Reactions multi-phase

Fauske, H. K., M. A. Grolmes, and J. C. Leung, "Multi-Phase Flow Considerations in Siting Emergency Relief Systems for Runaway Chemical Reactions," Multi-Phase Flow and Heat Transfer III-B, Applications, 899 (1984), Elsevier, Amsterdam, The Netherlands. 

The effect of physical processes on reactor performance is more complex than for two-phase systems because both gas-liquid and liquid-solid interphase transport effects may be coupled with the intrinsic rate. The most common types of three-phase reactors are the slurry and trickle-bed reactors. These have found wide applications in the petroleum industry. A slurry reactor is a multi-phase flow reactor in which the reactant gas is bubbled through a solution containing solid catalyst particles. The reactor may operate continuously as a steady flow system with respect to both gas and liquid phases. Alternatively, a fixed charge of liquid is initially added to the stirred vessel, and the gas is continuously added such that the reactor is batch with respect to the liquid phase. This method is used in some hydrogenation reactions such as hydrogenation of oils in a slurry of nickel catalyst particles. Figure 4-15 shows a slurry-type reactor used for polymerization of ethylene in a sluiTy of solid catalyst particles in a solvent of cyclohexane. 

Fauske, H. K, Venting of Runaway Chemical Reactions and Non-Equilibrium Effects, Paper Presented at the 4th Internationa Symposium on Multi-Phase Flow, Miami Beach, EL, December 15-17, 1986. 

Speed-up of mixing is known not only for mixing of miscible liquids, but also for multi-phase systems the mass-transfer efficiency can be improved. As an example, for a gas/liquid micro reactor, a mini packed-bed, values of the mass-transfer coefficient K a were determined to be 5-15 s [2]. This is two orders of magnitude larger than for typical conventional reactors having K a of 0.01-0.08 s . Using the same reactor filled with 50 pm catalyst particles for gas/Hquid/solid reactions, a 100-fold increase in the surface-to-volume ratio compared with the dimensions of laboratory trickle-bed catalyst particles (4-8 mm) is foimd. 

Worz et al. stress a gain in reaction selectivity as one main chemical benefits of micro-reactor operation [110] (see also [5]). They define criteria that allow one to select particularly suitable reactions for this - fast, exothermic (endothermic), complex and especially multi-phase. They even state that by reaching regimes so far not accessible, maximum selectivity can be obtained [110], Although not explicitly said, maximum refers to the intrinsic possibilities provided by the elemental reactions of a process under conditions defined as ideal this means exhibiting isothermicity and high mass transport. 

The selectivity issue has been related to multi-phase processing [31]. Nitrations include both organic and aqueous phases. Oxidation to phenol as one side reaction takes places in the organic phase, whereas all other reactions occur in the aqueous phase and are limited by organic solubility. For this reason, enhancing mass transfer by large specific interfaces is a key to affecting product selectivity. 

The performance of the Sonogashira reaction is claimed to be the first example of a homogeneously metal-catalyzed reaction conducted in a micro reactor [120], Since the reaction involves multi-phase postprocessing which is needed for the separation of products and catalysts, continuous recycling technology is of interest for an efficient production process. Micro flow systems with micro mixers are one way to realize such processing. 

Despite affecting conversion, mass transfer is known to impact enantio- and regioselectivity for many reactions [63]. For this reason, conventional micro-titration apparatus, typically employed in combinatorial chemistry of single-phase reactions, also often suffer from insufficient mixing when dealing with multi-phases [63, 66]. 

The high surface-to-volume ratio can also significantly improve both thermal and mass transfer conditions within micro-channels in two ways firstly, the convective heat and mass transfers, which take place at the multi-phase interface, are improved via a significant increase in heat and mass transfer area per unit volume. Secondly, heat and mass transfers within a small volume of fluid take a relatively short time to occur, enabling a thermally and diffusively homogeneous state to be reached quickly. The improvement in heat and mass transfer can certainly influence overall reaction rates and, in some cases, product selectivity. Perhaps one of the more profound effects of the efficient heat and mass transfer property of micro-reactors is the ability to carry potentially explosive or highly exothermic reactions in a safe way, due to the relatively small thermal mass and rapid dissipation of heat. 

Multiphase copolymers, Ziegler-Natta catalysts for, 26 535, 537-540 Multiphase laminar flow patterning, in microfluidics, 26 961 Multiphase reactions, in microbial transformations, 16 412-414 Multi-phase reactors, 21 333-335 Multiphoton effects, in photochemical technology, 19 109... 

The protection principle of pressure relief is based on limiting the pressure to which equipment might be exposed by the removal of gaseous or multi-phase material flows from, in the case of an explosion or a runaway reaction by allowing certain predetermined openings to be opened in such a way that the pressure in the vessel does not exceed a predetermined permitted value. 

These multi-phase approaches, however, suffer from the drawback that some of the reaction media must be separated from the catalyst or the reaction products. Therefore, the conditions of catalyst separation may be very different from those of the reaction such that catalyst decomposition may still occur. Furthermore, most approaches have two or three phases during the reaction, which may cause problems in controlling the phase equilibria and in controlling the distribution of the reacting components and the catalytically active component between the phases. Most effective agitation of the reaction mixture is often required. 

One of the benefits of SCCO2 for homogeneous catalysis is that rates or se-lectivities may be significantly higher than in other multi-phase systems or in conventional solvents, because mass transfer across interfaces is enhanced. An example is CO2 hydrogenation that simultaneously uses CO2 as both reaction medium and substrate [114]. 

Table 1 Multi-phase catalysis approaches with respect to both, reaction and separation conditions (S substrates, P products)...

Fig. 20 Reaction/separation strategies for multi-phase catalysis in SCCO2...

Fig. 1. Water/rock reaction diagramme illustrates fields of mapped high- and low-temperature isotopic alteration (see text) for rhyolites (R) and andesites (A) districts studied (DBL = Doyon-Bousquet-LaRonde). Solid and dashed curves evolution of rock and water, respectively. Multi-phase reaction history of the central Noranda district, shown schematically, generally corresponds to intrusive history. Approximate of modern high-temperature submarine vent fluid and estimated W/R after Shanks et al. (1995) and Bowers Taylor (1985), respectively.

These are the easiest to understand of all the multi-phase reaction systems and, probably for this reason, have received most attention in the... 

EM plays a crucial role in the development of thermodynamic data, especially for defective solids, multi-phase solids and solids with coexisting intergrowth structures. These microstructural details, which are essential to catalytic properties, cannot be revealed readily by other diffraction methods which tend to average structural information. The formation of anion vacancies in catalytic reactions and the resulting extended defects are described here, from which an improved understanding of the formation of CS planes and their role in catalysis can be obtained. These general results are applicable to other CS structures. 

Besides the effect on the solubility in multi-phase systems, the pressure can also directly increase the reactant concentrations in gas-phase reactions through the compressibility of gases. The reaction-rate increases because of the increased concentrations. The compression... 

Membrane reactors became an option for the retention of biocatalysts when the processing of membrane materials had progressed sufficiently to control thickness and pore structure and to manufacture a membrane that was defect-free. Besides its function as a retainer the membrane also serves other functions such as (i) to stabilize the phase boundary in case of multi-phase reactions (ii) as a consequence of (i), to transport dissolved 02 preferentially over gaseous 02 and (iii) to support purification and sterilization of air and other nutrients in fermentations. 

Multi-phase reactors are of interest in biocatalytic reactions if one or several components of the reaction are insoluble or insufficiently soluble in aqueous phases but if an aqueous phase has to be kept, if only for the biocatalyst. However, a two-phase system can be utilized advantageously for the shifting of an equilibrium this is demonstrated below. We analyze the simple reaction A <=> B in an organic-aqueous two-phase system with the assumption that reactant A and product B partition between the two phases. The partition coefficients Pw and Porg are de-... 

The degree of conversion has to be considerably higher than 50% to obtain the desired product in sufficiently high enantiomeric purity. The incomplete enantio-selectivity of PPL (E = 18) was countered by continuation of the reaction to 67% conversion. The productivity in a multi-phase reactor (Figure 13.24), in which the racemic ester was circulated in the lumen, was 17.6 g (h m2)-1 or 28.4 mmol (h mg enzyme)-1. The main problem of the multi-phase reactor is the low catalyst effectiveness factor, which is normally found to lie between 30 and 50%. 

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