In multiphase reactors

Flow Regimes in Multiphase Reactors. Reactant contacting, product separations, rates of mass and heat transport, and ultimately reaction conversion and product yields are strong functions of the gas and Hquid flow patterns within the reactors. The nomenclature of commonly observed flow patterns or flow regimes reflects observed flow characteristics, ie, armular, bubbly, plug, slug, spray, stratified, and wavy. 

The several industrial applications reported in the hterature prove that the energy of supersonic flow can be successfully used as a tool to enhance the interfacial contacting and intensify mass transfer processes in multiphase reactor systems. However, more interest from academia and more generic research activities are needed in this fleld, in order to gain a deeper understanding of the interface creation under the supersonic wave conditions, to create rehable mathematical models of this phenomenon and to develop scale-up methodology for industrial devices. 

Mathpati, C.S. and Joshi, J.B. (2007) Insight into theories of heat and mass transfer at the solid/fluid interface using direct numerical simulation and large eddy simulation. Joint 6th International Symposium on Catalysis in Multiphase Reactors/5th International Symposium on Multifunctional Reactors (CAMURE-6/ISMR-5-), 2007, Pune. 

Fuel industry is of increasing importance because of the rapidly growing energy needs worldwide. Many processes in fuel industry, e.g. fluidized catalytic cracking (FCC) [1], pyrolysis and hydrogenation of heavy oils [2], Fischer-Tropsch (FT) synthesis [3,4], methanol and dimethyl ether (DME) synthesis [5,6], are all carried out in multiphase reactors. The reactors for these processes are very large in scale. Unfortunately, they are complicated in design and their scale-up is very difflcult. Therefore, more and more attention has been paid to this field. The above mentioned chemical reactors, in which we are especially involved like deep catalytic pyrolysis and one-step synthesis of dimethyl ether, are focused on in this paper. 

Advances in multiphase reactors for fuel industry are discussed in this work. Downer reactors have some advantages over riser reactors, but suffer from some serious shortcomings. The coupled reactors can fully utilize the advantages of the riser and the downer. For fuel industry that involves gas-liquid-solid system, slurry bed reactors especially airlift reactors are preferred due to their performance of excellent heat control and ease of seale up. For high-pressure processes, the spherical reactor is promising due to its special characteristics. 

Mixing by mechanical agitation is almost always necessary in multiphase reactors... 

Microlevel. The starting point in multiphase reactor selection is the determination of the best particle size (catalyst particles, bubbles, and droplets). The size of catalyst particles should be such that utilization of the catalyst is as high as possible. A measure of catalyst utilization is the effectiveness factor q (see Sections 3.4.1 and 5.4.3) that is inversely related to the Thiele modulus (Eqn. 5.4-78). Generally, the effectiveness factor for Thiele moduli less than 0.5 are sufficiently high, exceeding 0.9. For the reaction under consideration, the particles size should be so small that these limits are met. 

Abdallah, R., Caravieilhes, S., Grenouillet, P., de Bellefon, C., Proceedings of 4th International Symposium on Catalysis in Multiphase Reactors, Lausanne, September 22-25, 2002, p. 43. 

In multiphase reactors we frequently exploit the density differences between phases to produce relative motions between phases for better contacting and higher mass transfer rates. As an example, in trickle bed reactors (Chapter 12) liquids flow by gravity down a packed bed filled with catalyst, while gases are pumped up through the reactor in countercurrent flow so that they may react together on the catalyst surface. 

Catalytic reactors are multiphase if sohd catalysts are used as discussed in Chapter 7. Reactions that form or decompose sohds were discussed in Chapter 9, and many polymerization reactors are multiphase, as discussed in Chapter 11. Biological reactions usually take place in multiphase reactors. 

Surface tension can be very important in deternhning drop and bubble sizes and shapes. This ultimately controls the size of drops and the breakup of films and drops. The presence of surface active agents that alter the interfacial tension between phases can have enormous influences in multiphase reactors, as does the surface tension of sohds and the wetting between solids and liquids. 

The mass balances [Eqs. (Al) and (A2)] assume plug-flow behavior for both the gas/vapor and liquid phases. However, real flow behavior is much more complex and constitutes a fundamental issue in multiphase reactor design. It has a strong influence on the reactor performance, for example, due to back-mixing of both phases, which is responsible for significant effects on the reaction rates and product selectivity. Possible development of stagnant zones results in secondary undesired reactions. To ensure an optimum model development for CD processes, experimental studies on the nonideal flow behavior in the catalytic packing MULTIPAK are performed (168). 

Krishna, R. (1993). Analogies in Multiphase Reactor Hydrodynamics. In Encyclopedia of Fluid Mechanics. Supplement 2. Advances in Multiphase Flow. Ed. N. P. Cheremisinoff. Houston Gulf Publishing. 

Another classification of chemical reactors is according to the phases being present, either single phase or multiphase reactors. Examples of multiphase reactors are gas liquid, liquid-liquid, gas solid or liquid solid catalytic reactors. In the last category, all reactants and products are in the same phase, but the reaction is catalysed by a solid catalyst. Another group is gas liquid solid reactors, where one reactant is in the gas phase, another in the liquid phase and the reaction is catalysed by a solid catalyst. In multiphase reactors, in order for the reaction to occur, components have to diffuse from one phase to another. These mass transfer processes influence and determine, in combination with the chemical kinetics, the overall reaction rate, i.e. how fast the chemical reaction takes place. This interaction between mass transfer and chemical kinetics is very important in chemical reaction engineering. Since chemical reactions either produce or consume heat, heat removal is also very important. Heat transfer processes determine the reaction temperature and, hence, influence the reaction rate. 

In multiphase reactors, the dispersed phase moves in one of two characteristic regimes, depending upon the nature of dispersion. The two regimes are homogeneous and heterogeneous. These regimes are commonly known as particulate and aggregative, respectively. 

A unified approach has been developed for the prediction of transition in multiphase reactors such as gas-hquid bubble columns, liquid-liquid spray columns, solid-liquid fluidized beds, gas-solid fluidized beds, and three-phase fluidized beds. 

Larachi, F., Chaouki, J., and Kennedy, G., 3-D mapping of solids flow fields in multiphase reactors with RPT. AIChE. J. 41(2), 439 (1995). 

F. Luck, M. Djafer, and M.M. Bourbigot, Catalytic wet air oxidation of biosolids in a monolithic reactor. Proceedings of the European Symposium on Catalysis in Multiphase Reactors, Lyon, France, 7-9 December 1994. 

P. Purwanto, H. Delmas, Paper presented at the symposium Catalysis in Multiphase Reactors, Lyon, France, Dec. 1994 Catal Today 1995, 24, 135. 

Rigby, G.D., Evans, G.M. and Jameson, G.J. (1997), Bubble breakup from ventilated cavities in multiphase reactor, Chem. Eng. Sci., 52, 3677-3684. 

Ranade, V.V. and Joshi, J.B. (1987), Transport phenomena in multiphase reactors. Proceedings of International Symposium on Transport Phenomena in Multiphase Systems, BHU Press, Varanasi, pp. 113-196. 

Jiang, Y., Khadilkar, M.R., Al-Dahhan, M.H., Dudukovic, M.P. etal. (2000a), CFD modeling of multiphase flow distribution in catalytic packed-bed reactors scale down issues, presented at 3rd International Symposium in Catalysis in Multiphase Reactors, Naples (Italy) also published in Catalyst Today, 66, 209-218 (2001). 

In multiphase reactor simulations the zero- and two equation models are very popular, only a few attempts have been made to implement Re3molds Stress Models, and LES models are still really rare. 

In the subsequent sections the averaging procedures most frequently used in multiphase reactor modeling are examined. Hence it follows that the basic principles of averaging are presented with emphasis on disperse two phase systems. 

In multiphase reactor flow simulations the impacts of the history forces are normally neglected as the present understanding of these phenomena is far from complete. 

In multiphase reactor simulations, the momentum, heat and species mass balances may thus contain several terms that can be placed in this framework. However, in these problem solvers, the generalized proportionality coefficient K is usually derived from the steady drag force, and the convective heat and mass transfer fluxes, respectively. 

The linear interpolation approach cannot accurately treat the abrupt changes of diffusivity that may occur in some locations in multiphase reactors where the phase fractions change rapidly (e.g., across the transition zone from the dense bed to the freeboard section in a bubbling fluidized bed). To improve the interpolation procedure, one seeks an approximation of the diffusivity that gives an accurate approximation of the diffusive flux for cases with large changes in the material properties. 

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P. H. M. R. Cramers, R. F. Duveen and L. L van Dierendonck, Process Intensification with Buss loop reactors , Proc. Eur. Symp. on Catalysis in Multiphase Reactors, 1994. 

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