C.M. van den Bleek

Characterization of fluidization regimes by time-series analysis of pressure fluctuations

Abstract This work compares time, frequency and state-space analyses of pressure measurements from fluidized beds. The experiments were carried out in a circulating fluidized bed, operated under ambient conditions and under different fluidization regimes. Interpretation of results in time domain, such as standard deviation of the pressure fluctuations, may lead to erroneous conclusions about the flow regime. The results from the frequency domain (power spectra) and state-space analyses (correlation dimension, D ML , and Kolmogorov entropy, K ML , together with a non-linearity test) of the pressure fluctuations are generally in agreement and can be used complementary to each other. The power spectra can be divided into three regions, a region corresponding to the macro-structure (due to the bubble flow) and, at higher frequencies, two regions representing finer structures that are not predominantly governed by the macro structure of the flow. In all fluidization regimes, the measured pressure fluctuations exhibited an intermittent structure, which is not revealed by power spectral analysis of the original signals. Fluctuations with pronounced peaks in the power spectrum and in the auto-correlation function, corresponding to passage of single bubbles through the bed, are non-linear with a low dimension ( D ML D ML D ML >5.5 both K ML (bits/cycle) and D ML are insensitive to changes in the distribution of energy in power spectra. Thus, the state-space analysis reflects that non-linearity is mostly found in the macro-structure of the flow. Fluidized bed time series treated in this work are available at http://www.entek.chalmers.se/∼fijo

CFD modelling and experimental validation of pressure drop and flow profile in a novel structured catalytic reactor packing

Abstract Packed beds of catalyst particles are normally described using models that contain a number of empirical parameters. The development of computer technology and CFD models makes it tempting to try to (1) fully simulate the flow in packed beds to obtain a more detailed understanding of the physical phenomena that take place in the bed, and (2) to use the CFD solutions to derive ‘simple’ correlations suitable for design purposes. In this paper it is shown that a commercial CFD code (CFX-5.3) can be used to predict, with an average error of about 10%, the pressure drop characteristics of packed beds of spheres that have a tube-to-particle-diameter ratio of 1.00 to 2.00. Packed beds with these unusually low tube-to-particle-diameter ratios can be used as unit cells in a novel type of structured catalytic reactor packing, proposed in this paper, that has very favorable pressure drop characteristics. The error of 10% in the pressure drop prediction by CFD is acceptable for design purposes. The CFD model is also able to predict local velocity profiles that were measured with LDA. The CFD results have been used to fit a simple two-parameter model that describes the experimental pressure drop data with an average error of about 20%. For a grid-independent CFD solution of laminar flow in a packed bed containing only 16 particles, already three million cells are required. However, it is anticipated that within five years from now the simulation of a packed bed containing a few hundred particles will be considered a ‘standard’ problem in terms of memory and calculation time requirements.

Characterization of regimes and regime transitions in bubble columns by chaos analysis of pressure signals

In this study it is shown that the transition from the homogeneous to the heterogeneous flow regime in bubble columns can be quantitatively found with high accuracy by analysing the chaotic characteristics of the pressure fluctuation signal (PFS). In previous work (van den Bleek and Schouten, 1993; Schouten et al., 1996), the authors have already applied this technique to time series from gas-solid fluid beds. Also, it was shown (Krishna et al., 1993, Ellenberger and Krishna, 1994) that hydrodynamics of bubble columns and fluid beds can be described in an analogous manner. Therefore in this work, the method of chaos analysis is applied to bubble columns. A distinctive feature of the pressure signal from bubble columns is that it is composed of two different parts: a low frequency part resulting from the motion of the large bubbles and a high frequency part resulting from all other processes (coalescence, collapse, breakup) that take place in the column. From the phase of the cross spectrum of two pressure probes, placed at different axial positions, it was possible to identify the bands in the spectrum of the PFS that show a significant time delay. This time delay is of the order of the passage time of bubbles between the measurement locations. This band in the spectrum of the PFS was used to estimate the Kolmogorov entropy to quantify the chaotic dynamics in the bubble column. The Kolmogorov entropy as a function of gas velocity indicates a sharp transition from the homogeneous to the churn-turbulent flow regime. From other methods considered (e.g. holdup and other properties of the signal such as variance), this transition was less clear. Therefore chaos analysis of PFSs is believed to be a powerful technique for on-line identification of flow regimes.

Zeolitic coatings and their potential use in catalysis

The formation of zeolitic coatings and their properties, such as the thickness, continuity and orientation of the crystals, are related to the presence and macro-organization of a precursor phase. Based on this view, preshaped zeolitic coatings can be prepared which may be either active catalysts themselves or an inert thin membrane on an existing catalyst. They can be applied in low pressure drop reactors, in adsorption units, in catalytic distillation units and in integrated reaction separation systems as zeolitic membranes. Coating preparation and particular aspects of the performance of the zeolitic coatings in a membrane configuration, in selective catalytic conversion of NOx, as an inert membrane on a catalyst phase, and model structured catalyst systems are discussed.

In situ synthesis of binderless ZSM-5 zeolitic coatings on ceramic foam supports

In situ synthesis of binderless ZSM-5 zeolite coatings on ceramic foam supports was studied. The effects were investigated of the rotation of the autoclaves during synthesis, the reaction mixtures SiO2/Al2O3 and dilution ratios, the reaction mixture-to-support surface area ratio, the synthesis temperature and time as well as the composition of the support material (α-Al2O3, SiC/Al2O3 and ZrO2/Y2O3 foam). X-ray diffraction patterns showed highly crystalline ZSM-5 structures superposed on the ceramic foam carrier pattern. The deposition of ZSM-5 was quantified by measuring mass increase and BET surface of the samples. Uniform and continuous ZSM-5 coatings with overall specific surface areas of up to 200 m2/g were obtained. SEM micrographs showed coffin-like, partly intergrown crystals of 5–10 μm that appeared to be attached firmly to the support.

Fluidization regimes and transitions from fixed bed to dilute transport flow

Characterization by means of Kolmogorov entropy shows that the dynamics of the bottom bed in small size circulating fluidized bed risers are significantly different from the dynamics of the dense bottom bed in large size risers and, as a consequence, two types of circulating regimes are introduced: the exploding bubble bed for large risers and the circulating ‘slugging’ bed for small risers, the latter at high superficial gas velocities. In a pictorial fluidization diagram ten gas—solid fluidization regimes are given, seven of which are experimentally identified with the Kolmogorov entropy by varying the superficial gas velocity, riser solids holdup and diameter (or width) of the riser: bubbling bed, slugging bed, exploding bubble bed, intermediate turbulent bed, circulating ‘slugging’ bed, intermediate dilute flow, and dilute transport flow. No transition could be identified between the exploding bubble bed at captive conditions and the exploding bubble bed at circulating conditions in the dense bottom bed of the two largest facilities in this study. This suggests that the dense bottom bed in large size risers can be considered as a bubbling bed. A turbulent bed was found in none of the facilities of this study with the Geldart B solids used. As well as by the Kolmogorov entropy (chaos analysis), the hydrodynamics have been characterized by amplitude of pressure fluctuations, while a solids distribution analysis has also been carried out. The study has been made in four (circulating) fluidized beds of different size and design, all operated with 0.30 mm silica sand. The dimensions of the fluidized bed risers are 1.47 × 1.42 × 13.5 m, 0.70 × 0.12 × 8.5 m, 0.12 m i.d. × 5.8 m, and 0.083 m i.d. × 4.0 m.

Selective catalytic reduction of NO with NH3 over Fe-ZSM-5 catalysts prepared by sublimation of FeCl3 at different temperatures

Fe-ZSM-5 catalysts were prepared by subliming FeCl3 into H-ZSM-5. The method used allowed Fe-ZSM-5 catalyst preparation by FeCl3 exchange at a desired sublimation temperature and was found to be more precise. The sublimation of FeCl3 into H-ZSM-5 was carried out at 320 and 700 °C. Fe-ZSM-5 prepared by sublimation of FeCl3 at 320 °C followed by rapid heating to 700 °C and the catalyst prepared by subliming FeCl3 at 700 °C were found to be more active for NO reduction with NH3 in the presence of simulated exhaust gases containing water vapor than catalysts prepared by subliming FeCl3 at 320 °C. To determine the active sites, the catalysts were characterized by H2-TPR, in situ DRIFTS of NO adsorption, NH3-TPD, XRD and chemical analysis methods. The observed NO conversion differences in selective catalytic reduction using NH3 could be correlated to the iron cation species present at different locations determined from diffuse reflectance infrared spectroscopy. Enhanced NO reduction activity was obtained when γ positions in Fe-ZSM-5, corresponding to Fe2+(NO) band at 1877 cm-1 in DRIFTS, were preferentially occupied.

Influence of elevated pressure on the stability of bubbly flows

The effect of elevated pressure on the stability of the homogeneous bubbly flow regime in a gas-liquid bubble column is examined. Experiments were performed in a 0.15 m diameter bubble column operated at pressures in the range 0.1–1.3 MPa with nitrogen as the gas phase and water as the liquid phase. The transition from homogeneous to heterogeneous flow regime was determined by two procedures. The first procedure involved visual examination of the swarm velocity vs gas velocity curve to determine the transition point. In the second procedure the transient pressure signals were monitored by high-frequency pressure transducers and the instability point determined by analysis of the chaotic features. The two major findings of the work were: (a) increased system pressure reduces the bubble swarm velocity of the homogeneous dispersion, (b) increased system pressure results in a significant increase in the gas holdup at the instability point. The stability theory of Batchelor (1988, J. Fluid Mech.193, 75–110) and Lammers and Biesheuvel (1996, J. Fluid Mech.328, 67–93) has been used to provide theoretical support to these observations.

A numerical comparison of alternative three-phase reactors with a conventional trickle-bed reactor. The advantages of countercurrent flow for hydrodesulfurization

Abstract A computer model was developed to compare a conventional cocurrent trickle-bed reactor with two novel countercurrent three-phase reactors: the three-levels-of-porosity reactor and the internally finned monolith reactor. The hydrodesulfurization of a vacuum gas oil was taken as a case study. It was found that the application of countercurrent flow in the novel reactors results in significant increase of conversion. Sensitivity for mass transfer and H2S inhibition were investigated. Reduced mass transfer compared to a conventional trickle-bed does not limit the reaction.

The effect of dilution on the degree of conversion in fixed bed catalytic reactors

Abstract Catalyst dilution is often used to effect isothermic kinetic measurements. It is demonstrated that this dilution influences the conversion. A stochastical model is developed quantitatively describing this influence. Furthermore a dilution criterion is introduced which makes it possible to determine the allowable degree of dilution and the minimum amount of catalyst required.