Kinetic homogenous liquid phase

Homogeneous liquid phase inorganic oscillatory reactions. D. O. Cooke, Prog. React. Kinet., 1978, 8,185-229(184). 

A brief reading of the literature would indicate that the liquid-phase reaction is what is technically known as an absorption with fast reaction and that gaseous CO2 physically dissolves in the liquid phase and reacts in a region close to the gas—liquid interface with dissolved NH3, according to second order in ammonia, first order in CO2 homogeneous liquid-phase kinetics... 

Evidence in support of a carbonium ion type of mechanism for low temperature polymerization was also obtained in an investigation of the kinetics of the homogeneous liquid phase polymerization of propene in the presence of aluminum bromide and hydrogen bromide at about —78° (Fontana and Kidder, 89). The rate of reaction is approximately proportional to the concentration of the promoter, no polymerization occurring in its absence. During the main portion of the reaction, the rate is independent of the monomer concentration toward the end, it decreases, due apparently to the low-concentration of the monomer, addition of more olefin resulting in an increase in the rate. It was concluded that the reaction involves an active complex, which may be regarded as a carbonium ion coupled with an anion ... 

As an alternative to investigating the kinetics of a gas-liquid reaction on a laboratory scale, the mass transfer resistance may be minimised or eliminated so that the measured rate corresponds to the rate of the homogeneous liquid-phase reaction. This method of approach will be considered after first describing those reactors giving rise to controlled surface exposure times. 

Modeling of Chemical Kinetics and Reactor Design ACID-BASE CATALYSIS HOMOGENEOUS LIQUID PHASE... 

In the above Da denotes the Damkohler number as the ratio of the characteristic process time H/V to the characteristic reaction time l/r0. The reaction rate r0 is a reference value at the system pressure and an arbitrary reference temperature, as the lowest or the highest boiling point. For catalytic reactions r0 includes a reference value of the catalyst amount. R is the dimensionless reaction rate R = r/r0. The kinetics of a homogeneous liquid-phase reaction is described in general as function of activities ... 

The rate of liquid-phase chemical reactions involving transfer of reactants from another phase depends on the homogeneous liquid-phase kinetics, physical mass transfer rates of reactants, and their thermodynamic equilibria at the phase boundaries. The interaction among these phenomena produces four distinct types of behavior depending on chemical reaction velocity. These will be examined in this paper. 

The analytical problems associated with differential reactors can be overcome by the use of the recirculation reactor. A simplified form, called a Schwab reactor, is described by Weisz and Prater . Boreskov.and other Russian workers have described a number of other modifications " . The recirculation reactor is equivalent kinetically to the well-stirred continuous reactor or backmix reactor , which is widely used for homogeneous liquid phase reactions. Fig. 28 illustrates the principle of this system. The reactor consists of a loop containing a volume of catalyst V and a circulating pump which can recycle gas at a much higher rate, G, than the constant feed and, withdrawal rates F. 

In the laboratory either integral or differential (see Sec. 4-3) tubular units or stirred-tank reactors may be used. There are advantages in using stirred-tank reactors for kinetic studies. Steady-state operation with well-defined residence-time conditions and uniform concentrations in the fluid and on the solid catalyst are achieved. Isothermal behavior in the fluid phase is attainable. Stirred tanks have long been used for homogeneous liquid-phase reactors and slurry reactors, and recently reactors of this type have been developed for large catalyst pellets. Some of these are described in Sec. 12-3. When either a stirred-tank or a differential reactor is employed, the global rate is obtained directly, and the analysis procedure described above can be initiated immediately. 

Simple homogeneous liquid-phase reactions can be described with the help of formal kinetic rate laws in which the reaction rate r depends on the concentrations of the reactants, on the temperature and possibly on the homogeneous catalyst only. Examples of such formal kinetic rate laws are presented in Table 4-1. 

The classical apparatus for homogeneous liquid-phase reaction systems is the stirred tank, which is preferentially operated in batch mode, since for normal reaction kinetics (order > 0) its continuous operation is disadvantageous an exception here is its use in a cascade. 

Neglecting fast transient effects, the quenching reaction in a homogeneous liquid phase follows quasi-first-order kinetics ... 

The simplest kinetic reactor model is the CSTR (continuous-stirred-tank reactor), in which the contents are assumed to be perfectly mixed. Thus, the composition and the temperature are assumed to be uniform throughout the reactor volume and equal to the composition and temperature of the reactor effluent However, the fluid elements do not all have the same residence time in the reactor. Rather, there is a residence-time distribution. It is not difficult to provide perfect mixing of the fluid contents of a vessel to approximate a CSTR model in a commercial reactor. A perfectly mixed reactor is used often for homogeneous liquid-phase reactions. The CSTR model is adequate for this case, provided that the reaction takes place under adiabatic or isothermal conditions. Although calculations only involve algebraic equations, they may be nonlinear. Accordingly, a possible complication that must be considered is the existence of multiple solutions, two or more of which may be stable, as shown in the next example. 

The 18-crown-6-cyclic polyether-KBr complex catalyses the homogeneous liquid phase molecular oxygen oxidation of ethylbenzene to the hydroperoxide. The macrobicyclic ligands (cryptands) are strong bases but the rates of proton transfer from ethyl nitroacetate to the free base cryptand and to the mono-protonated cryptand are ca, 10 and 100-fold smaller, respectively, than that of transfer to a normal base of similar basicity. This is attributed either to steric hindrance to proton transfer or to proton transfer occurring only to the thermodynamically unfavourable exo-nitrogen conformation. However, a large kinetic isotope effect = 3.9) is observed for the protonation of cryptand... 

On an industrial scale, BRs are primarily intended for homogeneous liquid-phase reactions and less frequently for gas-phase reactions. On a laboratory scale, however, BRs with a constant volume are often used for the determination of the kinetics of homogeneous gas-phase reactions. BRs are typically used industrially for the production of fine chemicals via organic liquid-phase reactions, such as drug synthesis, and the manufacture of paints, pesticides, and herbicides. 

This reaction is very slow and it has extraordinary autocatalytic kinetics (independent of chlorine concentration). The system can thus be treated as a homogeneous liquid-phase system, for which the mass transfer effects are negligible. 

The kinetics of high-energy radiation-induced polymerization in homogeneous liquid phase can be treated conventionally in terms of initiation, propagation and... 

Scheme 5.4 Kinetic scheme of free-radical chain polymerization in homogeneous liquid phase. M monomer P polymer.

The concentrations of reactants are of little significance in the theoretical treatment of the kinetics of solid phase reactions, since this parameter does not usually vary in a manner which is readily related to changes in the quantity of undecomposed reactant remaining. The inhomogeneity inherent in solid state rate processes makes it necessary to consider always both numbers and local spatial distributions of the participants in a chemical change, rather than the total numbers present in the volume of reactant studied. This is in sharp contrast with methods used to analyse rate data for homogeneous reactions in the liquid or gas phases. 

Since the free energy of a molecule in the liquid phase is not markedly different from that of the same species volatilized, the variation in the intrinsic reactivity associated with the controlling step in a solid—liquid process is not expected to be very different from that of the solid—gas reaction. Interpretation of kinetic data for solid—liquid reactions must, however, always consider the possibility that mass transfer in the homogeneous phase of reactants to or products from, the reaction interface is rate-limiting [108,109], Kinetic aspects of solid—liquid reactions have been discussed by Taplin [110]. 

This chapter is restricted to homogeneous, single-phase reactions, but the restriction can sometimes be relaxed. The formation of a second phase as a consequence of an irreversible reaction will not affect the kinetics, except for a possible density change. If the second phase is solid or liquid, the density change will be moderate. If the new phase is a gas, its formation can have a major effect. Specialized models are needed. Two-phase ffows of air-water and steam-water have been extensively studied, but few data are available for chemically reactive systems. 

In a bulk silica matrix that differs from the silica nanomatrix regarding only the matrix size but has a similar network structure of silica, several kinetic parameters have been studied and the results demonstrated a diffusion controlled mechanism for penetration of other species into the silica matrix [89-93]. When the silica is used as a catalyst matrix in the liquid phase, slow diffusion of reactants to the catalytic sites within the silica rendered the reaction diffusion controlled [90]. It was also reported that the reduction rate of encapsulated ferricytochrome by sodium dithionite decreased in a bulk silica matrix by an order of magnitude compared to its original reaction rate in a homogeneous solution [89], In gas-phase reactions in the silica matrix, diffusion limitations were observed occasionally [93],... 

As with homogeneous aldol reactions, simple power-type rate equations have been frequently used to describe the kinetics of solid-catalysed condensations. For several liquid phase reactions, second-order kinetics was established, viz. 

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