Modeling of Adsorption and Regeneration of Volatile Organic Compounds on Activated Carbon Fiber Cloth

A mathematical model is developed to predict continuous adsorption-regeneration cycling of volatile organic compounds (VOCs) on activated carbon fiber cloth (ACFC) at the indoor VOC concentration levels. The adsorption-regeneration model incorporates both the adsorption equilibrium and mass transfer fundamentals. It assumes local equilibrium between the gas-phase and the solid-phase, and axially dispersed-flow, film transfer, and intraparticle transport by surface and pore diffusion. Successful agreement between model simulations and experimental data was obtained and the kinetic properties of the adsorption/regeneration cycling on the ACFC were characterized. For the adsorption process, the film transfer is the dominant factor for mass transfer at low flow rates (45–184 L/min), and the intraparticle mass transfer rate controls over the gas-phase rate as the flow rates increase. The regeneration concentration profiles are most sensitive to the adsorption isotherms at the temperatures of interest, especially as desorption is initiated. The surface diffusivity also contributes to the shape of the regeneration profile: the tailing of desorption profile shifts up with the increase of surface diffusivity.     

Sorption of Tertiary Butyl Mercaptan to Indoor Materials in Contact with Air or Water

This study reports sorption of the malodorant 2-methyl-2-propanethiol, commonly known as tertiary butyl mercaptan (TBM), to selected indoor materials. The phase distribution of TBM in gas-solid and aqueous-solid systems was evaluated using batch reactors. Sorbents used in the study included two carpets, two wallpapers, a soil, and granular activated carbon (GAC). Sorption was studied for gaseous and aqueous TBM concentrations spanning three orders of magnitude and contact times ranging from 1 to 28 days. The phase distribution data were plotted and fitted using linear and Freundlich relationships. Results indicated that all solids sorbed environmentally significant quantities of TBM, with the likelihood of producing concentrations above the odor threshold during subsequent remediation using mechanical ventilation. TBM retention by sorbents was greater from air than from water. The malodorant partitioned readily into wallpapers and slowly into the carpet materials. Sorption was nonlinear in the case of GAC and the nonlinearity appeared to increase with sorption contact time. GAC sorbed TBM strongly from both air and water.     

室内空气中可吸入颗粒物的监测与研究

作者介绍室内空气中颗粒物的危害与来源。通过对居室空气中可吸入颗粒物（PM10）及总悬浮颗粒物（TSP）监测，应用监测结果分析了一般居室污染中可吸入颗粒物的污染情况及居住环境对室内PM10浓度的影响，同时研究了空气中PM10浓度与TSP浓度的关系。     

Adsorption of Perfluorinated Compounds onto Activated Carbon and Activated Sludge

The adsorption of perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) onto powdered activated carbon (PAC) was investigated in the presence and absence of effluent organic matter (EfOM) at an environmentally relevant concentration range (0.1–500??μg/L). Adsorption of PFOS and PFOA to PAC fitted the Freundlich model well (r2>0.98), and adsorption capacity of PFOS (KF = 17.48) and PFOA (KF = 10.03) in the absence of EfOM was more than one order of magnitude higher than that in the presence of EfOM (KF = 0.66 for PFOS, KF = 0.20 for PFOA), indicating that EfOM greatly reduces the adsorption capacity of PAC. Moreover, EfOM was characterized by ultrafiltration, and fractions of nominal molecular weights were obtained to investigate their effect on the PFOS and PFOA adsorption. The fraction of <1??kDa had greater effect on adsorption than the fraction of >30??kDa, indicating that the similar molecular size of target compounds was the major contributor to adsorption competition. Additionally, biosorption of PFOS and PFOA to activated sludge fitted the linear isotherm (r2>0.9) within a concentration range of 50–400??μg/L. On the basis of our data, the estimated partition coefficient, Kd, was 729??L/kg for PFOS and 154??L/kg for PFOA, respectively, suggesting that PFOS and especially PFOA have a low tendency to partition onto sludge.     

Adsorption onto Activated Carbon Cloths and Electrothermal Regeneration: Its Potential Industrial Applications

Activated carbon cloths (ACCs) are recently developed adsorbents that could be used in many air treatment applications. The use of the Joule effect heating as a regeneration process offers many possibilities in terms of desorption operating conditions to adapt the cloth process to the constraints of particular industrial applications. Hence, this paper presents ACC physical properties and adsorption–desorption cycles performed on a pilot plant to determine the influence of the desorption conditions on the desorbate quality. Two examples of activated carbon cloth processes for specific industrial applications are then presented. The role of the first one is to concentrate an industrial effluent upstream of recovery facilities. The second application is for indoor air treatment, where the desorbate quality is not the major interest but rather the development of a very simple system with an easily handled filter.     

Dynamic Modeling of Bentazon Removal by Pseudo-Moving-Bed Granular Activated Carbon Filtration Applied to Full-Scale Water Treatment

A model for the removal of pesticides by granular activated carbon (GAC) filtration in full-scale water treatment is presented. The model describes GAC filtration in a pseudo-moving-bed configuration, where two filters are operated in series and after breakthrough the first filter is regenerated and becomes the second filter. The influent of the second filter is changing due to gradual breakthrough of the first filter. Therefore, a dynamic model is developed based on kinetics, equilibrium, and mass balance equations. The model is calibrated and validated on data of full-scale and pilot plants. Operational strategies are evaluated of two different cases. From this study it can be concluded that a dynamic mathematical model can be successfully used to evaluate the performance and operation of full-scale GAC filters for pesticide removal and can be used for operational decision support. Data obtained from practice can be used for calibration without additional laboratory work.     

Assessment of Indoor Air Quality in Interior Alaskan Homes

Interior Alaska has indoor air quality issues similar to those of other far northern communities associated with long cold winters and reduced ventilation rates. Moreover, geological features in the hills around Fairbanks, Alaska increase the likelihood of elevated radon levels. To better quantify these features, our team has gathered data at 68 homes. We used CO2 concentrations as a surrogate for indoor air quality as well as to help determine ventilation rates to be used in subsequent modeling work. We found the CO2 data correlated well using basic mass conservation considerations for yearly averages. The summer CO values ranged from 1 to 9 ppm and the winter values from 0 to 23 ppm. For homes with woodstoves, we found the indoor PM10 concentrations rose after the stoves were fired up. Here PM10 refers to particles smaller than 10 μm. Over 40% of the tested homes in the hills around Fairbanks had radon levels>4?pCi/L compared to a nationwide average of 7%. Active subslab depressurization systems tested were very effective in reducing indoor radon concentrations (average reduction of over 90% for eight homes).     

室内污染的主要来源分析——室内空气污染

以往人们总是把环境污染归咎于工业生产和交通汽车等，据一些调查研究的结果显示，工业污染仅占41％，而室内污染却高达59％。为此，一些环保专家发出忠告：居室不是安乐窝，办公室可能是污染源。文章主要分析了造成室内污染的主要来源——空气污染。     

Large-Eddy Simulations on Indoor/Outdoor Air Quality Relationship in an Isolated Urban Building

The airflow, pollutant dispersion and penetration of outdoor pollutants indoor in an isolated urban building are investigated computationally. The computation involves solving the two-dimensional Navier–Stokes equations using large eddy simulations based on the Smagorinsky subgrid scale model. The simulations involve investigations of the phenomena of airflow and pollutant dispersion in both indoor and outdoor environments under different Reynolds numbers with distinct combinations of building height, floor height, and building width. Pollutants originate from outdoor, located at the datum level of the windward side of the building. The objective is to study how outdoor pollutants penetrate indoor and relate this pattern to different building geometric configurations. The gradual increase in Reynolds number significantly favors pollutant transportation into the indoor environment and thus eventually changes the indoor/outdoor (IO) air ratio. Moreover, the value of the IO air ratio is particularly found to be highly affected by building height due to the capability of climbing over and escaping from the roof of the building. Results also show that the IO air ratio is dependent on the position of the sampling, which has been ignored in many previous experimental works. The research work poses important questions of indoor air quality as well as field measurements of the IO parameter.     

Organic Vapor Recovery and Energy Efficiency during Electric Regeneration of an Activated Carbon Fiber Cloth Adsorber

An electrothermal-swing adsorption system was demonstrated on the bench scale for capture and recovery of organic vapors from air streams. Methyl propyl ketone (MPK), methyl ethyl ketone, n-hexane, acetone, and methylene chloride were removed and recovered at 200–1,020?ppmv in a 40.0 slpm air stream while using activated carbon fiber cloth (ACFC) adsorbent. Removal efficiencies were greater than 99.9%. Liquid recovery fractions increased with increasing relative pressure, ranging from 0.11 for methylene chloride (P/Psat = 2.1×10?3) to greater than 0.80 for MPK (P/Psat = 2.2×10?2). The electrical energy consumed during regeneration per mol of liquid organic compound recovered decreased with increasing relative pressure of the inlet gas stream, ranging from 4,698 kJ/mol for methylene chloride to 327 kJ/mol for MPK. Equilibrium ACFC adsorption capacity, throughput ratio, and length of unused bed were also evaluated. These results are encouraging for the development of a new technology to capture and readily recover a wide range of organic vapors from air streams.     

Characterizing Polyurethane Foam as a Sink for or Source of Volatile Organic Compounds in Indoor Air

Polyurethane foam (PUF) is widely used in indoor consumer products. Despite strong potential interactions with volatile organic compounds (VOCs), the effect of PUF on indoor concentrations of VOCs has not been examined. This study determines the behavior of PUF as a potential sink for or source of VOCs in indoor air. A flexible polyether-type foam and eight aromatic VOCs ranging in molecular weight from naphthalene to benzene were studied. Rapid determinations of PUF–air partition coefficient (K) and PUF–phase diffusion coefficient (D) were achieved using a dynamic microbalance procedure. A diffusion model was applied to interpret the experimental data. The PUF sample was assumed to conform to semi-infinite cylindrical geometry with solid-phase diffusion being the rate limiting step. The results indicate that sorption of VOCs by PUF is fully reversible. For the VOCs studied, K can be correlated with vapor pressure and D with molecular free surface area. Humidity appears to reduce the extent of sorption and slow the sorption kinetics. These findings should facilitate the prediction of the source/sink behavior of PUF and the related impact on VOC concentrations in the indoor environment.     

Evaluation of a Fast Large-Eddy-Simulation Model for Indoor Airflows

A three-dimensional large-eddy-simulation computational fluid dynamics (CFD) program, developed for studying the transport of smoke during a fire in an enclosure, is applied to four flow problems relevant to nonfire situations. This evaluation is relevant to the use of the program for indoor air quality modeling as well as its use in modeling the early phases of smoldering fires. The program uses finite-difference techniques to solve the Navier-Stokes equations, with an approach emphasizing high spatial resolution and efficient flow-solving techniques. Subgrid scale effects are addressed with the Smagorinsky model. The flow problems include simple geometries, with forced, natural, and mixed convection flows as well as a realistic test room with a displacement ventilation system and tracer gas release. Grid effects and computing time are investigated. Results are compared with the experimental data, and issues important to defining the problems in CFD are highlighted. In general the program predicts the experimental data reasonably well, with very fast computing times. However, care must be taken in defining convection from heated surfaces, and adequate grid resolution is needed to model the dispersion of a tracer gas in the enclosure.     

Effect of Media Grain Shape on Particle Straining during Filtration

This paper investigates how straining mechanisms of angular media (crushed limestone) provide improved filtration performance compared to rounded media (river stone). Columns of granular media were set in resin, sectioned, photographed, and digitized to produce a three-dimensional model of pore space geometry. This process was repeated for four filter media, each representing different grain shapes or packing densities. From measured pore throat distributions, a stepwise particle movement model was used to estimate the maximum volume of particles that could be stored in the bulk of the filter media. The results showed that the more angular the media, the wider the range of particle sizes that could be strained in the bulk of the filter. The stepwise model was applied only to individual particles; trapping of colloidal particles was not considered. However, when individual particles are too small to be strained, the same pore throat trapping contributes to the physical capture of avalanches and flocculates. Thus the findings of this work are relevant to deep bed filtration applications where headloss results from straining, such as storm-water best management practices or soil filters.     

Electrothermal Desorption Using Joule Effect on an Activated Carbon Monolith

A new adsorbing material, an activated carbon monolith, was used for the adsorption of toluene vapors and their desorption by electrothermal heating using Joule effect. The monolith appears under shape of a parallelepiped of square section including four hundred parallel channels. Equilibrium isotherms for toluene on a piece of this material at various temperatures were measured by the gravimetric method. The adsorption breakthrough fronts measured on the monolith show that it is effective for cleaning the gas. Desorption was performed by heating of the monolith by means of a continuous electric current flowing transversely to the channels. The curves of desorption show that the toluene is desorbed with a high factor of concentration. Desorption rate can be controlled by regulating electrical current and purge gas flow rate. An equilibrium model shows that the tail of the desorption curve is mainly due to the curvature of the isotherm.     

Modeling Particle Dispersion under Human Activity Disturbance in a Multizone Indoor Environment

Human activity is an important factor influencing particle resuspension in the indoor environment. This work studies the applicability of a multizone airflow and contaminant transport model (CONTAM 2.1) in the simulation of indoor dispersion of particles under human activity disturbance. An iterative method is suggested to complement CONTAM 2.1, by tracking the transient particle concentration on floor surface due to dynamic deposition and resuspension process. A three-zone building with a heating, ventilation, and air conditioning system is used as the simulation case to test the convergence and accuracy of this algorithm under different particle-release scenarios. The algorithm shows a very fast convergence speed in the simulation. Comparisons of calculation results between the multizone model and the analytical model show good agreement and verify the accuracy of the multizone model simulation. The airborne particle concentration profiles and human breathing dose are also analyzed for the three-zone building model.     

Evaluation of Antimicrobial Coatings for Cloth Media Filtration: Case Study

The results of an investigation of the effect of antimicrobial coatings on inactivating microbes on and within the media used in cloth media disk filtration systems are presented herein. Four filter fabrics were evaluated using a modified form of the American Association of Textile Chemists and Colorists Test Method 100 in 1996. The fabrics had been treated with two antimicrobial agents by two coating vendors. Treated fabrics were subjected to backwash cycles to examine whether the antimicrobial properties were retained during normal filter operation. Initially, the fabrics treated with Zoonocide exhibited significant microbial reduction whereas fabrics treated with AlphaSan exhibited little microbial reduction. Once the fabrics treated with Zoonocide were subjected to a backwash cycle, the biocidal properties of the fabrics were depleted. Lack of durability of the coating was either because it washed off easily or the antimicrobial properties were chemically depleted, rendering the treatment ineffective.     

Modeling Volumetric Clarifying Filtration of Particulate Matter for Transient Rainfall-Runoff Loadings

Volumetric clarification incorporating filtration with engineered granular media is increasingly used as a viable combined unit operation for separation of rainfall-runoff particulate matter (PM). Such combined unit operations are typically operated at the catchment-level for rainfall-runoff clarification of transient loadings, in contrast to centralized watershed or sewershed regional treatment. Using computational fluid dynamics (CFD), this study models the PM separation by a volumetric clarifying filter (VCF) subject to unsteady event-based hydrologic, hydraulic, mass, and particle size distribution (PSD) loadings. Modeled and measured physical model results indicate that the VCF is capable of PM load reductions and effluent concentrations at or below 30 mg/L. These results, with PM measured as suspended sediment concentration (SSC) represent reductions ranging from 83 to 97% on an event basis. CFD model results predict effluent PM mass loads. Modeled effluent median particle diameter (d50m), as an index for the filtered effluent PSD, reproduces the d50?m from the VCF physical model on an event basis. Filter head loss is examined as a function of flow rate. Despite geometric asymmetry of the multiple radial cartridge configuration tested, hydraulic loading for each individual cartridge is relatively uniform.     

Modeling Volatile Organic Compound Sorption in Activated Carbon. II: Multicomponent Equilibrium

A new approach is presented for modeling multicomponent volatile organic compound (VOC) sorption equilibrium in ultra- and supernanoporous activated carbons. The model uses “Dubinin–Astakhov thermal equation of equilibrium adsorption” (DA-TEEA) for single-component adsorption thermodynamics and “ideal/real adsorbed solution theories” (IAST/RAST) for the multicomponent mixing rules. Use of the Henry’s Law adsorption isotherm resolves the singularity of DA-TEEA at zero-coverage conditions. The introduced method predicts multicomponent adsorption equilibria of VOCs based on equilibrium data of only one similar component. Single and binary adsorption equilibria of acetone and benzene vapors in Kynol ACFC-5092-20 activated-carbon-fiber-cloth adsorbents are predicted with the presented models and compared with modeled and measured characterization data available in the literature. The Wilson model for nonideal binary solution mixtures is used to predict the activity coefficients needed in DA-TEEA/RAST. Modeled results are compared against measured characterization data. The selected Henry’s Law upper-bound pressure (HUBP) is found to be an important factor controlling the accuracy of the multicomponent equilibrium models. An optimum HUBP can generate highly accurate results from both DA-TEEA/IAST and DA-TEEA/RAST. The accuracy realized by applying this method to acetone–benzene mixtures is sufficient for engineering design and development purposes.     

Air Binding of Granular Media Filters

Air bubbles that form in water treatment filters create headloss and can form whenever the total dissolved gas pressure exceeds the local solution pressure. The location of potential bubble formation in filters can be predicted based on measurements of the clean bed headloss with depth, flow rate, and the influent total dissolved gas concentration. Bubble formation within filters can be reduced by increasing the pressure within the filter via greater submergence (water head above the media), lower hydraulic flow rate, or using a more porous media. Bubbles trapped in the bed can be released by “burping,” which can reduce the extent of headloss buildup. Burping is more significant at lower flow rates and within a lower density, higher porosity, hydrophobic anthracite layer.     

Volumetric Filtration of Rainfall Runoff. I: Event-Based Separation of Particulate Matter

As a unit operation, filtration generally requires flow equalization and primary clarification. This study examines the separation of runoff particulate matter (PM) in a volumetric clarifying filter (VCF). The VCF is a detention/retention vault integrating filtration after sedimentation. A paved source area (1,088?m2) directly loaded the vault (4.2?m3) with five radial filters (4?m2 of filtration surface area). PM separation was examined for 19 runoff events through monitoring of influent and effluent granulometric fractions. During the monitoring phase no maintenance was conducted and subsequent to the 19 events a measured material balance of the sedimentation vault and the radial cartridge filters generated a 94% recovery of PM. During 5 months of monitoring and PM mass, suspended sediment concentration (SSC) was reduced from 334 to 34 mg/L (90% reduction) with effluent sediment (>75?μm) of <3?mg/L, settleable of 14 mg/L, suspended PM (<25?μm) of 17 mg/L as event mean concentrations; with turbidity reduced from 96 to 23 NTU (76% reduction). Based on separate PM recovery from the vault and filters, 77% of the PM separation was sedimentation in the vault and 23% as filtration. Captured particle-size distributions are heterodisperse with a d50?m of 300 μm in the vault and a d50?m that ranged from 34 to 63 μm with filter depth. Filter forensics indicated PM capture was nonuniform, with the bottom and middle most heavily loaded by PM as compared to the upper third of the filter. While paired testing of automatic and manual sampling produced similar median effluent SSC, automatic sampling significantly misrepresented the median influent PM as SSC (p ? α = 0.05).