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Brazilian Journal of Chemical Engineering, Volume: 16, Número: 3, Publicado: 1999
  • Predicting diffusivities in dense fluid mixtures

    DARIVA, C.; COELHO, L.A.F.; OLIVEIRA, J. VLADIMIR

    Resumo em Inglês:

    In this work the Enskog solution of the Boltzmann equation, as corrected by Speedy, together with the Weeks-Chandler-Andersen (WCA) perturbation theory of liquids is employed in correlating and predicting self-diffusivities of dense fluids. Afterwards this theory is used to estimate mutual diffusion coefficients of solutes at infinite dilution in sub and supercritical solvents. We have also investigated the behavior of Fick diffusion coefficients in the proximity of a binary vapor-liquid critical point since this subject is of great interest for extraction purposes. The approach presented here, which makes use of a density and temperature dependent hard-sphere diameter, is shown to be excellent for predicting diffusivities in dense pure fluids and fluid mixtures. The calculations involved highly nonideal mixtures as well as systems with high molecular asymmetry. The predicted diffusivities are in good agreement with the experimental data for the pure and binary systems. The methodology proposed here makes only use of pure component information and density of mixtures. The simple algebraic relations are proposed without any binary adjustable parameters and can be readily used for estimating diffusivities in multicomponent mixtures.
  • Modelling and parameter estimation in reactive continuous mixtures: the catalytic cracking of alkanes - part II

    PEIXOTO, F. C.; MEDEIROS, J.L. de

    Resumo em Inglês:

    Fragmentation kinetics is employed to model a continuous reactive mixture of alkanes under catalytic cracking conditions. Standard moment analysis techniques are employed, and a dynamic system for the time evolution of moments of the mixture's dimensionless concentration distribution function (DCDF) is found. The time behavior of the DCDF is recovered with successive estimations of scaled gamma distributions using the moments time data.
  • Inulinase from Kluyveromyces marxianus: culture medium composition and enzyme extraction

    PESSOA JR, A.; VITOLO, M.

    Resumo em Inglês:

    K. marxianus DSM 70106 was cultivated for inulinase production in a medium containing 2.0 g/L of yeast extract, 5.0 g/L of peptone and salts. The addition of corn steep liquor did not increase enzyme production. Inulin, as the main carbon source, afforded higher inulinase production than glucose, fructose, sucrose, lactose, maltose and starch. Glucose, fructose and sucrose reduced enzyme production by 46, 58 and 71%, respectively. By using the best culture medium enzyme activity remained stable for 22 months at 4oC; while at -18oC it decreased by 10%. Maximal activity was found in the pH range of 3.5 to 5.0 and at temperatures from 50 to 60oC. Flocculation was used for cell separation. Shifting the pH was more efficient than using polyelectrolytes, CaCl2, bentonite and Fe2O3. Recovery of inulinase by AOT(sodium di-2-ethylhexyl sulfosuccinate)-reversed micelles yielded up to ~20%.
  • Phase rule calculations and the thermodynamics of reactive systems under chemical equilibrium

    PLATT, G. M.; MEDEIROS, J. L. de

    Resumo em Inglês:

    In this paper, we examine the resolution of some phase rule problems within the context of multiple chemical equilibrium reactions, using cubic equations of state and an activity coefficient model. Bubble and dew reactive surfaces, reactive azeotropic loci and reactive critical loci are generated and presented in graphical form. Also isobaric bubble and dew reactive enthalpy loci, which may be useful in the modeling of reactive distillation operations, are depicted. All the formalism here employed is developed within the coordinate transformation of Ung and Doherty, which is appropriate for equilibrium reactive or multireactive systems. The major contribution of this work is the determination of critical loci for reactive or multireactive equilibrium systems. Since it is known that for some class of chemical reactions the kinetics and product distribution exhibit high sensitivity to pressure near criticality, the present study may be useful as a predicting tool in these cases if the chemical equilibrium condition is not too far from the real phenomenon.
  • Using hybrid neural models to describe supercritical fluid extraction processes

    FONSECA, A. P.; STUART, G.; OLIVEIRA, J. V.; LIMA, E.

    Resumo em Inglês:

    This work presents the results of a hybrid neural model (HNM) technique as applied to modeling supercritical fluid extraction (SCFE) curves obtained from two Brazilian vegetable matrices. The serial HNM employed uses a neural network to estimate parameters of a phenomenological model. A small set of SCFE data for each vegetable was used to generate a semi-empirical extended data set, large enough for efficient network training, using three different approaches. Afterwards, other sets of experimental data, not used during the training procedure, were used to validate each approach. The HNM correlates well withthe experimental data, and it is shown that the predictions accomplished with this technique may be promising for SCFE purposes.
  • Pinch analysis based on rigorous physical properties

    WESTPHALEN, D. L.; MACIEL, M.R .WOLF

    Resumo em Inglês:

    Pinch Analysis is a methodology that allows the calculation of minimum hot and cold utilities consumption in a process from the knowledge of some data of the hot and cold streams. These energy targets are calculated through the Problem Table Method, that is based on the description of the process using constant heat capacity flowrates. This hypothesis is not valid in certain situations, particularly when phase changes are involved. A modification to Problem Table Method is proposed, that allows the use of enthalpies rigorously calculated from thermodynamic correlations. One case study is provided, where an error of 240% in hot target is verified if phase changes are neglected.
  • The influence of specific mechanical energy on cornmeal viscosity measured by an on-line system during twin-screw extrusion

    CHANG, Y. K.; MARTINEZ-BUSTOS, F.; PARK, T. S.; KOKINI, J .L.

    Resumo em Inglês:

    The influence of specific mechanical energy (SME) on cornmeal viscosity during the twin-screw extrusion at feed moisture contents of 25 and 30% and screw speeds in the range from 100 to 500 rpm was measured. Cornmeal was extruded in a co-rotating, intermeshing twin-screw coupled to a slit die rheometer. One approach to the on-line rheological measurement is to use a slit die with the extruder. In the present work it was show that shear viscosity decreased as a function of SME. The viscosity of cornmeal at the exit die was influenced by screw speed, rate of total mass flow, mass temperature inside the extruder and SME. An increase in screw speed resulted in an increase in SME and a decrease in viscosity. A reduction in slit die height and an increase in screw speed and mass temperature led to a remarkable macromolecular degradation of the starch, as evidenced by the decrease in viscosity.
  • Fractionation of benzene/n-hexane mixtures by pervaporation using polyurethane membranes

    CUNHA, V. S.; NOBREGA, R.; HABERT, A. C.

    Resumo em Inglês:

    In the present work polyurethane membranes obtained from different polyester/MDI-based polymers were used to separate benzene/n-hexane mixtures by pervaporation. In pervaporation experiments, with a 50% wt feed at room temperature, permeate fluxes in the range of 0.3 to 3.2 Kg/m2h (10 <FONT FACE="Symbol">m</font>m membrane thickness) and selectivity in the range of 3.8 to 5.6 were obtained. The permeate was always enriched in benzene. Taking into account the compromise between flux and selectivity, the best performance membrane was selected for complementary sorption and pervaporation experiments. Results show that selectivity increases and the permeation flux decreases when the benzene concentration in the feed decreases. In the present application, results also show that sorption is the main factor for selectivity. Using the distillation azeotropic mixture as feed, almost no influence of temperature on selectivity was observed in the range of 25oC to 56oC. The permeate flux increases seven-fold, while selectivity remains constant near 8.0.
  • EXAFS as a tool for catalyst characterization: a review of the data analysis methods

    NORONHA, F. B.

    Resumo em Inglês:

    A review of the EXAFS data analysis methods is presented. A detailed description of the EXAFS signal extraction and the Fourier transform of the data are discussed. The procedure for determining interatomic distances, coordination numbers and disorder effects from EXAFS data is described. This paper also discusses the data analysis statistics. Finally, one example of catalyst characterization by the EXAFS technique is reported.
  • Dense fluid self-diffusion coefficient calculations using perturbation theory and molecular dynamics

    COELHO, L. A. F.; OLIVEIRA, J. V.; TAVARES, F. W.

    Resumo em Inglês:

    A procedure to correlate self-diffusion coefficients in dense fluids by using the perturbation theory (WCA) coupled with the smooth-hard-sphere theory is presented and tested against molecular simulations and experimental data. This simple algebraic expression correlates well the self-diffusion coefficients of carbon dioxide, ethane, propane, ethylene, and sulfur hexafluoride. We have also performed canonical ensemble molecular dynamics simulations by using the Hoover-Nosé thermostat and the mean-square displacement formula to compute self-diffusion coefficients for the reference WCA intermolecular potential. The good agreement obtained from both methods, when compared with experimental data, suggests that the smooth-effective-sphere theory is a useful procedure to correlate diffusivity of pure substances.
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