J. Ruud van Ommen

Response characteristics of probe-transducer systems for pressure measurements in gas-solid fluidized beds: how to prevent pitfalls in dynamic pressure measurements

Abstract It is long known already that the pressure probe–transducer systems applied in gas–solid fluidized beds can distort the measured pressure fluctuations. Several rules of thumb have been proposed to determine probe length and internal diameter required to prevent this. Recently, Xie and Geldart [H.-Y. Xie, D. Geldart, Powder Technol. 90 (1997) 149] proposed 4 mm i.d. probes as a panacea for all practical situations encountered. However, almost no information is available in the literature that relates possible distortions to characteristics to be extracted from the pressure signal. This paper reports the influence of probe dimensions on the outcomes of different data analysis methods for fluidized bed pressure signals (spectral analysis, statistical analysis, and chaos analysis). It reviews the most important probe–transducer models and compares them on the basis of experiments with both noisy (i.e., highly turbulent gas phase) pressure time-series, and pressure time-series measured in a bench-scale fluidized bed. The comparison is carried out by determining the frequency response function in the frequency domain. It is shown, that the Bergh and Tijdeman model [H. Bergh, H. Tijdeman, Theoretical and experimental results for the dynamic response of pressure measuring systems, Report NLR-TR F.238, National Aero- and Astronautical Research Institute, Amsterdam, the Netherlands, 1965] is superior to all other models reported in literature. The Bergh and Tijdeman model, originally developed for wind-tunnel testing, is the only model that gives a good prediction of the frequency response characteristics of a probe–transducer system for a wide range of probe dimensions. In this paper, rules of the thumb supported by this model will be given. It is found that for statistical analysis and chaos analysis, probes up to 2.5 m length with an internal diameter ranging from 2 to 5 mm do not severely effect the analysis results, since these are mainly focused on frequencies up to about 20 Hz. However, in general, it is preferable to keep the probe length as short as possible. In the case of spectral analysis, the demands on the probe dimensions depend on the frequency range of interest: if one is interested in a frequency range up to 200 Hz (e.g., when studying the power-law fall-off in the power spectral density), the probe length should be limited to about 20 cm. The results reported in this paper are obtained using a transducer with an internal volume of 1500 mm3, but it is shown that the conclusions on the probe dimensions are valid for a wide range of transducer volumes. The experiments are carried out in an 80-cm i.d. bench-scale fluidized bed of sand (median diameter 470 μm, Geldart type B); for smaller particles and smaller scale installations, the frequency range of interest will shift to higher frequencies. In that case, the optimal probe diameter stays in the range from 2 to 5 mm, but it will become even more important to keep the probe length limited; this can be calculated with the Bergh and Tijdeman model [H. Bergh, H. Tijdeman, Theoretical and experimental results for the dynamic response of pressure measuring systems, Report NLR-TR F.238, National Aero- and Astronautical Research Institute, Amsterdam, the Netherlands, 1965]. The experiments presented in this paper are carried out at ambient pressure and temperature. However, since the Bergh and Tijdeman model contains no fitted parameters, it is expected to give a reliable estimate for the probe–transducer characteristics at other operating conditions as well; the effect of the temperature is shown in this paper.

Fischer–Tropsch reaction–diffusion in a cobalt catalyst particle: aspects of activity and selectivity for a variable chain growth probability

The reaction–diffusion performance for the Fischer–Tropsch reaction in a single cobalt catalyst particle is analysed, comprising the Langmuir–Hinshelwood rate expression proposed by Yates and Satterfield and a variable chain growth parameter α, dependent on temperature and syngas composition (H2/CO ratio). The goal is to explore regions of favourable operating conditions for maximized C5+ productivity from the perspective of intra-particle diffusion limitations, which strongly affect the selectivity and activity. The results demonstrate the deteriorating effect of an increasing H2/CO ratio profile towards the centre of the catalyst particle on the local chain growth probability, arising from intrinsically unbalanced diffusivities and consumption ratios of H2 and CO. The C5+ space time yield, a combination of catalyst activity and selectivity, can be increased with a factor 3 (small catalyst particle, dcat = 50 μm) to 10 (large catalyst particle, dcat = 2.0 mm) by lowering the bulk H2/CO ratio from 2 to 1, and increasing temperature from 500 K to 530 K. For further maximization of the C5+ space time yield under these conditions (H2/CO = 1, T = 530 K) it seems more effective to focus catalyst development on improving the activity rather than selectivity. Furthermore, directions for optimal reactor operation conditions are indicated.

Use of stress fluctuations to monitor wet granulation of powders

Abstract Control of granulation processes is usually achieved by measuring the torque and/or power draw of the mixer where agglomeration takes place. It is current industrial practice to determine the endpoint of granulation at a preset value of the torque or of the total electric power absorbed by the motor. While simple and easy to implement, this kind of control does not take into account explicitly the evolution of sizes during granule formation and growth. A somewhat different type of granulation monitoring is to measure stresses in the agglomerating mass by introducing a probe into the system. This method, while more direct, does not give significantly different results from torque measurements. The method proposed in the present work is based on direct measurements from a sensing element placed in the powder. It uses, however, stress fluctuations as input, instead of average stresses. The input signal is subsequently analyzed by applying Fourier transformations to get a qualitative understanding, while a quantitative analysis is carried out using a statistical method specially developed for this purpose. The statistics are based on the comparison of a reference time series of stress fluctuations with an ongoing time series measured during granulation. In this way, continuous monitoring (and control) can be achieved that takes granule growth dynamics into account. To demonstrate the feasibility of the method, granulation experiments were performed in a fluidized bed in which shear is induced by a rotating, rough inner cylinder. The experimental system (described in detail in Apicella et al. [AIChE J. 43 (1997) 1362.]), also known as a Fluidized Couette Device (FCD), consists of two concentric cylinders with particles in the annulus. The main advantage of the FCD is the constant shear distribution generated within the powder mass. Some model experiments were also performed using a fine powder into which large particles of known size were introduced that replaced actual granules. Comparison of stress fluctuation signals from model experiments and from actual granulations with small and large aggregates showed good reproducibility and a correlation between runs with similar size granules.

The role of the hydrogen bond in dense nanoparticle–gas suspensions

The effect of surface characteristics on the interaction between nanoparticles and their agglomeration in dense gas suspensions is still not fully understood. It is known that when the surface is covered with hydroxyl groups, the interaction between nanoparticles becomes substantially stronger than in the absence of these groups; this strengthening is typically attributed to the formation of capillary bridges between the particles. However, this work shows that part of the increase of the interaction is due to the direct hydrogen bonds formed between the surfaces of the polar particles. Dry nitrogen was used to fluidize polar (hydrophilic) and apolar (hydrophobic) SiO2, TiO2 and Al2O3 particles, with a size ranging from 13 to 21 nm. The dry polar particles showed smaller bed expansion and larger minimum fluidization velocity compared to their apolar counterparts, indicating stronger interparticle forces. The results show the importance of including the formation of hydrogen bonds in the modeling of the interaction between dry and polar nanoparticles.

Multidimensional Nature of Fluidized Nanoparticle Agglomerates

We show that fluidized nanoparticle agglomerates are hierarchical fractal structures with three fractal dimensions: one characterizing sintered aggregates formed during nanoparticle synthesis, one that is also found in stored agglomerates and represents unbroken agglomerates, and one describing the large agglomerates broken during fluidization. This has been possible by using spin-echo small-angle neutron scattering-a relatively novel technique that, for the first time, allowed to characterize in situ the structure of fluidized nanoparticle agglomerates from 21 nm to ∼20 μm. The results show that serial agglomeration mechanisms in the gas phase can generate nanoparticle clusters with different fractal dimensions, contradicting the common approach that considers fluidized nanoparticle agglomerates as single fractals, in analogy to the agglomerates formed by micron-sized particles. This work has important implications for the fluidization field but also has a wider impact. Current studies deal with the formation and properties of clusters where the building blocks are particles and the structure can be characterized by only one fractal dimension. However, fluidized nanoparticle agglomerates are low-dimensional clusters formed by higher-dimensional clusters that are formed by low-dimensional clusters. This multifractality demands a new type of multiscale model able to capture the interplay between different scales.

Scale-Up Study of a Multiphase Photocatalytic Reactor—Degradation of Cyanide in Water over TiO2

This paper provides an integrated view on various aspects of reactor design for photocatalytic reactions and presents a scale-up study of photocatalytic reactors. This study focuses on degrading organic pollutants in the effluent of an integrated gasification coal combustion plant over TiO2, with the target of degrading cyanide to below its allowable emission threshold set by European legislation. Here, we show the interplay of different efficiencies that affect the overall apparent photonic efficiency and the reactor volume required to achieve a certain objective in conversion. The chosen reactor configuration is rectangular slurry-bubble-columns-in-series to ensure a good mass transfer rate per photoreactor while approaching plug-flow behavior as a sum, and a high reactor surface-area-to-volume ratio for a good capture of incident photons. We consider a simple 1D photonic description of a photoreactor, in the direction of incident solar light, and implement a bidirectional scattering model for photocatalytic particles and bubbles to calculate the local rate of photon absorption and the photon absorption efficiency in the photoreactor. We show that, implementing the principles of process intensification, the large scale degradation of cyanide to below European emission limits is achievable.

Selectivity of the Fischer–Tropsch process: Deviations from single alpha product distribution explained by gradients in process conditions

Non-ASF product distributions in Fischer–Tropsch synthesis can occur with a gradient in process conditions at the particle or reactor scale, leading to a gradient in chain growth probability α. Weighted summation of local product distributions gives the proper, non-ASF product distribution.

A model to estimate the size of nanoparticle agglomerates in gas−solid fluidized beds

The estimation of nanoparticle agglomerates’ size in fluidized beds remains an open challenge, mainly due to the difficulty of characterizing the inter-agglomerate van der Waals force. The current approach is to describe micron-sized nanoparticle agglomerates as micron-sized particles with 0.1–0.2-μm asperities. This simplification does not capture the influence of the particle size on the van der Waals attraction between agglomerates. In this paper, we propose a new description where the agglomerates are micron-sized particles with nanoparticles on the surface, acting as asperities. As opposed to previous models, here the van der Waals force between agglomerates decreases with an increase in the particle size. We have also included an additional force due to the hydrogen bond formation between the surfaces of hydrophilic and dry nanoparticles. The average size of the fluidized agglomerates has been estimated equating the attractive force obtained from this method to the weight of the individual agglomerates. The results have been compared to 54 experimental values, most of them collected from the literature. Our model approximates without a systematic error the size of most of the nanopowders, both in conventional and centrifugal fluidized beds, outperforming current models. Although simple, the model is able to capture the influence of the nanoparticle size, particle density, and Hamaker coefficient on the inter-agglomerate forces.

Continuous production of nanostructured particles using spatial atomic layer deposition

In this paper, the authors demonstrate a novel spatial atomic layer deposition (ALD) process based on pneumatic transport of nanoparticle agglomerates. Nanoclusters of platinum (Pt) of ?1?nm diameter are deposited onto titania (TiO2) P25 nanoparticles resulting to a continuous production of an active photocatalyst (0.12–0.31?wt. % of Pt) at a rate of about 1?g min?1. Tuning the precursor injection velocity (10–40?m s?1) enhances the contact between the precursor and the pneumatically transported support flows. Decreasing the chemisorption temperature (from 250 to 100?°C) results in more uniform distribution of the Pt nanoclusters as it decreases the reaction rate as compared to the rate of diffusion into the nanoparticle agglomerates. Utilizing this photocatalyst in the oxidation reaction of Acid Blue 9 showed a factor of five increase of the photocatalytic activity compared to the native P25 nanoparticles. The use of spatial particle ALD can be further expanded to deposition of nanoclusters on porous, micron-sized particles and to the production of core–shell nanoparticles enabling the robust and scalable manufacturing of nanostructured powders for catalysis and other applications.

Monitoring fluidization dynamics for detection of changes in fluidized bed composition and operating conditions

In many industrial applications of gas-solids fluidized beds, it is worthwhile to have an on-line monitoring method for detecting changes in the hydrodynamics of the bed (due for example to agglomeration) quickly. In this paper, such a method, based on the short-term predictability of fluidized bed pressure fluctuations, is examined. Its sensitivity is shown by experiments with small step changes in the superficial gas velocity and by experiments with a gradual change in the particle size distribution of the solids in the bed. Furthermore, it is demonstrated that the method is well able to indicate if a stationary hydrodynamic state has been reached after a change in the particle size distribution (a ‘grade change’).