Bubbles In Bed Research Articles

Non-invasive imaging of shallow bubble columns using electrical capacitance tomography

This paper deals with application of non-invasive electrical capacitance tomography to study the hydrodynamics of shallow bed bubble columns. Two bubble columns with different height to diameter ratio were used. Air–kerosene system that represents dielectric two-phase mixture was investigated. The ECT provided good measurement of the gas holdup at different gas velocities compared to the classical pressure measurements. The ECT was able to provide the gas hold up and the bubble velocities distribution across the column diameter at different gas velocities. The study revealed that spatial gas holdup and bubble velocity distributions are sharp with parabolic shape in the small bubble column (HD/DC=5). However, in the large bubble column (HD/DC=4) the gas holdup and bubble velocity profiles were flatter indicating improvement in the mixing homogeneity and leading to well-mixed reactor. 3D graphical visualization of the flow regimes and transition points were also examined using the ECT. In the small bubble column flow regimes were heterogeneous to slugs flow especially at high flow rate, resulted in downward flow near the walls and imperfect mixing.

Liquid flow residence time in a fibrous fixed bed reactor with recycle

Waste removal efficiency of gas–liquid biofilter reactors for waste water treatment depends on its flow regime and residence time distribution (RTD) as key parameters of bio-reactor performance. The present study reports RTD regime in a fibrous fixed bed biofilm reactor related to a fluid velocity range appropriate for biofilm operation. The data from tracer experiments are correlated in terms of the one-parameter “tanks-in-series” model. The aerated fibrous bed reactor RTD function is found to be dependent on net liquid and gas phase superficial velocity U L and U G . Liquid internal recirculation exhibited small effect comparable with the effect of net liquid flow. A power law relationship relating the number of perfectly mixed cells with liquid and gas superficial velocity is elaborated. Assuming similarity of the prototype and real vessels’ flow fields, the equation as well as its corresponding range of fluid velocity can be used for bio-reactor design and scale-up. Comparison over the model parameters obtained in fixed bed bubble columns at low fluid velocity shows the results of this study to be comparable with previous data of mesh wire packing.

Identification of defluidization region in a gas-solid fluidized bed using a method based on pressure fluctuation measurements

Industrial applications that involve fluidized bed operations must prevent the undesirable phenomenon of partial or complete bed defluidization. Defluidization can be avoided by increasing the gas velocity and/or, in some cases, changing the solid feed conditions in the system, provided that the changes in the hydrodynamics of the flow are detected early enough. The use of a technique that can perform an early detection of the defluidization condition in industrial applications is important, in order to avoid the loss of efficiency or even an undesirable shutting down of the process. The objective of this work is to show the application of a method for early detection of the condition where the bed is tending to the defluidization, in a gas-solid fluidized bed flow. The method is based on pressure fluctuation measurements. Experimental tests are carried out using two solid particles: microcrystalline cellulose and sand. Results show that the proposed method is efficient in detecting the fluidization condition in a conventional bubbling bed regime. The potential of application of the technique is also shown for the control of the defluidization phenomenon in industry.

Flash pyrolysis of grape residues into biofuel in a bubbling fluid bed

The pyrolysis of two grape residues (grape skins and the mixture of grape skins and seeds) has been carried out in a pilot bubbling fluidized bed pyrolyzer operating under a range of temperature from 300 to 600 °C and three vapor residence time (2.5, 5, and 20 s), with the aim of determining their pyrolysis behavior including products yields and heat requirements. The composition of the product gases was determined, from which their heating value was calculated. The liquid bio-oil was recovered with cyclonic condensers and separated into two phases, an aqueous phase and an organic phase. The chemical composition of these liquid phases was characterized. In addition, the environmental parameters of the distilled fraction (85–115 °C) of the aqueous phase were tested, while the heating value of the organic phase was determined. Furthermore, the thermal sustainability of the pyrolysis process was estimated by considering the energy contribution of the product gases and of the liquid bio-oil in relation to the pyrolysis heat requirements. The optimum pyrolysis temperatures were identified in terms of maximizing the liquid yield, maximizing the energy from the product bio-oil, and maximizing the net energy from the product bio-oil after ensuring a self-sustainable process by utilizing the product gases and bio-oil as heat sources.

Measurement of bubble bed depth in dissolved air flotation using a particle counter

In the development of dissolved air flotation (DAF) technology, it is noteworthy that higher hydraulic loading rates are achieved while the reactors become deeper resulting in a smaller footprint. One of the reasons for the increased efficiency is claimed to be the deeper bubble bed depth in the flotation separation zone; however, there has been limited research on the characterization of the bubble bed. In this research, a method for locating the bubble bed interface using a commercially available particle counter is presented. The results compared remarkably well with observations by the naked eye at a pilot plant with a transparent acrylic wall. The method is then applied to locate the bubble bed interface at an operating DAF plant in Wonju, Korea. We can observe that the bubble bed profile changes according to the operational conditions and that higher effluent water quality is obtained with a deeper bubble bed. Based on these results, optimization of DAF process might be possible by monitoring the bubble bed profile in the reactor.

Interpretation of the hydrodynamic behaviour in a conical fluidized bed dryer

Wet batches of placebo pharmaceutical granule were dried at inlet superficial gas velocities of 0.64 and 1.3 m/s in a Glatt GPCG-1 fluidized bed. Using pressure fluctuation analysis, the hydrodynamic behaviour indicates a transition from a multiple bubbling regime to a coalescence dominated regime as drying proceeds. The transitional fluidization behaviour is linked to the physical mechanisms associated with the constant and falling rate periods of drying porous materials. Excess surface moisture present during the constant rate period increases interparticle forces through liquid bridging. These liquid bridges stabilize the bed structure which limits bubble formation in the bed. Once the falling rate period is reached, the liquid bridges cannot be maintained and bubble coalescence increases. The resulting bubbling bed hydrodynamics can be explained using the simple two-phase model proposed by Toomey and Johnstone [1952. Gas fluidization of solid particles. Chemical Engineering Progress 48, 220–226] using the full support velocity and bed voidage characteristics of the granule at varying moisture contents.

Experimental investigation of a rotating fluidized bed in a static geometry

Rotating fluidized beds in a static geometry are based on the new concept of injecting the fluidization gas tangentially in the fluidization chamber, via multiple gas inlet slots in its cylindrical outer wall. The tangential injection of the fluidization gas fluidizes the particles tangentially and induces a rotating motion, generating a centrifugal field. Radial fluidization of the particle bed is created by introducing a radially inwards motion of the fluidization gas, towards a centrally positioned chimney. Correctly balancing the centrifugal force and the radial gas–solid drag force requires an optimization of the fluidization chamber design for each given type of particles. Solids feeding and removal can be continuous, via one of the end plates of the fluidization chamber. The fluidization behavior of both large diameter, low density polymer particles and small diameter, higher density salt particles is investigated at different solids loadings in a 24 cm diameter, 13.5 cm long non-optimized fluidization chamber. Scale-up to a 36 cm diameter fluidization chamber is illustrated. Provided that the solids loading is sufficiently high, a stable rotating fluidized bed in a static geometry is obtained. This requires to minimize the solids losses via the chimney. With the polymer particles, a dense and uniform bed is observed, whereas with the salt particles a less dense and less uniform bubbling bed is observed. Solids losses via the chimney are much more pronounced with the salt than with the polymer particles. Slugging and channeling occur at too low solids loadings. The hydrostatic gas phase pressure profiles along the outer cylindrical wall of the fluidization chamber are a good indicator of the particle bed uniformity and of channeling and slugging. The fluidization gas flow rate has only a minor effect on the occurrence of channeling and slugging, the solids loading in the fluidization chamber being the determining factor for obtaining a stable and uniform rotating fluidized bed in a static geometry.

Analysis of the bubbling behaviour of 2D gas solid fluidized beds

Bubble characteristics are very important in the design of fluidized beds because they govern hydrodynamics and efficiency of the operation for which the bed is used. In this work, a digital image analysis technique has been developed to study the fluidization dynamics of a lab-scale two-dimensional bubbling bed. Digital image analysis may supply a great quantity of information; it is non-intrusive, capable of securing several properties simultaneously and cost effective. The image analysis method here developed allows for the simultaneous measurements of various significant bubble properties, i.e. bubble size and bubble velocity distributions, bed height and bubble-phase hold-up, by means of a purposely in-house developed software. Present results were compared with relevant literature correlations and resulted in sound agreement, thus confirming the large potential of the technique here developed.

Effect of surfactant on homogeneous regime stability in bubble column

Experiments were carried out to investigate the effect a surface active agent on homogeneous–heterogeneous flow regime transition in a laboratory scale bubble column. Air and water with various amount of CaCl 2 were the phases. The (voidage e) - (gas flow rate q) dependence was measured. The critical point where the homogeneous regime loses stability and the transition begins was evaluated by several methods. These methods are based on the slip speed concept and the drift flux model. The critical values of voidage and gas flow rate were taken as the quantitative measures of the homogeneous regime stability. They were plotted against the surfactant concentration. It was found that the surfactant has a dual effect on both the voidage and the regime transition: low concentration stabilizes and larger concentration destabilizes the homogeneous bubble bed. At present, we do not have an explanation to these observations. Possible physical mechanisms of the surfactant effect are expected to be revealed by further experiments, which are currently under way.

Abstract 3433: The Effect of Normal Pulmonary Arterial Bed Bubble Contrast Conductance and Latency on Bubble Contrast Echocardiography: Does “Any Bubbles” in the left atrium always mean Patent Foramen Ovale (PFO)?

BACKGROUND: The presence of a PFO remains a controversial possible mechanism for a significant number of cryptogenic strokes. However, unvalidated and arbitrary criteria of left heart contrast-bubble appearance on echocardiography form the basis for PFO diagnosis, treatment, and research. We postulated that variable conductance of contrast bubbles through the pulmonary artery (PA) vascular bed, as well as bubble latency can result in the misdiagnosis of PFO. METHODS: Consecutive pts (N=50) undergoing PFO closure by intra-cardiac echocardiography (ICE) guided imaging and right heart catheterization were studied. PULMONARY CONDUCTANCE: Prior to septal closure, ICE imaging of the left atrium was performed during selective PA bubble contrast injections (BCI) at baseline and after injection of 200micrograms of nitroprusside (NP). The appearance of left atrial bubbles was quantified by a validated 1–5 grading system (5–100 bubbles) at rest and following NP. A definite NP response was determined to be present if 2 observers agreed there was at least one grade level increase in bubble conductance from rest. PULMONARY LATENCY: 120 seconds after right atrial BCI, trans-tricuspid ICE pulmonic valve imaging was performed for count of residual bubbles as a correlate of bubbles remaining in the PA at rest. RESULTS: Age was 42 ± 18 years; 70% were female. Pulmonary conductance was present in most pts (84% =grade 1–2, 8% =grade 3– 4). Residual bubbles at the pulmonic valve at 2 minutes were common (range 0 –30 bubbles, mean 6.2 ± 6.5; 42% >5 bubbles). “Definite” increase in conductance after NP was present in 38% of pts. The difference between baseline and NP grade agreed with subjective “definitive” change and “no change” readings (1.26 vs −0.03; p=0.0001). Augmentation of pulmonary conductance by NP was significant (grade 1.32 ±.79 vs 1.74 ± 1.1; p=0.0001) and definitive change was more likely in women than in men (45.7% vs 20.0%; p=0.08). CONCLUSION: We conclude that: pulmonary conductance of bubbles is common, variable, modifiable, and more common in women. Bubble latency in the pulmonary system may erroneously augment Valsalva shunt evaluation. Caution should be exercised when equating lower bubble counts on echocardiography with the diagnosis of PFO.

Fluidized Bed Steam Gasification of Solid Biomass - Performance Characteristics of an 8 MWth Combined Heat and Power Plant

The work focuses on a dual fluidized bed gasification technology that is successfully operated for combined heat and power production at a scale of 8 MWth in Guessing/Austria since 2002. The reactor concept consists of a circulating fluidized bed system with a steam-fluidized bubbling bed integrated into the solids return loop. Accompanying the operation of the commercial scale plant, parameter models have been developed and validated by comparison to measured data. As the models naturally fulfill mass and energy balances, the simulation also allows the validation of measurement data. The behaviour of the plant is studied by carrying out variations of selected parameters. Evaluation of different plant operation cases yields correlations between process variables. The solids circulation rate is shown versus riser exit velocity. Fuel water content and gasification temperature significantly influence global plant performance. Simulation predicts the efficiency of the existing power plant in optimized operation. Finally, part load behaviour is investigated and performance maps of the CHP plant are presented. High fuel water content at high gas engine load results in high gas velocities in the riser (erosion limit) and higher heat ratio in the produced energy. It is concluded that CHP-concepts based on fluidized bed steam gasification can reach high electric efficiencies and high overall fuel utilization rates even at small plant capacities of about 10 MWth.

Numerical prediction of particle mixing behavior in a bubbling fluidized bed

In this article the hard-sphere discrete particle model (DPM) is used to study the mixing behavior of particles in the 2-D fluidized bed. Different flow patterns in the bed for two kinds of inlet configurations, namely free bubbling and jet bubbling mode, are captured by the numerical model, under specific superficial gas velocities. To examine the degree of particle mixing, the Fan index is applied. The numerical results show that the rate of particle mixing is larger in the jet bubbling than that in the free bubbling mode. The gross circulations of particles in the jet bubbling bed give a higher degree of mixing because of the involvement of a greater number of particles.

A local transient approach to monitoring fluidization quality in freely bubbling beds

In fluidization processes, the optimal contacting regime is dictated by the physical or chemical process in question, and can vary significantly from application to application. Even within the specified regime, it is often necessary to place constraints on the hydrodynamic behavior in order to meet operational requirements. While the ability to do this is very important, there is a lack in current technologies' ability to identify the operational state via direct measurement. This work adopts a local transient approach towards addressing this problem. In order to study the local fluid bed dynamics, needle-type capacitance probes were employed to gather local transient solids fraction data in a bench-scale bubbling bed. The time-series were then subject to analysis in order to extract various signal invariants. The parametric effects of axial measurement location, superficial gas velocity, and mean particle size were examined in order to identify favorable heuristic trends in the signal invariants with respect to global parameters governing fluidization quality (i.e., superficial gas velocity and mean particle size). This work demonstrates how these invariants then constitute the basis for a novel regime map based monitoring scheme for quality of fluidization with potential application to feedback control.

Effect of solid characteristics on hydrodynamic and mass transfer in a fixed bed reactor operating in co-current gas–liquid up flow

In order to understand better which parameters control the mass transfer efficiencies in biofilters, hydrodynamic and mass transfer parameters have been investigated for seven different packings in a three phase fixed bed reactor operating in co-current gas–liquid up flow. For this purpose, the packing characteristics (bed porosity, particle size and particle shape) and hydrodynamics parameters have been determined. Thus, the pressure drops, global gas hold-up, static gas fraction, slip velocities and bubble size have been investigated. For each type of solid, volumetric oxygen mass transfer coefficients have been measured under different operating conditions of gas (air) and liquid (tap water). The experiments were performed in a cylindrical PVC column with co-current up-flow of gas and liquid at laboratory temperature and atmospheric pressure. Air was injected at the bottom of the column using a porous disc diffuser. The superficial gas velocity ranged from 2.3E−3 m/s to 2.5E−2 m/s, the superficial liquid velocity varied from 1.2E−3 m/s to 3.3E−3 m/s. The column of 0.15 m in diameter and 4 m height was filled with packing. The following values were determined: • Pressure drop through the bed height was measured using differential manometers. • The average values of the global gas hold-up and static gas fraction were determined in the experimental set up by a new technique which is based on a monitoring of liquid flow. • The bubble sizes and slip velocities were determined by image acquisition and data treatment systems on a particular glass cell. • The volumetric oxygen mass transfer coefficient was determined by gas and liquid mass balance techniques. These different techniques applied on the fixed bed bubble column enabled for each type of solid a complete characterisation of hydrodynamic and mass transfer to be described. These experimental tools were used to compare and evaluate the influence of solid characteristics on the reactor performances and to initiate a mass transfer model.

Integration of ECT measurements with hydrodynamic modelling of conventional gas–solid bubbling bed

The electrical capacitance tomography (ECT) provides fast images of the cross-sectional concentration distribution of solid–gas flow in a confined volume. This information can be integrated with numerical simulation to estimate some of the most important hydrodynamic quantities in solid–gas flow, such as the particles velocity, interstitial gas velocity and particle–particle contact forces. In this study, using the two-fluid approach, momentum and energy balance equations, along with the appropriate boundary conditions, have been solved by integrating the numerical procedure with the experimental data of the fluidised bed pressure drop and pixel distribution of particle concentration available from the ECT measurements. Preliminary results of time-dependent hydrodynamic features of the bed are presented. These results were analysed and assessed using the available experimental literature data on conventional bubbling fluidised bed. In general, it is demonstrated that the integration of ECT measurements with numerical modelling offers a unique and promising technique for comprehensive non-intrusive information on gas–solid flow systems.

Fluidized bed combustion of a diesel fuel: A modeling interpretation for micro-explosions

The present paper deals with the combustion of liquid fuels in bubbling fluidized bed. The focus is on micro-explosions that were experimentally observed when burning a commercial bio-diesel in the intermediate temperature range 873–1023 K. The measured micro-explosions frequency fell in the range 2.4–3.8 Hz. A simplified model for the combustion of a rising fuel-bubble was purposely developed. It applies to a single non-premixed bubble that can undergo coalescence with other bed bubbles. A mechanistic approach is also included in the model in order to relate the frequency of micro explosions to the frequency by which relatively well-premixed bubbles approach the bed surface. The number of coalescences experienced by the endogenous bubble during its residence time is less than one, as predicted by the model with reference to the experimental conditions reported in the paper. Even with large simplifications, the model is able to explain the temperature dependence of the micro-explosive behavior. The accordance between the experimental frequency of micro-explosions and the predicted frequency of coalescence-free bubbles reaching the bed surface is rather good.

Bubble distribution and behaviour in bubbling fluidised beds

Bubble spatial distribution is a prime determinant of the process properties within a fluidised bed, and is coupled to and governed by the flow supply. Bubbles induce heterogeneity and the gas within them rises through the bed without much interaction with other phases. As more flow generates more bubbles, process efficiency can be severely affected. A planar gas–solid fluidised bed was used for observation of the bubbling phase directly and explicitly using real-time vision instrumentation. Frequency domain and statistical analyses of the bed bubble void fraction led to several descriptions of the bed dynamics. Spatial and temporal imaging analysis was used to process image data of the bed and has produced a visual indication of the bubble residency time history patterns for a given state. The results of the analysis further supported and consolidated the relationship between bubble spatial distribution and bed behaviour. The frequency and standard deviation of local changes in the state of the bed are demonstrated to be useful for inferring the bed fluidisation quality. It was also shown that the interaction between the bubbles and the vessel walls affected bubble placements in the bed immediately after introduction until their departure at the top. Subsequently, it was observed that at a given time the spatial distribution of bubbles governed the behaviour of successive bubbles, their properties as well as their positions further up the bed. Also, it is proposed that the introduction of bubbles completely alters the bed dynamics compared with those for smooth fluidisation, and this dominates the whole process.

Preparation of TiCl 4 with the Titanium Slag Containing Magnesia and Calcia in a Combined Fluidized Bed

This paper describes a new method for producing TiCl 4 by chloridizing materials of high content CaO and MgO, in which a combined fluidized bed is used as a reactor to avoid agglomeration between particles caused by molten CaCl 2 and MgCl 2. The combined fluidized bed consists of at least a riser tube and a semi-circulating fluidized bed. Two kinds of high titanium slag, in which the total mass content of CaO and MgO is 2.03% and 9.09% respectively, are employed to examine the anti-agglomeration effect and the conversion of the materials when the temperature ranges are between 923.15K and 1073.15K, gas apparent velocity 0.7–1.1m·s −1, and inlet amount of solid materials is 4.6–7.0kg·h −1. It is found that the anti-agglomeration effect in the combined fluidized bed is satisfactory and the new method can achieve a TiCl 4 production capacity of 14.0–75.4t·m −2·d −1 in relation to 25.0–40.0t.m −2·d −1 from the conventional bubble bed. Furthermore, low-temperature chloridization, for example, at 923 K or 973K, can also be used to produce TiCl 4 and avoid agglomeration.

Characterizing particle dispersion by image analysis in ICFB

A new particle image technique was developed to analyze the dispersion of tracer particles in an internally circulating fluidized bed (ICFB). The movement course and the concentration distribution of tracer particles in the bed were imaged and the degree of inhomogeneity of tracer particles was analyzed. The lateral and axial dispersion coefficients of particles were calculated for various zones in ICFB. Results indicate that the lateral diffusion coefficient in the fluidized bed with uneven air distribution is significantly higher than that in uniform bubbling beds with even air distribution. The dispersion coefficients are different along bed length and height.

Modeling of Sorption-Enhanced Steam Reforming in a Dual Fluidized Bubbling Bed Reactor

This paper highlights the use of a dual fluidized bed reactor system for producing hydrogen by sorption-enhanced steam methane reforming. Hydrogen concentrations of >98% are predicted for temperatures of ∼600 °C and a superficial gas velocity of 0.1 m/s, using a simple two-phase bubbling bed model for the reformer. The kinetics of the steam methane reforming and water-gas shift reactions are based on literature values, whereas experimentally derived carbonation kinetics are used for the carbonation of a dolomite. It is shown that the reformer temperature should not be <540 °C or >630 °C for carbon capture efficiencies to exceed 90%. Operating at relatively high solids circulation rates to reduce the need for fresh sorbent is predicted to give higher system efficiencies than for the case where fresh solid is added. This finding is attributed to the additional energy required to decompose both CaCO3 and MgCO3 in fresh dolomite.