Correlation-based approach to the optimization of fixed-bed sorption units

The sorption of copper, zinc, and cadmium ions onto bone char in fixed-bed columns has been studied. The effects of process variables such, as bed height, now rate, initial concentration, percentage breakthrough, and particle size have been investigated. The results have been used to predict the effect of parameter changes on the system by using the bed depth service time (BDST) approach. After the BDST results are obtained, the system variables can further be optimized based on the adsorbent exhaustion rate and the empty bed residence time (EBRT). The correlations for operating lines of the EBRT plot have been proposed and tested on three metal ion sorption systems. The mechanism of the metal ion sorption on bone char was also investigated

Fixed bed studies for the sorption of metal ions onto peat

In this study, peat has been used as an adsorbent for the removal of heavy metals in a fixed bed system. Experiments have been performed to investigate the effect of flowrate and bed depth on the peat-metal ion system. A film-pore diffusion model has been used to predict the breakthrough behaviour. The three major parameters in the mathematical model are the external mass transfer coefficient, k, the effective pore diffusivity, D-eff, and the solid-phase loading capacity, q(e). One of the main problems in this type of modelling is the prediction of q(e), particularly for sorbent-solute systems which take a long time to achieve equilibrium saturation. Some researchers have used the equilibrium isotherm capacity, some utilize a fixed fraction of the isotherm capacity and other workers perform the numerical or graphical mass balances at the breakthrough curves, which is time-consuming and tedious. The present method incorporates a novel empirical solution for q(e), which is correlated with the service time. Good agreement between the predicted theoretical breakthrough curves and the experimental results is observed

A branched pore kinetic model applied to the sorption of metal ions on bone char

A slightly modified form of the branched pore model of Peel, Benedek and Crowe was successfully applied to describe the batch sorption kinetics of three metal ions-cadmium, copper and zinc-on bone char. In comparison with an analytical film-surface solution, the additional parameters of the branched pore model were observed to produce a significant improvement in correlating the experimental results. The ranges of the values of the model parameters derived were deemed reasonable and the branched pore sorption capacities of two of the three metal ions were comparable (ca 0.16 mmol g(-1)). Given that the surface diffusivities of the metal ions were observed to vary with averaged surface loading, a number of correlations were examined for their accuracy in describing this behaviour. The exponential expression of Neretnieks resulted in the smallest total error when the data for all three metal ions were considered together. (c) 2005 Society of Chemical Industry

Application of the concentration-dependent surface diffusion model on the multicomponent fixed-bed adsorption systems

The ability of bone char to adsorb three metal ions, namely, cadmium, copper and zinc, from effluents in fixed beds has been studied. Two binary metal ion sorption systems, Cd + Cu and Cu + Zn, have been investigated. The variables studied include metal ion solution flowrate, initial metal ion concentration, and bone char particle size bed height. The experimental breakthrough curves for each binary system were measured at five bed heights. A multicomponent film-surface diffusion model has been developed to predict the breakthrough curves by incorporating the IAS for both the Langmuir and the Sips equations, since they both correlate the single component equilibrium isotherm data well. A novel development is the modification of the solution methodology, previously restricted to a constant diffusivity, to incorporate a variable diffusivity correlated with adsorbent coverage by the constant self-diffusivity. The self-diffusivities for the metal ions have been evaluated. (c) 2005 Elsevier Ltd. All rights reserved

Film-pore diffusion model for the fixed-bed sorption of copper and cadmium ions onto bone char

The sorption of copper and cadmium ions onto bone char in single component systems has been studied using fixed-bed column adsorbers. The effects of solution flowrate, initial metal ion concentration and bone char particle size have been studied. A film-pore diffusion model has been developed to predict the fixed-bed breakthrough curves for the two metal ions. A sensitivity analysis has been carried out to investigate the influence of the external mass transfer coefficient (film resistance), the effective diffusion coefficient (pore diffusion) and the solid phase loading capacity. It is round that under the experimental conditions employed in the study, film diffusional resistance was tow and the Biot numbers were relatively high. Furthermore. a constant effective pore diffusivity was not sufficient to correlate the breakthrough curves accurately and a variable dependent effective diffusivity was required; suggesting a possible contribution front surface diffusion. Since the metal ion-bone char systems take a long time to reach equilibrium. the solid phase loading capacity as predicted by the "best-fit" equilibrium isotherm. was not Suitable for use in the diffusional mass transport model and the mass balance solid phase loading was utilised instead. (C) 2001 Elsevier Science Ltd. All rights reserved

Production of active carbons from waste tyres - a review

A review of the production of activated carbons from waste tyres is presented. The effects of various process parameters, particularly, temperature and heating rate, on the pyrolysis stage are reviewed. The influence of activating conditions, physical and chemical, nature of the activation chemicals, on the active carbon properties are discussed. Under certain process conditions several active carbons with BET surface areas over 1000 m(2)/g have been produced with extensive micropore volumes, over 40\% of the total pore volume. A review is carried out of the reaction kinetic modeling applied to the pyrolysis of tyres and the chemical activation of tyres. The models cover one step and two step pyrolysis models, plus more recent models which are based on the actual chemical components such as natural rubber. SBR and other additives. (C) 2004 Elsevier Ltd. All rights reserved

Mass transport model for the fixed bed sorption of metal ions on bone char

The ability for the sorption of the three metals onto bone char was demonstrated by a series of equilibrium isotherm studies. The Sips isotherm equation was found to give the most accurate correlation of the equilibrium relationship between these metal ions and bone char, and the equilibrium adsorption capacities of bone char for copper, zinc, and cadmium ion were determined to be approximately 0.80, 0.50, and 0.55 mmol/g, respectively. A film surface diffusion model, which is based on the assumption that the sorbate penetration rate is controlled by the external film diffusion and surface diffusion within the sorbent, has been developed to simulate the fixed bed sorption of copper, zinc, and cadmium ions onto bone char. A computer program based on a numerical solution using the finite difference method has been constructed and successfully simulated the metal ion-bone char column behavior. A new external mass transfer correlation was proposed and applied in the model. The effect of axial dispersion was tested and found to be not significant in these systems

Optimised correlations for the fixed-bed adsorption of metal ions on bone char

The sorption of two divalent metal ions, namely, copper and cadmium ions, has been studied due to their toxicity in nature and extensive use in industry. Fixed-bed column experiments with different feed concentrations, flowrates and adsorbent particle sizes have been carried out, evaluating sorption of these two metal ions on bone char. Pilot-plant experimental studies have been performed and two simplified design models, namely, the BDST model and EBRT analysis, have been used to analyse the data. New correlations, incorporating a time-dependent term, have been developed to compensate for the time required for the bed to achieve equilibrium sorption capacity. Finally, predictions of the operating lines on the EBRT plot have been correlated using the two metal ion sorption systems. (C) 2000 Elsevier Science Ltd. All rights reserved

A branched pore model analysis for the adsorption of acid dyes on activated carbon

A new branched-pore adsorption model has been developed using an external mass transfer coefficient, K-f, an effective diffusivity, D-eff, a lumped micropore diffusion rate parameter, K-b, and the fraction of macropores, f, to describe sorption kinetic data from initial adsorbent-adsorbate contact to the long-term adsorption phase. This model has been applied to an environmental pollution problem-the removal of two dyes, Acid Blue 80 (AB80) and Acid Red 114 (AR114), by sorption on activated carbon. A computer program has been used to generate theoretical concentration-time curves and the four mass transfer kinetic parameters adjusted so that the model achieves a close fit to the experimental data. The best fit values of the parameters have been determined for different initial dye concentrations and carbon masses. Since the model is specifically applicable to fixed constant values of these four parameters, a further and key application of this project is to see if single constant values of these parameters can be used to describe all the experimental concentration-time decay curves for one dye-carbon system. The error analysis and best fit approach to modeling the decay curves for both dye systems show that the correlation between experimental and theoretical data is good for the fixed values of the four fitted parameters. A significantly better fit of the model predictions is obtained when K-f, K-b and f are maintained constant but D-eff is varied. This indicates that the surface diffusivity may vary as a function of surface coverage

Applications of multipore model for the mechanism identification during the adsorption of dye on activated carbon and bagasse pith

A solution to the intraparticle diffusion model has been developed on the basis of the pore structure of adsorbents. This global model incorporates the mechanism of external film mass transport, intraparticle macropore surface diffusion, and adsorption at the surface in the micropores limited by a first-order reaction kinetic rate constant. To assess the model, it has been applied to the adsorption of acid and basic dyes onto two adsorbents, active carbon and bagasse pith. The two adsorbents were selected because of their differences: active carbon has a large BET surface area and microporous structure, whereas bagasse pith has a small surface area and only a spongelike macropore structure. The branched pore model developed and tested is able to distinguish these physical-structural characteristic differences between the two adsorbents using two very different adsorbates