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.     

Dynamic Mathematical Modeling of an Isothermal Three-Phase Reactor: Model Development and Validation

A catalytic reactor model (CatReac) that describes the transport and series reactions of compounds in a three-phase fixed-bed catalytic reactor is developed for the purpose of describing the volatile assembly reactor system within the potable water processor on-board the International Space Station. CatReac includes these mechanisms: advective flow, axial dispersion, gas-to-liquid and liquid-to-solid mass transport, intraparticle mass transport with pore and surface diffusion, and series reactions of multiple compounds. A dimensional analysis of CatReac revealed the following seven dimensionless groups may be used to determine the controlling transport and/or reaction mechanisms: (1) the Peclet number is the ratio of the advective to the dispersive transport; (2) the Stanton number is the ratio of the external mass transfer rate to the advective rate; (3) the Damk?hler number compares the reaction rate to the advective transport rate; (4) the surface diffusion ratio equals the rate of transport by surface diffusion divided by the rate of transport by advection; (5) the pore diffusion modulus is the ratio of the rate of transport by pore diffusion to the rate of transport by advection; (6) the ratio of the gas to liquid advective rates; and, (7) the Biot numbers for surface and pore diffusion compare the external mass transfer rate to the intraparticle mass transfer rate. These dimensionless numbers are used to evaluate the impacts of the different mechanisms on the overall performance of the reactor. The numerical solution using orthogonal collocation was validated for a wide range of controlling mechanisms by comparing model simulations with several analytical solutions: (1) Gas-to-Liquid mass transfer controlling the overall mass transfer-reaction mechanisms, for a wide range of Pe number values; (2) Liquid-phase dispersion controlling the overall process; (3) Liquid-to-solid mass transfer resistance controlling the overall mass transfer-reaction process; (4) Reactions in series with two possibilities (4a): No intraparticle mass transfer resistance, and (4b): Significant intraparticle mass transfer resistance; (5) Langmuir isotherm (5a): single compound, no mass transfer resistance, and (5b): multicomponent competitive adsorption without mass transfer resistance; (6) Unsteady state operation: Plug flow with mass transfer and no reaction. These validations systematically examine all the mechanisms that are included in the general model and examine the model limitations based on the controlling mechanisms.     

Application of simplified rapid equilibrium models in simulating experimental breakthrough curves from fixed bed biosorption reactors

A modelling approach for a fixed bed biosorption column is presented. The Advection–Dispersion–Reaction (ADR) equation has been applied as the basic modelling equation for the special case of Local Equilibrium (LE). The model implements the minimum parameters for describing a fixed bed biosorption column, employing the geometrical dimensions of the bed, the packing arrangement, the operating conditions of the system, and the sorptive characteristics of the biosorbent material. An apparent axial dispersion coefficient has been used as a key parameter of the model. The authors compare model predictions to selected examples of experimental biosorption breakthrough curves reported from pilot scale work. Although the main assumption of the model is that biosorption equilibrium is rapid, the use of an apparent overall dispersion coefficient makes the model applicable for the cases where mass transfer resistances are present in the liquid and solid phases.     

Batch uptake of lysozyme: effect of solution viscosity and mass transfer on adsorption

In this study, solid-phase adsorption by macroporous and hyper-diffusive resins was investigated in a batch uptake adsorption system to quantify solid-phase diffusion rates as a function of bulk phase viscosity. The performance of chromatographic resins used for adsorption of proteins is dependent on several factors including solid and liquid-phase diffusivity, boundary layer mass transfer, and intraparticle mass transfer effects. Understanding these effects is critical to process development and optimization of both packed and fluidized bed adsorption systems. The macroporous resin used here was Streamline SP, and the hyper-diffusive resin was S-HyperD LS. Both have been frequently used in fluidized bed adsorption of proteins; however, factors that affect uptake rates of these media are not well quantified. Adsorption isotherms were well represented by an empirical fit of a Langmuir isotherm. Solid-phase diffusion coefficients obtained from simulations were in agreement with other models for macroporous and hyper-diffusive particles. S-HyperD LS in the buffer system had the highest uptake rate, but increased bulk phase viscosity decreased the rate by approximately 50%. Increases in bulk phase viscosity increased film mass transfer effects, and uptake was observed to be a strong function of the film mass transfer coefficient. Uptake by Streamline SP particles was slower than S-HyperD in buffer, due to a greater degree of intraparticle mass transfer resistance. The effect of increased film mass transfer resistance coupled with intraparticle mass transfer resistances at an increased bulk phase viscosity resulted in a decrease of 80% in the uptake rate by Streamline SP relative to S-HyperD.     

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.     

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.     

Effects of Particle Stratification on Fixed Bed Absorber Performance

The effects of adsorbent particle size distribution (PSD) and the layering of particles in stratified and reverse stratified modes on the performance of fixed bed adsorber were investigated. Using trichloroethylene as the adsorbate and granular activated carbon as the adsorbent, experimental studies were conducted in stratified beds for different flow rates and influent concentrations. The homogeneous solid diffusion model was modified to take into account PSD and was used to simulate breakthrough curves. The PSD-based model was validated using experimental data and was found to be more accurate in predicting the breakthrough curves than the non-PSD-based model. The validated model was used to conduct simulations to examine the effects of key variables on performance in the stratified and reverse stratified modes. In the reverse stratified mode, the adsorbent particle size decreases gradually in the direction of flow. Model simulations indicate that this mode of operation increases breakthrough time, decreases the time to reach saturation, and thereby increases the overall adsorbent capacity utilization. The mass transfer zone for the reverse stratified bed was found to be narrower and sharper than that for the stratified bed. These model predictions have important ramifications to the water and wastewater industry in terms of reducing the overall cost of treatment using granular activated carbon adsorption.     

Effect of Adsorbent Particle Layering on Performance of Conventional and Tapered Fixed-Bed Adsorbers

Experimental and modeling studies were conducted for the adsorption of phenol from aqueous solutions onto activated carbon in fixed beds with the adsorbent particles layered according to particle size. In the conventional stratified cylindrical adsorber (SCA), the particles were layered according to natural stratification, and increased in size with column depth. In the reverse stratified tapered adsorber (RSTA), the particle size decreased with column depth, and the fluid velocity decreased in the direction of flow. Experimental data indicate that for a uniform particle size distribution, the breakthrough time for the RSTA was about 60% higher than for the SCA under identical carbon loading and flow conditions. The homogeneous solid phase diffusion model with Linear-Freundlich isotherm was used to model the layered adsorbers. It provides excellent predictions for breakthrough curves at various column depths. Bed capacity utilization can be increased with the RSTA due to the sharpening of the solute front, and this will translate into lower capital and operating costs for the carbon adsorption system due to the smaller unit required, lower carbon inventory, and lower pumping costs.     

Sulfur Impregnation on Activated Carbon Fibers through H2S Oxidation for Vapor Phase Mercury Removal

Sulfur was impregnated onto activated carbon fibers (ACFs) through H2S oxidation catalyzed by the sorbent surface in a fixed-bed reactor. By changing the temperature and duration of the sulfur impregnation process, ACFs with different sulfur contents were developed. Characterization of ACFs before and after sulfur impregnation was conducted by surface area analysis, energy dispersive X-ray analysis, thermogravimetric analysis, X-ray photoelectron spectroscopy, and temperature programmed desorption. Vapor phase mercury adsorption experiments were carried out in a fixed-bed reactor. Sulfur was impregnated mainly as elemental sulfur and the amount of sulfur deposited on the ACF increased with an increase in impregnation temperature. Higher temperature leads to more uniform sulfur distribution inside the sorbent pores. The impregnation process can be explained by a combination of pore filling and monolayer adsorption, with the former mechanism predominating at low temperatures. In the absence of sulfur, the mercury adsorption capacity can be correlated with surface area and pore volume.     

Rate analysis of sorption of Ce3+, Sm3+, and Yb3+ ions from aqueous solution using Dowex 50W-X8 as a sorbent in a continuous flow reactor

Kinetic analysis of removal of three rare earth elements metals, Ce³⁺, Sm³⁺, and Yb³⁺ ions from aqueous solutions in a continuous flow fixed bed reactor using Dowex 50W-X8 ion-exchange resin was conducted. The performance of the fixed bed sorption was evaluated using the concept of the sorption breakthrough process. Parameters characteristic of a fixed bed sorption such as breakthrough times, saturation times, critical reactor lengths, and lengths of mass transfer zone were inferred from the metal ion concentration breakthrough curves. The sorption capacity of Dowex 50W-X8 ion-exchange resin for Ce³⁺, Sm³⁺, and Yb³⁺ are 191, 252, and 294 mg/g, respectively. The sorption kinetics were evaluated using a zero-order, first-order and second-order reaction models. The kinetics of the sorption process follows a zero-order model which has not been reported before. The rate constants of sorption using the zero-order kinetic model are obtained. Two different analysis were conducted to identify whether the diffusion is intraparticle or film diffusion. Both analysis confirms that the film diffusion is the controlling mechanism in reactor bed.     

树脂吸附铜锌离子的数值模拟

镀金原料氰化亚金钾的生产过程含氰废液中锌（Ⅱ）离子和铜离子（Ⅱ）浓度较高，为探究树脂吸附铜、锌离子的规律，运用Aspen Adsorption数值模拟软件对Φ10 mm×200 mm的树脂固定床吸附过程进行数值模拟，采用一阶迎风差分法作为偏微分方程的离散方法，研究不同条件下的吸附穿透曲线和吸附负荷分布曲线.结果表明:增加进料流量和进料浓度均可提高吸附效率，传质系数（K_S）大于临界值时，吸附穿透曲线不再受其影响，K_S（Zn2+）=4.3×10-3s-1, K_S（Cu2+）=1.00×10-2s-1.      

Adsorption of Sulfur Dioxide on Activated Carbon from Oil-Palm Waste

Adsorption of sulfur dioxide (SO2) onto activated carbons prepared from oil-palm shells was investigated in this paper. Experimental results showed that the adsorption temperature and SO2 concentration significantly determined the amount of SO2 adsorbed and the equilibrium time. However, sample particle size had minimum effect on the equilibrium time. For a fixed SO2 concentration, the adsorption rate and adsorption kinetic parameters (activation energy and frequency factor) were obtained. A linearly proportional relationship between the Brunauer-Emmett-Teller surface area and the adsorptive capacity of the activated carbon from oil-palm shells was observed. An intraparticle Knudsen diffusion model based on a Freundlich isotherm was developed for predicting the amount of SO2 adsorbed and the SO2 concentration profile within the particle. Based on the estimated isotherm parameters and diffusion coefficients by experimental data fitting, this model could predict the amount adsorbed under different concentrations very well. However, this model was unsuitable for the activated carbon prepared from oil-palm shells by chemical activation because of the occurrence of chemisorption, which was related to the nature of the sample surface functional groups.     

含氮有机物对活性碳纤维还原吸附银的影响

用H3PO4、ZnCl2等活化剂分别制备了两种化学活化的活性碳纤维（HPSACF和ZCSACF）,并通过水蒸汽活化制备了水蒸气活化活性碳纤维（SACF）。研究了它们对水溶液中Ag(NH3)2^ 的还原吸附性能,并与活性碳（AC）的还原吸附进行了比较。不同方法制备的活性碳纤维（ACF）对Ag(NH3)2^ 的还原吸附能力有显著的差异,以磷酸活化法制备的ACF的还原能力最强,而AC有一定的还原吸附能力。含氮有机物的吸附对活性碳纤维或活性碳的还原吸附能力有很大的影响,一般而言,吸附至碳吸附剂上的对硝基苯酚和苯胺可促进AC、SACF和HPSACF的还原能力,且苯胺的促进作用大于硝基苯酚的促进作用。     

Equilibrium versus Nonequilibrium Treatment Modeling in the Optimal Design of Pump-and-Treat Groundwater Remediation Systems

The present work proposes that the incorporation of granular activated carbon (GAC) treatment model that accounts for nonequilibrium adsorption into the optimal design of pump-and-treat systems will result in more realistic costs and better-engineered remediation systems. It was found that, when nonequilibrium GAC adsorption effects are considered, the predicted cost of optimal remediation strategies increases consistently when compared to costs obtained assuming equilibrium GAC adsorption, for a wide range of cleanup goals. This finding implies that when simpler equilibrium models are used for GAC adsorption, cleanup costs will be underestimated. GAC treatment costs are shown to be particularly sensitive to the degree of mass transfer limitations in the aquifer–contaminant system, especially when nonequilibrium GAC adsorption is accounted for. Time-varying pumping rates are shown to produce more efficient remediation solutions; the increase in efficiency is even more pronounced when nonequilibrium GAC adsorption is accounted for. Further results show that the optimal remediation designs can be significantly more efficient when the number of GAC adsorber units is selected through optimization.     

Removal of Toluene Vapor with Dispersed Activated Carbon

This work considers the mechanisms of mass transfer in a process of dispersed sorbent injection. During experiments, an air supply was dosed with toluene vapor, at partial pressures between 4 and 15 Pa. Powdered activated carbon (PAC) was added to remove the toluene from the air, and the resulting mixture was passed through a 3-m-long, tubular, aluminum test section. Toluene concentrations were measured at seven axial locations within the test section. Comparing the measurements with mathematical models indicated the importance of adsorption kinetics. At reduced toluene inlet concentrations the PAC removed a slightly bigger fraction of toluene from the air stream. This fraction increased with PAC concentration. The effect on removal of varying the air temperature between 25 and 85°C was small. Alternative models incorporating either pore diffusion or surface diffusion were fitted to the results. The quality of the fits was fair only, but sufficient to show that the pore diffusivity that gave the best fit was far larger than would be expected from the Knudsen diffusivity in the pores. That is, surface diffusion was an important part of the intraparticle mass transfer.     

Lattice Boltzmann model for simulation on leaching process of weathered elution-deposited rare earth ore

The lattice Boltzmann model with coupled chemical reaction was proposed to simulate the ion exchange process of rare earth leaching and verified by comparison with both empirical correlation of mass transfer coefficient and unreacted-core shrinking model. By simulation, the zonation phenomenon of leaching reagent in the leaching column was presented, and the breakthrough curve of leaching reagent was obtained. When t=50 s, there existed the saturated and exchange zones, and the leaching reagent concentration decreased gradually from 20 to 9.3 g/L. In accordance with the breakthrough curve, the breakthrough capacity of ion-type rare earth ore and the adsorbed ion concentration of leaching reagent were derived, the time of t=25 s was the breakthrough point of ammonium ion in leaching reagent and the breakthrough capacity of the rare earth ore was 125 g/L. Besides, the chemical kinetics parameters used for the solute transfer process of rare earth leaching were obtained by the simulation and then were used to determine the rate-limiting steps of rare earth leaching process.     

Radial Pore Diffusion with Nonuniform Intraparticle Porosities

Effects of radially dependent intraparticle pore sizes on solute fate and transport are examined for batch systems with spherical particles using a recently developed numerical model. The model can accommodate multiple particles distributed in size, mass transfer resistance at particle boundaries, intraparticle reversible sorption kinetics, and first-order decays. Two applications are examined. In the first application, random or deterministic intraparticle porosities across a spherical particle are considered. In the second application, multiple particles distributed in sizes with particle size-dependent intraparticle porosities are studied. Results from these applications indicate that concentration profiles are largely determined by interplays between B, η, and ε that incorporate the effects of intraparticle pore structures. Steady-state concentration values in both applications are determined by the volume-averaged intraparticle porosities. These results could be useful for understanding solute tailing behavior in natural porous media and the design of synthetic sorbents for treatment of contaminated waters.     

Adsorptive Removal of Parts per Million Level Selenate Using Iron-Coated GAC Adsorbents

Selenate removal by adsorption using iron-coated granular activated carbons (Fe-GACs) is reported in this study. Adsorption kinetics and equilibrium experiments with initial selenium concentration of 1 mg/L were conducted under three different ionic strengths to study selenate adsorption behavior. Selenate adsorption reached equilibrium within 48 h with more than 85% of the equilibrium capacities being obtained within the first 6 h. High removal efficiency (i.e., >75%) was achieved for pH range of 2–5. Acid-base titration experiments showed point of zero charge (pHpzc) at pH 7.5 for the tested Fe-GAC. Pseudo-second-order kinetic model characterized selenate adsorption kinetics well (R2 = 0.999) and the rate constant decreased with ionic strength. Adsorption capacity decreased significantly with increasing ionic strength, which was not observed in selenite adsorption with the same adsorbent. Competitive adsorption with other four oxyanions (SiO32?, SO42?, PO43?, and CO32?) showed that selenate removal efficiency was reduced to various degrees in the presence of each individual anion. Competitive adsorption of binary adsorbates (selenite and selenate) showed a decreasing trend of selenite adsorption capacity with decreasing ionic strength, indicating stronger competition of selenate against selenite under the low ionic strengths. The Sheindorf-Rebuhn-Sheintuch multiadsorbate competitive adsorption model was applied to quantify the binary competitive adsorption between selenate and selenite.     

Modeling of Aqueous BTEX Adsorption in Column and Multistage Adsorbers

Theoretical and experimental investigations have been conducted on the adsorption characteristics of benzene, toluene, ethylbenzene, and xylene (BTEX) by macroreticular resins. A theoretical model is proposed for describing the BTEX breakthrough curves of the adsorption column. The column adsorption model contains two model parameters, τ and K, that are conveniently estimated using the observed breakthrough data. The model was tested for benzene adsorption with a flow rate between 12 and 30 mL∕min, inlet concentration between 200 and 500 mg∕L, and temperature between 30 and 60°C. Excellent fit of the model to the measured data was obtained. The proposed model hence offers a convenient means for accurate predictions of the adsorption capacity and breakthrough point of a column BTEX adsorption process. Also developed are the mass balance equations representing countercurrent multistage adsorption process. The countercurrent multistage adsorption process has been shown to save a significant amount of adsorption resin in achieving the same BTEX removal as the single-stage process. Experimental tests for a two-stage example verify that up to 60% of the Ambersorb resin can be saved over a single-stage process.     

PAC-Membrane Filtration Process.?I: Model Development

Adsorption is modeled when powdered activated carbon (PAC) is applied in continuous-flow reactors followed by membrane filtration units operated without carbon wastage between backwash events. Four reactor configurations are studied: (1) A membrane reactor dosed with a step input of PAC; (2) a continuous-flow stirred tank reactor dosed with a step input of PAC and followed by a membrane reactor; (3) a plug-flow reactor dosed with a step input of PAC and followed by a membrane reactor; and (4) a membrane reactor dosed with a pulse input of PAC at the beginning of the filtration cycle. A steady-state operation is considered to describe the adsorption process through the continuous-flow stirred tank reactor and plug-flow reactor, whereas adsorption in the membrane reactor is modeled as a non-steady-state process. Adsorption kinetics is assumed to occur by homogeneous surface diffusion, and adsorption equilibrium is described with the Freundlich isotherm model. Analytical solutions of the homogeneous surface diffusion model with no external mass transfer limitation are used to evaluate adsorbate concentrations in the solid phase as a function of time. Part II of this study presents model simulations and verification with experimental data obtained in a bench-scale apparatus.