Mass transfer introduction

The effect may be reduced by the introduction of surfactants which tend to concentrate at the interface where they exert a stabilising influence, although they may introduce an interface resistance and substantially reduce the mass transfer rate. Thus, for instance, hexadecanol when added to open ponds of water will collect at the interface and substantially reduce the rate of evaporation. 

Carey van P (1992) Liquid-vapor phase-change phenomena. An introduction to the thermophysics of vaporization and condensation processes in heat transfer equipment. Hemisphere, New York Celata GP, Cumo M, Mariani A (1997) Experimental evaluation of the onset of subcooled flow boiling at high liquid velocity and subcoohng. Int J Heat Mass Transfer 40 2979-2885 Celata GP, Cumo M, Mariani A (1993) Burnout in highly subcooled water flow boiling in small diameter tubes. Int J Heat Mass Transfer 36 1269-1285 Chen JC (1966) Correlation for boiling heat transfer to saturated fluids in convective flow. Ind Eng Chem Process Des Develop 5 322-329... 

An overview of this kind is, of necessity, limited in detail. Readers interested in a more thorough development of mass transfer principles are encouraged to consult the references listed at the end of the chapter. In particular, Cussler s excellent textbook on diffusion is an accessible introduction to the subject geared toward the physical scientist [11], Those with a more biological orientation may prefer Friedman s text on biological mass transfer [12], which is also exceptional. A classic reference in the field is Crank s Mathematics of Diffusion [13], which contains solutions to many important diffusion problems. 

Smith, Van Ness, and Abbott Introduction t f Cherhical Engineering Thermodynamics Treybal Mass Transfer Operations ... 

Volumes 1, 2 and 3 form an integrated series with the fundamentals of fluid flow, heat transfer and mass transfer in the first volume, the physical operations of chemical engineering in this, the second volume, and in the third volume, the basis of chemical and biochemical reactor design, some of the physical operations which are now gaining in importance and the underlying theory of both process control and computation. The solutions to the problems listed in Volumes 1 and 2 are now available as Volumes 4 and 5 respectively. Furthermore, an additional volume in the series is in course of preparation and will provide an introduction to chemical engineering design and indicate how the principles enunciated in the earlier volumes can be translated into chemical plant. 

The beneficial effect that the introduction of water has on the extraction of ethylene from low-density polyethylene is shown in Fig. 18. The reduction in the exit concentration of ethylene is substantial, especially at the higher pressures, and dramatically illustrates the increase in mass transfer rates which results from the introduction of an immiscible stripping agent. 

Although HPLC column technology is considered to be a mature field now, improvements and new developments are being made continuously in the stationary phases. One of the improvements has been the reduction in particle sizes. Smaller particles help to improve mass transfer and provide better efficiency. Manufacturers are producing particles down to 1.5 J,m in diameter, although 3- and 5- J,m particles are still the most popular. Because of the smaller particle sizes, the backpressure increases proportionally to the inverse of the square of the particle size. Most commercially available HPLC systems cannot accommodate the pressures required to operate these columns at optimum flow rates. This has led to the introduction of systems that run at high pressures. 

As noted in the Introduction, one of the defining characteristics of any fuel-cell model is how it treats transport. Thus, these equations vary depending on the model and are discussed in the appropriate subsections below. Similarly, the auxiliary equations and equilibrium relationships depend on the modeling approach and equations and are introduced and discussed where appropriate. The reactions for a fuel cell are well-known and were introduced in section 3.2.2. Of course, models modify the reaction expressions by including such effects as mass transfer and porous electrodes, as discussed later. Finally, unlike the other equations, the conservation equations are uniformly valid for all models. These equations are summarized below and not really discussed further. 

A factor closely related to the catalyst loading is the efficiency or utilization of the electrode. This tells how much of the electrode is actually being used for electrochemical reaction and can also be seen as a kind of penetration depth. To examine ohmic and mass-transfer effects, sometimes an effectiveness factor, E, is used. This is defined as the actual rate of reaction divided by the rate of reaction without any transport (ionic or reactant) losses. With this introduction of the parameters and equations, the various modeling approaches can be discussed. 

The topics of heat, mass and momentum transfer, known collectively as transport processes, are fully examined in the books by Welty et al. [21] and Bird et al. [22]. There is a useful introduction to fluid mechanics and heat transfer by Kay and Nedderman [23], while mass transfer is fully discussed by Treybal [24] and Sherwood et al. [25]. Coulson and Richardson [26] also give clear introductions to these subjects. 

MASS TRANSFER ACROSS A PHASE BOUNDARY 6.5.1 Introduction... 

The introduction of large pore supports, which are able to increase particle permeability and hence mass transfer, represented a turning point in protein analysis [113]. Around 1990, anew HPLC technique, using nonporous supporting materials characterized by presenting two set of pores, was introduced by Afeyan et al. [114]. A brief description of this HPLC technique, which is gaining still more popularity in protein analysis, is discussed in the next section. 

A short guard column containing the same stationary phase as the analytical column is placed before the analytical column to protect it from contamination with particles or irreversibly adsorbed solutes. A high-quality pump provides smooth solvent flow. The injection valve allows rapid, precise sample introduction. The column is best housed in an oven to maintain a reproducible temperature. Column efficiency increases at elevated temperature because the rate of mass transfer between phases is increased. Mass spectro-metric detection provides quantitative and qualitative information for each substance eluted from the column. Ultraviolet detection is most common and it can provide qualitative information if a photodiode array is used to record a full spectrum of each analyte. Refractive index detection has universal response but is not very sensitive. Evaporative light scattering responds to the mass of each... 

Incropera. F.P. and D.P. Dewitt Introduction to Heat and Mass Transfer, 3rd Edition, John Witey Sons. tnc New York NY, 2000-Rolle, K.C. Heat ami Muss Transfer. Prentice-Hall, Inc., Upper Saddle River. NJ. 1999. 

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