Interphase reactions

In the latter part of stage 1, the cement particles in the paste become fiilly coated with a layer of hydrate products. This protective layer hinders the diffusion of the reacting species in and out of the reaction interphase, thus sharply reducing the rate of the various reactions. The system enters into a period of latency referred to as the induction or dormant period processes initiated during stage I, however, will continue throughout the induction period. 

It is clear from previous work and from the papers in this symposium that models are much more sensitive to assumptions in some areas than in others. For very slow reactions, rates become controlled by chemical kinetics and insensitive to whatever hydro-dynamic assumptions are adopted (14,48) For intermediate reactions, interphase transfer generally becomes the key factor controlling the reactor performance, with the distribution of gas between phases also playing a significant role. As outlined above, advances have been made in understanding both areas, but models have generally been slow to adopt changes in the basic assumptions used in early bubble models. For fast reactions, the... 

LIQUID-PHASE MASS BALANCES WITH CHEMICAL REACTION, INTERPHASE TRANSPORT,... 

Very fast reaction Interphase mass transfer control... 

We shall now apply the methods developed in the previous chapters to model PTC reactions in liquid-liquid and solid-liquid systems, including solid-supported systems. For a more detailed account of these methods, reference may be made to the articles, among others, of Naik and Doraiswamy (1998), and Yadav and collaborators (1995,2004). The rate of the overall PTC cycle is dependent on the relative rates of the different steps in the PTC cycle. Thus, when the basic conservation equations for mass balance are written for a PTC system, the individual steps that comprise the PTC cycle must be accounted for. These steps are the ion-exchange reaction, interphase mass transfer of both inactive and active forms of the phase transfer (PT) catalyst, partitioning of the catalyst between the two phases (in liquid-liquid systems), and the main organic phase reaction. When these are considered, the normal assumption of pseudo-first-order kinetics (Equation 16.1) is no longer valid. 

Eq.2.31) can be reexpressed by combining with the expression obtained for SO2 concentration at the reaction interphase from the grain model (Eq.2.5). 

F reaction interphase-area per unit pellet volume (Eq.2.19) surface reaction rate constant (m/s)... 

The mechanism of chemical adhesion is probably best studied and demonstrated by the use of silanes as adhesion promoters. However, it must be emphasized that the formation of chemical bonds may not be the sole mechanism leading to adhesion. Details of the chemical bonding theory along with other more complex theories that particularly apply to silanes have been reviewed [48,63]. These are the Deformable Layer Hypothesis where the interfacial region allows stress relaxation to occur, the Restrained Layer Hypothesis in which an interphase of intermediate modulus is required for stress transfer, the Reversible Hydrolytic Bonding mechanism which combines the chemical bonding concept with stress relaxation through reversible hydrolysis and condensation reactions. 

The effect of physical processes on reactor performance is more complex than for two-phase systems because both gas-liquid and liquid-solid interphase transport effects may be coupled with the intrinsic rate. The most common types of three-phase reactors are the slurry and trickle-bed reactors. These have found wide applications in the petroleum industry. A slurry reactor is a multi-phase flow reactor in which the reactant gas is bubbled through a solution containing solid catalyst particles. The reactor may operate continuously as a steady flow system with respect to both gas and liquid phases. Alternatively, a fixed charge of liquid is initially added to the stirred vessel, and the gas is continuously added such that the reactor is batch with respect to the liquid phase. This method is used in some hydrogenation reactions such as hydrogenation of oils in a slurry of nickel catalyst particles. Figure 4-15 shows a slurry-type reactor used for polymerization of ethylene in a sluiTy of solid catalyst particles in a solvent of cyclohexane. 

This involves knowledge of chemistry, by the factors distinguishing the micro-kinetics of chemical reactions and macro-kinetics used to describe the physical transport phenomena. The complexity of the chemical system and insufficient knowledge of the details requires that reactions are lumped, and kinetics expressed with the aid of empirical rate constants. Physical effects in chemical reactors are difficult to eliminate from the chemical rate processes. Non-uniformities in the velocity, and temperature profiles, with interphase, intraparticle heat, and mass transfer tend to distort the kinetic data. These make the analyses and scale-up of a reactor more difficult. Reaction rate data obtained from laboratory studies without a proper account of the physical effects can produce erroneous rate expressions. Here, chemical reactor flow models using matliematical expressions show how physical... 

According to the solid electrolyte interphase (SEI) model presented by Peled [8], the reaction products of the lithium and the... 

The Diels-Alder reaction of methyl methacrylate with cyclopentadiene was studied [72] with solutions from three different regions of the pseudophase diagram for toluene, water and 2-propanol, in the absence and in the presence of surfactant [sodium dodecyl sulfate (SDS) and hexadecyltrimethylammonium bromide (HTAB)]. The composition of the three solutions (Table 6.11) corresponds to a W/O-fiE (A), a solution of small aggregates (B) and a normal ternary solution (C). The diastereoselectivity was practically constant in the absence and in the presence of surfactant a slight increase of endo adduct was observed in the C medium in the presence of surfactant. This suggests that the reaction probably occurs in the interphase and that the transition state has a similar environment in all three media. 

Many semibatch reactions involve more than one phase and are thus classified as heterogeneous. Examples are aerobic fermentations, where oxygen is supplied continuously to a liquid substrate, and chemical vapor deposition reactors, where gaseous reactants are supplied continuously to a solid substrate. Typically, the overall reaction rate wiU be limited by the rate of interphase mass transfer. Such systems are treated using the methods of Chapters 10 and 11. Occasionally, the reaction will be kinetically limited so that the transferred component saturates the reaction phase. The system can then be treated as a batch reaction, with the concentration of the transferred component being dictated by its solubility. The early stages of a batch fermentation will behave in this fashion, but will shift to a mass transfer limitation as the cell mass and thus the oxygen demand increase. 

Chapter 11 treats reactors where mass and component balances are needed for at least two phases and where there is interphase mass transfer. Most examples have two fluid phases, typically gas-liquid. Reaction is usually confined to one phase, although the general formulation allows reaction in any phase. A third phase, when present, is usually solid and usually catalytic. The solid phase may be either mobile or stationary. Some example systems are shown in Table 11.1. 

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