Diffusion effects in rotating rotary kilns

A novel approach to the modeling of mass transfer in rotary kilns has been described (Heydenrych et al, 2001). It considers the mass transfer to occur by the inclusion of gas in the interparticle voids in between the particles that move concentrically with the kiln. By doing so, the rate of mass transfer was found to be dependent on bed fill and the ratio of reaction rate constant to angular velocity (k/ ). The model was found to be valid at slow to medium fast reactions. For fast reactions it under-predicted mass transfer. Therefore in this paper, the model will be extended to include diffusion effects. An additional dimensionless number is necessary then to describe the system. This can either be a Peclet number (R2/De) or a Thiele modulus (kR2/De)1/2. The solution of the 2-dimensional partial differential equations that describe the extended model gives a handle on the effect of scale-up in rotary kilns. For industrial-scale kilns, the Peclet number is large, which means that diffusion within the lower (passive) layer of the bed is unimportant for slower rates. With high reaction rates, iso-concentration lines are closely stacked near the surface of the bed, implying that it is important to model the active layer rather than the bed as a whole in these circumstances. However, the stiff differential equations are not easily solved then, and other methods of solution are advisable

Mass-fraction of oxygen as a predictor of HHV of gaseous, liquid and solid fuels

The higher heating value (HHV) of gaseous, liquid and solid fuels is demonstrated to be a strong function of the mass fraction of oxygen required for combustion, and suitable correlations are proposed to describe this relationship accurately. A 4th order correlation was found to be the best for estimating HHV as an all-purpose correlation with a root-mean-square error (RMSE) of 1.2 MJ/kg, coefficient of determination (R2) value of 0.9900 and mean bias error (MBE) of 0.40 %. A 3rd order correlation was as accurate with an RMSE of 1.2 MJ/kg, R2 value of 0.9898 and an MBE of – 0.16 %. In addition, the linear relationship between the HHV and oxygen required for combustion on a mole basis is demonstrated and an alternative mole-based correlation is proposed. A total of 311 HHV data from various sources are used to validate these correlations and 13 other correlations available in the literature are used for further comparison.The Paper Manufacturers Association of South Africa (PAMSA), as well as the guidance and resources provided by the Future Energy Cluster at the Mäladalens Högskola, Sweden.http://www.elsevier.com/locate/procediaam2018Chemical Engineerin

Pyrolysis oil composition and catalytic activity estimated by cumulative mass analysis using Py-GC/MS EGA-MS

Please read abstract in the article.The Paper Manufacturers Association of South Africa (PAMSA)http://www.elsevier.com/locate/energy2021-11-28hj2021Chemical Engineerin

Mechanistic model for dispersion coefficients in bubble column

A mechanistic model describing the mass and momentum exchange in bubble columns is proposed that is based on the observations that the gas distribution in a bubble column is not uniform across the column and that as a consequence large liquid recirculation cells are formed in such columns. The model provides insight into the mechanism of mass and momentum exchange in bubble columns and predicts turbulent viscosities in bubble columns that are similar to those reported in the literature. It also predicts axial and radial dispersion coefficients that are of the same order of magnitude as the reported data. Whereas the model is based on a description of the underlying physical phenomena, its validity and extrapolation is expected to be more reliable than that of empirical correlations.http://www.saiche.co.za/article.asphb201

Oxygen consumption as the definitive factor in predicting heat of combustion

Please read abstract in the article.The Paper Manufacturers Association of South Africa (PAMSA), as well as the guidance and resources provided by the Future Energy Cluster at the Mäladalens Högskola, Sweden.http://www.elsevier.com/locate/apenergy2020-02-01hj2018Chemical Engineerin

Development of a mesoporous silica-supported layered double hydroxide catalyst for the reduction of oxygenated compounds in E. grandis fast pyrolysis oils

Biomass fast pyrolysis oil is a potential renewable alternative to fossil fuels, but its viability is constrained by its corrosiveness, low higher heating value and instability, caused by high oxygenate concentrations. A few studies have outlined layered double hydroxides (LDHs) as possible catalysts for the improvement of biomass pyrolysis oil characteristics. In this study, the goal was to reduce the concentration of oxygen-rich compounds in E. grandis fast pyrolysis oils using CaAl- and MgAl- LDHs. The LDHs were supported by mesoporous silica, synthesised at different pHs to obtain different pore sizes (3.3 to 4.8 nm) and surface areas (up to 600 m2/g). The effects of the support pore sizes and use of LDHs were investigated. GC/MS results revealed that MgAl-LDH significantly reduced the concentrations of ketones and oxygenated aromatics in the electrostatic precipitator oils and increased the concentration of aliphatics. CaAl-LDH had the opposite effect. There was little effect on the oxygenate concentrations of the heat exchanger oils, suggesting that there was a greater extent of conversion of the lighter oil compounds. Bomb calorimetry also showed a marked increase in higher heating values (16.2 to 22.5 MJ/kg) in the electrostatic precipitator oils when using MgAl-LDH. It was also found that the mesoporous silica support synthesised at a pH of 7 was the most effective, likely due to the intermediate average pore width (4 nm).https://www.mdpi.com/journal/catalystsam2022Chemical Engineerin

Mass transfer in rolling rotary kilns: a novel approach

A novel approach to modeling mass transfer in rotary kilns or rotating cylinders is explored. The movement of gas in the interparticle voids in the bed of the kiln is considered, where particles move concentrically with the geometry of the kiln and gas is entrained by these particles. The approach considers a differential section along the length of a rotary kiln where the gas concentration in the freeboard is assumed to be uniform in that section. A reactor modelling approach has been used to derive effectiveness factors for the bed as a function of bed fill, reaction kinetics and rotation speed. In many cases, the entrained gas becomes depleted within the bed, leading to a simplified model for the bed effectiveness factor. Experimental data confirms the validity of this model for slower rates. At faster rates, mass transfer can be much higher than the model predicts, indicating that other mechanisms, such as dispersion or diffusion are also important in these conditions

Energy metrics of fuel juxtaposed with mass yield metrics

Please read abstract in the article.The Paper Manufacturers Association of South Africa (PAMSA)http://www.elsevier.com/locate/renene2021-05-15hj2020Chemical Engineerin