Scielo RSS http://bibliotecadigital.uns.edu.ar/rss.php?pid=0327-079320120033&lang=es vol. 42 num. 3 lang. es http://bibliotecadigital.uns.edu.ar/img/en/fbpelogp.gif http://bibliotecadigital.uns.edu.ar http://bibliotecadigital.uns.edu.ar/scielo.php?script=sci_arttext&pid=S0327-07932012003300001&lng=es&nrm=iso&tlng=es A Fe-Mn mixed oxide catalyst for ethanol combustion was optimized using different Fe:Mn atomic ratios and varying the calcination temperature (500°C, 650°C and 800°C). The results indicated that the increase of the calcination temperature caused a decrease of the catalytic activity due to a strong decrease of the specific surface area. Then, among the catalysts calcined at 500°C, the most active one was the catalyst with lower amount of iron. The results of X-ray diffraction (XRD) and temperature programmed reduction (TPR) indicated that a small amount of iron in the Fe-Mn mixed catalyst favored the formation of dispersed surface amorphous MnOx species. These species are easily reducible and very active in total oxidation reactions. http://bibliotecadigital.uns.edu.ar/scielo.php?script=sci_arttext&pid=S0327-07932012003300002&lng=es&nrm=iso&tlng=es Lignins were precipitated from Kraft black liquor to different acidic conditions; subsequently were subjected to oxidative treatment with KMnO4. The oxidized material was characterized using IR spectroscopy for structural elucidation, and potentiometry in aqueous medium for determination of carboxylic groups. Among the trends, it found that the proportion of carboxylic groups in lignins increases with the oxidative treatment of the lignin. In addition with the IR were identified functional groups characteristic of lignin and showed also the changes caused by oxidative modification confirming the increase of oxidized groups when comparing the spectra of the different materials. Known the effects of oxidation on the lignins was evaluated adsorptive capacity of the oxidized lignin, showing that the oxidized lignin was able to adsorb up to 98% of Ni available initially, which corresponds to an increase in this ion sorption capacity of the double with respect to which only was precipitated. For its part, the lignin used to adsorb V decreased its adsorptive capacity, reaching only 19% of V fed. Based on this result was improved the experimental method of adsorption and the pH of process was controlled, thus it was obtained an increase of the adsorptive capacity of the material, reaching an adsorption of 91% of V fed. These results agree with the proposed mechanisms on how the metal ions studied in this work can be adsorbed. http://bibliotecadigital.uns.edu.ar/scielo.php?script=sci_arttext&pid=S0327-07932012003300003&lng=es&nrm=iso&tlng=es Activated carbons are universal adsorbents that can be obtained from a wide variety of raw materials, and have been found to be very effective for mercury removal from water. This contribution presents a comparative adsorption in column study of activated carbons obtained by physical activation of carbonized eucalyptus wood with water vapour, as adsorbents of mercury in liquid phase. Two samples were treated with sulphuric acid and with carbon disulphide, in order to introduce sulphur as a heteroatom onto the adsorbent surface. Adsorption in column assays was performed and the breakthrough curves were used to compare their performance. The sulphurization treatments produce important variations on the textural properties of the solid, on the zero charge point, and on the number of acid and basic surface groups. From the analysis of these factors, the dependency of the adsorption capacity of a solid with the pH of the solution can be explained. http://bibliotecadigital.uns.edu.ar/scielo.php?script=sci_arttext&pid=S0327-07932012003300004&lng=es&nrm=iso&tlng=es The equilibrium moisture content of malting barley grains (cv. Scarlett) was determined by gravimetric-static method in the range 20-50°C using saturated salt solutions (LiCl, MgCl2.6H2O, K2CO3, NaBr, NaNO2, KI, NaNO3, NaCl, (NH4)2SO4) to provide water activities for each temperature between 0.11 and 0.813. Desorption isotherms were obtained at 20, 30, 40, and 50°C. All the sorption curves were found to be Type II, increasing moisture content with the increase of water activity from approximately 5.5-7.45% d.b. (dry basis) for different temperatures when the water activity was 0.111. Different models (Modified Henderson, Modified Chung-Pfost, Modified Halsey, Modified Oswin and GAB) were fit to the data and it was found that the best results corresponded to the Modified Henderson equation. The isosteric sorption heat varied from 3479 kJ/kg (20°C) to 2393 kJ/kg (50°C) decreasing with increasing moisture content, thus indicating that reducing the moisture content increases the energy required to remove water. http://bibliotecadigital.uns.edu.ar/scielo.php?script=sci_arttext&pid=S0327-07932012003300005&lng=es&nrm=iso&tlng=es This work presents a methodology for control structure design based on the Singular Value Decomposition (SVD) of the Hankel matrix, constructed from the observability and controllability matrices. A weight index is proposed as a measure of the impact of each input and output variable and, based on it, an input-output pairing selection is performed. The methodology is validated using the Tennessee Eastman process and the result is compared to the relative gain method in frequency. http://bibliotecadigital.uns.edu.ar/scielo.php?script=sci_arttext&pid=S0327-07932012003300006&lng=es&nrm=iso&tlng=es The capability of first principles models and neural networks for predicting the main state variables (biomass and substrate concentrations) in a novel depollution bioprocess has been tested. Experimental data recorded from batch sequential cultures of anaerobic bacteria and yeast to transform organic nitrogen and carbonaceous substrates into useful feed material were used to train the net and validate the first principle model. Both modeling approaches were tested for a number of experiments carried out under different conditions (maximum growth rate cultures, high pH conditions and starving nutrient conditions). The results indicate that the performance of a simple well-trained neural network model was equivalent or better than the first principles model but showed some limitations for providing insight into the mechanism governing the bioprocess. Limitations of both modeling approaches are finally discussed. http://bibliotecadigital.uns.edu.ar/scielo.php?script=sci_arttext&pid=S0327-07932012003300007&lng=es&nrm=iso&tlng=es The Shortest-path problem with time-windows and capacity constraints (SPPTWCC) is a problem used for solving vehicle-routing and crew-scheduling applications. The SPPTWCC occurs as a sub-problem used to implicitly generate the set of all feasible routes and schedules in the column-generation formulation of the vehicle routing problem with time windows (VRPTW) and its variations. The problem is NP-hard in the strong sense. Classical solution approaches are based on a non-elementary shortest-path problem with resource constraints using dynamic-programming labeling algorithms. In this way, numerous label-setting algorithms have been developed. Contrarily to this approach and with the aim to obtain elemental and optimal solutions, we propose a new mixed integer-linear formulation to the SPPTWCC. Some valid inequalities that can be used to strengthen the linear relaxation of the SPPTWCC are also proposed. Numerical experiments on some VRPTW instances taken from Solomon's benchmark problems show that (near) optimal solutions are easily obtained in spite of the considerable problem size. Also the number of generated columns is kept at a very low level. http://bibliotecadigital.uns.edu.ar/scielo.php?script=sci_arttext&pid=S0327-07932012003300008&lng=es&nrm=iso&tlng=es In this paper the numerical results of temperature profiles and flow patterns for natural convection in a two-dimensional partially open tilted cavity are studied. The cavity holds the opposite wall to the aperture at a constant temperature, while the remaining walls are thermally insulated. The most important assumptions in the mathematical formulation are: the flow is laminar, two-dimensional and the Boussinesq approximation is valid. The results are obtained for Rayleigh numbers from 10(6) to 10(8) and inclination angles from 90° to 150°, for the steady state. The comparison between the Nusselt numbers of the fully open cavity and partially open cavity, indicates that the greatest difference is 55.4% (Ra=10(8) and 135°) and the smallest difference is 4.6% (Ra=10(6) and 90°), with an average percentage difference of 29%. http://bibliotecadigital.uns.edu.ar/scielo.php?script=sci_arttext&pid=S0327-07932012003300009&lng=es&nrm=iso&tlng=es The photocatalytic degradation of phenol was studied using slurry and immobilized TiO2 as catalysts in order to compare the specific reaction rate constants, k s and k p. Losses from phenol evaporation, the time necessary to reach adsorption equilibrium, irradiation effects and photolysis rates were quantified. k s was determined under rate-optimizing operating conditions, and k p was determined using the amount of immobilized catalyst mass that gave the maximum pollutant degradation rates. k s was two times larger than k p when the calculations were made on catalyst mass basis. Experiments performed to study catalyst deactivation showed that k p was approximately one-half of its initial value after 18 hours of phenol photodegradation. Also, among the photocatalytic degradation reactions of rhodamine B, tetracycline and phenol, the ratio of k s to k p was between 2.1 and 5.3 when the calculations were made on catalyst mass basis. http://bibliotecadigital.uns.edu.ar/scielo.php?script=sci_arttext&pid=S0327-07932012003300110&lng=es&nrm=iso&tlng=es A finite element formulation to deal with the friction contact problem between an elastic body and a rigid foundation is presented. An augmented Lagrangian method, which incorporates two Lagrange multipliers and a slack variable in the constraint equation, is developed. In this method, the solution is achieved through a Newton-Raphson monolithic iterative strategy and leads to simple implementation. Examples are provided to demonstrate the accuracy of the proposed algorithm. http://bibliotecadigital.uns.edu.ar/scielo.php?script=sci_arttext&pid=S0327-07932012003300111&lng=es&nrm=iso&tlng=es Peracetic acid (PAA) is a powerful sanitizer used by food industries all over the world. The main disadvantage of PAA is its decomposition rate. The main purpose of this paper is to study the decomposition of PAA between 20 and 40 °C with 0.0, 2.5 or 5.0 g•L-1 of beer, milk or tomato juice in its solutions. The observed decomposition of PAA is a first-order reaction for solutions free of organic matter with observed rate constants from 1.24•10-3 to 5.59•10-3•h-1 and an activation energy (Ea) of 62.11 kJ•mol-1. However a pseudo first-order reaction was observed in the presence of organic matter and the rate constants between 1.13•10-3 and 2.18•10-2 h-1. The Ea calculated by the observed decomposition rate constants are 42.3 and 61.2 kJ•mol-1 when there was 2.5 and 5.0 g/L beer in solution, respectively. PAA solutions contaminated with 2.5 and 5.0 g•L-1 milk have showed Ea values between 9.7 and 55.3 kJ•mol-1, respectively. For tomato juice solutions, the Arrhenius equation slope was not constant. The different Ea obtained indicate that different mechanisms are taking place at the various tested operating conditions. This work also proposes a mathematical equation to estimate the PAA concentration for solutions free of organic matter. http://bibliotecadigital.uns.edu.ar/scielo.php?script=sci_arttext&pid=S0327-07932012003300112&lng=es&nrm=iso&tlng=es Based on a mechanism composed by 372 reversible chemical reactions among 56 reactive species for the oxidation of ethanol, we propose a reduction strategy to obtain a sixstep kinetic mechanism for the methanol and a seven-step mechanism for the ethanol. A threestep kinetic mechanism results for the carbon monoxide and two-step for the hydrogen. The reduction strategy consists of four steps: 1) estimate the order of magnitude of the rates of chemical reaction, 2) define the main chain, 3) apply the steady-state and partial equilibrium assumptions and 4) justify the assumptions by asymptotic analysis. The main advantage of the obtained reduced mechanisms is the de-crease of the work needed to solve the system of chemical equations. Such decrease is proportional to the order of the number of elementary reactions present in the complete mechanism. http://bibliotecadigital.uns.edu.ar/scielo.php?script=sci_arttext&pid=S0327-07932012003300113&lng=es&nrm=iso&tlng=es In the present study, a microwave-assisted extraction (MAE) technique has been developed for extraction of phenolic and flavonoid compounds from eucalyptus leaves. Various experimental conditions, such as solvent type, ethanol concentration, MAE time, liquid/solid ratio and pre-leaching time before MAE were examined to optimize the extraction. Among the solvents tested, 50% aqueous ethanol extracted the highest phenolic and flavonoid contents from eucalyptus leaves. The extraction of phenolic and flavonoid compounds with MAE for 5 min was equivalent with ultrasound-assisted extraction (UAE) (60 min) and traditional extraction (24 h) methods. Time of extraction at room temperature and UAE was about 288 and 12 times higher than the needed with MAE, respectively. Due to the considerable saving in time, MAE was more effective than the traditional and UAE methods. http://bibliotecadigital.uns.edu.ar/scielo.php?script=sci_arttext&pid=S0327-07932012003300114&lng=es&nrm=iso&tlng=es This research deals on a new low-cost porous sorbent named calcareous chitin (CaCh). CaCh is prepared by alkaline treatment of crustacean exoskeleton to produce a porous matrix of chitin and calcium carbonate free of original proteins. SEM-EDX, X-ray, FT-IR were used to characterize the material. CaCh structure is more porous than that of chitin. Adsorption studies demonstrated its capability to remove Cd(II) from waters. The Langmuir equation fitted the adsorption isotherms well. Due to its easy, rapid and low cost preparation and its adsorption characteristics calcareous chitin can be used as low cost sorbent of heavy metals ions like cadmium for wastewater treatment. http://bibliotecadigital.uns.edu.ar/scielo.php?script=sci_arttext&pid=S0327-07932012003300115&lng=es&nrm=iso&tlng=es Causticizing system is an important part of the recuperation cycle of the Kraft pulp production process. In this system, the Na2CO3 in green liquor from the recovery boiler is converted in NaOH which is a component of the digester cooking liquor. The system is composed by one extinction reactor, where CaO is added to green liquor, followed by three reactors. Three semi-empirical models of CENIBRA's causticizing system are proposed to simulate the stationary state. The mass balance is developed considering three types of reaction rate equations, whose coefficients were estimated using industrial data. The following assumptions are used: CSTR reactors, instantaneous slaking reaction, constant temperature and rate reactions as functions of NaOH and Na2CO3 concentrations. The premise is that all non-modeled factors and the system specificities are included in the estimated coefficients. The obtained models were validated using new industrial data and good agreement was achieved.