Draft-tube Bubble Column Research Articles

Prediction of gas hold-up for alcohol solutions in a draft-tube bubble column

This paper deals with the prediction of the overall gas hold-up (?g), in the diluted solutions of C -C alcohols in draft - tube bubble column, by applying several newly proposed correlations and some of the well-known equations. Experiments were carried out in a column, consisting of two coaxial glass tubes, with a single sparger. Gas phase was air, while the liquid phases were aqueous solutions of alcohols, in concentrations of 0.5% w/w and 1% w/w. Overall gas hold-up was determined by applying volume expansion technique. The following order for ?g values was observed: water < methanol < ethanol < n-propanol < n-butanol. Concentration of the applied alcohol appeared to be less significant than the .sort of alcohol itself. The best newly proposed correlation enables predicting of our experimental data with the average square deviation of empirical formula: s2=0.58 E-04.

Dynamic modelling of a bubble column for particle formation via a gas–liquid reaction

This paper presents a dynamic model of a bubble column reactor with particle formation, accomplished by adopting a hybrid CFD-reaction engineering approach. CFD is employed for estimating the hydrodynamics and is based on the two-phase Eulerian–Eulerian viewpoint. The reaction engineering model links the penetration theory to a population balance that includes particle formation and growth with the aim of predicting the average particle size. The model is then applied to the precipitation of CaCO 3 via CO 2 absorption into Ca(OH) 2aq in a draft tube bubble column and draws insight into the phenomena underlying the crystal size evolution.

Three-dimensional simulation of a three-phase draft-tube bubble column

Three-dimensional simulations of a three-phase flow in conical-bottom draft-tube bubble columns were performed using a finite-volume flow simulation technique. Simulated gas volume fraction and column circulation times agree with the experimental observations of Pironti et al. ( Chemical Engineering Journal, 60, 155–160). Furthermore, the simulations showed the same loss of column circulation as observed experimentally when the column was operated with the draft-tube in its highest position. The simulation code employed an unstructured grid method along with a multifield description of the multiphase flow dynamics. Modeled physical phenomena included momentum exchange and turbulence due to both shear and slip among phases.

Continuous hydrogenation of adiponitrile on Raney nickel in a slurry bubble column

An experimental study of the catalytic hydrogenation of adiponitrile in a mini-plant with a continuously operated slurry draft-tube bubble column is discussed. The purpose of this work is to determine the influence of operating parameters on the performance of the reactor, and to develop a complete model to describe its behaviour. The following basic information was first obtained: • •hydrodynamic data; a mock-up of the same scale as the reactor is built to measure the characteristics of the fluid flow. • •intrinsic kinetics data; the Langmuir-Hinshelwood model is chosen, supposing surface reaction limitation and competitive adsorption between hydrogen and nitriles. This correctly describes the experimental data. From this first information, a model supposing well-mixed liquid phase is developed. It is not a completely predictive model, since two parameters have to be estimated: the volumetric liquid-phase mass transfer coefficient and the deactivation parameter. This complete model is available representing experimental data with good accuracy.

Determination of volumetric oxygen transfer coefficient by off-gas analysis

In various aerobic bioreactors including activated sludge aeration tanks, the volumetric mass transfer coefficient K L a is frequently used as an estimate of the rate of oxygen dissolution into the liquid phase. The K L a measurement in such bioreactors is widely applied with the aid of sodium sulfite (Na 2SO 3) as an oxygen-consuming substance used to maintain low dissolved oxygen concentration. In the present study, the effect of the addition of Na 2SO 3 on K L a, determined by an off-gas analysis, was investigated specifically from the viewpoint of variations in the size of air bubbles and the enhancement factor associated with the change in sulfite concentration. Experiments were conducted in a draft-tube bubble column, using a zirconia electrode oxygen analyzer for measurement of the O 2 mole fraction in the exhaust gas and a dual electrical resistivity probe for measurement of the bubble size. It was found that the increase in the specific gas-liquid interfacial area, resulting from bubble size reduction effected by Na 2SO 3 functioning as an electrolyte, is more pronounced than the enhancement of the absorption rate through the interface. The upper limit of Na 2SO 3 concentration for sustaining physical absorption, in the absence of any catalyst, ranges from 30 to 70 mol/m 3, while that for preventing the average bubble size from decreasing is about 15 mol/m 3. Furthermore, to secure a reliable K L a measurement, the K L a value should not exceed 50 h −1 for the liquid depth of 3 m even when the limiting conditions are not exceeded. The off-gas analysis proposed in this study for K L a determination is expected to be extremely useful provided that the above conditions are fulfilled, since it only requires moderate addition of the sulfite as the oxygen-consuming substance and will not interrupt the reactor operation as long as oxygen uptake occurs in the system.

The effect of static mixer on mass transfer in draft tube bubble column and in external loop column

Measurement were carried out for pressure drops, liquid circulation velocities, minimum gas velocities for complete suspension of particles, rates of mass transfer from gas to liquid and from liquid to solid in bubble slurry columns with and without static mixers (motionless mixers). Two types of column, a draft tube bubble column (internal type) and an external loop column (external type), were compared. The static mixers were found to be effective to increase rates of mass transfer when a single nozzle was used as the gas distributor. When an optional pump was used in the external loop column, the mass transfer rate could be enhanced significantly.

EFFECT OF STATIC LIQUID HEIGHT ON GAS-LIQUID MASS TRANSFER IN A DRAFT-TUBE BUBBLE COLUMN AND THREE-PHASE FLUIDIZED BED

Experiments are performed under batch-liquid operating conditions to investigate the effect of static liquid height on the gas-liquid mass transfer coefficient (KLa) in a draft-tube bubble column (DTBC) and a draft-tube three-phase fluidized bed (DTFB). In addition, the effects of column diameter, gas-distributor, and draft-tube diameter are studied. The results indicate that for a given system with a porous plate gas-distributor at low superficial gas velocities (<70 m/hr), increasing static liquid height decreases KLa. At high gas velocities, KLa is independent of the static liquid height. For systems with a perforated gas-distributor, there is no effect of static liquid height on KLa. The formation of small dispersed bubbles at low gas velocities in the porous plate distributor system accounts for the considerably high KLa values and the observed effect of liquid height. On the other hand, the formation of large spherical-cap bubbles and the bubble coalescence at high gas velocities reduce the performa...

Flow dynamics in a draft-tube bubble column using various liquids

Gas hold-up and liquid circulation rates in a draft-tube bubble column (0.22 m diameter, 85 l capacity) fitted with various diameter draft-tubes are measured for water and aqueous solutions of ethanol, glycerol and carboxymethylcellulose (CMC), respectively. The data are analyzed in terms of a development of the simple energy balance incorporating flow contraction coefficients to quantify deviations from ideal flow. Ethanol and glycerol enhance the entrainment of gas bubbles into the downcomer compared with pure water and inhibit liquid circulation. In CMC solutions, however, large coalesced bubbles are easily disengaged at the top of the column with little entrainment into the downcomer and promote the highest liquid circulation rate. Bubble column geometry, i.e. draft-tube diameter, affects both bubble entrainment and the flow contraction coefficients, and interacts with the fluid properties to affect both gas hold-up and liquid circulation.

Mass transfer in bubble slurry column with static mixer in draft tube.

Systematic investigations are reported for pressure drops, minimum gas velocities for complete suspension of particles, and rates of mass transfer from gas to liquid and from liquid to solid in a draft-tube bubble column with and without a static mixer

MASS TRANSFER AND REACTION IN BUBBLE COLUMN SLURRY REACTOR WITH DRAFT TUBE

Gas-liquid and liquid-solid mass transfer coefficients were obtained in a draft tube bubble column slurry reactor (abbreviated as DTBCSR) from the measurements of gas absorption and ion-exchange, respectively. Oxidation of sulfur dioxide on activated carbon was carried out in the same reactor. The effects of gas flowrates, diameters of draft tube and solid concentrations were investigated. These results were compared with those in a bubble column slurry reactor without draft tube (abbreviated as BCSR). Minimum gas velocities for complete suspension of solid particles in DTBCSR were lower than those in BCSR. Gas-liquid and liquid-solid mass transfer coefficients in DTBCSR were higher than those in BCSR. Both coefficients were almost independent of solid loadings. Reaction rates in DTBCSR were higher than those in BCSR and a stirred slurry reactor (abbreviated as SSR) except for lower gas flow rates and smaller particles.

On the prediction of liquid circulation in a draft-tube bubble column

Analyse de la circulation du liquide qui inclut les effets du profil de fraction de vide dans le tube d'aspiration

Liquid circulation in a draft-tube bubble column

A study of liquid circulation in a draft-tube bubble column (250 mm diameter, 60l. capacity) using a flow-follower technique is reported. It was observed that annulus liquid circulation velocity increased to a maximum with air flowrate and a maximum in the volumetric liquid circulation rate occurred at a tube: column diameter ratio of about 0.5.A simple model based on air-lift pump theory is used to predict liquid circulation velocity using either pressure drop or air flowrate data. Reasonable agreement is found for narrow draft-tubes (≤ 121 mm) at low gas flowrates (≤ 400 ml./s) but with significant deviations occurring with larger draft-tube diameters. These are thought to be due to secondary liquid flow within the draft-tube reducing primary liquid circulation.At higher gas flowrates bubble entrainment into the annulus becomes increasingly significant at all diameter ratios and the simple air-lift model is then inapplicable.