Multiphase reaction product

Explicit expressions for the ratio (k /k ) of a multiphase reaction product layer have been presented in the literature, see, for example, [H. Schmalzried (1981)]. If k(2) of the second kind, which depends only on the properties of phase p, is calculated or measured for every phase p individually, it is possible to derive (from all NiiP, A p, and the molar volumes Vp) the rational rate constant k p] of the first kind, and thus eventually k in Eqn. (6.41). 

Fig, 6-7. Schematic plot of the activity of a reactant in a multiphase reaction product layer under the assumption that (1) local thermodynamic equilibrium is maintained, (2) local thermodynamic equilibrium is not maintained. 

A Xi is the reaction layer thickness in the case where it is formed in a reaction starting with the pure reactants. A Xj, on the other hand, is the reaction layer thickness in the case of a reaction where the reaction layer under consideration is formed from the saturated adjacent phases. These definitions hold for a one-phase reaction product as well as for a multiphase reaction product. Since the average diffusion coefficients in a certain reaction layer (phase k) cannot depend upon the starting material if local equilibrium prevails, one may return to eqs. (7-35) and (7-36) in order to obtain a relation between the reaction rate constants of the first and second kind. By taking into account the definitions, the relation between the two rate constants for a reaction product with phases of very narrow ranges of homogeneity can be shown to be ... 

Obviously, there are many good reasons to study ionic liquids as alternative solvents in transition metal-catalyzed reactions. Besides the engineering advantage of their nonvolatile natures, the investigation of new biphasic reactions with an ionic catalyst phase is of special interest. The possibility of adjusting solubility properties by different cation/anion combinations permits systematic optimization of the biphasic reaction (with regard, for example, to product selectivity). Attractive options to improve selectivity in multiphase reactions derive from the preferential solubility of only one reactant in the catalyst solvent or from the in situ extraction of reaction intermediates from the catalyst layer. Moreover, the application of an ionic liquid catalyst layer permits a biphasic reaction mode in many cases where this would not be possible with water or polar organic solvents (due to incompatibility with the catalyst or problems with substrate solubility, for example). 

In Section 4.3.3 the use of hydrotropes for intensifying multiphase reactions and making them more selective was covered. The key advantage of an aqueous solution of a hydrotrope is that the solute can be recovered by diluting the aqueous solution, after extraction, to a hydrotope concentration below the critical hydrotrope concentration, when a major part of the product will separate out. The diluted solution can then be reconcentrated at reduced pressure to the original concentration for recycle. Thus a number of products of commercial value, such as phenyl ethyl alcohol, can be recovered (Friberg et al 1996 Gaikar and Phatak, 1999). 

See also Biotransformations Microbial oxidations Microbial reductions applications of, 76 396-399 biocatalyst selection in, 76 404-409 biocatalysts in, 76 409-414 for drug metabolite production, 76 398-399 further advances in, 76 414 in hydrolysis, 76 400-401 multiphase reactions in, 777 412-414 scale-up of, 76 414 systematic studies of, 76 398 technique overview for, 76 403-414 timing of substrate additions in, 76 411-412 uses for, 777 400-403 Microbial waxes, 26 203 Microbiocides, triorganotins as, 24 817 Microbiological culture media, agar in, 73 68... 

In the 1990s, reaction calorimetry has found extensive use in industrial scale-up operations. The editor s limited exposure to it suggests that the commercially available equipment has indifferent agitation and may in consequence handle multiphase reactions poorly anathema though they be to academic kineticists, industry finds slurries attractive. However it is a simple technique by which to acquire data on the kinetics of heat production. Publications have become very numerous, mostly publicity from testing houses offering the service. Some very recent work is found in [6], [7],... 

Modem concretes often incorporate a mixture of chemical and mineral admixtures, each of which may interact with the various constituents of cements and influence cement hydration reactions. The admixture-cement interactions may in fact be viewed as the reaction between two complex chemical systems - the multicomponent, multiphasic inorganic materials in the cement and the organic compounds of multicomponent admixture systems. For example, lignosulfonate water-reducers are intrinsically complex mixtures of chemical compounds derived from the chemical degradation of lignin, while synthetic admixtures such as superplasticizers contain species with a broad distribution of molecular weights, reaction products, or other chemicals added for a specific purpose [125]. The performance of an admixture in concrete is highly dependent on many... 

Schuth s group developed in the past a number of reactors similar to conventional testing methods with different degrees of sample integration. For multiphase reactions a 25-fold stirrer vessel reactor was developed [70] and for heterogeneous gas-phase reactions a 16-fold fixed-bed reactor was presented [71], which was later followed by a 49-fold parallel reactor [135], The reactor in Figure 3.42 was used for methanol production from Syngas at up to 50 bar and was essentially an improved version of the 49-fold reactor described in [135],... 

CSTR may also be used for liquid-phase reactions. However, there can be severe problems with heat and mass transfer in such systems, and sampling of products can cause difficulties. For multiphase reactions it is necessary to ensure good contact between the gas and the liquid as well as between the liquid and the catalyst. 

Aliquiat 336 Pd/C Et3N, KOH, k2co3, NaHC03 100 °C. Multiphasic reaction system aryliodides react up to 10 times faster in the presence of Aliquat 336 product extracted with iso-octane. [60]... 

The film theory was originally proposed by Whitman,195 who obtained his idea from the Nernst117 concept of the diffusion layer. It was first applied to the analysis of gas absorption accompanied by a chemical reaction by Hatta.85,86 It is a steady-state theory and assumes that mass-transfer resistances across the interface are restricted to thin films in each phase near the interface. If more than one species is involved in a multiphase reaction process, this theory assumes that the thickness of the film near any interface (gas-liquid or liquid-solid) is the same for all reactants and products. Although the theory gives a rather simplified description of the multiphase reaction process, it gives a good answer for the global reaction rates, in many instances, particularly when the diffusivities of all reactants and products are identical. It is simple to use, particularly when the... 

FIG. 7-16 Concentration profiles for the general reaction A(g) + bB(l) —> products with the rate r = kCamCbn. [Adapted from Mills, Ramachandran, arid Chaudhari, Multiphase Reaction Engineering for Fine Chemicals and Pharmaceuticals,1 1 Rev. Chem. Eng. 8(1—2) 1 (1992), Figs. 19... 

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. 

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