Hydrodynamics 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). 

The UASB tractor was modeled by the dispensed plug flow model, considering decomposition reactions for VFA componaits, axial dispersion of liquid and hydrodynamics. The difierential mass balance equations based on the dispersed plug flow model are described for multiple VFA substrate components considaed... 

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. 

DESIGNER also contains different hydrodynamic models (e.g., completely mixed liquid-completely mixed vapor, completely mixed liquid-vapor plug flow, mixed pool model, eddy diffusion model) and a model library of hydrodynamic correlations for the mass transfer coefficients, interfacial area, pressure drop, holdup, weeping, and entrainment that cover a number of different column internals and flow conditions. 

Knowledge of the hydrodynamics of liquid flow and particle movement are required for scale-up and optimization of expanded-bed processes. Residence time distribution (RTD) analysis i.e., a plot of the dimensionless tracer concentration in the effluent stream versus the dimensionless time, can determine whether the liquid flow in the expanded bed is plug flow or well mixed. Using the method described by Levenspiel,6 values of mean residence time in the expanded bed (t), the dimensionless variance of the RTD curve,... 

Determinations of Peclet number were carried out by comparison between experimental residence time distribution curves and the plug flow model with axial dispersion. Hold-up and axial dispersion coefficient, for the gas and liquid phases are then obtained as a function of pressure. In the range from 0.1-1.3 MPa, the obtained results show that the hydrodynamic behaviour of the liquid phase is independant of pressure. The influence of pressure on the axial dispersion coefficient in the gas phase is demonstrated for a constant gas flow velocity maintained at 0.037 m s. 

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... 

Hydrodynamic regime is close to a plug-flow. Segregation of gas is strong. 

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. 

Pr = 0 and Pr —> oo. Pr = 0 in this case means that the viscosity vanishes, but the thermal conductivity is finite. As no friction forces act in the fluid, the velocity at the inlet remains constant. This type of flow is known as plug flow. In the limiting case Pr —> oo, because the viscosity is large in comparison to the thermal diffusivity, the flow is hydrodynamically but not thermally fully developed. At the limit Pr —> oo equation (3.258) yields Nume/Num = 1. 

---