Reactors hydrodynamic plug-flow

A second level of integration (reactor integration) is to run the reactions in a single reactor. This implies that the hydrodynamics (plug flow or well mixed) are the same, but not necessarily the reaction conditions. These could still be separated in time (in a batch reactor) or space (in a continuous reactor). Compartmentalization is also possible, meaning the two catalysts could be separated and replaced at different times as required (Figure 20.5b). 

Concerning the hydrodynamics and the dimensioning of the test reactor, some rules of thumb are a valuable aid for the experimentalist. It is important that the reactor is operated under plug-flow conditions in order to avoid axial dispersion and diffusion limitation phenomena. Again, it has to be made clear that in many cases testing of monolithic bodies such as metal gauzes, foam ceramics, or monoliths used for environmental catalysis, often needs to be performed in the laminar flow regime. 

After the investigation of hydrodynamics and mass transfer, the next step is the examination of the reactor model. For example, let us consider here the two-phase model with plug flow of gas in both bubble and emulsion phase and first-order reaction (see Section 3.8.3). The first step at this stage is to transform its equations to dimensionless forms. 

This study, which contributes towards the understanding of hydrodynamic behaviour of gas-liquid reactors at elevated pressure, has shown the influence of pressure on the gas flow in a packed column through the axial dispersion coefficient. The gas flow diverges from plug flow when the pressure increases. As for the gas hold-up, an important parameter for the calculation of the reactional volume of a reactor, the pressure has no effect on this parameter in the studied range. This result allows to extrapolate gas hold-up values obtained... 

None of the above studies, however, deals with the detailed hydrodynamics in a membrane reactor. It can be appreciated that detailed information on the hydrodynamics in a membrane enhances the understanding and prediction of the separation as well as reaction performances in a membrane reactor. All the reactor models presented in Chapter 10 assume very simple flow patterns in both the tube and annular regions. In almost all cases either plug flow or perfect mixing is used to represent the hydrodynamics in each reactor zone. No studies have yet been published linking detailed hydrodynamics inside a membrane reactor to reactor models. With the advent of CFD, this more complete rigorous description of a membrane reactor should become feasible in the near future. 

The performances of pyrolysis reactors are often con ared on the basis of the fractions of the different recovered products (gases, oils, char). In addition to the maximum biomass throughputs, the comparison criteria usually rely on the values of the residence times and temperatures, two parameters that are often poorly known and defined, and that may be very different for each of the phases involved. The hydrodynamics are often very different from plug flows and generally the true reaction temperatures cannot be accurately measured. 

Exelus has developed a novel structured catalytic system that allows one to meet all four criteria in a single catalytic system Hydrodynamic tests reveal that the HyperCat has similar gas hold-up as a slurry bubble column reactor but with a much lower liquid axial-dispersion coefficient. Cold-flow studies appear to indicate that the heat-transfer coefficient of this new system is similar to a bubble column reactor. Catalyst performance tests reveal that the performance of the HyperCat is similar to that of a powder catalyst when used in a plug-flow reactor. 

The first hydrodynamic model proposed for fluid-bed reactor design (see Davidson and Harrison, 1963) is simple but is the basis of most models developed since. A sketch of the model appears in Figure CS5.1a. Three main groups are involved U for fluidization, for reaction, and Y for mass transfer. Equations can be derived both for plug flow and mixed flow of emulsion gas. The simpler mixed-flow model is usually adequate (with predictions close to those of the plug-flow model) and is given by... 

Proper description of hydrodynamic effects and the momentum balance is often neglected in reactor modeling today. Assumptions of plug flow or perfectly mixed are common and simplify the calculations tremendously. A trend towards full calculation of flow and momentum profiles is starting to take shape in the literature, but it is still hampered by excessive computing times. 

The design of such gas-solid catalytic reactors can be approximated by a pseudo-homogeneous model with gas phase in plug flow. In the case of very exothermic reactions accounting for radial dispersion of heat and mass might be useful to prevent excessive particle overheating. The reaction time must find a compromise with the hydrodynamic design, namely the maximum gas velocity and pressure drop. 

Experiments with laboratory monoliths of small cross-section area can lead to biased results due to an uneven flow distribution in the channels, especially close to the reactor wall. The wash-coat of the outer broken chaimels should be scraped away, and the void between the reactor wall and the monolith should be carefully plugged. To minimize wall effects, the diameter of the monolith should be ten tunes the chaimel diameter at least. Plug flow must prevail in a packed bed of crushed catalyst. The bed length and radius should be more than 50 and 10 particle diameters respectively, the flow resistance of the bed support must be unifonn throughout its cross-section, and the particle size distribution must be as narrow as possible. Otherwise, there can be oy-passes or dead vohunes. These hydrodynamic problems are overcome in a recycle loop reactor because the same physical and chemical conditions prevail everywhere. 

The selectivity and conversion of a chemical reactor depend not only on the kinetics of the reaction but also on the time for which the reaction partners are available for the reaction, that is, the hydrodynamic behavior. By determining the residence-time behavior of a reactor, one can determine its deviation from ideal hydrodynamic behavior (boundary cases plug flow and complete backmixing) and hence decide with which reactor model the real reactor can best be modeled. 

The process of theoretical calculation and reactor design selection, in accordance with process kinetics, is carried out on the basis of idealised mixing and plug flow models, which are determined by the hydrodynamic structure of liquid flows. 

Cellular and diffusion models are usually used to calculate the efficiency of longitudinal mixing (turbulence) in a reactor and the related degree of deviation of the liquid flow hydrodynamic structure from perfect mixing and plug flow modes [4, 38,121-123]. 

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