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.     

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.     

Modeling TOC Breakthrough in Granular Activated Carbon Adsorbers

Linear regression techniques were used to develop practical models to predict total organic carbon (TOC) breakthrough in bituminous granular activated carbon (GAC) adsorbers. Models were developed for two field-scale GAC sizes (8×30 and 12×40?mesh) and two empty bed contact times (EBCTs) (10 and 20 min). Model input parameters include two water quality variables, influent TOC concentration (TOC0) and pH, that impact performance. The dependent variables for the models were normalized breakthrough time, throughput in bed volumes, to six fractional (TOC/TOC0 = 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7) and three mass (TOC = 1.0, 1.5, and 2.0 mg/L) effluent concentrations. Model development was performed using small-scale breakthrough data from 35 different source waters; external model validation was performed with small-scale breakthrough data from 14 source waters; a sensitivity analysis was performed to ensure that the models effectively capture expected breakthrough trend; and a scalability test was performed to verify the models’ ability to predict breakthrough for field-scale GAC adsorbers.     

Naphthalene and o-Xylene Adsorption onto High Carbon Fly Ash

Adsorption is an effective remediation technique for petroleum hydrocarbons because of its ease of use and high efficiency. The utilization of high-carbon content industrial by-products in such applications can present significant economic and environmental advantages. In this study, batch adsorption tests and petrographic analyses were used to investigate the adsorption of two nonpolar petroleum contaminants, naphthalene and o-xylene, onto seven fly ashes with varying carbon contents, with powdered activated carbon (PAC) as a control. Six equilibrium isotherm models were used to evaluate the batch data. The results yielded nonlinear sorption isotherms characterized by high sorption capacity at low concentrations. The naphthalene and o-xylene adsorption capacity of the fly ashes was correlated with the unburned carbon content, specific surface area of the sorbent, and the percentage of the anisotropic and isotropic carbon content of the ash. On the basis of the Polanyi-Dubinin-Manes model, a pore-filling mechanism is the dominant mechanism for the adsorption of the nonpolar organic chemicals onto PAC, whereas the adsorption onto fly ash is likely to be governed by the unburned carbon content and the specific surface area of the ash.     

Adsorption of Methylene Blue and Phenol by Wood Waste Derived Activated Carbon

The phosphoric acid activated carbon (PAC) was derived from waste wooden pallets by a two-step chemical activation technique, carbonization and phosphoric acid activation in sequence. A widely used commercial activated carbon, Calgon Filtrasorb 400 (F400), was studied in parallel for comparison. The physical properties and surface chemistry of the activated carbons were characterized using BET-N2 adsorption, elemental analysis, Boehm’s titration, Fourier transform infrared spectroscopy, and x-ray photoelectron spectroscopy. PAC possessed physical properties (surface area and pore volume) that were comparable to those of F400, but displayed distinct surface chemistry in terms of pHPZC, surface acidity and basicity, and surface functional groups. Batch studies were conducted to evaluate the methylene blue (MB) and phenol adsorption capacity of PAC and F400 and their dependence on pH, contact time, and initial adsorbate concentration. Experimental results showed that the solution pH slightly influenced the adsorption of MB and phenol on F400, whereas it had no effect on their adsorption on PAC. Equilibrium adsorption data were fitted with an Langmuir isotherm equation. In comparison with F400, PAC showed a higher adsorption capacity for MB but lower for phenol. Given the comparable physical properties of PAC and F400 and the polar nature of MB and phenol, surface chemistry of the two carbons appeared to determine the adsorption mechanism and capacity. The strongly negative surface of PAC, due to phosphoric acid activation, facilitated the adsorption of positively charged MB, whereas the presence of oxygen-containing functional groups on PAC inhibited phenol adsorption.     

Adsorption and Desorption of Volatile Organic Compounds in Fluidized Bed

A model of adsorption and thermal desorption was built and validated from experiments performed under various operating conditions. The abatement increased with the reduction in the inlet concentration and with the increase of bed height. Particles at the end of adsorption step were saturated although their pores were not completely filled with acetone. Adsorption yielded an increase of the bed temperature at the beginning of the tests, where the separation rate had the maximum value nearly equal to 100%, but the temperature rise of the bed remained much smaller than that obtained in fixed-bed adsorption for similar conditions. Simulations of a process operating with successive cycles of adsorption and thermal desorption was then performed. Desorption appeared to have a reasonable duration compared to adsorption. Calculations proved that a third reactor for transient states from adsorption to desorption or from desorption to adsorption that would achieve the cooling or the heating of particles was not necessary. Results show the feasibility of the adsorption–desorption process of volatile organic compounds in fluidized beds of granular activated carbon.     

Modeling the Temperature Dependence of Adsorption Equilibriums of VOC(s) onto Activated Carbons

In order to optimize the efficiency of the removal of volatile organic compounds (VOCs) by adsorption onto activated carbon beds, process simulations taking into account exothermicity effects are helpful. Significant temperature increases may arise in the bed during the VOC adsorption cycle, especially when high concentrations have to be treated. Consequently, reliable and easy-to-handle isotherms remain a key hurdle to build realistic models. In this study, adsorption models were tested to describe a set of experimental data obtained for three VOCs (acetone, ethyl formate, and dichloromethane) adsorbed onto five commercial activated carbons at four different temperatures (20, 40, 60, and 80°C). A new expression of the Freundlich equation [qe = (a1T+a2T2)Ce(1/nf)] was shown to be statistically the most efficient to describe the adsorption isotherms of VOCs, single or in mixtures. A second-order polynomial temperature-dependence was introduced in this expression. The so-adapted Freundlich relationship gave a mean coefficient of determination of 0.97 for single-component adsorption and a correlation coefficient of 0.98 for binary mixtures.     

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.     

Repeated Reductive and Oxidative Treatments of Granular Activated Carbon

Fenton oxidation and reductive treatment solutions were applied to granular activated carbon (GAC) to chemically regenerate the adsorbent. No adsorbate was present on the GAC so physicochemical effects from chemically aggressive regeneration could be distinguished from the potential effects of accumulation of reaction byproducts. Fifteen sequential oxidation treatments with hydrogen peroxide (H2O2) and fifteen sequential reduction/oxidation treatments with hydroxylamine and H2O2 on Fe-amended GAC were evaluated. The GAC Iodine number, N2 Brunauer–Emmett–Teller surface area, microporosity, and total porosity declined with sequential treatments, but meso- and macroporosity essentially remained unchanged. Similar changes in Iodine number, surface area, and pore volume distribution suggest that the effects of treatment are functionally dependent on oxidation and independent of hydroxylamine reduction. An inverse relationship was established between the number of chemical treatments and contaminant (methyl tert-butyl ether, 2-chlorophenol, trichloroethylene) adsorption. Loss in sorptive capacity was attributed to the combined and undifferentiated effects of reductions in microporosity and surface area, alterations in surface chemistry (overabundance of surface oxides), and to a lesser degree, micropore blockage by iron oxides.     

Activated Carbon Addition to an Activated Sludge Model Reactor for Color Removal from a Cotton Textile Processing Wastewater

Color removal from cotton textile processing wastewater by addition of powdered activated carbon (PAC) into a lab-scale activated sludge system was examined. The activated sludge system was continuously operated in different sludge ages (SRTs) and hydraulic retention times (HRTs). SRT = 30?d and HRT = 1.6?d operation resulted in up to 36% color removal and 94% COD removal. PAC was added 100, 200, and 400 mg/L into the activated sludge system under these operating conditions. The results indicated that 100 mg/L PAC was sufficient to remove the maximum color measured (up to 50 m?1) from the wastewater. The addition of PAC did not affect chemical oxygen demand (COD) removal significantly. Oxygen uptake rate (OUR) tests were also performed to investigate the microbial activities controlling the system performance. The average OUR was 74.1 mg/L/h without PAC addition while it was 70 mg/L/h with PAC addition. Adsorbable organic halogens of the effluent wastewater decreased from 400 to 50 μg/L with the addition of PAC. Toxicity dilution factor decreased from 2 to 1.5 with the PAC addition into the activated sludge system.     

Using the Kinetics of Biological Flocculation and the Limiting Flux Theory for the Preliminary Design of Activated Sludge Systems. II: Experimental Verification

Current activated sludge models consider that the removal of biodegradable organics by suspended growth includes: rapid enmeshment of the organic particles in the microbial floc, hydrolysis of the complex organic molecules into readily biodegradable organic substances, and oxidation of dissolved organic substances. All of the models assume that hydrolysis is the rate-limiting step, but none considers the role that the kinetics of biological flocculation and the sludge-settling characteristics may play in defining the activated sludge operating parameters. Several researchers have studied the kinetics of biological flocculation, and have analyzed its role on the removal of particulate chemical oxygen demand in suspended growth reactors. It has been demonstrated that a large proportion of the organic matter present in sewage can be removed by biological flocculation using short hydraulic retention times and subsequent settling. The first paper demonstrates that the one-dimensional limiting flux theory may be useful for coupling the sludge-settling properties with the aeration tank behavior, and the second paper presents experimental evidence that the proposed model is a reasonable first approximation that can be used for activated sludge system design and operation.     

Adsorption of Carbon Disulfide (CS2) in Water by Different Types of Activated Carbon—Equilibrium, Dynamics, and Mathematical Modeling

Adsorption equilibrium and kinetics of carbon disulfide in water by granular activated carbon (GAC), powdered activated carbon (PAC), and activated carbon fiber (ACF) were investigated and compared in an effort to elucidate the fundamentals for optimizing the control process design. It has been shown that the BET expression can satisfactorily describe the adsorption equilibrium of carbon disulfide (CS2) on GAC, PAC, and ACF and the corresponding kinetic experimental data properly correlated with the second-order kinetic model, which indicates that the CS2 adsorption is the rate-limiting step. A two-phase mathematical model was developed to simulate CS2 transfer in fixed-bed operation filled with the GAC, PAC, and ACF, and the equilibrium and kinetics information is subsequently used in the model to characterize the dynamics of adsorption. The model includes mechanisms such as axial dispersion, advection, liquid-to-solid mass transport, and intraparticle mass transport by pore and surface diffusion. It is manifested that the model was able to predict the dynamic breakthrough curve of CS2 in a fixed-bed adsorption column filled with GAC, PAC, and ACF at varied conditions (standard deviations for 1.5?cm/min is 12.13% and for 2.2?cm/min is 16.12%), based on BET-3 equilibrium and second-order kinetics, which indicates that the methodology proposed by this work could be employed for adsorbents selection, adsorption design, and process optimization for CS2 waste-water emission control.     

Disinfection By-Product Precursor Adsorption as Function of GAC Properties: Case Study

Four different granular activated carbons (GACs) were tested at the bench scale for the adsorption of disinfection by-product (DBP) precursors and were found to be spent at different rates for the Lincoln (Nebraska) water system. This study examined the value of several physical and chemical tests for ranking the potential of different GACs for DBP precursor removal for one water utility. The surface area in the micro- and mesopore range and tannin adsorption were found to be useful indicators of DBP precursor adsorption potential. GACs with the largest surface in the 5?to?50?? pore-width range were able to treat the largest amount of water before being spent. A high value obtained in the tannin adsorption test was observed for the GACs that treated large water volumes.     

Equilibrium and Kinetic Study on Reactive Dyes Adsorption by Palm Kernel Shell-Based Activated Carbon: In Single and Binary Systems

The adsorption of two reactive dyes, Reactive Black 5 and Reactive Red E, onto palm kernel shell-activated carbon (PKSAC) was studied. The effect of the presence of more than one dye in solution on the equilibrium and kinetics of adsorption was investigated. Equilibrium isotherm models were applied to describe the adsorption capacities of single and binary systems. Adsorption of reactive dyes for single system can be represented by the Freundlich and the Redlich-Peterson models. For binary system, the equilibrium was described successfully by the modified extended Freundlich model. Experimental data showed that competitive adsorption for active sites on the carbon surface resulted in a reduction in the overall uptake capacity of the reactive dyes. The rates of adsorption in single system were found to agree well with the pseudosecond-order kinetic model. Finally, the chemical oxygen demand (COD) of the treated reactive dye solutions from single and binary systems showed that a minimum of 4 g/L dosage of PKSAC was needed to reduce the COD to an acceptable level according to the Water Quality Guidelines and the Pollutant Fact Sheets Guidelines.     

Control of PAHs from Incineration by Activated Carbon Fibers

The aim of this research was to use activated carbon fibers (ACFs) to adsorb 16 polycyclic aromatic hydrocarbon (PAH) species from flue gas emissions during incineration. The operation conditions included the presence of three activated carbon fibers, the adsorption temperature (200, 300, and 340°C), and the weight of the ACFs. The removal efficiencies of the gaseous and solid-state PAHs were evaluated respectively. It was found that the BET surface area did not affect PAH removal when the BET surface area was enough for PAH removal and micropore volume was the determinant parameter for PAHs removal. The best adsorption temperature in this study was 300°C. The removal efficiency of PAHs was proportional to the weight of ACFs.     

Nitrification in Pure Oxygen Activated Sludge Systems

Mixed liquor pH and temperature are two parameters that affect the growth rate of nitrifying bacteria and therefore the minimum solids retention time required to achieve nitrification. The objective of this study was to determine the consequence of low mixed liquor pH, and to determine if pH depression could be alleviated by recovering alkalinity through denitrification in a pure oxygen activated sludge system. The study was conducted at the University of Manitoba using laboratory scale, pure oxygen activated sludge reactors, fed with primary effluent. The results indicated that when denitrification was not included in the process, the concentration of CO2 in the headspace of the pure oxygen reactors increased to as high as 15% due to carbon oxidation and endogenous respiration. The high CO2 concentration in the headspace combined with low alkalinity caused by nitrification resulted in bulk mixed liquor pHs below 5.5. In order to maintain complete nitrification at a temperature of 24°C and a mixed liquor pH of 5.5, a solids retention time (SRT) of 12 days was required. In comparison, when denitrification was included in the process the pH of the mixed liquor was increased to 6.4 allowing for full nitrification at an SRT of 5.6 days at a temperature of 24°C. The increase in pH in the denitrification trains was attributed to three factors: recovery of alkalinity through the denitrification process, the conversion of influent carbon to CO2 in the anoxic reactor allowing the CO2 to escape to the atmosphere, and the recycle of mixed liquor super saturated with CO2 from the pure oxygen reactor to the anoxic reactor allowing the CO2 to escape to the open atmosphere. It was determined that the nitrifier growth rate at 12°C was approximately 50% of the rate measured at 24°C. At mixed liquor pHs between 6.0 and 6.3 at a temperature of 12°C, the specific nitrifier growth rate was between 0.12 and 0.15?d?1, while at 24°C, the specific nitrifier growth rate was between 0.25 and 0.30?d?1 at pHs ranging from 5.0 to 6.1     

Assessing Parameter Identifiability of Activated Sludge Model Number 1

The main difficulties reported in the identification of biokinetic models describing the activated sludge process are related to poor convergence or nonconvergence of the identification algorithms and nonuniqueness of the parameter estimates (i.e., different values for the parameters produce approximately the same response from the model). In the present paper, we assessed the identifiability of the Activated Sludge Model Number 1 parameters for a simulated full-scale WWTP calibrating situation, using both noise-free and noise-corrupted simulated data, and analyzed the efficiency of different optimization methods in the identification process. We began by comparing the performance, in terms of the rate of convergence, for different identification algorithms based on three distinct optimization methods. Finally, a procedure based on the information content of the Fisher and covariance matrices was applied in order to define the set of best identifiable parameters in different calibration situations.     

Modeling VOC Emissions in the High-Purity Oxygen Activated Sludge Process

Mass balances for volatile organic compounds (VOCs) were added to a structured mathematical model of the high-purity oxygen activated sludge (HPO-AS) process. The model was sized to correspond to two large existing HPO-AS treatment plants. The stripping of ten different VOCs was modeled and compared to stripping from conventional air activated sludge process. The results show that the covered aeration tanks can reduce stripping by more than 90%, depending on the specific VOC. If biodegradation is considered, the HPO-AS process degrades more than the conventional process due to the higher liquid-phase concentrations that result because of reduced stripping. The increase in biodegradation depends on the VOCs degradability but should increase to nearly 100% for highly volatile but biodegradable VOCs.     

Characteristics of Ground Granular Activated Carbon for Rapid Small-Scale Column Tests

The rapid small-scale column test (RSSCT) has become a popular method for sizing granulated activated carbon (GAC) systems and columns for water treatment facilities. In this procedure, the GAC is ground and a specific size fraction is used for the RSSCT. Since GAC is produced and activated using different processes from different starting materials (e.g., bituminous, lignite, wood, etc.), the possibility exists that the extent of activation and, hence, the adsorptive capacity and surface reactivity may vary throughout the GAC particles. This would be the case if there were less activated inner cores in the GAC particles. If there is a variation in the sorption properties throughout the GAC particles, then grinding the GAC may result in smaller particles that have different properties than the bulk GAC. This study was carried out to test this commonly assumed hypothesis that the limited-sized ground particles represent the same adsorptive properties as the bulk GAC. Four activated carbons (manufactured from different source carbons) were studied. Gas adsorption tests determined the physical morphology, Boehm’s titrations checked the chemical nature of the surface oxides, and the Mohs hardness test was performed on all bulk GACs and ground fractions. No apparent differences were found in the total surface area, cumulative pore volume, or pore size volume of fractions generated by grinding activated carbons. In addition, the Boehm technique did not identify any significant differences in the chemical nature of the surfaces of the various size fractions of GAC. The Mohs hardness test did not indicate any variations in the hardness of the bulk GAC, the ground fractions, and the unground core. Based on the methods and materials used, the underlying assumption in the RSSCT analysis—that there are no variations in the different size fractions of the ground GAC—appears to be correct.