Multiphase reactors temperature

Having considered reactor temperature and pressure, we are now in a position to judge whether the reactor phase will be gas, liquid, or multiphase. Given a free choice between gas- and liquid-phase reactions, operation in the liquid phase is usually preferred. Consider the single reaction system from Eq. (2.19) ... 

Emulsion Polymerization in a CSTR. Emulsion polymerization is usually carried out isothermally in batch or continuous stirred tank reactors. Temperature control is much easier than for bulk or solution polymerization because the small (. 5 Jim) polymer particles, which are the locus of reaction, are suspended in a continuous aqueous medium as shown in Figure 5. This complex, multiphase reactor also shows multiple steady states under isothermal conditions. Gerrens and coworkers at BASF seem to be the first to report these phenomena both computationally and experimentally. Figure 6 (taken from ref. (253)) plots the autocatalytic behavior of the reaction rate for styrene polymerization vs. monomer conversion in the reactor. The intersection... 

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. 

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. 

Compared to fixed bed multiphase reactors additional modes of operation are available with slurry reactors as also the solid phase can be feeded continuously and eventually recirculated temperature control can be assured by internal cooling coils or/ and external circulation through heat exchangers (for the gas as well as for the liquid phase). Furthermore also partial evaporation and external condensation with recirculation of the liquid phase can support the cooling. 

Similarly, oxidation reactions are also highly exothermic. If the reaction selectivity is affected by temperature, temperature control must be given due importance. Most gas-dispersed multiphase reactors yield fairly high heat transfer coefficients ( 400W/ m °C). However, as the reactor is scaled up, the heat transfer area may not be sufficient and additional heat transfer area must be provided. As an example, heat transfer area per unit volume for kettle-type reactors is given by (jacket heat transfer area/volume) = (4 X or (4/7). Most large stirred reactors face this problem... 

Geometrically similar systems are thermally similar when the corresponding temperature differences bear a constant ratio to one another and when the systems if moving are also kinematically similar. For multiphase reactors, an additional requirement will be that of hydrodynamic regime and turbulence similarity since the turbulence structure decides the rate of heat transfer. 

The process has been briefly described by Turunen (1997) as an example of process intensification activities. Most of the hydrogen peroxide production is nowadays based on the anthraquinone method. The differences between the technologies include mainly differences in solvents, catalysts and equipment types and details. The process has less than ten main unit operations including two multiphase reactors, liquid-liquid extraction, gas desorption, distillation and filtration. The process conditions do not include high temperatures or pressures. The necessary properties are not readily available from literature because of the large number of components in the process liquid. However, the measurement of the most of the properties is relatively easy because of the mild conditions. The number of components which take part in the main production reactions and separation steps is small. Therefore it was possible to develop reliable models for most of the unit operations and to base the design on these nnodels. However, the side reactions and by-products involve complicated chemistry and... 

Optimal reactor design is critical for the effectiveness and economic viability of AOPs. The WAO process poses significant challenges to chemical reactor engineering and design, due to the (i) multiphase nature of WAO reactions (ii) temperatures and pressures of the reaction and (iii) radical reaction mechanism. In multiphase reactors, complex relationships are present between parameters such as chemical kinetics, thermodynamics, interphase/intraphase intraparticle mass transport, flow patterns, and hydrodynamics influencing reactant mass transfer. Complex models of WAO are necessary to take into account the influence of catalyst wetting, the interface mass-transfer coefficients, the intraparticle effective diffusion coefficient, and the axial dispersion coefficient. " ... 

On occasion, a reaction takes place in more than one phase of a multiphase reactor. An example is the so-called catalytic combustion. If the temperature is high enough, a hydrocarbon fuel such as propane can be oxidized catalyticaUy, on the surface of a heterogeneous catalyst, at the same time that a homogeneous oxidation reaction takes place in the gas phase. This situation calls for two separate definitions of the reaction rate, one for the gas phase and the other for the heterogeneous catalyst. 

Why are the CSTRs worth considering at all They are more expensive per unit volume and less efficient as chemical reactors (except for autocatalysis). In fact, CSTRs are useful for some multiphase reactions, but that is not the situation here. Their potential justification in this example is temperature control. BoiUng (autorefrigerated) reactors can be kept precisely at the desired temperature. The shell-and-tube reactors cost less but offer less effective temperature control. Adiabatic reactors have no control at all, except that can be set. 

Direct measurement of particle velocity and velocity fluctuations in fluidized beds or riser reactors is necessary for validating multiphase models. Dudukovic [14] and Roy and Dudukovic [28] have used computer-automated radioactive particle tracking (CARPT) to foUow particles in a riser reactor. From their measurements, it was possible to calculate axial and radial solids diffusion as well as the granular temperature from a multiphase KTGF model. Figure 15.10 shows one such measurement... 

Fig. 5.4.1 Schematic drawing of the NMR-compatible multiphase catalytic reactor for MRI studies at elevated temperatures ...

As discussed in previous chapters, the phase behavior with changing temperature and pressure may be strongly influenced by small concentration gradients in multi-component systems already. Therefore, experimental control should take this into account. It is a common practice to use reactors with glass or sapphire windows. The transition of an inhomogeneous multiphase system to a homogeneous one can be observed visually as cloud point (Sect. 2.2, with the pressure and temperature values being monitored. 

---