Homogeneous liquid-phase reactions

Homogeneous reactions are those in which the reactants, products, and any catalysts used form one continuous phase (gaseous or liquid). Homogeneous gas phase reactors are almost always operated continuously, whereas liquid phase reactors may be batch or continuous. Tubular (pipeline) reactors arc normally used for homogeneous gas phase reactions (e.g., in the thermal cracking of petroleum of dichloroethane lo vinyl chloride). Both tubular and stirred tank reactors are used for homogeneous liquid phase reactions. 

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

Homogeneous, liquid-phase reactions may also be important in trickle beds, and a strictly homogeneous term has been included in Equation (11.42) to note this fact. There is usually no reaction in the gas phase. Normally, the gas phase merely supplies or removes the gaseous reactants (e.g., H2 and H2S in hydrodesulfurization). ... 

An nth-order homogeneous liquid phase reaction is carried out in a batch tank reactor. 

Three special cases of equation 9.2-18 arise, depending on the relative magnitudes of the two mass-transfer terms in comparison with each other and with the reaction term (which is always present for reaction in bulk liquid only). In the extreme, if all mass-transfer resistance is negligible, the situation is the same as that for a homogeneous liquid-phase reaction, ( rA)im = kAcAcB. 

Figure 12.3 illustrates some modes of operation of semibatch reactors. In Figure 12.3(a), depicting a homogeneous liquid-phase reaction of the type A + B - products, reactant A is initially charged to the vessel, and reactant B is added at a prescribed rate, as reaction proceeds. In Figure 12.3(b), depicting a liquid-phase reaction in which a... 

Adapted from Bhaumik et al. 244). Reaction conditions reaction time, 12 h reactant H202 = 1 1 catalyst (TS-1, Si/Ti = 29), 20 wt% with respect to reactant temperature, 353 K. a Tri solid catalyst + two immisible liquid phases (organic reactant + H202 in water) bi solid catalyst + one homogeneous liquid phase (organic reactant + aqueous H202 + CH3CN as cosolvent). 

Esterification is the first step in PET synthesis but also occurs during melt-phase polycondensation, SSP, and extrusion processes due to the significant formation of carboxyl end groups by polymer degradation. As an equilibrium reaction, esterification is always accompanied by the reverse reaction being hydrolysis. In industrial esterification reactors, esterification and transesterification proceed simultaneously, and thus a complex reaction scheme with parallel and serial equilibrium reactions has to be considered. In addition, the esterification process involves three phases, i.e. solid TPA, a homogeneous liquid phase and the gas phase. The respective phase equilibria will be discussed below in Section 3.1. 

Transesterification is the main reaction of PET polycondensation in both the melt phase and the solid state. It is the dominant reaction in the second and subsequent stages of PET production, but also occurs to a significant extent during esterification. As mentioned above, polycondensation is an equilibrium reaction and the reverse reaction is glycolysis. The temperature-dependent equilibrium constant of transesterification has already been discussed in Section 2.1. The polycondensation process in the melt phase involves a gas phase and a homogeneous liquid phase, while the SSP process involves a gas phase and two solid phases. The respective phase equilibria, which have to be considered for process modelling, will be discussed below in Section 3.1. 

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. 

The reaction is performed in a homogeneous liquid phase by use of less than 2 mol% of heteropolyacids with respect to acetylated aldohexose, and 70-90% yields and 60-98% mol% / -anomer selectivity are obtained. 

This section is concerned with the activation of hydrocarbon molecules by coordination to noble metals, particularly palladium.504-513 An important landmark in the development of homogeneous oxidative catalysis by noble metal complexes was the discovery in 1959 of the Wacker process for the conversion of ethylene to acetaldehyde (see below). The success of the Wacker process provided a great stimulus for further studies of the reactions of noble metal complexes, which were found to be extremely versatile in their ability to catalyze homogeneous liquid phase reaction. The following reactions of olefins, for example, are catalyzed by noble metals hydrogenation, hydroformylation, oligomerization and polymerization, hydration, and oxidation. 

When a reactant is gaseous or very light the driving force for reaction is lower compared with a homogeneous liquid-phase reaction at higher pressure. 

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 ... 

Weckhuysen and coworkers (Mesu et al., 2005 Tinnemans et al., 2006) also presented a combination of UV-vis (fiber optics) and XAFS (energy dispersive) spectroscopy for the analysis of homogeneous liquid-phase reactions. Both methods are performed in transmission, with the beams crossing in a cuvette (5-mm path length each) with quartz windows. 

It is probably necessary to make a primary operational distinction of reaction classes based on the phase (or phases) of matter involved thus (1) homogeneous, liquid phase (2) gas phase (3) solid phase (4) heterogeneous. A basic subclassification distinguishes between reactions in which the reactants are chemically different from the prodncts, as in equations (1) and (2), and reactions in which the reactants and prodncts involve the same chemical species, as in equations (3) and (4) when (N4) = ( N4). Eqnations (1) and (2) are examples of cross electron-transfer reactions (or cross-reactions), while eqnations (3) and (4) are examples of self-exchange electron-transfer reactions when (N4) = ( N4). More generally, subclassifications of the primary classes are commonly based on energy or free energy considerations such as ... 

Ionization reactions in homogeneous liquid phase usually proceed as reactions between closed-shell molecules. According to the functional approach, these reactions are regarded as EPD-EPA reactions in the strict sense of the word. Owing to the inherently high stability of closed-shell molecules or ions, these reactions usually lead to heterolytic bond cleavage. 

Using the definitions above, the reaction of sodium hydroxide with tert-butyl bromide (TBB) can be described as an irreversible, homogeneous, liquid-phase reaction which is first-order with respect to ferr-butyl bromide, zero-order with respect to sodium hydroxide, overall first-order, and nonelementaty. 

Co and Cr have been found to be incorporated into the lattice of aluminiumphosphates in a well dispersed manner [159]. Both elements assume two oxidation states in the lattice depending upon the pretreatment procedures. While it seems certain that during synthesis incorporation can be achieved and that these tetrahedrally coordinated atoms are stable in gas phase reactions, conclusive evidence is lacking that leaching is not an important side reaction in liquid phase studies. Indeed, it seems that for several reactions the highly active complexes that are leached out of the lattice and homogeneously dissolved in the reactant/solvent mixture dominate the catalytic properties. 

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