Scielo RSS <![CDATA[Brazilian Journal of Chemical Engineering]]> http://www.scielo.br/rss.php?pid=0104-663220020001&lang=en vol. 19 num. 1 lang. en <![CDATA[SciELO Logo]]> http://www.scielo.br/img/en/fbpelogp.gif http://www.scielo.br <![CDATA[<B>THE DEVELOPMENT AND EXPERIMENTAL TESTING OF A FUZZY CONTROL SYSTEM FOR BATCH DISTILLATION</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322002000100001&lng=en&nrm=iso&tlng=en The present work describes the development and implementation of fuzzy control algorithms in order to control on-line the overhead product composition of a batch distillation column. Firstly, the influence of design parameters was evaluated through computational simulations and then the algorithms were experimentally tested by monitoring a pilot column. Binary mixtures of n-hexane/n-heptane were distilled. Temperature measurements and vapor-liquid equilibrium data are the basis for the inference of overhead and bottom compositions. Two different operational strategies were used for the experimental runs: constant overhead product composition and previously determined set-point trajectory. Using the first strategy, the performance of the fuzzy controllers is compared to the performance of conventional feedback digital controllers. Experimental results show that fuzzy control presents a better performance than the conventional digital feedback control and also that fuzzy controllers were able to deal successfully with variable set-point strategy, albeit using constant design parameter values. Under conventional control, the average reflux rate implemented was higher than the average reflux rate implemented with fuzzy algorithms. Consequently, the process becomes less time- and energy-consuming under fuzzy control. Since fuzzy methodology is a promising new way of looking at process control problems and their solutions, the results of this work could provide control system designers with a better evaluation of the potential worth of fuzzy control. <![CDATA[<B>ROBUST MPC FOR STABLE LINEAR SYSTEMS</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322002000100002&lng=en&nrm=iso&tlng=en In this paper, a new model predictive controller (MPC), which is robust for a class of model uncertainties, is developed. Systems with stable dynamics and time-invariant model uncertainty are treated. The development herein proposed is focused on real industrial systems where the controller is part of an on-line optimization scheme and works in the output-tracking mode. In addition, the system has a time-varying number of degrees of freedom since some of the manipulated inputs may become constrained. Moreover, the number of controlled outputs may also vary during system operation. Consequently, the actual system may show operating conditions with a number of controlled outputs larger than the number of available manipulated inputs. The proposed controller uses a state-space model, which is aimed at the representation of the output-predicted trajectory. Based on this model, a cost function is proposed whereby the output error is integrated along an infinite prediction horizon. It is considered the case of multiple operating points, where the controller stabilizes a set of models corresponding to different operating conditions for the system. It is shown that closed-loop stability is guaranteed by the feasibility of a linear matrix optimization problem. <![CDATA[<B>SIMULTANEOUS SCHEDULING AND OPERATIONAL OPTIMIZATION OF MULTIPRODUCT, CYCLIC CONTINUOUS PLANTS</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322002000100003&lng=en&nrm=iso&tlng=en The problems of scheduling and optimization of operational conditions in multistage, multiproduct continuous plants with intermediate storage are simultaneously addressed. An MINLP model, called TSPFLOW, which is based on the TSP formulation for product sequencing, is proposed to schedule the operation of such plants. TSPFLOW yields a one-order-of-magnitude CPU time reduction as well as the solution of instances larger than those formerly reported (Pinto and Grossmann, 1994). Secondly, processing rates and yields are introduced as additional optimization variables in order to state the simultaneous problem of scheduling with operational optimization. Results show that trade-offs are very complex and that the development of a straightforward (rule of thumb) method to optimally schedule the operation is less effective than the proposed approach. <![CDATA[<B>LIQUID-LIQUID EQUILIBRIA OF THE ACETIC ACID-WATER-MIXED SOLVENT (CYCLOHEXYL ACETATE-CYCLOHEXANOL) SYSTEM</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322002000100004&lng=en&nrm=iso&tlng=en Mixtures of cyclohexyl acetate and cyclohexanol were used as a mixed solvent to study liquid-liquid equilibria (LLE) of the acetic acid-water-cyclohexanol-cyclohexyl acetate quaternary system. The solubility diagram and tie-line data were determined at 298±0.20 K and atmospheric pressure, using various compositions of mixed solvent. Reliability of the data was ascertained by making Othmer-Tobias and Hand plots. <![CDATA[<B>GROWTH AND ENZYME PRODUCTION DURING CONTINUOUS CULTURES OF A HIGH AMYLASE-PRODUCING VARIANT OF <I>Aspergillus Oryzae</B></I>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322002000100005&lng=en&nrm=iso&tlng=en Growth and product formation by a selected variant of Aspergillus oryzae showing high alpha-amylase production was studied in continuous cultivations carried out at six different specific growth rates, using glucose as the growth-limiting nutrient. The analysis of the steady-state data revealed that the variant and wild-type strains were similar with respect to glucose uptake system and stoichiometric coefficients. However, the variant was capable of maintaining an enzyme production as high as 40 FAUgDW-1h-1 at a dilution rate of 0.2 h-1, while the wild-type strain reached a maximum specific alpha-amylase production rate of 17 FAUgDW-1h-1 at a dilution rate of 0.1 h-1. Using a morphologically structured model originally proposed for the wild-type strain, it was possible to describe enzyme production, biomass formation and glucose consumption after modification of a few parameters to adjust the model to the characteristics of the selected variant. <![CDATA[<B>ANALYSIS OF TRICKLE BED AND PACKED BUBBLE COLUMN BIOREACTORS FOR COMBINED CARBON OXIDATION AND NITRIFICATION</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322002000100006&lng=en&nrm=iso&tlng=en Biological removal of nitrogen and carbon by combined nitrification-oxidation in gas-liquid trickle-bed reactors (TBRs) and packed bubble columns (PBCs) was analyzed theoretically using a transient two-dimensional model. The model describes TBR and PBC performances at steady state as well as their transient response to a pulse or step increase in inlet methanol and NH4+-nitrogen concentrations. The hydrodynamic parameters were determined from residence time distribution measurements, using an imperfect pulse method for time-domain analysis of nonideal pulse tracer response. A transient diffusion model of the tracer in the porous particle coupled with the piston-dispersion-exchange model was used to interpret the residence time distribution curves obtained. Gas-liquid mass transfer parameters were determined by a stationary method based on the least-squares fit of the calculated concentration profiles in gas phase to the experimental values. Analysis of steady-state performances showed that under like operating conditions, the TBR outperforms the PBC in terms of conversions. A pulse change in the inlet methanol or NH4+-nitrogen concentration causes a negligible transient change in the outlet methanol concentration and a negligible or high transient change in the outlet NH4+-nitrogen concentration. A step change in the inlet methanol concentration causes the negligible transient change in the methanol outlet concentration and a relatively important transient change in the NH4+-nitrogen outlet concentration. A step increase in the NH4+-nitrogen inlet concentration induces a drastic transient change in the NH4+-nitrogen outlet concentration but a negligible transient change in the methanol outlet concentration. <![CDATA[<B>A COMPARISON OF DIFFERENT MODELING APPROACHES FOR THE SIMULATION OF THE TRANSIENT AND STEADY-STATE BEHAVIOR OF CONTINUOUS EMULSION POLYMERIZATIONS</B> <B>IN PULSED TUBULAR REACTORS</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322002000100007&lng=en&nrm=iso&tlng=en Dynamic mathematical models are developed to simulate styrene emulsion polymerization reactions carried out in pulsed tubular reactors. Two different modeling approaches, the tanks-in-series model and the axial dispersion model, are compared. The models developed were validated with experimental data from the literature and used to study the dynamics during transient periods, e.g., the start-up of the reactor and the response to disturbances. The effect of the Peclet number on process variables such as conversion and particle concentration was also verified. <![CDATA[<B>ELECTRODEPOSITION OF COPPER IONS ON FIXED BED ELECTRODES</B>: <B>KINETIC AND HYDRODYNAMIC STUDY</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322002000100008&lng=en&nrm=iso&tlng=en The kinetic and hydrodynamic behaviour of a fixed-bed electrochemical reactor was studied in terms of current efficiency (CE) and energy efficiency (EE). In the kinetic experiments the effects of fixed bed thickness (L), current density (i) and initial concentration of copper (C0) were studied. In the hydrodynamic experiments the permeability (k) of the electrode and the coefficient for inertial forces (c) were also studied as functions of the applied current density. At low current densities and bed thicknesses greater than 1.0 cm, negative CE and EE were observed as a consequence of the dissolution of the porous matrix. At high current densities low CE and EE were observed and a powdery deposit was formed on the surface of the particles. From the results of the kinetic study bed thickness and the range of current densities employed in the hydrodynamic experiments were chosen. In these experiments the electrodeposition process continued until the whole electrode had been clogged and no more electrolyte could pass through it. The relationship between pressure drop and flow rate was well described by the Forchheimer equation. It was observed that the reduction in porosity due to copper electrodeposition causes the flow rate to decrease because of the decrease in electrode permeability, but it had no influence on current efficiency. <![CDATA[<B>ELEVATION ON BOINLING POINT OF COFFE EXTRACT</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322002000100009&lng=en&nrm=iso&tlng=en The rise in boiling point of coffee extract was experimentally measured at soluble solids concentrations in the range of 9.2 to 52.4ºBrix and pressures between 5.8 × 10³ and 9.4 × 10(4) Pa (abs.). Different approaches to representing experimental data, including the Dühring's rule, the Antoine equation and empirical models proposed in the literature were tested. In the range of 9.2 to 16.2ºBrix, the rise in boiling point was nearly independent of pressure, varying only with extract concentration. Considerable deviations of this behavior began to occur at concentrations higher than 16.2ºBrix. Experimental data could best be predicted by adjusting an empirical model which consists of a single equation that takes into account the dependence of rise in boiling point on pressure and concentration.