Minimum Gas Velocity Research Articles

CaptiveSpray Differential Ion Mobility Spectrometry Device with Enhanced Ion Transmission and Improved Resolving Power

A performance enhanced CaptiveSpray differential ion mobility device was designed and constructed by incorporating a circular channel and a gas flow homogenizing channel (GFHC) between the CaptiveSpray ion source and planar differential ion mobility spectrometry (DMS). The GFHC was used to reduce gas flow heterogeneity prior to the entrance of the DMS device. The optimal flared entrance greatly reduces gas flow velocity at the inlet region owing to its relatively large gas inlet interface, which assists in reducing disparities between the minimum and maximum gas velocity along the x-axis. The circular electrode was machined with channels along the x- and y-axis for the passage of auxiliary gas and was applied with a potential to focus the incoming ions from the CaptiveSpray source into the DMS channel. Using reserpine as a reference standard, substantial signal enhancement was achieved with a concomitant reduction of the peak width in the ionogram.

Effect of louver baffle on the stability and separation performance of the gas-solid separation fluidized bed

Bed density stability is often the dominant factor restricting the efficiency of coal beneficiation in gas-solid separation fluidized beds (GSSFB). Here, in order to improve the fluidization quality, a louver baffle was introduced into a conventional GSSFB, converting it into a so-called louver baffle separation fluidized bed (LBSFB). The influence of the louver baffle on the fluidization characteristics and coal separation efficiency of the GSSFB were investigated using a combination of experiments and numerical simulations. The minimum fluidized gas velocity, pressure drop, and cross section density of the bed were measured when baffle was placed at two different positions (H=40 mm and H=80 mm). Meanwhile, the evolution of bubble generation and particle flow were simulated when the baffle was placed at H= 40 mm and the operating gas velocity was 7.52 cm/s. The results showed that the use of a single guide baffle could effectively break up the bubbles in the bed, inhibit bubble growth and particle backmixing, and restore the uniform distribution of the updraft. When the louver baffle was installed at a height of H = 40 mm and the superficial gas velocity was 7.52 cm/s, the standard deviation of radial density fluctuation in the bed was reduced to 0.028 g/cm3, which is more than 35% lower than that of the conventional GSSFB, and that of the axial density fluctuation was reduced to 0.146 g/cm3. Under these conditions, the separation of 6–13 mm raw coal in a LBSFB resulted in a clean coal yield of 69.97% and a probable error, E, value of 0.082 g/cm3

Hydrodynamic characteristics of a rectangular gas-driven inverse liquid-solid fluidized bed

The hydrodynamic characteristics of a rectangular gas-driven inverse liquid-solid fluidized bed (GDFB) using particles of different diameters and densities were investigated in detail. Rising gas bubbles cause a liquid upflow in the riser portion, enabling a liquid downflow that causes an inverse fluidization in the downer portion. Four flow regimes (fixed bed regime, initial fluidization regime, complete fluidization regime, and circulating fluidization regime) and three transition gas velocities (initial fluidization gas velocity, minimum fluidization gas velocity, and circulating fluidization gas velocity) were identified via visual observation and by monitoring the variations in the pressure drop. The axial local bed voidage (ε) of the downer first decreases and then increases with the increase of the gas velocity. Both the liquid circulation velocity and the average particle velocity inside the downer increase with the increase of the gas velocity in the riser, but decrease with the particle loading. An empirical formula was proposed to successfully predict the Richardson-Zaki index “n”, and the predicted ε obtained from this formula has a ±5% relative error when compared with the experimental ε.

Flooding Characteristics and Countermeasures in a PEM Fuel Cell System

PEM fuel cell systems in automotive application face challenging power requirements, as they must provide full load as well as part load efficiently. Especially in systems without an external humidifier, relatively low cathode stoichiometries and increased operating pressures are the key to prevent membrane dry-out by increasing the local relative humidity. During low load operation at increased temperature and pressure, low gas velocities within the flow field occur. Too low gas velocities are known to result in a transition of the two-phase flow from film flow to slug flow regime [1]. This transition within the flow field channel structure can result in a local or global blockage of the reactant paths by liquid water droplets.While the flow regime transition within simple channel geometries is well known in literature, there is a lack of experimental data for full stack situations in fuel cell systems. Some work regarding the stationary minimal flow limitation exists [2], but countermeasures enabled by dynamic operation of the whole system are seldom in focus of systematic studies. However, well guided dynamic operation enables robust low load capability. The negative impact of flooding on stack power and lifetime can be minimized and a possible dry-out can be prevented, while maintaining a relatively low required compressor energy consumption.In the work to be presented, targeted experiments conducted with an automotive fuel cell system confirm the existence of a minimum cathode gas velocity for a stable operation and its relevance in application-like operation. It is shown that during low load operation of a non-humidified system, the targets of sufficient membrane humidification and liquid water drainage are incompatible. Because of a response time of multiple seconds until flooding or dry-out occurs, an intermittent operation with periodic switching between two operating points becomes feasible. During the first phase, the membrane is humidified due to a relatively low cathode stoichiometry and an increased cathode inlet pressure. As the liquid water accumulation becomes critical, the pressure is decreased and the stoichiometry is increased for the second phase, where droplets are blown out by utilizing a high gas velocity, as shown in figure 1.The exemplary results suggest that dynamic water management analysis can widen the operating range of a fuel cell system towards lower loads due to dynamic control. It is expected that a deeper understanding of the underlying processes will allow the development of optimized control strategies in the future.[1] M. Grimm, E.J. See, S.G. Kandlikar. Modeling gas flow in PEMFC channels: Part I – Flow pattern transitions and pressure drop in a simulated ex situ channel with uniform water injection through the GDL. Int. Journal of Hydrogen Energy 37, 12489-12503 (2012).[2] D. Jenssen, O. Berger, U. Krewer. Anode flooding characteristics as design boundary for a hydrogen supply system for automotive polymer electrolyte membrane fuel cells. Journal of Power Sources 298, 249-258 (2015).Figure 1: Normalized current, high frequency resistance, cell voltages, cathode stoichiometry, cathode inlet pressure and cathode inlet velocity, measured on a system test rig during intermittent low load operation. The operating point is periodically switched between membrane humidification (green) and liquid water drainage (orange). Figure 1

Bubble hydrodynamics of adding fine particles in 2D pulsed fluidized bed

The bubbling behavior of powders in the range of 30–2600 µm is well understood. However, the bubbling behavior in binary-medium pulsed fluidized beds is complex and has received little attention. Therefore, Geldart C and B magnetite powders were mixed to form a dense medium. The effects of C particles and frequency on the minimum fluidization gas velocity, bubble area, number, shape factor, and aspect ratio were studied. The results show that the introduction of pulsating energy can increase the bubble area but decrease the number of bubbles. Under an appropriate content of fine particles, the pulsating energy increases the bubble shape factor and reduces the aspect ratio. A bubble size growth model was developed. A pulsed fluidized bed mixed-medium minimum fluidization velocity model was developed with 10% error. A pulsed fluidized bed bubble size model was developed with a prediction error of 10% or less.

Separation performance of 0.5–1 mm fine coal in a gas–solid fluidized bed without dense medium

The polluting elements in coal are mainly concentrated in gangue, and it is difficult for existing dry coal preparation technologies to separate fine gangue below 1 mm effectively. Because 0.5–1 mm raw coal particles belonging to the Geldart B group can form a stable bubbling fluidized bed, the fluidization and segregation of simulated coal particles composed of 0.5–1 mm quartz sand and clean coal were investigated in this study. The results showed that the optimum operation gas velocity was slightly higher than the minimum fluidization gas velocity when the content of quartz sand was less than 60%; when it was higher than 60%, the optimum gas velocity was somewhat less than or equal to the minimum fluidization velocity. When the content of quartz sand was 60%, the separation effect was the most obvious, and the segregation standard deviation was 0.37. The initial bed height was in the range of 150–210 mm, and the separation effect was good. In addition, the separation tests of three raw coal samples showed that the research results of binary-simulated coal particles composed of quartz sand and clean coal were valid. Under the optimal conditions, the ash content of clean coal products of three raw coal samples can be reduced by 75.65%, 79.03%, and 83.03%, respectively.

Experimental study of fluidization characteristics of Geldart-D particles in pressurized bubbling fluidized bed

The pressurized bubbling fluidized bed shows great advantage in retreating municipal solid waste because it could effectively capture CO2 and enhance the reaction rate of the process of combustion and gasification. In the present work, fluidization characteristics of Geldart-D particles at elevated pressure were experimentally investigated, such as flow pattern, pressure drop, minimum fluidization gas velocity. At the same fluidization gas velocity, as elevating operating pressure, the fluidization of Geldart-D particles became more intense, the bubbles got larger, the standard deviation and the power density of dominant frequency of the pressure drop signal increased. While, under the same fluidization number, as increasing operating pressure, the fluidization of Geldart-D particles became smoother, the bubble size decreased, both the standard deviation and the power density of dominant frequency of the pressure drop signal decreased. It seems that, under elevated pressure, the fluidization behavior of Geldart-D particles would transition to that of Geldart-B particles. Finally, the minimum fluidization velocity of the Geldart-D particles was found decreased with the increase of the operating pressure. A new correlation for the prediction of the minimum fluidization velocity of Geldart-D particles at elevated pressure was also formulated based on the present experimental results.

Study on Fluidization Characteristics of Magnetically Fluidized Beds for Microfine Particles

The bed pressure drop, minimum fluidized gas velocity, bed density, and bed expansion rate are important parameters characterizing the fluidization characteristics of gas-solid fluidized beds. By analyzing these parameters, the advantages and disadvantages of the fluidization state can be known. In this study, experiments were conducted to study the fluidization characteristics of a gas-solid magnetically fluidized bed for microfine particles by changing the magnetic field strength, magnetic field addition sequence, and static bed height. The experimental results show that when the magnetic field strength increased from 0 KA/m to 5 KA/m, the minimum fluidized gas velocity of particles increased from 4.42 cm/s to 10.32 cm/s, while the bed pressure drop first increased and then decreased. When the magnetic field strength is less than 3.4 KA/m, the microfine particles in the bed are mainly acted on by the airflow; while when the magnetic field strength is greater than 3.4 KA/m, the microfine particles are mainly dominated by the magnetic field. The magnetic field addition sequence affects the fluidization quality of microfine particles. The fluidized bed with ‘adding magnetic field first’ shows a more stable fluidization state than ‘adding magnetic field later’. Increasing of the static bed height reduces the bed expansion rate. The bed expansion rate is up to 112.5% at a static bed height of h0 = 40 mm and H = 5 KA/m. This will broaden the range of density regulation of a single magnetic particle and lay the advantage of gas-solid magnetically fluidized bed for microfine particles in the field of separation of fine coal.

Fluidization and separation characteristics of a gas–solid separation fluidized bed in the presence of an acoustic field

The bed fluidization characteristics, such as pressure drop, minimum fluidization gas velocity, density, and voidage, play a vital role in understanding the hydrodynamic behavior of fluidization technologies. In this work, the variation in the bed pressure drop was analyzed in presence and absence of an acoustic field. Meanwhile, the minimum fluidization gas velocity was investigated for five different fine particle (−0.074 mm) contents as the acoustic source varied between 90 and 150 Hz, and the sound pressure level was within a range of 100–135 dB. The experimental results show that, upon an increase in the fine-grained particle (−0.074 mm) content, the sound frequency and sound pressure level have different effects on the minimum fluidization gas velocity. The introduction of an acoustic wave can effectively ensure the stability of the bed density, thus improving the separation performance of a gas–solid separation fluidized bed, with a lower probable error E.

Study on the influence of different carrier gases on the fluidization properties of glass bead and FCC powders

The powder flow was influenced by carrier gas in the unit operation, such as pneumatic conveying and hopper discharge, in the pressurized entrained flow gasification technology. Carrier gases play a vital role in fluidization/ defluidization of powders. In this paper, we investigate the fluidization/ defluidization of absorbent Fluid Catalytic Cracking (FCC) and non-absorbent glass bead with two different carrier gases, N2 and CO2. The flowability of aerated powders with different carrier gases were obtained in the fixed bed region whose superficial gas velocity was lower than minimum fluidization gas velocity and fluidization region. The powder flowability was characterized by basic flow energy, flowability rate index and aeration energy with the help of FT4 powder Rheometer. The influence of different carrier gases on fluidization/ defluidization of powder bed was further discussed. Transition line of A/C powders with different carrier gases was revealed. Effect of gas type and interparticle force were calculated, compared with the flowability of aerated powder bed. In addition, combined with bed stress analysis, different bed fluidization mechanism with different gas velocities were revealed. Our findings might help people understand the influence of carrier gases on the fluidization/ defluidization of powders in the relative chemical unit operations.

Self-excited gas–solid two-phase flow using non-uniform soft fins

The purpose of this study is to reduce the conveying velocity and power consumption in gas–solid two-phase flows. Four pieces of soft fins with different (non-uniform) lengths, which are fixed on the horizontal central plane of the inlet, are employed to self-excite the gas–solid flow. A horizontal acrylic pipe, approximately 5 m long and with an inside diameter of 80 mm, is selected as the test pipeline. The test solid materials are spherical polyethylene particles with average diameter and density of 2.3 mm and 978 kg/m3, respectively. The solid mass flow rate and average gas velocity are 0.10–0.47 kg/s and 10–17 m/s, respectively. Compared with non-fin and previous uniform soft fin gas–solid flows, the minimum gas velocity, pressure drop, additional pressure drop, and power consumption are reduced when a lower gas velocity is achieved with the use of non-uniform soft fins. The highest reduction rates in power consumption and minimum velocity are approximately 5.87% and 10.04%, respectively. The particle concentration in flows with non-uniform soft fins is higher than that in the non-fin flow in the upper part of the pipe of the acceleration region and lower than those in non-fin and uniform fin flows near the pipe bottom. By employing particle image velocimetry measurement, the time-averaged axial particle velocity and fluctuating particle velocity in all non-uniform fin flows are found to be higher than those in non-fin and uniform fin flows, especially near the upper part of the pipe.

Comparison of different drag models in CFD-DEM simulations of spouted beds

Spouted beds are commonly simulated through the Computational Fluid Dynamics – Discrete Element Method approach. The choice of the drag model is still a matter of debate, as they feature peculiar operative conditions. In this work, we simulated two spouted beds containing Geldart-D particles. We tested seven drag models: three are classic models, while four are developed through advanced computational techniques. The results indicate that the key variable is the ratio between the operative and the minimum spouting gas velocity (u/ums). At u = ums only the Gidaspow model can always predict fluidisation, but at low u/ums values the Beetstra model is the best compromise. For higher values, the Rong and Di Felice models behave better, while the others overestimate the particles' velocity. These results can be useful to identify the best performing model and show there is a need for more appropriate models for spouted beds.

Characterization and Applications of Non-Magnetic Rotating Gliding Arc Reactors - A Brief Review

Gliding arc discharge (GAD) reactors are known for high energy efficiency and good chemical selectivity compared to non-thermal plasmas such as glow discharge, corona and dielectric barrier discharge. Reported literature identified that planar diverging GAD have non-uniform gas treatment (e.g. only 20% of gas processed by plasma depending on electrode configuration). Further requirement of minimum limit gas velocity to drag the arc results in lower gas residence time.
 This paper attempts to investigate the GAD performance and preliminary studies to overcome some of the identified drawbacks, by using only fluid mechanics without magnetic field (rotating gliding arc (RGA)) inside the plasma reactor developed in various research laboratories. This article discusses the applications of GAD and also focuses on bringing out the performance and comparing with the results from the existing non-magnetic rotating gliding arc reactors. The paper also summarizes results from literature in such reactor designs.

Behaviour of biomass particles in a bubbling fluidized bed: A comparison between wood pellets and wood chips

For successful operation and design of a bubbling fluidized bed reactor handling a specific biomass, in-depth knowledge about the bed behaviour is paramount. This study compares the behaviour of a bed of sand containing wood pellets with that containing wood chips at different gas velocities and biomass proportions in a cold fluidized bed of diameter, 10.4 cm. The density and volume-equivalent spherical particle diameter of the pellets are 1139 kg/m3 and 8.96 mm, respectively while those of the wood chips are 423 kg/m3 and 6.87 mm, respectively. The results show that at low gas velocities, wood chips segregate upwards while the pellets segregate downwards in their respective beds. The spread of biomass towards the walls is higher in the bed with wood chips than in that with wood pellets. As the biomass load increases, the bubble diameter increases and the transition from bubbling to slugging regime gets smoother, resulting in an increase in the minimum slugging velocity. The minimum gas velocity for effective solids mixing is less dependent on the bed height, but increases with increase in the biomass load and decreases with increase in the bed diameter. However, when slugs flow in the bed, the biomass layer at the bed surface plugs, preventing mixing of particles to be achieved at the desired gas velocity. A mechanistic model is developed for predicting the minimum gas velocity required to achieve an effective mixing at the surface of a segregated bed. Although this study is conducted in a cold bed, this same model is considered important for a hot bed reactor since devolatilization enhances the upward flow of biomass due to reduction of the biomass density.

Sand minimum transport conditions in gas–solid–liquid three-phase stratified flow in a horizontal pipe at low particle concentrations

Sand production in the life of oil and gas reservoirs is inevitable, as it is co-produced from reservoirs. Its deposition in petroleum pipelines poses considerable risk to production and can lead to pipe corrosion and flow assurance challenges. Therefore, it is important that pipe flow conditions are maintained to ensure sand particles are not deposited but in continuous motion with the flow. The combination of minimum gas and liquid velocities that ensure continuous sand motion is known as the minimum transport condition (MTC). This study investigates the effect both of sand particle diameter and concentration on MTC in gas/liquid stratified flow in a horizontal pipeline. We used non-intrusive conductivity sensors for sand detection. These sensors, used for film thickness measurement in gas/liquid flows, were used for the first time here for sand detection. We found that MTC increases with increase in particle diameter for the same concentration and also increases as the concentration increases for the same particle diameter. A correlation is proposed for the prediction of sand transport at MTC in air–water flows in horizontal pipes, by including the effect of sand concentration in Thomas’s lower model. The correlation accounts for low sand concentrations and gave excellent predictions when compared with the experimental results at MTC.

Comparison of adsorption capacity of mono-ethanolamine and di-ethanolamine impregnated activated carbon in a multi-staged fluidized bed reactor for carbon-dioxide capture

The increase of carbon dioxide (CO2) concentration in the atmosphere leads to global warming and climate change. In our present study, a four stage fluidized bed reactor has been designed and developed for removal of CO2.Two different amine impregnated activated carbons have been prepared and used as adsorbents for capture of CO2. By changing the three hydrodynamics factors like gas velocity, solid flow rate and weir height, the highest percentage (%) removal of CO2 has been found out. Sampling and analysis has been performed by using Orsat apparatus. Among the two adsorbents i.e. Monoethanolamine impregnated activated carbon (MEA-AC) and Diethanolamine impregnated activated carbon (DEA-AC), the % removal of CO2 by Monoethanolamine impregnated activated carbon of impregnation ratio 0.6 (MEA-AC(0.6)) shows the highest, due to high degree stability of primary zwitterions formed than secondary zwitterions.Monoethanolamine impregnated activated carbon (MEA-AC) shows the highest % removal of CO2 (95%), at minimum gas velocity (0.188 m/s), maximum solid flow rate (4.12 kg/h) and at maximum weir height (50 mm) at initial concentration of CO2, 3000 ppm. MEA impregnated activated carbon was found to be suitable adsorbent for removal of CO2 from the flue gas in a multistage fluidized bed reactor.

Flow pattern transition and coal beneficiation in gas solid fluidized bed with novel secondary distributor

Gas solid fluidized bed (GSFB) is an effective method of dry coal separation. In this study, porous sponge was introduced into a typical gas solid fluidized bed as secondary air distribution layer (PSFB) to stabilize the fluidized bed layer. The difference between PSFB and GSFB in flow pattern transition process was studied. Compared with GSFB, the minimum gas velocity and bed density fluctuation decreased while bed expansion ratio increased in PSFB. Furthermore, the distribution of bubble phase and emulsion phase were more homogeneous in PSFB. Under the operational conditions, the results of coal preparation in a PSFB showed that the ash content of clean coal was 10.25% .The probable error (E) was 0.095 g/cm3, indicating that PSFB could provide a novel way for a good performance of dry coking coal beneficiation.

Characteristic gas velocity and fluidization quality evaluation of vibrated dense medium fluidized bed for fine coal separation

In order to remove incombustible impurity minerals, a vibrated dense medium fluidized bed (VDMFB) can be adopted for fine coal dry separation, while Geldart B magnetite powder can serve as the medium. The influence of vibration on characteristic gas velocity in flow pattern transition stage was experimentally investigated at a vibration amplitude range of 0.5–4 mm and frequency range of 5–35 Hz. The experimental results demonstrate that at a low frequency (f < 10 Hz), the vibration effect results in a denser bed and a slightly smaller initial fluidization gas velocity. As the vibration frequency increases to a level similar to the bed’s natural frequency, the minimum fluidization gas velocity reduces sharply. The minimum fluidization gas velocity correlation in the VDMFB is obtained by means of theoretical deduction and experimental data fitting. Furthermore, a method is proposed for evaluating the effects of vibration on improving fluidization quality. Based on that, using the coal separation probable error Ep⩽0.1 as the evaluation index, a suitable effective operating gas velocity range for coal separation under different vibration parameters is determined. The ratio of the boundary operating gas velocity to Umfc is 0.67–2.28. Thus, a uniform and stable fluidization environment is provided for dry fine coal separation.

Collaborative optimization of vibration and gas flow on fluidization quality and fine coal segregation in a vibrated dense medium fluidized bed

In a vibrated dense medium fluidized bed (VDMFB), the uniform and stable gas-solid fluidized bed where medium particles belong to the Geldart B, was formed and considered suitable for 1–6mm fine coal separation under the combined effects of both vibration energy and air flow. Driven by the excitation force, the particles in the VDMFB had a higher kinetic activity and lower minimum fluidization gas velocity. Prior to the bed fluidizing initiation (fluidization number, N<1), the solid particles absorbed energy during inelastic collisions with the gas distributor. Subsequently to the bed uniform fluidization, the vibration energy was continuously transferred to the quasi-elastic bed in the form of waves. The vibration amplitude and frequency played a different role in the gas-solid contact. When the operating gas velocity, Ui, was 6.26cm/s and the vibration acceleration level, K, was lower than 7.1, it could have been quite effective for the vibration energy transfer in the bed to be enhanced by the vibration frequency increase. As the frequency increased, the transmission efficiency of vibration energy decreased. When K>7.1, the vibration amplitude effect on the vibration energy transmission in the bed was quite significant. In the study of the vibration energy effective range, it was discovered that the bed could be utilized for the fine coal effective separation in the area below 60mm for the VDMFB with 80mm of static bed height, where the vibration energy was continuously and steadily transmitted and attenuated along the bed height. Regarding the raw coal with a washability variety, the separation density could be flexibly regulated in the VDMFB. The separation results demonstrated that the ash content of three clean coal products was decreased exceeding 50%, compared to the ash content of the raw coal when the separation density was set at 1.49g/cm3, 1.67g/cm3 and 1.78g/cm3 respectively, indicating that the VDMFB could effectively separate fine coal by a dry method.

Exploring the tension between energy consumption, light provision and CO2 mass transfer through varying gas velocity in the airlift bioreactor

The airlift principle is a key operating principle of many aerated vertical tube photobioreactors and the aeration arm of horizontal tube reactors. It can also be incorporated into low cost hanging bag reactors. In each of these reactor systems, it is well recognised that algal productivity is typically limited by the supply of light and the rate of CO2 supply. It is desirable to maximise algal productivity by optimising these factors while minimising energy consumption in the reactor. In this paper, the relationship between gas velocity and light supply is investigated. Experimental algal growth in 3.2L internal airlift reactors showed that when CO2 supply was sufficient, light was the limiting factor. Productivities of 0.186gL−1d−1 at 148μmolm−2s−1 light, and 0.254gL−1d−1 at 300μmolm−2s−1 were obtained at a superficial gas velocity of 0.02ms−1. Reductions in algal concentration and productivity at lowered superficial gas velocity, even when the critical CO2 supply rate was met, suggested poor light distribution when liquid circulation and mixing were reduced. This was supported by simulated algal growth, modelled using a realistic light history obtained from Positron Emission Particle Tracking (PEPT) and a light attenuation model. This is the first reported use of PEPT in algal reactors, used here to determine realistic trajectories of algal cells through the airlift system, thereby allowing the cell's light history to be mapped as a function of gas velocity. Critical minimum superficial gas velocities were determined for light distribution and CO2 supply, dependent on the light intensity provided. Algal productivity in the airlift system can thus be maximised on the basis of energy efficiency.