Scielo RSS <![CDATA[Brazilian Journal of Chemical Engineering]]> http://www.scielo.br/rss.php?pid=0104-663220000004&lang=en vol. 17 num. 4-7 lang. en <![CDATA[SciELO Logo]]> http://www.scielo.br/img/en/fbpelogp.gif http://www.scielo.br <![CDATA[<B>Preface</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400001&lng=en&nrm=iso&tlng=en <![CDATA[<b>Automatic differentiation tools in the dynamic simulation of chemical engineering processes</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400002&lng=en&nrm=iso&tlng=en Automatic Differentiation is a relatively recent technique developed for the differentiation of functions applicable directly to the source code to compute the function written in standard programming languages. That technique permits the automatization of the differentiation step, crucial for dynamic simulation and optimization of processes. The values for the derivatives obtained with AD are exact (to roundoff). The theoretical exactness of the AD comes from the fact that it uses the same rules of differentiation as in differential calculus, but these rules are applied to an algorithmic specification of the function rather than to a formula. The main purpose of this contribution is to discuss the impact of Automatic Differentiation in the field of dynamic simulation of chemical engineering processes. The influence of the differentiation technique on the behavior of the integration code, the performance of the generated code and the incorporation of AD tools in consistent initialization tools are discussed from the viewpoint of dynamic simulation of typical models in chemical engineering. <![CDATA[<B>Mathematical modeling of dispersion polymerizations study of the styrene polymerization in ethanol</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400003&lng=en&nrm=iso&tlng=en A mathematical model for prediction of monomer conversion, of particle number and of the evolution of the particle size distribution (PSD) in dispersion polymerization is developed. Despite being completed very early during the polymerization process (monomer conversion <1%), nucleation of new particles is the most important factor affecting the PSD. In order to describe the particle nucleation phenomena, the mechanism of homogeneous coagulative nucleation is considered. According to this mechanism, polymer chain aggregates can either coagulate and grow, to give birth to new polymer particles (particle nucleation), or be captured by existing polymer particles. Two sets of population balance equations are used: one for the aggregates, and a second one for the stable polymer particles. It is shown that the model is able to describe the dispersion polymerization of styrene in ethanol and the formation of micron-size monodisperse polymer particles. <![CDATA[<B>Mathematical modeling of polystyrene particle size distribution produced by suspension polymerization</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400004&lng=en&nrm=iso&tlng=en Particle size distribution (PSD) of polystyrene particles produced by suspension polymerization is of fundamental importance in determining suspension stability and product quality attributes. Within a population balance framework, a model is proposed for suspension polymerization reactors to describe the evolution of the PSD. The model includes description of breakage and coalescence rates in terms of reaction kinetics and rheology of the dispersed phase. The model is validated with experimental data of styrene suspension polymerization. <![CDATA[<B>Wastewater treatment by flotation</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400005&lng=en&nrm=iso&tlng=en This work deals with the performance analysis of a separation set-up characterized by the ejector-hydrocyclone association, applied in the treatment of a synthetic dairy wastewater effluent. The results obtained were compared with the results from a flotation column (cylindrical body of a hydrocyclone) operated both batch and continuously. As far as the experimental set-up studied in this work and the operating conditions imposed to the process, it is possible to reach a 25% decrease in the total effluent chemical oxygen demand (COD). This corresponds approximately to 60% of the COD of the material in suspension. The best results are obtained for ratios air flow rate-feed flow rate (Qair/Q L) greater then 0.15 and for ratios underflow rate-overflow rate (Qu/Qo) lower than 1.0. <![CDATA[<b>The inclusion of surface characteristics in equations for turbulence closure</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400006&lng=en&nrm=iso&tlng=en The possibility of the inclusion of the charactheristics of surfaces in the formulation of closure equations for the turbulent fluxes is considered. The general formulation is presented in the form of an expression which, with respect to the fluid motion, it is a general constitutive equation as proposed by Noll, and to which, it was added the explicit dependence upon the wall configuration. Restrictions imposed by frame indifference, and by the symmetry group of the surface are examined. <![CDATA[<B>Control and optimization of baths for electrodeposition of Co-Mo-B amorphous alloys</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400007&lng=en&nrm=iso&tlng=en Optimization and control of an electrodeposition process for depositing boron-containing amorphous metallic layer of cobalt-molybdenum alloy onto a cathode from an electrolytic bath having cobalt sulfate, sodium molybdate, boron phosphate, sodium citrate, 1-dodecylsulfate-Na, ammonium sulfate and ammonia or sulfuric acid for pH adjustments has been studied. Detailed studies on bath composition, pH, temperature, mechanical agitation and cathode current density have led to optimum conditions for obtaining satisfactory alloy deposits. These alloys were found to have interesting properties such as high hardness, corrosion resistance, wear resistance and also sufficient ductility. A voltammetric method for automatic monitoring and control of the process has been proposed. <![CDATA[<B>Soft sensors with white- and black-box approaches for a wastewater treatment process</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400008&lng=en&nrm=iso&tlng=en The increasing degradation of water resources makes it necessary to monitor and control process variables that may disturb the environment, but which may be very difficult to measure directly, either because there are no physical sensors available, or because these are too expensive. In this work, two soft sensors are proposed for monitoring concentrations of nitrate (NO) and ammonium (NH) ions, and of carbonaceous matter (CM) during nitrification of wastewater. One of them is based on reintegration of a process model to estimate NO and NH and on a feedforward neural network to estimate CM. The other estimator is based on Stacked Neural Networks (SNN), an approach that provides the predictor with robustness. After simulation, both soft sensors were implemented in an experimental unit using FIX MMI (Intellution, Inc) automation software as an interface between the process and MATLAB 5.1 (The Mathworks Inc.) software. <![CDATA[<b>Application of pulse combustion technology in spray drying process</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400009&lng=en&nrm=iso&tlng=en The paper presents development of valved pulse combustor designed for application in drying process and drying tests performed in a specially built installation. Laser technique was applied to investigate the flow field and structure of dispersed phase during pulse combustion spray drying process. PDA technique was used to determine initial atomization parameters as well as particle size distribution, velocity of the particles, mass concentration of liquid phase in the cross section of spray stream, etc., in the drying chamber during drying tests. Water was used to estimate the level of evaporation and 5 and 10% solutions of sodium chloride to carry out drying tests. The Computational Fluid Dynamics technique was used to perform theoretical predictions of time-dependent velocity, temperature distribution and particle trajectories in the drying chamber. Satisfactory agreement between calculations and experimental results was found in certain regions of the drying chamber. <![CDATA[<B>Performance analysis and design of small diameter cyclones</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400010&lng=en&nrm=iso&tlng=en In this work, the effect of the configuration on the collection efficiency and pressure drop in small diameter cyclones is evaluated based on neural networks (Functional Link Networks). The experiences were conducted at LSP/COPPE Laboratory in a Stairmand high efficiency prototype (Dc = 5 cm) with variable overflow diameter. Three different configurations were tested, and it is possible to observe a significant increase in the collection efficiency with the reduction of the overflow diameter. <![CDATA[<B>Viscosities and excess energy of activation for viscous flow for binary mixtures of tetrahydrofuran with 1-butanol, 2-butanol and 1-chlorobutane at 283.15, 298.15 and 313.15 K</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400011&lng=en&nrm=iso&tlng=en Kinematic viscosities of binary mixtures composed of tetrahydrofuran with 1-butanol, 2-butanol and 1-chlorobutane have been measured at 283.15, 298.15 and 313.15 K and atmospheric pressure for the whole range of compositions. The dynamic viscosity, the excess viscosity and the excess energy of activation for viscous flow were also calculated. The excess viscosity shows negative deviations from ideal behavior for the mixtures tetrahydrofuran with 1-butanol and 2-butanol and a small positive deviation for the binary tetrahydrofuran + 1-chlorobutane system. The experimental results have also been used to test some empirical and semiempirical equations adopted previously to correlate viscosity - composition data. <![CDATA[<STRONG>Control of a loop polymerization reactor using neural networks</STRONG>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400012&lng=en&nrm=iso&tlng=en or multivariable non linear predictive control implementations, a hybrid-neural model (lumped model) was successfully used for modeling a loop-tubular polymerization reactor (a lumped or distributed model, depending on recycle ratio). Bifurcation diagrams were computed in order to investigate the agreement between process and model, of paramount importance for model based controller implementation purposes. Performance was evaluated considering the nonlinear model predictive control of both a loop tubular reactor (lumped) SISO problem and a tubular reactor (distributed) MIMO problem. <![CDATA[<B>Optimization of annatto<I> </I>(<I>Bixa orellana </I>L.) drying in fixed bed</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400013&lng=en&nrm=iso&tlng=en The drying of annatto seeds (Bixa orellana L.), red piave cultivate, was studied in a fixed bed dryer. The best conditions were estimated to minimize the loss of coloring and to obtain final moisture of the seeds in appropriate levels to its conservation and maintenance of quality. The quantification of the influence of entrance variables in the final contents of bixin and moisture seeds and the identification of the optimal point was performed through the techniques of factorial design, response surfaces methodology, canonical analysis and desirability function. It was verified that the final moisture of the seeds may be estimated by a second-order polynomial model and that the final content of bixin is only significantly influenced by the time of drying being described properly by a linear model, for the seeds used in this study. <![CDATA[<strong>Salt effect in the solubility of hydrogen in n-alcohols at pressures up to 10 MPa and temperatures up to 498.15 K</strong>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400014&lng=en&nrm=iso&tlng=en Gas-liquid solubility of hydrogen in methanol and ethanol systems with electrolytes was studied in the temperature range from 305.15 K to 498.15 K and pressures from 4 to 10 MPa. The experimental method used was the Total Pressure Method, which eliminates sampling and analysis of the phases, determining their composition at equilibrium using the following experimental data: moles of solute and solvent in the system; pressure, temperature and volume of the system at equilibrium; together with thermodynamic equations for fluid-phase equilibria. The salts used were lithium chloride and potassium acetate. The solubility of hydrogen increases with increasing temperature and pressure and the presence of salts causes a decrease in the solubility of hydrogen, when compared with the results of systems without salts, characterizing a "salting-out" effect, which is greater in conditions of lower temperature and pressure, specially for potassium acetate. <![CDATA[<B>SMB chromatography applied to the separation/purification of fructose from cashew apple juice</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400015&lng=en&nrm=iso&tlng=en The simulated moving-bed (SMB) technology has been successfully used in separations in petrochemical, food and fine chemical industries. This work is intended to show a potencial economic alternative for the industrial processing of the cashew apple juice. The cashew tree is a native tropical plant abundant in Northeastern Brazil, whose commercial value relies mainly on the processing of its nut. The penduncle of the fruit is called the cashew apple. Despite its high nutrition value, around 90% of the crop spoils on the soil. Simulation and experimental results are presented for SMB separation of fructose from glucose, both present (<FONT FACE="Symbol">~</FONT>40 kg/m³) in the aqueous phase of comercial cashew apple juice. Kinetic and equilibrium data for fructose and glucose on packed columns using cation-exchange resins are reported. Experimental results for SMB operation indicate close to 90% purity in each product (fructose-rich extract and glucose-rich raffinate). Simulated unit performance and internal profiles agree well with experimental values. To increase the added-value and versatility of the products, either a step of isomerization of the raffinate or diverse SMB fluid-solid flowrate ratios may be applied. By this way, a wide range of products may be obtained, from nearly pure fructose to 42%, 55% and 90% solutions, which are the standard high fructose syrup concentrations. If solids content is conveniently raised to the usual HFCS (high fructose corn syrup) comercial standards, these products may be used as food additives, thus confirming a potentially attractive use of cashew apple juice. <![CDATA[<b>N</b><strong>eural networks for predicting mass transfer parameters in supercritical extraction</strong>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400016&lng=en&nrm=iso&tlng=en Neural networks have been investigated for predicting mass transfer coefficients from supercritical Carbon Dioxide/Ethanol/Water system. To avoid the difficulties associated with reduce experimental data set available for supercritical extraction in question, it was chosen to use a technique to generate new semi-empirical data. It combines experimental mass transfer coefficient with those obtained from correlation available in literature, producing an extended data set enough for efficient neural network identification. With respect to available experimental data, the results obtained to benefit neural networks in comparing with empirical correlations for predicting mass transfer parameters. <![CDATA[<b>Pinch analysis of evaporation systems</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400017&lng=en&nrm=iso&tlng=en Evaporation systems are separation processes widely used in chemical industries. Some guidelines can be found in the literature for the process integration of multiple effect evaporators. In the published methodologies some aspects are neglected as boiling point rise, effect of pressure on latent heat of water, sensible heat of liquid streams, heat of mixing, effects configuration and inclusion of accessories. In this work, a new graphical representation for the integration of multiple effect evaporators was developed, using rigorous physical properties. From this representation, an algorithm for optimization of bleed streams was conceived using the concepts of Pinch Analysis. As a case study, a crystal glucose plant was optimized using this new methodology. The optimization of bleed streams showed as result a steam consumption 16% smaller than a similar previous study. From energy and capital costs, it is shown that the integrated evaporator exhibits a total cost 14% smaller than the non-integrated configuration. <![CDATA[<strong>Generalized drying curves in conductive/convective paper drying</strong>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400018&lng=en&nrm=iso&tlng=en This work presents a study related to conductive/convective drying of paper (cellulose) sheets over heated surfaces, under natural and forced air conditions. The experimental apparatus consists in a metallic box heated by a thermostatic bath containing an upper surface on which the paper samples (about 1 mm thick) are placed. The system is submitted to ambient air under two different conditions: natural convection and forced convection provide by an adjustable blower. The influence of initial paper moisture content, drying (heated surface) temperature and air velocity on drying curves behavior is observed under different drying conditions. Hence, these influence is studied through the proposal of generalized drying curves. Those curves are analyzed individually for each air condition exposed above and for both together. A set of equations to fit them is proposed and discussed. <![CDATA[<strong>Optimizing dissolved air flotation design system</strong>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400019&lng=en&nrm=iso&tlng=en Dissolved Air (Pressure) Flotation-DAF, is a well-established separation process that employs micro-bubbles as a carrier phase. This work shows results concerning bubble generation at low working pressures in modified DAF-units to improve the collection of fragile coagula by bubbles. DAF of Fe (OH)3 (as model) was studied as a function of saturation pressure in the absence and presence of surfactants in the saturator. DAF was possible at 2 atm by lowering the air/water surface tension. This fact, which leads to substantial energy savings, was explained in terms of decreasing the "minimum" energy required for bubble nucleation and cavity in the nozzle. More, bubbles-fragile coagula attachment was improved by dividing the recycling water into two: 1) the inclined inlet to the cell (traditional) and 2) inside the separation tank through a water flow inlet situated below the floating bed using a "mushroom" type diffuser. Because of the reduction observed in the degree of turbulence in the conventional collection zone, DAF performance improved yielding high precipitate recoveries. <![CDATA[<B>Leak detection in pipelines through spectral analysis of pressure signals</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400020&lng=en&nrm=iso&tlng=en The development and test of a technique for leak detection in pipelines is presented. The technique is based on the spectral analysis of pressure signals measured in pipeline sections where the formation of stationary waves is favoured, allowing leakage detection during the start/stop of pumps. Experimental tests were performed in a 1250 m long pipeline for various operational conditions of the pipeline (liquid flow rate and leakage configuration). Pressure transients were obtained by four transducers connected to a PC computer. The obtained results show that the spectral analysis of pressure transients, together with the knowledge of reflection points provide a simple and efficient way of identifying leaks during the start/stop of pumps in pipelines. <![CDATA[<b>Advanced control of propylene polimerizations in slurry reactors</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400021&lng=en&nrm=iso&tlng=en The objective of this work is to develop a strategy of nonlinear model predictive control for industrial slurry reactors of propylene polymerizations. The controlled variables are the melt index (polymer quality) and the amount of unreacted monomer (productivity). The model used in the controller presents a linear dynamics and a nonlinear static gain given by a neuronal network MLP (multilayer perceptron). The simulated performance of the controller was evaluated for a typical propylene polymerization process. It is shown that the performance of the proposed control strategy is much better than the one obtained with the use of linear predictive controllers for setpoint tracking control problems. <![CDATA[<b>A planning model for petroleum refineries</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400022&lng=en&nrm=iso&tlng=en The main objective of this paper is to develop a nonlinear planning model for refinery production. The model described here represents a general petroleum refinery and its framework allows the implementation of nonlinear process models as well as blending relations. This model assumes the existence of several processing units, producing a variety of intermediate streams, with different properties, that can be blended to constitute the desired kinds of products. Two real-world applications are developed, one for the planning of diesel production in the RPBC refinery in Cubatão (SP) and the other for the general production planning in the REVAP refinery in S. José dos Campos (SP). In both cases, different market scenarios were analyzed using the planning optimization and the results were compared with the current situation, in which there is no extensive use of planning decision support tools. Results revealed that there is a very large potential of profitability embedded in the planning activity, reaching several millions of dollars per year. <![CDATA[<b>A hybrid feedforward neural network model for the cephalosporin C production process</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400023&lng=en&nrm=iso&tlng=en At present, direct on-line measurements of key bioprocess variables as biomass, substrate and product concentrations is a difficult task. Many of the available hardware sensors are either expensive or lack reliability and robustness. To overcome this problem, indirect estimation techniques have been studied during the last decade. Inference algorithms rely either on phenomenological or on empirical models. Recently, hybrid models that combine these two approaches have received great attention. In this work, a hybrid neural network algorithm was applied to a fermentative process. Mass balance equations were coupled to a feedforward neural network (FNN). The FNN was used to estimate cellular growth and product formation rates, which are inserted into the mass balance equations. On-line data of cephalosporin C fed-batch fermentation were used. The measured variables employed by the inference algorithm were the contents of CO2 and O2 in the effluent gas. The fairly good results obtained encourage further studies to use this approach in the development of process control algorithms. <![CDATA[<B>Spinning process variables and polymer solution effects in the die-swell phenomenon during hollow fiber membranes formation</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400024&lng=en&nrm=iso&tlng=en During hollow fiber spinning many variables are involved whose effects are still not completely clear. However, its understanding is of great interest because the control of these variables may originate membranes with the desired morphologies and physical properties. In this work, the phase inversion process induced by the immersion precipitation technique was applied to prepare hollow fibers membranes. It was verified that some of the variables involved, can promote a visco-elastic polymer solution expansion, called die-swell phenomenon, which is undesired since it may lead to low reproducibility of the permeation properties. The effects of the distance between spinneret and precipitation bath, the bore liquid composition, and the polymer solution composition were analyzed and discussed in order to avoid this phenomenon. According to the results, it was verified that the parameters investigated might promote a delay precipitation, which restrained the visco-elastic expansion. <![CDATA[<B>Development of a hydrodynamic model for air-lift reactors</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400025&lng=en&nrm=iso&tlng=en In this paper, a 1D hydrodynamic model has been developed for gas hold-up and liquid circulation velocity prediction in air-lift reactors. The model is based on momentum balance equations and has been adjusted to experimental data collected on a pilot plant reactor equipped with two types of gas distributors and using water and water/butanol as the liquid phase. Different techniques of signal analysis have also been applied to pressure fluctuations in order to extract information about flow regimes and regime transitions. A good knowledge of the flow pattern is essential to establish adequate correlations for the hydrodynamic model. <![CDATA[<B>Inference of conversion and purity for ETBE reactive distillation</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400026&lng=en&nrm=iso&tlng=en Reactive distillation (RD), an unconventional and attractive technique, has been applied in fuel ether production. A typical application of RD is the synthesis of the widely used methyl tert-butyl ether (MTBE). RD has also been found to have potential to produce high quality ethyl tert-butyl ether (ETBE), a potential alternative to MTBE. A RD process integrates conventional reaction and separation into a single unit, resulting in extra complexity and dual process objectives, i.e. maximization of reactant conversion and purity of products. The conversion and the purity are thus important variables to be controlled in RD of ETBE. Unfortunately, both of them are not economically and reliably available for closed-loop control. This study aims to develop an effective method to infer the conversion and the purity from multiple temperature measurements that are easily available on-line and in real time. Nonlinear inferential models are recommended for ETBE synthesis with a ten-stage pilot scale RD column. The models are two-variable third-order regressive models, in which the temperature measurements of the reboiler and the bottom reactive section are employed. Experimental design, model identification, and model testing are also investigated. <![CDATA[<B>Application of genetic algorithms to the adjustment of the supports of fuzzy sets in a mamdani controller</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400027&lng=en&nrm=iso&tlng=en The development of control systems based on fuzzy rules facilitates the solving of problems when insufficient phenomenological information is available. The most common way of grouping fuzzy rules to form a controller is known as Mamdani controller. This controller consists of a set of rules with two premises, the error and the error variation, and one conclusion, the control action variation. One of the most delicate phases of the project of fuzzy systems is the definition of the supports (range) of each fuzzy qualifiers. This work apply genetic algorithms, together with some model of the system, to the adjustment of the supports of the fuzzy sets used in a Mamdani controller. The results show that the automatic adjustment is faster and more efficient that the manual one. Finally, the results are compared with a PID that was also adjusted with genetic algorithms. <![CDATA[<B>Adaptive control of paste drying in spouted bed using the GPC algorithm</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400028&lng=en&nrm=iso&tlng=en Spouted bed is an equipment reasonable investigated for drying of many and different materials ranging from granules to pastes and suspensions. This work presents an implementation and tests of an advanced control strategy (GPC) on a spouted bed dryer. Water was used as the ideal paste for tests. A microcomputer PC-486 type was used to maintain humidity and temperature set points of air in the dryer by manipulating electric power of heat exchanger and paste feed flow rate. An instrumentation was set-up with different sensors, interface and final control elements for the process. The tests were carried out with pure water as ideal paste. Stability and performance analysis of the control strategy were accomplished. A stable controller was obtained which encouraged us to investigate this control strategy in the drying of other pastes too. <![CDATA[<B>Operational cost minimization in cooling water systems</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400029&lng=en&nrm=iso&tlng=en In this work, an optimization model that considers thermal and hydraulic interactions is developed for a cooling water system. It is a closed loop consisting of a cooling tower unit, circulation pump, blower and heat exchanger-pipe network. Aside from process disturbances, climatic fluctuations are considered. Model constraints include relations concerning tower performance, air flowrate requirement, make-up flowrate, circulating pump performance, heat load in each cooler, pressure drop constraints and climatic conditions. The objective function is operating cost minimization. Optimization variables are air flowrate, forced water withdrawal upstream the tower, and valve adjustment in each branch. It is found that the most significant operating cost is related to electricity. However, for cooled water temperatures lower than a specific target, there must be a forced withdrawal of circulating water and further makeup to enhance the cooling tower capacity. Additionally, the system is optimized along the months. The results corroborate the fact that the most important variable on cooling tower performance is not the air temperature itself, but its humidity. <![CDATA[<B>Energy and capital targets for constrained heat exchanger networks</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400030&lng=en&nrm=iso&tlng=en A new procedure for estimating area and capital cost targets of constrained heat exchanger networks is presented. The method allows for match constrained networks and exchangers with more than one tube pass. The procedure is based on modelling the problem as a non-linear formulation where the forbidden exchanger matches are included as constraints and the temperature difference correction due to multipass exchangers is included in the model. The difficulty of converging to a solution due to the additional non-linear constraints imposed by the multipass exchangers required the use of a two-level approach: at the inner level, the area targets for simple pass exchangers are obtained, and at the outer level the temperature difference required for multipass exchangers are computed and fed back to the inner level. The procedure is repeated until an appropriate tolerance between two iterations was achieved. A comparison between the estimated exchanger areas and costs estimated by the new procedure and the area and costs obtained from the final heat exchanger design shows a very good agreement. <![CDATA[<B>Sensor locations and noise reduction in high-purity batch distillation control loops</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400031&lng=en&nrm=iso&tlng=en The influence of the sensor locations on the composition control of high-purity batch distillation columns has been investigated. Using concepts of the nonlinear control theory, an input-output linearizing controller was implemented to keep the distillate composition constant at a desired value by varying the reflux ratio. An Extended Kalman Filter was developed to estimate the compositions required in the control algorithm using temperature measurements. In the presence of measurement noise, the control performance depended greatly on the sensor locations. Placing the sensors further from the top stages reduced the detrimental effects of noise but increased the inference error. To achieve accurate composition control, both noise reduction and composition estimate accuracy should be considered in the selection of the sensor locations. <![CDATA[<B>Applications of an alternative formulation for one-layer real time optimization</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400032&lng=en&nrm=iso&tlng=en This paper presents two applications of an alternative formulation for one-layer real time structure for control and optimization. This new formulation have arisen from predictive controller QDMC (Quadratic Dynamic Matrix Control), a type of predictive control (Model Predictive Control - MPC). At each sampling time, the values of the outputs of process are fed into the optimization-control structure which supplies the new values of the manipulated variables already considering the best conditions of process. The variables of optimization are both set-point changes and control actions. The future stationary outputs and the future stationary control actions have both a different formulation of conventional one-layer structure and they are calculated from the inverse gain matrix of the process. This alternative formulation generates a convex problem, which can be solved by less sophisticated optimization algorithms. Linear and nonlinear economic objective functions were considered. The proposed approach was applied to two linear models, one SISO (single-input/single output) and the other MIMO (multiple-input/multiple-output). The results showed an excellent performance. <![CDATA[<B>Simulation and optimization of an industrial PSA unit</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400033&lng=en&nrm=iso&tlng=en The Pressure Swing Adsorption (PSA) units have been used as a low cost alternative to the usual gas separation processes. Its largest commercial application is for hydrogen purification systems. Several studies have been made about the simulation of pressure swing adsorption units, but there are only few reports on the optimization of such processes. The objective of this study is to simulate and optimize an industrial PSA unit for hydrogen purification. This unit consists of six beds, each of them have three layers of different kinds of adsorbents. The main impurities are methane, carbon monoxide and sulfidric gas. The product stream has 99.99% purity in hydrogen, and the recovery is around 90%. A mathematical model for a commercial PSA unit is developed. The cycle time and the pressure swing steps are optimized. All the features concerning with complex commercial processes are considered. <![CDATA[<B>Recovery of aroma compounds from orange essential oil</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400034&lng=en&nrm=iso&tlng=en The objective of this work was to study the recovery of aroma compounds present in the orange essential oil using experimental data from CUTRALE (a Brazilian Industry of Concentrated Orange Juice). The intention was to reproduce the industrial unit and afterwards to optimize the recovery of aroma compounds from orange essential oil by liquid-liquid extraction. The orange oil deterpenation was simulated using the commercial software PRO/II 4.0 version 1.0. The UNIFAC model was chosen for the calculation of the activity coefficients. <![CDATA[<B>Supercritical extraction of oleaginous</B>: <B>parametric sensitivity analysis</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400035&lng=en&nrm=iso&tlng=en The economy has become universal and competitive, thus the industries of vegetable oil extraction must advance in the sense of minimising production costs and, at the same time, generating products that obey more rigorous patterns of quality, including solutions that do not damage the environment. The conventional oilseed processing uses hexane as solvent. However, this solvent is toxic and highly flammable. Thus the search of substitutes for hexane in oleaginous extraction process has increased in the last years. The supercritical carbon dioxide is a potential substitute for hexane, but it is necessary more detailed studies to understand the phenomena taking place in such process. Thus, in this work a diffusive model for semi-continuous (batch for the solids and continuous for the solvent) isothermal and isobaric extraction process using supercritical carbon dioxide is presented and submitted to a parametric sensitivity analysis by means of a factorial design in two levels. The model parameters were disturbed and their main effects analysed, so that it is possible to propose strategies for high performance operation. <![CDATA[<B>Salt effect of KBr on the liquid-liquid equilibrium of the water/ethanol/1-pentanol system</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400036&lng=en&nrm=iso&tlng=en Liquid-liquid equilibrium data for the water/ethanol/1-pentanol/potassium bromide systems were experimentally determined at 25° C and 40ºC. The experimental data were correlated through the NRTL and UNIFAC-Dortmund models for the activity coefficient, with the estimation of new binary interaction parameters for both models, corresponding to the salt-solvent and solvent-solvent interactions for the NRTL model and the ion-ion and solvent-ion interactions for the UNIFAC-Dortmund model. The results obtained have shown that the NRTL model was more able to represent equilibrium data for the studied systems. <![CDATA[<B>The influence of heat exchanger design on the synthesis of heat exchanger networks</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400037&lng=en&nrm=iso&tlng=en Heat exchanger network (HEN) synthesis has been traditionally performed without accounting for a more detailed unit design, which is important since the final HEN may require unfeasible units. Recently, publications on this matter have appeared, as well as softwares that simultaneously perform synthesis and units design. However, these publications do not clearly show the influence of the new added features on the final HEN. Hence, this work aims at showing that units' design can strongly affect the final HEN. Improvements on heat transfer area and total annual cost estimations, which influence the HEN structural evolution, are the main responsible for that. It is also shown the influence of some design bounds settings, which can indicate an unfeasible unit design and, therefore, the need for a new match search or the maintenance of a loop. An example reported in the literature is used to illustrate the discussion. <![CDATA[<B>Thermodynamic analysis of multicomponent distillation columns</B>: <B>identifying optimal feed conditions</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400038&lng=en&nrm=iso&tlng=en A new methodology for the optimisation of feed conditions as well as the calculation of minimum reflux ratio of distillation columns is presented. The reversible profile approach used for saturated liquid feeds is extended to consider other feed conditions. For flashed feed, the liquid fraction of the feed stream is used to compute the column pinch conditions and the minimum reflux ratio. The modifications required for subcooled liquid and superheated vapor feed are discussed, and a procedure to estimate the minimum reflux for those conditions is proposed. The methodology presented allows the identification of the optimal feed condition, without having to resort to a full stage-by-stage procedure. <![CDATA[<B>Simulation and thermodynamic modeling of the extraction of tocopherol from a synthetic mixture of tocopherol, squalene and CO<SUB>2</B></SUB>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400039&lng=en&nrm=iso&tlng=en Soybean oil is the most consumed vegetable oil in the world, representing 54% of the total world production. Brazil is the second country in the world that produces and export soybean seeds, almost 20%. One of the most important by-product of the soybean oil is the deodorizer distillate, commonly known as soybean sludge. This residue is rich in many high value compounds as tocopherols, squalene and sterols. Tocopherols are the major components in the deodorized distillated due to their characteristics as an antioxidant agent. So, the objective of this work is to study the concentration of tocopherols presented in this raw material, using the operational conditions obtained from the equilibrium data and using supercritical carbon dioxide as a solvent. The deodorizer distillate is a complex mixture of more than 200 components, so a synthetic mixture was chosen to represent the deodorizer distillate. The synthetic mixture used in this work is composed by tocopherols, fatty acids and squalene. The simulation was carried out using ASPEN+ simulator and the LCVM thermodynamic model was used to correlate the available equilibrium data. <![CDATA[<B>Calculation of mixture critical diagrams using an equation of state based on the lattice fluid theory</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400040&lng=en&nrm=iso&tlng=en A modified form of the Hicks and Young algorithm was used with the Mattedi-Tavares-Castier lattice equation of state (MTC lattice EOS) to calculate critical points of binary mixtures that exhibit several types of critical behavior. Several qualitative aspects of the critical curves, such as maxima and minima in critical pressure, and minima in critical temperature, could be predicted using the MTC lattice EOS. These results were in agreement with experimental information available in the literature, illustrating the flexibility of the functional form of the MTC lattice EOS. We observed however that the MTC lattice EOS failed to predict maxima in pressure for two of the studied systems: ethane + ethanol and methane + n-hexane. We also observed that the agreement between the calculated and experimental critical properties was at most semi-quantitative in some examples. Despite these limitations, in many ways similar to those of other EOS in common use when applied to critical point calculations, we can conclude that the MTC lattice EOS has the ability to predict several types of critical curves of complex shape. <![CDATA[<B>Solubility of carbon dioxide and ethane in lemon oil at elevated pressures</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400041&lng=en&nrm=iso&tlng=en The solubility of ethane and carbon dioxide in lemon oil at elevated pressures has been measured using a variable volume cell, over temperature ranges of 298 - 308 K and 303 - 313 K, respectively, at pressures from 0.44 to 8.75 MPa. Partial liquid miscibility was not observed at any concentration measured. Molar densities of the saturated liquid mixtures are also reported. The Soave<FONT FACE="Symbol">-</FONT> Redlich<FONT FACE="Symbol">-</FONT> Kwong equation of state was used to fit experimental vapor<FONT FACE="Symbol">-</FONT> liquid equilibria of near critical light component<FONT FACE="Symbol">-</FONT> lemon oil mixtures. An appropriate representation of the experimental data was obtained by using a binary interaction parameter in the combinatorial rule for the cross energy parameter a ij. The application of a quadratic mixing rule for the co<FONT FACE="Symbol">-</FONT> volume b with a binary interaction coefficient l ij did not improve greatly the performance of the model. <![CDATA[<B>Multivariable low order structured-controller design by frequency response approximation</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400042&lng=en&nrm=iso&tlng=en The method presented here offers an effective and time saving tool for multivariable controller design. The relation between controller complexity and closed loop performance can easily be evaluated. The method consists of five steps: 1. A desired behavior of the closed loop system is specified. Considering the nonminimum phase part of the process model the closed loop attainable performance is determined. 2. The process model and the attainable performance are scaled by the RPN-scaling procedure. 3. This defines an "ideal" scaled controller, which is usually too complex to be realized. 4. The frequency response of the ideal scaled compensator is approximated by a simpler one with structure and order chosen by the user. 5. Since the approximation in frequency response is performed with the scaled system, it is necessary to return to the original system’s units. <![CDATA[<b>Phenomenological model of the clavulanic acid production process utilizing <i>Streptomyces clavuligerus</i></b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400043&lng=en&nrm=iso&tlng=en The kinetics of clavulanic acid production process by Streptomyces clavuligerus NRRL 3585 was studied. Experiments were carried out in a 4 liters bioreactor, utilizing 2 complex media containing glycerol as the carbon and energy source, and peptone or Samprosoy 90NB (soybean protein) as nitrogen source. Temperature was kept at 28°C and the dissolved oxygen was controlled automatically at 40 % saturation value. Samples were withdrawn for determination of cell mass (only peptone medium), glycerol and product concentrations. Gas analyzers allowed on line determination of CO2 and O2 contents in the exit gas. With Samprosoy, cell mass was evaluated by determining glycerol consumption and considering the cell yield, Y X/S, as being the same for both cases. Oxygen uptake and CO2 production rates were strongly related to growth and substrate consumption, allowing determination of stoichiometric constants in relation to growth, substrate, oxygen, product and carbon dioxide. <![CDATA[<B>Purification of <FONT FACE="Symbol">a</FONT>-galactosidase from seeds of S<I>esbania marginata</B></I>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400044&lng=en&nrm=iso&tlng=en Alpha-galactosidase taken from a raw extract of Sesbania marginata legume seeds was purified by partitioning in aqueous two-phase systems (ATPS). Initially, galactomannan/dextran 2,000,000 systems were used for the purification, and the partition coefficients of alpha -galactosidase varied from 1.5 to 4.0. However, mass transport in these systems was poor due to the high viscosity of the employed polymers. Therefore, partitioning in polyethyleneglycol (PEG)/ sodium phosphate systems and the effect of sodium chloride upon the enzyme purification and the yield of alpha -galactosidase were also investigated. The purification achieved in a single-step was 5.7 with a recovery of 144% of alpha -galactosidase, possibly due to the removal of materials which inhibited alpha -galactosidase activity before the purification. The removal of the main protein contaminants and the highest yields were achieved in PEG 4,000/ sodium phosphate + 6% NaCl system at pH 5.0. Further purification by preparative on-exchange chromatography was also developed. <![CDATA[<b>Studies on the rheology and oxygen mass transfer in the clavulanic acid production by <i>Streptomyces clavuligerus</i></b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400045&lng=en&nrm=iso&tlng=en In the present work rheological characteristics and volumetric oxygen transfer coefficient (kLa) were investigated during batch cultivations of Streptomyces clavuligerus NRRL 3585 for production of clavulanic acid. The experimental rheological data could be adequately described in terms of the power law model and logistic equation. Significant changes in the rheological parameters consistency index (K) and flow behavior index (n) were observed with the fermentation evolution. Interesting correlations between the consistency index (K)/biomass concentration (C X) and the flow behavior index (n)/biomass concentration were proposed. Volumetric oxygen mass transfer coefficient (kLa) was determined by the gas balance method. Classical correlation relating the volumetric oxygen mass transfer coefficient to the operating conditions, physical and to transport properties, including apparent viscosity (muap), could be applied to the experimental results. <![CDATA[<B>A simplified kinetic model for the side reactions occurring during the enzymatic synthesis of ampicillin</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400046&lng=en&nrm=iso&tlng=en This work presents a kinetic study of the side reactions of the ampicillin enzymatic synthesis, from phenylglycine methyl ester and 6-aminopenicillanic acid using penicillin G acylase immobilized on agarose. A Michaelis-Menten model with competitive inhibition was fitted to initial rates of ester and antibiotic hydrolysis, at pH 6.5 and 25ºC. Inherent kinetic parameters were estimated for low enzymatic loads, to assure that diffusional resistance was not important. It was observed that ampicillin inhibits the hydrolysis of PGME, but the inhibitory effect of the ester on ampicillin hydrolysis was almost negligible. The obtained parameters were: k cat1= 0.025 mM/UI min, Km1 = 155.4mM, K AE = 16.18mM, k cat2= 4.67x10-3 mM/UI min, Km2 = 11.47, K EA = 0.68 mM. Parameter values are in the range reported in the literature, except for Km1, which is much higher. The large confidence interval for this parameter denotes that the model presents low sensitivity with respect to it. <![CDATA[<B>Thermal stability and energy of deactivation of free and immobilized cellobiase</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400047&lng=en&nrm=iso&tlng=en Commercial cellobiase has been immobilized in controlled pore silica particles by covalent binding with the silane-glutaraldehyde method with protein and activity yields of 67% and 13.7%, respectively. Thermal stability of the free and immobilized enzyme (IE) was determined with 0.2% w/v cellobiose solution, pH 4.8, temperatures from 40 to 70°C for free enzyme and 40 to 75°C for IE. Free cellobiase maintained its activity practically constant for 240 min at temperatures up to 55°C. The IE has shown higher stability retaining its activity in the same test up to 60° C. Half-lives for free enzyme were 14.1, 2.1 and 0.17 h at 60, 65 and 70°C, respectively, whereas the IE at the same temperatures had half-lives of 245, 21.3 and 2.9 h. The energy of thermal deactivation was 80.6 kcal/mol for the free enzyme and 85.2 kcal/mol for the IE, confirming stabilization by immobilization. <![CDATA[<B>Evaluation of inorganic matrixes as supports for immobilization of microbial lipase</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400048&lng=en&nrm=iso&tlng=en Candida rugosa was immobilized by physical adsorption on several inorganic supports using hexane as coupling medium. The enzymatic activities of the different derivatives were determined by both hydrolysis of olive oil and esterification of n-butanol with butyric acid. The results were compared to previous data obtained by using a controlled porous silica matrix. The goal was to contribute in searching inexpensive supports for optimum lipase performance. All supports examined exhibited good properties for binding the enzyme lipase. Zirconium phosphate was the best support, giving the highest percentage of protein fixation (86%) and the highest retention of lipase activity after immobilization (34%). The operational stability performance for niobium oxide derivative was improved by previously activated the support with silane and glutaraldehyde. Thermal stabilities were also examined by thermal gravimetric analysis (TG). <![CDATA[<B>Influence of substrate partition coefficient on the performance of lipase catalyzed synthesis of citronellyl acetate by alcoholysis</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400049&lng=en&nrm=iso&tlng=en The enzymatic synthesis of selected low molecular weight esters such as acetate esters by direct esterification using acetic acid as acyl donor usually display low yields. The acetic acid changes the polarity of the reaction medium, which in this turn modifies the partitioning of water between the solid phase (enzyme preparation) and the liquid phase (substrate), resulting in its accumulation on the enzyme solid phase. This may reduce the enzyme´s local pH. Therefore, the enzyme active site is modified and the reaction became nearly impossible. Our previous work showed that there is a negative relationship between enzyme activity and substrate partition coefficient (Ps); that is, the higher the substrate partition coefficient the lower the amount of product formed. This work investigated the feasibility of minimizing this inhibition by replacing the esterification reaction for alcoholysis reaction using several acetate esters. This approach enhanced the reaction yields to 46%, which is about 3 times higher than that one obtained in the esterification route. <![CDATA[<B>Thermal stability and deactivation energy of free and immobilized invertase</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400050&lng=en&nrm=iso&tlng=en The thermal stability and the energy of deactivation of free invertase and the immobilized enzyme (IE) was measured at temperatures in the range of 35 to 65°C for the hydrolysis of a 5% w/v sucrose solution. The free enzyme at pH 5.0 is stable up to 50°C for a period of 4 h. Invertase immobilized in controlled pore silica by the silane-glutaraldehyde covalent method is stable up to 55ºC, in pH 4.5 for the same period. For higher temperatures the enzyme deactivation follows the exponential decay model and half-lives are 0.53, 1.80, and 13.9 h for free invertase, at 65, 60, and 55ºC, respectively. For the IE half-lives are 0.48, 1.83, and 20.9 h, at 65, 60, and 55ºC, respectively. The IE is more stable than the free invertase; the energy of deactivation being 83.1 kcal/mol for the IE and 72.0 kcal/mol for the free enzyme. <![CDATA[<b>Characterization of free and immobilized invertase regarding activity and energy of activation</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400051&lng=en&nrm=iso&tlng=en Invertase from NOVO Nordisk has been immobilized in controlled pore silica particles (diameter: 0.351 mm and mean pore size: 37.5 nm) by covalent binding with the silane-glutaraldehyde method. The activity of the free and immobilized enzyme (IE) was determined with 5% (w/v) sucrose, at 35 to 65ºC and pH from 3 to 7. Maximum activities were found in the pH range from 5 to 6 for free invertase, and pH 4.5 for the IE. Activity yield for the IE was 24%. The Energy of Activation (Ea) was found to be a function of pH, giving for free invertase, Ea = 7.0 and 6.86 kcal/mol at pH 5.0 and 5.5, respectively, whereas for the immobilized enzyme, Ea = 6.55 and 5.93 kcal/mol at pH 4.5 and 5.0, respectively. <![CDATA[<B>A pseudobiospecific hollow fiber cartridge for <I>in vitro</I> adsorption of autoantibodies from pathological serum</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400052&lng=en&nrm=iso&tlng=en The affinity filtration technique using histidine as a pseudobiospecific ligand immobilized on poly(ethylene vinyl alcohol) hollow fiber membranes (His-PEVA) was used to remove autoantibodies from serum of patients with autoimmune disease. The effects of buffering solution conditions on the efficiency of autoantibodies removal was studied. The removal of anti-dsDNA, anti-SS-A/Ro, anti-Sm, anti-Sm/RNP and anti-cardiolipin autoantibodies present in the serum was investigated, comparing the efficiency between Hepes and Tris-HCl buffers. The results showed the potential of the membrane to remove all the autoantibodies studied. Anti SS-A/Ro was removed more efficiently in Tris-HCl buffer system rather than with the Hepes buffer. <![CDATA[<B>Optimization of pipe networks including pumps by simulated annealing</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400053&lng=en&nrm=iso&tlng=en The objective of this work is to present an application of the simulated annealing method for the optimal design of pipe networks including pumps. Although its importance, the optimization of pumped networks did not receive great attention in the literature. The proposed search scheme explores the discrete space of the decision variables: pipe diameters and pump sizes. The behavior of the pumps is describe through the characteristic curve, generating more realistic solutions. In order to demonstrate the versatility of the approach, three different examples of optimal design problems are solved. <![CDATA[<B>Predictive control based on neural networks</B>: <B>an application to a fluid catalytic cracking industrial unit</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400054&lng=en&nrm=iso&tlng=en Artificial Neural Networks (ANNs) constitute a technology that has recently become the focus of great attention. The reason for this is due mainly to its capacity to treat complex and nonlinear problems. This work consists of the identification and control of a fluid cracking catalytic unit (FCCU) using techniques based on multilayered ANNs. The FCC unit is a typical example of a complex and nonlinear process, possessing great interaction among the operation variables and many operational constraints to be attended. Model Predictive Control is indicated in these occasions. The FCC model adopted was validated with plant data by Moro (1992); and was used in this work to replace the real process in the generation of data for the identification of the ANNs and to test the predictive control strategy. The results of the identification and control of the process through ANNs indicate the viability of the technique. <![CDATA[<B>Removal of phenol by enzymatic oxidation and flotation</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400055&lng=en&nrm=iso&tlng=en This work presents a process for phenol removal comprising a reaction step in which phenol is polymerized in the presence of an enzyme followed by a separation step involving dissolved air flotation (DAF). A crude preparation from horseradish roots was used as a low purity source of the enzyme horseradish peroxidase (HRP). The technical feasibility of the process was studied at bench scale using 1 to 10 mM synthetic phenol solutions. Experimental results showed the potential of the proposed technique. A phenol conversion higher than 99 % was observed at the polymerization step and an efficiency higher than 94 % was achieved at the separation stage. Despite the use of a low purity source of HRP, which increases the input of organic matter, the chemical oxygen demand (COD) decreased by 50 %. <![CDATA[<B>Comparison of a reduced order model for packed separation processes and a rigorous nonequilibrium stage model</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400056&lng=en&nrm=iso&tlng=en The large dimensionality of the system of algebraic (and differential) equations of the models for packed distillation columns makes their solution difficult to be achieved. Aiming the solution of this problem, a reduced order model was developed for steady state packed separation processes. The model was developed transforming the differential equations of the rigorous model into algebraic equations through the use of the orthogonal collocation technique. The heat and mass transfer rates through the vapor-liquid interface were rigorously computed. The performance of this reduced order model was assessed comparing its results with those obtained with a rigorous rate based model. The results of the reduced order model were in good agreement with those from the rigorous model. Furthermore, the solution of the new model was the easiest to be obtained due to the robustness of the reduced order model. <![CDATA[<B>Characterizing the flow of stirred vessels with anchor type impellers</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400057&lng=en&nrm=iso&tlng=en Despite its importance in chemical industries, there are few works which studies anchor type impellers and only a fraction of the works investigate these systems under a computational approach. The great majority refers to turbine impellers, specially Rushton turbines, under turbulent flow. Anchor impellers are used specially for highly viscous flow, typical of polymer reactions. The viscosity is normally in the range 1000-10000 cp. Since this range of viscosity describe highly viscous flows, the reactions for anchor agitated systems are normally carried out under laminar flow. This work presents a detailed computational fluid dynamics (CFD) approach to study the behaviour of stirred vessels using anchor impellers. The axial plane of the tank, which is being modelled, is divided into small control volumes, which collectively is referred to as the mesh, or grid. In each of these cells the momentum balance, energy and mass conservation, which describes the model, are rewritten algebraically using the finite volumes method to relate such variables as velocity, pressure and temperature to values in neighbouring cells. The equations are then solved numerically, and the results yield the flow corresponding to the model. Since the geometry of a vessel with anchor impellers strictly calls for a three dimensional method, an approximation is made to account for the effect of the blades (Kuncewics, 1992). The main objective of this work is to give a detailed description of the flow generated by this axial impeller with a view to indicate ways in which the design and operation of these systems can be improved. <![CDATA[<B>Effects of Cu over Pd based catalysts supported on silica or niobia</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400058&lng=en&nrm=iso&tlng=en Palladium and palladium-copper catalysts supported on silica and niobia were characterized by H2 chemisorption and H2-O2 titration. Systems over silica were also analyzed by transmission electron microscopy and EXAFS. The metallic dispersion decreased from 20% to 7% when the content of Pd was increased from 0.5wt.-% to 3wt.-% in monometallic catalysts. The addition of 3 wt.-% Cu to obtain Pd-Cu catalysts caused a remarkable capacity loss of hydrogen chemisorption. TPR analysis suggested an interaction between the two metals and EXAFS characterization of the catalyst supported on silica confirmed the formation of Pd-Cu alloy. Pd/Nb2O5 catalysts showed turnover numbers higher than those obtained with the Pd/SiO2 systems in the cyclohexane dehydrogenation. However, the bimetallic catalysts showed very low turnover numbers. <![CDATA[<B>Recent extensions of the residence time distribution concept</B>: <B>unsteady state conditions and hydrodynamic model developments</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400059&lng=en&nrm=iso&tlng=en Two recent extensions of the residence time distribution concept are developed. The first one concerns the use of this method under transient conditions, a concept theoretically treated but rarely confirm by relevant experiments. In the present work, two experimental set-ups have been used to verify some limits of the concept. The second extension is devoted to the development of hydrodynamic models. Up to now, the hydrodynamics of the process are either determined by simple models (mixing cells in series, plug flow reactor with axial dispersion) or by the complex calculation of the velocity profile obtained via the Navier-Stokes equations. An alternative is to develop a hydrodynamic model by use of a complex network of interconnected elementary reactors. Such models should be simple enough to be derived easily and sufficiently complex to give a good representation of the behavior of the process. <![CDATA[<B>Evaluation of drying and degradation kinetics using neurocomputing</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400060&lng=en&nrm=iso&tlng=en Application of artificial neural network (ANN) in chemical engineering with special reference to drying process is discussed in the paper. Two types of networks: RBF and MLP, which are important for the description of a process dynamics, are presented. As an example drying and degradation of ascorbic acid in agricultural products are considered. The final conclusion supported with experimental data states that the type of ANN should be carefully selected because the real capability of the ANN model for a given dynamic problem is expressed in recurrent working mode. <![CDATA[<B>Robust predictive control of a gasoline debutanizer column</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400061&lng=en&nrm=iso&tlng=en This paper studies the application of Model Predictive Control to moderately nonlinear processes. The system used in this work is an industrial gasoline debutanizer column. The paper presents two new formulations of MPC: MMPC (Multi-Model Predictive Controller) and RSMPC (Robust Stable MPC). The approach is based on the concepts of Linear Matrix Inequalities (LMI), which have been recently introduced in the MPC field. Model uncertainty is considered by assuming that the true process model belongs to a convex set (polytope) of possible plants. The controller has guaranteed stability when a Lyapunov type inequality constraint is included in the MPC problem. In the debutanizer column, several nonlinearities are present in the advanced control level when the manipulated inputs are the reflux flow and the reboiler heat duty. In most cases the controlled outputs are the contents of C5+ (pentane and heavier hydrocarbons) in the LPG (Liquefied Petroleum Gas) and the gasoline vapor pressure (P VR). In this case the QDMC algorithm which is usually applied to the debutanizer column has a poor performance and stability problems reflected in an oscillatory behavior of the process. The new approach considers several process models representing different operating conditions where linear models are identified. The results presented here show that the multimodel controller is capable of controlling the process in the entire operating window while the conventional MPC has a limited operating range. <![CDATA[<B>Optimal policies for activated sludge treatment systems with multi effluent stream generation</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400062&lng=en&nrm=iso&tlng=en Most industrial processes generate liquid waste, which requires treatment prior to disposal. These processes are divided into sectors that generate effluents with time dependent characteristics. Each sector sends the effluent to wastewater treatment plants through pumping-stations. In general, activated sludge is the most suitable treatment and consists of equalization, aeration and settling tanks. During the treatment, there is an increase in the mass of microorganisms, which needs to be removed. Sludge removal represents the major operating costs for wastewater treatment plants. The objective of this work is to propose an optimization model to minimize sludge generation using a superstructure in which the streams from pumping-stations can be sent to the equalization tank. In addition, the aeration tank is divided into cells that can be fed in series and parallel. The model relies on mass balances, kinetic equations, and the resulting Nonlinear Programming problem generates the best operational strategy for the system feed streams with a high substrate removal. Reductions of up to 30 % can be achieved with the proposed strategy maintened BOD efficiency removal upper than 98 %. <![CDATA[<B>State and parameter estimation based on a nonlinear filter applied to an industrial process control of ethanol production</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400063&lng=en&nrm=iso&tlng=en Most advanced computer-aided control applications rely on good dynamics process models. The performance of the control system depends on the accuracy of the model used. Typically, such models are developed by conducting off-line identification experiments on the process. These experiments for identification often result in input-output data with small output signal-to-noise ratio, and using these data results in inaccurate model parameter estimates [1]. In this work, a multivariable adaptive self-tuning controller (STC) was developed for a biotechnological process application. Due to the difficulties involving the measurements or the excessive amount of variables normally found in industrial process, it is proposed to develop "soft-sensors" which are based fundamentally on artificial neural networks (ANN). A second approach proposed was set in hybrid models, results of the association of deterministic models (which incorporates the available prior knowledge about the process being modeled) with artificial neural networks. In this case, kinetic parameters - which are very hard to be accurately determined in real time industrial plants operation - were obtained using ANN predictions. These methods are especially suitable for the identification of time-varying and nonlinear models. This advanced control strategy was applied to a fermentation process to produce ethyl alcohol (ethanol) in industrial scale. The reaction rate considered for substratum consumption, cells and ethanol productions are validated with industrial data for typical operating conditions. The results obtained show that the proposed procedure in this work has a great potential for application. <![CDATA[<b>Practical evaluation of hold-up for passive decoupling in distillation column</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400064&lng=en&nrm=iso&tlng=en Decisions made during the steady-state design of distillation columns have a large influence on the dynamic characteristics of the operating unit. Conventionally, steady-state design is often completed before dynamics and control issues are considered. Additional complexity in the active control system must as a result be used to compensate for any resulting shortcomings in dynamic performance. Here, our aim is to improve control system performance not through more complex active control but through passive improvements arising from more favourable equipment design. An extra liquid hold-up, the middle vessel, was placed in the middle of the column by connecting the feed to an external vessel via a circulating system. To exploit and demonstrate the potential performance of this new modified design, our study was focussed on an existing pilot scale distillation column at Nottingham University. This new design was shown to be very effective in rejecting both feed composition and feed flowrate step changes. <![CDATA[<B>Retrofit design of multipurpose batch plants with multiple production routes</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400065&lng=en&nrm=iso&tlng=en This work analyzes possible alterations of configuration of an existing multipurpose batch plant for which new production targets and selling profits have been specified. New equipment can be added to the batch plant in the form of a retrofit in order to become a more economical and optimal design. The retrofit problem is posed as a mixed-integer formulation (MILP). The application of the proposed model is illustrated using three examples where can be verified that the method is efficient and of easy application, when compared of the others strategies to solve the retrofit problem. <![CDATA[<B>Dynamic modelling of catalytic three-phase reactors for hydrogenation and oxidation processes</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322000000400066&lng=en&nrm=iso&tlng=en The dynamic modelling principles for typical catalytic three-phase reactors, batch autoclaves and fixed (trickle) beds were described. The models consist of balance equations for the catalyst particles as well as for the bulk phases of gas and liquid. Rate equations, transport models and mass balances were coupled to generalized heterogeneous models which were solved with respect to time and space with algorithms suitable for stiff differential equations. The aspects of numerical solution strategies were discussed and the procedure was illustrated with three case studies: hydrogenation of aromatics, hydrogenation of aldehydes and oxidation of ferrosulphate. The case studies revealed the importance of mass transfer resistance inside the catalyst pallets as well as the dynamics of the different phases being present in the reactor. Reliable three-phase reactor simulation and scale-up should be based on dynamic heterogeneous models.