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Principles Of Mass Transfer And Separation Proc...



Mass transfer is the net movement of mass from one location (usually meaning stream, phase, fraction or component) to another. Mass transfer occurs in many processes, such as absorption, evaporation, drying, precipitation, membrane filtration, and distillation. Mass transfer is used by different scientific disciplines for different processes and mechanisms. The phrase is commonly used in engineering for physical processes that involve diffusive and convective transport of chemical species within physical systems.




Principles Of Mass Transfer And Separation Proc...



Some common examples of mass transfer processes are the evaporation of water from a pond to the atmosphere, the purification of blood in the kidneys and liver, and the distillation of alcohol. In industrial processes, mass transfer operations include separation of chemical components in distillation columns, absorbers such as scrubbers or stripping, adsorbers such as activated carbon beds, and liquid-liquid extraction. Mass transfer is often coupled to additional transport processes, for instance in industrial cooling towers. These towers couple heat transfer to mass transfer by allowing hot water to flow in contact with air. The water is cooled by expelling some of its content in the form of water vapour.


In astrophysics, mass transfer is the process by which matter gravitationally bound to a body, usually a star, fills its Roche lobe and becomes gravitationally bound to a second body, usually a compact object (white dwarf, neutron star or black hole), and is eventually accreted onto it. It is a common phenomenon in binary systems, and may play an important role in some types of supernovae and pulsars.


Mass transfer finds extensive application in chemical engineering problems. It is used in reaction engineering, separations engineering, heat transfer engineering, and many other sub-disciplines of chemical engineering like electrochemical engineering.[1]


The driving force for mass transfer is usually a difference in chemical potential, when it can be defined, though other thermodynamic gradients may couple to the flow of mass and drive it as well. A chemical species moves from areas of high chemical potential to areas of low chemical potential. Thus, the maximum theoretical extent of a given mass transfer is typically determined by the point at which the chemical potential is uniform. For single phase-systems, this usually translates to uniform concentration throughout the phase, while for multiphase systems chemical species will often prefer one phase over the others and reach a uniform chemical potential only when most of the chemical species has been absorbed into the preferred phase, as in liquid-liquid extraction.


While thermodynamic equilibrium determines the theoretical extent of a given mass transfer operation, the actual rate of mass transfer will depend on additional factors including the flow patterns within the system and the diffusivities of the species in each phase. This rate can be quantified through the calculation and application of mass transfer coefficients for an overall process. These mass transfer coefficients are typically published in terms of dimensionless numbers, often including Péclet numbers, Reynolds numbers, Sherwood numbers and Schmidt numbers, among others.[2][3][4]


There are notable similarities in the commonly used approximate differential equations for momentum, heat, and mass transfer.[2] The molecular transfer equations of Newton's law for fluid momentum at low Reynolds number (Stokes flow), Fourier's law for heat, and Fick's law for mass are very similar, since they are all linear approximations to transport of conserved quantities in a flow field. At higher Reynolds number, the analogy between mass and heat transfer and momentum transfer becomes less useful due to the nonlinearity of the Navier-Stokes equation (or more fundamentally, the general momentum conservation equation), but the analogy between heat and mass transfer remains good. A great deal of effort has been devoted to developing analogies among these three transport processes so as to allow prediction of one from any of the others.


Many of these separation processes have certain fundamental and basic principles or mechanisms in common. For example, the mechanism of diffusion or mass transfer occurs in drying, membrane separation, absorption, distillation, and crystallization. Heat transfer occurs in drying, distillation, and evaporation. The following classification of a more fundamental nature is often made, according to transfer or transport processes.


1. Momentum transfer. This is concerned with the transfer of momentum that occurs in moving media, such as in the separation processes of fluid flow, sedimentation, mixing, and filtration. Momentum transfer is commonly called fluid mechanics in other disciplines.


2. Heat transfer. In this fundamental process, we are concerned with the transfer energy in the form of heat from one place to another. It occurs in the separation processes of drying, evaporation, distillation, and many others.


Part 1: Transport Processes: Momentum, Heat, and Mass. These fundamental principles are covered extensively in Chapters 1 through 21 in order to provide the basis for study of separation processes in Part 2 of this text.


There are a number of elementary engineering principles, mathematical techniques, and laws of physics and chemistry that are basic to a study of the principles of momentum, heat, and mass transfer, and the separation processes. These are reviewed for the reader in this first chapter. Some readers, especially chemical engineers, agricultural engineers, civil engineers, and chemists, may be familiar with many of these principles and techniques, and may wish to omit all or parts of this chapter.


Separation Process Principles, 3rd edition provides review chapters on thermo and mass transfer, comprehensive discussion of many separation processes, photos, diagrams, and descriptions of process equipment, and challenging, realistic problems. Improved clarity, study questions, and boxed equations and examples in this 3rd edition are especially helpful for students encountering separation processes for the first time.


Mass transfer along with separation processes is an area that is often quite challenging to master, as most volumes currently available complicate the learning by teaching mass transfer linked with heat transfer, rather than focusing on more relevant techniques. With this thoroughly updated second edition, Mass Transfer and Separation Processes: Principles and Applications presents a highly thoughtful and instructive introduction to this sophisticated material by teaching mass transfer and separation processes as unique though related entities. In an ever increasing effort to demystify the subject, with this edition, the author- * Avoids more complex separation processes * Places a greater emphasis on the art of simplifying assumptions * Conveys a greater sense of scale with the inclusion of numerous photos of actual installations * Makes the math only as complicated as necessary while reviewing fundamental principles that may have been forgotten The book explores essential principles and reinforces the concepts with classical and contemporary illustrations drawn from the engineering, environmental, and biological sciences. The theories of heat conduction and transfer are utilized not so much to draw analogies but rather to make fruitful use of existing solutions not seen in other texts on the subject. Both an introductory resource and a reference, this important text serves environmental, biomedical, and engineering professionals, as well as anyone wishing to gain a grasp on this subject and its increasing relevance across a number of fields. It fills a void in traditional chemical engineering literature by providing access to the principles and working practices that allow mass transfer theory to be applied to separation processes.


The number of transfer units (NTU) required is a measure of the difficulty of the separation. A single transfer unit gives the change of composition of one of the phases equal to the average driving force producing the change. The NTU is similar to the number of theoretical trays required for trayed column. Hence, a larger number of transfer units will be required for a very high purity product.


The height of a transfer unit (HTU) is a measure of the separation effectiveness of the particular packings for a particular separation process. As such, it incorporates the mass transfer coefficient that we have seen earlier. The more efficient the mass transfer (i.e. larger mass transfer coefficient), the smaller the value of HTU. The values of HTU can be estimated from empirical correlations or pilot plant tests, but the applications are rather restricted.


and KY is the overall gas-phase mass transfer coefficient. "a" is the packing parameter that we had seen earlier (recall the topic on column pressure drop, e.g. Table 6.3) that characterize the wetting characteristics of the packing material (area/volume). Normally, packing manufacturers report their data with both KY and "a" combined as a single parameter. Since KY has a unit of mole/(area.time.driving force), and "a" has a unit of (area/volume), the combined parameter KY a will have the unit of mole/(volume.time.driving force), such as kg-mole/(m3.s.mole fraction). As seen earlier, other than mole fraction, driving force can be expressed in partial pressure (kPa, psi, mm-Hg), wt%, etc.


(x1* - x1) is the concentration difference driving force for mass transfer in the liquid phase at point 1 (bottom of column) and (x2* - x2) is the concentration difference driving force for mass transfer in the liquid phase at point 2 (top of column).


The present paper on the supported liquid membrane (SLM) deals with the general principles and applications, followed by the uphill transportation characteristic of SLM. The liquid-liquid extraction with supported liquid membrane is one of the best alternate and promising technologies for the extraction of metal ions from solutions over other hydrometallurgical separation processes. The salient features of the supported liquid membrane (SLM) technique such as simultaneous extraction and stripping, low solvent inventory, process economy, high efficiency, less extractant consumption, and operating costs are discussed in detail. The supported liquid membrane of hollow fiber type provides high interfacial surface area for achieving maximum metal flux. Also the use of different organic extractants for SLM has been discussed. 041b061a72


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