Chemical reaction rates liquid phase

According to the film theory, in reactive-absorption processes the resistance to mass transfer is concentrated in a small region near the gas/liquid interface. The ratio between tbe rate of chemical reaction and liquid-phase mass transfer is given by the Hatta number. For a second-order reaction (12.1), the Hatta number is defined as ... 

Increase in interfacial area. The total surface area for diffusion is increased because the bubble diameter is smaller than for the free-bubbling case at the same gas flow rate hence there is a resultant increase in the overall absorption rate. The overall absorption rate will also increase when the diffusion is accompanied by simultaneous chemical reaction in the liquid phase, but the increase in surface area only has an appreciable effect when the chemical reaction rate is high the absorption rate for this case is then controlled by physical diffusion rather than by the chemical reaction rate (G6). 

The liquid-liquid interface is not only a boundary plane dividing two immiscible liquid phases, but also a nanoscaled, very thin liquid layer where properties such as cohesive energy, density, electrical potential, dielectric constant, and viscosity are drastically changed along with the axis from one phase to another. The interfacial region was anticipated to cause various specific chemical phenomena not found in bulk liquid phases. The chemical reactions at liquid-liquid interfaces have traditionally been less understood than those at liquid-solid or gas-liquid interfaces, much less than the bulk phases. These circumstances were mainly due to the lack of experimental methods which could measure the amount of adsorbed chemical species and the rate of chemical reaction at the interface [1,2]. Several experimental methods have recently been invented in the field of solvent extraction [3], which have made a significant breakthrough in the study of interfacial reactions. 

Both the mass transfer kinetic parameters (diffusion in the phases, D, D j, surface renewal frequency, s) and chemical reaction rate constants (kg, kj) strongly influence enhancement of the absorption rate. The particle size, dp, the dispersed liquid holdup, e and the partition coefficient, H can also strongly alter the absorption rate [42-44,46,48]. Similarly, the distance of the first particle from the gas-liquid interface, 6q is an essential factor. Because the diffusion conditions are much better in the dispersed phase (larger solubility and, in most cases, larger diffusivity, as well) the absorption rate should increase with the decrease of the (5g value. 

In this section, the various methods which have been developed to treat chemical reaction rates between solutes in solution are discussed, with specific concern for those reactions where the rate of reaction of encounter pairs is of comparable magnitude to the rate of diffusive formation of encounter pairs. Some of the detailed comments on the partially reflecting boundary condition are discussed, the effects of angular variation of the reaction rate and the possibility of using a sink term to represent chemical reaction rather than a boundary condition are presented. Such comments are contrasted with the relatively few instances where experimental data has been obtained for the rate of the concomitant chemical reaction. Recently, attention has been given to a development of aspects of gas-phase reaction rate theory to be applied to reactions in liquids. 

S.S. Cherry et al, Identification of Important Chemical Reactions in Liquid Propellant Rocket Engines , Pyrodynamics 6 (3—4), 275—96 (1969) CA 70,. 98394 (1969) [The authors state that the kinetics of nonequilibrium expansion of the propint system N204/A-50 (UDMH 49 plus hydrazine 51 wt%) can be described by the following gas phase reactions with an accuracy such that not more than 0.5 lb force-sec/lb mass variation in specific impulse (at a nozzle expansion rate of 40) is produced, as compared to the results of a full kinetic analysis ... 

The design of packed column reactors is very similar to the design of packed columns without reaction (Volume 2, Chapter 12). Usually plug flow is assumed for both gas and liquid phases. Because packed columns are used for fast chemical reactions, often the gas-side mass transfer resistance is significant and needs to be taken into account. The calculation starts on the liquid side of the gas-liquid interface where the chemical reaction rate constant is compounded with the liquid side mass transfer coefficient to give a reaction-enhanced liquid-film mass transfer... 

Ray Kapral came to Toronto from the United States in 1969. His research interests center on theories of rate processes both in systems close to equilibrium, where the goal is the development of a microscopic theory of condensed phase reaction rates,89 and in systems far from chemical equilibrium, where descriptions of the complex spatial and temporal reactive dynamics that these systems exhibit have been developed.90 He and his collaborators have carried out research on the dynamics of phase transitions and critical phenomena, the dynamics of colloidal suspensions, the kinetic theory of chemical reactions in liquids, nonequilibrium statistical mechanics of liquids and mode coupling theory, mechanisms for the onset of chaos in nonlinear dynamical systems, the stochastic theory of chemical rate processes, studies of pattern formation in chemically reacting systems, and the development of molecular dynamics simulation methods for activated chemical rate processes. His recent research activities center on the theory of quantum and classical rate processes in the condensed phase91 and in clusters, and studies of chemical waves and patterns in reacting systems at both the macroscopic and mesoscopic levels. 

The liquid-liquid interface formed between two immissible liquids is an extremely thin mixed-liquid state with about one nanometer thickness, in which the properties such as cohesive energy density, electrical potential, dielectric constant, and viscosity are drastically changing from those of bulk phases. Solute molecules adsorbed at the interface can behave like a 2D gas, liquid, or solid depending on the interfacial pressure, or interfacial concentration. But microscopically, the interfacial molecules exhibit local inhomogeneity. Therefore, various specific chemical phenomena, which are rarely observed in bulk liquid phases, can be observed at liquid-liquid interfaces [1-3]. However, the nature of the liquid-liquid interface and its chemical function are still less understood. These situations are mainly due to the lack of experimental methods required for the determination of the chemical species adsorbed at the interface and for the measurement of chemical reaction rates at the interface [4,5]. Recently, some new methods were invented in our laboratory [6], which brought a breakthrough in the study of interfacial reactions. 

In the case that the reactants are already in the phase surrounding the catalyst pellet, the components only have to be transported through this single phase, being either liquid or gas. Reactant A is assumed to be converted according to a first-order reaction. For convenience a chemical reaction rate R is defined per unit of external surface ... 

Mass Transfer Effects on Liquid-Phase Chemical Reaction Rates... 

Once this interpretation has been established, MODEL.LA. (a) generates all the requisite modeling elements and (b) constructs the modeling relationships, such as material balances, energy balance, heat transfer between jacket and reactive mixture, mass transport between the two liquid phases, equilibrium relationships between the two phases, estimation of chemical reaction rate, estimation of chemical equilibrium conditions, estimation of heat generated (or consumed) by the reaction, and estimation of enthalpies of material convective flows. In order to automate the above tasks, MODEL.LA. must possess the following capabilities ... 

Knowledge of interfacial areas, drop size distributions, and dispersed phase coalescence rates is essential for accurate description and prediction of mass transfer and chemical reaction rates in liquid-liquid dispersions. In this section, a review of the experimental methods and techniques developed for describing and measuring interfacial area, drop size distributions, and coalescence rates will be given in addition, summaries of important results and correlations are presented. 

Turbulence models are generally limited to fully developed high-Reynolds number flows. Gas-phase flows are normally characterized by 5c 1, while for liquid phase flows, 5c > 1. The value of this Damkohler number indicates the relative rates of the mixing and chemical reaction rate time scales. Reactive flows might thus be divided into three categories Slow chemistry (Da/ -C 1), fast chemistry (Da/ 1), and finite rate chemistry (Da/ 1). 

Transition state theory (TST) [1—4] is a widely used method for calculating rate constants for chemical reactions. TST has a long history, which dates back 70 years, including both theoretical development and applications to a variety of reactions in the gas phase, in liquids, at interfaces, and in biological systems. Its popularity and wide use can be attributed to the fact that it provides a theoretical framework for understanding fundamental factors controlling chemical reaction rates and an efficient computational tool for accurate predictions of rate constants. 

In homogeneously catalyzed gas/liquid-phase reactions the overall reaction rate is determined by the actual chemical reaction rate and by mass transfer processes [lb]. Depending on the magnitude of the rates of the catalytic reaction and of the transfer rate of the gaseous reactants, severe concentration gradients may exist near the gas-liquid interface. These phenomena are shown in Figure 1 for the reaction... 

The above discussion indicates the importance of the size of the catalyst particles-a very small size is required if a technically useful rate of production is to be achieved. The range of sizes of the catalyst particles that can be employed is, however, dominated by the reactor in which the catalyst must operate. Most of the reactions performed in the fine-chemical industry involve liquid-phase processes, normally reactions between two different dissolved compounds. Often one of the reactants is a gaseous compound, which dissolves in the liquid and migrates to the surface... 

Analogously to batch distillation and the RCM, the simplest means of reactive distillation occurs in a still where reaction and phase separation simultaneously take place in the same unit. Additionally, we can choose to add a mixing stream to this still, and the overall process thus consists of three different phenomena chemical reaction, vapor liquid equilibrium, and mixing. Such a system is referred to as a simple reactive distillation setup. This setup is shown in Figure 8.1 where a stream of flowrate F and composition Xp enters a continuously stirred tank reactor (CSTR) in which one or more chemical reaction(s) take place in the liquid phase with a certain reaction rate r =f(kf, x, v) where v represents the stoichiometric coefficients of the reaction. Reactants generally have negative stoichiometric coefficients, while products have positive coefficients. For example, the reaction 2A + B 3C can... 

Sundmacher and Qi (Chapter 5) discuss the role of chemical reaction kinetics on steady-state process behavior. First, they illustrate the importance of reaction kinetics for RD design considering ideal binary reactive mixtures. Then the feasible products of kinetically controlled catalytic distillation processes are analyzed based on residue curve maps. Ideal ternary as well as non-ideal systems are investigated including recent results on reaction systems that exhibit liquid-phase splitting. Recent results on the role of interfadal mass-transfer resistances on the attainable top and bottom products of RD processes are discussed. The third section of this contribution is dedicated to the determination and analysis of chemical reaction rates obtained with heterogeneous catalysts used in RD processes. The use of activity-based rate expressions is recommended for adequate and consistent description of reaction microkinetics. Since particles on the millimeter scale are used as catalysts, internal mass-transport resistances can play an important role in catalytic distillation processes. This is illustrated using the syntheses of the fuel ethers MTBE, TAME, and ETBE as important industrial examples. 

The chemical reaction rate for a reaction of arbitrary order with respect to both the gas- and liquid-phase components is given by... 

In this Sect.4.9 we discuss Eqs. (4.156), (4.171) concerning chemical reactions in a regular linear fluids mixture (see end of Sect. 4.6), i.e. with linear transport phenomena. This model gives the (non-linear) dependence of chemical reaction rates on temperature and densities (i.e. on molar concentrations (4.288)) only (4.156), which is (at least approximately) assumed in classical chemical kinetics [132, 157]. Here, assuming additionally polynomial dependence of rates on concentrations, we deduce the basic law of chemical kinetics (homogeneous, i.e. in one fluid (gas, liquid) phase) called also the mass action law of chemical kinetics, by purely phenomenological means [56, 66, 79, 162, 163]. 

Similar to the liquid-liquid system, the volume-surface diameter of dispersed phase particles in a liquid-gas flow is determined by the initial size of the bubbles at the gas input points [81-83]. An increase of the liquid-gas flow rate leads to an increase of the shear deformation influence of the dispersed phase particles and therefore, to a decrease in the diameter of the gas bubbles in the input area of the device. Finally, it leads to the formation of a finely dispersed system in a device with an increase of the liquid-gas flow rate (Figure 2.23). Fast chemical reactions in two-phase gas-liquid systems usually occur in a gas-phase excess, so it is reasonable to analyse the influence of the gas content in a flow on the change of phase contact surface. 

A detailed and very fundamental analysis of mass-transfer with chemical reaction in liquid-liquid dispersions has been published recently by Tavlarides and Stamatoudis (10). These authors point out that the design and analysis problem depends on the phase in which the reaction occurs, whether multiple reactions are involved, the relative magnitudes of the rates of mass transfer and reaction, and upon macromixing processes of the dispersed and continuous phases. Hence, they first examine dispersion phenomena such as coalescence and breakage of droplets, and drop size distribution. The topics discussed by the authors (10)... 

Reversible Reaction. This type of absorption is characterized by the occurrence of a chemical reaction between the gaseous component being absorbed and a component in the liquid phase to form a compound that exerts a significant vapor pressure of the absorbed component. An example is the absorption of carbon dioxide into a monoethanolamine solution. This type of system is quite difficult to analyze because the vapor-liquid equilibrium curve is not linear and the rate of absorption may be affected by chemical reaction rates. 

Hence, for deep desulfurization, a two-phase reactor (oil externally pre-saturated with H2 and solid catalyst) could be an alternative to the trickle bed. The H2-recycle is then redundant, and scale-up problems do not occur. In addition, the two-phase technology utilizes the maximum intrinsic chemical reaction rate as pore diffusion does not play a role in the slow desulfurization of refractory compounds left in predesulfurized feedstocks. For a trickle bed this rate is an upper limit, which caimot, or only hardly, be reached with regard to improper gas-liquid distribution and/or wetting of the catalyst. A laboratory-scale tricHe-bed and two-phase reactor with pre-saturation are compared in Figure 6.8.10 for a model oil. 

We have been introduced to various aspects of solvent extraction in the following sections Section 3.3.7.2, liquid-liquid equilibria in aqueous-organic, organic-organic, aqueous-aqueous systems Section 3.4.1.2, flux expressions in liquid-liquid systems Section 3.4.3.2, solute transport in phase barrier membranes Section 4.1.3, separation achieved in a closed vessel Section 5.2.2, role of chemical reaction in liquid extraction Section 5.3.2, rate controlled aspects of chemical reaction in liquid-liquid systems Section G.3.2.2, bulk flow parallel to force Section 6.4.1.2, mixer-settler, CSTS system. 

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