Darko R. Jaćimovski

Carbon microspheres have been synthesized by the hydrothermal method with fructose and a phosphoric acid solution at two different concentrations, which were used as precursors. The obtained materials were characterized by elemental analysis, X-ray powder diffraction (XRPD) analysis, scanning electron microscopy (SEM), nitrogen adsorption/desorption measurements, and Fourier transform infrared (FTIR) spectroscopy. Batch sorption experiments were performed to remove methylene blue (MB) from aqueous solutions by varying the initial concentration of MB (C₀) from 50 to 500 mg/dm³, contact period, solution pH value, and temperature. Prepared sorbents consisted of microsphere particles with diameters in the range of 0.6–2.7 µm. The synthetic route was found to govern the microporous–mesoporous structure and surface acidic functional groups of the final product. A phosphoric acid concentration of 40 wt.% gave carbon material with a specific surface area of 932 m²/g and a total pore volume of 0.43 cm³/g. It was found that the extent of MB sorption by the obtained carbon microspheres increased with initial dye concentration, contact time, and especially solution pH but slightly decreased with increasing temperature. Kinetic studies showed that the dye sorption process followed pseudo-second-order kinetics.

Adsorption method has been used for flow visualisation and determination of the local and average mass transfer coefficients around a horizontal cylinder in a liquid-solid fluidized bed. The obtained concentration fields on the adsorbent (silica-foils) represent a clear qualitative flow pattern around the cylinder. By comparison of the concentration fields around the cylinder in the single-phase flow and in the fluidized bed, a significant disturbance of the boundary layer by fluidized particles can be observed. Local mass transfer coefficients were shown to be dependent on the angular position around the cylinder with the maximum determined at the angle 140? (measured from the stagnant point 0?). Selected correlations were applied to predict the average mass transfer coefficients and the two best have shown deviations from the experimental data up to 5 %.

One of the ways to intensify the process is the use of rotating elements, which increase the mass transfer coefficients many times over. In this study, the mass transfer coefficient was determined by adsorption and electrochemical methods, using a rotating disk (RD) to study the effect of rotation on the mass transfer intensity. For adsorption method, a dilute solution of methylene blue was used, and silica gel foils on rotation disk were used as adsorbents. The measurements were performed under diffusion-controlled conditions, and the color intensity of the surface was determined using SIGMA Scan Pro. The study of mass transfer by electrochemical method was performed in batch systems with a volume of 20 dm3 and 0.8 dm3 , using a solution of sulfuric acid and copper sulfate, and potassium ferricyanide (III), potassium ferricyanide (II), and potassium chloride. Disks with diameters of 0.8 cm, 1 cm, 2 cm, 5.6 cm, and 20 cm were used in the rotation range up to 800 rpm. After determining the operating voltage, the current intensity was measured at different rpm and the mass transfer coefficients were calculated. It was found that the mass transfer coefficient increases as the rotation speed of the RD increases, and that is does not depend on the diameter of RD. It is shown that the current density increases with the increase of the RD speed, while the diffusion layer thickness decreases. A comparative analysis of the results obtained by the adsorption and electrochemical methods was made and the results were compared with literature correlations.

In this work, the coefficient of fluid-wall mass transfer in an inverse fluidized bed was determined using the adsorption method. The experiments were carried out in a column with a diameter of 45 mm with spherical and non-spherical particles of polypropylene and polyethylene with a diameter of 3.3?4.9 mm and a density of about 930 kg m-3. A diluted solution of methylene blue was used as a fluidization medium, which was adsorbed on part of the surface of the column on silica gel. The obtained results showed that the presence of particles during inverse fluidization does not contribute significantly to mass transfer compared to the influence of particles on transfer in conventional fluidized beds. Therefore, the pseudo-fluid concept was introduced into the analysis and an empirical correlation was performed to determine the mass transfer coefficient. The obtained results were compared with literature correlations for inverse and conventional fluidized beds.

In this work the theoretical model for heat transfer from a wall to a liquid-solid fluidized bed by liquid convective mechanism has been proposed and developed. The model is based on thickness of boundary layer and film theory. The key parameter in the model is the distance between two adjacent particles which collide with the wall. According to the proposed model, the liquid convective heat transfer in a fluidized bed is 4 to 5 times more intense than in a single-phase flow. Additionally, the wall-to-bed heat transfer coefficient has been measured experimentally in water?glass particles fluidized bed, for different particle sizes. Comparison of the model prediction with experimental data has shown that size of the particles strongly influences the mechanism of heat transfer. For fine particles of 0.8 mm in diameter, the liquid convective heat transfer model represents adequately the experimental data, indicating that particle convective mechanism is negligible. For coarse particles of 1.5?2 mm in diameter, the liquid convective heat transfer mechanism accounts for 60 % of the overall heat transfer coefficient.