Single interfaces between phases

Our interest is in solution kinetics, so we will concern ourselves only with homogeneous reactions, which take place in a single phase. Heterogeneous reactions take place, at least in part, at interfaces between phases.) Further, we will mainly work with closed systems, those in which matter is neither gained nor lost during the period of observation. 

The CFD models considered up to this point are, as far as the momentum equation is concerned, designed for single-phase flows. In practice, many of the chemical reactors used in industry are truly multiphase, and must be described in the context of CFD by multiple momentum equations. There are, in fact, several levels of description that might be attempted. At the most detailed level, direct numerical simulation of the transport equations for all phases with fully resolved interfaces between phases is possible for only the simplest systems. For... 

Much interesting chemistry, and biology, occurs not in a single phase (gas, liquid or solid) but at a surface or an interface between phases [1, Chap. 23 2, Chap. 10]. The term surface is generally used when one of the phases is gaseous, the other liquid or solid whereas interface is generally used for the boundary between two... 

Multiphase reactors are much more prevalent in industrial practice than single-phase reactors. The behavior of multiphase systems can be very complex. It is not always straightforward to determine whether the reaction takes place in one phase, more than one phase, or at the interface between phases. However, there is one very important case where the locus of reaction is well understood. 

One should make a distinction between homogeneous catalysis, in which the entire reaction occurs in a single phase, and heterogeneous catalysis, in which the reaction takes place at the interfaces between phases. In heterogeneous catalysis, the reactants and the catalyst are present in different phases. Reactant molecules are adsorbed on the surface of the catalyst, and the reaction takes place on that surface. In the so-called chemical adsorption (or chemisorption), the molecules are held to the surface by bonds that are of approximately equal strength as those in chemical compounds. The adsorbed molecules undergo changes so that some bonds may be stretched and thus weakened, or even broken. 

The interfadal energy between the filler and polymer also affeas the compatibility or ease of dispersing the filler within the host matrix. This is a property often exploited for the localization of particles at the interface between phases or in one of the phases in polymer hlend composites. However, for composites made with a single polymer matrix, incompatibility between the filler and matrix makes it difficult to achieve uniform dispersions. Even so, numerous strategies for improving filler dispersion can be adopted, as will be disarssed in Section 7.17.7.1. 

It was made clear in Chapter II that the surface tension is a definite and accurately measurable property of the interface between two liquid phases. Moreover, its value is very rapidly established in pure substances of ordinary viscosity dynamic methods indicate that a normal surface tension is established within a millisecond and probably sooner [1], In this chapter it is thus appropriate to discuss the thermodynamic basis for surface tension and to develop equations for the surface tension of single- and multiple-component systems. We begin with thermodynamics and structure of single-component interfaces and expand our discussion to solutions in Sections III-4 and III-5. 

Many reactions take place in heterogeneous systems rather than in a single homogeneous phase. These reactions often occur at the interface between the two phases. In such cases it is appropriate to define the reaction rate in terms of the interfacial area (S ) available for reaction. 

In non-electrochemical heterogeneous catalysis, the interface between the catalyst and the gas phase can often be characterized using a wide variety of spectroscopic probes. Differences between reaction conditions and the UHV conditions used in many studies have been probed extensively 8 as have differences between polycrystalline and single-crystalline materials. Nevertheless, the adsorbate-substrate interactions can often be characterized in the absence of pressure effects. Therefore, UHY based surface science techniques are able to elucidate the surface structures and energetics of the heterogeneous catalysis of gas phase molecules. 

The value of S0 decreases with increasing elevation. Zao, the interface between air and the LNAPL phase, may or may not coincide with Zu, the upper boundary of the aquifer. Typically, the saturation of the LNAPL phase extends over two distinct regions (see Figure 5.10). These are (1) water and LNAPL phase zone, and (2) water, LNAPL phase, and air zone. When a single homogeneous stratum is considered, O can be assumed constant. In a stratified medium, however, saturation discontinuities generally exist due to the variation in soil characteristics, and the determination of LNAPL volume based on Equation 6.22 may become much more involved. 

Unfortunately it is always impossible to change the potential only across a single interface - potentials are always changed between two points in two bulk phases. This means that the observed electrical... 

In the preceding sketches the reactors are coupled through the interface between them. In a single-phase system the only ways that a species could change were by flow in entrance and exit pipes and by reaction. Now we also have the possibihty of a species entering or leaving the reactor by transferring between phases within the reactor. 

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