial area a. The non-dimensional Sherwood number Sh = kL/D relates k to diffusivity D and a characteristic length L. Correlations with Reynolds and Schmidt numbers (Re, Sc) are used to estimate Sh. Film theory, which assumes a stagnant liquid film around a phase, and the penetration theory, which emphasizes pore or tortuous paths, are two classical models for gas-liquid mass transfer.

Common processes include absorption, stripping, distillation, extraction, drying, and crystallization; in environmental engineering, mass transfer governs pollutant uptake and transfer between air, water, and soil. Mass transfer is often the rate-limiting step in design, and engineers use experimental data or empirical correlations to determine transfer coefficients.

Mass transfer is related to but distinct from heat transfer and momentum transfer; like those phenomena, it is described by transport equations and often requires similar modeling techniques, but governed by concentration instead of temperature or velocity fields.