Basic Mass-Transfer Concepts

In this chapter, we study various correlations for gas-liquid mass transfer, interfacial area, bubble size, gas hold-up, agitation power consumption, and volumetric mass-transfer coefficient, which are vital tools for the design and operation of fermenter systems. Criteria for the scale-up and shear sensitive mixing are also presented. First of all, let s review basic mass-transfer concepts important in understanding gas-liquid mass transfer in a fermentation system. 

When the concentration of a component varies from one point to another, the component has a tendency to flow in the direction that will reduce the local differences in concentration. 

Molar flux of a component A relative to the average molal velocity of all constituent A is proportional to the concentration gradient dCA/dz as 

Molar flux relative to stationary coordinate NA is equal to 

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It is assumed that readers have some knowledge of basic mass transfer concepts either from Chapter 1S or from other sources (e.g., Cussler. 1997 Geankoplis. 2003 McCabe et al.. 2005 Taylor and Krishna. 1993). 

We do not intend in this chapter to present an extensive analysis of mass transfer concepts but, rather, to summarize the basics of mass transfer as required in the design and analysis of polymer processing operations. In this regard, we give only an extensive overview of the estimation techniques for the diffusivity, solubility, and permeability of solvents in polymers. The laws of diffusional mass transfer, as well as the relationships for convective mass transfer, remain the same as applied to any material. The books by Perry and Chilton (1973), Reid et al. (1977), and Brandmp and Immergut (1989) provide an extensive overview of experimental data and formulas for the calculation of diffusivity, solubility, and permeability of various polymer systems. 

Explain the basic concepts underlying the two-film iheory for mass transfer across a phase boundary, and obtain an expression for film thickness. 

Basically, a concept for microbial transformations in sewer networks should cover soluble and particulate components and relevant processes in the water phase, in the biofilm and in the sewer sediments. In addition, mass transfer between these phases and an air-water transfer of oxygen should be taken into account (Figures 1.3 and 5.2). Although only the aerobic microbial activity will be focused on in the concept presented in this chapter, anoxic and anaerobic processes should be considered possible extensions (cf. Chapter 6). 

In addition to the three basic FF designs mentioned, various new FF channel concepts (e.g., biomimetic or fractal flow fields, improved mass transfer channels with variable channel cross section, etc.) have been proposed recently [275]. In all cases, the DL requirements and design will depend on the type of FF design. Therefore, it is critical to understand the relationship between any flow field design and the corresponding DL. 

Chapter 1 reviews the concepts necessary for treating the problems associated with the design of industrial reactions. These include the essentials of kinetics, thermodynamics, and basic mass, heat and momentum transfer. Ideal reactor types are treated in Chapter 2 and the most important of these are the batch reactor, the tubular reactor and the continuous stirred tank. Reactor stability is considered. Chapter 3 describes the effect of complex homogeneous kinetics on reactor performance. The special case of gas—solid reactions is discussed in Chapter 4 and Chapter 5 deals with other heterogeneous systems namely those involving gas—liquid, liquid—solid and liquid—liquid interfaces. Finally, Chapter 6 considers how real reactors may differ from the ideal reactors considered in earlier chapters. 

This chapter describes the different types of batch and continuous bioreactors. The basic reactor concepts are described as well as the respective basic bioreactors design equations. The comparison of enzyme reactors is performed taking into account the enzyme kinetics. The modelhng and design of real reactors is discussed based on the several factors which influence their performance the immobilized biocatalyst kinetics, the external and internal mass transfer effects, the axial dispersion effects, and the operational stabihty of the immobilized biocatalyst. 

It is hoped that the next twenty-five years will see the development of chemical engineering techniques for predicting solvent dosages and operating characteristics for extraction processes"which will utilize basic physical chemical properties and the fundamental concepts of mass transfer involving individual film resistances and driving forces. 

The transfer of mass from one phase to another because of a concentration difference (or, in this case, because of vapor pressure) is called diffusion. Diffusion, or mass transfer, although analogous to heat transfer, must be set apart from the basic concepts of heat transfer. 

This chapter gives an introduction to the subject of chemical reaction engineering. The first part introduces basic definitions and concepts of chemical reaction engineering and chemical kinetics and the importance of mass and heat transfer to the overall chemical reaction rate. In the second part, the basic concepts of chemical reactor design are covered, including steady-state models and their use in the development... 

The basic concepts in forming a molecularly imprinted polymer are therefore rather simple. Indeed this apparent simplicity has misled some would-be users of this approach who have failed to appreciate that realising this in practice, particularly with any degree of efficiency, has proved enormously difficult. Not the least, most polymer chemists would appreciate that to produce a crosslinked polymeric network sufficiently rigid to retain some memory of an imprint molecule, and yet allow ready mass transfer of molecules to and from the memory cavities, is no small undertaking. The early workers in the field have made enormous efforts to bring the technique to a point where materials capable of application and exploitation are now becoming available, and this is as much a tribute to their tenacity as it is to their scientific invention. 

Heat and mass transfer is a basic science that deals with the rate of transfer of thermal energy. It has a broad application area ranging from biological systems to common household appliances, residential and commercial buildings, industrial processes, electronic devices, and food processing. Students are assumed to have an adequate background in calculus and physics. The completion of first courses in thermodynamics, fluid mechanics, and differential equations prior to taking heat transfer is desirable. However, relevant concepts from these topics are introduced and reviewed as needed. 

This chapter is divided into three parts. In the first, basic definitions and their consequences for homogeneous chemistry are presented. The second deals with the fundamental aspects of electrode phenomena, whereas the third discusses the problem of mass transfer at electrodes and its consequences for electrochemical kinetics. The particular problems and concepts associated with preparative-scale electrolysis are presented in a special chapter (Chapter 3). 

Oxygen can be supplied to the culture by directly sparging the supply gas into the bioreactor. The basic concepts describing mass transfer in gas-sparged bioreactors are reviewed in Moo-Young and Blanch (1981). Sparger aeration offers... 

Many heat and mass transfer problems can be solved using the balance equations and the heat and mass transfer coefficients, without requiring too deep a knowledge of the theory of heat and mass transfer. Such problems are dealt with in the first chapter, which contains the basic concepts and fundamental laws of heat and mass transfer. The student obtains an overview of the different modes of heat and mass transfer, and learns at an early stage how to solve practical problems and to design heat and mass transfer apparatus. This increases the motivation to study the theory more closely, which is the object of the subsequent chapters. 

The specific problems discussed in this book require the use of fundamental concepts and equations from various fields like kinetic theory of gases, kinetics of chemical reactions, thermodynamics and mass transfer. This chapter presents some basic relationships relevant to these problems. From the very beginning, the studies of gas-phase radiochemistry of heavy metallic elements have been largely motivated by the quest for new man-made chemical elements. It necessitated experimentation with very short-lived nuclides on one-atom-at-a-time basis. We will pay much attention to this direction of research. Accordingly, we will consider microscopic pictures (at the atomic and molecular level) of the processes underlying the experimental methods and concrete techniques, and follow individual histories of the molecules. 

Fairly rigorous formulas for the interfacial heat and mass transfer terms are defined in sect 3.3 for the different averaging methods commonly applied in chemical reactor analysis. However, since the modeling concepts are mathematically similar for the different averages, we choose to examine these constitutive equations in the framework of the volume averaging method described in sect 3.4.1. This modeling framework is used extensively in chemical reactor analysis because the basic model derivation is intuitive and relatively easy to understand. 

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