Heterogeneous flow reactions

Solid acid catalysts, such as ion exchange resins, montmorillonites, zeolites, heteropoly acids, and silica gels, are widely used in heterogeneous reactions. Some of them are used in industrial processes and can be employed under the flow systems. In 1932, Karnatz and Whitmore reported dehydration of 3-pentanol with a phospholic acid catalyst on silica gel at high temperature under the flow conditions, which might be the first example of a heterogeneous flow reaction [55]. Although they made clear that the chemical transformation could be completed, the effldency of the reaction was fairly limited. 

Clearly, these groupings are not mutually exclusive. The chief distinctions are between homogeneous and heterogeneous reactions and between batch and flow reactions. These distinctions most influence the choice of equipment, operating conditions, and methods of design. 

If the flow is accompanied with CBA decomposition, the G value in Eq. (5) should be substituted with its time function, G(t). In the general case, thermal decomposition of a solid substance with gas emission is a heterogeneous topochemical reaction [22]. Kinetic curves of such reactions are S -shaped the curves representing reaction rate changes in time pass a maximum. At unchanging temperature, the G(t) function for any CBA is easily described with the Kolrauch exponential function [20, 23, 24] ... 

Equation 8.3.4 may also be used in the analysis of kinetic data taken in laboratory scale stirred tank reactors. One may directly determine the reaction rate from a knowledge of the reactor volume, flow rate through the reactor, and stream compositions. The fact that one may determine the rate directly and without integration makes stirred tank reactors particularly attractive for use in studies of reactions with complex rate expressions (e.g., enzymatic or heterogeneous catalytic reactions) or of systems in which multiple reactions take place. 

For gas phase heterogeneous catalytic reactions, the continuous-flow integral catalytic reactors with packed catalyst bed have been exclusively used [61-91]. Continuous or short pulsed-radiation (milliseconds) was applied in catalytic studies (see Sect. 10.3.2). To avoid the creation of temperature gradients in the catalyst bed, a single-mode radiation system can be recommended. A typical example of the most advanced laboratory-scale microwave, continuous single-mode catalytic reactor has been described by Roussy et al. [79] and is shown in Figs. 10.4 and... 

This study was carried out to simulate the 3D temperature field in and around the large steam reforming catalyst particles at the wall of a reformer tube, under various conditions (Dixon et al., 2003). We wanted to use this study with spherical catalyst particles to find an approach to incorporate thermal effects into the pellets, within reasonable constraints of computational effort and realism. This was our first look at the problem of bringing together CFD and heterogeneously catalyzed reactions. To have included species transport in the particles would have required a 3D diffusion-reaction model for each particle to be included in the flow simulation. The computational burden of this approach would have been very large. For the purposes of this first study, therefore, species transport was not incorporated in the model, and diffusion and mass transfer limitations were not directly represented. 

Malmstrom, M. E., G. Destouni and P. Martinet, 2004, Modeling expected solute concentration in randomly heterogeneous flow systems with multicomponent reactions. Environmental Science and Technology 38,2673-2679. 

Desai, B. and Kappe, C.O. (2005). Heterogeneous Hydrogenation Reactions Using a Continuous Flow High Pressme Device, J. Comb. Chem., 7, 641-643. 

Suppose that the reaction takes place in a slurry bubble column and that it is desirable to work in the heterogeneous flow regime. Moreover, assume that the liquid is batch and well mixed and the flow of the gas approximates the ideal plug flow. 

Hydraulic analysis Using Figure 3.27, the appropriate gas superficial velocity and the column diameter for the heterogeneous flow regime can be selected. An appropriate choice for the reactor diameter and the superficial gas velocity is 0.5 m and 0.1 m/s, respectively. The height to diameter ratio in columns is greater than unity and a value of 5 is reasonable. Therefore, the value of 2.5 m has been selected for the column height. As a result, the reactor volume is equal to 0.49 m3. This volume is occupied by the reaction mixture, which is the gas, the liquid, and the solid phase. 

The most frequently applied technique for in situ MAS NMR investigations of heterogeneously catalyzed reactions under flow conditions is based on the injection... 

The diene cyclization shown in Eqn. 2, has been reported to take place only over RhCl3 and Wilkinson s catalyst (ref. 6). We have found that it also occurs when run over supported Rh catalysts. The heterogeneously catalyzed reaction is particularly sensitive to the nature of the solvent used. With alcohols or other solvents which can adsorb on the catalyst, there is an apparent competition with the adsorption of the double bonds and the cyclization does not take place. In alkane solvents, which do not interact with the catalyst, the reaction occurs with reasonable facility. This cyclization is run routinely at 145°C in a flow system with a decane solution of 5 passing through a small column containing a Rh/Al2Oj catalyst. The product composition was related to the time 5 was in contact with the catalyst. With fast flow rates (short contact times) 6 was the primary product of the reaction but the isomerized species, 7 and 8, were produced when slower flow rates were used. This indicates that 6 was the primary product of the reaction but that it was isomerized over the catalyst to 7 and 8. 

In most of the investigations described below the reaction rates were measured by the circulation flow method proposed in 1950 (4). This method offers a possibility of realizing a steady-state heterogeneous catalystic reaction without any concentration and temperature gradients i.e., it belongs to the group of methods which were called nongradient (5). The scheme of the circulation flow system is shown in Fig. 1. 

The circulation flow method is applicable for the studies of almost any heterogeneous catalytic reaction. At high pressures steel equipment is used instead of glass. The first circulation flow reactor for high pressures was designed by Sidorov (6) the gas mixture is circulated in this reactor by means of steel bellows that are actuated by a rod introduced into the reactor also through bellows, without any packing. 

Frequently, particularly from the viewpoint of the technological application of a heterogeneous catalytic reaction, the conditions of experiments in a flow reactor are characterized by space velocity or contact time values. Space velocity, V, is the ratio to the volume of the catalyst bed of the volume of a gas mixture, reduced to the normal conditions (0°C, 760 Torr), passed through the reactor per hour. If the reaction involves a volume change, inlet and outlet space velocities should be distinguished. The reciprocal of V is of the dimension of time. Contact time ( conventional contact time), rc, is a value proportional to V l. It is defined as the ratio of the catalyst volume to the volume of the gas mixture passed per unit time, the gas volume being not under normal conditions but at temperature and pressure in the reactor. Usually, tc is expressed in seconds. It follows from the definitions given that... 

Fig. 8.19 Schematic diagram of a flow-type reactor for heterogeneous photocatalytic reaction of NO,.

In general, if heterogeneous catalytic reactions are to be conducted isothermally, the reactor design must provide for heat flow to or from the particles of catalyst so as to keep the thermal gradients small. Otherwise, temperatures within the catalyst bed will be non-uniform. The differential reactor and the various forms of the gradientless reactors are advantageous in this regard. 

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. 

Mass transfer is an important issue in the reaction processes in EL-ALRs, especially in the heterogeneous flow regime. The volumetric mass transfer rate is determined... 

Chiral oxazoborolidine systems have been regularly used as catalysts for the enantioselective Diels-Alder reaction [173]. Such a catalyst has been immobilized by copolymerization of a sulfonamide-modified styrene monomer to produce polystyrene beads and subsequently reacted with borane to furnish catalyst 49 (Scheme 4.77). A column of 49 was cooled to —30 °C and a 1 1.5 methylacrolein cyclopentadiene solution was flowed in. Following aqueous workup and column chromatography, the desired product 50 was isolated to yield 138 mmol of product in 95% yield and 71% ee by using only 5.7 mmol of catalyst. This result was found to be comparable with the heterogeneous batch reactions that were also attempted [174]. 

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