Homogeneous and heterogeneous reactors

A further classification of homogeneous and heterogeneous reactors is based on the nature of the involved reacting phases—... 

Equation (45) can be solved by applying different photoreactor models. The literature reports several photochemical reactor models for both homogeneous and heterogeneous reactors [11,108,109]. In practice, annular photoreactors are often used (see Fig. 5) therefore, models for this type of reactor are considered here. For other types of reactors, attention should be given to other publications [109]. 

The latter model type describes the experimentally determined relations between dependent and independent variables with the help of statistical methods and neglects the known physicochemical relations. Such models are primarily used on reactor types difficult to describe deterministically. The cell models are composed of specific networks of mixing cells (e.g. stirred reactor cascades) or of combinations of mixing cells and transport cells (ideal tube reactors). The so-called continuum models, however, handle each phase as a continuum. The continuum models are further distinguished as homogeneous and heterogeneous reactor models. In the heterogeneous reactor model, the fluid phases and the solid phase (catalyst) are considered and mathematically described as individual items. 

Overall, tracers have been and continue to be used in all homogeneous and heterogeneous reactor types. They remain an irreplacable tool in scaleup and cold flow modeling on which all prudent design engineers rely. The broad spectrum of their use is illustrated by this review. 

Table 12.1 Pseudo-homogeneous and heterogeneous reactor model classification...

Mitsui Toatsu Chemical, Inc. disclosed a similar process usiag Raney copper (74) shortiy after the discovery at Dow, and BASF came out with a variation of the copper catalyst ia 1974 (75). Siace 1971 several hundred patents have shown modifications and improvements to this technology, both homogeneous and heterogeneous, and reviews of these processes have been pubHshed (76). Nalco Chemical Company has patented a process based essentially on Raney copper catalyst (77) ia both slurry and fixed-bed reactors and produces acrylamide monomer mainly for internal uses. Other producers ia Europe, besides Dow and American Cyanamid, iaclude AUied CoUoids and Stockhausen, who are beheved to use processes similar to the Raney copper technology of Mitsui Toatsu, and all have captive uses. Acrylamide is also produced ia large quantities ia Japan. Mitsui Toatsu and Mitsubishi are the largest producers, and both are beheved to use Raney copper catalysts ia a fixed bed reactor and to sell iato the merchant market. 

The general picture of the relative merits of homogeneous and heterogeneous processes has not yet emerged clearly. The homogeneous catalyst system may offer advantages in chemical efficiency but lead to difficulties of catalyst separation and recovery, or catalysts may tend to plate out in the reactor due to slight instability. Materials of construction may have to be different for the two rival plants. All these factors will have to be considered in an economic assessment and detailed studies made of the complete process networks in both cases. 

The system can be operated in the parallel mode, discontinuously (batch-wise) with each reactor as an independent unit, semi-continuously or as a reactor cascade. Both homogeneous and heterogeneous reactions as well as product and catalyst separation and catalyst recycling are possible. 

De Wasch and Froment (1971) and Hoiberg et. al. (1971) published the first two-dimensional packed bed reactor models that distinguished between conditions in the fluid and on the solid. The basic emphasis of the work by De Wasch and Froment (1971) was the comparison of simple homogeneous and heterogeneous models and the relationships between lumped heat transfer parameters (wall heat transfer coefficient and thermal conductivity) and the effective parameters in the gas and solid phases. Hoiberg et al. (1971)... 

The principal difference between homogeneous and heterogeneous reaction rates is that the latter is based on mass, volume, or more rarely, on the area of the solid and not on the fluid-phase volume or reactor volume. The reactor volume or liquid-phase volume is of secondary significance in heterogeneous reactions since the reaction takes place on the solid rather than throughout the reactor volume. Moreover, the mass of the solid is usually used instead of the solid volume or surface, because it is the most easily measured property. 

D.K. Zerkle, M.D. Allendorf, M. Wolf, and O. Deutschmann. Understanding Homogeneous and Heterogeneous Contributions to the Partial Oxidation of Ethane in a Short Contact Time Reactor. J. Catalysis, 196 18-39,2000. 

Chemat and co-workers have developed a continuous flow microwave reactor that is suitable for homogeneous and heterogeneous reactions127. Although their system had only a small cavity (66 ml), it was possible to treat significant quantities of reagents (30 -330 ml/min). This underscored the feasibility of scaling up the manufacture of fine... 

Chemat, E, Poux, M., Di Martino, J.L. andBerlan, J., A new continuous-flow recycle microwave reactor for homogeneous and heterogeneous chemical reactions, Chem. Eng. Tech., 1996,19(5), 420. 

The reaction of methane oxidation with hydrogen peroxide under pressure was studied on an automated micropilot flow unit with integral reactor based on the standard double reactor OL 105/02 system. The OL 105/02 system is usually used in studies of pressurized homogeneous and heterogeneous processes in gas and liquid [123]. The micropilot unit has two equal reactors of 250 cm3 volume and is equipped with standard metering, recording and control instruments. 

Plotting residue curves maps (RCM) allows the designer to anticipate problems by the separation of nonideal mixtures, namely when dealing with homogeneous and heterogeneous azeotropes. By reactor selection, it may foresee problems incurred by the recycle of some reactants. 

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