Convective drying of particulate solids – Packed vs. fluid bed operation

Experimental study and simulation of vibrated fluidized bed drying

Mass transfer in fluidized beds is an important operation for separation processes. Two effects can be achieved by using fluidized beds in mass transfer processes increasing interface area and relative movement between the phases. These effects are both desirable because they lead to greater process rates. This paper presents an experimental investigation regarding mass transfer from the wall of a column to the fluid in a fluidized bed of inert spherical particles. The experiments were conducted in column 40 mm in diameter with spherical particles 0,8-3 mm in diameter and water as one fluidizing fluid. The method of dissolution of benzoic acid was used to provide very low mass flux. The average wall-to-fluid mass transfer coefficients were determined for two systems: single-phase fluid flow and a fluidized bed of inert particles The measurements encompassed a Reynolds number range from 100-4000 for single-phase flow and 600-4000 in fluidized beds. The mass transfer coefficients for both systems were calculated from weight loss of benzoic acid. The effects of superficial liquid velocity and particle diameter on the mass transfer coefficient were investigated. It was found that mass transfer was more intensive in the fluidized bed in comparison with single phase flow. The best conditions for mass transfer were reached at a minimum fluidization velocity, when the mass transfer coefficient had the greatest value. The experimental data were correlated in the form: jd = f(Re), where jd is the dimensionless mass transfer factor and Re the Reynolds number.

Experimental mass and heat transfer at a serpentine tube heat exchanger located at the wall of a square stirred tank reactor

Rates of mass transfer between a liquid and a surface which is axially immersed in fluidized beds are measured. Liquid-solid and gas-liquid-solid beds of both inert and active conducting particles are considered. Variables used in the experiments are liquid superficial velocity, particle size, and gas superficial velocity. The wall-to-liquid mass transfer coefficients in liquid-solid fluidized beds are increased with the liquid superficial velocity and particle size. The liquid holdup at which the maximum mass transfer occurs, is decreased with particle size. The mass transfer coefficients in gas-liquid-solid fluidized beds are slightly decreased with the gas superficial velocity. The mass transfer coefficients in terms of Chilton-Colburn factor can be well correlated with the modified Reynolds number.

One of the high-intensity methods for removing both free and bound moisture is filtration drying. This is due to the fact that during filtration drying, the heat agent is filtered through the porous structure of the wet material, which is placed on the perforated partition in the direction "wet material–perforated partition". The velocity of the heat agent in the pores and channels of a stationary layer of wet material determines the thickness of the boundary layer (hydrodynamic, thermal, and diffusion) and, accordingly, the values of the heat and mass transfer coefficients. In addition, the surface of heat and mass transfer is the total surface of the pores and channels through which the heat agent is filtered. The filtration rate of the heat agent is determined based on technical and economic considerations, given that its increase affects the growth of pressure loss. Moreover, the actual speed of the heat agent relative to the layer elements is much higher than in the case of drying by any other methods (in a fluidized bed, during drying in pneumatic transport dryers, etc.). Large heat and mass transfer surfaces and the speed of the heat agent in the pores and channels of the stationary layer of wet material provide high heat and mass transfer coefficients and, accordingly, the intensity of filtration drying. This article presents the results of the study of external heat exchange between the heat agent and dry particles of organic materials of fibrous structure, in particular raw cotton and its stems, and the mechanism of filtration drying.

Adsorption of a two-component gas mixture of acetone and methylbenzene was tested in a fixed-bed apparatus, and a coupled model of heat and mass transfer was developed according to the theory of Soret and Dufour effects. The numerical simulation results agreed well with the experimental results. Using this mathematical model, the coupled effects of heat and mass transfer were studied in a fixed-bed adsorption simulation, and numerical analysis results were used to discuss the influences of heat transfer coefficients, mass transfer coefficients, and the coupled effect of heat and mass transfer on the fixed-bed process. These results show that heat transfer coefficients have little effect on mass transfer. In contrast, mass transfer coefficients have some impact on heat transfer. The mass transfer effect caused by the temperature gradient is more obvious than the heat transfer effect caused by the concentration gradient.

The influence of liquid-side mass transfer on heat transfer and selectivity during surface and nucleate boiling of liquid mixtures in a falling film

The performance of the absorber significantly affect the overall efficiency of any absorption refrigeration system. One of the major reasons for the poor design of the absorber for ammonia-water refrigeration systems is the neglect of mass transfer resistances in the vapour or liquid phase in favour of only heat transfer considerations. Heat removal is critical to the absorption process, however, the ammonia gas is constantly absorbed by the liquid ammonia-water solution. This means that two-phase mass and heat transfer operations take place during the absorption process. Therefore, this implies that, for any proper absorber design, both heat and mass transfer considerations are paramount. Accurate absorber design is further predicated on accurate properties correlations. The correlations required for the absorber design include liquid and vapour enthalpies, saturated vapour pressure, specific heat capacity of water and ammonia vapour, liquid solution dynamic viscosity, liquid solution density, surface tension, liquid solution thermal conductivity, liquid-liquid diffusivity and vapour-liquid diffusivity. Procedures for calculating these parameters have been presented in this paper and their accuracy with previous studies compared. Results showed relatively low percentage deviations and so can be applied for the design and simulation of the absorber. Designs of the absorber for various cooling capacities have been carried in this paper. The variation of ammonia vapour temperatures, liquid ammonia-water solution temperatures, cooling water temperatures, and flow rates of ammonia-water solution and ammonia vapour across the absorber is investigated for the four cooling capacities. For refrigeration systems with cooling capacities of 1 kW, 2 kW, 3 kW and 4 kW, the calculated lengths for a water-cooled coiled absorber are 4.484 m, 8.1467 m, 12.50459 m and 14.78329 m respectively. The calculated overall heat transfer coefficients for the absorption process for the four cooling capacities (1 kW, 2 kW, 3 kW and 4 kW) were 0.4427, 0.4376, 0.4208 and 0.4309 kWm-2K-1 while the mass transfer coefficients were 3.721 x10-9, 4.206 x10-9, 4.556 x10-9and 4.856 x10-9 m2s-1 respectively. Concurrent flow is considered in this study, but the procedure can also be applied to countercurrent flow. KEYWORDS: Absorption Refrigeration, Ammonia Water Heat and Mass Transfer, Design

Effects of net-type spacers on heat and mass transfer in direct contact membrane distillation and comparison with ultrafiltration studies

A critical comparison between local heat and mass transfer coefficients of horizontal cylinders immersed in bubbling fluidised beds

Fluidization is a well-established technique for processing particulate materials, characterized by intensive interactions between solid particles and the fluid flow. Among the various design parameters of fluidized bed dryers, bed height plays a critical role, directly influencing flow resistance, heat and mass transfer, and consequently, the system’s energy efficiency and overall performance. A thorough understanding of the mechanisms governing fluidized bed dynamics is therefore essential for process optimization. In this study, a three-dimensional gas-solid fluidized bed was simulated using a coupled Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) framework. This approach enables precise tracking of particle motion and distribution, allowing the temporal evolution of bed height to be quantified. Simultaneously, flow characteristics such as gas velocity, pressure, and particle distribution were analyzed under various operating conditions. Before performing a sensitive simulation study, the model was validated against data from previously published studies, showing strong agreement with established empirical correlations and confirming the reliability of the numerical framework. Building on this validation, a parametric study was conducted to examine the effects of key operating variables - namely, inlet gas velocity and bed height - on the hydrodynamic behavior of the system. The obtained results demonstrate that these parameters have a significant impact on flow uniformity and overall bed stability.

Heat Transfer in Fluidized Beds

The mass transfer coefficient between fluid and an immersed sphere in liquid packed and fluidized beds of inert spherical particles have been studied experimentally using a column 40 mm in diameter. The mass transfer data were obtained by studying the transfer of benzoic acid from the immersed sphere to flowing water using the dissolution method. In all runs, the mass transfer rates were determined in the presence of inert glass particles 0.50-2.98 mm in diameter. The influence of different parameters, such as: liquid velocity, particles size and bed void age, on the mass transfer in packed and fluidized beds is presented. The obtained experimental data for mass transfer in the packed and particulate fluidized bed were correlated by a single correlation, thus confirming the similarity between the two systems.

This study was aimed to improve the SO2 desorption effect of basic aluminum sulfate SO2-rich solution. Based on the desorption mechanism, we first used a falling film evaporation method to intensify heat transfer and mass transfer. Based on the falling film desorption heat and mass transfer model, we analyzed the falling film evaporation and desorption of basic aluminum sulfate SO2-rich solution inside the converging–diverging tube and the smooth tube and investigated the heat and mass transfer rules under different conditions. It was found as the quantity of SO2-rich solution increased, the heat transfer coefficient and mass transfer coefficient of falling film evaporation both increased, but the SO2 desorption efficiency decreased. With the rise of the heating temperature, the three indices all increased. With the rise of the inlet sulfur concentration, the three indices all increased. With the rise of the aluminum concentration, the three indices all gradually declined. With the rise of the alkalinity, the three indices all gradually declined. Comparative analysis showed the heat transfer coefficient and mass transfer coefficient of falling film evaporation and SO2 desorption efficiency were 17%–29%, 33%–69%, and 6.7%–16.3% larger inside the converging–diverging tube than the smooth tube, respectively, indicating the basic aluminum sulfate SO2-rich solution significantly outperforms inside the converging–diverging tube in terms of heat transfer and mass transfer. At the heating temperature of 108 °C, liquid film flow rate of 0.005 kg/s, sulfur concentration of 0.06 mol/L, aluminum concentration of 20 g/L, and basicity of 20%, the SO2 desorption efficiency inside the converging–diverging tube could reach a high level 94.2%, compared with only 83.7% inside the smooth tube. Moreover, correlations were obtained to predict the heat and mass transfer coefficients.

Heat and mass transfer in combined convective and far-infrared drying of fruit leather