Juliana Piña

Use of a heterogeneous two-dimensional model to improve the primary steam reformer performance

The reforming units are basically furnaces containing burners (which provide a large amount of heat by fuel combustion) and tubes packed with supported nickel catalyst. Due to the high heat input through the reformer tube wall and the endothermic reforming reactions, the catalyst tubes are exposed to significant axial and radial temperature gradients. For this reason, a two-dimensional mathematical model that takes into account the diffusion reaction phenomena inside the particles rigorously has been used to represent the reactor. Strong radial temperature gradients in the reformer tube have been found, particularly close to the reactor entrance. These temperature differences cause significant variations in the methane reaction rate along the radial position, being the catalyst close to the reforming tube center poorly used. For this reason, the reforming tube diameter and the catalyst activity distribution were modified to use the catalyst more efficiently. The tube diameter has an important influence on the reformer performance, considerable higher conversions and reactor capacities per tube (i.e. closer equilibrium approaches) have been observed for the tubes with smaller diameters. The catalyst activity distribution also strongly impacts the reactor operation. The use of two catalysts of different activity, adequately distributed along the axial and radial directions, allowed to significantly decrease the maximum tube wall temperature and simultaneously minimize the reactor volume fraction packed with the catalyst of higher activity.

Fundamental Study of Thermal Treatment of Soil

Morphological and chemical changes exhibited by different types of soils heated to different final temperatures are reported. Beds of soils were heated (in a helium atmosphere) from ∼20°C to final temperatures, ranging from between 200 to 900°C, simulating ex situ thermal treatments in a nonoxidizing media. Structural changes exhibited by the soil samples during the treatments were analyzed by SEM, measurement of surface area, and measurement of particle porosity. The soil chemical transformations were quantified by means of soil weight loss, light gases yields, and carbon conversion. Soils with low organic matter content do not undergo important structural and chemical changes during the thermal treatment. On the other hand, soils with high organic carbon content suffer significant chemical modifications and, as a consequence, noticeable structural transformations. Indeed, for thermal treatments of about 900°C, weight losses as high as 22u2009wt%, final surface area of one order of magnitude higher than its original value (untreated soil), and changes of porosity as high as 27% were found for soils of high organic matter content. Simple mathematical equations are proposed to predict the soil weight loss and particle porosity as a function of the treatment temperature. The models provide a good fit to the experimental data.

Population balance discretization for growth, attrition, aggregation, breakage and nucleation

Abstract This paper presents a new discretization method to solve one-dimensional population balance equations (PBE) for batch and unsteady/steady-state continuous perfectly mixed systems. The numerical technique is valid for any size change mechanism (i.e., growth, aggregation, attrition, breakage and nucleation occurring alone or in combination) and different discretization grids. The developed strategy is based on the moving pivot technique of Kumar and Ramkrishna and the cell-average method of Kumar et al. A novel contribution is proposed to numerically handle the growth and attrition terms, for which a new representation of the number density function within each size class is developed. This method allows describing the number particle fluxes through the class interfaces accurately by preserving two sectional population moments. By comparing the numerical particle size distributions with analytical solutions of one-dimensional PBEs (including different size change mechanisms and particle-size dependent kinetics), the accuracy of the proposed numerical method was proved.

Microencapsulation of Phytosterols by Spray Drying

Abstract Phytosterols (PS) are vegetable sterols with a similar structure to cholesterol. Although PS are poorly absorbed into the blood stream, they are widely recognized as lowering absorption of cholesterol and their serum levels. It has been found that PS exert their hypocholesterolemic effect if they are dispersed. Indeed, the PS must be administered finely divided in order to facilitate their exposure to the bile salts, preferably in particles smaller than 25xa0μm to reduce the sandy mouth feel and favor their incorporation into the micellar phase in the intestine. Several authors focused on particle-size reduction to improve the PS dispersibility by several techniques. However, all of them are complex and time- and energy consuming because they require more than one step (homogenization or milling, including cooling or heating) to obtain the desired particle size. Furthermore, for solid PS, several abrasive effects of the homogenizer valves or parts of the milling equipment have been found. PS and their derivatives (stanols, esters, and stanol esters) have been included in fat- or oil-based food products, which are clearly restricted in diets for hypercholesterolemia. Therefore, the incorporation of PS in aqueous-based formulations (like beverages, soups, and others) is an attractive field of application. The hydrophobic and water-insoluble nature of PS, which make them poor candidates for stable dispersions, hinder their applicability on intermediate or final aqueous-based products. In this context, the microencapsulation by spray drying appears as a good choice to provide a physical barrier between the PS and the aqueous medium. In this chapter, the microencapsulation of PS by spray drying was studied to obtain particles (microcapsules) with small sizes (lower than 25xa0μm) in order to keep the suspension stable. The feed suspensions to be spray dried were analyzed in details (e.g., composition, treatment conditions, rheological behavior, and interfacial properties). The influence of these factors on the microcapsule structure and on the particle size of PS in suspension was particularly investigated. The main operating variables were also rationally studied in order to identify the optimum set in terms of low particles size and high process yield, encapsulation efficiency, and PS retention in the powder of microcapsules. Several parameters involved in the microcapsules formation were affected by the spray-drying conditions. Finally, the microcapsules of PS were incorporated in aqueous matrices and the stability of the product suspension was analyzed. The microencapsulation of PS by spray drying using a mixture of Arabic gum and maltodextrin was successfully achieved. Moreover, by including a surface active material in low concentrations (especially sodium lauryl sulfate at 2%, m/v), better results were obtained (low mean particle size and relatively high encapsulation efficiency and PS retention). Infrared spectroscopy, powder X-ray diffraction, and differential scanning calorimetry of the raw materials and microcapsules indicated that no changes in chemical structure nor strong interaction between microcapsules components took place.