Scielo RSS <![CDATA[Latin American applied research]]> http://www.scielo.org.ar/rss.php?pid=0327-079320060004&lang=es vol. 36 num. 4 lang. es <![CDATA[SciELO Logo]]> http://www.scielo.org.ar/img/en/fbpelogp.gif http://www.scielo.org.ar <![CDATA[A CP approach to the scheduling of resource-constrained multiproduct continuous facilities]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932006000400001&lng=es&nrm=iso&tlng=es This contribution presents a new Constraint Programming (CP) formulation to address the optimal operation of multiproduct continuous processes. In particular, facilities having continuous stages, which manufacture intermediate or final products and a limited number of intermediate storage tanks, are tackled. The problem involves finding (i) the optimal sequence of production runs and (ii) the optimal agenda of usage of each storage tank. The proposed formulation is able to handle different features found in industrial environments such as dissimilar machines, sequence-dependent changeover times, forbidden campaign-machine assignments, production runs' length constraints, etc. Moreover, one of the main contributions of this paper is the handling of constrained resources of various types, such as intermediate storage tanks of limited capacity and qualified manpower required at some stages. <![CDATA[Langrangean decomposition applied to multiperiod planning of petroleum refineries under uncertainty]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932006000400002&lng=es&nrm=iso&tlng=es This work presents a stochastic multiperiod model for representing a petroleum refinery. Uncertainty is taken into account in parameters such as demands, product sale prices and crude oil prices. In the present work, uncertainty is considered as a set of discrete scenarios, each representing a possible shifting of market expectations. Every environment is weighted through an expected probability of occurrence. Previous work revealed that the computational effort of uncertain multiperiod refinery production planning models grows exponentially with the number of time periods and scenarios. Therefore, in order to reduce the computational effort over uncertain long-planning horizons, special techniques must be employed. The proposal is to apply Lagrangean Decomposition, which exploits the block-diagonal structure of the problem, to reduce solution time by decomposing the model on a temporal basis. Solution of the proposed algorithm showed a significant reduction in computational effort with respect to the full-scale outer approximation solver. <![CDATA[Optimal programming of batch distillation: vessel network operations]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932006000400003&lng=es&nrm=iso&tlng=es This work presents an alternative structure for multivessel batch distillation and focuses on the determination of an optimal programming for it. The system is composed by a set of heated or cooled vessels connected by total reflux distillation cascades. This structure is able for separating multicomponent mixtures achieving simultaneously desirable specifications for all components, without loss of material and avoiding complex control strategies. An optimization problem is formulated in order to maximize a profit function assigned to a separation task. Transformed decision variables together with a smooth price function, are used to pose the problem according to an unconstrained form. The decision variables are the vapor molar flow rate and the operating time of each column. Batch separations of ideal quaternary mixtures are optimally programmed regarding purity specifications under fixed number of stages in each cascade and with an upper bound on the total rate of heat consumption. <![CDATA[Closed vessel experiment modelling and ballistic parameter estimation of gun propellants for lifetime prediction]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932006000400004&lng=es&nrm=iso&tlng=es A closed vessel is an apparatus with very thick walls, in which propellants under test are burnt. After ignition, gaseous products and heat are produced. The time history of pressures is measured by piezoelectric transducers, and recorded in either an analog or digital way. For the design and simulation of a gun performance, it would be rather important if one could use the thousands of P versus t points generated by a single shot for the estimation of as many as possible ballistic parameters. The present work is devoted to the development of predictive models for the closed vessel, comparing the performance to real experimental data. Using known regression procedures (Maximum Likelihood, for instance), ballistic parameters are fitted and lifetime of the propellant can be predicted. This can be done once such parameters are related to its ageing process, which consists of a loss of volatile components. <![CDATA[Optimization of emulsion copolymerization reactions: Experimental validation]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932006000400005&lng=es&nrm=iso&tlng=es Time optimal monomer and reactor temperature set-point profiles were computed and implemented experimentally for the control of copolymer composition and minimization of reaction time in emulsion copolymerization systems. Iterative Dynamic Programming (IDP) was used for the off-line calculation of the optimal profiles. This approach can deal with constrained optimization of systems described by complex mathematical models, as those needed for the emulsion copolymerization. The optimization procedure was applied to vinyl acetate / butyl acrylate emulsion copolymerization under industrial-like conditions, which includes safety constraints normally found in the operation of large-scale reactors. The dynamics of the reactor heat exchange system were also taken into account in the mathematical model. The optimal trajectories obtained by the IDP procedure were implemented in a jacketed reactor with automated control of temperature and monomers feed flow rates. The real-time implementation of the optimal trajectories was successfully carried out resulting in a significant reduction in reaction time and the production of homogeneous copolymer composition. <![CDATA[Air quality monitoring network design to control PM<SUB>10</SUB> in Buenos Aires city]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932006000400006&lng=es&nrm=iso&tlng=es An air-quality monitoring network properly designed is a key component of any air quality control programme. This paper presents an objective procedure to determine the minimum number of monitoring sites needed to detect the occurrence of background air pollutant concentrations greater than a reference concentration level (C L) in an urban area. We propose an air quality monitoring network design based on the analysis of the results of atmospheric dispersion models, in order to identify the grid cells (in which the city is divided) where the air pollutant concentration exceeds C L. At present, Buenos Aires city has not an air quality monitoring network. This paper also describes the application of the proposed methodology to design a monitoring network to control PM10 levels in Buenos Aires. Results show that four monitoring stations are required to detect the occurrence of PM10 24h-concentrations greater than C L=0.150 mg m-3. <![CDATA[Effect of composite edible coating from Amaranthus cruentus flour and stearic acid on refrigerated strawberry (Fragaria ananassa) quality]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932006000400007&lng=es&nrm=iso&tlng=es A composite coating from Amaranthus cruentus flour and stearic acid (10 g stearic acid/100 g flour, 26 g glycerol/100 g flour and stirring speed at 12000 rpm) was applied on fresh strawberries in order to verify the effect on its quality. Other treatments were effected to comparison: PVC film, bilayer coating of the optimized formulation, optimized formulation without stearic acid and a control group of fruits (not coated). Fruit quality was evaluated by weight loss, mold spoilage, firmness retention and surface color development. The weight loss increased with the storage time for all treatments and the firmness decreased, however, the optimized coating was the most effective for the firmness retention of fruits among the edible coatings studied (excepting PVC film). The same trend was observed for the surface color development; the optimized coating resulted in a minor increase in the ratio of chromaticity parameters (a/b) as a function of storage time. <![CDATA[An MILP framework for dynamic vehicle routing problems with time windows]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932006000400008&lng=es&nrm=iso&tlng=es A key issue in logistics is the efficient management of a vehicle fleet servicing a set of customers with known demands. Every vehicle route must start and finish at the assigned depot, each customer is to be visited by a single vehicle and vehicle capacities must not be exceeded. These are the constraints for the capacitated vehicle routing problem (VRP) whose objective is usually the minimization of the travel distance. When every customer has an associated time window, we are dealing with the vehicle routing problem with time windows (VRPTW), an NP-hard problem extensively studied. In the static VPRTW, all the problem data are given. A more challenging subject is the dynamic VRPTW (DRVPTW) where routes must be periodically updated because of new service requests. In DVRPTW, the information on the problem is time-dependent since the data are in part given a priori and in part dynamically updated. As a result, the best solution must be periodically revised. There are two classes of DVRPTW solution methodologies: the immediate assignment that updates vehicle routes as soon as a new service request is received, and the deferred assignment retaining the new service calls for a certain time period before dispatching them all at once. The latter type has been adopted in this paper. At the time of revising their routes, the vehicles are already on duty and some nodes have already been visited. The remaining old customers that have designated vehicles are either being serviced or awaiting service. The customers to be considered in the DVRPTW include not only old customers still to be serviced but also new visit requests. The DVRPTW is tackled by solving a series of static VRPTW problems, with each one being defined every time the input data is updated. The approach assumes that each vehicle will start its new route at the location where it is servicing or to which it is traveling. <![CDATA[Risk analysis and safe distances calculation considering atmospheric parameters uncertainlty]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932006000400009&lng=es&nrm=iso&tlng=es This work presents a computational tool, useful in risk assessment of hazardous material releases. It is based on a methodology that takes into account the stochastic uncertainty of atmospheric parameters. This is relevant for calculating risk of hazardous gases, particulate matter or bioaerosols diffusion when an accidental release or continuous emission occurs. This methodology can be applied to risk analysis of static sources (a stack or a fixed tank in a facility) or transportation accidents (road, train, maritime and pipeline transport) involving different scenarios. After carrying out a stochastic simulation based on well-known diffusion models (heavy and light gases or particulate matter), downwind concentrations are obtained so that individual and societal risks can be computed. This work contributes mainly to the formal presentation of the procedure, and the real application of the stochastic simulation avoiding large (prohibitive) computational effort, by introducing a translation algorithm and the Stochastic Reference Emitter (SRE) definition. This new approach also shows to be a fundamental step to organize a software prototype for risk assessment. Besides, taking advantage of the new computational capability a SRE-based algorithm for safe distances calculation is introduced. Finally, an example shows the main capabilities of the new tool. <![CDATA[Population balance equations for particle size distributions in semibatch emulsion polymerizations]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932006000400010&lng=es&nrm=iso&tlng=es In a previous work of this group, theoretical studies and stability measurements were performed in order to describe the particle coagulation provoked by electrolytes. The work of the current paper concerns the development of mathematical models based on the DLVO theory and on population balance equations (PBE), which involve highly complex integro-differential equations. This model includes particle coagulation, particle nucleation, the polymerization kinetics and monomer partitioning among the many phases of the system. The model ensured a proper description of butyl acrylate/methyl methacrylate emulsion copolymerizations reactions, and related phenomena such as coagulation and particle nucleation in the presence of large seed particles. These latter systems are very important to the multimodal latexes production. Finally, simulation studies were performed in order to test the model ability to follow the competitive growth of polymer particles during semibatch operations. <![CDATA[Monte Carlo simulation of binary mixtures adsorbed on heterogeneous surfaces]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932006000400011&lng=es&nrm=iso&tlng=es Most of the solids used in adsorption processes are heterogeneous and most isotherm models are not derived to use information about the solid structure and energy heterogeneity. A possible alternative is to use molecular simulation techniques, which turn the thermodynamic model more microscopic. Then, starting from information on the solid structure, it is possible to develop appropriate solid models for each type of real system. Therefore, the objective of this work is to use the grand canonical Monte Carlo technique of molecular simulation for correlating pure-component data and predicting adsorption of multicomponent mixtures. To decrease the computational effort, we use a simplified solid model that is represented by a two-dimensional square lattice. The solid heterogeneity is represented by a random distribution of different kinds of sites, characterized by distinct energy levels. The methodology developed by Cabral et al. (2003a) is used here to correlate pure components and for predicting binary mixtures of ethane, ethylene, isobutane, and carbon dioxide on zeolite 13X with good results. <![CDATA[Biodiesel production via esterification reactions catalyzed by lipase]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932006000400012&lng=es&nrm=iso&tlng=es The synthesis of ethyl hexadecanoate was carried out by esterification of palmitic acid with ethanol in a solvent-free system. A commercial immobilized lipase (Lipozyme RM-IM) was mixed with the reagents, in a 15 mL closed batch reactor with constant stirring and coupled with a condenser. The effects of palmitic acid/ethanol molar ratio (0.16 to 1.84), reaction temperature (65 to 75ºC) and enzyme concentration (0.48 to 5.52% w/w) on the initial reaction rate of ethyl hexadecanoate were determined using central composite design 2³ with six central points. Statistical analysis indicated that the enzyme concentration and palmitic acid/ethanol molar ratio had been found to be the most significant variables affecting the initial reaction rate. The best result was obtained under the following experimental conditions: palmitic acid/ethanol molar ratio of 0.50, temperature of 67ºC, and enzyme concentration of 4.50% (w/w). <![CDATA[Modeling an simulation of an autothermal reformer]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932006000400013&lng=es&nrm=iso&tlng=es The steady-state operation of the autothermal reformer is simulated as two reactors in series by means of a detailed mathematical model. An input-output model is selected to describe the upper combustion chamber. The possible occurrence of homogeneous steam reforming and water-gas-shift (WGS) reactions is considered. The catalyst bed is represented through a one-dimensional heterogeneous model, which allows calculating the axial variations of composition, temperature and pressure of the process gas stream. The strong intraparticle diffusional limitations are taken into account by rigorous solution of the mass balances inside the catalyst particle. The gas-solid heat-transfer resistances are also evaluated. An intrinsic kinetics is used for the steam reforming and WGS reactions. Heat losses to the environment are considered. The influence of the main operating variables on the reactor performance is studied. The proposed mathematical has been checked successfully against data available in the open literature. <![CDATA[Technological risk planning as part of environmental management]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932006000400014&lng=es&nrm=iso&tlng=es Although industries operate under safe conditions, an industrial accident may occur, threatening human life, property or environment. In the context of environmental management, the importance of taking into account the risk of major technological accidents is concerned not only with the definition of the extent of it consequences but with the measures taken to mitigate unwanted impacts. With regard to these objectives, the consequence analysis performed in Bahía Blanca industrial area are shown. Bahía Blanca is a middle-size populated town with one of the most important Argentinean petrochemical sites, where dangerous goods are processed, transported and stored. In particular, this work presents the results of evaluating the effects of about 200 possible accidental scenarios involving toxic and flammable substances releases. Hazard areas defined by means of this analysis have been employed to improve the local emergency plan. Finally, an easily understandable document was elaborated in order to achieve the effective risk communication to the local community. <![CDATA[Emulsion copolymerization of acrylonitrile and butadiene in a train of CSTRS: Intermediate addition policies for improving the product quality]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932006000400015&lng=es&nrm=iso&tlng=es A continuous industrial emulsion copolymerization of acrylonitrile (A) and butadiene (B) carried out in a train of 8 continuous stirred-tank reactors (CSTRs) at 10 ºC is theoretically investigated, with the aim of producing nitrile rubber (NBR) of a quality similar to that obtained in an equivalent batch process. The rubber quality is specified by the latex characteristics (number particle concentration and number-average particle diameter), and by the molecular parameters (average values of the copolymer composition, the molecular weights, and the degree of branching). A mathematical model of the NBR industrial batch process is here extended for simulating the operation of the CSTR train. Two NBR grades of a different chemical composition are investigated. Compared with the batch product, the continuous Normal Steady State (SS) operation, with the complete recipe fed into the first reactor of the train, produces a deteriorated rubber (higher polydispersity and higher degree of branching). Such undesirable effects can be corrected through intermediate additions of A and chain transfer agent (CTA) along the continuous train. Finally, the reduction of the off-spec product generated during the change of grade between Normal SSs is also investigated. To this effect, "bang-bang" strategies for the comonomers and the CTA feeds in the first reactor of the train allow reducing up to 30 % of the off-spec product generated when a simple step change in all the feeds is applied. <![CDATA[A mathematical model for the styrene - methyl methacrylate copolymerization in the presence of polybutadiene]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932006000400016&lng=es&nrm=iso&tlng=es The batch copolymerization of styrene (St) and methyl methacrylate (MMA) in the presence of polybutadiene (PB) is theoretically and experimentally investigated. A mathematical model that enables to calculate the main global variables and the macromolecular structure of the evolving polymer mixture is presented. The model is an extension of that developed by Estenoz et al. (1999) for the homopolymerization of St in the presence of PB. It was validated with experiments that considered diluted solution and bulk copolymerizations carried out at a constant low temperature (65 º C), with THF as solvent, benzoyl peroxide as initiator, and a St-MMA ratio close to the azeotropic composition. For comparison, the solution copolymerization without PB was also considered. All experimental runs were adequately simulated using a single set of kinetic parameters. <![CDATA[New material as support for nickel boride catalyst]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932006000400017&lng=es&nrm=iso&tlng=es The main objective of this work is to study the feasibility of new materials to be used as support for boron-nickel catalysts. Potential support materials such as: silica gel, alumina, hydrothermal modified Perlites and zeolite 4A, were characterized by BET, TPR and DRX. After the addition of Ni and B, their catalytic activity evaluated with the nitrobenzene hydrogenation model reaction. The influence of operational parameters during the impregnation process such as order of reactants, speed of agitation, time of aging and it influences from the previous thermal treatment were evaluated. The results show that the adequate materials to be used as catalysts supports are the commercial silica and Rehydroxilated Perlite. It is due to the existence of superficial OH groups, which allow the anchorage of the nickel boride catalyst. <![CDATA[Effect of sodium butyrate and zinc sulphate supplementation on recombinant human IFN-β production by mammalian cell culture]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932006000400018&lng=es&nrm=iso&tlng=es Recombinant human interferon-&beta;1 (rhIFN-&beta;1) glycoprotein is used as a therapeutic agent for a variety of diseases, such as multiple sclerosis and hepatitis. In the present work, different strategies were applied to produce rhIFN-&beta;1a in mammalian cell cultures. Transfected population of CHO-K1, CHO dhfr-, BHK and HEK cells were compared for their ability to produce rhIFN-&beta;1a, and clones of the most promising cell line (CHO-K1) were isolated by the limit dilution method. Likewise, different culture conditions were assayed by changing the amounts of fetal calf serum, sodium butyrate and/or ZnSO4, to improve cell productivity. The presence of each additive increased the rhIFN-&beta;1a yield ranging from 2 to 8 times, depending on the tested cell clone, but when these components were simultaneously added to the medium, the rhIFN-&beta;1a concentration in the supernatants was even greater. <![CDATA[Thermodynamic approach for optimal design of heat and power plants: Relationships between thermodynamic and economics solutions]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932006000400019&lng=es&nrm=iso&tlng=es In this paper, a new procedure for the design and analysis of Heat and Power Plants is presented. In this formulation, thermodynamic solutions obtained by maximizing the plant efficiency are used to find the economic design of the plant. In fact, by applying the Karush-Kuhn-Tucker conditions it is possible to relate thermodynamic and economic solutions. A formal context for the use of thermodynamic models in solving complex optimization problems that arise in the area of synthesis and design of chemical processes is introduced. The proposed methodology has been successfully applied for two plant arrangements. Rigorous models have been used to model the plant equipments. The obtained results are presented in order to illustrate the proposed procedure. <![CDATA[Atmospheric dispersion of emissions due to leakages in pressurized natural gas ducts]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932006000400020&lng=es&nrm=iso&tlng=es A simplified approach is presented to the transient atmospheric dispersion of accidental releases of natural gas, originated by leakages in pressurized ducts on sea level. In this scenario, shut-off valves are used for instantaneous shutdown of tube operation, isolating the harmful inventory, whose transient release is simulated providing estimations of gas instantaneous atmospheric concentration. The analysis further covers the transient behavior inside the ducts through a leakage model and the occurrence of multiple ruptures, synchronized or not, with known spatial distribution. The time-space dispersion model employed accounts for: (i) atmospheric conditions, (ii) wind speed, (iii) transient conditions of gas release into the atmosphere, and (iv) plume rise. The process of continuous release is approximated by a finite sequence of pulses, known as "puffs". Discrete puffs have volume dependent on release intensity, which depends on the actual time instant, as inventory decreases due to emission.