Multiphase media reaction

The term "heterogeneous" as applied to the atmosphere refers to chemistry that occurs in or on ambient condensed phases that are in contact with the gas phase aerosols, clouds, surface waters, etc. It is important to distinguish between heterogeneous processes that occur on the surface of the solid, and multiphase chemical reactions that take place in the bulk of the liquid medium. In the latter case, it is assumed that the reaction takes place after the molecule has been incorporated in the bulk liquid medium, such as occurs by wet deposition, where a species is ultimately removed from the atmosphere, especially in the troposphere. 

Heterogeneous reactors are multiphase reactors in which the reaction medium is a multiphase medium. Heterogeneous reactors are broadly classified as reactors in which multiphase non-catalytic reactions take place and reactors in which multiphase catalytic reactions take place. Principles of multiphase reaction kinetics and design of multiphase reactors are discussed in this chapter. 

In PEMFCs, the membrane electrode assembly (MEA, Eig. 15.2a) is a multilayer sandwich composed of catalytic layers (CLs) where electrochemical reactions take place, gas-diffusion media providing access of gases to the CLs, and a proton exchange membrane (PEM) such as Nafion . The CL is a multiphase multicomponent medium comprising ... 

Emulsion Polymerization in a CSTR. Emulsion polymerization is usually carried out isothermally in batch or continuous stirred tank reactors. Temperature control is much easier than for bulk or solution polymerization because the small (. 5 Jim) polymer particles, which are the locus of reaction, are suspended in a continuous aqueous medium as shown in Figure 5. This complex, multiphase reactor also shows multiple steady states under isothermal conditions. Gerrens and coworkers at BASF seem to be the first to report these phenomena both computationally and experimentally. Figure 6 (taken from ref. (253)) plots the autocatalytic behavior of the reaction rate for styrene polymerization vs. monomer conversion in the reactor. The intersection... 

Slurry Reactors Slurry reactors are akin to fluidized beds except the fluidizing medium is a liquid. In some cases (e.g., for hydrogenation), a limited amount of hydrogen may be dissolved in the liquid feed. The solid material is maintained in a fluidized state by agitation, internal or external recycle of the liquid using pipe spargers or distributor plates with perforated holes at the bottom of the reactor. Most industrial processes with slurry reactors also use a gas in reactions such as chlorination, hydrogenation, and oxidation, so the discussion will be deferred to the multiphase reactor section of slurry reactors. 

The atmosphere is a complex medium in which several phases coexist gas, aerosol particles, condensed water, liquid, and ice particles. All of the interactions that may occur between these various phases are included in the term multiphase or heterogeneous chemistry. Clouds favor the development of atmospheric multiphase chemistry, as they are composed of all three atmospheric phases (i.e., gas, liquid, and solid phases that stimulate a full set of reactions). Moreover, clouds modify radiative properties by diffusion of short-wavelength radiation coming from... 

Multiphase reactors are reactors in which two or more phases are necessary to carry out the reaction. The majority of multiphase reactors involve gas and liquid phases which contact a solid. In the case of the slurry and trickle bed reactors, the reaction between the gas and the liquid takes place on a solid catalyst Sluface (see Table 12-2). However, in some reactors the liquid phase is an inert medium for the gas to contact the solid catalyst. The latter situation arises when a large heat sink is required for highly exothermic reactions. In many cases the catalyst life is extended by these milder operating conditions. 

Micellar catalysis enables water to be employed as a reaction medium, to gain not only an enhancement in the rate of reaction but also improvements in selectivity and sometimes even to allow the catalyst to be recycled in a simple manner (cf. Section 3.1.1.1). A micellar system is a multiphase system in a colloidal dimension and allows the microheterogenization of a catalyst under special conditions [7]. 

The other distinct mode of operation for a tubular reactor occurs in applications where more than one phase is present in the reaction mixture, e.g., gas and liquid reactants. The products from the reaction can be gases, liquids, or solids where the latter can exist as crystalline or amorphous materials. Either aqueous or organic-based solvents are sometimes included in the reaction medium to control the concentrations of reaction species, to provide increased thermal capacity for highly exothermic systems, or to alter solubility properties for subsequent catalyst recovery or product separations and recovery. This type of reaction is often termed a multiphase reaction, owing to the presence of multiple interacting phases in the reaction environment. In most practical applications of this mode, either a soluble organometallic complex or a solid heterogeneous catalyst is utilized to transform the reactants into the desired product or products. 

It is noteworthy to mention that tliis reaction is not a homogeneous single phase system as neither reactant is soluble in aqueous alkaline reaction medium. We believe that the selective absorption of microwaves by polar molecules and intermediates in a multiphase system could substitute as a phase transfer catalyst witliout using any phase transfer reagent, thereby providing the observed acceleration as has been observed for ultrasonic irradiation. ... 

In three-phase reactors, one of the main problems is often the mass transport limitations, which may reflect internal as well as external mass transfer resistances. The use of filamentous catalytic materials for multiphase reactions may help reduce or even avoid mass transfer limitations [63,132,133]. Filamentous woven cloths made of glass, composite mixed oxides, metallic alloys, or activated carbon (Figure 18) can be used as supports for active components such as platinum, palladium, or transition metal oxides. The diameters of the filaments are of the order of several micrometers and correspond to the typical diameters of catalysts that are suspended in the reaction medium. By using such small diameters, internal mass transfer limitations can be avoided. 

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