G.P. Sakellaropoulos

Reduction of CO2 emissions by a membrane contacting process

A membrane-based gas–liquid contacting process was evaluated in this work for CO2 removal from flue gases. The absorption of CO2 from a CO2–N2 mixture was investigated using a commercial hollow fiber membrane contactor and water or diethanolamine as absorbing solvents. Significant CO2 removal (up to 75%) was achieved even with the use of pure water as absorbent. By using aqueous amine solutions and chemical absorption, mass transfer improved, and CO2 removal was nearly complete (∼99%). A mathematical model was developed to simulate the process and it was validated with experimental data. Results show that membrane contactors are significantly more efficient and compact than conventional absorption towers for acid gas removal.

As(V) removal from aqueous solutions by fly ash

Abstract The present work examines the possible use of fly ash, a by-product of coal power stations, as a means of removing arsenic (V) from water, or equivalently, of restricting its movement in the solid wastes or the soil. Kinetic and equilibrium experiments were performed in order to evaluate the removal efficiency of lignite-based fly ash. Both adsorption and desorption experiments were done at three pH levels, namely 4, 7 and 10. The results indicated that arsenic can be removed from water by fly ash, yet the degree of removal depended markedly on the pH. Removal at pH 4, as demonstrated by the adsorption isotherms, was significantly higher than that at the other two pH values. For 80% removal of arsenic, the solid phase concentration at pH 4 was up to 4 times greater than that at the other two pH levels. During the desorption studies only a small amount of the pre-adsorbed arsenic was released into the water. This amount was practically independent of the initial fly ash loading. This indicates that adsorption of arsenic on fly ash is almost irreversible and, therefore, there are good prospects for arsenic fixation on fly ash in practical applications.

Gas permeation through PSF-PI miscible blend membranes

The permeation rates of He, H2, CO2, N2 and O2, are reported for a series of miscible polysulfone-polyimide (PSF-PI) blend membranes synthesized in our laboratory. For gases which do not interact with the polymer matrix (such as He, H2, N2 and O2), gas permeabilities in the miscible blends vary monotonically between those of the pure polymers and can be described by simple mixture equations. In the case of CO2, which interacts with PI, blend permeabilities decrease somewhat, compared to pure PSF and PI. This, however, is accompanied by a two-fold improvement in the critical pressures of plasticization vs. polyimide. Permselectivities of CO2N2 and H2CO2 in the blends deviate from mixing theory predictions, in contrast to selectivities of gas pairs which do not interact with PI. Differential scanning calorimetry measurements of pure and PSF/PI blend membranes show one unique glass transition temperature, supporting the miscible character of the PSF/PI mixture. Optical micrographs of the blend membranes clearly indicate perfect homogenization and no phase separation. Frequency shifts and absorption intensity changes in the FTIR spectra of the blends, as compared with those of the pure polymers, indicate mixing at the molecular level. This compatibility in mixing PSF and PI, results essentially in a new blend polymer material, suitable for the preparation of gas separation membranes. Such membranes combine satisfactory gas permeation properties, reduced cost, advanced resistance to harsh chemical and temperature environments, and improved tolerance to plasticizing gases.

Kinetic studies of elemental mercury adsorption in activated carbon fixed bed reactor

Activated carbons are suitable materials for Hg(0) adsorption in fixed bed operation or in injection process. The fixed bed tests provide good indication of activated carbons effectiveness and service lives, which depend on the rates of Hg(0) adsorption. In order to correlate fixed bed properties and operation conditions, with their adsorptive capacity and saturation time, Hg(0) adsorption tests were realized in a bench-scale unit, consisted of F400 activated carbon fixed bed reactor. Hg(0) adsorption tests were conducted at 50 degrees C, under 0.1 and 0.35 ng/cm(3) Hg(0) initial concentrations and with carbon particle sizes ranging between 75-106 and 150-250 microm. Based on the experimental breakthrough data, kinetic studies were performed to investigate the mechanism of adsorption and the rate controlling steps. Kinetic models evaluated include the Ficks intraparticle diffusion equation, the pseudo-first order model, the pseudo-second order model and Elovich kinetic equation. The obtained experimental results revealed that the increase in particle size resulted in significant decrease of breakthrough time and mercury adsorptive capacity, due to the enhanced internal diffusion limitations and smaller external mass transfer coefficients. Additionally, higher initial mercury concentrations resulted in increased breakthrough time and mercury uptake. From the kinetic studies results it was observed that all the examined models describes efficiently Hg(0) breakthrough curves, from breakpoint up to equilibrium time. The most accurate prediction of the experimental data was achieved by second order model, indicating that the chemisorption rate seems to be the controlling step in the procedure. However, the successful attempt to describe mercury uptake with Ficks diffusion model and the first order kinetic model, reveals that the adsorption mechanism studied was complex and followed both surface adsorption and particle diffusion.

SIMULATION OF MULTICOMPONENT GAS SEPARATION IN A HOLLOW FIBER MEMBRANE BY ORTHOGONAL COLLOCATION — HYDROGEN RECOVERY FROM REFINERY GASES

Abstract Modeling of hollow fiber asymmetric membranes can provide useful guidelines to achieve desirable separations of multicomponent gas mixtures. Especially in cases of high commercial interest, such as hydrogen recovery from refinery streams, the accurate prediction of membrane separation performance is important. In this work, the appropriate model equations are solved by orthogonal collocation to approximate differential equations, and to solve the resulting system of non-linear algebraic equations by the Brown method. This technique is applied for the first time in a multicomponent gas separation by hollow fiber membranes and offers minimum computational time and effort, and improved solution stability. The predictions of the mathematical model are compared with experimental results for the separation and recovery of hydrogen from a typical gas oil desulfurization unit for various feed pressures, temperatures and stage cuts. In general, there is a very good agreement between simulation and experimental results. Further application of the developed mathematical model to various refinery gas streams of interest reveals that high permeate purity (99.95+), and high recovery (0.6–0.9), can be achieved even in a one-stage membrane unit. The reported experimental results and the theoretical analysis demonstrate the potential which polymer membrane technology has for the separation of hydrogen from refinery gas streams.

Influence of ozonation on the in vitro mutagenic and toxic potential of secondary effluents

Reclamation of municipal effluents by advanced treatment processes is an attractive perspective for facing certain water shortage problems. However, the application of tertiary techniques should be thoroughly examined for their potential hazardous effects. Ozonation is an efficient chemical oxidation method, often used in wastewater reclamation, which may result in by-products that may alter the toxic and mutagenic properties of effluents. In this study, Ames test and Microtox test were used for the evaluation of ozonation efficiency to upgrade secondary effluents quality. In general, the toxic response and mutagenic effect without metabolic activation of test species were influenced mainly by the ozone dose and ozonation duration, whereas the mutagenic effect with metabolic activation was influenced mainly by ozone dose, indicating that ozone conditions strongly affect the formation of by-products. In most cases, the toxicity was increased and reached up to 100% (in relation to that of secondary effluent) after ozonation with 8.0 mg O3/L for 5 min. On the contrary, in most cases the mutagenic activity towards strain TA98 without metabolic activation was reduced, when ozone dose and contact time increased. However, the mutagenicity was also increased after ozonation at low ozone doses and for contact times less than 5 min. The mutagenic activity of treated effluents towards strain TA98 with metabolic activation remained about the same or was reduced, compared to that of secondary effluent, and was even eliminated after ozonation with 8.0 mg O3/L for contact times higher than 5 min.

The effect of mineral matter and pyrolysis conditions on the gasification of Greek lignite by carbon dioxide

Coal specimens with different mineral matter contents were produced from Greek lignite using various acid treatment conditions. Ash content and chemical composition of mineral matter depended on the type of acid used and the sequence of treatment stages. Gasification rates of coals were investigated by thermogravimetric analysis in a carbon dioxide atmosphere in the temperature range 700–900°C. The combined effects of inorganic constituents and carbonization conditions such as heating rate and final temperature were determined. Gasification rates of chars with high ash content were higher than those of similarly prepared char with low ash content, due to the presence of catalytically active inorganic constituents. An almost proportional increase of gasification rate with Mg concentration was found, but such correlation was not evident for Ca, Na and K, possibly due to the chemical form of these elements in the organic structure. Slow carbonization led to the production of chars with higher reaction rates than those of chars prepared by rapid carbonization. The gasification rate increased with the concentration of CO2 in the reaction gases. The effects of heating rate and CO2 concentration on char gasification rate were more pronounced for samples from untreated lignite than for those from acid-washed lignite.

Mineral matter effects in lignite gasification

Greek lignite samples with different mineral contents were gasified after drying, without any other treatment, in H2 and CO2 atmospheres, in order to study mineral matter effects on lignite gasification. Tests were performed in a fixed-bed reactor operating at ambient pressure. The alkali index (AI) was calculated for each lignite sample in an effort to correlate it with the gasification rate. The inorganic constituents in Greek lignite seem to play a controlling role in determining gasification reactivity. A reasonably good correlation exists between the alkali index and the gasification rate, which, in the studied region of alkali indices, varied almost linearly with the alkali index. For both H2 and CO2, gasification rate increased proportionately to Ca concentration, but such correlation was not evident for Na and K, possibly due to the low content and the chemical form of these elements in the organic structure. An initial increase and a subsequent decrease of gasification rate with Mg concentration was found for both H2 and CO2 gasification. As for the Fe, hydrogasification rate seems to be almost unaffected (a slight decrease is actually observed with Fe concentration), while no correlation was evident for CO2 gasification. D 2002 Elsevier Science B.V. All rights reserved.

Comparative study of phenol and cyanide containing wastewater in CSTR and SBR activated sludge reactors.

The objectives of this work were the examination of the performance of two bench scale activated sludge systems, a conventional Continuous Stirring Tank Reactor (CSTR) and a Sequential Batch Reactor (SBR), for the treatment of wastewaters containing phenol and cyanides and the assessment of the toxicity reduction potential by bioassays. The operation of the reactors was monitored by physicochemical analyses, while detoxification potential of the systems was monitored by two bioassays, the marine photobacterium Vibrio fischeri and the ciliate protozoan Tetrahymena thermophila. The reactors influent was highly toxic to both organisms, while activated sludge treatment resulted in the reduction of toxicity of the influent. An increased toxicity removal was observed in the SBR; however CSTR system presented a lower ability for toxicity reduction of influent. The performance of both systems was enhanced by the addition of powdered activated carbon in the aeration tank; activated carbon upgraded the performance of the systems due to the simultaneous biological removal of pollutants and to carbon adsorption process; almost negligible values of phenol and cyanides were measured in the effluents, while further toxicity reduction was observed in both systems.

An investigation on the physical, chemical and ecotoxicological characteristics of particulate matter emitted from light-duty vehicles

Particulate matter (PM) emitted from three light-duty vehicles was studied in terms of its physicochemical and ecotoxicological character using Microtox bioassay tests. A diesel vehicle equipped with an oxidation catalyst emitted PM which consisted of carbon species at over 97%. PM from a diesel vehicle with a particle filter (DPF) consisted of almost equal amounts of carbon species and ions, while a gasoline vehicle emitted PM consisting of approximately 90% carbon and approximately 10% ions. Both the DPF and the gasoline vehicles produced a distinct nucleation mode at 120 km/h. The PM emitted from the DPF and the gasoline vehicles was less ecotoxic than that of conventional diesel, but not in direct proportion to the emission levels of the different vehicles. These results indicate that PM emission reductions are not equally translated into ecotoxicity reductions, implying some deficiencies on the actual environmental impact of emission control technologies and regulations.