Complex mass transfer phenomena

Regarding the chemical processes, sediments are heterogeneous at various sample, aggregate, and particle scales. Adherent or entrapped nonaqueous-phase liquids and combustion residue particulate carbon (e.g., chars, soot, and ashes) can also function as sorbents. Complex assemblages of these constituents can cause complex mass transfer phenomena, and the term sequestration refers to some combination of diffusion limitation, adsorption, and partitioning (Luthy et al. 1997). Some geosorbents exhibit typical nonlinear sorption behavior (Farrell and Reinhard 1994 Huang and Weber 1998). 

Besides these essential questions various criteria such as location of the reactive zone and catalyst mass must be taken into account. In the case of reactive distillation, the column height is influenced not only by the separation efficiency but also by the required residence time. Further difficulties in scale-up calculations arise from complex mass-transfer phenomena and hydrodynamic effects. 

In the film theory description of the mass-transfer process occurring between two fluid phases or between a solid and a fluid phase, the complex mass-transfer phenomenon is substituted by the notion of simple molecular diffusion of the species through a stagnant fluid fUm of thickness <5. The actual concentration profiles of species A being transferred from phase 2 to phase 1 are shown in Figures 3.1.6 (a) and (b) in one phase only for a solid-liquid and a gas-iiquid system, respectively. The concentration of A in the liquid phase at the solid-liquid or the gas-Uquid interface is C. Far away from the interface it is reduced to a low value in the liquid phase. In turbulent flow, the curved profile of species A shown would correspond to the time-averaged value (Bird et al, 1960, 2002). According to the... 

Not only catalytic activity but also selectivity can be influenced by mass transfer phenomenon. Analytical treatment is available for simple reactions and as the complexity increases, numerical treatment becomes necessary. A few very basic reaction types will be analyzed following the interpretation of Ioffe et al. (I.I. Ioffe, V.A. Reshetov, A.M. Dobrotvorskii, Heterogeneous catalysis, Chimia, 1985). 

Not only catalytic activity, but also selectivity can be influenced by mass transfer phenomenon. Analytical treatment is available for simple reactions and as the complexity... 

From Eq. 3.9, the greater the thickness (5) of each phase the larger the resistance to solute transfer. Although the use of 5 is convenient for modeling and conceptualizing WBL resistance, it is largely fictitious, as complex hydrodynamics control resistance to mass transfer across the WBL in environmental exposures (see Section 3.6.5. for a more in- depth discussion on this phenomenon). 

Mass transfer One more difficulty arises from the fact that there are two phases in the reactor (i) hydrocarbon and (ii) acid. The reaction occurs in the acid phase while reactants are feed in hydrocarbon phase. This implies that, in order to reaction occurs, there is mass transfer from hydrocarbon to acid phase. The mass transfer is a very complex phenomenon which can involve the reaction-diffusion equation. However, such a phenomenon is beyond of the goal of this chapter. Both isobutane and propilene are feed in hydrocarbon phase. Solubility of propylene in acid phase is very... 

Droplet vaporization is a phenomenon occurring in a gas-liquid system, although only recently have serious efforts been made towards understanding the various hquid-phase processes and their influence on the overall behavior. The problem is a complex, yet interesting and important one. Fundamental research in the interdisciplinary areas of fluid mechanics, chemical kinetics, phase equilibrium analysis, and heat and mass transfer are required to achieve a good understanding of the problem. The following discussions may substantiate this point and stimulate future research efforts. 

Chromatography is a complex phenomenon, which results from the superimposition of a number of different effects. A mobile phase percolates through a bed of porous particles. It carries the components of a mixture that interact to different degrees with the stationary phase. These components diffuse in and out of the particles, undergo molecular interactions with the stationary phase or form transient chemical bonds with it, and are eventually swept out of the colimm. Fluid dynamics, mass transfer phenomena, and equilibriiun thermodynamics play an... 

All cases of practical importance in liquid chromatography deal with the separation of multicomponent feed mixtures. As shown in Chapter 2, the combination of the mass balance equations for the components of the feed, their isotherm equations, and a chromatography model that accounts for the kinetics of mass transfer between the two phases of the system permits the calculation of the individual band profiles of these compounds. To address this problem, we need first to understand, measure, and model the equilibrium isotherms of multicomponent mixtures. These equilibria are more complex than single-component ones, due to the competition between the different components for interaction with the stationary phase, a phenomenon that is imderstood but not yet predictable. We observe that the adsorption isotherms of the different compounds that are simultaneously present in a solution are almost always neither linear nor independent. In a finite-concentration solution, the amount of a component adsorbed at equilib-... 

PTC incorporated with other methods usually greatly enhances the reaction rate. Mass transfer of the catalyst or the complex between different phases is an important effect that influences the reaction rate. If the mass transfer resistance cannot be neglected, an improvement in the mass transfer rate will benefit the overall reaction rate. The application of ultrasound to these types of reactions can be very effective. Entezari and Keshavarzi [12] presented the utilization of ultrasound to cause efficient mixing of the liquid-liquid phases for the saponification of castor oil. They used cetyltrimethylammo-nium bromide (CTAB), benzyltriethylammonium chloride (BTEAC), and tetrabutylammonium bromide (TBAB) as the catalysts in aqueous alkaline solution. The more suitable PT catalyst CTAB can accumulate more at the liquid-liquid interface and produces an emulsion with smaller droplet size this phenomenon makes the system have a high interfacial surface area, but the degradation of CTAB is more severe than that of BTEAC or TBAB because of more accumulation at the interface of the cavity under ultrasound. 

Knowledge of the equilibrium is a fundamental prerequisite for the design of non-reactive as well as reactive distillation processes. However, the equilibrium in reactive distillation systems is more complex since the chemical equilibrium is superimposed on the vapor-liquid equilibrium. Surprisingly, the combination of reaction and distillation might lead to the formation of reactive azeotropes. This phenomenon has been described theoretically [2] and experimentally [3] and adds new considerations to feasibility analysis in RD [4]. Such reactive azeotropes cause the same difficulties and limitations in reactive distillation as azeotropes do in conventional distillation. On the basis of thermodynamic methods it is well known that feasibility should be assessed at the limit of established physical and chemical equilibrium. Unfortunately, we mostly deal with systems in the kinetic regime caused by finite reaction rates, mass transfer limitations and/or slow side-reactions. This might lead to different column structures depending on the severity of the kinetic limitations [5], However, feasibility studies should identify new column sequences, for example fully reactive columns, non-reactive columns, and/or hybrid columns, that deserve more detailed evaluation. 

Fouling is an extremely complex phenomenon. From a fundamental point of view, it may be characterized as a combined momentum, heat, and mass transfer problem. In many instances, chemical kinetics are involved as well as solubility characteristics and corrosion technology. 

As mentioned above, heat and mass transfer in textile materials is a complex phenomenon that includes several mechanisms. Textile material properties significantly influence these mechanisms. Several researchers studied the effect of these properties on heat and mass transfers at three different levels (1) the microscopic level (chemical composition, morphological characteristics, fineness, cross-section, porosity, and water content of the component fibers), (2) the mesoscopic level (yam structure and properties), and (3) the macroscopic level (the fabric s physical and stmctural characteristics and finishing treatments) [3,22,23]. Thus, in the following section, heat transfer properties, such as thermal conductivity, thermal resistance, thermal absorptivity, and thermal emissivity and mass transfer properties, such as water vapor transmission and liquid water transmission, will be defined at fiber, yam, and fabric levels. 

Mass transfer of chemical or electrochemical specie j, known as ions can be a complex phenomenon because the solution (fluid) containing the ions may be strongly influenced by turbulent flow, and to a lesser extent to laminar flow, diffusion and an electrical field. For a stationary chemical or electrochemical system, such as a tank, pipe or battery, as observed by a stationary observer, under internal laminar flow the mass transfer is quantified by the molar or mass flux. For a one-dimensional treatment in the x-diiection, the mass transfer... 

Recently, terms flow-assisted corrosion and flow-accelerated corrosion (FAC) have been used to describe the erosion (or thinning) of carbon steel in nuclear and fossil power plants where there is no threshold solution velocity. FAC is a complex phenomenon that is a function of many parameters of water chemistry, material composition and hydrodynamics. FAC involves the electrochemical aspects of general corrosion plus the effects of mass transfer and momentum transfer. 

This chapter discusses the very complex phenomenon of combined heat and mass transfer. In some cases, these two transfer operations may take place in opposite directions across the interface between phases. At dmes, the mass transfer may involve more than one component. Methods for design of such columns will be developed despite the absence of rigorous theory for many of these engineering applications. This chapter deals with these complicated situations through empirical adjustments to heat transfer coefficients, based upon extensive practical experience. 

As mentioned in Chapter 2, when textile materials are immersed in the dye solution, the rate at which the dye is taken up is dependent upon the extent to which the liquor is agitated, and tends to approach a maximum value when the stirring is vigorous. This phenomenon, unfortunately, cannot be described in any simple fashion within fibrous assembhes, because of the extreme complexity of defining the flow of liquor through the textile materials. The dyeing of fibres, yams or cloth can be treated as mass transfer in a porous medium, defined by convective dispersion eqiratioa ... 

---