Stirred Tank Adsorber Research Articles

Hydrodynamic effects on the overall adsorption rate of phenol on activated carbon cloth through the advection-diffusion model application

In this work, a mathematical formulation based on fundamental principles of momentum and mass transport with a kinetic adsorption model at the liquid-solid interface is proposed. This formulation was numerically solved and used for the interpretation of adsorption rate data of phenol onto an activated carbon cloth in a stirred tank adsorber under different agitation rates. The mathematical model solution was compared with the obtained by mathematical models that neglect the local velocity and concentration profiles in the solution; external mass transfer model (EMTM) and the first-order kinetic model (FOM). The results showed that under all studied stirring speeds (30−200rpm) the proposed model was able to capture the dynamics of the concentration decay curves, while EMTM and FOM models were only able to interpret the data at high stirring speeds, indicating that velocity profiles play an important role during phenol adsorption. Particular “wake zones” were identified behind the mobile adsorbent, which importantly promotes the transport of solute toward the activated carbon cloth. As a result, it was found that convective transport is much more important than diffusive transport in the solution.

Basic dyestuffs removal from textile effluents using feathers: equilibrium, kinetic and column studies

This research work has been focused in the study of gallinaceous feathers, a waste that may be valorised as sorbent, to remove the Dark Blue Astrazon 2RN (DBA) from Dystar. This study was focused on the following aspects: optimization of experimental conditions through factorial design methodology, kinetic studies into a continuous stirred tank adsorber (at pH 7 and 20ºC), equilibrium isotherms (at pH 5, 7 and 9 at 20 and 45ºC) and column studies (at 20ºC, at pH 5, 7 and 9). In order to evaluate the influence of the presence of other components in the sorption of the dyestuff, all experiments were performed both for the dyestuff in aqueous solution and in real textile effluent. The pseudo-first and pseudo-second order kinetic models were fitted to the experimental data, being the latter the best fit for the aqueous solution of dyestuff. For the real effluent both models fit the experimental results and there is no statistical difference between them. The Central Composite Design (CCD) was used to evaluate the effects of temperature (15 - 45ºC) and pH (5 - 9) over the sorption in aqueous solution. The influence of pH was more significant than temperature. The optimal conditions selected were 45ºC and pH 9. Both Langmuir and Freundlich models could fit the equilibrium data. In the concentration range studied, the highest sorbent capacity was obtained for the optimal conditions in aqueous solution, which corresponds to a maximum capacity of 47± 4 mg g-1. The Yoon-Nelson, Thomas and Yan’s models fitted well the column experimental data. The highest breakthrough time for 50% removal, 170 min, was obtained at pH 9 in aqueous solution. The presence of the dyeing agents in the real wastewater decreased the sorption of the dyestuff mostly for pH 9, which is the optimal pH. The effect of pH is less pronounced in the real effluent than in aqueous solution. This work shows that feathers can be used as sorbent in the treatment of textile wastewaters containing DBA.

Modeling of arsenic adsorption kinetics of synthetic and contaminated groundwater on natural laterite

A simple shrinking core model is applied to predict the adsorption kinetics of arsenite and arsenate species onto natural laterite (NL) in a stirred tank adsorber. The proposed model is a two-resistance model, in which two unknown parameters, external mass transfer coefficient ( K f ) and pore diffusion coefficient ( D e ) are estimated by comparing the simulation concentration profile with the experimental data using a nonlinear optimization technique. The model is applied under various operating conditions, e.g., initial arsenic concentration, NL dose, NL particle size, temperature, stirring speed, etc. Estimated values of D e and K f are found to be in the range of 2.2–2.6 × 10 −11 m 2/s and 1.0–1.4 × 10 −6 m/s at 305 K for different operating conditions, respectively. D e and K f values are found to be increasing with temperature and stirrer speed, respectively. Calculated values of Biot numbers indicate that both external mass transfer and pore diffusion are important during the adsorption. The model is also applied satisfactorily to predict the arsenic adsorption kinetics of arsenic contaminated groundwater–NL system and can be used to scale up.

Metal biosorption by algae Gelidium derived materials from binary solutions in a continuous stirred adsorber

Continuous metal biosorption from Pb(II)/Cu(II) and Pb(II)/Cd(II) solutions onto algae Gelidium and granulated algal waste was performed in a continuous stirred tank adsorber (CSTA). Biosorption was simulated by a mass transfer model including intraparticle and film resistances and equilibrium described by the Langmuir binary equation. The breakthrough curves present an initial “knee” suggesting a film resistance to mass transfer. Biosorption of Pb(II) is significantly reduced by the presence of Cu(II), which indicates that competition exists for the same binding sites. In the binary system Pb(II)/Cd(II), an “overshoot” region was observed, for the cadmium concentration profile, due to the higher affinity of lead ions to the binding sites. A fast and efficient desorption of Cu(II) and Pb(II) was obtained using a 0.1 M HNO 3 solution, as eluent. Results suggest that desorption is based on ion exchange between the metal ions , loaded in the biomass, and H + in solution. Desorption was adequately predicted by a mass transfer model considering intraparticle diffusion resistance and equilibrium given by the mass action law.

Lead and copper biosorption by marine red algae Gelidium and algal composite material in a CSTR (“Carberry” type)

Adsorption of lead and copper ions by algae Gelidium and algal composite material was studied in a continuous stirred tank adsorber (CSTR). Danckwerts curves show that dead volumes and by-passes do not exist inside the reactor. The average residence time for cadmium ions was τ = 15.2 ± 0.2 min, and the useful adsorber volume Vr = 540 cm3. Varying stirring velocity in the range 100–270 rpm the breakthrough curves show that a resistance to the mass transfer exists in the film of fluid around particles. The analysis of the effect of the flow rate on the breakthrough curves leads to the conclusion that the curves sharpness increases with flow rate. As expected, the larger the initial lead concentration, the faster the breakthrough of the metal, as the binding sites became more quickly saturated. The slope of the breakthrough curve increases when increasing the influent concentration. For higher initial pH values, the breakthrough time increases since metal uptake capacity increases. Biosorbents regeneration can be achieved by metal ions desorption with nitric acid as eluant. Desorption is an ion exchange process between metal ions loaded in the biomass and H+ ions in the solution. Two mass transfer models were developed to simulate biosorption and desorption performances. These models were able to predict the experimental results in a quite good manner.

Molecular Simulation of Adsorption Processes. 1. Isothermal Stirred-tank Adsorber

A new molecular simulation method is proposed to solve the governing equations for a multicomponent, isothermal stirred-tank adsorber under equilibrium controlled conditions. The technique is formulated in the Gibbs ensemble, but is more appropriately viewed as a hybrid of a molecular simulation and continuum modeling. For the general case of an arbitrary multicomponent mixture, the total number of molecules of each species is allowed to fluctuate so that the ensemble average satisfies the macroscopic material balances to the adsorber. It is shown that if an analytical equation of state for the fluid phase is known, the simulation procedure can be considerably simplified and acquires many characteristics of a Monte Carlo simulation conducted in the grand canonical ensemble. The technique is thoroughly validated and its usefulness is demonstrated through application to a gas separation problem encompassing the major steps of practical value to batch adsorption processes.

Adsorption and mass transfer characteristics of metsulfuron-methyl on activated carbon

Removal of a synthetic organic herbicide, metsulfuron-methyl (MSM), from aqueous solutions has been studied in batch and stirred tank adsorbers charged with granular activated carbon particles. Two kinetic parameters, film mass transfer and intraparticle diffusion coefficients, were estimated from concentration decay curves obtained in the batch adsorber. Based on these kinetic parameters, the concentration profiles measured in the stirred tank adsorber were simulated. From experimental and simulated results, it was proven that the film mass transfer at external surfaces of carbon particles controls the overall mass transfer, particularly at low mixing (rotation of blades), during the adsorption of MSM by granular activated carbon particles.

Linear driving force approximation in cyclic adsorption processes: Simple results from system dynamics based on frequency response analysis

The linear driving force (LDF) approximation for cyclic adsorptive processes is discussed on the basis of model equivalence with the homogeneous diffusion model (HDM), the pore diffusion model (PDM) and the intraparticle diffusion and convection model (IDCM). Model equivalence is based on the frequency response of the adsorbent particle, namely on the equality of the amplitude ratio and the phase-lag functions. The analysis of the continuous stirred tank adsorber (CSTA) and of the plug flow adsorber (PFA) is addressed.

Modelling of solid-liquid adsorption: effects of adsorbent heterogeneity

Effects of adsorbent heterogeneity on the adsorption of cobalt phthalocyanine dye on activated carbon have been studied. Adsorption experiments were carried out by varying the temperature and adsorbent mass in batch adsorbers and, in addition, the adsorbent particle size and fluid flow rate in a continuous stirred tank reactor (CSTR)-type adsorber in order to investigate the equilibrium and the kinetics of adsorption. The Brunauer-Emmett-Teller (BET), Langmuir with uniform distribution (LUD) and Langmuir-Freundlich equations are able to represent the equilibrium data with similar accuracy within the range of measurements. Reasonably large values of the heterogeneity parameter (2.69–2.86) show that the carbon surface is energetically heterogeneous. A mathematical model that describes the adsorption dynamics, including film-, pore- and concentration-dependent surface diffusion on an energetically and structurally heterogeneous adsorbent, is presented here and fitted to the experimental concentration vs. time curves obtained in the continuously stirred tank adsorber. Structural heterogeneity of the carbon, if not accounted for in the kinetic model, can be responsible for the very strong concentration dependence of the surface diffusion coefficient and for the variation in the parameter D o with particle size and adsorber porosity as shown in this work.

Determination of intraparticle transport parameters for multicomponent adsorption

This paper is devoted to the problem of determining intraparticle transport coefficients for multicomponent adsorption from experimental kinetic curves. Due to thermodynamic and kinetic interactions between the mixture's components, this task is more difficult than for single component systems. As an example, the kinetics of adsorption on activated carbon of binary mixtures of phenol and indole from aqueous solutions was studied in a continuous stirred tank adsorber (CSTA). To describe the intraparticle transport of this system, the surface diffusion model using a linear driving force (LDF) was applied. For determining the matrix of the unknown kinetic parameters of the LDF-model, the application of three independent mathematical methods is discussed. Firstly, a semi-analytical calculation (SA) of the intraparticle transport parameters by solving a system of linear equations on the basis of an empirical description of experimental kinetic curves is put forward. Secondly, a non-linear regression (NR) method for parameter estimation was applied. The third method is a non-linear regression which, in contrast to the SA and NR methods, guarantees the stability of the system of differential equations in the steady state of the process (NRS).

Determination of intraparticle kinetic parameters for adsorption of binary solutions on activated carbon

The matrix of intraparticle mass transfer coefficients for competitive adsorption of two dissolved organic components on activated carbon is determined from kinetic curves obtained in a continuous stirred tank adsorber using a nonlinear regression method. Stability considerations are included in the parameter estimation procedure. The applied mathematical approach guarantees the stability of the system of differential equations in the steady state of the considered process. The results for the sorption of phenol and indole demonstrate the important role of cross-term coefficients for the kinetics of this process.