Margaritis Kostoglou

Green Adsorbents for Wastewaters: A Critical Review

One of the most serious environmental problems is the existence of hazardous and toxic pollutants in industrial wastewaters. The major hindrance is the simultaneous existence of many/different types of pollutants as (i) dyes; (ii) heavy metals; (iii) phenols; (iv) pesticides and (v) pharmaceuticals. Adsorption is considered to be one of the most promising techniques for wastewater treatment over the last decades. The economic crisis of the 2000s led researchers to turn their interest in adsorbent materials with lower cost. In this review article, a new term will be introduced, which is called “green adsorption”. Under this term, it is meant the low-cost materials originated from: (i) agricultural sources and by-products (fruits, vegetables, foods); (ii) agricultural residues and wastes; (iii) low-cost sources from which most complex adsorbents will be produced (i.e., activated carbons after pyrolysis of agricultural sources). These “green adsorbents” are expected to be inferior (regarding their adsorption capacity) to the super-adsorbents of previous literature (complex materials as modified chitosans, activated carbons, structurally-complex inorganic composite materials etc.), but their cost-potential makes them competitive. This review is a critical approach to green adsorption, discussing many different (maybe in some occasions doubtful) topics such as: (i) adsorption capacity; (ii) kinetic modeling (given the ultimate target to scale up the batch experimental data to fixed-bed column calculations for designing/optimizing commercial processes) and (iii) critical techno-economical data of green adsorption processes in order to scale-up experiments (from lab to industry) with economic analysis and perspectives of the use of green adsorbents.

A conductance probe for measuring liquid fraction in pipes and packed beds

Al~tract--The performance of ring electrodes measuring the conductance of gas--liquid mixtures in pipes and packed beds is studied experimentally and theoretically. With relatively closely spaced pairs of rings, one can detect liquid segregation, i.e. stratified and annular distribution, as well as a uniform liquid distribution if the mean liquid fraction is available. Conversely, reliable cross-sectionally-averaged holdup data can be obtained if the liquid distribution pattern is known. Measurements in packed beds and pipes are qualitatively very similar. Existing theoretical expressions are employed to interpret the conductance measurements. Moreover, a new analytical solution for an annular liquid distribution is presented, which is particularly helpful in assessing the spatial probe sensitivity in the axial direction. The latter is found to be quite satisfactory. In general, agreement between the data and theoretical predictions is fair to excellent, providing the necessary confidence for practical applications. Two successful applications of the technique to packed beds are reported.

Reciprocating flow regeneration of soot filters

Particulate filter systems will most likely be required for compliance of diesel-powered vehicles and equipment to future emission legislation. The major issue with such systems is that their reliable and cost-effective regeneration (through oxidation of the collected soot particles) is not currently possible under all engine operating conditions without additional external thermal energy. In the present work we exploit the autothermal properties of the reverse flow reactor and study its application for the improvement of the regeneration process of diesel particulate filters. The reciprocating flow regeneration of soot filters is investigated with the aid of a mathematical model for the regeneration process, after the model has been assessed against conventional experimental regeneration data. The numerical results confirm the capability of the new technique to provide a clean filter where conventional regeneration fails. In this way the operating limits of current regeneration techniques (thermal or catalyst-assisted) can be extended substantially, and the stage is set for the construction of an industrial prototype.

Evolution of aggregate size and fractal dimension during Brownian coagulation

Fractal aggregate coagulation is described within a general framework of multivariate population dynamics. The effect of aggregate morphology on the coagulation rate, is taken into account explicitly, introducing in addition to aggregate particle size, the aggregate fractal dimension, as a second independent variable. A simple constitutive law is derived for determining the fractal dimension of an aggregate, resulting from a coagulation event between aggregates with different fractal dimensions. An efficient Monte Carlo method was implemented to solve the resulting bivariate Brownian coagulation equation, in the limits of continuum and free molecular flow regimes. The results indicate that as the population mean fractal dimension goes from its initial value towards its asymptotic value, the distribution of fractal dimension remains narrow for both flow regimes. The evolution of the mean aggregate size in the continuum regime is found to be nearly independent of aggregate morphology. In the free molecular regime however, the effects of aggregate morphology, as embodied in its fractal dimension, become more important. In this case the evolution of the aggregate size distribution cannot be described by the traditional approach, that employs a constant fractal dimension.

Multi-channel simulation of regeneration in honeycomb monolithic diesel particulate filters

In recent years advanced computational tools of diesel particulate &lter (DPF) regeneration have been developed to assist in the systematic and cost-e5ective optimization of next generation particulate trap systems. In the present study, we employ a previously validated, state-of-the-art multichannel DPF simulator to study the regeneration process over the entire spatial domain of the &lter. Particular attention is placed on identifying the e5ect of inlet cones and boundary conditions, &lter can insulation and the dynamics of “hot spots” induced by localized external energy deposition. Lateral heat losses through the insulation and the periphery of the &lter can, as captured by the magnitude of the Nusselt number,Nu, are detrimental to the e5ectiveness of the regeneration process. A &lter can Nu number less than 10 and preferably less than 5 is a good design target for high regeneration e=ciency. For the case studied, insulation of the inlet cones can lead to a gain of 30% in regeneration e=ciency by eliminating radial temperature gradients at the inlet &lter face. The multichannel simulator provides an instructive illustration of the well-appreciated e5ects of localized hot spot on &lter regeneration: hot spots play a more signi&cant role (spread over) when located near the entrance of the &lter. ? 2003 Elsevier Ltd. All rights reserved.

On the steady-state size distribution of dispersions in breakage processes

Breakage processes are considered in the absence of agglomeration or coagulation. A new method is proposed, based on a population balance type of formulation, applicable to systems (such as dispersions) that may be characterized by a maximum stable particle size. In this method, considerable simplification is achieved by means of a transformation that effectively eliminates the breakage frequency, thus allowing the convenient computation of steady state through solution of an integral equation. To compute the steady state, apart from the maximum size and the breakage kernel, only an estimate of the initial distribution is required. Two functional forms of binary breakage kernels which can represent a large variety of possible breakage mechanisms are proposed (by an appropriate selection of parameter values). For the sake of completeness, analytical solutions are also presented for several, relatively simple kernels. Finally, a study is made to assess the influence of initial conditions on the steady-state size distribution, which is helpful in tackling the inverse problem of determining the breakage kernel using limited experimental data.

Relating Interactions of Dye Molecules with Chitosan to Adsorption Kinetic Data

The scope of the present work is the study of the adsorption behavior of two dyes of different nature/class on several chitosan derivatives. The adsorbents used were grafted with different functional groups (carboxyl, amido, sulfonate, N-vinylimidazole) to increase their adsorption capacity and cross-linked to improve their mechanical resistance. This complete kinetic analysis was realized at 25, 45, and 65 degrees C to observe the effect of temperature on adsorption rates for each adsorbent-adsorbate system. Activated carbon was also used as an adsorbent for reference/comparison. The experimental equilibrium data were successfully fitted to the Langmuir-Freundlich (L-F) isotherms, presenting high correlation coefficients (R(2) approximately 0.998). A detailed pore-surface diffusion with local adsorption-desorption model has been developed to describe the adsorption kinetics in chitosan adsorbents. The existence of kinetic data in several temperatures assists in recognizing the diffusion mechanism in the adsorbent particles. The findings on diffusion mechanisms and the corresponding coefficients, from using the model to match the experimental data, are compatible with the expected adsorbent-dye interactions based on their chemical structure.

N-(2-Carboxybenzyl) grafted chitosan as adsorptive agent for simultaneous removal of positively and negatively charged toxic metal ions.

A chitosan material, cross-linked and grafted with N-(2-carboxybenzyl) groups, was evaluated as adsorbent to sufficiently remove both positively charged ions (Cu(II), Ni(II)) and negatively charged ones (Cr(VI), As(V)) from aqueous solutions. After complete characterization study with FTIR, SEM and BET analysis, the evaluation of adsorption was done with experiments at pH range 2-12 and then varying the initial concentrations of ions. Langmuir-Freundlich equation was used to fit the adsorption data and showed maximum adsorption capacities 308 mg/g at 25 °C for copper and 381 mg/g for nickel ions. The adsorption capacity of As(V) was 208 mg/g and 175 mg/g for Cr(VI). Also, a mechanistic (phenomenological) model of adsorption kinetics is employed to fit the kinetic data found using parameters with physical meaning, and clearly understand the process dynamics. The regeneration of adsorbents was presented in ten cycles of reuse (adsorption-desorption), showing the strong reuse potential of the adsorbent used.

An assessment of low-order methods for solving the breakage equation

The population balance is a necessary vehicle for modeling processes that involve size reduction due to fragmentation (grinding) of solid particles. The online control and optimization of such processes require efficient algorithms for the numerical solution of the breakage equation. However, the plethora of available methods, relying on discretization of the integrodifferential breakage equation, is characterized by computational inefficiency in tackling complicated spatially dependent problems. For such cases, the method of moments, which transforms the continuous breakage equation to one with a few degrees of freedom, seems to be advantageous. The accuracy of several versions of the method of moments is examined in the present work by comparing them with analytical solutions of the breakage equation for typical cases. The results of this work allow the selection of the best method for a particular problem and the a priori estimation of the error associated with the use of a specific method. Finally, by revealing the weaknesses of the existing methods, the present results set the basis for pursuing improvements.

Performance of a double drum dryer for producing pregelatinized maize starches

The response of an industrial scale double drum dryer to variation of steam pressure, drums rotation speed and level (height) of the gelatinization pool between the drums is presented. To our knowledge, this is the first time that the gelatinization pool level is treated as an input variable. The output variables are the products moisture content, mass flow rate and specific load (equivalent to the products film thickness). The effect of the drum surface temperature and width of the gap between the drums on the behavior of the output variables is examined. A theoretical analysis is presented for the qualitative assessment of the basic process variables that control the film thickness of the product. The role of the thermal inertia of the drum wall to the response of the dryer is discussed. Changes in the thermal efficiency of the dryer are inferred from overall heat transfer coefficients.