Particle formation mass transfer

Liquid-solid and liquid-liquid mass transfer is highly dependent upon surface area, or particle size. Mass transfer is involved in simple wetting, dissolution, hydration, swelling of product components, ion exchange, electric double layer formation,... 

Sources of atmospheric aerosol particles are bulk-to-particle conversion, gas-to-particle conversion, and combustion processes. Bulk-to-particle conversion includes the formation of sea salt, dust, and biogenie partieles. Gas-to-particle formation involves either new particle formation from aerosol precursor gases, or growth of preexisting particles by mass transfer processes between the gas and the partiele phase. Particles derived from gas-to-partiele eonversion processes are also called secondary aerosol particles. Other partieles, sueh as from bulk-to-partiele eonversion processes or combustion particles (soot, fly ash), are called primary aerosol particles. 

Each stage of particle formation is controlled variously by the type of reactor, i.e. gas-liquid contacting apparatus. Gas-liquid mass transfer phenomena determine the level of solute supersaturation and its spatial distribution in the liquid phase the counterpart role in liquid-liquid reaction systems may be played by micromixing phenomena. The agglomeration and subsequent ageing processes are likely to be affected by the flow dynamics such as motion of the suspension of solids and the fluid shear stress distribution. Thus, the choice of reactor is of substantial importance for the tailoring of product quality as well as for production efficiency. 

Several reported chemical systems of gas-liquid precipitation are first reviewed from the viewpoints of both experimental study and industrial application. The characteristic feature of gas-liquid mass transfer in terms of its effects on the crystallization process is then discussed theoretically together with a summary of experimental results. The secondary processes of particle agglomeration and disruption are then modelled and discussed in respect of the effect of reactor fluid dynamics. Finally, different types of gas-liquid contacting reactor and their respective design considerations are overviewed for application to controlled precipitate particle formation. 

The mass transfer effect is relevant when the chemical reaction is far faster than the molecular diffusion, i.e. Ha > 1. The rapid formation of precipitate particles should then occur spatially distributed. The relative rate of particle formation to chemical reaction and/or diffusion can as yet be evaluated only via lengthy calculations. 

Rate of Formation of Primary Precursors. A steady state radical balance was used to calculate the concentration of the copolymer oligomer radicals in the aqueous phase. This balance equated the radical generation rate with the sum of the rates of radical termination and of radical entry into the particles and precursors. The calculation of the entry rate coefficients was based on the hypothesis that radical entry is governed by mass transfer through a surface film in parallel with bulk diffusion/electrostatic attraction/repulsion of an oligomer with a latex particle but in series with a limiting rate determining step (Richards, J. R. et al. J. AppI. Polv. Sci.. in press). Initiator efficiency was... 

In a biphasic solid-liquid medium irradiated by power ultrasound, major mechanical effects are the reduction of particles size leading to an increased surface area and the formation of liquid jets at solid surfaces by the asymmetrical inrush of the fluid into the collapsing voids. These liquid jets not only provide surface cleaning but also induce pitting and surface activation effects and increase the rate of phase mixing, mass transfer and catalyst activation. 

The adsorbent typically consists of the selectively adsorbing zeoHte and a binder. The binding material is required to make parHcles of the zeoHtes of a large enough size to allow for practical use in a commercial appHcation. The particle formation method and resulting characteristics of the binder/zeoHte combinaHon is of criHcal importance, especially as it relates to mass transfer through the particle and the characteristics of the zeoHte [30]. The practical aspects of forming and... 

The issue of bioavailability is further clouded by the physical characteristics of soil and the role of a possible mass transfer limitation. Soil constituents are not simply flat surfaces with free and equal access to all bacterial species. The formation of aggregates from sand-, silt-, and clay-sized particles results in stable structures which control microbial contact with the substrate (Figure 2.7). Discussion of sorption mechanisms and binding affinities must include the possible impact of intra-aggregate transport of the substrate. If the substrate is physically inaccessible to the microorganism then both desorption from soil constituents and diffusion to an accessible site are necessary. The impact of intra-aggregate diffusion on degradation kinetics has been modeled for y-hexachlorocyclohexane (Rijnaarts et al., 1990) and naphthalene (Mihelcic Luthy, 1991). 

Self-supported MIP membranes can be seen as an alternative format to the traditional MIP particles for applications in separation and sensor technology, avoiding the limitations of mass transfer across conventional MIP materials. Two main approaches have been used for the preparation of membranes composed of an MIP in situ polymerisation and polymer solution phase inversion. 

Continued advances in source/interface designs for the coupling of LC and MS/MS have increased ion formation and transfer into the mass spectrometer. Improved optics (focusing lenses, etc.) have increased ion transmission through the MS and to the detector. These improvements have resulted in a significant increase in sensitivity, and as a result, quantification at the parts per trillion concentration level is routine today and was not possible a decade ago. Advances in source/interface design to increase the abundance of ion formation, transfer, and transmission through the MS may lead to even more sensitive instruments. In addition, the use of smaller particle sizes in LC (e.g., UPLC) also increases resolution, selectivity, and sensitivity. 

The effectiveness of the gas-solid mass transfer in a circulating fluidized bed (see Chapter 10) can be reflected by the contact efficiency, which is a measure of the extent to which the particles are exposed to the gas stream. As noted in Chapter 10, fine particles tend to form clusters, which yield contact resistance of the main gas stream with inner particles in the cluster. The contact efficiency was evaluated by using hot gas as a tracer [Dry et al., 1987] and using the ozone decomposition reaction with iron oxide catalyst as particles [Jiang etal., 1991], It was found that the contact efficiency decreases as the particle concentration in the bed increases. At lower gas velocities, the contact efficiency is lower as a result of lower turbulence levels, allowing a greater extent of aggregate formation. The contact efficiency increases with the gas velocity, but the rate of increase falls with the gas velocity. 

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