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mass transferA process occurring in close binary stars when one of the stars has expanded to such an extent that mass can be transferred to its companion. This mass may strike the more compact companion directly or, more usually, may orbit the star and eventually form a ring of matter – an accretion disk. The mass transfer takes place through the inner Langrangian point, lying between the stars, and occurs when the larger star fills its Roche lobe (see equipotential surfaces).
the spontaneous, irreversible transfer of mass of a given component in a space with a nonuniform field of chemical potential of the component. In the simplest case, the process takes place in a space with a nonuniform concentration or partial pressure field of the component. In the case of thermal diffusion, mass transfer also results from a temperature difference. Mass transfer between a moving medium and an interface with another medium is called mass emission. Mass-transfer processes usually involve multiple stages and include the transfer of matter both within a phase and through a phase surface.
Mass transfer is the basis of many technological processes, including fractional distillation, extraction, absorption, adsorption, drying, and isotope exchange, which are widely used for separating substances and removing harmful or inert impurities from them.
When a flow of substance D, in which the concentration of a diffuse component varies from y1 to y2, passes through an apparatus, the quantity of the substance G = D(y1 - y2) passing through the interface F over time r is given by the mass transfer equation
G = KΔcFr
in which Δc is the average difference between the effective and equilibrium phase concentrations, which is the driving force of the mass-transfer process and may be expressed as the difference in chemical potential or partial pressure, and K is the mass transfer coefficient, which is quantitatively determined by the physicochemical properties of the contacting phases, the design of the apparatus, and the hydrodynamic conditions for the process. In engineering calculations, the concept of a bulk mass-transfer coefficient is often used as the actual phase contact surface is unknown.
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