Scielo RSS <![CDATA[Latin American applied research]]> vol. 34 num. 3 lang. es <![CDATA[SciELO Logo]]> <![CDATA[Catalyst systems for the oxidation of phenol in water]]> The oxidation of phenol solutions (5 g/L) has been studied in a semibatch autoclave reactor. Activity screening experiments were performed over two commercial catalysts: CuO+ZnO (Topsoe) and CuO+NiO (Engelhard), and CuO/Al2O3 prepared in our laboratory. Isothermal runs between 393 and 423 K were carried out under an oxygen pressure of 7 bar. Phenol conversion as well as pH and Chemical Oxygen Demand (COD) measurements were recorded. Almost complete phenol conversion was obtained for all catalysts. However, conversion to CO2 was only partial. Laboratory prepared catalysts gave the best performance in terms of activity and selectivity with the lowest induction period. Stability tests showed that catalysts remained moderately active after 60 h of operation. A simple kinetic model fits experimental results. <![CDATA[Nonlinear H∞ control of an experimental pH neutralization system]]> The classical control theory is based on the design of linear controllers for systems described by linear models. However, there exist some situations where it is not recommended, or even impossible, to use a linear controller. One of those situations arises when the magnitude of the process gain experiences a dramatic variation within the operating range of interest. A classic example of a chemical process where this situation occurs is the pH control around the neutralization point in a continuous stirred tank. In this work, the pH control for a strong acid - strong base system is addressed. To solve this problem, a nonlinear H∞ control law is derived based on a nonlinear model previously developed. The attainment of that control law is done with the help of recent mathematical results from the authors concerning the solution of Hamilton-Jacobi-Isaacs inequalities. The nonlinear controller is implemented on an experimental reactor and its performance is compared with a PID control law tuned according to the classical minimum error integral criteria. The obtained results show that the nonlinear H∞ control theory can be a good alternative to solve this difficult SISO (Single Input - Single Output) control problem. <![CDATA[Effects of solution and soil chemistry on the distribution of oil residual in patagonian soil]]> The distribution coefficient of oil residuals between water and the soil under equilibrium conditions is, in the case of Patagonian soils, strongly dependent on the clay contents and humidity of the soil. Other variables such as, the soil salinity and the environmental exposure conditions, also affect the interactions between the phases. The oil residuals are generally accompanied by water spills, that are extracted together with the oil and which, frequently, have high salinity. These salts stay on soils during long times and finally they became part of the soil. The resulting oil aqueous concentrations and the soil-water distribution ratio (Kd) may be strongly influenced by these factors. Based on experimental data of several parameters determined in different regional field samplings, a semi-empiric model was developed that allows prediction of the Kd dependence with the exposure time, the salinity of the equilibrium aqueous concentration and the clay contents of the soil. The Monte Carlo simulation was used in this work. Distribution values of soil conductivity, clays content, and assumed fixed values of the age of the oil spill were used in the model. A set of calculated values of Kd was obtained and the results show a distribution that was probabilistically analyzed. The Kd values increase with increasing age of spill and soil salinity and decrease when the salinity of the initial aqueous concentration is greater than the soil salinity. The determined parameters are useful for modeling of the environmental impact on polluted soils and for the design of remediation techniques. <![CDATA[Optimal operating points in alkaline pulping]]> The problem of optimizing the alkaline impregnation of wood chips is posed and solved under usual restrictions. The cost to be optimized balances opposite criteria by taking economics and product quality into account, and is conditioned by the system dynamics. Evolution is modeled from typical transport phenomena equations. Optimization is attacked in the lines of variational calculus, although the final treatment involves numerical methods. Cost function design is provided for: alkali consumption, thermal energy consumption, product quality, and total production; each one affected by a preference-weighting coefficient. A new parameter, the "Deacetylation Index", is introduced as an observable quantity for tracking the end of the digestion stage in pulping processes. This index turns out to be significant even at low temperatures. Cost terms depend essentially on three design variables: (i) alkaline bulk concentration, (ii) digester temperature, and (iii) total duration of the process. An algorithm to ascertain the optimal values of these variables is devised. Numerical results provide insight in deciding changes on design variables in case they are allowed within a certain extent to be manipulated. <![CDATA[Improved activity and stability of Ce-promoted Ni/γ -Al2O3 catalysts for carbon dioxide reforming of methane]]> The CO2 reforming of CH4 was carried out over Ni catalysts supported on γ -Al2O3 and CeO2-promoted γ -Al2O3. The catalysts were characterized by means of surface area measurements, TPR, CO2 and H2 chemisorption, XRD, SEM, and TEM. The CeO2 addition promoted an increase of catalytic activity and stability. The improvement in the resistance to carbon deposition is attributed to the highest CO2 adsorption presented by the CeO2 addition. The catalytic behavior presented by the samples, with a different CH4/CO2 ratio used, points to the CH4 decomposition reaction as the main source of carbon deposition. <![CDATA[Enhancement to the LuGre model for global description of friction phenomena]]> The LuGre model of friction -a bristle based model- predicts important friction phenomena useful for control of mechanical systems. This model accurately describes the behavior of control systems for small initial conditions. This paper proposes a simple but fundamental modification of the model in order to improve the reliability in a global sense. This improvement increases the comprehension of experimental evidences in control of mechanisms. <![CDATA[Decoupling with stability of linear multivariable systems: An algebraic approach]]> The result that a linear multi-variable system is decouplable with stability if and only if its associated stable interactor is diagonal, is proved in this paper using an algebraic approach. As it will be shown, this condition is actually equivalent to the coincidence between the infinite and unstable global structure (infinite and unstable zeros) and the row infinite and unstable structure of the system. Two procedures are presented to compute a state feedback which decouples the system with stability, the first one based on the solution of a polynomial matrix equation, and the second one based on the static left kernel of a strictly proper rational matrix. Illustrative examples are also presented. <![CDATA[A hybrid neural model for the production of sorbitol and gluconic acid using immobilized Zymomonas mobilis cells]]> Only ten years were enough for hybrid neural network-first principle models (HNM) reach a status of a standard industrial tool. This modeling strategy is employed here to represent the production of sorbitol and gluconic acid from glucose and fructose, using permeabilized and immobilized Zymomonas mobilis cells. Mass component balances are derived for the substrate concentrations. A multilayered neural network is used to represent the reaction rate. Experimental results were used to develop and validate the model. The HNM allows the elucidation of the phenomena involved in the process. It is observed from the results that the resistance for mass transfer from the liquid to the particles is increased at higher substrate concentrations and that the reaction rate depends on the concentrations of substrate and product in the particles. Additionally, it may be stated that the flexibility of the HNM allows the development of a model that would otherwise be difficult, if based solely on phenomenological principles. <![CDATA[Rheometry and scanning electron microscopy study of casein curds added with mesquite seed gum and soy proteins]]> The rheological and micro-structural characteristics of curds added with mesquite seed gum (MESG) and soy protein (SP) were studied. Two types of curds were prepared either with 100% raw commercial milk or with a 50% low-fat milk and the effect of the addition of 0.15% (w/w) and 0.45% (w/w) of MESG and/or 0.3% (w/w) and 0.6% (w/w) soy protein content was analyzed. The addition of MESG and SP results in moisture of up to 64.0% compared to 43.2% in the control sample. The rheometric functions G' (storage modulus) and G'' (loss modulus), measured in the frequency sweep from 6.28 to 62.8 rad/s tend to decrease for curds prepared with 100% raw milk and the addition of MESG and/or SP. In the case of low-fat milk curds the opposite behavior was observed. The micro-morphology of the samples added with MESG-SP present a cavernous appearance that was not present in the control curd.