Study of gas-fines in decreasing gas velocity in a packed bed

It has been well recognized now that the blast furnace aerodynamics can be represented more accurately under the decreasing gas velocity condition. Therefore, gas-fines study has been carried out in a packed bed under the decreasing gas velocity condition. Gas and fines flow equations have been developed and solved, for two-dimensional case using finite volume method. To take into account the turbulence, k-e turbulent flow model has also been developed in two-dimension. The model's predictions have been validated against the published experimental data for the increasing gas velocity case, as no experimental data are available in open literature for the decreasing gas velocity. This study shows the difference in the results for increasing and decreasing gas velocity cases under various conditions which have been reported here. Implication of the results to the blast furnace condition has also been discussed.

Gas dispersion measurements in industrial flotation cells

For nearly 100 years, the flotation plant metallurgist has often wondered what is happening 'beneath the froth'. To assist in unravelling this mystery, new technology has been developed as part of the Australian Mineral Industries Research Association (AMIRA) P9 project, to measure gas dispersion characteristics (such as gas hold-up, superficial gas velocity and bubble size) in industrial flotation cells. These measurements have been conducted in a large number of cells of different types and sizes by researchers from the Julius Kruttschnitt Mineral Research Centre (JKMRC) and JKTech. A large database has been developed and the contents of this database are described in this paper. Typical cell characterization measurements show a wide spread in values, even in the same cell types and sizes performing similar duties. In conventional flotation cells, the typical gas hold-up values range from 3% to 20%, bubble sizes range between I and 2 mm, and superficial gas velocity ranges from 1 to 2.5 cm/s. The ranges of cell characterization measurements given in this paper will enable plant personnel to compare their operation to other similar types of operations from around Australia and the rest of the world, giving opportunities for further improvement to flotation plant operations. (C) 2005 Elsevier Ltd. All rights reserved.

Gas dispersion measurements in industrial flotation cells

For nearly 100 years, the flotation plant metallurgist has often wondered what is happening ‘beneath the froth’. To assist in unravelling this mystery, new technology has been developed as part of the Australian Mineral Industries Research Association (AMIRA) P9 project, to measure gas dispersion characteristics (such as gas hold-up, superficial gas velocity and bubble size) in industrial flotation cells. These measurements have been conducted in a large number of cells of different types and sizes by researchers from the Julius Kruttschnitt Mineral Research Centre (JKMRC) and JKTech. A large database has been developed and the contents of this database are described in this paper. Typical cell characterisation measurements show a wide spread in values, even in the same cell types and sizes performing similar duties. In conventional flotation cells, the typical gas hold-up values range from 3 - 20 per cent, bubble sizes range between 1 and 2 mm, and superficial gas velocity ranges from 1 to 2.5 cm/s. The ranges of cell characterisation measurements given in this paper will enable plant personnel to compare their operation to other similar types of operations from around Australia and the rest of the world, giving opportunities for further improvement to flotation plant operations.

Gas-liquid simulation of an airlift bubble column reactor

The simulation of two-phase flow for an experimental airlift reactor (32-l volume) using commercially available software from Fluent Incorporated is presented here (http://www.fluent.co.uk). Data from the simulation is compared with the experimental data obtained by the tracking of a magnetic particle and analysis of the pressure drop to determine the gas hold-up. Comparisons between vertical velocity and gas hold-up were made for a series of experiments where the superficial gas velocity in the riser was adjusted between 0.01 and 0.075 m s-1. © 2003 Elsevier B.V. All rights reserved.

Gas-liquid-solid flow modelling in a bubble column

An alternative approach to the modelling of solid-liquid and gas-liquid-solid flows for a 5:1 height to width aspect ratio bubble column is presented here. A modified transport equation for the volume fraction of a dispersed phase has been developed for the investigation of turbulent buoyancy driven flows (Chem. Eng. Proc., in press). In this study, a modified transport equation has been employed for discrete phase motion considering both solid-liquid and gas-liquid-solid flows. The modelling of the three-phase flow in a bubble column was achieved in the following case: injecting a slug of solid particles into the column for 10 s at a velocity of 0.1 m s-1 and then the gas phase flow was initiated with a superficial gas velocity of 0.02 cm s-1. © 2003 Elsevier B.V. All rights reserved.

Flow regime transition criteria for two-phase flow at reduced gravity conditions

Flow regime transition criteria are of practical importance for two-phase flow analyses at reduced gravity conditions. Here, flow regime transition criteria which take the friction pressure loss effect into account were studied in detail. Criteria at reduced gravity conditions were developed by extending an existing model with various experimental datasets taken at microgravity conditions showed satisfactory agreement. Sample computations of the model were performed at various gravity conditions, such as 0.196, 1.62, 3.71, and 9.81 m/s2 corresponding to micro-gravity and lunar, Martian and Earth surface gravity, respectively. It was found that the effect of gravity on bubbly-slug and slug-annular (churn) transitions in a two-phase flow system was more pronounced at low liquid flow conditions, whereas the gravity effect could be ignored at high mixture volumetric flux conditions. While for the annular flow transitions due to flow reversal and onset of dropset entrainment, higher superficial gas velocity was obtained at higher gravity level.

A simple and low-cost airlift photobioreactor for microalgal mass culture

A simple, low-cost, and efficient airlift photobioreactor for microalgal mass culture was designed and developed. The reactor was made of Plexiglas, and composed of three major parts: outer tube, draft tube and air duct. The fluid-dynamic characteristics of the airlift reactor were studied. The system proved to be well suited to the mass cultivation of a marine microalga, Chlorella sp. In batch culture, the biomass volumetric output rate of 0.21 g l(-1) d(-1) was obtained at the superficial gas velocity of 4 mm s(-1) in the draft tube.

Experimental aspects of biomass fuels in a bubbling fluidized bed combustor

This work presents experimental information relevant to the combustion of biomass in a bubbling fluidized bed. The biomass distribution in a fluidized bed was studied through tests performed in a cold bed, while the volatiles released in the biomass pyrolysis, the burning rate of the resulting charcoal, and the combustion control regime, were studied through tests performed in a high temperature bed.Visual examination of photographs taken from a transparent walls bed, with a rectangular cross-section, showed that the large fuel particles, typical of biomass processing, were distributed in the bubbles, in the splash zone, and in the emulsion phase. The occurrence of biomass in the emulsion phase was favored by burning biomass particles of greater density and smaller size-expetimentally determined in each case. Decreasing the fuel particle size improved the biomass distribution inside the bed. The same was accomplished by increasing the superficial gas velocity as high as possible, compatibly with the acceptable elutriation.Burning tests showed that the biomass fuels have the advantage of reaching the diffusional regime at temperatures that can be lower than 1000 K, which ensures that the biomass fuels burn in a stable regime. (C) 2007 Elsevier B.V. All rights reserved.

Parâmetros de dispersão de ar em células mecânicas de flotação em usinas brasileiras

O objetivo desse trabalho é apresentar e discutir a determinação dos parâmetros de dispersão de ar em células mecânicas de flotação, em circuitos industriais brasileiros. Foram investigados os circuitos rougher e scavenger das usinas RPM, Taquari-Vassouras, Copebras e Fosfértil-Catalão (atual Vale Fertilizantes). Nas usinas RPM e Taquari-Vassouras, foram determinados a velocidade superficial do ar (Jg), o hold-up do ar (εg), o diâmetro médio de bolhas (d32) e o fluxo de área superficial de bolhas (Sb). Para as outras usinas, foram determinados somente o Jg e εg. Os resultados mostraram que, com exceção da RPM, todos os circuitos investigados apresentaram baixos valores para os parâmetros de dispersão do ar. Taquari-Vassouras apresentou os mais baixos valores médios para Jg e εg, respectivamente, 0,05 cm/s e 5%. Os maiores valores de Jg foram determinados no circuito rougher da Copebras, 1,17 cm/s, ao passo que a RPM apresentou os maiores valores para o hold-up e tamanho de bolha, respectivamente, 31% e 2,61mm. Os valores obtidos, para Sb, situaram-se abaixo daqueles reportados na literatura.

Medidas de dispersão de gás em células mecânicas industriais e seus condicionantes.

A qualidade da dispersão de gás em células de flotação é comumente caracterizada através de parâmetros como velocidade superficial do gás (Jg), hold-up do gás (?g), distribuição de tamanho de bolha (db ou D3,2) e fluxo de superfície de bolha (Sb). Sendo um processo de separação de minerais que é dependente da interação (colisão + adesão) entre partículas hidrofóbicas e bolhas de ar, a flotação tem seu desempenho dependente de uma dispersão de gás apropriada na polpa de minério. Desta forma, este trabalho objetivou caracterizar o estado da dispersão de gás de duas células em um banco composto por quatro células Wemco de 42,5 m³ (subaeradas), operando em série na usina da Vale Fertilizantes (Cajati-SP). Realizaram-se três campanhas de medidas que foram conduzidas sob diferentes condições operacionais: a) Diâmetro do rotor (D) de 1,09 m e rotação (N) entre 145 RPM e 175 RPM; b) D = 0,99 m e N entre 110 RPM e 190 RPM; c) D = 0,99 m e N de 120 RPM e de 130 RPM. Observaram-se os seguintes valores de dispersão de gás: 0,7 <= Jg <= 5,4 cm/s, 7 <= ?g <= 15%, 1,6 <= D3,2 <= 2,4 mm e Sb na faixa de 24 a 162 s-1. A magnitude de Jg medida na 1ª e 2ª campanhas mostrou-se acima dos valores reportados pela literatura, indicando necessidade de modificação de condições operacionais dos equipamentos, assim como cuidadosa manutenção. Posteriormente, a 3ª campanha indicou maior conformidade dos parâmetros de dispersão de gás em relação à literatura, constatando-se uma considerável melhora de desempenho do processo de flotação.

Hydrodynamics and mass transfer coefficient in activated sludge aerated stirred column reactor: Experimental analysis and modeling

The aerated stirred reactor (ASR) has been widely used in biochemical and wastewater treatment processes. The information describing how the activated sludge properties and operation conditions affect the hydrodynamics and mass transfer coefficient is missing in the literature. The aim of this study was to investigate the influence of flow regime, superficial gas velocity (U-G), power consumption unit (P/V-L), sludge loading, and apparent viscosity (pap) of activated sludge fluid on the mixing time (t(m)), gas hold-up (epsilon), and volumetric mass transfer coefficient (kLa) in an activated sludge aerated stirred column reactor (ASCR). The activated sludge fluid performed a non-Newtonian rheological behavior. The sludge loading significantly affected the fluid hydrodynamics and mass transfer. With an increase in the UG and P/V-L, the epsilon and k(L)a increased, and the t(m), decreased. The E, kLa, and tm,were influenced dramatically as the flow regime changed from homogeneous to heterogeneous patterns. The proposed mathematical models predicted the experimental results well under experimental conditions, indicating that the U-G, P/V-L, and mu(ap) had significant impact on the t(m) epsilon, and k(L)a. These models were able to give the tm, F, and kLa values with an error around +/- 8%, and always less than +/- 10%. (c) 2005 Wiley Periodicals, Inc.

Hydrodynamics and mass transfer coefficient in three-phase air-lift reactors containing activated sludge

This study was to investigate the impacts of operating conditions and liquid properties on the hydrodynamics and volumetric mass transfer coefficient in activated sludge air-lift reactors. Experiments were conducted in internal and external air-lift reactors. The activated sludge liquid displayed a non-Newtonian rheological behavior. With an increase in the superficial gas velocity, the liquid circulation velocity, gas holdup and mass transfer coefficient increased, and the gas residence time decreased. The liquid circulation velocity, gas holdup and the mass transfer coefficient decreased as the sludge loading increased. The flow regime in the activated sludge air-lift reactors had significant effect on the liquid circulation velocity and the gas holdup, but appeared to have little impact on the mass transfer coefficient. The experimental results in this study were best described by the empirical models, in which the reactor geometry, superficial gas velocity and/or power consumption unit, and solid and fluid properties were employed. (c) 2006 Elsevier B.V. All rights reserved.

Downdraft gasification of biomass

The objectives of this research were to investigate the parameters affecting the gasification process within downdraft gasifiers using biomass feedstocks. In addition to investigations with an open-core gasifier, a novel open-topped throated gasifier was designed and used. A sampling system was designed and installed to determine the water, tar and particular content of the raw product gas. This permitted evaluation of the effects of process parameters and reactor design on tar and particular production, although a large variation was found for the particulate measurements due to the capture of large particles. For both gasifiers, the gasification process was studied in order to identify and compare the mechanisms controlling the position and shape of the reaction zones. The stability of the reaction zone was found to be governed by the superficial gas velocity within the reactor. A superficial gas velocity below 0.2 Nms-1 resulted in a rising reaction zone in both gasifiers. Turndown is achieved when the rate of char production by flaming pyrolysis equals the rate of char gasification over a range of throughputs. A turndown ratio of 2:1 was achieved for the hybrid-throated gasifier, compared to 1.3:1 for the open-core. It is hypothesized that pyrolysis is a surface area phenomenon, and that in the hybrid gasifier the pyrolysis front can expand to form a dome-shape. The rate of char gasification is believed to increase as the depth of the gasification zone increases. Vibration of the open-core reactor bed decreased the bed pressure drop, reduced the voidage, aided solids flow and gave a minor improvement in the product gas energy content. Insulation improved the performance of both reactors by reducing heat losses resulting in a reduced air to feed ratio requirement. The hybrid gasifier gave a higher energy conversion efficiency, a higher product gas heating value, and a lower tar content than the open-core gasifier due to efficient gas mixing in a high temperature tar cracking region below the throat and reduced heat losses.

Three-dimensional gas-liquid simulation of an airlift bubble column reactor

Basic hydrodynamic parameters of an airlift reactor with internal loop were estimated experimentally and simulated using commercially available CFD software from Fluent. Circulation velocity in a 32-dm(3)-airlift reactor was measured using the magnetic tracer method, meanwhile the gas hold-up was obtained by analysis of the pressure drop using the method of inverted U-tube manometers. Comparison of simulated (in two and three dimensions) and experimental data was performed at different superficial gas velocities in the riser.