Axial gas dispersion in a fluidized bed of polyethylene particles

Gas mixing behavior was investigated in a residence time distribution experiment in a bubbling fluidized bed of 0.07 m ID and 0.80 m high. Linear low density polyethylene (LLDPE) particles having a mean diameter of 772 Μm and a particle size range of 200-1,500 Μm were employed as the bed material. The stimulus-response technique with CO₂ as a tracer gas was performed for the RTD study. The effects of gas velocity, aspect ratio (H₀/D) and scale-up on the axial gas dispersion were determined from the unsteady-state dispersion model, and the residence time distributions of gas in the fluidized bed were compared with the ideal reactors. It was found that axial dispersion depends on the gas velocity and aspect ratio of the bed. The dimensionless dispersion coefficient was correlated with Reynolds number and aspect ratio.     

Axial gas phase dispersion in a molten salt oxidation reactor

Gas phase axial dispersion characteristics were determined in a molten salt oxidation reactor (air-molten sodium carbonate salt two phase system). The effects of the gas velocity (0.05–0.22 m/s) and molten salt bed temperature (870–970 °C) on the gas phase axial dispersion coefficient were studied. The amount of axial gas-phase dispersion was experimentally evaluated by means of residence time distribution (RTD) experiments using an inert gas tracer (CO). The experimentally determined RTD curves were interpreted by using the axial dispersions model, which proved to be a suitable means of describing the axial mixing in the gas phase. The results indicated that the axial dispersion coefficients exhibited an asymptotic value with increasing gas velocity due to the plug-flow like behavior in the higher gas velocity. Temperature had positive effects on the gas phase dispersion. The effect of the temperature on the dispersion intensity was interpreted in terms of the liquid circulation velocity using the drift-flux model.     

RTD studies in trickle bed reactors packed with porous particles

The residence time distribution (RTD) for liquid phase in a trickle bed reactor (TBR) has been experimentally studied for air-water system. Experiments were performed in a 15.2 cm diameter column using commerical alumina extrudates with D/d_p ratio equal to 75 to eliminate the radial flow differences. The range of liquid and gas flow rates covered was 3.76 < Re_L < 9.3 and 0 < Re_G < 2.92. The axial dispersion model was used to compute axial dispersion coefficient. The effect of liquid and gas flow rates on total liquid holdup and axial dispersion was investigated. The total liquid holdup has been correlated to liquid and gas flow rates.     

Influence of high density particles in reduction of axial dispersion in liquid fluidized bed with residence time distribution curve studies

iquid phase RTD curves were investigated in classical fixed and fluidized bed regimes with high density particles. The effect of liquid velocity was studied on bed hydrodynamics. Using an impulse tracer injection technique in a column of 5 cm inner diameter and 1.2 m height, liquid RTD, mean residence time (MRT), axial dispersion coefficient (ADC) and vessel dispersion number (N _D ) were determined. ADC increases with liquid superficial velocity. It varied from 4.63 to 20.7 cm²/s for the particle Reynolds number of 43 to 279, respectively. The experimental results show that the hight density particles cause less ADC than the low density particles at an identical Reynolds number.     

Liquid backmixing in oscillatory flow through a periodically constricted meso-tube

This paper deals with the measurement and modelling of axial liquid dispersion in a 4.5 mm internal diameter tube provided with smooth-periodic constrictions (meso-tube) in steady and oscillatory flow conditions. The residence time distribution (RTD) in the meso-tube was monitored for a range of fluid oscillation frequency (f) and amplitude (x₀) at laminar flow. The RTD response was modelled with three hydrodynamic models: (i) tanks-in-series, (ii) tanks-in-series with backflow and (iii) plug flow with axial dispersion. The steady flow through the meso-tube at flow rates up to 21.30 ml/min resulted in broad RTDs, mainly due to the parabolic velocity profile. The use of fluid oscillations allowed a fine-control of the axial liquid dispersion in the meso-tube due to generation of secondary flow in the regions between the constrictions. The axial dispersion coefficient D was reduced by up to 13-fold in comparison with the steady flow situation. Values of x₀ ≤ 1 mm and f = 10 Hz generally resulted in a maximum reduction in axial dispersion through, therefore maximum improvements in RTD. The tanks-in-series model was generally not capable of predicting RTDs in the meso-tube. The potential of this platform for the continuous, sustainable production of added-value products is herein demonstrated.     

Liquid phase axial mixing in bubble columns operated at high pressures

Liquid phase axial mixing was measured in a 100 mm i.d. bubble column operated in the pressure range of 0.1-0.5 MPa. Water, ethanol and 1-butanol were used as the liquid phase and nitrogen as the gas phase. The temperature and superficial gas velocity were varied in the range of 298-323 K and 0.01-0.21 m/s, respectively. The axial dispersion coefficient increased with an increase in the gas density due to pressure. The temperature had surprisingly a small effect. A CFD model was developed for the prediction of flow pattern in terms of mean velocity and eddy diffusivity profiles. The model was further extended for the prediction of residence time distribution and hence the axial dispersion coefficient (D_L). The predictions of axial dispersion coefficient agree favorably with all the experimental data collected in this work as well as published in the literature. The model was extended for different gas-liquid systems. The predicted values of axial dispersion coefficient were found to agree very well with all the experimental data.     

The effect of adsorption on mass transfer in fluidized bed catalytic reactors

Tracer gas residence time distributions (RTD) in a laboratory scale fluidized bed system have been measured for pulses of three different tracer gases (methane, ethane and propane) at different temperatures in the range 323 to 435 K. The fluidized solid was a commercial zeolite based FCC catalyst (CBZ‐2), and measurements were carried out in a superficial air velocity range of 0.01 to 0.04 m/s. The data were interpreted with two‐phase dense phase dispersion models for adsorptive tracers, available in the literature. In addition, modified models were considered by assuming a stationary dense phase and neglecting axial dispersion in this phase. Mean residence time, μ₁, and the variance of the residence time, σ², of RTD data were calculated for each experimental run. Applying the moment technique in the Laplace domain, the differential equations for all models considered were analytically solved. Mass transfer coefficients obtained from dynamic experiments were compared with the values estimated from the relations available in the literature. It was found that methods considering convective flux alone between the bubble and emulsion phases give closer values to the experimental ones than the methods also including the diffusive flux.     

Hydrodynamics,Mass Transfer and Gas Scrubbing in a Co‐current Droplet Column Operating at High Gas Velocities

The main objective of this work was to propose a new process for household fume incineration treatment: the droplet column. A feature of this upward gas‐liquid reactor which makes it original, is to use high superficial gas velocities (13 m s^–1) which allow acid gas scrubbing at low energy costs. Tests were conducted to characterize the hydrodynamics, mass transfer performances, and acid gas scrubbing under various conditions of superficial gas velocity (from 10.0 to 12.0 m s^–1) and superficial liquid velocity (from 9.4·10^–3 to 18.9·10^–3 m s^–1). The following parameters characterized the hydrodynamics: pressure drops, liquid hold‐ups, and liquid residence time distribution were identified and investigated with respect to flow conditions. To characterize mass transfer in the droplet column, three parameters were determined: the gas‐liquid interfacial area (a), the liquid‐phase volumetric mass transfer coefficient (k_La) and the gas‐phase volumetric mass transfer coefficient (k_Ga). Gas absorption with chemical reaction methods were applied to evaluate a and k_Ga, while a physical absorption method was used to estimate k_La. The influence of the gas and liquid velocities on a, k_La, and k_Ga were investigated. Furthermore, tests were conducted to examine the utility of the droplet column for the acid gas scrubbing, of gases like hydrogen chloride (HCl) and sulfur dioxide (SO₂). This is a process of high efficiency and the amount of pollutants in the cleaned air is always much lower than the regulatory European standards imposed on household waste incinerators.     

Axial dispersion of gas in a circulating fluidized bed

An axial dispersion of gas in a circulating fluidized bed was investigated in a fluidized bed of 4.0 cm I.D. and 279 cm in height. The axial dispersion coefficient of gas was determined by the stimulus-response method of trace gas of CO₂. The employed particles were 0.069 mm and 0.147 mm silica-sand. The results showed that axial dispersion coefficients were increased with gas velocity and solid circulation rates as well as suspension density. The experimentally determined axial dispersion coefficients in this study were in the range of 1.0-3.5 m²/s.     

Residence time distribution and modeling of the liquid phase in an impinging stream reactor

Based on some experimental investigations of liquid phase residence time distribution (RTD) in an impinging stream reactor, a two-dimensional plug-flow dispersion model for predicting the liquid phase RTD in the reactor was proposed. The calculation results of the model can be in good agreement with the experimental RTD under different operating conditions. The axial liquid dispersion coefficient increases monotonously with the increasing liquid flux, but is almost independent of gas flux. As the liquid flux and the gas flux increase, the liquid dispersion coefficient of center-to-wall decreases. The axial liquid dispersion coefficient is much larger than that of center-to-wall, which indicates that the liquid RTD is dominated mainly by axial liquid dispersion in the impinging stream reactor.     

Residence time distribution and modeling of the liquid phase in an impinging stream reactor

Based on some experimental investigations of liquid phase residence time distribution (RTD) in an impinging stream reactor, a two-dimensional plug-flow dispersion model for predicting the liquid phase RTD in the reactor was proposed. The calculation results of the model can be in good agreement with the experimental RTD under different operating conditions. The axial liquid dispersion coefficient increases monotonously with the increasing liquid flux, but is almost independent of gas flux. As the liquid flux and the gas flux increase, the liquid dispersion coefficient of center-to-wall decreases. The axial liquid dispersion coefficient is much larger than that of center-to-wall, which indicates that the liquid RTD is dominated mainly by axial liquid dispersion in the impinging stream reactor.     

Estimation of liquid phase mixing due to solids in a turbulent bed contactor

The mixing in two-phase gas-liquid and three-phase gas-liquid-solid system (turbulent bed contactor) is evaluated through residence time distribution (RTD) studies in terms of Peclet number. RTD experiments are conducted for various gas and liquid velocities, and number of stages for two- and three-phase systems. Since the mean residence time is very short in both the systems, a mixed flow tank with exponential decay RTD is used in series. After deconvolution, the RTD of the system is obtained. The experimental RTD curves are satisfactorily compared with the axial dispersion model and Peclet numbers are evaluated for all the experiments. The axial dispersion coefficients are calculated from Peclet numbers. With this study, it is thought that liquid phase mixing may be controlled by changing the quantity of solid particles in the bed.     

Gas backmixing in the dense region of a circulating fluidized bed

The gas backmixing characteristics in a circulating fluidized bed (0.1 m-IDx5.3-m high) have been determined. The gas backmixing coefficient (D_ba) from the axial dispersion model in a low velocity fluidization region increases with increasing gas velocity. The effect of gas velocity onD _ba in the bubbling bed is more pronounced compared to that in the Circulating Fluidized Bed (CFB). In the dense region of a CFB, the two-phase model is proposed to calculate D_bc from the two-phase model and mass transfer coefficient (k) between the crowd phase and dispersed phase. The gas backmixing coefficient and the mass transfer coefficient between the two phases increase with increasing the ratio of average particle to gas velocities (U_p/U_g).     

多级鼓泡塔中液相返混系数与交换速度的研究

采用示踪方法对高2 000 mm,内径282 mm多级筛板鼓泡塔内液相返混系数进行测量研究,并通过扩散-返混模型以及RTD曲线给出鼓泡塔内筛板上下二侧液体交换速度,同时考查了表观气速、开孔率等因素对轴向扩散系数与液体交换速度的影响.根据实验得出鼓泡塔内轴向返混系数以及液体交换速度与表观气速、开孔率有很大关系,均随表观气...     

短接触旋流反应器导叶位置对气相流动的影响

采用雷诺应力模型对不同导叶位置下的短接触旋流反应器内气相流动进行数值模拟,并且用组分输运方程研究了气体在旋流反应器内停留时间的分布规律,分析了导叶位置对反应器内气相停留时间分布、气相流场以及排气管入口短路流的影响。结果表明:不同导叶位置的旋流反应器排剂口处气相停留时间分布曲线相似,而混合反应区内气相停留时间分布曲线仅在时间轴上发生微小偏移;叶片离反应器入口越远,混合反应区内切向速度越小,而排气管下方分离反应区内外旋流切向速度显著增大及准自由涡范围减小;叶片与排气管入口距离减小可以降低附近短路流率,减小催化剂的“跑损”,但也增大了排气管内气流旋转强度,造成不必要的能量损耗增大。     

Axial Dispersion and Wall Effects in Narrow Fixed Bed Reactors: A Comparative Study Based on RTD and NMR Measurements

Axial dispersion and wall effects in narrow fixed beds with aspect ratios < 10 were investigated, both by classical methods and by NMR imaging. The residence time distribution (RTD) in the center and at the wall was measured, system water/NaCl‐solution as tracer, and subsequently compared with radial velocity profiles based on NMR imaging. The influence of the aspect ratio and Re_p on dispersion and on the degree of non‐uniformity of the velocity profile was studied. The NMR results are consistent with the RTD and also with literature data of numerical simulations. For low aspect ratios, dispersion/wall effects have a strong influence on the reactor behavior, above all, in cases where a low effluent concentration is essential, as proven by breakthrough experiments with the reaction of H₂S with ZnO.     

Recent developments on catalytic membrane for gas cleaning

Catalytic membrane, a novel membrane separation technology that combines catalysis and separation, exhibits significant potential in gas purification such as formaldehyde, toluene and nitrogen oxides(NO_x). The catalytic membrane can remove solid particles through membrane separation and degrade gaseous pollutants to clean gas via a catalytic reaction to achieve green emissions. In this review, we discussed the recent developments of catalytic membranes from two aspects: preparation of catalytic membrane and its application in gas cleaning.Catalytic membranes are divided into organic catalytic membranes and inorganic catalytic membranes depending on the substrate materials. The organic catalytic membranes which are used for low temperature operation(less than 300 °C) are prepared by modifying the polymers or doping catalytic components into the polymers through coating, grafting, or in situ growth of catalysts on polymeric membrane. Inorganic catalytic membranes are used at higher temperature(higher than 500 °C). The catalyst and inorganic membrane can be integrated through conventional deposition methods, such as chemical(physical) vapor deposition and wet chemical deposition. The application progress of catalytic membrane is focused on purifying indoor air and industrial exhaust to remove formaldehyde, toluene, NO_x and PM2.5, which are also summarized. Perspectives on the future developments of the catalytic membranes are provided in terms of material manufacturing and process optimization.     

Mixing–segregation phenomena of binary system in a fluidized bed

A study on mixing–segregation phenomena in a gas fluidized bed of binary density system was performed by analysis of the residence time distribution and mixing degree. The effect of particle mixing on the residence time distribution and solid mixing was studied in a binary particle system with different densities. Residence time distribution curve and mean residence time of each particle were measured according to the flotsam particle size, mixing ratio and gas velocity in a gas fluidized bed (0.109 m I.D., 1.8 m height). The characteristics of residence time distribution and the deviation of mean residence time of each particle are consistent with previous mixing index based on the axial concentration of jetsam. From this study, mixing index of binary particle system with different densities should be considered by not only axial concentration distribution of jetsam particle but also characteristics of residence time distribution. This result suggests that the solid movement by fluidization gas is more important than solid axial dispersion.     

Radial gas mixing characteristics in a downer reactor

In a downer reactor (0.1 m-I.D.x3.5 m-high), the effects of gas velocity (1.6-4.5 m/s), solids circulation rate (0–40kg/m²s) and particle size (84, 164 Μm) on the gas mixing coefficient have been determined. The radial dispersion coefficient(D _r ) decreases and the radial Peclet number (Pe_r) increases as gas velocity increases. At lower gas velocities, D_r in the bed of particles is lower than that of gas flow only, but the reverse trend is observed at higher gas velocities. Gas mixing in the reactor of smaller particle size varies significantly with gas velocity, whereas gas mixing varies smoothly in the reactor of larger particle size. At lower gas velocities, D_r increases with increasing solids circulation rate (G_s), however, D_r decreases with increasing G_s at higher gas velocities. Based on the obtained D_r values, the downer reactor is found to be a good gas-solids contacting reactor having good radial gas mixing.     

Fe-doped LaCoO<Subscript>3</Subscript> perovskite catalyst for NO oxidation in the post-treatment of marine diesel engine’s exhaust emissions

New post-treatment process for marine diesel engine exhaust emissions was proposed by combining NO oxidation and wet scrubbing technology for the simultaneous removal of SO_X, NO_X and PM. NO, insoluble in aqueous scrubbing absorbent, is preferentially oxidized to NO₂, which then turns fully soluble in it. Fe substituted LaCo_1-xFe_xO₃ perovskite catalysts were developed for NO oxidation to NO₂. The catalysts were prepared by co-precipitation method and analyzed with XRD, XRF, BET, FT-IR, NO-TPD and XPS techniques. Crystal structure change from rhombohedral to orthorhombic was observed with the increased amount of Fe substituted in the B site of the perovskite by XRD analysis. From FT-IR and NO-TPD analysis, nitrate on perovskite species was found to be the active species for NO oxidation. Quantitative analysis was performed within the prepared catalysts. Catalytic activity was measured using a packed bed reactor operated at 150–400 °C, atmospheric pressure and with gas hourly space velocity (GHSV) of 20,000 h^-1 using a simulated exhaust gas composed of NO 400 ppm, O₂ 10% balanced with N2. Formation of Fe⁴⁺ cation enhanced the redox property as well as the mobility of the lattice oxygen present in the perovskite catalysts, confirmed by XPS analysis. Reaction mechanism of NO oxidation on Fe substituted LaCo_1-xFe_xO₃ was discussed based on Mars-van Krevelen mechanism.