Water-to-Air Mass Transfer of VOCs: Laboratory-Scale Air Sparging System

Nonequilibrium air-water mass transfer experiments for six volatile organic compounds (VOCs) were conducted using a bench-scale air sparging system. VOCs used were carbon tetrachloride, trichloroethylene, tetrachloroethylene, chloroform, dichloromethane, and toluene. The average particle size of the porous media used ranged from 0.278 to 1.71 mm. The air-water mass transfer coefficients were estimated by fitting the experimental data to a lumped parameter model. The model assumed that the saturated porous media under air sparging conditions consisted of two zones. In the “mass transfer” zone, VOCs were directly impacted by the flow of air in the air channels, while in the “bulk water” zone, VOCs were not directly affected by the air flow in the air channels. The estimated air-water mass transfer coefficients (KGa) were found to increase for higher injected air flow rates and for larger mean particle sizes of porous media but were inversely proportional to the Henry's law constant of the VOCs. An empirical correlation was developed by correlating the Sherwood number with the Péclet number, the Henry's law constant, and the mean particle size of porous media. The estimated fraction (F) of the total volume of the porous media directly affected by air channels was between 5 and 20% for fine sand, indicating that a small fraction of the porous media was affected by the advecting air stream.     

Remediation of TCE-Contaminated Groundwater in a Sandy Aquifer Using Pulsed Air Sparging: Laboratory and Numerical Studies

The objective of this study was to investigate, through laboratory and numerical investigations, the effectiveness of a pulsed air sparging system for remediation of groundwater contaminated with trichloroethylene (TCE) in a sandy aquifer. In laboratory experiments, air was pulsed into TCE source zone on a daily basis in order to remediate TCE-contaminated groundwater. Most dissolved TCE was removed at the end of experiments although its concentrations fluctuated due to the air pulsing. The measured gaseous phase TCE concentration increased whereas the aqueous phase TCE concentration decreased during air sparging pulses. Experimental data were assessed by using a numerical code STOMP (subsurface transport over multiphases) with some modification based on the TCE dissolution kinetics. The unmeasured residual TCE mass was predicted through numerical simulations. Results show that aqueous concentrations for TCE are still much higher than the maximum contaminant level in spite of successful removal of 95% of residual TCE. It may imply that it would be more appropriate to apply air sparging combined with other remediation technologies such as bioremediation for remediation of TCE-contaminated groundwater.     

Removal of Dissolved- and Free-Phase Benzene Pools from Ground Water Using In Situ Air Sparging

This paper presents the results of a laboratory investigation performed to study the use of air sparging to remediate dissolved-phase and free-phase [or non-aqueous-phase liquid (NAPL)-phase] benzene pools from ground water. The specific objectives of the study were (1) to assess how air injection rate affects the mass transfer and transport of dissolved- and NAPL-phase pools; and (2) to determine the effect of ground-water flow on the removal of dissolved- and NAPL-phase pools during the application of air sparging. A total of five 2D physical model tests were performed in a homogeneous coarse sand profile subjected to both static ground water and ground-water flow conditions. Three different air injection rates were used in a static ground-water condition, and two different air flow rates were used in soil profiles subjected to ground-water flow (hydraulic gradient = 0.011). All tests were performed with similar initial dissolved- and NAPL-phase benzene conditions. Injected air traveled within a parabolic zone of influence (in channel mode) when subjected to both static ground water and ground-water flow conditions, indicating that ground-water flow (for the ground-water velocities tested) did not affect the injected air zone of influence. An increase in air injection rate led to faster contaminant removal; however, at higher air injection rates, a threshold rate of removal was reached above which further increases in injection rate are a waste of effort. Additionally, air injected into the soil profile reduced the hydraulic conductivity within the zone of influence. This in turn led to lower ground-water flow rates, allowing for effective interception and treatment of a migrating NAPL plume. Higher air injection rates led to further reductions in hydraulic conductivity, allowing for substantial control of the NAPL plume in the downgradient direction. Overall, this study showed that air sparging can be used to effectively remediate dissolved- and NAPL-phase benzene.     

Benzene Nonaqueous Phase Liquids Removal under Air-Sparged Conditions

The dissolution, diffusion, and volatilization of a free-phase benzene glob in the presence of air sparging were measured in laboratory-scale air sparging reactors and modeled using a dissolution–diffusion–volatilization (DDV) model. The estimated dissolution rate coefficients (Kf) from the DDV model ranged from 0.0050 to 0.017 mm/min while the estimated volatilization rate coefficients (KL) ranged from 0.012 cm/min to 0.029 mm/min. The DDV model fitted well the aqueous phase migration of benzene for air channels spaced at 45 and 60 mm. For air channels spaced at 30 mm, the model fitted the aqueous migration at most locations except above the benzene glob where the model underestimated the experimentally determined concentrations. However, the mass removed using the gas phase concentrations as predicted by the model were 65% of the actual mass removed. These observations suggest that other mass transport mechanisms may influence benzene mass removal, especially when the air channels are close to the benzene glob.     

Effects of Soil Heterogeneity on Airflow Patterns and Hydrocarbon Removal during In Situ Air Sparging

This paper presents the results of a laboratory investigation performed to study the effect of soil heterogeneity on the removal of benzene from six different homogeneous and heterogeneous soil profiles using in situ air sparging. Air injected in homogeneous coarse sand profiles traveled in channels within a parabolic zone. Within fine gravel, injected air traveled in bubbles and was confined to a smaller zone of influence than within the sand profiles. Heterogeneous soil profiles were subjected to airflow patterns that were combinations of patterns observed in homogeneous soil profiles. When hydraulic conductivity differences between adjacent soil layers or inclusions were less than a factor of 10, air freely entered into the lower permeability soil regions. However, when differences were greater than a factor of 10, the injected air followed a path of least resistance and avoided regions of lower permeability. Regions that were subjected to high airflow were remediated rapidly as a result of efficient vapor-phase partitioning due to volatilization. Regions that were not subjected to high airflow, whether the result of locations outside of the zone of influence or airflow diversion due to permeability differences between adjacent soil layers∕lenses, did not experience rapid contaminant removal and required rate-limiting diffusion for contaminant removal. Overall, the importance of determining the degree of soil heterogeneity at a given site with a detailed site investigation prior to implementing air sparging is demonstrated.     

Neural Network Prediction of Air Stripping KLa

Design of air stripping packed towers used to remove volatile organic compounds requires an estimate of the overall mass transfer coefficient (KLa), which is frequently obtained via physically based parametric correlations. Parametric correlations have some shortcomings and produce predictions with relatively large deviations for full-scale, modern application of air stripping towers. In this study, neural network (NN) technology, a powerful new nonparametric approach, is used to analyze mass transfer characteristics in air stripping towers and to simulate KLa. A large database that is representative of current applications of air stripping towers was assembled for this analysis. The KLa predictions by neural networks were superior to both the Onda model [Onda, K., Takeuchi, H., and Okumoto, Y. (1968). “Mass transfer between gas and liquid phases in packed columns.” J. Chem. Eng. Jpn., 1, 56–62.] and an improved Onda model [Djebber, Y. and Narbaitz, R. M. (unpublished)], the best existing parametric models for air stripping applications. The average absolute error for the validation, as well as for the development data, were found to be less than 19%. The NN model was able to simulate the sudden increase in KLa at high gas loading rates. Also, it simulated more realistically the effect of the packing depth and liquid flow.     

Interaction of Soil Air Permeability and Soil Vapor Extraction

A new theoretical model to analyze the measurements obtained from a typical soil column venting experiment is proposed. The principles of mass transfer, Darcy's law, and air compressibility in the form of pressure-volume relationships were coupled to calculate the contaminant concentration in the gas phase (air), and the rate of contaminant removal. The proposed model relates soil air permeability with the contaminant removal and is capable of calculating the variation of soil air permeability with time during the venting process. The contaminated sand sample was idealized as a system of straight capillary tubes in the direction of flow, lined by the liquid contaminant. A closed-form solution for radial diffusion of the contaminants in a cylinder, coupled with axial advection of air, was used to model contaminant removal. The results from the mass transfer model were then used to trace the change of soil air permeability with time. The model also uses, as an alternative approach, a modified form of Darcy's law for compressible flow.     

生物通风修复甲苯污染土壤过程中挥发和生物降解模拟研究

A two-dimensional numerical model is developed to simulate the flow, transport and biodegradation of toluene during bioventing (BV) processes in the unsaturated zones. The simulation for a single well BV system is used to illustrate the effect of air injection rate on remediation efficiency. The air is injected into the vadose zone to create a positive pressure. Simulation results show that air injection rate is a primary parameter governing the dispersal,redistribution and surface loss of contaminant. At injection rates of 81.504 m3.d-1(Run 1) and 407.52 m3.d-1(Run 2), the total removed mass of toluene is 169.14 kg and 170.59 kg respectively. Ratios of volatilization to biodegradation in Run 1 and Run 2 are 0.57 : 1 and 0.89 : 1, respectively, indicating that lower air injection rate enhances the biodegradation efficiency greatly. Air injection rate should be optimized to meet oxygen demand and to minimize the operational cost.     

Removal of Mercury from Low-Concentration Aqueous Streams Using Chemical Reduction and Air Stripping

Field, laboratory, and engineering data confirmed the efficacy of chemical reduction and air stripping as a low concentration mercury treatment concept for water containing Hg(II). The process consists of dosing the water with low levels of stannous chloride [Sn(II)] to convert the mercury to elemental mercury (Hg0). Hg0 can easily be removed from the water by air stripping or sparging. We studied this concept for groundwater containing initial mercury concentrations of approximately 138 ng/L (0.00069 μmol/L). In undosed samples, sparging removed 0% of the initial mercury. Removal in the treated samples varied by reagent dose. Low reagent doses, with Sn:Hg stoichiometric ratios <1, showed little removal. High reagent doses, with Sn:Hg stoichiometric ratios greater than about 5 to 25, showed relatively complete removal (>94%) and yielded final mercury concentrations <10 ng/L (<0.00005 μmol/L). At intermediate doses, mercury removal was a function of the dose. A kinetic study indicated that addition of the Sn(II) reagent resulted in rapid reduction of Hg(II) to Hg0. When combined with standard supporting engineering techniques (e.g., treating the purge air) as needed, a simple system of chemical reduction and stripping may be useful and cost effective.     

气刀对热镀锌锅流场影响的数值模拟

气刀喷吹是带钢连续热镀锌生产的重要工序,理解气刀喷吹过程对锌锅内部流场和表面流场规律的影响对提高带钢质量具有重要意义.建立了带钢连续热镀锌过程中气刀喷吹三维流场数学模型及锌锅流场三维数学模型,将气刀喷吹在锌锅表面的压力作为动量源项加载到锌锅流场数学模型的动量方程中,分析气刀喷吹压力及带钢速度对锌锅内部及表面流场的影响,...     

Leaching characteristics of nickeliferous pyrrhotite tailings from the Sudbury,Ontario area

Mineralogical characterisation showed the deportment of Ni to be similar in the Vale and Glencore tailings, with 60% of the total Ni locked in pyrrhotite, and the balance 40% associated with pentlandite. Nickel leaching was correlated with the dissolution extents of pyrrhotite and pentlandite as functions of four leaching regimes: ‘anoxic acid’ (with and without pH control), ‘oxic acid’ (oxygen sparging), ‘oxic acid’ (air sparging), and ‘oxic ferric’ (air sparging). The results showed that the maximum Ni dissolution was obtained during the pH controlled oxic acid leach with oxygen sparging at pH 1.5, while the anoxic acid leach at pH 1.5 resulted in minimum Ni dissolution (10–15%) from pyrrhotite. An overall Ni mass balance showed that pyrrhotite and pentlandite dissolve simultaneously in the presence of Fe(III) and oxygen, in contrast to the preferential dissolution of pyrrhotite in the absence of Fe(III). Elemental sulphur yield increased with increasing temperature, but no observable trend could be linked to ferric or ferrous ion concentration.     

Long-Term AS∕SVE for Petroleum Removal in Low-Permeability Piedmont Saprolite

A 3.5-year pilot test of air sparging∕soil vapor extraction (AS∕SVE) was carried out to determine whether the heterogeneity of the Piedmont saprolite would allow adequate soil vapor velocities and effective vapor-phase extraction rates for petroleum hydrocarbon (PHC) remediation. The objectives were to compare: (1) the effectiveness of pulsed SVE versus pulsed AS∕SVE operation; (2) benzene, toluene, ethylbenzene, and xylene (BTEX) versus PHC removal; and (3) biological versus physical removal of PHC. Stack exhaust gas, SVE wells, and soil vapor probes were monitored for total combustible hydrocarbons (TCH), BTEX, O2, CO2, temperature, and flow rate using handheld meters and gas chromatography. The majority of contaminant recovered was removed from the vadose zone via SVE. BTEX and TCH were both effectively removed from the more-permeable and highly contaminated unsaturated-saturated zone interface. Bioremediation accounted for 23% of total removal. Overall, the AS∕SVE system physically removed an estimated 18,800 kg of PHC and 5,300 g of BTEX with an average rate of 70 kg d?1 and 0.4 g d?1, respectively, which was consistent with other AS∕SVE studies in sandy media.     

Determination of C1-C5 alkyl nitrates in rain, snow, white frost, lake, and tap water by a combined codistillation head-space gas chromatography technique. Determination of Henry's law constants by head-space GC

Alkyl nitrates with a chain length up to five carbon atoms have been determined in snow, white frost, and surface water. The samples were taken in the vicinity of Ulm, Germany, a region in central Europe. The determination of C1-C5-alkyl nitrates in water samples was achieved with a new water codistillation enrichment technique directly coupled with on-column head-space gas chromatography. The concentrations of the short chain alkyl nitrates in the different forms of wet deposition range from 89 ng L-1 for 1-propyl nitrate down to 35 ng L-1 for 1-pentyl nitrate. C1-C5-alkyl nitrates in wet depositions were also directly determined by static head-space gas chromatography. Gas-water partition coefficients KGW (Henry's law constant H) were determined by head-space gas chromatography and secondly by calculating the Henry's law constant by the ratio of vapor pressure to water solubility. The gas-water partition constants (dimensionless) or Henry's law constants range from KGW = 0.038 (H = 93 Pa m3 mol-1) for 1-propyl nitrate up to KGW = 0.122 (H = 302 Pa m3 mol-1) for 2-pentyl nitrate.     

Gas-Liquid Mass Transfer along Small Sewer Reaches

Municipal and industrial sewers may be localized sources of volatile organic compound (VOC) emissions to the ambient atmosphere. Previous studies of VOC emissions from sewers have focused on sewers with large diameters that are often characterized as having mild channel slopes and as conveying relatively large wastewater flow rates. The study described in this paper was completed to better understand VOC emissions from sewer reaches with small diameters, steep channel slopes, and relatively low wastewater flow rates (e.g., as might be typical for building laterals, street sewers, and on-site industrial sewers). Mathematical models were developed to investigate the nature of mass transfer kinetics and equilibrium conditions in such sewers. A series of 20 experiments were then completed to determine liquid-phase and gas-phase mass transfer coefficients for a range of sewer operating conditions and chemical properties. Experiments were completed in an experimental sewer reach (60 m length, 0.2 m diameter) using five volatile chemicals (acetone, ethyl acetate, toluene, ethylbenzene, and cyclohexane, listed in order of increasing Henry's law constants). Experimental stripping efficiencies were as high as 47% for cyclohexane and as low as 0.3% for acetone. Experimental and mathematical results indicate that VOCs with low Henry's law constants (e.g., acetone) can reach equilibrium conditions rapidly in sewers. However, emissions of VOCs with high Henry's law constants (e.g., cyclohexane) are kinetically limited, allowing for the sewer to be treated as an “open” system. The findings described herein suggest that a large fraction of VOCs with high Henry's law constants may be emitted to the ambient atmosphere in the near vicinity to the point of discharge.     

Hybrid bilayer membranes in air and water: infrared spectroscopy and neutron reflectivity studies

In this report we describe the fabrication and characterization of a phospholipid/alkanethiol hybrid bilayer membrane in air. The bilayer is formed by the interaction of phospholipid with the hydrophobic surface of a self-assembled alkanethiol monolayer on gold. We have characterized the resulting hybrid bilayer membrane in air using atomic force microscopy, spectroscopic ellipsometry, and reflection-absorption infrared spectroscopy. These analyses indicate that the phospholipid added is one monolayer thick, is continuous, and exhibits molecular order which is similar to that observed for phospholipid/phospholipid model membranes. The hybrid bilayer prepared in air has also been re-introduced to water and characterized using neutron reflectivity and impedance spectroscopy. Impedance data indicate that when moved from air to water, hybrid bilayers exhibit a dielectric constant and thickness that is essentially equivalent to hybrid bilayers prepared in situ by adding phospholipid vesicles to alkanethiol monolayers in water. Neutron scattering from these samples was collected out to a wave vector transfer of 0.25 A(-1), and provided a sensitivity to changes in total layer thickness on the order of 1-2 A. The data confirm that the acyl chain region of the phospholipid layer is consistent with that observed for phospholipid-phospholipid bilayers, but suggest greater hydration of the phospholipid headgroups of HBMs than has been reported in studies of lipid multilayers.     

A two-phase model for controlled drug release from biphasic polymer hydrogels

A comprehensive two phase model is developed to describe the sustained release of a solute or drug from a biphasic hydrogel substrate. Such a material consists of a continuous hydrophilic phase (polymer backbone in water) and a dispersion of spherical microdomains made of the hydrophobic side chains of the polymer organised in a micelle like fashion. The solute or drug is assumed to be encapsulated within the dispersed microdomains, and to diffuse from the interior to the surface of the microdomain where it exchanges following a Langmuir isotherm. Mass transfer to the bulk phase occurs by desorption of the drug from the surface through a driving force that is proportional to the difference of surface and bulk concentration. Accordingly the drug is released to the surroundings by diffusion through the bulk. Depending on the values of the Langmuir constant and assuming well stirred behaviour in the interior of the microdomain, the present model results in either of the two asymptotic models developed in previous studies. The results of a parametric study show that the desired steady state flux of a specific drug to the surroundings may be obtained given appropriate values of structural properties of the material. This conclusion is further supported when using this model to simulate earlier experimental results. The polymer structural properties can be manipulated easily during the fabrication of dispersed-phase networks, as indicated by preliminary experiments.     

板坯连铸异钢种连浇混浇坯长度及成分变化模型的开发及应用

基于建立的连铸中间包及结晶器内钢液混合过程的物理模型,开发了板坯连铸异钢种连浇过程混浇坯长度及成分变化模型。以某钢厂单流板坯连铸机220 mm×1560 mm断面Q235与Q335Ti钢的混浇过程为研究对象,采用水模型试验结合数值模拟确定模型的关键参数,并通过开展现场试验对混浇坯取样验证模型的准确性。结果证明:混浇坯成分取样与模型预测的成分偏差小于5%,且模型预测的混浇坯长度与人工确定的一致。故采用该模型可跟踪不同混浇工况下中间包内及铸流上钢液的混合行为,准确预测混浇坯的长度以及成分变化规律。采用该模型研究了拉速及中间包内剩余钢液质量对混交坯长度及不同浇注长度铸坯C元素质量分数变化的影响规律。发现当拉速保持不变时,中间包内剩余钢液越多,混浇坯越长;当中间包内剩余钢液质量保持不变时,拉速越大混浇坯越短。相比而言,中间包内剩余钢液质量比拉速对混浇坯长度的影响更大。另外当拉速不变时,随着中间包内剩余钢液质量的增加,C元素质量分数由0.16%变化到0.18%的速率减慢;当中间包内剩余钢液质量不变时,随着拉速的增加,C元素质量分数由0.16%变化到0.18%的速率增加。因此异钢种连浇过程,适当提高拉速以及减少中间包内剩余钢液质量,可有效减少混浇坯长度,成分变化速率降低。      

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In order to research the stirring effect on bottom blowing gas supply mode in combined blowing converter, the effect of bottom blowing flow rate distribution, switching frequency and grouping mode on mass transfer between slag- metal interface and mixing time was investigated. The study indicates that both mass transfer coefficient and mixing time change with the bottom blowing flow rate distribution, switching and grouping mode. Under continuous mode with distribution ratio as 4, the mass transfer effect reaches best; when set distribution ratio as 5,mixing time reaches smallest.Under spaced mode with distribution ratio as 3, the mass transfer effect reaches best; when set distribution ratio as 5, mixing time reaches smallest. Mass transfer coefficient decreases with increases of switching frequency. Distribution ratio has greater effect on mass transfer coefficient under continuous mode than spaced mode. Both modes are almost identical on mixing effect. In continuous mode, bottom blowing leads to uneven erosion, large flow side accelerates the lining erosion and two- phase emulsion process, while the small flow side is opposite.     

新型高炉喷煤模拟燃烧实验装置设计

设计了一种适合模拟高炉喷煤燃烧的实验装置,满足热风既高温又高速的煤粉喷吹条件,可以模拟氧气高炉条件下的煤粉喷吹.喷吹瞬间流场的数值模拟结果表明,当喷吹气体速度达到最大值时,直吹管气体平均速度为162.35 m·s^-1.利用该装置研究了氧气高炉条件下煤粉的燃烧规律.结果表明:煤粉的燃烧率随O/C原子比的增加而增加;在低O/C原子比条件下,煤粉的燃烧率较低,但其增幅比较明显;无烟煤的燃烧率低于烟煤.