Enhancement of the liquid feed distribution in gas solid fluidised beds by non‐mechanically induced nozzle pulsations

The effect of non‐mechanically induced nozzle pulsations was investigated in the current work, and it was found that appropriately tailored spray nozzles pulsations resulted in the dramatic improvement of the liquid feed spray distribution on particles of a fluidised bed. Non‐mechanically induced pulsations were imposed on the liquid spray, using liquid and gas circuits that favour the development of beneficial pulsations. The resulting effect on liquid dispersion on the fluidised bed particles was determined with a conductance method. © 2013 Canadian Society for Chemical Engineering     

Statistical analysis of the phase holdup characteristics of a gas–liquid–solid fluidized bed

Experiments have been carried out to study the individual phase holdup characteristics in a cocurrent three‐phase fluidized bed. An antenna type modified air sparger has been used in the gas–liquid distributor section, for uniform mixing of the fluids with the gas moving as fine bubbles to the fluidizing section. This arrangement also reduces the pressure drop encountered through a conventional distributor used for the purpose. To overcome the non‐uniformity of flow through the column (i.e., the central region), a distributor plate with 20% open area has been fabricated with concentric circular punched holes of increased diameter from centre to the wall. Model equations have been developed by factorial design analysis for predicting various individual phase holdups.     

Phase distributions in a gas–liquid–solid circulating fluidized bed riser

The distributions of the three phases in gas–liquid–solid circulating fluidized beds (GLSCFB) were studied using a novel measurement technique that combines electrical resistance tomography (ERT) and optical fibre probe. The introduction of gas into a liquid–solid circulating fluidized bed (LSCFB), thus forming a GLSCFB, caused the increase of solids holdup due to the significantly decreased available buoyancy with the lower density of the gas, even with a somewhat increased liquid velocity due to the decreased liquid holdup giving space for the gas holdup. The gas passed through the riser in the form of bubbles, which tended to flow more through the central region of the riser, leading to more radial non‐uniformity in radial holdup of the phases. The gas velocity has the most significant effect on the gas phase holdup. While the gas velocity also has an obvious effect to the solids holdups, the liquid flow rate had a much more considerable effect on the phase holdups. The solids circulation rate also had a significant effect on the phase holdups, with increasing solids circulation rate causing much more increased solids holdup in the central region than close to the wall. A correlation was developed for the relative radial distributions of solids holdup in GLSCFB, as such radial profiles were found similar over a wide range of operating conditions, like those in a typical gas–solid circulating fluidized beds (GSCFB). Finally, the axial solids profiles in a GLSCFB was found to be much closer to those in an LSCFB which are very uniform, than those found in a GSCFB which are less uniform and sometime having a S shape. Water was used as the continuous and conductive phase, air was the gas phase and glass bead and lava rock particles were used as the solid and non‐conductive phase.     

Minimum fluidization velocity and gas holdup in gas–liquid–solid fluidized bed reactors

Experiments were performed to study the hydrodynamics of a cocurrent three‐phase fluidized bed with liquid as continuous phase. Based on the 209 experimental data (with four liquid systems and five different particles) along with 115 literature data from six different sources on minimum fluidization velocity, a unique correlation for the estimation of minimum fluidization velocity in two‐phase (u_g = 0) as well as in three‐phase systems is developed. A data bank consisting of 1420 experimental measurements for the fractional gas phase holdup data with a wide range of variables is used for developing empirical correlations. Separate correlations are developed for two flow regimes observed in this present work. The proposed correlations are more accurate and simpler to use. © 2002 Society of Chemical Industry     

The impacts of standard wall function and drag model on the turbulent modelling of gas–particle flow in a circulating fluidised bed riser

A transient turbulence model was applied to simulate the gas–particle system in a circulating fluidised bed riser. The k–epsilon turbulent equations coupled with the fluctuating energy equation were used to simulate the gas–particle system in a riser. The simulation results were validated by the experimental data of a CFB system. A grid study was implemented to examine the impact of grid discretisation. A comparison between the conventional drag models and the EMMS model was also conducted. Other factors, like the restitution coefficient particle to particle, was also found to have a significant impact on the turbulence model. © 2013 Canadian Society for Chemical Engineering     

Realization and control of multiple temperature zones in liquid‐containing gas–solid fluidized bed reactor

Fluidized bed reactors (FBRs) have been developed to establish multiple temperature zones for various industrial processes. To overcome the common weakness, this work proposed to spray liquid into bottom and upper zones, respectively, to realize multiple temperature zones FBR (MTZFBR). Temperature, pressure, and acoustic emission techniques were applied to fully characterize liquid interaction and hydrodynamics. Compared with the bottom liquid‐spraying approach, the upper liquid‐spraying approach showed higher temperature difference (ΔT) and better fluidization stability, thus was selected for further control studies. Effects of liquid flow rate, static bed height, and inlet gas temperature on MTZFBR were studied systematically. The results showed that increasing liquid evaporation behavior or decreasing liquid bridge behavior enhance ΔT and fluidization stability and vice versa. G–L–S fluidization pattern depended mostly on the liquid behaviors and fluidization stability, and thus the stabilized MTZFBR could be regarded as a coexisted mode of two distinctive G–L–S fluidization patterns. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1454–1466, 2016     

Enhancement of the liquid feed distribution in gas‐solid fluidized beds by nozzle pulsations (induced by solenoid valve)

In this work, it was found that spray nozzles pulsations greatly improved the liquid feed spray distribution on fluidized bed particles. Pulsating a spray nozzle doubled its nozzle performance index at various operating conditions. The objective of this study was to impose fluctuations of well‐defined frequency and amplitude on the liquid spray to investigate potentially beneficial effects of fluctuations on the liquid feed distribution on the particles in the fluidized bed. Three sets of experiments were conducted to study the quality of the spray jet‐bed interaction using a conductance probe method. The jet penetration for each experiment was calculated theoretically. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Friction on a solid sphere exposed to gas–liquid and gas–liquid–solid flow in bubble column and fluidized bed reactors

Local velocity gradients on a solid spherical surface have been studied in a bubble column and in two- and three-phase fluidized beds, in order to clarify the influence of gas flow. The electrochemical method, measuring apparent local mass transfer coefficients, was verified and used to obtain the local velocity gradients, shear stresses and total frictional forces. The observed mass transfer rate was independent of liquid velocity, owing to a non-changing flow structure around the particles and not to averaging opposing effects. The identity in flow structure also held for three-phase fluidized beds up to a superficial gas velocity of 5 cm s^?1. The dramatic increase in velocity gradient on gas introduction was not a result of decreased homogenous density, but was caused by a change in the turbulent structure around a particle, leaving a larger portion of the total drag as frictional drag, thus improving the mass transfer characteristics of the bed. Use of velocity gradient measurements, including span of fluctuations and exposure time, to predict biomass growth and mechanical degradation in a reactor is also discussed.     

Characterization on hydrodynamic behavior in liquid‐containing gas‐solid fluidized bed reactor

In many industrial processes involving gas–solid fluidized bed rectors, the addition of a liquid phase significantly alters the hydrodynamics. To fully characterize the hydrodynamics in the fluidized bed, pressure and acoustic measuring techniques were applied to study the behavior of gas bubbles and particles. A camera was used to take pictures to verify the pressure and acoustic results. During the liquid‐addition process, the pressure technique captured the bubble size variation and bubble motion while the acoustic technique reflected particle motion and particle size growth. Hurst and V‐statistics analyses of acoustic emission were used for the first time to detect periodic behavior during the injection process. The new break formation and change trend of V_max were used as the criteria to judge occurrence of abnormal fluidization states, such as agglomeration and gas channeling formation. These measurement techniques are beneficial in the elimination of adverse effects caused by the addition of liquid. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1056–1065, 2013     

Performance evaluation of the anaerobic fluidised bed system: I. Substrate utilisation and gas production

COD removal efficiencies in the range 75 to 98% were achieved in an anaerobic fluidised bed system designed for the recovery of methane from liquid wastes, when evaluated at COD loadings of between 5.8 to 108 kg m^?3 day^?1, hydraulic retention times of between 4.45 to 8 h, and feed COD concentrations of beween 480 to 9 000 mg dm^?3. More than 90% of feed COD could be removed up to COD loadings of about 40 kg m^?3 day^?1. Up to around 300 dm² of methane were produced per kg COD removed and this methane production rate was independent of the COD loadings applied in this investigation. Volatile acid concentration in the reactor increased sharply at a COD loading of about 40 kg m^?3 day^?1 and therefore, sufficient alkalinity should be provided to prevent pH from dropping to the undesirable level. The anaerobic fluidised bed system can be operated at a significantly higher liquid throughputs while maintaining its excellent efficiency.     

Axial liquid dispersion in a liquid–solid circulating fluidized bed

Although axial liquid dispersion has been studied extensively in particulate fluidized beds, no data has been reported previously in a liquid–solid circulating fluidized bed (LSCFb). In this work, the axial liquid dispersions at various radial positions were measured in an LSCFB of 76 mm in diameter and 3.0 m in height using a dual conductivity probe. The results reveal that the axial liquid dispersion is affected not only by the operating conditions but by the radial positions as well. A local axial dispersion model is proposed to describe the axial liquid dispersion at various radial positions. The local axial liquid dispersion coefficients determined by the proposed model are greater at the axis than near the wall region of the riser. This nonuniformity of axial liquid dispersion is believed to be caused by the radial nonuniform distribution of liquid velocity, and bed voidage in the LSCFB can significantly affect the axial liquid dispersion.     

Bubble dynamics and its effect on the performance of a jet fluidised bed gasifier simulated using CFD

A computational fluid dynamic modelling study of a jet fluidised bed gasifier has been carried out. The modelling was based on the Eulerian-Eulerian models for gas and solid flows, which take into account the hydrodynamics, mass and heat transfer, and heterogeneous and homogeneous reactions. The bubble dynamics was simulated in detail, enabling its effect on temperature distributions, gasification reactions and gas compositions in the bed to be examined. The results revealed that jet growth, bubble rise, and the associated convective flow play a significant role in the heat exchange and mass transfer, and in turn, affect the gasification reactions.     

Hydrodynamic characterisation of liquid–solid two–phase fluidised beds: Vibration signature and pressure fluctuations analyses

Hydrodynamic behaviour of a two–phase liquid–solid fluidised bed was investigated over a wide range of liquid velocities by means of simultaneous vibration and pressure fluctuations analyses. The liquid velocities were set in a way that covered two most important hydrodynamic events in the bed, namely minimum fluidisation and circulating‐solid regime. To prevent solids from being carried out of the bed, the maximum liquid velocity was kept lower than the terminal velocity of solids. Statistical analysis on the vibration signatures of bed shell proved to be a strong representative for minimum fluidisation characterisation and solid regime change. The minimum fluidisation velocity can be obtained from the intersection of two linear parts in the standard deviation of vibration fluctuation signals. Moreover, the kurtosis of vibration signals could predict the minimum fluidisation and approximate solid regime transition successfully. Meanwhile, statistical parameters, such as standard deviation, skewness, and kurtosis as well as newly‐introduced parameters, namely the energy and average cycle frequency of pressure signals, determined both of minimum fluidisation condition and circulating‐solid flow regime. © 2011 Canadian Society for Chemical Engineering     

Hydrodynamic modeling on the external liquid–solid wetting efficiency in a trickling flow reactor

A three‐region model was proposed, which considers the bed cross section being composed of a stagnant liquid region, a liquid film region, and a rivulet flow region. To estimate the fractions of the three regions, the fraction of film flow was evaluated first, by transforming the complex trickling flow texture into pure liquid film flow. Through the measurements of liquid holdup and pressure drop for the film flow, a relationship between relative permeability and gas saturation was established, and from which the fraction of film flow region was obtained. It shows packing size is most important to the faction of rivulet flow. The external wetting efficiency of the packing was correlated as the sum of two‐third power of the liquid film fraction and the rivulet flow fraction, besides, a correlation based on Reynolds and Galileo numbers of the two phases in the form of was proposed. © 2012 American Institute of Chemical Engineers AIChE J, 59: 283–294, 2013     

Experimental investigation of electrostatic effect on bubble behaviors in gas‐solid fluidized bed

Electrostatics and hydrodynamics in the fluidized bed are mutually affected, and excess accumulation of electrostatic charges has a severe impact on hydrodynamics. However, there is a serious lack of experimental investigation of electrostatic effect on hydrodynamics. This work provides a first insight into the electrostatic effects on bubble behaviors experimentally by injecting a trace of liquid antistatic agents (LAA) into a fluidized bed. Different amounts of LAA (0–50 ppm) were injected to make the electrostatic charges vary in a wide range and the bubble behaviors were investigated simultaneously. Results showed that the charges on particles decreased with increasing amount of LAA, which resulted in larger bubble sizes, stronger fluctuations of dynamic bed height, and less wall sheeting, respectively. The maximum reduction ratio of bubble sizes due to electrostatic effect was 21%. When particles were charged, the bubble sizes were significantly smaller than those estimated from the classical correlation. This discrepancy was attributed to the neglect of electrostatic effect in classical correlation. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1160–1171, 2015     

Minimum fluidization velocity in gas‐liquid‐solid minifluidized beds

The initial fluidization characteristics of gas‐liquid‐solid minifluidized beds (MFBs) were experimentally investigated based on the analyses of bed pressure drop and visual observations. The results show that U_Lmf in 3–5 mm MFBs can not be determined due to the extensive pressure drop fluctuations resulting from complex bubble behavior. For 8–10 mm MFBs, U_Lmf can be confirmed from both datum analyses of pressure drop and Hurst exponent at low superficial gas velocity. But at high superficial gas velocity, U_Lmf was not obtained because the turning point at which the flow regime changes from the packed bed to the fluidized bed disappeared, and the bed was in a half fluidization state. Complex bubble growth behavior resulting from the effect of properties of gas‐liquid mixture and bed walls plays an important role in the fluidization of solid particles and leads to the reduction of U_Lmf. An empirical correlation was suggested to predict U_Lmf in MFBs. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1940–1957, 2016     

Experimental study on gas-solids flows in a circulating fluidised bed using electrical capacitance tomography

It is difficult to measure the gas-solids flow in a circulating fluidised bed (CFB) because of the complicated and rapid transient process. Electrical capacitance tomography (ECT), a cross-sectional imaging technique, has been used to measure the dilute flow in a large square CFB. A sensor has been specifically designed for the measurement and a new algorithm has been developed for image reconstruction. Flow conditioning parts (internals) are designed and placed inside the CFB, aiming to enhance the contact between gas and particles in the dilute gas-solids flow. The dynamic characteristics and detailed information were obtained on two sections in the bed at different height. The performance of the internals is related to their size, combination, height in the bed and the superficial gas velocity. It has been confirmed that a particular combination of internals can increase the solids concentration in the central area of a cross-section, and can improve the probability density distribution (PDD) with a moderate gas velocity. Using a combination of a large internal at an upper location and a small one at a lower location can optimise the flow in the CFB.     

Influence of an orifice on liquid–liquid two phase flow

The present study aims an in depth investigation of liquid–liquid horizontal flow through an orifice. Initial studies have been directed to observe the influence of the orifice plate on the phase distribution of the two liquids in the pipe. The flow patterns have been identified using an optical probe along with photographic technique. The probability density function (PDF) analysis of the random signals obtained from the optical probe has been adopted to quantify the observations. The cross‐correlation function between the probe signals upstream and downstream of the orifice has been estimated to check the repeatability of the phenomenon. The inception of dispersion in the downstream section has been observed to occur in the stratified region of the upstream. The use of an orifice as a homogenizer as well as a feasible flow‐metering device for liquid–liquid flow has been encouraged by experimental results.