Survey of heat transfer mechanisms in a slurry bubble column

Heat transfer mechanisms in the bulk and distributor regions of a slurry bubble column are investigated based on the measurements of local heat transfer in a 0.28 m diameter Plexiglas column. The gas, liquid and solid phases used are oil‐free compressed air, tap water and 35 μm glass beads. The slurry concentration and superficial gas velocity are varied from 0 to 40 vol% and 0.05 to 0.30 m/s respectively. Measurements have been made with a fast response heat flux probe which provided local instantaneous heat transfer coefficients. The time‐averaged heat transfer coefficients in the bulk region were on average about 50% higher than the distributor region of the column. The wall region heat transfer coefficients are well predicted by the correlation of Deckwer et al. (1980). Heat transfer mechanism in column centre can be adequately described by the consecutive film and surface renewal model.     

Gas-liquid mass transfer in a slurry bubble column operated at gas hydrate forming conditions

Gas-liquid interphase mass transfer was investigated in a slurry bubble column under CO₂ hydrate forming operating conditions. Modeling gas hydrate formation requires knowledge of mass transfer and the hydrodynamics of the system. The pressure was varied from 0.1 to 4 MPa and the temperature from ambient to 277 K while the superficial gas velocity reached 0.20 m/s. Wettable ion-exchange resin particles were used to simulate the CO₂ hydrate physical properties affecting the system hydrodynamics. The slurry concentration was varied up to 10%vol. The volumetric mass transfer coefficient (k_la_l) followed the trend in gas holdup which rises with increasing superficial gas velocity and pressure. However, k_la_l and gas holdup both decreased with decreasing temperature, with the former being more sensitive. The effect of solid concentration on k_la_l and gas holdup was insignificant in the experimental range studied. Both hydrodynamic and transport data were compared to best available correlations.     

Effect of pressure fluctuations on the heat transfer characteristics in a pressurized slurry bubble column

The hydrodynamics and heat transfer characteristics were investigated in a slurry bubble column reactor whose diameter was 0.0508 m (ID) and 1.5 m in height. Effects of gas velocity (0.025–0.1 m/s), pressure (0.1–0.7MPa), solid concentration (0–20 vol%) and liquid viscosity (1.0–38.0 mPa s) on the hydrodynamics and heat transfer characteristics were examined. The pressure difference fluctuations were analyzed by means of attractor trajectories and correlation dimension to characterize the hydrodynamic behavior in the column. The gas holdup increased with increasing gas velocity or pressure, but decreased with increasing solid concentration or liquid viscosity. It was found that the attractor trajectories and correlation dimension of pressure fluctuations were effective tools to describe the hydrodynamic behaviors in the slurry bubble column. The heat transfer coefficient increased with increasing pressure or gas velocity, but decreased with increasing solid concentration or viscosity of slurry phase in the slurry bubble column. The heat transfer coefficient value was well correlated in terms of operating variables and correlation dimension of pressure fluctuations in the slurry bubble column.     

Improvement of oxygen mass transfer estimation from oxygen concentration measurements in bubble column reactors

An original procedure has been established for estimating the overall volumetric mass transfer coefficient using the oxygen concentration curves resulting from the usual gassing-in and gassing-out method. This procedure was applied to experimental data obtained in a small scale bubble column using both tap water and a coalescence-inhibiting liquid mixture that represents the coalescence behavior of biological media. It is based on the analysis of the characteristics times of the system, including those of the hydrodynamics of the two phases, the sensor dynamics and the system inertia when the gas composition is modified. A numerical procedure was developed to estimate the characteristic time of the system inertia t_i, using the assumption that this inertia is nearly independent of superficial gas velocity U_G. The calculations confirmed that the optimized t_i value was nearly independent of U_G and of the coalescence behavior of the liquid phase. Additionally, the resulting K_La_L values for tap water were closer to the correlation of Shah et al. [1982. Design parameters estimations for bubble column reactors. A.I.Ch.E. Journal 28, 353-379] than those of other conventional models. Finally, the original procedure was also reported to reduce significantly the square sum deviation between the predicted and the measured oxygen response curves.     

Liquid-Solid mass transfer in a slurry bubble column and a gas-liquid-solid three-phase fluidized bed

Mass transfer coefficients between particles and liquids in a slurry bubble column and a three-phase fluidized bed containing small size particles were obtained with two mass transfer systems: (1) K⁺ –Na⁺ ion-exchange in cation-exchange resin bead beds, including anion-exchange resin beads as inert particles; (2) zinc dissolution by HCl in zinc-plated glass bead beds, and in beds of non-plated glass beads. Operating parameters were gas velocity, liquid velocity, particle diameter, and particle concentration. The dependence of mass transfer coefficients on these parameters is discussed from the viewpoint of the energy supplied into the systems. Correlations of the experimental data using dimensionless groups are compared to previous correlations.     

A statistical approach of heat transfer coefficient analysis in the slurry bubble column

Heat transfer plays an important role in slurry bubble column reactors (SBCRs). Design of heat transfer equipments for SBCRs is a limiting step in sizing and scale up of them. In order to study the convection heat transfer coefficient (CHTC) in industrial SBCRs, a proper column was designed and manufactured in pilot scale with a special convection heat transfer coefficient measurement probe (CHTC MP). In this study, influence of effective parameters such as solid fraction, superficial gas velocity, radial and vertical position of the column was investigated on CHTC. The design of experiments was performed using a full factorial method including 3¹ × 2² × 9¹ = 108 experiments, to determine the main effects, binary and ternary interactions of variables. Study of the curvature functionality of CHTC versus gas velocity and effect of flow regime transition from homogeneous to heterogeneous flow was the other goal of this work. A statistical model including the main variables and their significant binary and ternary interactions was explained for CHTC with a good fitness with experimental data. Interactions of four effective parameters including solid fraction, superficial gas velocity, radial and vertical position of the column was studied for the first time in the SBCRs. A dimensionless correlation for local Stanton number was developed as a function of Pr, Re, Fr, non-dimensional radial position and non-dimensional distance from the sparger. Considering local positions in the correlation is a novel work and this correlation has good agreement with experimental data.     

Simultaneous effect of particle size and solid concentration on the hydrodynamics of slurry bubble column reactors

The simultaneous effect of particle size and concentration on the total gas holdup of slurry bubble column reactors was investigated in this work. The total gas holdup was measured for air–water–glass beads systems. Three solid concentrations and three particle diameters were used. It was found that increasing particle size at high constant concentration decreases gas holdup. Moreover, increasing solid concentration decreases gas holdup and this decreasing effect is higher for larger particles. Also, solid particles have two effects on hydrodynamics, namely, changing the viscosity and density of the liquid phase as well as hindering the bubbles from rising within the column by the collision phenomenon. Therefore, a novel correcting factor was introduced to correct the gas holdup. The hindering factor considers both the collision efficiency affected by the particle size as well as the solid concentration. A novel correlation was developed to predict the experimental data of the three-phase gas holdup.     

Hydrodynamic characterization of a needle sparger rectangular bubble column: Homogeneous flow, static bubble plume and oscillating bubble plume

In this work a detailed experimental hydrodynamic characterization of a needle sparger rectangular bubble column has been performed. The liquid velocity profiles and bubble plume oscillation frequency have been measured by means of laser Doppler anemometry (LDA), and the bubble velocity map by particle image velocimetry (PIV). In this way, the influence of the superficial gas velocity, liquid height and aeration pattern on the column flow structure was analysed. A highly uniform upward flow structure with down flow near the walls was obtained by means of a full-length aeration pattern. This flow structure was preserved even for high gas fractions values. The partial-length aeration patterns with the aerated zone (defined as the aerated width divided by the column width) larger than 0.7 provide a bubble plume and two pure liquid vortical structures in the column bottom, although they are static in nature. With aerated zones lower than 0.6, an oscillating bubble plume is obtained. A non-dimensional analysis of bubble plume oscillation frequency shows a dependence of bubble plume behaviour with the aerated zone. In this way, two different types of bubble plume oscillations, namely confined bubble plume oscillation and free bubble plume oscillation, are introduced and analysed.     

Mathematical modeling of gas-liquid mass transfer rate in bubble columns operated in the heterogeneous regime

In this paper, a multi-scale approach is followed to study gas-liquid mass transfer in bubble columns. First, a single bubble of equivalent diameter d is considered. Its morphology and its gas to liquid relative velocity are related to the bubble diameter through the use of known correlations. Then, the gas-liquid mass transfer between the bubble and the surrounding liquid is studied theoretically. An equation describing the transport of the transferred species in the viscous boundary layer around the bubble is solved. In a second step, a bubble column of 6-10 m height is studied experimentally. The gas phase in the column is characterized experimentally by means of a gammametric technique. Finally, the two studies are linked, yielding a 1D mathematical model able to predict the gas-liquid mass transfer rate in a bubble column operated in the heterogeneous regime.     

Influence of bubble column diameter on local heat transfer and related hydrodynamics

Heat transfer coefficients measured in a 0.15 m ID bubble column are compared with similar studies in larger diameter columns to identify influence of column diameter. Gas phase used is oil free compressed air and its flow rate is varied from 0.03 to 0.35 m/s. Tap water is the liquid phase and the solid particles used are 49 μm glass beads and their concentration is varied up to 20 vol%. The observed increase in heat transfer coefficients can be related to increase in liquid circulation velocity with column diameter which in turn is related to increase in large bubbles rise velocity. A simplified scale-up procedure is presented based on available data and suitably modified literature correlations for heat transfer coefficient.     

Determination of heat transfer formulas for gas flow in fin-and-tube heat exchanger with oval tubes using CFD simulations

Plate fin-and-tube heat exchangers operate in a cross-flow arrangement with the complex path of gas flow, hence in order to determine the velocity field and heat transfer characteristics, the numerical methods must be used. The CFD codes allow obtaining local values of the heat transfer coefficient, however it is impossible to incorporate these values into the analytical formulas for the overall heat transfer coefficient, that is fundamental for the designing procedure of the cross-flow heat exchangers. Therefore this paper presents a method for determination of the average heat transfer coefficient for gas flow in a plate fin-and-tube heat exchanger using the CFD simulations. The values of the heat transfer coefficient obtained using the heat transfer formulas for the Nusselt number, determined with the CFD simulations, can be directly implemented in the thermal designing procedure of the cross-flow heat exchangers. The results of the numerical computations are validated experimentally.     

Bubble column with internals: Effects on hydrodynamics and local heat transfer

Local heat transfer and column hydrodynamics are investigated in a bubble column in presence of internals of different configurations. The liquid phase used is tap water and the gas phase is oil-free compressed air. The gas velocity is varied over a wide range of 0.03–0.35 m/s. The heat transfer variations are measured with a fast response probe capable of capturing bubble dynamics as well as detect local flow direction and deduce local liquid velocity. Measurements obtained in presence of internals are compared with those without internals to elucidate the effects of different internals design. Comparisons are based on average values and temporal variations obtained with the fast response probe. The average gas holdup, local liquid velocity and bubble fractions holdups obtained with and without internals are also compared to further point out the differences. The observed differences are discussed based on the insights provided by these comparisons. The results obtained show influence of internals design on column hydrodynamics which need to be considered for their proper design and modeling.     

流化床浸没换热管传热研究进展

论述了前人对流化床与浸没换热管间传热规律的理解和认识,介绍了常用的传热模型及其实用条件,总结了近年来在实验研究等方面所取得的进展。     

Effect of particles on hydrodynamics and mass transfer in a slurry bubble column: Correlation of experimental data

The effects of particle concentration and size on hydrodynamics and mass transport in an air–water slurry bubble column were experimentally studied. When the particle concentration α_s increased from 0% to 20%, the averaged gas holdup decreased by ~30%, gas holdup of small bubbles and gas–liquid volumetric mass transfer coefficient decreased by up to 50%, while the gas holdup of large bubbles increased slightly. The overall effect of particle size was insignificant. A liquid turbulence attenuation model which could quantitatively describe the effects of particle concentration and size was first proposed. Semi-empirical correlations were obtained based on extensive experimental data in a wide range of operating conditions and corrected liquid properties. The gas holdup and mass transfer coefficient calculated by the correlations agreed with the experimental data from both two-phase and three-phase bubble columns, with a maximum error <25%.     

Oxygen mass transfer coefficient in bubble column slurry reactor with ultrafine suspended particles and neural network prediction

The gas–liquid volumetric mass transfer coefficient was determined by the dynamic oxygen absorption technique using a polarographic dissolved oxygen probe and the gas–liquid interfacial area was measured using dual‐tip conductivity probes in a bubble column slurry reactor at ambient temperature and normal pressure. The solid particles used were ultrafine hollow glass microspheres with a mean diameter of 8.624 µm. The effects of various axial locations (height–diameter ratio = 1–12), superficial gas velocity (u_G = 0.011–0.085 m/s) and solid concentration (ε_S = 0–30 wt.%) on the gas–liquid volumetric mass transfer coefficient k_La_L and liquid‐side mass transfer coefficient k_L were discussed in detail in the range of operating variables investigated. Empirical correlations by dimensional analysis were obtained and feed‐forward back propagation neural network models were employed to predict the gas–liquid volumetric mass transfer coefficient and liquid‐side mass transfer coefficient for an air–water–hollow glass microspheres system in a commercial‐scale bubble column slurry reactor. © 2012 Canadian Society for Chemical Engineering     

Pressure drop and heat transfer study in tube-in-tube helical heat exchanger

In the present work attempts were made to investigate the hydrodynamics and heat transfer characteristics of tube-in-tube helical heat exchanger at the pilot plant scale. The experiments were carried out in counter current mode operation with hot fluid in the tube side and cold fluid in the annulus area. The outer tube was fitted with semicircular plates to support the inner tube and also to provide high turbulence in the annulus region. Overall heat transfer coefficients were calculated and heat transfer coefficients in the inner and outer tube were determined using Wilson plots. A commercial Computational Fluid Dynamics package [FLUENT User's Guide, release 6.0, Fluent Inc., Lebnon, NH, 1994] was used to predict the flow and thermal development in tube-in-tube helical heat exchanger. The Nusselt number and friction factor values in the inner and outer tubes were compared with the experimental data collected in the present study as well as reported in the literature. The CFD simulations were in agreement with the present experimental data. In case of literature data a reasonable comparison was found even though the boundary conditions in the present work were different.     

Experimental and numerical investigations of scale-up effects on the hydrodynamics of slurry bubble columns

Experiments and simulations were conducted for bubble columns with diameter of 0.2 m(180 mm i.d.), 0.5 m(476 mm i.d.) and 0.8 m(760 mm i.d.) at high superficial gas velocities(0.12–0.62 m·s-1) and high solid concentrations(0–30 vol%). Radial profiles of time-averaged gas holdup, axial liquid velocity, and turbulent kinetic energy were measured by using in-house developed conductivity probes and Pavlov tubes. Effects of column diameter, superficial gas velocity, and solid concentration were investigated in a wide range of operating conditions. Experimental results indicated that the average gas holdup remarkably increases with superficial gas velocity, and the radial profiles of investigated flow properties become steeper at high superficial gas velocities. The axial liquid velocities significantly increase with the growth of the column size, whereas the gas holdup was slightly affected. The presence of solid in bubble columns would inhibit the breakage of bubbles, which results in an increase in bubble rise velocity and a decrease in gas holdup, but time-averaged axial liquid velocities remain almost the same as that of the hollow column. Furthermore, a 2-D axisymmetric k–ε model was used to simulate heterogeneous bubbly flow using commercial code FLUENT 6.2. The lateral lift force and the turbulent diffusion force were introduced for the determination of gas holdup profiles and the effects of solid concentration were considered as the variation of average bubble diameter in the model. Results predicted by the CFD simulation showed good agreement with experimental data.     

Investigation of the unsteady two-phase flow with small bubbles in a model bubble column using phase-Doppler anemometry

The unsteady two-phase flow of water laden with small air bubbles in a model bubble column is investigated experimentally. Phase-Doppler anemometry (PDA) is used for measuring the velocities of water and bubbles. The measured sizes of reflecting tracers in the water and of the air bubbles are used to discriminate between water and bubble data. The investigations are focussed on the unsteady behaviour of the flow and on the interaction between the two phases. The measurement of relative (slip) velocities between bubbles and water reveals information about the dynamic behaviour of the two-phase system under the action of buoyancy on the disperse phase. The evaluation of time series of bubble velocities yields insight into typical frequencies at which the flow fluctuates. It is shown that, at all locations in the flow field, the velocity probability density functions of bubbles and liquid can be described by two superimposed Gaussian functions. The bubbles belonging to the two Gaussians exhibit different slip velocities. The probability for the occurrence of bubble collisions is quantified on the basis of the PDA data.