Diffusional Mass Transfer at Sediment–Water Interface of Cylindrical Sediment Oxygen Demand Chamber

Diffusional mass transfer of dissolved substances across the sediment–water interface in coastal waters is an important factor for realistic determination of sediment oxygen demand (SOD) and nutrient recycle. The benthic diffusive boundary layer inside a cylindrical chamber commonly deployed for in situ measurements of sediment oxygen demand is studied. In a series of laboratory experiments, the SOD is measured with the chamber operated in both continuous flow and batch modes, and a microelectrode is employed to measure the near bed dissolved oxygen (DO) profile for different chamber flows and sediment types. The dependence of the diffusive boundary layer thickness and the sediment–water mass transfer coefficient on the hydraulic parameters are quantified. Using the derived mass transfer coefficient, it is shown that for a given sediment type, the SOD is a function of the bulk DO concentration and chamber flowrate. The theoretical predictions are validated by both laboratory and field SOD data.     

Stochastic Model for Landfill Gas Transport and Energy Recovery

Predicting the amount of landfill gas (LFG) that will be recovered at a sanitary landfill is generally associated with a high level of uncertainty, which is primarily due to the uncertainty in the definition of the parameters that control the LFG generation rate and LFG transport. To quantify these uncertainties, a three-dimensional stochastic model for the generation and transport of LFG is proposed. Using Monte Carlo simulations, multiple realizations of key input parameters are generated. For each realization, LFG transport is simulated and then used to evaluate probabilistically the rates and efficiency of energy recovery. For demonstration, the stochastic model is applied to the Kemerburgaz landfill in Istanbul, Turkey. Uncertainty in the definition of three key parameters, namely: the LFG production rate, the waste gas permeability and the soil cover gas permeability were accounted for. Modeling results suggest that the collection system is sufficient to capture most of the generated gas, but that uncertainty in the factors controlling LFG production is the main source of uncertainty in the amount of energy that will be recovered.     

Gas Transport Parameters for Compacted Reddish-Brown Soil in Sri Lankan Landfill Final Cover

Gas exchange through the compacted final cover soil at landfill sites plays a vital role for emission, fate, and transport of toxic landfill gases. This study involved measuring the soil-gas diffusivity (Dp/Do, the ratio of gas diffusion coefficients in soil and free air) and air permeability (ka) for differently compacted soil samples (reddish-brown soil) from the final cover at the Maharagama landfill in Sri Lanka. The samples were prepared by either standard Proctor compaction or hand compaction to dry bulk densities of 1.60–1.94??g?cm-3. Existing and modified models for predicting Dp/Do and ka were tested against the measured data. The simple, single-parameter Buckingham model predicted measured Dp/Do values across compaction levels equally well or better than a dry bulk density (DBD) dependent model and a soil-water retention (SWR) dependent model. The measured ka values for differently compacted samples were highly affected by the compaction level and the sample moisture preparation method. Also, for air permeability, a single-parameter Buckingham-type ka model was most accurate in predicting ka in the differently compacted soil samples. Equivalent air-filled pore diameters (the effective diameter of the drained pores active in leading air through the sample) for gas flow, deq, were calculated from the measured Dp/D0 and ka values. The deq increased with compaction level, suggesting that a very high compaction level creates well-connected macropores in the reduced total pore space of the cover soil. This is an important consideration when designing cover soils for optimally low water and high oxygen exchange while minimizing climate and toxic gas emissions from the waste layer to the atmosphere.     

Mass Transfer of Ozone in a Novel In Situ Ozone Generator and Reactor

A porous tubular reactor that also served as an electrode for ozone generation was studied in this research to determine the effects of in situ ozone generation on mass transfer and reaction rates. Experimental data over a range of gas flow rates and ozone generation rates gave KLa values in the range 0.77–1.14?min?1. These values are more than double the values typically reported for bubble columns, and about 30% higher than that for packed beds. The specific power requirement for the laboratory-scale in situ reactor is an order of magnitude lower than that for bubble columns and stirred tank reactors that are used for ozone dissolution. A compartments-in-series fluid flow model was developed to describe the reactor system, and this model provides a good comparison to the experimental data for dissolved ozone and off-gas concentrations in the reactor. Sensitivity analyses indicate that the dissolved and off-gas ozone profiles are most sensitive to the gas–liquid partition coefficient and the overall mass transfer coefficient.     

Application of Fuzzy Logic to Estimate Flow of Methane for Energy Generation at a Sanitary Landfill

This paper proposes the use of a multicriteria assessment technique to evaluate the methane flow during gas extraction from a sanitary landfill. A number of parameters determine the gas generation and the feasibility for its extraction from a landfill. These parameters form a complex set of information with unknown mathematical interrelationships making potential gas flow evaluations difficult and elusive. In addition, the data available for a particular landfill are very often imprecise, uncertain, or subjective, making it even more difficult to evaluate the potential for gas extraction without conducting pilot tests. The method proposed in this paper uses fuzzy composite programming that allows for the use of imprecise information. A landfill gas potential index has been defined, which can be determined by easily obtainable climatological, geological, and landfill parameters. The landfill gas (LFG) potential index is related to the landfill gas flow using an empirical equation. The LFG potential model was calibrated and verified using data obtained from 61 landfills where gas extraction is being conducted. A sensitivity analysis was done to study the impact of variations in the input data on model output.     

Modeling of Biodegradation in an Old Unregulated European Landfill

In this note an example of modeling of biodegradation processes of an old abandoned municipal solid waste landfill for its simulation is illustrated using the landfill dynamic simulation tool MODUELO. In this program the waste biodegradation model is based on the quantification of organic matter, its chemical composition, biodegradability, accessibility to microorganisms, and the ratio nonbiodegradable leachable organic matter to gasifiable matter. Data from a characterization campaign, presented elsewhere, were used to determine these parameters. The experimental information was completed with safety factors (to compensate for sampling uncertainty) and literature values. The degradation rates were determined, after having calibrated the hydrological model, by fitting the temporal series of pollutants’ concentration measured in the leachate and the biogas composition. The achieved fit of the simulated series compared to the measured data is reported as the result of this work. Given the limited information available, the obtained simulation model is considered an acceptable tool to study the future evolution of the landfill in different circumstances.     

Mass Transfer Effects on Kinetics of Dibromoethane Reduction by Zero-Valent Iron in Packed-Bed Reactors

Mass transfer effects on the kinetics of 1,2-dibromoethane (EDB) reduction by zero-valent iron (ZVI) in batch reactors, a laboratory scale packed-bed reactor, and a pilot scale packed-bed reactor are described. EDB was debrominated by ZVI to ethylene and bromide. EDB sorption to the cast iron surface was nonlinear and was described by a Langmuir equation. Laboratory scale column studies showed a nonlinear dependence of EDB removal on flow rate and initial EDB concentration. A nonequilibrium model of EDB sorption and reaction dependent on mass transfer was constructed using the laboratory scale data. The model was verified using data from a larger pilot scale packed-bed reactor that was used to remove EDB from contaminated groundwater. The data showed two distinct removal processes, an initial rapid phase dominated by mass transfer followed by a slower phase where surface reactions dominated. The model successfully predicted the transition from mass transfer controlled to surface reaction controlled conditions in the pilot scale data.     

Measuring Aqueous-Air and Sorbed-Aqueous Mass Transfer Coefficients for Application in Soil Vapor Extraction

Soil vapor extraction (SVE) is a common remediation practice typically implemented without a rigorous design process because of insufficient or unspecific design data. Field observations have shown that the mass of contaminant removed by SVE often tails off over time. Significant time has been spent modeling SVE to gain a better understanding of the governing processes and the cause of tailing. Studies have shown that improper mass transfer coefficients affect modeling accuracy. As a result, considerable effort has been spent studying the mass transfer coefficients directly related to the non-aqueous-phase liquid (NAPL), with less emphasis on aqueous air and sorbed-aqueous mass transfer coefficients, despite affecting the observed tailing behavior. Accordingly, a laboratory and modeling study was undertaken with toluene to determine the appropriate aqueous air and sorbed-aqueous mass transfer coefficients. SVE column data generated in laboratory experiments were used to back-calculate the mass transfer coefficients by using FRACMAT, a numerical model. The developed experimental protocol allowed the placement of toluene contamination in the unsaturated soil environment without the development of a NAPL phase. The data generated by the SVE column with soils with organic matter and without organic matter, showed that the aqueous-air mass transfer coefficient was exponential, with the aqueous concentration the independent variable. For the zero to moderate organic matter content soils tested, the aqueous-air mass transfer coefficient varied from 1??s-1 to 0.001??s-1. Some sorbed contamination was also observed, requiring a sorbed-aqueous mass transfer coefficient. Numerical modeling with FRACMAT showed that the best sorbed-aqueous mass transfer coefficient was a constant value of 0.01??s-1. The aqueous-air mass transfer coefficient was observed to be the controlling rate limitation in SVE when no NAPL was present in the soil with the zero to moderate organic matter content soil. Studies with silty loam soil showed that additional mass transfer resistances occurred, which could be attributed to the increase in organic matter content and decrease in particle size.     

Operation and Performance of Horizontal Wells for Leachate Control in a Waste Landfill

This paper presents data on gas and leachate flow rates and leachate levels obtained during a 600?day pumping trial on three retrofitted horizontal wells in a domestic waste landfill at Rainham, United Kingdom. The changes in gas and leachate flow rate with time and atmospheric pressure, and the interaction between the two flows, are discussed. The spatial variability of the response of the leachate levels within the landfill is explored with reference to the anisotropy and heterogeneity of the permeability of the waste. It is shown that horizontal wells can be an effective means of controlling leachate heads near the base of a landfill, and that leachate levels must be measured using piezometers with a discrete response zone rather than fully screened observation wells if meaningful results are to be obtained. It is argued that the large amounts of gas pumped from the nominally saturated zone of the landfill must have come from the ongoing degradation of the waste within the zone of influence of the well.     

Minimizing Mass Transfer Effects in Granular Iron Batch Tests Using GEM Reactors

The glass encased magnet (GEM) batch reactor was designed to permit reactivity studies of granular iron with good mixing, and minimal abrasion. This study compared the GEM reactor with two conventional batch test designs (for granular iron studies). In the GEM reactor experiments, the reduction of 4-chloronitrobenzene with Connelly granular iron was found to proceed at about twice the maximum rate of the reaction mixed by orbital shaking or rolling, indicating that mass transfer was minimized in the GEM tests. In addition, the reproducibility of GEM tests was superior, possibly due to abrasion or other variations to the grain surfaces that occurred in the other types of tests. The biases and uncertainties in kinetic parameters estimated from the shaker and roller experiments were shown to be significant compared to the GEM derived parameters.     

Mass Transfer Correlation Coefficients for Two-Phase Systems: A General Review for Liquid-Liquid

A general review of the mass transfer correlation coefficients available in the literature was done. The emphasis was for liquid-liquid phases and the main mathematical forms of the correlations accessible are reported. In general, there is a necessity of general models. More work has to be done in this area in order to establish more general models that permit reporting only their parameters for a given system and/or equipment.     

Nonequilibrium Nonaqueous Phase Liquid Mass Transfer Model for Soil Vapor Extraction Systems

Soil vapor extraction is a popular soil remediation technology that is hampered by less than optimal performance in the field due to mass transfer limitations. Therefore, laboratory column venting experiments were completed to quantify mass transfer limitations for the removal of multicomponent nonaqueous phase liquid (NAPL) contaminants from a silt loam soil at three water contents. The observed mass transfer limitations were quantified using a four phase multicomponent, nonequilibrium contaminant transport model based on first-order mass transfer kinetics. The overall mass transfer coefficient Kga was treated as a variable and modeled as a linear function of the NAPL volumetric fraction using two adjustable parameters (m, the slope parameter and Kgamin, the intercept). Both were back calculated from column venting data. The agreement between the calibrated model and experimental results were favorable for the removal of single and binary contaminants under conditions ranging from near equilibrium to severe mass transfer limitations and extended tailing. A strong dependency of Kga on water content was evident by the differences in Kgamin and to a lesser extent, m, at the three water contents investigated. A single expression Kga captured the performance of both components in the binary mixture. For the quaternary venting experiments a single expression for Kga captured the performance of all four components well under air dry conditions. However, the agreement between the hexane model versus the experimental result deteriorated significantly as the water content increased. This difference is attributed to hexane’s lower affinity for the water phase relative to the other three components in the mixture.     

Synchrotron Radiation for Detecting Movement of a Volatile Soil Liquid

Heat and volatile liquid transport in soils are physically coupled processes. Soil water movement driven by thermal gradients has received considerable attention both experimentally and via models. This is appropriate since the thermal regime of underground electric power cables, pipelines, agricultural soils, and nuclear-waste repositories influences their ability to function effectively. However, available experimental liquid concentration data are of relatively low resolution, generally about one measurement per cm. By using synchrotron x rays we were able to nondestructively obtain high (50 μm) spatial resolution, time-lapsed data for the thermally driven movement of dibromomethane (DBM) in soil. Under repeated temperature cycling during a 22 h period, DBM movement within a sealed, 25-mm-long quartz sand (D50 = 0.25?mm) soil column was highly repeatable. About four complete scans of the column were completed each hour. Average DBM vol?% within the column as a whole was 4.0 and the porosity was 0.31. Measurements of DBM concentration at the same position and equivalent times during different cycles usually differed by less than 0.2 vol?%. Experimental data of this precision will permit better comparison with detailed model predictions.     

Periodic Diffusional Mass Transfer near Sediment/Water Interface: Theory

Time-variable (periodic) flow over a lake bed, and the associated boundary layer development, have the potential to control or at least influence rates of mass transfer across the sediment/water interface. An analysis for instantaneous and time averaged flux of a material across the sediment/water interface for infinite supply in the water and infinite sink in the sediment is presented. The water flow above the interface is characterized by the shear velocity (U?) which is a periodic function of time with a maximum amplitude of (U?0) as may be typical of an internal seiche (internal standing wave) motion in a density stratified lake. The relationship between the shear velocity on the lake bed and the wind shear on the lake surface is illustrated for an extremely simplified two-layered lake of constant depth. For a less restrictive analysis, shear velocities on a lake bed have to be obtained either from field measurements or from a three-dimensional lake circulation model driven by atmospheric forcing including wind. Smaller and wind-sheltered lakes will have lower (U?0) and periodicities (T). The response of the diffusive boundary layer was related to the period of the periodic motion (T), Schmidt number (Sc), and shear velocity (U?). The vertical diffusive flux at the sediment/water interface was expressed by a Sherwood number (Sh), either instantaneous or time averaged. The mean Sherwood number (Shave) varies with shear velocity of the wave motion over the sediment bed, Schmidt number (Sc) and the period (T) due to the response of the diffusive boundary layer to the time variable water velocity. Effective diffusive boundary layers develop only at low shear velocities. Where they do, maximum and minimum boundary layer thickness depends on all three independent variables (T, Sc, and U?0). The diffusive boundary layer strongly affects sediment/water mass transfer, i.e., Sherwood numbers. Mass transfer averaged over a period can be substantially less than that produced by steady-state flow at the same U?0 and Sc. At Sc = 500, typical for dissolved oxygen, the mass transfer ratio can be reduced to 60% of steady state, depending on the internal wave period (T).     

Role of Mass Transfer Resistance in Overall Substrate Removal Rate in Upflow Anaerobic Sludge Bed Reactors

A kinetic model (incorporating intrinsic Haldane kinetics) and an empirical model (incorporating apparent Haldane kinetics) for phenol degradation in upflow anaerobic sludge bed (UASB) reactors were used. UASB-reactor performance data were also generated for model verification. From the independent batch study together with statistical analyses, the apparent Haldane kinetic constants k′ and Ki′ did not differ from the intrinsic k and Ki, but the apparent Ks′ was significantly larger than the intrinsic Ks (i.e., greater internal mass transfer resistance). From the calculated results (of overall effectiveness factor, Thiele modulus, and Biot number) together with parametric sensitivity analyses, the internal mass transfer resistance would play a more influential role than the external mass transfer resistance on the overall substrate removal rate in UASB reactors. The simulated residual phenol concentrations using the empirical model were in good agreement with the experimental data; the simulated results using the empirical model were also close to those using the kinetic model. Accordingly, the empirical model that can properly describe the overall substrate removal rate should be acceptable for process design of UASB reactors.     

Axial Dispersion and Mass Transfer Controlled Simulation Study of Chromium (VI) Adsorption onto Tree Leaves and Activated Carbon

In this paper, the feasibility and efficacy of chromium (Cr(VI)) removal using three different kinds of tree leaves viz. Emblica officinalis, Azadirachta indica, Eucalyptus agglomerata, and the activated carbon is examined through batch and continuous flow experiments. Pretreatments were given to the selected tree leaf powders to remove the natural pigments and lignin present. Batch and continuous flow experiments have been conducted to study the kinetics of adsorption, effects of pH, adsorbent dose, contact time, bed depth, flow rate, and initial Cr(VI) concentration on Cr(VI) adsorption onto the selected adsorbents. The adsorption capacity is observed higher for Emblica officinalis followed by Eucalyptus agglomerata and Azadirachta indica. The adsorption equilibrium is reached in less than 30 min and the maximum Cr(VI) uptake occurred at pH 3.0 under the test conditions. The results are also compared with the commercially available activated carbon. A mathematical model incorporating diffusion, advection, and mass transfer mechanisms available in the literature has been simplified and is then tested to simulate the laboratory and literature data. A simple method for the determination of saturation Cr(VI) concentration along the length of column has been presented. The study reveals that the model incorporating the molecular diffusion and the mass transfer mechanisms simulates better the Cr(VI) adsorption onto tree leaf powders than the literature model and the advection term plays only a negligible role due to low flow rates applied during the experiments. The model parameters, i.e., axial dispersion coefficient, “DL” and the external mass transfer coefficient, “kf” are found in the order of 10?5–10?6?m2/s and 10?9–10?11?m/s, respectively.     

RH-MFB传质特性的物理模拟研究

采用NaOH稀溶液吸收CO2实验来模拟120tRH—MFB顶吹O2条件下真空脱碳反应过程的传质现象。在1：5．45的水模型中,考察了提升气体流量、顶吹气体流量、顶枪枪位、插入管内径、浸入深度、喷嘴个数与气泡行程等因素对传质的影响,测定不同参数组合条件下的容量传质系数,进而考察容量传质系数与气体分压等的关系。结果表明,容量传质系数随提升气体流量、顶吹流量、气相分压、插入管内径和浸入深度的增大而增大。     

转炉烟道内二次燃烧和传热传质过程的研究

采用非预混燃烧模型和离散传播辐射模型,对转炉烟道内的流动、传热传质和燃烧反应进行三维数值模拟,研究炉气转变为烟气的规律和烟道内烟气浓度分布特点.结果表明,烟道内大体可分为3个区域:炉口燃烧区,卷吸的空气在烟罩附近与炉气发生燃烧反应;烟道混合区,燃烧后的烟气与未燃炉气逐渐混合,但仍有较大的成分波动;弯头混合区,烟气进一步...     

Heat Transfer and Its Effect on Solidification in Combined Mould

SymbolList cp,cps,cpl———Specificheat,specificheatofsolidsteel andmoltensteel,(J·kg-1·K-1);C2———EmpiricalcoefficientoflowReynoldsκεturbu lencemodel;Dl———Darcyconstant;fl,fs———Fractionofmoltensteelandsolidsteel;f2———Empiricalcoefficientre