Gas Phase Residence Time Research Articles

Tailoring electrochemically assisted absorbers for the efficient removal of volatile organic compounds

A new electrochemical cell is conceptualized, designed, manufactured using 3D printing technology, tested, and evaluated for the treatment of VOCs (for which benzene was used as model pollutant). This cell combines into a single device, a packed absorption column, where the pollutant is absorbed into an absorbent liquid, and an electrochemical cell equipped with a membrane-electrode assembly (MEA), for the continuous regeneration of the absorbent. The use of MEA technology allows to use of absorbents with low salt concentration. During operation, oxidants are produced on the electrodes of the electrochemical cell and distributed both in the liquid and gas phase, effectively degrading benzene. Computational Fluid Dynamics (CFD) modeling and experimental characterization of residence time distribution and mass transfer coefficient were also conducted. The results demonstrated that the use of electrochemically assisted absorbers in a single unit maximizes the utilization of the produced oxidants. The maximum removal efficiency of nearly 60% is reached working at 50 mA·cm−2. High current densities lead to pollutant desorption due to the significant increase in the gas phase outflow, resulting in reduced gas phase residence time and the pollutant leaving the system without undergoing reaction.

Up-rotating plasma gasifier for waste treatment to produce syngas and intensified by carbon dioxide

Industrial and medical wastes are substantial and extremely dangerous. In order to effectively tackle and utilize the wastes, an up-rotating plasma gasifier with a treatment capacity of 60 t/d is designed. It improves the structures of conventional plasma gasifier by the improved feeders, retrofitted installation of the plasma torch, and setting flue gas outlet with a slope. This provides an up-rotating flow field to improve the gasification performance. The up-rotating plasma gasifier affords to produce syngas. A 3D model is developed to characterize the performance of the gasifier. The temperature field, velocity field, and gas concentration field in the gasifier are analyzed. The overall temperature is determined above 1200 °C and the gas phase residence time is above 5 s. CO2 and steam are introduced to improve the gasification performance. It is found that the molar fraction of syngas reaches 0.775 and cold gas efficiency reaches 75.68% at CO2 ratio of 0.15, inlet gas flux ratio of 1.0, and steam to CO2 ratio of 1.0.

Hydrodynamic CFD-DEM model validation in a gas–solid vortex unit

Process intensification in gas–solid fluidization processes can be achieved by working in a centrifugal rather than a gravitational field. In this regard, the gas–solid vortex unit (GSVU) is an ideal candidate for heterogeneously catalyzed processes. A four-way coupled CFD-DEM model describing the hydrodynamics in the GSVU with an unprecedented level of detail is validated using 2D particle image velocimetry (PIV) experimental data on both azimuthal and radial particle velocity components. It captures high and low velocity regions, both qualitatively and quantitatively. Gas-solid slip velocities several times higher than those obtainable in a gravitational field are achieved, greatly enhancing heat and mass transfer rates. Furthermore, the gas-phase residence time distribution in the GSVU is shown to be narrow. This developed model presents a powerful tool for a better understanding and a detailed design aimed at enhancing the non-reactive and reactive process intensification capabilities of the gas–solid vortex technology.

Effect of secondary gas-phase reactions (SGR) in pyrolysis of carbon feedstocks for anisotropic carbon materials production – 2: Effects of SGR on tars produced from varying ranks of coal

This work explores the extent to which secondary gas-phase reactions (SGR) in pyrolysis can be used to modify coal tar chemistries from coals of different ranks in favor of improved anisotropy formation in their respective pitches. Pyrolysis was performed on four different coals of varying bituminous rank (Utah Sufco, Wyoming PRB Black Thunder, Illinois #6, and West Virginia Flying Eagle), with SGR temperatures ranging from 800 to 900° C and nominal SGR gas-phase residence times from 1 to 2.5 s. The oxygen content, aliphatic content, and molecular weight distributions of the coal tar samples were measured to indicate the changes with increasing levels of SGR, and microscopy was also used to measure changes in anisotropy formation in the resulting pitch samples. Generally, for all coals tested, increased levels of pyrolysis SGR led to decreased oxygen and aliphatic content, increased molecular weight sizes, and improved anisotropy formation. However, it was clear that the extent of these property changes depended on the chemistry and rank of the starting coal feedstock. Despite the relatively high rank of the Illinois #6 coal, its respective pitch samples performed poorly in improving anisotropy formation, due to its high sulfur content. The PRB, Sufco, and Flying Eagle coals performed better in their anisotropy formation than Illinois #6, depending on their respective coal ranks. Statistical analysis (analysis of variance, ANOVA) performed on the sample characterization data also suggests that SGR temperature is consistently the most dominant and significant effect on the resulting coal products.

Experimental measurement and theoretical analysis on bubble dynamic behaviors in a gas-liquid bubble column

Bubble dynamic behaviors have significant effects on the interfacial area between gas and liquid phases, the gas-phase residence time and the efficiency of heat and mass transfer. This study mainly investigated the single bubble dynamic behaviors and the radial distribution patterns of bubbly flow in a bubble column through Particle Tracking Velocimetry (PTV) and High Speed Imaging (HSI). As the results indicate, with the bubble size increasing, bubble deformation and bubble rising velocity are intensified and bubble trajectories change accordingly. Meanwhile, theoretically calculated values of single bubble terminal velocities are in accord with experimental values. According to the study, gas-liquid interaction has a noticeable impact on multiple radial distribution patterns of bubbly flow, including bubble size, velocity, the number of small bubbles and the width of gas-liquid interaction region. Finally, a correlation is proposed in this work to predict the width of gas-liquid interaction region based on a dimensionless analysis.

Scale-up of a downflow bubble column: Experimental investigations

Co-current downflow bubble columns provide advantages like high gas phase residence time and small bubble diameters, which result in high gas-liquid interfacial area and high mass transfer rates. The precise knowledge of gas fraction, bubble size distribution, interfacial area (a_) and volumetric mass transfer coefficient (kLa_) data is essential for the design of such downflow bubble columns. The present work investigates the effect of superficial gas velocity, liquid velocity and gas sparging location on the volume averaged gas fraction, axial and chordal gas fraction profiles, Sauter-mean bubble size, kLa_, and a_ in a downflow bubble column modified to operate via an array of downward pointing micro-jets. Experiments were performed for air-water system with 20 PPM SDS in two-different scale downflow bubble columns: (1) volume of 5 L (Height (H) = 0.6 m, Diameter (D) = 0.1 m, aspect ratio (H/D) = 6) and (2) volume of 120 L (H = 1.5 m, D = 0.3 m, H/D = 5). The experimental results of axial and chordal gas fraction profiles and bubble size distributions are presented. Further, the empirical correlations have been proposed to predict the gas fraction (∈G), Sauter-mean bubble diameter (dB) and effective gas-liquid interfacial area a_ for better understanding the scale-up process.

Biomass Fast Pyrolysis Using a Novel Microparticle Microreactor Approach: Effect of Particles Size, Biomass Type, and Temperature

Biomass fast pyrolysis is emerging as a front-running approach for the generation of renewable chemical and fuel resources. The pyrolysis temperature, solid- and gas-phase residence times, and biomass particle size and type have a substantial impact on char, oil, and gas yields. A laboratory-scale fast pyrolysis technique was demonstrated using manufactured biomass microspheres. A unique single-particle (∼10 μg) microreactor technology coupled to a millisecond response flame ionization detector was used to investigate the effects of relevant particle and process parameters and to capture the dynamics of real-time microscale single-particle pyrolysis for the first time. The manufactured biomass microspheres were produced by spray-drying finely milled microcrystalline cellulose, switchgrass (Panicum virgatum), and tall fescue straw (Festuca arundinacea) flour.

Pilot-scale biofiltration at a materials recovery facility: The impact on bioaerosol control

This study investigated the performance of four pilot-scale biofilters for the removal of bioaerosols from waste airstreams in a materials recovery facility (MRF) based in Leeds, UK. A six-stage Andersen sampler was used to measure the concentrations of four groups of bioaerosols (Aspergillus fumigatus, total fungi, total mesophilic bacteria and Gram negative bacteria) in the airstream before and after passing through the biofilters over a period of 11 months. The biofilters achieved average removal efficiency (RE) of 70% (35 to 97%) for A. fumigatus, 71% (35 to 94%) for total fungi, 68% (47 to 86%) for total mesophilic bacteria and 50% (-4 to 85%) for Gram negative bacteria, provided that the inlet concentration was high (103–105 cfu m−3), which is the case for most waste treatment facilities. The performance was highly variable at low inlet concentration with some cases showing an increase in outlet concentrations, suggesting that biofilters had the potential to be net emitters of bioaerosols. The gas phase residence time did not appear to have any statistically significant impact on bioaerosol removal efficiency. Particle size distribution varied between the inlet and outlet air, with the outlet having a greater proportion of smaller sized particles that represent a greater human health risk as they can penetrate deep into the respiratory system where gaseous exchange occurs. However, the outlet concentrations were low and would further be diluted by wind in full scale applications. In conclusion, this study shows that biofilters designed and operated for odour degradation can also achieve significant bioaerosol control in waste gas.

Soot formation during biomass pyrolysis: Effects of temperature, water-leaching, and gas-phase residence time

Soot particles from biomass incomplete combustion, as a major source of particulate matter, arouse significant concerns due to its harmful impacts on environment and human health. In this paper, the effects of pyrolysis temperature, volatile residence time and water-leaching of biomass on soot formation during biomass pyrolysis were studied in a lab-scale furnace. The physicochemical characteristics of soot particles were comprehensively analyzed. Results show that soot is formed by the secondary reactions of volatiles from biomass thermal-cracking requiring a certain high temperature and long volatile residence time. With a short volatile residence time of 0.2 s, no soot is formed at 900–1200 °C. When the residence time increases to 2 s, soot is significantly formed at >1000 °C, when onion-like shell/core structures is observed. With pyrolysis temperature increasing, the soot particle size decreases and becomes uniform, the carbon content in soot increases, and the ordered structure of carbonization is enhanced, which leads to the degradation of soot oxidation reactivity. The water-leaching pretreatment for biomass leads to the increases of soot yield, particle size, and carbon content, as well as the KCl absence in soot and significantly lowered oxidation reactivity of soot. This indicates strong catalysis of potassium on soot production and consumption. Both gas and tar analyses support soot formation mechanism: (1) H2 yield increases due to formation of soot, but C2H2 yield reaches peak at 1000–1100 °C; (2) tar is dominated by PAHs, which molecules grow with the increase of temperature and residence time.

Special issue: Plasma and Liquids

Special issue: Plasma and Liquids

Correlation for kLa Prediction of Airlift Loop Reactors Including the Gas Phase Residence Time Effect

AbstractExperimental mass transfer data and literature data of airlift loop reactors are analyzed concerning the effect of the gas phase residence time on the volumetric mass transfer coefficient kLa. It becomes evident that a neglect of the oxygen depletion can be regarded as a diminishment of the mass transfer driving force. The latter is found to be an important cause of the mass transfer dependency on the actual apparatus height of airlift loop reactors. Therefore, a new correlation is presented that describes the volumetric mass transfer coefficients of airlift loop reactors by including the effect of gas phase transient component concentration depletion.

Geographical and diurnal features of amine‐enhanced boundary layer nucleation

AbstractAmines have recently been found to be an important ingredient in the nucleation and initial growth of atmospheric aerosols; however, global estimates of the spatial and temporal extent of amine‐enhanced nucleation are currently missing. We utilize two recently published laboratory data sets of amine‐sulfuric acid nucleation to evaluate the accuracy of previously published nucleation parameterizations and to produce a new amine‐enhanced new particle formation (NPF) parameterization that better reproduces the laboratory observations at atmospherically relevant sulfuric acid concentrations. We implement and compare the amine‐enhanced NPF parameterizations and a kinetic nucleation parameterization within the global aerosol‐climate model ECHAM‐HAMMOZ and find that the spatial features of amine‐enhanced and kinetic NPF are clearly different. Amine‐enhanced NPF is limited to areas near the source regions of amine due to its short gas phase residence time of 6.9 h, whereas kinetic nucleation (which depends only on sulfuric acid concentration) produces particles more uniformly across the globe due to long‐range transport of SO2. The notably stronger land‐sea contrast in amine‐enhanced nucleation simulations is in line with relatively rare atmospheric observations of NPF over open oceans. However, when the uptake of gas phase amine molecules to aerosol particles is limited according to previously published estimates (0.2% of collisions leading to uptake), the amine‐enhanced NPF parameterization predicts in some regions unrealistically high NPF rates (∼1000 cm−3 s−1) compared to typical observations. Our results indicate that amine‐enhanced nucleation may be an important particle formation mechanism near amine source regions but also highlights the need for more tightly defined constraints on the spatial and temporal distribution of amine emissions, gas‐to‐particle partitioning mechanisms of amines, and condensation and coagulation sinks in global models.

Production of aromatic green gasoline additives via catalytic pyrolysis of acidulated peanut oil soap stock

Catalytic pyrolysis was used to generate gasoline-compatible fuel from peanut oil soap stock (PSS), a high free fatty acid feedstock, using a fixed-bed reactor at temperatures between 450 and 550°C with a zeolite catalyst (HZSM-5). PSS fed at 81gh−1 along with 100mLmin−1 inert gas was passed across a 15g catalyst bed (WHSV=5.4h−1, gas phase residence time=34s). Results indicate that fuel properties of PSS including viscosity, heating value, and O:C ratio were improved significantly. For PSS processed at 500°C, viscosity was reduced from 59.6 to 0.9mm2s−1, heating value was increased from 35.8 to 39.3MJL−1, and the O:C ratio was reduced from 0.07 to 0.02. Aromatic gasoline components (e.g., BTEX), were formed in concentrations as high as 94% (v/v) in catalytically-cracked PSS with yields ranging from 22% to 35% (v/v of PSS feed).

Numerical Analysis of the Effect of Gas Sparging on Bubble Column Hydrodynamics

A discrete bubble model (DBM) has been used to study the effect of gas sparger properties on the hydrodynamics in a bubble column. As a first step the performance of the model was evaluated by comparison with experimental data. Subsequently, four different perforated plates with different sparged areas were used as a gas sparger. Distributions of liquid velocity, turbulent kinetic energy, and void fraction in the central plane were compared for the four different systems. Furthermore, the effect of the sparger location was also investigated. It was found that the liquid-phase circulation becomes more pronounced as the sparged area location is more distant from the center of the bottom plate. Finally, gas-phase residence time distributions (RTD) were obtained from the simulations. By employing standard axial dispersion model, the gas-phase mixing in the bubble column was characterized. Results show that the extent of mixing increased when the sparged area decreased. The axial dispersion coefficient increased as the sparged area was shifted to the edge of the bottom plate.

Intensification of G/L absorption in microstructured falling film. Application to the treatment of chlorinated VOC's - part II: Modeling and geometric optimization

The falling film microabsorber developed by the Institut für Mikrotechnik Mainz is used to study its performance for the treatment of VOC-containing gases. The technique used is the gas/liquid absorption. The gas (mixture of air and tetrachloroethylene) and the solvent (Di(ethyl-2-hexyl) adipate, DEHA) flow counter-currently in the microabsorber. The solvent streams by gravity on a plate structured by microchannels. This technique makes it possible to miniaturize the process by increasing the mass transfer flow per volume unit as explained in Part I. Experiments are presented and a model is developed from equations of mass and momentum transfers. Results show a competition between two transport phenomena: diffusion and convection. The mass transfer rate is also studied with respect to the structure of microabsorber. It is also shown that it is possible “to optimize” the geometry of the apparatus in order to intensify the transfer so as to miniaturize the process. In the case of physical absorption, mass transfer intensification ( Murphree Efficiency) increases when the number of transfer unit NTU G (= τ/ t transfer ) increases too. It is mainly due to the small volume of the apparatus where the gas phase residence time must be taken into account in the study of the mass transport towards the gas/liquid interface: the radial and the axial distances for the mass transfer and the mass transport have the same order of magnitude.

Heterogeneous Chemistry of Cl2O and HOCl on Frozen Natural Sea Salt, Recrystallized Sea Salt, KCl and NaCl Solutions at 200 and 215 K

Abstract The HOCl heterogeneous reaction on frozen natural (NSS) and recrystallized (RSS) sea salt, KCl and NaCl solutions was studied using a low pressure flow reactor in order to measure the uptake coefficient γ and products of reaction. The HOCl sample used in these experiments always contained up to 25% Cl2O which was also studied separately as a pure gas in order to understand the heterogeneous chemistry of both gases. By performing HOCl uptake on frozen NSS solution at 200 K and a gas-phase residence time of (1.6±0.6) s we obtained a steady state uptake coefficient γHOCl on NSS = (2.5±0.7)×10-3 and γCl2O on NSS = (2.8±0.8)×10-3. On frozen KCl solution at 200 K we obtain γHOCl on KCl = (2.8±1.3)×10-3, identical to NSS, and γCl2O on KCl = (4.6±0.8)×10-4. The main product formed during the uptake on frozen NSS solution is Cl2 which is sustained for at least one hour. In contrast, only a transient Cl2 flow (pulse) decreasing on the time scale of 100 s was observed on frozen KCl (NaCl) solution. 25±10 % of the HOCl taken up on all chloride-containing frozen substrates at 200 K react to produce Cl2 at high HOCl concentration (4.5×1011 molecule cm-3) and at a residence time of 1.6 s in comparison with twice that for Cl2O. For smaller concentrations such as [HOCl] = 3.7×1010 molecule cm-3 and/or a shorter residence time (0.137±0.004s), HOCl uptake did not generate Cl2 in contrast to Cl2O. A single Br2 burst event was monitored when a Cl2O or HOCl/Cl2O mixture is taken up on fresh frozen NSS solution during the first uptake at 200 K. Further Cl2O or HOCl/Cl2O uptake on the same sample, even after annealing at 240 K does not show an additional Br2 pulse. This Br2 release may be significant in the autocatalytic ozone destruction mechanism in the troposphere during polar sunrise. Some of the atmospheric implications of the present results are highlighted with emphasis on the preequilibrium Cl2O(ads) + H2O(ice) ↔ 2 HOCl(ads) between adsorbed HOCl and Cl2O, with the latter being the gateway to reactive uptake of HOCl at low temperatures.

Measurement of gas phase characteristics using a monofibre optical probe in a three-phase flow

The study of gas–liquid–solid systems structure requires reliable measurement tools. In this paper, preliminary results on the potential use of a monofibre optical probe to investigate such flow are presented. This probe, manufactured at LEGI, allows the simultaneous measurement of the gas phase residence time and gas phase velocity. This specificity makes this probe more interesting than classical single tip probes (which measure only the gas residence time) or double tip probes (which are more intrusive). Although extensively used in two-phase gas–liquid, this probe was never used in gas–liquid–solid systems. First, the probe signal response is studied for three-phase flow conditions in the presence of solids. Results show that for soft solids, the probe tips can be contaminated when the probe pierces the solid. The signal processing procedure was modified accordingly to take into account these events. Second, the probe results are validated by comparing global results (global void fraction, gas flowrate) deduced from profile measurements with measurements performed by independent means. Lastly, void fraction profiles and interfacial area are studied more in detail. Depending on the solid loading, these profiles exhibit different behaviours. These features are associated to characteristics of the flow such as the transition from an homogeneous regime to an heterogenous regime, and are consistent with global observation performed by independent means. This demonstrates the ability of the probe to connect local information to the global behaviour and structure of the flow.

Dynamic Measurement of Carbon Dioxide Volumetric Mass Transfer Coefficient in a Well-Mixed Reactor Using a pH Probe: Analysis of the Salt and Supersaturation Effects

This paper presents a new method for the evaluation of volumetric mass transfer coefficients (kLa) using a pH probe response recorded during the absorption of carbon dioxide in a well-mixed reactor. For this method of evaluation, it is not necessary to know the reaction equilibrium constant, the experiment start time, or the initial and final steady-state pH probe readings. The experimental procedure is based on Hill's recent publication (Ind. Eng. Chem. Res. 2006, 45, 5796). Hill reported a decrease in kLa following the addition of salt during carbon dioxide absorption in a well-mixed reactor and explained it as the result of ionic charge effects reducing the ability of carbon dioxide molecules to diffuse away from the surface. Generally, oxygen absorption from air is preferred because the solubility of oxygen is low and, thus, so will be the gas-phase depletion. In such experiments, the mass transfer coefficients are less affected by gas-phase residence time distribution and by bubble-size distribution....

Effect of wall conditions on the self-limiting deposition of metal oxides by pulsed plasma-enhanced chemical vapor deposition

Pulsed plasma-enhanced chemical vapor deposition has been engineered to deliver self-limiting growth (i.e., ∼Å∕pulse) of metal oxides such as Ta2O5 and Al2O3. In this process the reactor walls are alternately exposed to atomic oxygen and metal precursors. The degree of adsorption in the latter step can dramatically influence both deposition rates and film quality. The impact of precursor adsorption on the plasma and gas-phase composition in these systems was quantified using optical emission spectroscopy and quadrupole mass spectrometry, respectively. It is shown that the time scale for a complete adsorption on the chamber walls is much greater than gas-phase residence times. Adsorbed compounds significantly alter the reactor composition, particularly at the initiation of each plasma pulse. As a consequence, careful attention must be paid to reactor design and operation to control deposition rates and maintain film quality.

Product Analysis and Thermodynamic Simulations from the Pyrolysis of Several Biomass Feedstocks

The pyrolysis of southern pine, red oak, and sweet gum sawdust is reported. Pyrolysis experiments were conducted under either a helium or nitrogen atmosphere at ∼371−871 °C, to determine the balance between liquid and gas products. Gas- and liquid-phase pyrolysis products were identified using gas chromatography (GC) and GC/mass spectrometry (MS). A total of 109 liquid and 40 gas compounds were identified. A total of 59 chemical compounds (35 liquids and 24 gaseous products) were quantitatively determined. The influence of the gas-phase residence time and biomass feed particle size were studied. The gas residence time determined the extent of secondary reactions. Very short residence times enhanced liquid production versus gas production. Particle sizes (d < 105 μm, 105 μm < d < 149 μm, 149 μm < d < 297 μm, and d > 297 μm) did not have a pronounced effect on either the yield or product distributions, indicating that heat-transfer limitations within the particles were negligible. The pyrolysis of pine, red oak, and sweet gum sawdust yielded similar product distributions. Simulations were conducted using the ASPEN/SP software package based on Gibbs energy minimization. At high temperatures, dominant species were hydrogen and carbon monoxide, while at lower temperatures, methane, carbon dioxide, and water were the predominant species. Above 871 °C, further increases in the temperature did not affect the product distribution. Lower gasification temperatures and higher steam/carbon ratios resulted in higher hydrogen and carbon monoxide production.