Chapter 14 Workbook

Chapter 14 Workbook - Chapter 14 Diffusion Mass Transfer...

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Chapter 14 – Diffusion Mass Transfer Introduction Being knowledgeable in mass transfer phenomena is one of the distinguishing features of a good chemical engineer. Other engineering disciplines rarely consider mass transfer effects – the only exception being environmental engineering, which is really chemical engineering anyway. You have already covered mass transfer by convection. In this chapter, the goal is to cover mass transfer by diffusion. Diffusion mass transfer is caused by a species concentration gradient in a mixture. A chemical species will diffuse from an area of high concentration towards lower concentration. This phenomenon is extremely important in applications for industry, biochemistry, biotechnology, and daily life. In industry, diffusion can be used to an advantage (e.g., mixing), or it can be a severe disadvantage - especially in separations when you are trying to get a highly concentrated pure product. Much energy and money are spent on counteracting the bad effects of diffusion in product purification. In the physiology of living systems, much energy is spent on counteracting diffusion – this is known as “active transport.” Without active transport, the cardiovascular system would fail, your eyes would shrivel up like raisins, and your blood would become filled with toxins. In a chemical engineering career, most of the applications of mass transfer are in product purification and separations. You should be concerned with product purification because approximately 75% of the capital cost in the typical production facility is dedicated to purification unit operations. In your Separations class that you took earlier, you assumed equilibrium between phases. Sometimes that is a good assumption (e.g., distillation columns). But for separation processes that are used for more complex molecules such as pharmaceuticals and biotechnology products – precipitation, centrifugation, extraction, absorption, chromatography, membrane filtration, etc. – the assumption of equilibrium is often a poor one. Another area where mass transfer rates are important is solid phase catalytic reactions. Understanding the balance between reaction rates and mass transfer rates of the reactants and products to and from the porous catalytic surface is extremely important to the optimization of the process.
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Introduction to Mass Transfer o Mass transfer refers to mass in transit due to a species concentration gradient in a mixture. ± Must have a mixture ± Driving Force – species concentration gradient ± Equations describing mass transfer by diffusion can be analogous to those describing heat transfer by conduction. o Physical Origins of diffusion ± Random molecular motion ± Consider two species A and B at the same T and p , but initially separated by a partition.
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This note was uploaded on 03/11/2011 for the course CHEME 333 taught by Professor Anthony during the Spring '11 term at UNL.

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Chapter 14 Workbook - Chapter 14 Diffusion Mass Transfer...

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