On the selection of primary variables in numerical formulation for modeling multiphase flow in porous media

Selecting the proper primary variables is a critical step in efficiently modeling the highly nonlinear problem of multiphase subsurface flow in a heterogeneous porous-fractured media. Current simulation and ground modeling techniques consist of (1) spatial discretization of mass and/or heat conservation equations using finite difference or finite element methods; (2) fully implicit time discretization; (3) solving the nonlinear, discrete algebraic equations using a Newton iterative scheme. Previous modeling efforts indicate that the choice of primary variables for a Newton iteration not only impacts computational performance of a numerical code, but may also determine the feasibility of a numerical modeling study in many field applications. This paper presents an analysis and general recommendations for selecting primary variables in simulating multiphase, subsurface flow for one-active phase (Richards' equation), two-phase (gas and liquid) and three-phase (gas, water and nonaqueous phase liquid or NAPL) conditions. In many cases, a dynamic variable switching or variable substitution scheme may have to be used in order to achieve optimal numerical performance and robustness. The selection of primary variables depends in general on the sensitivity of the system of equations to the variables selected at given phase and flow conditions. We will present a series of numerical tests and large-scale field simulation examples, including modeling one (active)-phase, two-phase and three-phase flow problems in multi-dimensional, porous-fractured subsurface systems.     

填料塔中Na2CO3吸收高浓度H2S的传质特性

在填料吸收塔中考察了Na2CO3溶液吸收高浓度H2S气体的气液传质特性。通过测量填料塔进出口气体中H2S浓度计算了Na2CO3溶液吸收高浓度H2S气体的总体积传质系数（KGa）,并研究了进气流速、吸收液流量、吸收温度和吸收液浓度对KGa的影响。结果表明,KGa随Na2CO3浓度、吸收液流量的增加而增加,随吸收温度、进气流速的升高而降低;在高浓度H2S吸收过程中液相传质阻力不能忽略。     

Thermal venting to recover less-volatile hydrocarbons from the unsaturated zone, 1. Theory

Thermal venting is a remediation technique suitable to the liquid unsaturated zone to enhance recovery of less-volatile residual hydrocarbon contaminants. Thermal venting is different to traditional soil venting because heated air instead of air at ambient conditions is applied to the contaminated zone. The vapor pressure of a less-volatile contaminants is typically increased by temperature causing the gas-phase concentrations to increase by three- to five-fold over a temperature increase of 20–30°C. The work described in this first paper provides the theoretical framework of analysis related to thermal venting. The analysis included nonisothermal gas flow, thermal energy transport and multicomponent mass transport in a multiphase porous medium. The transient gas flow analysis included the effect of temperature on fluid properties and gas compressibility. The heat energy transport analysis was performed under the thermodynamic equilibrium condition with phase-summed effective thermal properties. Multi-component mass transport was performed under local equilibrium for partitioning between phases. Model verification was performed to the extend possible using analytical and available experimental data for different physical processes. The second paper of this two-part series will demonstrate the applicability of thermal venting technique through numerical simulations of hypothetical laboratory and field-scale scenarios.     

Simulation of organic liquid flow in porous media using estimated and measured transport properties

Many numerical models which describe the movement of a separate organic liquid phase in the subsurface require information about the relationships between capillary pressure and saturation, and between relative permeability and saturation. An evaluation of the information available for these relationships suggests that substantial discrepancies may be introduced into simulations if estimated, rather than measured, data are employed. The purpose of this study was to quantify these deviations. Two-phase displacement simulations were performed in one and two dimensions for several organic liquid-water systems. Both constant-head and constant-flux boundary conditions were employed at a variety of flow rates and time scales, using both measurements and estimates of capillary pressure and relative permeability for a sandy aquifer material. The results demonstrate that the use of estimated transport relationships produces significantly different predictions of organic liquid migration. The magnitude of the deviations between predictions may be as high as 25% or more after relatively short displacement periods, depending on the boundary conditions of the simulated scenario, as well as on the physical characteristics of the two-phase system. For the systems examined, most of the deviations resulted from the estimates for relative permeability to the organic liquid. Thus, improved methods for the estimation of the relative permeability to the organic liquid are needed to reduce the uncertainty in displacement simulations.     

Kinetic modeling of microbially-driven redox chemistry of radionuclides in subsurface environments: coupling transport, microbial metabolism and geochemistry

Microbial reactions play an important role in regulating pore water chemistry as well as secondary mineral distribution in many subsurface systems and, therefore, may directly impact radionuclide migration in those systems. This paper presents a general modeling approach to couple microbial metabolism, redox chemistry, and radionuclide transport in a subsurface environment. To account for the likely achievement of quasi-steady state biomass accumulations in subsurface environments, a modification to the traditional microbial growth kinetic equation is proposed. The conditions for using biogeochemical models with or without an explicit representation of biomass growth are clarified. Based on the general approach proposed in this paper, the couplings of uranium reactions with biogeochemical processes are incorporated into computer code BIORXNTRN Version 2.0. The code is then used to simulate a subsurface contaminant migration scenario, in which a water flow containing both uranium and a complexing organic ligand is recharged into an oxic carbonate aquifer. The model simulation shows that Mn and Fe oxyhydroxides may vary significantly along a flow path. The simulation also shows that uranium(VI) can be reduced and therefore immobilized in the anoxic zone created by microbial degradation.     

氧化镁烟气脱硫反应特性研究

利用实验室规模的鼓泡式反应装置,对比了碳酸钙、氧化镁和氧化镁/硫酸镁脱硫剂的反应活性,证实脱硫液中高浓度硫酸镁的存在是保证镁法脱硫效率高于钙法的重要因素,并考察了硫酸镁浓度、脱硫剂(氧化镁)浓度、烟气量、SO2浓度和吸收液温度等因素对脱硫效率的影响。结果表明,脱硫反应可以根据pH分为2个不同阶段;反应过程中脱硫效率随着硫酸镁浓度的增加而显著升高;烟气量增加将会导致脱硫效率有所下降;入口SO2浓度升高,脱硫效率下降;氧化镁浓度、温度对脱硫效率影响不显著。结合实验现象进行推断,氧化镁脱硫的反应过程受SO2在气液两相界面的传质扩散和其水解产物在液相的扩散控制。     

改性活性炭吸附净化黄磷尾气中的PH_3

通过钢瓶配气模拟和气相色谱GC-14C测定的方法,研究了改性活性炭吸附净化黄磷尾气中PH3的相关问题。得到了吸附反应的最佳吸附温度、氧含量、气体流量、改性液浓度、活性炭粒径和焙烧温度。再生方法可行,可通过再生液回收磷酸。热重差热分析和孔径分布初步证明吸附质为磷和磷氧化物。     

On the hydro-dispersive equivalence between multi-layered mineral barriers

In the context of municipal solid waste and hazardous waste disposal, the notion of "equivalence" between different barrier designs appears in regulatory documents from several industrialized countries. While in the past, equivalence has been thought of mainly in terms of contaminant travel times, in recent years it has been defined more in terms of the magnitude of a disposal site's potential impact on groundwater resources. This paper presents some original analytical solutions to the problem of contaminant migration through a multi-layered mineral barrier. The solutions account for the two major mechanisms of subsurface contaminant migration, namely, advection and diffusion-dispersion. An example application using the proposed solutions and a numerical model illustrates how one multi-layered mineral barrier can be considered superior to another from a strictly hydro-dispersive viewpoint. The influence of partial saturation of the mineral barrier is investigated using a numerical solution to the Richards equation for unsaturated flow. It is emphasized that conclusions relative to the superiority of one multi-layered barrier, with respect to another, should not only consider hydro-dispersive aspects, but also other processes such as the mechanical and chemical evolutions of the different barrier components. Although such phenomena are poorly addressed by existing models, failure to take them into account, at least in a qualitative fashion, may lead to unconservative conclusions with respect to barrier equivalence.     

改性活性炭吸附净化黄磷尾气中的PH3

通过钢瓶配气模拟和气相色谱GC-14C测定的方法，研究了改性活性炭吸附净化黄磷尾气中PH₃的相关问题。得到了吸附反应的最佳吸附温度、氧含量、气体流量、改性液浓度、活性炭粒径和焙烧温度。再生方法可行，可通过再生液回收磷酸。热重差热分析和孔径分布初步证明吸附质为磷和磷氧化物。     

The effects of scale and spatial heterogeneities on diffusion in volcanic breccias and basalts: Amchitka Island, Alaska

Knowledge of the factors that influence the diffusion of contaminants, such as the diffusivity and the connected porosity, is crucial to modeling the long-term fate and transport of contaminants in subsurface systems with small or negligible advective flow, such as in fractured crystalline rock. Fractured rock is naturally heterogeneous, and hence, understanding the diffusivity of a molecule through this material (or the formation factor of the medium) becomes a complex problem, with critical concerns about the scale of laboratory measurements and about the spatial variability of these measurements relative to the scale needed for fate and transport modeling. This study employed both electrical and tracer-based laboratory methods to investigate the effects of scale and pore system connectivity on the diffusivity for volcanic matrix rock derived from the study site, a former underground nuclear test site at Amchitka Island, Alaska. The results of these investigations indicate a relatively well-connected pore system with scale effects generally limited to approximately 6 cm lengths and well-correlated to observed heterogeneous features. An important conclusion resulting from this study, however, is that there is a potential for the estimated diffusivity to be misrepresented by an order of magnitude if multiple samples or longer sample lengths are not used. Given the relatively large number of measurements resulting from these investigations, an analysis of the probability density function (PDF) of the diffusivity was possible. The PDF of the diffusivity was shown to generally follow a normal distribution for individual geologic layers. However, when all of the geologic layers are considered together, the distribution of the subsurface as a whole was shown to follow a lognormal distribution due to the order of magnitude differences amongst the layers. An understanding of these distributions is essential for future stochastic modeling efforts.     

Gas production and migration in landfills and geological materials.

Landfill gas, originating from the anaerobic biodegradation of the organic content of waste, consists mainly of methane and carbon dioxide, with traces of volatile organic compounds. Pressure, concentration and temperature gradients that develop within the landfill result in gas emissions to the atmosphere and in lateral migration through the surrounding soils. Environmental and safety issues associated with the landfill gas require control of off-site gas migration. The numerical model TOUGH2-LGM (Transport of Unsaturated Groundwater and Heat-Landfill Gas Migration) has been developed to simulate landfill gas production and migration processes within and beyond landfill boundaries. The model is derived from the general non-isothermal multiphase flow simulator TOUGH2, to which a new equation of state module is added. It simulates the migration of five components in partially saturated media: four fluid components (water, atmospheric air, methane and carbon dioxide) and one energy component (heat). The four fluid components are present in both the gas and liquid phases. The model incorporates gas-liquid partitioning of all fluid components by means of dissolution and volatilization. In addition to advection in the gas and liquid phase, multi-component diffusion is simulated in the gas phase. The landfill gas production rate is proportional to the organic substrate and is modeled as an exponentially decreasing function of time. The model is applied to the Montreal's CESM landfill site, which is located in a former limestone rock quarry. Existing data were used to characterize hydraulic properties of the waste and the limestone. Gas recovery data at the site were used to define the gas production model. Simulations in one and two dimensions are presented to investigate gas production and migration in the landfill, and in the surrounding limestone. The effects of a gas recovery well and landfill cover on gas migration are also discussed.     

Gas Cleaning in a Wetted Butterfly Valve

An attempt was made to demonslrate the utility of a wetted butterfly valve for cleaning flue gases from industrial wastes. Butterfly valves are extensively in use to control the gas pressures in various processes. A small pilot plant with a maximum airflow rate of 400 cfm and water flow rate of 4 gpm was constructed to provide the desired testing conditions. The results indicate that for a pressure drop greater than 30 in. water a collection efficiency of greater than 95% for submicron particles can be obtained. For particles larger than 1.7;u diameter efficiencies of greater than 99% were obtained. The exceptional suitability of wet butterfly valves for gas cleaning is demonstrated by the flexibility to adjust to large variations of gas and liquid flow rates.     

Henry's law constants and mass transfer coefficients for methyl bromide and 1,3-dichloropropene applied to Florida sandy field soil

Methyl bromide, a pre-emergent soil fumigant, is scheduled to be phased out in the US by 2005, with exceptions for critical use. Comparison of some of the physical constants related to distribution and retention for methyl bromide (MBr) to other fumigants yields a useful quantification of possible alternatives. In this study, the atmospheric and subsurface dissipation of methyl bromide as well as (Z)- and (E)-1,3-dichloropropene (1,3-D) isomers in Telone II were examined. The Henry's law constants of the three chemicals at soil temperature and their mass transfer coefficients for movement through an agricultural mulch of UV-resistant, high-density polyethylene (PE) were evaluated using field data. At the soil temperature of 16.4 degrees C, calculated Henry's law constant gave a fumigant ranking of MBr (0.21)>(Z)-1,3-D (0.041)>(E)-1,3-D (0.027). Since rapid subsurface distribution of a fumigant is highly dependent on the amount in the gas phase, the greater value for Henry's law constant implies faster distribution throughout the soil. After distribution through the soil, retention of the fumigant becomes imperative. Calculation of the fumigant's mass transfer coefficients through PE from field data gave a ranking of the three chemicals: MBr (1.08 cm/h)<(E)-1,3-D (3.25 cm/h)<(Z)-1,3-D (4.13 cm/h). With mass transfer coefficients of this magnitude, it was concluded that PE film was an inadequate barrier for retaining these fumigants in an agricultural setting.     

A partially coupled, fraction-by-fraction modelling approach to the subsurface migration of gasoline spills

The subsurface spreading behaviour of gasoline, as well as several other common soil- and groundwater pollutants (e.g. diesel, creosote), is complicated by the fact that it is a mixture of hundreds of different constituents, behaving differently with respect to e.g. dissolution, volatilisation, adsorption and biodegradation. Especially for scenarios where the non-aqueous phase liquid (NAPL) phase is highly mobile, such as for sudden spills in connection with accidents, it is necessary to simultaneously analyse the migration of the NAPL and its individual components in order to assess risks and environmental impacts. Although a few fully coupled, multi-phase, multi-constituent models exist, such models are highly complex and may be time consuming to use. A new, somewhat simplified methodology for modelling the subsurface migration of gasoline while taking its multi-constituent nature into account is therefore introduced here. Constituents with similar properties are grouped together into eight fractions. The migration of each fraction in the aqueous and gaseous phases as well as adsorption is modelled separately using a single-constituent multi-phase flow model, while the movement of the free-phase gasoline is essentially the same for all fractions. The modelling is done stepwise to allow updating of the free-phase gasoline composition at certain time intervals. The output is the concentration of the eight different fractions in the aqueous, gaseous, free gasoline and solid phases with time. The approach is evaluated by comparing it to a fully coupled multi-phase, multi-constituent numerical simulator in the modelling of a typical accident-type spill scenario, based on a tanker accident in northern Sweden. Here the PCFF method produces results similar to those of the more sophisticated, fully coupled model. The benefit of the method is that it is easy to use and can be applied to any single-constituent multi-phase numerical simulator, which in turn may have different strengths in incorporating various processes. The results demonstrate that the different fractions have significantly different migration behaviours and although the methodology involves some simplifications, it is a considerable improvement compared to modelling the gasoline constituents completely individually or as one single mixture.     

石灰与石灰石作湿法脱硫剂的比较

石灰、石灰石是最常用的湿法脱硫剂。从反应机理、运行控制指标、液气比和钙硫比（化学过量比）、浆液循环池等方面对其性能作了比较。两者的脱硫机理、运行控制指标不同,受温度的影响程度也不同。两系统的最小液气比分别为10．0和5．6,化学过量比分别为1．05～1．15和1．25～1．60。前者的脱硫效力优于后者。     

生物膜填料塔对低浓度甲苯废气的净化性能研究

在扩大的实验装置上，考察研究了入口气体甲苯浓度、气体流量、液体喷淋量、填料层高度、操作方式等因素对生物膜填料塔净化低浓度甲苯废气性能的影响，取得了一定的基础数据，为下一步的工业应用提供依据。     

Partitioning of mono- and polycyclic aromatic hydrocarbons in a river sediment adjacent to a former manufactured gas plant site

The equilibrium distributions, between water and coal-tar contaminated sediment, of 16 monocyclic and polycyclic aromatic hydrocarbons were measured and evaluated for consistency with a Raoult's Law-based quantitative relationship. The quantitative relationship calculates the pore water concentration as the product of the aqueous solubility (or for compounds that are solid at room temperature, the aqueous super-cooled liquid solubility) and the mole fraction concentration of the compound within the liquid coal tar. Sediment was collected at five locations at two depths within a 120 m stretch of a river adjacent to a former manufactured gas plant, and all samples contained non-aqueous phase liquid (NAPL) coal tar. Although the amount of coal tar varied between samples by over an order of magnitude, the Raoult's Law-based NAPL-water partition coefficients for each monocyclic or 2- or 3-ring polycyclic aromatic hydrocarbon measured in this study generally varied within a factor of 2 over all sediments.     

A description of a test plant for combustion on a pilot scale

A plant for combustion on a pilot scale is described.The plant is comprised of two shaft furnaces connected to a common afterburning chamber. Sawmill chips and LP-gas are used as supporting fuel. The plant is provided with equipment for continuous feeding-in of both solid and liquid wastes.The products of decomposition and residual concentrations resulting from combustion of the wastes are analysed with respect to different organic compounds. These analyses are made on ingoing product, slag, dust and fuel gases. In addition, temperature, transit time, gas composition (CO, CO₂, O₂), fuelgas flow and - in some cases - parameters such as total hydrocarbon and nitrogen oxides are investigated.     

生物膜填料塔净化低浓度苯乙烯废气的实验研究

研究了进口气体中苯乙烯浓度、气体流量和液体流量等3个因素对生物膜填料塔净化苯乙烯废气的影响.研究结果表明,当进口气体中苯乙烯浓度为1000 mg/m3以下、气体流量为200L/h、循环液流量为10L/h的操作条件下,废气中苯乙烯的去除率可达90%以上.     

生物膜填料塔净化低浓度苯乙烯废气的实验研究

研究了进口气体中苯乙烯浓度、气体流量和液体流量等3个因素对生物膜填料塔净化苯乙烯废气的影响。研究结果表明，当进口气体中苯乙烯浓度为1000mg／m^3以下、气体流量为200L／h、循环液流量为10L／h的操作条件下，废气中苯乙烯的去除率可达90％以上。