H. de Lasa

A novel calibration procedure for a fiber optic solids concentration probe

Optical fiber probes for the measurement of local solids concentration must be accurately calibrated with defined limits and, in such a way, optimized for the flow densities to be measured. In this study, a multi-fiber optical reflection probe was uniquely calibrated in a downer to obtain quantitatively precise solids holdup. An iteration procedure was utilized to modify the initial calibration curves, which was verified both theoretically and practically. Calibrations using two different batches of FCC particles, with different carbon contents, indicate that these probes are sensitive to minor variations of particle colour and reflective properties; however, the characteristic shape of the calibration curves remains unchanged and a simple linear relationship exists between the curves from different batches of FCC particles.

Experimental evaluation of photon absorption in an aqueous TiO2 slurry reactor

This study presents an experimental evaluation of photon absorption in a TiO2 slurry medium using an annular photoreactor. This photoreactor was equipped with several windows placed at equidistant axial positions. Tubular black collimators and inner polished-aluminum collimators were attached to these windows to measure the total transmitted radiation and the transmitted non-scattered radiation. Experiments were developed with six TiO2 commercial powders having differences in particle size, agglomerate particle size in water suspension, and specific surface area. Modeling allowed to establish that the forward-transmitted radiation can be represented by the difference of two exponential decay functions accounting respectively for the total transmitted radiation and the transmitted non-scattered radiation. These two exponentials are functions of particle concentration and extinction coefficients, with the extinction coefficient for the transmitted radiation being strongly affected by the particle agglomerate size.

Diffusion and catalytic cracking of 1,3,5 tri-iso-propyl-benzene in FCC catalysts

Abstract The present study describes catalytic cracking experiments developed in a novel CREC Riser Simulator using 1,3,5-Tri-iso-propyl-benzene and two FCC catalysts with different crystal sizes (0.4 and 0.9 μm diameter). The experiments are modeled using an unsteady state model for both gas and catalyst phases. It is found that a quasi-steady state approximation can be used for the catalyst and changes in the gas phase can be accounted, under the allowed model simplifications, with a relatively simple unsteady state equation. The model is completed using two catalytic decay models, with one of them involving a decay function based on “reactant converted”. Experimental and modeling observations point towards an overall cracking reaction rate controlled by diffusion at 350–450°C with this rate shifting to one being controlled by the intrinsic cracking reaction at 500–550°C.

ZnOCr2O3 + ZSM-5 catalyst with very low Zn/Cr ratio for the transformation of synthesis gas to hydrocarbons

Abstract Three Zn Cr mixed oxide plus ZSM-5 compound catalysts with different compositions covering a wide range of Zn/Cr atomic ratios (0.064 to 1.913), aimed at the direct conversion of synthesis gas into hydrocarbons, were tested under different experimental conditions: temperature ranged from 356 to 410°C, pressure from 3.60 to 4.49 MPa and space velocity from 0.2 to 3.0 mmol reactants/(g cat min). Reaction products included carbon dioxide, water and hydrocarbons with methanol conversion (through which hydrocarbons are formed) being complete. The catalyst with the least content of zinc gave the highest yields of liquid hydrocarbons (up to 74% of total hydrocarbons). Different crystalline phases (ZnO, ZnCr 2 O 4 and Cr 2 O 3 ) were found in the methanol synthesis component as a function of Zn/Cr ratio. The low Zn/Cr catalyst was characterized by X-ray diffraction, differential thermal analysis, nitrogen adsorption, temperature-programmed reduction, Infrared and X-ray photoelectron spectroscopy. The only phases observed in this catalyst were ZnCr 2 O 4 and Cr 2 O 3 . Calcination temperature had an influence in both physical and chemical catalyst properties. After calcination, Cr VI and Cr III species could be seen on the catalyst surface, but only Cr III species were observed after reaction or reduction. The evidences gathered suggest that Cr 2 O 3 is mainly responsible for methanol synthesis, while ZnCr 2 O 4 contributes to increase the specific surface area of the catalyst and influences gas product distributions. Compound catalysts with the mixed oxide (Zn Cr) as the methanol synthesis component showed to be more active than those with the individual (Cr or Zn) oxides.

Photo-catalytic conversion of air borne pollutants: Effect of catalyst type and catalyst loading in a novel photo-CREC-air unit

Abstract This study reports the design, the development and the application of a novel photo-CREC-air reactor. This photo-catalytic unit operates in a batch mode and features a basket with immobilized TiO 2 . The TiO 2 is supported on a fiberglass mesh and is illuminated externally by eight Pen-Ray ® lamps housed in parabolic reflectors. Flow patterns in the illuminated section show good contact between the treated air stream and the photo-catalyst. Two photo-catalysts, Degussa P25 and Hombikat UV-100, were employed with the catalytic activity being tested using acetone as a model pollutant. Special emphasis was assigned to the determination of the influence of several operating parameters including catalyst type, catalyst loading and catalyst attachment to the support. Experimental results showed that, in addition to the catalyst loading, the photo-conversion rate strongly depends on the physical properties of the photo-catalyst such as particle–particle agglomeration and particle–mesh interaction.

Simulation and multiplicity of steady states in fluidized FCCUs

Abstract Multiple steady states are conditions with important consequences for both operation and control of industrial scale FCCs. In order to predict these conditions and to develop strategies for preventing unsteady states, the present study considers a phenomenological model including both fluidized regenerator and riser cracker.

Particle clustering in down flow reactors

This study describes a new optical sensor, the CREC-GS-Optiprobe. This sensor can be used in gas–solid reactors for characterizing clustering phenomena with minimum intrusion effects. Theoretical considerations allow to anticipate a narrow focal region at 4.5 mm away from the sensor tip. These theoretical predictions are confirmed with calibrations and experimental testings of the novel sensor. It is the goal of the present study to offer evidence, using this novel CREC-GS-Optiprobe, of cluster formation in down flow reactors. It is demonstrated through experimental studies that in down flow systems (dp=65 μm FCC particles, solid fluxes=3–7 kg m−2 s) that catalyst evolves, as strings of particles, with a relatively large size distribution: average cluster sizes are comprised between 2 and 6 dp with most of these strings having 3.5 dp. Using theoretical considerations and solving the equations of motion, it is proven that hydrodynamic interactions between particles lead, in down flow reactors, to formation of particle agglomerates (clusters). Key parameters in this process are particle approaching times and particle approaching velocities. It is shown that particle agglomeration is a very rapid process taking place in downer units in a time scale much smaller than the few seconds expected residence times.

Eggshell catalysts for Fischer–Tropsch synthesis: Modeling catalyst impregnation

Abstract This study presents a successful methodology to produce CoZr on silica eggshell catalysts. It is shown that parameters such as impregnation time, metal solution concentration, solution viscosity, as well as the state of the support before impregnation, strongly influence the evolution of the thickness and the final state of the metal in the eggshell catalyst. It is demonstrated that the wet impregnation, using low metal concentration solutions improves metal dispersion by producing a more progressive eggshell profile than the dry impregnation. Mathematical models were used to describe both the dry impregnation, effected with low and high metal concentration solutions, as well as the wet impregnation, effected with low metal concentration solutions. These models include either no adjustable parameter (dry impregnation) or a single adjustable parameter (wet impregnation) and lead to reliable predictions of the formation of the eggshell.

Photocatalytic degradation of water organic pollutants : Pollutant reactivity and kinetic modeling

Photocatalytic reactors can play an increasing role in novel technologies for the purification of water polluted with organic chemicals. These organic pollutants may strongly adsorb on the TiO 2 -mesh system leading to an overall process influenced by the combined adsorption/reaction phenomena. In the present study, chlorinated aromatics frequently found as water contaminants (2-chlorophenol and 2-4-dichlorophenol) are selected as model pollutants. It is demonstrated, developing experiments in a novel photocatalytic reactor (Photo-CREC unit) that addition of chlorine atoms to the phenol molecule has a significant impact on both adsorption and photoreaction. Due to catalyst deactivation and drop in lamp power, kinetic constants are corrected to conditions of new lamp and fresh catalyst. It is foreseen that using these corrected kinetic constants, high-energy efficiencies (PTEF) can be achieved in Photo-CREC.

Eggshell catalyst in Fischer–Tropsch synthesis: Intrinsic reaction kinetics

Abstract The present study reports the synthesis of paraffinic hydrocarbons in the C 10 –C 20 range using a Co/Zr Fischer–Tropsch (FT) eggshell catalyst supported on silica. Operating conditions selected for the experimental studies, in a Berty-type reactor, are representative of industrial operation. Five kinetic models are considered: one empirical model and four variations of a Langmuir–Hinselwood representation. All models considered incorporate a strong inhibition due to CO adsorption. Kinetic parameters are determined using non-linear regression and are validated by means of statistical analysis.