Mass transfer general aspects

The mini-column should be as small as possible to minimise dispersion and increase sample throughout, yet large enough to ensure efficient mass transfer. The aspect ratio (length intemal diameter) is therefore an important parameter to optimise. In general, a mini-column is assembled... 

In slow reactions in liquid/liquid dispersions, the reaction phase has to be the continuous one. In rapid reactions, that are dependent on mass transfer, several aspects may affect the choice. The rate of mass transfer can be very different depending on which phase is dispersed. Since the mass transfer coefficient in the dispersed phase is usually smaller, the phase with the highest solubility of the transferred reactant might be taken as the continuous phase. But diere is another aspect mass transfer from a dispersed liquid phase to the continuous one is generally more rapid than vice versa, because of surface tension effects (see section 4.6.4). 

In the case of ionic liquids, these general aspects for all fluid-fluid reactions are of particular importance, since mass transfer into an ionic liquid layer is generally slower than into an organic or aqueous medium. This is because ionic liquids usually have much higher viscosities than organic solvents. The least viscous ionic liquids are somewhat similar to ethylene glycol as demonstrated in Table 7.2. However, many ionic liquids used in liquid-liquid biphasic catalysis are significantly more viscous. 

The most common and appropriate methods used to determine the mass transfer coefficient and the problems inherent in each are presented in the following sections. The methods are discussed from a practical viewpoint for the direct determination of the ozone mass transfer coefficient. However, it may be impractical, even impossible to use ozone as the transferred species, because of fast reactions which cause mass transfer enhancement etc. Then the oxygen mass transfer coefficient can be used to indirectly determine the ozone mass transfer coefficient. The procedure is described below and special aspects of oxygen mass transfer experiments are referred to in the following sections whenever necessary or of general importance. 

A brief chronological review of the more important theoretical and experimental investigations pertinent to the flow of liquids in thin films is given in the Appendix. In general, only those heat and mass transfer investigations which throw light on some aspect of the film flow problem have been included. 

An SLM extraction can be seen as a combination of extraction into an organic solvent followed by a back-extraction into a second aqueous phase. However, as these two extraction steps occur simultaneously, the mass transfer kinetics will be different, and generally more efficient, compared to the situation when the steps are performed in sequence in separation funnels. The general mass transfer theory for SLM extraction in flow systems has been described in detail [45], with some additional aspects described more recently [46]. 

Crittenden observes that there are generally a minimum temperature below which fouling will not be observed and a critical upper temperature above which the deposition rate falls away because of the changes in the underlying reaction. Two aspects of fluid velocity have to be appreciated in connection with the effects of velocity on chemical reaction fouling heat and mass transfer, both of which are dependent... 

This chapter covers all important aspects of distillation column rating or desiga. It follows generally the requence of the steps noted immediately above. The common case for discussion is conventional distillation operated in the continuous mode, Exceptions to this mode, such as extractive distillation or betch distillation, will be handled separately. Much of the material in Sections 5.7-5.10, dealing with hydraulics and mass transfer in columns, can also be used in the design and analysis of aheorption columns. Perusal of Chapter 6 will show the reader bow the connection may be made. 

In general, polymer dissolution differs from dissolution of a non-polymeric material in two aspects. Polymers require an induction time before starting to dissolve, while non-polymeric materials dissolve instantaneously. Also, polymer dissolution can be controlled either by the disentanglement of the polymer chains or by the diffusion of the chains through a boundary layer adjacent to the solvent-polymer interface. However, the dissolution of non-polymeric materials is generally controlled by the external mass transfer resistance through a liquid layer adjacent to the solid-liquid interface. 

In cases where hydrodynamic dispersion and the corresponding broadening of residence-time distributions deteriorate the performance of a process, the question arises as to which channel design minimizes dispersion. Already from the analysis of Taylor and Aris it becomes clear that an enhanced mass transfer perpendicular to the main flow direction reduces the broadening of concentration tracers. Such a mass-transfer enhancement can be achieved by the secondary fiow occurring in a curved channel. This aspect was investigated by Daskopoulos and Lenhoff [78] for ducts of circular cross section. They assumed the diameter of the duct to be small compared to the radius of curvature and solved the convection-diffusion equation for the concentration field numerically. More specifically, a two-dimensional problem defined on the cross-sectional plane of the duct was solved based on a combination of a Fourier series expansion and an expansion in Chebyshev polynomials. The solution is of the general form... 

For many processes performed in supercritical fluids, the transport properties of the medium will play an important role. For polymerizations, this includes mass transfer for mixing reactants and to allow proper contact between monomer and catalyst. Polymerization reactions are usually highly exothermic, so that the heat of reaction needs to be absorbed and transported through the supercritical fluid. Virtually aU studies described in the literature have been performed on a relatively small scale, and scale-up aspects, for which mass and heat transfer are major issues, have generally been disregarded. This chapter will describe an experimental study of some aspects of mass and heat transfer in supercritical CO2 (SCCO2), and a comparison will be made with the behavior of standard liquid systems. 

General aspects of fluid-fluid reactions are discussed in detail in Section 2.4 within the context of homogeneous catalytic reactions in biphasic systems. Mostly, the reaction takes place only in one phase and the reactant must be transferred from the nom-eactant phase, for example, the gas phase to the reaction phase. In consequence, the mass transfer between the different phases plays an important role on the overall kinetics and may strongly influence the... 

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