Phase reactions, homogeneous

Due to existence of an isothermal region, temperature of both entrance and outlet was rather lower than that of intermediate section. Where temperature was high, the reaction was sever and fast. So, at site 200-300 mm away from entrance, the temperature was highest, the scale layer was thickest and the whisker column was longest there. The reaction route in this zone could be described as phase reaction homogeneous nucleation — coagulation. When... 

Chemical Temperature, pressure, and mechanical stress effects on reactions Transport of reaction products Media for heterogeneous phase reactions Homogeneous phase reactions, solutions and gases... 

The nature of electrode processes can, of course, be more complex and also involve phase fonnation, homogeneous chemical reactions, adsorption or multiple electron transfer [1, 2, 3 and 4],... 

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. 

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. 

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. 

If at high temperatures (> 600 K) the volatilized NH3 and HC104 are prevented from leaving the heated zone by the presence of an inert gas, decomposition in the homogeneous phase follows. This is the high temperature (gas phase) reaction in which there is complete conversion of the reactant to volatile products and no residue remains. 

Explicit mechanisms attempt to include all nonmethane hydrocarbons believed present in the system with an explicit representation of their known chemical reactions. Atmospheric simulation experiments with controlled NMHC concentrations can be used to develop explicit mechanisms. Examples of these are Leone and Seinfeld (164), Hough (165) and Atkinson et al (169). Rate constants for homogeneous (gas-phase) reactions and photolytic processes are fairly well established for many NMHC. Most of the lower alkanes and alkenes have been extensively studied, and the reactions of the higher family members, although little studied, should be comparable to the lower members of the family. Terpenes and aromatic hydrocarbons, on the other hand, are still inadequately understood, in spite of considerable experimental effort. Parameterization of NMHC chemistry results when NMHC s known to be present in the atmosphere are not explicitly incorporated into the mechanism, but rather are assigned to augment the concentration of NMHC s of similar chemical nature which the... 

A homogeneous gas-phase reaction that follows a third-order kinetic scheme is... 

Suppose a homogeneous, gas-phase reaction occurs in a constant-volume batch reactor. Assume ideal gas behavior and suppose pure A is charged to the reactor. 

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. 

Hinshelwood and Green d Studied the homogeneous, gas-phase reaction ... 

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). ... 

In the case of the reaction between N-acryloyloxazolidin-2-one and cy-clopentadiene, both catalysts showed activities and enantioselectivities similar to those observed in homogeneous phase. However, a reversal of the major endo enantiomer obtained with the immobilized 6a-Cu(OTf)2 catalyst, with regard to the homogeneous phase reaction, was noted. Although this support effect on the enantioselectivity remains unexplained, it resembles the surface effect on enantioselectivity of cyclopropanation reaction with clay supports [58]. 

In the following, the impact of the micro-channel diameter on the temperature rise due an exothermic gas-phase reaction is investigated. For simplicity, a homogeneous reaction A —> B of order n with kinetic constant k is considered. Inside the micro channel, the time evolution of the radially averaged species concentration c and temperature T is governed by the equations... 

GP 11] ]R 19] The suppression of explosive homogeneous gas-phase reactions is not due simply to thermal quenching as a result of the heat losses from a micro reactor, but rather to radical quenching [9]. The micro reactor will therefore be safe even when heat losses from the reaction micro channel are reduced by design modifications. 

Similarly, a catalytic route to indigo was developed by Mitsui Toatsu Chemicals (Inoue et al, 1994) to replace the traditional process, which dates back to the nineteenth century (see earlier), and has a low atom efficiency/high E factor (Fig. 2.15). Indole is prepared by vapour-phase reaction of ethylene glycol with aniline in the presence of a supported silver catalyst. The indole is selectively oxidised to indigo with an alkyl hydroperoxide in the presence of a homogeneous molybdenum catalyst. 

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

Extending the formalism for ET in homogeneous phase, reactions at liquid-liquid interfaces can be described in terms of a series of elementary steps initiated by the approach of reactants to the interfacial region and the formation of the ET precursor complex [1,5,60],... 

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