Scielo RSS <![CDATA[Latin American applied research]]> http://www.scielo.org.ar/rss.php?pid=0327-079320030002&lang=es vol. 33 num. 2 lang. es <![CDATA[SciELO Logo]]> http://www.scielo.org.ar/img/en/fbpelogp.gif http://www.scielo.org.ar <![CDATA[Optimisation of peroxidase adsorption on concanavalin A-agarose]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200001&lng=es&nrm=iso&tlng=es Many publications describe methods for peroxidase purification from plant material. When the goal was to obtain a high purity enzyme every purification method included an affinity chromatography step using Concanavalin A as the ligand. However, the adsorption step was carried out under quite different conditions with regard to pH, ionic strength and metal cation content in the binding buffer, thus leading to a rather confuse situation. To establish the best conditions for purification of peroxidases from horseradish root (HRP) and soybean hull (SBP), equilibrium adsorption isotherms were fitted to the Langmuir-type model, where ionic strength, pH and cation concentration were chosen as the variables. For SBP, our results showed that Kd rounded 10-6 M in all cases (pH 5.0 - 7.0, 1 and 5 mM Mn2+ / Ca2+, 0 - 0.75 M NaCl). For HRP, Kd ranged between 10-5 M and 10-6 M depending on the parameters. Under optimised binding conditions, 84.3% SBP was recovered after elution carried out with 0.74 M a -methyl-D-mannopyranoside and 0.37 M NaCl. For HRP, the recovery was lower (75%) and 0.36M a -methyl-D-mannopyranoside was necessary for the elution step. <![CDATA[Production of hydrogen employing Ni-Rh catalysts in membrane reactors]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200002&lng=es&nrm=iso&tlng=es Rh, Ni and Rh-Ni catalysts were prepared by wet impregnation of La2O3. The Rh content was modified but the Ni loading was kept constant at 2 wt%. The solids were calcined at 823 K in air flow and were reduced at 973 K in flowing hydrogen. The activity, stability, and residue formation during the CO2 reforming of CH4, of all the catalysts were tested in a fixed-bed reactor. Those with low carbon deposition were used in a Pd-Ag membrane reactor. The Rh solids showed the highest activity at 823 K. In order to understand the Rh effect, the catalysts were characterized through XRD and TPR. No evidence of crystalline Ni phase through XRD was observed in either the calcined or used catalysts. On the other hand, both the TPR and XPS results suggest a significant metal-support interaction. The bimetallic solids presented higher carbon deposition. The carbonaceous species formed on stream were characterized through thermal analysis. At least two types of carbons were detected on the catalyst surface with different structural properties which did not affect the catalytic activity. A low carbon deposition was observed on Rh/La2O3 which exhibited a stable activity after 100 h on stream. The monometallic Rh solids were tested in the membrane reactor, varying the pressure drop across the Pd-Ag alloy. A significant increase in reactant conversions was obtained when the membrane reactor was used. <![CDATA[Effect of the acid-base properties of Mg-Al mixed oxides on the catalysts deactivation during aldol condensation reactions]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200003&lng=es&nrm=iso&tlng=es The effect of chemical composition of Mg-Al mixed oxides on both the acid-base properties and the deactivation process during the gas phase self-condensation of acetone was studied. The activity and selectivity for acetone oligomerization depended on the catalyst acid-base properties. Mg-rich catalysts selectively yielded mesityl oxides whereas Al-rich Mg yAlOx oxides produced mainly isophorone. The initial deactivation rate, increased linearly with the density of surface basic sites, thereby suggesting that although Mg yAlOx oxides promote the self-condensation of acetone by both acid- and base-catalyzed mechanisms, the deactivation rate would be closely related to the surface basic properties. The Mg yAlOx activity declines in the acetone oligomerization reaction due to a blockage of both base and acid active sites by a carbonaceous residue formed by secondary reactions. The amount and the nature of the carbon deposits were characterized by temperature-programmed oxidation technique. Mg yAlOx and Al2O3 formed more and heavier coke than pure MgO but the latter deactivates faster. The deactivation rate and coke composition are defined by the nature of the active site involved in the coke-forming reactions at different catalyst compositions rather than by the carbon amount or polymerization degree. <![CDATA[Kinetic parameters for thermal inactivation of cut green beans lipoxigenase using unsteady-state methods]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200004&lng=es&nrm=iso&tlng=es Most raw vegetables can be stored for only a short time even at -20ºC. Blanching is the primary pre-freezing means of inactivating undesirable enzymes present in the vegetable. Lipoxygenase is widely distributed in vegetables and evidence is mounting to support its involvement in off-flavour development and colour loss. In order to optimize the blanching process of vegetables, it is essential to establish a kinetic model of the inactivation of the indicator enzyme. Isothermal and dynamic thermal approaches have been used to determine kinetic parameters. The unsteady-state procedure is more flexible and can be applied to uniform and non-uniform heating situations and generally a food medium, rather than an aqueous buffer solution, is always used for determining kinetic parameters. Kinetic parameters describing lipoxygenase inactivation during heating of cut green beans were determined using two unsteady-state procedures. The model used an analytical solution for heat conduction in a finite cylinder to predict time-temperature profiles, and a trial and error and a nonlinear regression of experimental lipoxygenase retentions to estimate kinetic parameters, rate constant, k and activations energy, Ea. Thermal diffusivity, a , and convective heat transfer coefficient, h, were experimentally determined, but thermal conductivity, l , was estimated. Mean values obtained, with standard deviations between parenthesis, were k76ºC = 27.2 (9.4) s-1, k82ºC = 92.9 (7.5) s-1; k88ºC = 212.1 (52.7) s-1; k94ºC = 407.8 (56.7) s-1; Ea = 160.7 (8.1) KJ/mol using the trial and error procedure; k85ºC = 150 (26.3) s-1 and Ea = 164 (4.7) KJ/mol using the nonlinear regression method. Predicted and observed lipoxygenase retentions showed good agreement. <![CDATA[Kinetic model of pectin demethylation]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200005&lng=es&nrm=iso&tlng=es Pectins are polysaccharides that act as a cellular binder in the peel of many different fruits and vegetables. An important feature of pectins is the esterification of the galacturonic acid residues with methanol. The degree of methylation is defined as the number of moles of methanol per 100 moles of galacturonic acid. The objective of the present work was to study the acid demethylation of apple pectin at different temperatures, by following changes in the degree of methylation. A solution of high methoxyl apple pectin was demethylated with concentrated HCl (pH = 0.5). The processes were carried out for 120 min at T = 80°C and 400 min at T = 65°C and T = 50°C. Anhydrogalacturonic acid content, degree of methylation and intrinsic viscosity as a function of reaction time were determined. Results showed pectin demethylation followed a second order kinetics and demethylation rate increased exponentially with temperature. Results also showed pectin "purity", as anhydrogalacturonic acid content, increased with demethylation. However, intrinsic viscosity reduced with reaction time. This behavior was associated with the hydrolysis of non-polyuronic material during the acid treatment, lowering in that way the average molecular weight. <![CDATA[Low resolution 1H-Pulsed NMR for sugar cristallization studies]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200006&lng=es&nrm=iso&tlng=es The stability of food and other labile biological systems containing sugars has been related to the amorphous characteristics of the matrices which they formed, being inversely related to the degree of crystallization. It is thus necessary to explore the applicability of simple, non-destructive and reliable methods to analyze sugar crystallization. Solid fat content is a well-stablished AOCS method to study solid content in lipid systems. However, there are no literature reports on the use of this method to analyze sugar crystallization. Crystallization kinetics of concentrated trehalose and trehalose/salts solutions was followed by proton pulse nuclear magnetic resonance (¹Hp -NMR). Three different concen-trations (X T= 0.60, 0.63 and 0.66) of trehalose solutions were crystallized to 25, 20, 15, 10 and 5°C and the degree of crystallization was investigated by following the index of solids with time. Crystallization rate was determined by a combined effect of supercooling and molecular diffusion. By analyzing trehalose systems by ¹H p-NMR, it was possible to confirm the effect of divalent cations on retarding sugar crystallization. <![CDATA[Supercritical extraction of solid matrices. Model formulation and experiments]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200007&lng=es&nrm=iso&tlng=es Nowadays, supercritical extraction of solid matrices is used frequently to obtain valuable substances especially by food and pharmaceutical industries. Most often, solid particles are arranged in a fixed bed configuration and the supercritical fluid flows through the bed extracting the desired solute. In this study, a mathematical model is set to simulate this process. The influence on the extraction yield of processing variables such as temperature, pressure and flow velocity as well as some process parameters like particle size, void fraction and bed height are predicted by the model. This model was used to simulate the extraction of essential oil from clove buds with supercritical CO2. Experiments were carried out at different pressures (90-120 bar) at a fixed temperature and CO2 flow. A good agreement between experimental and theoretical results was obtained. <![CDATA[Polyethylenes modified by irradiation and organic peroxide treatment: rheological study]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200008&lng=es&nrm=iso&tlng=es A series of polyethylenes was obtained by modification of two commercial high-density polyethylenes. The organic peroxide concentrations and irradiation dosis used are below the critical ones that produce molecular networks. The molecular weights of the polymers increase, and the molecular weight distribution gets wider, as the concentration/dose increases. These results are a consequence of the large molecular weight molecules generated during the chain-linking modification processes. By the time traces of gel begin to appear in the samples, the peroxide modified materials display larger molecular weights and smaller vinyl concentrations than the irradiated ones. The rheological behavior of these materials is analyzed as a function of the molecular structure of the polymers and the concentration/dose used in the modification processes. All the modified polymers show a complex thermo-rheological behavior associated to the presence of long-branched macromolecules. The peroxide treatment generates the polymers with the largest rate of change of flow activation energy. <![CDATA[Emulsion copolymerization of acrylonitrile and butadiene in an industrial reactor: Mathematical modeling, estimation, and control of polymer quality variables on the basis of calorimetric measurements]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200009&lng=es&nrm=iso&tlng=es An industrial emulsion copolymerization of acrylonitrile and butadiene carried out in a batch (or semibatch) reactor for the production of nitrile rubber (NBR) is investigated. The effect of variable amounts of deactivating impurities on the predictions of a global polymerization model is analyzed; and the advantages of using the model predictions in combination with on-line calorimetric measurements are shown. The adjusted polymerization model allows to adequately monitor main quality variables of the process, such as: monomer conversion, average copolymer composition, average molecular weights, and degree of branching. The semibatch addition of the comonomers and/or the chain transfer agent along the reaction was investigated with the aim of controlling the polymer quality characteristics and increasing the productivity. It is possible to obtain a uniform-composition copolymer with pre-specified profiles of the average molecular weights or the degrees of branching. Furthermore, the NBR productivity can be increased by about 5%, without deteriorating the final copolymer quality with respect to that produced in the more conventional batch operation. <![CDATA[Optimal design of stable processes]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200010&lng=es&nrm=iso&tlng=es Open loop dynamic stability is a major feature of operability. An optimization approach for open loop stable process design is presented. It is based on Lyapunov's stability theory and formulated as an eigenvalue optimization problem. The resultant non-linear semi-definite programming problem is reformulated into an interior-point / logarithmic-barrier- transformation programming problem. The proposed methodology is applied to the design of a three states stirred tank reactor. <![CDATA[An optimal approach to the multiple-depot heterogeneous vehicle routing problem with time window and capacity constraints]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200011&lng=es&nrm=iso&tlng=es The vehicle routing problem (VRP) has become a crucial industrial issue for its impact on product distribution costs. Though quite important in practice, the time-constrained version of the VRP accounting for several types of vehicles and m- depots, called the extended VRP with time windows (m-VRPTW), has received less attention. Since it is an NP-hard problem, most of the current approaches to m-VRPTW are heuristic, thus providing good but not necessarily optimal solutions. This work presents a novel MILP mathematical framework for the m-depot heterogeneous-fleet VRPTW problem. The new optimization approach permits to find both the optimal vehicle route/schedule and the fleet size by choosing the best set of preceding nodes for each pick-up point. To get a significant reduction on the problem size to tackle larger m-VRPTW problems, some elimination rules have been embedded in the MILP framework. When applied to a pair of examples, it was observed a remarkable saving in computer costs with regards to prior VRPTW optimization methods. <![CDATA[Model reduction and identification of wastewatertreatment plants - A subspace approach]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200012&lng=es&nrm=iso&tlng=es In this paper, a low-order linear time-invariant (LTI) state-space model that describes the nitrate concentrations in both anoxic and aerobic zones of an activated sludge wastewater treatment plant (WWTP), for biological treatment of municipal sewage, is identified around a given operating point (a model with lumped parameters). Several subspace identification methods, such as CCA, N4SID, MOESP and DSR are applied and their performance are compared, based on performance quality criteria, in order to select the best-reduced model. The selected model is validated with a data set not used in the identification procedure and it describes well the complex dynamics of the process. This model is asymptotically stable and it can be used for control, optimization, prediction and monitoring purposes. In this work the ASWWTP-USP benchmark is used as a data generator. This benchmark simulates the biological, chemical and physical interactions that occur in a complex activated sludge plant. <![CDATA[A hybrid methodology for optimization of multi-stage flash-mixer desalination systems]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200013&lng=es&nrm=iso&tlng=es In this paper, a new strategy involving an evolutionary algorithmic procedure for the optimization of Multiple Stage Flash (MSF-M) Systems is presented. A "detailed model" of an MSF-M System is developed according to rigorous material, momentum and energy balances for each stage. The model of the MSF-M System is represented as a complex NLP, which incorporates a high number of nonlinear constraints that difficult the global optimum determination. Here we present a hybrid methodology that uses optimal solutions obtained from a "thermodynamic method" to find the "economic global optimal solution". A pre-processing stage (solving successive NLPs) is used to initialize the final NLP problem. A Case Study and a discussion of the results are presented. <![CDATA[Use of wiener nonlinear MPC to control a CSTR with multiple steady state]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200014&lng=es&nrm=iso&tlng=es In this paper a Nonlinear Model Predictive Control based on a Wiener Model with a Piecewise Linear gain is presented. The major advantages of this algorithm is that it retains all the interesting properties of the classical linear MPC and the computations are easy to solve due to the canonical structure of the nonlinear gain. The proposed control scheme is applied to a nonlinear CSTR that presents multiple steady states. <![CDATA[Dynamic simulation of high-index models of batch distillation processes]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200015&lng=es&nrm=iso&tlng=es The dynamic modeling of batch distillation columns frequently leads to a mixed system of differential and algebraic equations (DAEs) with differential index greater than one, and this particular feature has many implications on the resolution strategy adopted. As the number of stages and components can be arbitrarily high, those mathematical models can be large scale systems, and the analysis of the system prior to numerical resolution can be cumbersome. Additionally, the consistent initialization step can pose a nontrivial numerical task. For the numerical resolution of this model it is employed the computational code under development at PEQ/COPPE/UFRJ and DEQUI/UFRGS. This code employs Automatic Differentiation (AD) tools to perform index reduction and consistent initialization with minimum interference of the user. The resulting consistency system is solved and the numerical integration of the final index-one DAE is accomplished by means of the integration code DASSLC. <![CDATA[Optimal solvent cycle design in supercritical fluid processes]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200016&lng=es&nrm=iso&tlng=es This work addresses the determination of optimal solvent cycle scheme for supercritical fluid processes. The optimization model, which includes reliable thermodynamic predictions and rigorous process models, has been extended to include potential units that constitute the solvent recovery cycle. Capital cost functions, based on graphical correlations from the literature, have been derived for each unit in the superstructure. A mixed integer nonlinear programming model has been formulated, where the objective function is net profit maximization. Process and solvent cycle design has been performed for the deterpenation of lemon peel oil. <![CDATA[Modeling and understanding different types of process design activities]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200017&lng=es&nrm=iso&tlng=es One of the major tasks addressed by the chemical industry is the design and revamping of production processes. Increasingly powerful computer-aided tools are available to face these complex tasks. Nevertheless, most design knowledge still rests in the minds of experienced designers. It is desirable to make it part of a computer support environment. Therefore, it is necessary to have a model of the design process. This contribution addresses this objective by introducing a model, based on the identification of three types of design activities: Synthesis, Analysis, and Decision. We discuss the intrinsic characteristics of each type of activity from two different view points: characteristic subactivities and products. Every type of activity consists of typical subordinate subactivities which are distinctive for the type and determine its behavior. On the other hand, activities operate on the results or products of the design process, called product data, including requirements, the representation of the design artifact itself, and arguments. These products also allow a specification of the three types. These ideas are exemplified by modeling the design of a separation system. <![CDATA[Optimization of the operating conditions of azeotropic distillation columns with pervaporation membranes]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200018&lng=es&nrm=iso&tlng=es The main objective of this work is the optimization of the operating conditions of azeotropic distillation columns in hybrid distillation/pervaporation systems. In the configuration analyzed, a liquid side stream extracted from the azeotropic distillation column with the distributing non-key component is treated in a pervaporation membrane while the retentate is recycled to the column. The pervaporation membrane separates the pure distributing non-key component from the mixture, thus helping to improve the purity in the top and/or bottom products of the distillation column. The case study shows the hybrid process of a distillation column combined with pervaporation membranes, compared to the classical two-column process for Methyl tert-Butyl Ether production. In both cases, the operating conditions of the distillation columns such as reflux ratio, product and side draw flowrates are selected optimally. The numerical results show a significant reduction in operating cost obtained by using a pervaporation membrane instead of the second column in the original process. <![CDATA[Estimation of the molecular weight distribution of linear homopolymer blends from linear viscoelasticity for bimodal and high polydisperse samples]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200019&lng=es&nrm=iso&tlng=es This work is concerned with the approximate solution of the problem generated by the integral of first kind relating the shear relaxation modulus of entangled, linear and flexible homopolymer blends and the molecular weight distribution (MWD). Procedures are proposed to estimate the density distribution function (DDF) of the MWD from numerical solutions of the theoretical model composed by the double reptation mixing rule and a law for the relaxation time of chains in polydisperse matrixes. One procedure uses the expansion of the DDF through orthogonal polynomial functions. This expansion is formulated for two cases: a) Hermite polynomials associated with the normal-DDF and b) Laguerre polynomials associated with the gamma-DDF. The other procedure uses the mean value theorem of continuum functions, which turns out the integral problem into a differential form. Calculations are carried out with dynamic rheometric data of linear viscoelasticity for samples of polydimethylsiloxane, polypropylene and polybutadiene. High values of polydispersity are considered. The predictions of the DDF through these procedures compare well with experimental data of size exclusion chromatography (SEC) <![CDATA[Carbochlorination of molybdenum trioxide: kinetic treatment]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200020&lng=es&nrm=iso&tlng=es The kinetics of MoO3 chlorination in the presence of carbon is analyzed. The experimental data were obtained in a fixed-bed isothermal reactor, operated at atmospheric pressure between 543 and 603 K, and at different feed flows and Cl2 concentrations. The following global reaction was obtained by identification of the reaction products: 2 MoO3(s) + 2 Cl2(g) + C(s) ® 2 MoO2Cl2(g) + CO2(g). On the basis of the analysis of experimental data, thermodynamic results and observations by other authors, three basic stages are distinguished: 1) Formation of the chlorinating agent; 2) Chlorination of molybdenum trioxide; 3) Carbon oxidation. This mechanism allows to explain the greater reactivity of carbochlorination reactions at low temperatures as compared to direct chlorination. A detailed scheme is proposed for the kinetic treatment, which reveals that 22 stages are involved in this complex reaction. The experimental data were processed with different kinetic models, and it was found that the"nucleation and growth"model, conveniently adapted to non catalyzed heterogeneous reactions is the one that better matches the experimental data. <![CDATA[Simulation of an industrial packed column for reactive absorption of CO2]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932003000200021&lng=es&nrm=iso&tlng=es The steady-state simulation of a reactive absorption column is presented. The absorber is used in a large-scale ammonia plant to remove CO2 from the process gas stream. To enhance the absorption process, high pressures and low temperatures are commonly used (T = 45-80 °C, P = 30-40 bar). At the outlet of the absorber, the CO2 content in the process gas must be reduced to less than 500 ppm (dry basis) to avoid an excessive temperature rise in the methanation reactor (downstream of the absorption section). To represent the gas-liquid system, a rigorous mathematical model based on the two-film theory is considered. The heat effects are taken into account. The behaviour of different process variables for a reference operating condition is analyzed. The influence of changes in some operating variables is evaluated.