Journal of Environmental Engineering

Design and development of new sorption technologies for control of volatile organic compound (VOC) laden air discharges from industrial sources require characterization of proposed novel processes and systems. Extensive capital, labor, and time are needed to obtain multicomponent adsorption characteristic data through laboratory experiments. As a substitute for laboratory characterization experiments, this paper presents steps for computationally predicting VOC sorption equilibrium and nonequilibrium processes in activated carbon. The method of predicting sorption equilibrium has been described in detail, since it is critical for modeling even nonequilibrium sorption dynamic processes that are constructed as successions of equilibria steps. An accurate single-component “thermal equation of equilibrium adsorption” (TEEA) is presented by modifying the Dubinin–Astakhov (DA) adsorption isotherm. DA-TEEA is capable of accurately predicting thermodynamics of single-component vapor adsorption in ultra- and supernanoporous adsorbents. The adsorption affinity factor, determined from dispersion interaction theory, enables the DA-TEEA to predict adsorption equilibrium characteristics of VOCs from adsorption equilibrium data of only one similar reference adsorbate.

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.

Adsorption isotherms for alkane, aromatic, and ketone vapors were determined for activated carbon fiber cloth, tire-derived activated carbon and coal-derived activated carbon adsorbents. Physical and chemical properties of the vapors and adsorbents were used to interpret these results that were obtained from 20 to 50°C, with a more limited data set at 125 and 175°C and relative pressures between 0 and 0.99. Fitted isotherms using the Freundlich and Dubinin–Radushkevich adsorption models had mean total relative errors <5.6 and 9.2% for the microporous and mesoporous/macroporous adsorbents, respectively, at the temperature range from 20 to 50°C. The predictive direct quantitative structure activity relationship model had mean total relative errors <9.7 and 61% for the microporous and mesoporous/macroporous adsorbents, respectively, at the temperature range from 20 to 50°C without requiring experimental input.

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.

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.

Activated-carbon-fiber cloth (ACFC) is an alternative adsorbent to granular activated carbon (GAC) for removing and recovering organic vapors from gas streams. Electrothermal desorption (ED) of ACFC provides rapid regeneration while requiring less energy compared to traditional regeneration techniques used with GAC. This paper provides proof-of-concept results from a bench-scale ACFC adsorption system. The automated system captured 1,000 ppmv of hazardous air pollutants/volatile organic compounds (HAPs/VOCs) from air streams and demonstrated the use of ED, using ac voltage, to recover the HAP/VOC as a pure liquid. The desorbed HAP/VOC condensed onto the inner walls of the adsorber and was collected at the bottom of the vessel, without the use of ancillary cooling. Seventy percent of the HAP/VOC was collected per cycle as condensate, with the balance being retained in the regenerated adsorber or recycled to the second adsorber. ED with in-vessel condensation results in minimal $N_2$ consumption and short regeneration cycle times allowing the process to be cost competitive with conventional GAC-based adsorption processes. This technology extends the application of carbon adsorption systems to situations that were previously economically and physically impractical.

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 $200-1\text{,}020{\mathrm{ppm}}_v$ 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 $(P/P_{\mathrm{sat}}=2.1\times {10}^{-3})$ to greater than 0.80 for MPK $(P/P_{\mathrm{sat}}=2.2\times {10}^{-2})\text{.}$ 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.

This study investigates the effects of adding anionic and nonionic surfactants to the scrubbing liquid during the absorption of naphthalene (Nap) using a wet scrubber. Both batch and continuous experiments were performed on a laboratory-scale packed tower scrubber by adding surfactants to the scrubbing liquid. An anionic surfactant sodium dodecyl sulfate (SDS) and two nonionic polyoxyethylene surfactants $C_{10}{E}_4$ [tetraethylene glycol mono(decyl ether)] and $C_{14}{E}_8$ [octaethylene glycol mono(tetradecyl ether)] were used. The concentrations of the surfactant solutions all exceeded the critical micelle concentration. The results of continuous experiments indicate that the efficiencies of removal of naphthalene by $C_{10}{E}_4$ and $C_{14}{E}_8$ were 75.0 and 71.9%, respectively, at a concentration of $1.0\times {10}^{-2}M\text{.}$ The removal efficiency of naphthalene by SDS ranged from 6 to 39% at concentrations from $1.0\times {10}^{-2}$ to $1.0\times {10}^{-1}M$ under continuous scrubbing. In the batch experiments, the batch naphthalene absorption capacities were estimated to be $31.8\mu g \mathrm{Nap}/g{C}_{10}{E}_4\text{,}$ $12.9\mu g \mathrm{Nap}/g{C}_{14}{E}_8\text{,}$ and 2.4 μg Nap/g SDS. A comparison was also made among SDS, $C_{10}{E}_4\text{,}$ and $C_{14}{E}_8$ in terms of cost, foaming, impact of wastewater, removal efficiency, and absorption capacity.

A hybrid bioreactor, combining an activated sludge process (ASP) and a rotating drum biofilter (RDB), was developed and evaluated for the treatment of volatile organic compounds (VOCs) in waste gas streams. The effects of the influent VOC concentration and the organic loading rate on the VOC removal efficiency and on the pattern of biomass accumulation were investigated. Toluene was used as the model VOC, the flow rate of the waste gas stream was 0.59 L/s, and the empty-bed retention time (EBRT) in the ASP portion was 46 s with an actual retention time of about 2 s. The EBRT in the RDB portion was 38 s based on the drum volume. When the VOC feed concentration increased from 221 to 884 mg toluene/m³ (from 57.2 to 229 ppm), correspondingly the organic loading rate of the hybrid bioreactor increased from 1.58 to 6.32 kg chemical oxygen demand/m³/day (from 0.505 to $2.02\mathrm{kg} \mathrm{toluene}/m^3/\mathrm{day})$ based on the drum volume, both the ASP and RDB decreased, and the overall toluene removal efficiency declined from 99.8 to 74.1%. Biomass accumulation at different medium depths became more even when the organic loading rate was increased. Part of the applied VOC was biodegraded by the ASP, which suggests that this hybrid bioreactor could achieve longer runs between medium cleanings and higher VOC removal efficiencies than a single RDB bioreactor without an ASP portion at the same organic loading rate.

Biofiltration, a process in which contaminated air is passed through a biologically active bed, can be used to remove volatile organic pollutants from air streams. While most biofilters rely on bacteria to degrade organic pollutants, biofilters utilizing a fungal biofilm have the potential to be an efficient and robust treatment alternative. In this study, two surfactants were evaluated for their ability to activate the spores of the fungus Exophiala lecanii-corni and reduce the start-up period typically observed in fungal vapor-phase bioreactors. Sodium dodecyl sulfate, an anionic surfactant, was found to inhibit growth of E. lecanii-corni. Polyoxyethylene sorbitan monolaurate (Tween 20), a nonionic surfactant, stimulated bud formation and enhanced toluene degradation in E. lecanii-corni cultures. Tween 20 was also found to enhance inoculum development by shortening the lag period during culture growth. However, when bioreactors were presoaked in medium containing Tween 20, washout of the fungal cells occurred during inoculation, inhibiting the initial fungal biofilm development. After a seven-day start-up period, no detrimental effects on reactor performance due to residual Tween 20 were observed, and toluene elimination capacities of up to $150g/m^3\cdot h$ were achieved.

Although biofiltration has been used successfully to remove and biodegrade a wide variety of gas-phase organic contaminants generated by industrial facilities and environmental remediation efforts, the ability of conventional biofilters to maintain high removal efficiency during short-term, unsteady-state, elevated loading conditions is limited. A promising alternative for improving biofilter performance during transient elevated loading conditions while minimizing the disadvantages of conventional treatment technologies is utilization of adsorption packing media and implementation of sequencing batch operating strategies. In the studies described herein, a continuous-flow biofilter (CFB) and a sequencing batch biofilter (SBB) were operated for more than 300 days to treat a methyl ethyl ketone (MEK) contaminated gas stream. The packing medium for both biofilters consisted of activated carbon coated polyurethane foam cubes. Both biofilters exhibited stable long-term performance with greater than 99% removal of the influent $106{\mathrm{ppm}}_v$ MEK concentration during “normal” loading conditions. To assess performance during transient loading conditions, on a regular basis the influent MEK concentration of each biofilter was temporarily increased by a factor of 10 to $1\text{,}060{\mathrm{ppm}}_v$ MEK for a duration of either 45 or 60 min. Results are presented which demonstrate how the operational flexibility of SBB systems can be utilized to minimize or eliminate contaminant emissions from biofilters during unsteady-state loading conditions. The SBB was able to remove more than 99% of the influent MEK at a transient loading rate of $380g m^{-3} h^{-1}$ and 83% of the influent MEK at a transient loading rate of $760g m^{-3} h^{-1}\text{.}$ In comparison, the CFB exhibited lower MEK removal efficiency.

The deodorization efficiency of a suspended-growth bioscrubber was characterized from an experimental and theoretical approach, in order to optimize such systems for the treatment of polluted air from wastewater low lift station. A model of prediction of volatile compound removal efficiencies was developed according to operating conditions and contact mode (packed and spray column). The predictive ability of the model was validated from transfer data obtained with two representative molecules (ethanol and hydrogen sulfide) on a laboratory scale device. The theory takes into account the hydrodynamic characteristics of the fluids flowing in the contactor, which were defined from a previous experimental residence time distribution study. A study of parametric sensitivity of the model was then conducted to evaluate the influence of operating conditions (gas and liquid flow rates, contact mode, washing solution characteristics), hydrodynamic parameters of each flow (liquid holdup in the column, hydrodynamic behavior of the liquid flow, axial dispersion of the gas flow), and biodegradation step on the deodorization efficiency of a bioscrubber applied to the treatment of a polluted gas containing ethanol. The assumptions of sizing and optimization were confirmed on a suspended-growth bioscrubber used for the deodorization of an exhaust gas emitted by a wastewater low lift station.

The performance of a field-scale biotrickling filter was investigated for the treatment of styrene vapors released from a bathtub manufacturing process. The two-stage biotrickling filter was operated in series with an average gas flow rate of 350 m³ h⁻¹ corresponding to an overall empty bed gas contact time of 84 s. Daily average values of styrene removal efficiency varied from 40 to 90% with inlet concentrations ranging between 0.4 and 1.7 g m⁻³. System performance was not significantly affected by changes in temperature and was moderately susceptible to 3-day starvation or complete system shutdown. After 7 months of styrene treatment, toluene contaminated air was fed to the system and experiments were performed in which styrene and toluene were fed alternately at 3-h intervals. While styrene elimination remained unchanged over the cycles, the elimination capacity of toluene increased with the number of cycles, indicating some adaptation of the process culture to the new contaminant. Overall, the results suggest that biotrickling filters for air pollution control can be successful even under greatly varying operating conditions.

Metal oxide and supported-Pt catalysts were developed for complete oxidation of volatile organic compounds (VOCs) and other solvent-derived organic vapors (OVs) in air at relatively low temperatures. The goal for this work is to produce a simple, cost-effective technology for reducing the concentration of organic contaminants in air to acceptable levels before the air is released into the atmosphere or recirculated. Specific applications include ventilated work spaces for spray painting and engine maintenance, indoor air decontamination, dry cleaning, food processing, fume hoods, residential use, and solvent-intensive industrial processes. Catalyst powders and monolith-supported catalysts were screened for conversion of 1-butanol, toluene, and methyl ethyl ketone to carbon dioxide and water. The concentration of OVs in the feedstream was maintained at approximately 100 ppmv, and the space velocity was between 6,000 and 18,000 h⁻¹. Metal oxide catalysts without Pt generated complete conversion of 1-butanol to ${\mathrm{CO}}_2$ at 150°C, 69% conversion at 100°C, and 15% conversion at 80°C. For toluene, complete conversion was achieved at 200°C, and greater than 75% conversion at 150°C. Addition of Pt to the metal oxide compositions typically lowered the temperature for a given OV oxidation rate by at least 20–50°C. Catalysts deposited onto standard commercial cordierite monoliths retained their composition and activity, and were stable in humid air, as well as nitrogen- and chlorine-containing OVs. However, the catalysts quickly deactivated in the presence of sulfur and phosphorus.

CHA Corporation installed a fume hood adsorber (scrubber) unit onto an operating fume hood and microwave regeneration system in the Safety and Risk Management Building at the Advanced Technology Park in Bozeman, Montana. The microwave-based fume hood exhaust purification process is designed to capture hazardous air pollutants in the fume hood exhaust using granular activated carbon (GAC) and natural zeolite, and subsequently either recover or destroy desorbed pollutants by microwave energy during regeneration of the saturated adsorbents. Onsite testing of this prototype microwave system was conducted using a variety of air borne pollutants including toluene, ethyl acetate, methylene chloride and hydrochloric acid. Test results showed that microwave energy, at a frequency of 2,450 MHz, restored the original adsorption capacity of saturated GAC and saturated natural zeolite. The results also demonstrated that microwave-induced catalytic oxidation was capable of providing oxidation efficiencies greater than 99% with oxidation temperatures below $100°C\text{.}$

This study investigates the feasibility of using nonthermal plasma produced in a dielectric barrier discharge reactor to destroy 1,1,1-trichloroethane (TCA) in a stream of air. The effects of various operating parameters on the destruction and removal efficiency (DRE) of TCA were examined. The experiments indicated that the water vapor concentration greatly influenced the destruction of TCA and the relative amount of oxidation by-products. DRE as high as 99.9% could be achieved at very low relative humidity (RH) conditions. Analysis of the $\mathrm{CO}/{\mathrm{CO}}_2$ concentrations in the reactor effluent indicated a decrease in the amount of CO generated as the RH was increased. The lowest $\mathrm{CO}/{\mathrm{CO}}_2$ ratio, 1:3, was observed at 88% RH. The estimated cost and energy requirement for operation were also determined. The calculated energy density values (β) varied with respect to the RH, and ranged from 1,478 to 3,010 J/L over a RH range of 0–88%.

Various mesoporous catalysts with titanium loadings between 0.5 and 4 Ti wt. % and surface areas between 600 and 1,600 m²/g were synthesized using the molecular designed dispersion technique. These catalysts were tested using toluene oxidation in a fixed bed reactor at temperatures between 300 and 550°C. The reaction products were found to be ${\mathrm{CO}}_2$ and CO with selectivity towards ${\mathrm{CO}}_2$ above 80% for all catalysts. The catalytic activity of the catalysts increases with titanium loading. The total conversion at 550°C was not affected by the textural porosity, but increased textural porosity did significantly reduce the ignition temperature by up to 50°C. The Thiele modulus was calculated to be much less than one for all these materials indicating that the reaction rate is not diffusion limited.