Rate of liquid-phase reactions

Increasing the pressure of irreversible vapor-phase reactions increases the rate of reaction and hence decreases reactor volume both by decreasing the residence time required for a given reactor conversion and increasing the vapor density. In general, pressure has little effect on the rate of liquid-phase reactions. 

The effect of external pressure on the rates of liquid phase reactions is normally quite small and, unless one goes to pressures of several hundred atmospheres, the effect is difficult to observe. In terms of the transition state approach to reactions in solution, the equilibrium existing between reactants and activated complexes may be analyzed in terms of Le Chatelier s principle or other theorems of moderation. The concentration of activated complex species (and hence the reaction rate) will be increased by an increase in hydrostatic pressure if the volume of the activated complex is less than the sum of the volumes of the reactant molecules. The rate of reaction will be decreased by an increase in external pressure if the volume of the activated complex molecules is greater than the sum of the volumes of the reactant molecules. For a decrease in external pressure, the opposite would be true. In most cases the rates of liquid phase reactions are enhanced by increased pressure, but there are also many cases where the converse situation prevails. 

The rates of liquid-phase reactions can generally be obtained by measuring the time-dependent concentrations of reactants and/or products in a constant-volume batch reactor. From experimental data, the reaction kinetics can be analyzed either by the integration method or by the differential method ... 

There is a large body of literature on the mechanisms of liquid phase reactions, but theory is not as highly developed as it is for gas phase reactions. The rates of liquid phase reactions are influenced by bulk properties of the medium, in contrast with the gas phase. The theory of solvent effects is an active research topic at the present time. In addition, fast liquid phase reactions are rate limited by reactant diffusion rates. Various aspects of liquid phase reactions have been covered in several previous volumes in this series. 

Material and energy balances are based on the conservation law, Eq. (7-69). In the operation of liquid phase reactions at steady state, the input and output flow rates are constant so the holdup is fixed. The usual control of the discharge is on the liquid level in the tank. When the mixing is adequate, concentration and temperature are uniform, and the effluent has these same properties. The steady state material balance on a reacdant A is... 

In catalysis, adsorbed CO may retard some reactions such as olefin hydrogenation, fuel cell conversion, and enantioselective hydrogenation. For instance, Lercher and coworkers observed the deactivation of Pt/Si02 in the liquid-phase hydrogenation of crotonaldehyde, and ascribed this deactivation to the decomposition of crotonaldehyde on platinum surface to adsorbed CO [138]. Blaser and coworkers found that the addition of a small amount of formic acid decreases the rate of liquid-phase hydrogenation of ethyl pyruvate on cinchonidine-modified Pt/Al203 catalyst, which they explained as the decomposition of formic acid on the catalyst to adsorbed CO. Interestingly, the addition of acetic acid does not decrease the reaction rate, but whether acetic acid decomposes on the catalyst as formic acid does was not mentioned [139]. 

Measurements of kinetic parameters of liquid-phase reactions can be performed in apparata without phase transition (rapid-mixing method [66], stopped-flow method [67], etc.) or in apparata with phase transition of the gaseous components (laminar jet absorber [68], stirred cell reactor [69], etc.). In experiments without phase transition, the studied gas is dissolved physically in a liquid and subsequently mixed with the liquid absorbent to be examined, in a way that ensures a perfect mixing. Afterwards, the reaction conversion is determined via the temperature evolution in the reactor (rapid mixing) or with an indicator (stopped flow). The reaction kinetics can then be deduced from the conversion. In experiments with phase transition, additionally, the phase equilibrium and mass transport must be taken into account as the gaseous component must penetrate into the liquid phase before it reacts. In the laminar jet absorber, a liquid jet of a very small diameter passes continuously through a chamber filled with the gas to be examined. In order to determine the reaction rate constant at a certain temperature, the jet length and diameter as well as the amount of gas absorbed per time unit must be known. 

The theoretical treatment of liquid-phase reaction kinetics is limited by the fact that no single universal theory on the liquid state exists at present. Problems which have yet to be sufiiciently explained are the precise character of interaction forces and energy transfer between reacting molecules, the changes in reactivity as a result of these interactions, and finally the role of the actual solvent structure. Despite some hmitations, the absolute reaction rates theory is at present the only sufficiently developed theory for processing the kinetic patterns of chemical reactions in solution [2-5, 7, 8, 11, 24, 463-466]. According to this theory, the relative stabilization by solvation of the initial reactants and the activated complex must be considered cf. Section 5.1). 

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. 

In his more recent paper, Beck d compares the reaction rates of liquid-phase hydrogenation reactions with electrochemical reductions. The comparison is somewhat arbitrary, as Beck concedes. Thus the electrochemical reduction rates become potential-independent at higher overpotential, and it is this transport-limited current that is used as one half of the comparison. The liquid-phase hydrogenations also appear to be mass-transport controlled. However, given that the conditions, in either case, are reasonable relative to their respective technologies, it is interesting to note that the ratio of electrochemical to gas-phase reaction rates (per unit catalyst area) ranges from 4.5 1 to 20 1. ... 

The rate of a homogeneous reaction, r, should not depend on the scale of the reactor in which the reaction is carried out. When this happens, a diffu-sional process must be suspected. In the preceding section, it was seen how diffusional limitations could change the rate of liquid-phase chain reactions by affecting the rate of termination. In the gas phase at low pressures,... 

Although the accurate computation of liquid-phase reactions remains difficult due to numerical issues, Aold et al. [82] performed simulations of various model reaction systems, allowing relative comparisons. In particular, they studied the effects of the width of the fluid lamellae and the rate constants on reactant conversion and product selectivity. Qualitatively, the results reveal a strong dependence of the product selectivity on the lamellar width. From these CFD simulations, a model was developed that relates lamellar width, rate constants and product selectivity for various multiple reactions and reaction conditions. 

If the volumetric flow rate is constant, which is approximately true in the case of liquid-phase reactions and isothermal gas-phase reactions with ij = 0, in continuous... 

The concentrations and volumetric flowrates are obtained from Equations 3.91 and 3.98 in the case of ideal gases in the case of liquid-phase reactions with constant volumetric flow rates, the concentrations are given by Equation 3.111. 

We call Equation (2-6) the differential form of the design equation for a batch reactor because we have written the mole balance in terms of conversion. The differential forms of the batch reactor mole balances. Equations (2-5) and (2-6), are often used in the interpretation of reaction rate data (Chapter 7) and for reactCHS with heat effects (Chapters 11-13), respectively. Batch reactors are frequently used in industry for both ga.s-phase and liquid-phase reactions. The lidmratory bomb calorimeter reactor is widely used for ol ning reaction rate data Liquid-phase reactions are frequently carried out in batch reactors when small-scale production is desired or operating difficulties rule out the use of continuous Row systems. 

More often than not the rate at which residual absorbed gas can be driven from the liqmd in a stripping tower is limited by the rate of a chemical reaction, in which case the liquid-phase residence time (and hence, the tower liquid holdup) becomes the most important design factor. Thus, many stripper-regenerators are designed on the basis of liquid holdup rather than on the basis of mass transfer rate. 

As discussed later, the reaction-enhancement factor ( ) will be large for all extremely fast pseudo-first-order reac tions and will be large tor extremely fast second-order irreversible reaction systems in which there is a sufficiently large excess of liquid-phase reagent. When the rate of an extremely fast second-order irreversible reaction system A -t-VB produc ts is limited by the availabihty of the liquid-phase reagent B, then the reac tion-enhancement factor may be estimated by the formula ( ) = 1 -t- B /VCj. In systems for which this formula is applicable, it can be shown that the interface concentration yj will be equal to zero whenever the ratio k yV/k B is less than or equal to unity. 

Although they are termed homogeneous, most industrial gas-phase reactions take place in contact with solids, either the vessel wall or particles as heat carriers or catalysts. With catalysts, mass diffusional resistances are present with inert solids, the only complication is with heat transfer. A few of the reactions in Table 23-1 are gas-phase type, mostly catalytic. Usually a system of industrial interest is liquefiea to take advantage of the higher rates of liquid reactions, or to utihze liquid homogeneous cat ysts, or simply to keep equipment size down. In this section, some important noncatalytic gas reactions are described. 

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