For steady state and constant area of mass transfer na,z constant. Mass transfer basic theories for mass transfer coefficients 921 these simple models for fluidfluid interfacespretend that fluid motion is incorporated in diffusion and everything is treated as a thin film or semiinfinite slab problem. As usual, the basic study first focuses on homogeneous nonreacting systems with welldefined boundaries not only in mass transfer, but in heat transfer and in fluid mechanics, touching upon. Diffusion is a key part of the undergraduate chemical engi. Jul 06, 2015 diffusion mass transfer in fluid systems 1. Total mass transfer happens by contribution of two methods. Mass transfer by ordinary molecular diffusion occurs because of a concentration difference or gradient. Little mention is made of the alternative, but less well developed. Diffusion and mass transfer play a significant role in many materials processing operations. Diffusion is the macroscopic result of random molecular motion on a microscopic scale. Often, the transfer takes place between two phases across an interface. In principle, these two extreme cases should bracket all possible geometries. But there are important differences between both subjects. Burghardt encyclopedia of life support systems eolss gases and liquids are all associated with mass transfer.
Mass transfer through membranes involves the concepts of permeation and diffusion. Mass transfer is commonly described as diffusional phenomena in the presence of convective motion. Diffusion mass transfer in fluid systems linkedin slideshare. What is the difference between mass transfer and diffusion.
Understanding how equilibrium and diffusion helps mass transfer between phases will let you understand how several equipment works, such as flashing drums, absorbers, distillation columns, extractors, etc. It is a parameter that is used to describe the ratio between the actual. Mass transfer is the transport of mass from one place to another. Mass transfer is the basis for many chemical and biological processes such as the removal of sulfur dioxide from the flue gas, a chemical process, or the design of an artificial kidney, a biological. Ficks laws of diffusion describe diffusion and were derived by adolf fick in 1855. Apr 02, 2020 ppt introduction to mass transfer and diffusion chemical engineering notes edurev is made by best teachers of chemical engineering. The usual way to make the best of both approaches is to first consider heat transfer. The main difference between mass transfer and diffusion is that mass transfer may or may not occur across a concentration gradient whereas diffusion occurs across a concentration gradient. The objective is to solve the differential equation of mass transfer under steady state conditions at different conditions chemical reaction, one dimensional or more etc. Introduction to diffusion and mass transfer in mixtures convection and diffusion and agitation or stirring moves material over long distances exposing new fluid elements diffusion mixes newly adjacent material because diffusion is slow, it operates only over short distances reference. Sometimes, but not always, this is similar to heat transfer by conduction.
Such unit operations like absorption, rectification, extraction and adsorption, in which mass transfer plays the. Encyclopedia of life support systems eolss component velocity the mass flux of component i can be determined as kgm. The diffusion coefficient or mass diffusivity for a gas may be. Pdf cussler e diffusion mass transfer in fluid systems. The brownian diffusion of small particles and ficks law are first discussed. The flux through diffusion takes place perpendicular to the concentration isosurfaces, i. This document is highly rated by chemical engineering students and has been viewed 1543 times. Mass transfer and reaction in a packed bed section 15. The concentration isosurfaces reveal mass transfer through diffusion and convection. As usual, the basic study first focuses on homogeneous nonreacting systems with welldefined boundaries not only in mass transfer, but. Chemical engineering and chemical process technology vol.
Molecular diffusion, ficks law mass balance transport analogies. The main difference between mass transfer and diffusion is that mass transfer may or may not occur across a concentration gradient. Interphase mass transfer the transport of mass within a single phase depends directly on the concentration gradient of the transporting species in that phase. Mass transfer operations i introduction video youtube. Mass transfer and diffusion request pdf researchgate. Diffusion and mass transfer mass transfer wiley online. The driving force for mass transfer is the concentration difference. This is followed by the presentation of a number of applications. Mass transfer coefficients introduction to mass transfer coefficient 1skm 10 equimolar counter diffusion of a and b na nb 2skm 11 correlation for convective mass transfer coefficient 3skm 12 correlation of mass transfer coefficients for single cylinder 4skm theories of mass transfer 5skm 14 penetration theory, surface. Explain saturated flow and unsaturated capillary flow. Molecular diffusion is caused by the random motion of the fluid molecules or solid microscopic particles. Difference between mass transfer and diffusion definition.
Steadystate molecular diffusion this part is an application to the general differential equation of mass transfer. It deals with the description of diffusion processes in terms of solutions of the differential equation for diffusion. The mass transfer rate is proportional to the area normal to the direction of mass transfer and not to the volume of the mixture. Ahmed daham 4 chapter 10 in volume 1 diffusion the term diffusion mass transfer is used to denote the transference of a component in a mixture from a region where its concentration is high to a region where the concentration is lower. Determination of limiting situations from reactionrate data section 15. The total mass concentration is the density of the mixture which is the total mass of the mixture per unit volume. Mass transfer coefficients introduction to mass transfer coefficient 1skm 10 equimolar counterdiffusion of a and b na nb 2skm 11 correlation for convective mass transfer coefficient 3skm 12 correlation of mass transfer coefficients for single cylinder 4skm theories of mass transfer 5skm 14 penetration theory, surface. The mass transfer process in porous media includes the following two aspects shao et al. Introduction to mass transfer, its importance and role in separation processes. The basic law governing mass transfer at the molecular diffusion level is known as ficks law. Mass transfer in fluid systems brings unsurpassed, engaging clarity to a complex topic.
Mass is transferred by the random motion of molecules across a concentration gradient. Mass transfer theories for mass transfer coefficients. Chemical engineering, molecular motiondiffusion, velocities and fluxes of mass transfer, average mixture velocities, ficks law of diffusion, binary. They can be used to solve for the diffusion coefficient, d. The general differential equation for mass transfer of component a, or the equation of continuity of a, written in rectangular coordinates is initial and boundary conditions to describe a mass transfer process by the differential equations of mass transfer the initial and boundary conditions must be specified. In mass transfer, molar quantities are more convenient to use as compared to mass units, because mass transfer is due to the movement of molecules as discrete quantities. Understand the process of convective mass transfer as due to bulk flow added to diffusion or dispersion. Definition of diffusional flux and reference velocity of diffusion. Multicomponent diffusion coefficients in the gas phase example 2.
Mass diffusion in this section the mass transfer process is described. The common denominator in mass transfer processes is that species fluxes occur within a phase when there are compositionvariations within that phase. The total mass concentration is related to the species mass concentration by. Analyzing diffusion with mass transfer coefficients requires assuming that changes in concentration are limited to that small part of the systems volume near its boundaries. Diffusion coefficients binary diffusion coefficients in the gas phase.
Permeation involves transferring components from an upstream fluid phase to a downstream fluid phase. Many physical situations occur in nature where two. Diffusion and heat transfer university of wisconsin. For example, in the absorption of one gas into a liquid, we assume that gases and liquids are well mixed, except near the gasliquid interface. Cussler e diffusion mass transfer in fluid systems. Diffusion is the even distribution of solutes throughout the system. Mass transfer introduction, brownian diffusion 19 types of mass transfer. Department of chemical and biomolecular engineering. Ppt introduction to mass transfer and diffusion chemical. Ficks law mass transfer fouriers law heat transfer newtons law mom. Both heat and mass are transferred from the more concentrated regions to the less concentrated. It corresponds to the heat conduction mechanism in the heat transfer process.
Besides, heat and mass transfer must be jointly considered in some cases like evaporative cooling and ablation. Mass transfer is a general term, and diffusion is a form of mass transfer. The basic concepts and laws of mass, momentum and energy transport have been presented in three separate chapters. Explain saturated flow and unsaturated capillary flow in a porous solid 6. The book includes mass transfer effects in polymers, which are very important in many industrial processes. Chemical reaction and thermal diffusion effects on mass. Mass may also transport from one phase to another, and this process is called interphase mass transfer. Thus, the absorption by a liquid of a solute from a gas involves mass transfer of the. Pdf the general subject of mass transfer may be divided into four broad areas of particular interest and importance. After studying this chapter, you will be able to describe diffusion and reaction, determine when internal diffusion limits the overall rate of reaction for. Mass transfer is general term and diffusion is a part of this. Mass transfer volumetric coefficient and the sauter mean diameter of near the droplet were measured by varying the impeller position and liquid height using the alkaline hydrolysis reaction of esters. One dimensional mass transfer independent of chemical reaction.
Jul 16, 20 diffusion is the transport of material due to concentration gradients or more precisely due to gradients in the chemical potential. We already have encountered the mass transfer coefficient, defined in a manner analogous to the heat transfer coefficient. Have an idea of the relative rates of the different modes of mass transfer. Other mass transfer operations include distillation, humidification, liquidliquid extraction, adsorption and ion exchange, and so on. Diffusion is a key part of the undergraduate chemical engineering curriculum and at the core of understanding chemical puri. This is similar to the fourier heat conduction law. There have been few studies on heat transfer boundary layer flow in the literature however mass transfer flow. Diffusion mass transfer in fluid systems third edition diffusion. The general subject of mass transfer may be divided into four broad areas of particular interest and importance. For example, in the absorption of one gas into a liquid, we assume that gases and liquids. Analyzing diffusion with mass transfer coefficients requires assuming that changes in concentration are limited to that small part of the systems volume near its. Ficks first law can be used to derive his second law which in turn is identical to the diffusion equation a diffusion process that obeys ficks laws is called normal diffusion or fickian diffusion. Diffusion mass transfer in fluid systems third edit ion diffusion. In module 2 conduction, the fourier equation was introduced, which relates the heat transfer to an existent temperature.