Tubular multiphase reactors

This review paper is restricted to stirred vessels operated in the turbulent-flow regime and exploited for various physical operations and chemical processes. The developments in the field of computational simulations of stirred vessels, however, are not separated from similar developments in the fields of, e.g., turbulent combustion, flames, jets and sprays, tubular reactors, and multiphase reactors and separators. Fortunately, there is a strong degree of synergy and mutual cross-fertilization between these various fields. This review paper focuses on aspects specific to stirred vessels (such as the revolving impeller, the resulting strong spatial variations in turbulence properties, and the macroinstabilities) and on the processes carried out in them. 

Conclusions. In tubular multiphase reactors with an exothermic reaction where one phase with a high throughput serves to carry the heat of reaction out of the reactor, a sudden flow reduction in this phase (whether accompanied by a similar reduction in the other phases or not) can lead to a considerable transient temperature rise, well above the new steady state temperature. The maximum excess temperature depends in a complex way upon the rate of the flow reduction, the flow rates in the different phases, the heat capacities and the reaction rates of the system. 

Example 9.11 Modeling of a nonisothermal plug flow reactor Tubular reactors are not homogeneous, and may involve multiphase flows. These systems are called diffusion convection reaction systems. Consider the chemical reaction A -> bB described by a first-order kinetics with respect to the reactant A. For a nonisothermal plug flow reactor, modeling equations are derived from mass and energy balances... 

Cini P., and M. P. Harold. Experimental Study of The Tubular Multiphase Reactor . AIChE... 

In spite of this enticing come-on, we will not solve this problem for the moment, being content with its illustration of a typical two-phase reactor balance formulation using the PFR model. We hasten to add, however, that the solution to the set of equations (7-140) and (7-141) with the initial and boundary conditions given is identical to that for the much simpler set of (7-54) and (7-139). In the following sections we shall pursue in detail the developments using the by-now familiar dispersion model for tubular reactors, and in Chapter 8 will treat a number of other multiphase reactor models. 

As in the situation for tank-type reactors, we need first to define the characteristic time quantities associated with the reactor design. The characteristic diffusion time, tj), is given in equation (8-207), and the extent-of-reaction time, is given in equation (8-208). The third time here is tp, the length of time an element of fluid remains in the reactor. This is reminiscent of the exit-age distribution function developed for homogeneous tubular-flow reactors, but the development of the theory for multiphase reactors has been different. " ... 

The tubular multiphase hollow membrane wall reactor briefly described before and sketched in Figure 24.1 h is a multiphase reactor design very similar to the trickle-bed reactor. In a regular trickle-bed reactor, the liquid flows over a partially wetted pellet as a thin film and supplies the liquid-phase reactant to the catalyst pores. This action, however, has the effect of hindering pore access to... 

Tubular reactors, as previously stated, are also advantageous for high-pressure reactions where smaller-diameter cylindrical vessels can be used to allow thinner vessel walls. Tubular reactors should be avoided when carrying out multiphase reactions, since it is often difficult to achieve good mixing between phases. 

A useful classification of lands of reaclors is in terms of their concentration distributions. The concentration profiles of certain limiting cases are illustrated in Fig. 7-3 namely, of batch reactors, continuously stirred tanks, and tubular flow reactors. Basic types of flow reactors are illustrated in Fig. 7-4. Many others, employing granular catalysts and for multiphase reactions, are illustratea throughout Sec. 23. The present material deals with the sizes, performances and heat effects of these ideal types. They afford standards of comparison. 

In a certain range of process conditions, column reactors for multiphase processes behave as a tubular reactor with respect to gaseous reactants and as an ideally mixed tank reactor with respect to condensed phases. 

Chemical Kinetics, Tank and Tubular Reactor Fundamentals, Residence Time Distributions, Multiphase Reaction Systems, Basic Reactor Types, Batch Reactor Dynamics, Semi-batch Reactors, Control and Stability of Nonisotheimal Reactors. Complex Reactions with Feeding Strategies, Liquid Phase Tubular Reactors, Gas Phase Tubular Reactors, Axial Dispersion, Unsteady State Tubular Reactor Models... 

Quite new ideas for the reactor design of aqueous multiphase fluid/fluid reactions have been reported by researchers from Oxeno. In packed tubular reactors and under unconventional reaction conditions they observed very high space-time yields which increased the rate compared with conventional operation by a factor of 10 due to a combination of mass transfer area and kinetics [29]. Thus the old question of aqueous-biphase hydroformylation "Where does the reaction takes place " - i.e., at the interphase or the bulk of the liquid phase [23,56h] - is again questionable, at least under the conditions (packed tubular reactors, other hydrodynamic conditions, in mini plants, and in the unusual,and costly presence of ethylene glycol) and not in harsh industrial operation. The considerable reduction of the laminar boundary layer in highly loaded packed tubular reactors increases the mass transfer coefficients, thus Oxeno claim the successful hydroformylation of 1-octene [25a,26,29c,49a,49e,58d,58f], The search for a new reactor design may also include operation in microreactors [59]. 

Finally, a nonpermselective membrane can be used in multiphase applications. The original idea belongs to Harold and Cini [137]. Their reactor consisted of a supported catalytically active tubular membrane (Pd/y-A Oi on a two-layer a-Al203 porous support) separating the two reactants the gas flows at the tube side (membrane side), the liquid at the shell side (support side). Capillary forces let the liquid penetrate the pores of the... 

Tubular reactors are also used to carry out some multiphase reactions. Wamecke et al. (1999) reported use of a computational flow model to simulate an industrial tubular reactor carrying out a gas-liquid reaction (propylene oxide manufacturing process). In this process, liquid is a dispersed phase and gas is a continuous phase. The two-fluid model discussed earlier may be used to carry out simulations of gas-liquid flow through a tubular reactor. Warnecke et al. (1999) applied such a model to evaluate the influence of bends etc. on flow distribution and reactor performance. The model may be used to evolve better reactor configurations. In many tubular reactors, static mixers are employed to enhance mixing and other transport processes. Computational flow models can also make significant contributions to understanding the role of static mixers and for their optimization. Visser et al. (1999) reported CFD... 

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. 

Multiphase reactions that occur in the presence of a heterogeneous catalyst can be realized using a variety of tubular reactor types that can be classified according to the state of motion of the catalyst. When the catalyst remains stationary inside the tube and has the shape of a... 

The tubular reactor is a vessel through which the flow is continuous. There are several configurations of tubular reactors suitable for multiphase work, e.g. for liquid-solid and gas-liquid-solid compositions. The flow patterns in these systems are complex. A fixed bed reactor is packed with catalyst, typically formed into pellets of some shape, and if the feed is single phase, a simple tubular plug-flow reactor may suffice (Figure 1.1). Mixed component feeds can be handled in modifications to this. 

M. Reif, Tubular inorganic catalytic membrane reactors Advantages and performance in multiphase hydrogenation reactions, Catal. Today 2003, 79-80, 139-149. 

The approach to be followed in the determination of rates or detailed kinetics of the reaction in a liquid phase between a component of a gas and a component of the liquid is, in principle, the same as that outlined in Chapter 2 for gas-phase reactions on a solid catalyst. In general the experiments are carried out in flow reactors of the integral type. The data may be analyzed by the integral or the differential method of kinetic analysis. The continuity equations for the components, which contain the rate equations, of course depend on the type of reactor used in the experimental study. These continuity equations will be discussed in detail in the appropriate chapters, in particular Chapter 14 on multiphase flow reactors. Consider for the time being, by way of example, a tubular type of reactor with the gas and liquid in a perfectly ordered flow, called plug flow. The steady-state continuity equation for the component A of the gas, written in terms of partial pressure over a volume element dV and neglecting any variation in the total molar flow rate of the gas is as follows ... 

Extent of reaction specified Two-phase, chemical equilibrium Multiphase, chemical equilibrium Continuous-stirred tank reactor Plug-flow tubular reactor Pump or hydraulic turbine Compressor or turbine Pressure drop in a pipe Stream multiplier Stream duplicator... 

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