Volume 16, Issue 11 - Special Issue-International Energy Conference-Mexico, Guest Editors: Castillo-Araiza, Carlos O. / González-Brambilla, Margarita

The main goal of this paper is to critically review current microkinetics available for Fischer-Tropsch synthesis (FTS) modelling, in order to propose the best way to follow this set of complex reactions; therefore a microkinetic model was developed for FTS, accomplishing surface chemistry, heterogeneous kinetics, and single-event previous development for Co-based catalysts. Model starts simulating CO activation on catalyst surface, and then formation of methane, ethane and subsequent chain growth. Reaction rates were derived following the formalism of Langmuir-Hinshelwood-Hougen-Watson (LHHW); surface steps were proposed in consequence of feasibility. Chain growth was modelled by single-event steps, taking into account geometrical conformation explicitly. Number of growth steps of 1-olefins was rectified as requiring one more active site than mechanisms proposed previously; it was found that formation of these olefins exhibits a fast drop in chain growth; this phenomenon is explained in terms of probable geometrical conformations that lead to the number of single events. Experimental results in literature about isothermal synthesis of hydrocarbons in the gasoline range were simulated in a fixed-bed laboratory reactor; thermodynamic consistency was derived from chemical equilibrium over all reactions occurring during FTS. Equilibrium constants were evaluated as function of Gibbs free energy, and partial pressures of reactants and products.

Ethanol was produced using mucilage juice residues from processed cocoa with Pichia kudriavzevii in batch fermentation. Experimental results showed that maximum ethanol concentration was 13.8 g/L, ethanol yield was 0.50 g-ethanol/g glucose with a productivity of 0.25 g/L h. Likewise, a novel phenomenological model based on the mechanism of multiple parallel coupled reactions was used to describe the kinetics of substrate, enzyme, biomass and product formation. Model parameters were optimized by applying the Levenberg-Marquardt approach. Analysis of results was based on statistical metrics (such as confidence interval), sensitivity and by comparing calculated curves with the experimental data (residual plots). The efficacy of the proposed mathematical model was statistically evaluated using the dimensionless coefficient for efficiency. Results indicated that the proposed model can be applied as a way of augmenting bioethanol production from laboratory scale up to semi-pilot scale.

The problem of the optimization of selective treatment systems of wastewater streams contaminated with hexavalent chromium [Cr(VI)] is investigated. In order to comply with the Mexican environmental norm of Cr(VI) for treated wastewater streams at minimum cost, a nonlinear programming (NLP) model for the electrochemical reduction of hexavalent chromium to trivalent chromium was developed. The model incorporates a variable reaction rate, which is a function of the Cr(VI) concentration and the electrical current density of the electrochemical process. For this purpose, a basic superstructure of the effluent treatment is proposed. The superstructure is composed of three continuous electrochemical reactors without recirculation, and it may produce either a series and/or parallel design topology. The NLP model was used to minimize the objective function, defined as the total annual cost (TAC), which includes the capital cost of each electrochemical reactor, the electrical energy cost and the cost of the treatment of the wastewater streams. In order to investigate the solution set of the proposed NLP model, i. e., to improve the possibilities of obtaining optimum solutions based on economic criteria, a multi-start algorithm was implemented. Two example problems are used to show the versatility of the model and different local optimal solutions were obtained for each case study. The results show that a selective treatment of wastewater streams based on the search of local optimal solutions yields significant savings with respect to a centralized treatment design.

Cheese whey is a byproduct of the cheese industry and contains high concentrations of organic matter. Anaerobic digestion (AD) technology is an attractive solution to whey disposal since it allows the reduction of organic matter and simultaneously generates energy via biogas. The biological degradation of cheese whey is characterized by an unstable operation. A critical operational issue in the AD treatment of cheese whey is the tendency of rapid acidification of the waste requiring robust monitoring and control systems for reliable and efficient operation. Recent studies show that techniques based on fractal analysis of time series can be used for the indirect monitoring of critical variables of AD process (i. e., COD, VFA and methane production) for agro-industrial wastewaters. In this work, the application of the fractal analysis of pH time series obtained from an up-flow digester for cheese whey treatment is presented. The results suggest that fractal analysis can be applied to the indirect monitoring of a representative and high strength dairy wastewater. Furthermore, although the complex phenomena underlying in pH in the AD of cheese whey, the fractal analysis can unveil correlations of fractal parameters with key process variables.

Increasing the emissions of greenhouse gases (GHG) due to fossil fuel consumption has led to problems such as global warming, climate change, loss of biodiversity, and urban pollutions. Bioethanol production especially from different biomass such as wheat straw has been specified as one of the sustainable solutions to deal with energy crisis. Bioethanol logistics network optimization will reduce total costs of supply chain management and improves its competency with fossil fuels. In this paper, a mixed-integer linear programming (MILP) model is proposed to integrate and optimize bioethanol logistics network design problem. The proposed model is a multi-period and multi-echelon including feedstock supply centers, collection centers, bio-refineries, and customer centers. The proposed model is applied in a real case in Iran. The results justify the applicability and performance of the model in efficient design of bioethanol logistics network problems.

Edible oils, used in restaurants and households, have become a potential source of environmental pollution because their residuals are indiscriminately poured into rivers and lakes. One cost-effective and sustainable way to treat this waste is using this biomass in the production of biofuels, such as biodiesel. The main reactions for obtaining biodiesel are catalyzed in a homogeneous phase, using basic or acid solutions (NaOH or H 2 SO 4 , respectively) or in a heterogeneous phase, using a porous material with or without metals. One interesting reaction, owing to its low energy consumption, is carried out using biocatalysts of enzymes immobilized in porous materials. In this work, a porcine pancreatic lipase (PPL) was immobilized in a zirconium-pillared clay (Zr-PILC) by means of two syntheses: adsorption (PPL/Zr-PILC ADS) and cross-linking (PPL/Zr-PILC CL). The biocatalysts were used in the transesterification of canola oil. The amount of methyl esters was produced in the order Zr-PILC ≈ PPL ≪ PPL/Zr-PILC CL ≪ PPL/Zr-PILC ADS. According to these results, the activity and selectivity are a function of the method of synthesis and show the potential of these biocatalysts to transform waste oil in biodiesel at low cost by means of a sustainable process.

A theoretical model that describe the effect of design and operational conditions on current density distribution in a bioelectrochemical reactor used as microbial electrolysis cell (MEC) is described in this study. This model is proposed considering an approach where a direct electron transfer mechanism from the biofilm to the electrode surface takes place (mechanism present in most of microbial systems) and is governed by a dual donor-acceptor Nernst-Monod bioelectrochemical kinetic expression. The bioelectrochemical reactor is modelled considering two flow electrochemical reactor designs (a reactor design based in literature reports and a modified system proposed by the authors) operating at different flow inlet velocities and electrical overpotentials. Results obtained from the numerical solution shows that flow distribution is an essential aspect that impact the reactor performance, since concentration profiles and electrical potential-current distributions are strongly dependent on flow regime. Modified inlet configuration displays a more homogeneous fluid distribution and this behavior directly affects the mass transport and current density performance, as a result higher current density values are obtained for such configuration. Finally, it is expected that the information obtained from the analysis carried out in this report will provide us with a theoretical basis to realize the construction of a bioelectrochemical reactor prototype to develop the MEC concept.

This work presents the adsorption process of nitrogen compounds, namely quinoline (Q), pyridine (Pyr), and indole (In), from liquid fuels such as gasoline and diesel containing dibenzothiophene (DBT) as sulfur-containing molecules. These compounds were adsorbed on mesoporous materials, namely SBA-15 and SBA-16, in calcined form in batch mode using dodecane as a solvent represent to a diesel mixture. The main conclusion of this research is that SBA-15 showed a higher nitrogen adsorption capacity than SBA-16 for all molecules containing nitrogen and sulfur. A comparative study of nitrogen and sulfur adsorption confirms that selective removal of nitrogen compounds from fuels using SBA-15 was better than that of sulfur compounds. Moreover, an increase in the adsorption of Q was found with SBA-15 material compared to SBA-16. To explain this behavior, the solids were characterized using X-ray diffraction (XRD), nitrogen physisorption, and High-Resolution Transmission Electron Microscopy (HRTEM). A pseudo-second-order kinetic model, rather than a first-order one, fitted the nitrogen adsorption data best. Moreover, the Langmuir model was suitable for describing the adsorption of nitrogen compounds from simulated diesel fuel, instead of the Freundlich model, which means that nitrogen compounds are adsorbed in a monolayer.

This work shows the study of two alumina materials synthesized from aluminum sulfate with different purity by hydrolysis-precipitation route. The main difference between the aluminum salt precursors was the lower cost of one of them which was due to the higher percentage of magnesium species as impurity. Both materials showed different mesoporous structure nano-fibrillar. The physic-chemical properties of these materials were studied by several characterization techniques as XRD, XRF, BET, 27 Al MAS NMR, Pyridine adsorption FT-IR, FE-SEM/EDX, TEM and XPS. Furthermore, these materials were used as supports in the formulations of NiMo-based catalysts which were obtained by impregnation by the incipient wetness method of Ni and Mo salts in 3.3 and 15 wt. % respectively. The two NiMo/γ-Al 2 O 3 sulfide catalysts were evaluated as catalysts in the reaction of hydrodesulphurization (HDS) of dibenzothiophene (DBT), using a high-pressure batch reactor at 350 °C and 3.1 MPa and time reaction of 5 h −1 . The NiMo/γ-Al 2 O 3 catalyst prepared by aluminum sulfate of low purity and lower cost exhibited the highest HDS efficiencies, 95 %, respectively, which were mainly ascribed to the presence of Mg (0.9 wt. %) as impurity.

This is a theoretical study about the influence of turbulence on momentum and heat transport in a packed-bed with low tube to particle diameter ratio. The hydrodynamics is given here by the time-averaged Navier-Stokes equations including Darcy and Forchheimer terms, plus a κ-ε two-equation model to describe a 2D pseudo-homogeneous medium. For comparison, an equivalent conventional flow model has also been tested. Both models are coupled to a heat transport equation and they are solved using spatial discretization with orthogonal collocation, while the time derivative is discretized by an implicit Euler scheme. We compared the prediction of radial and axial temperature observations from a packed-bed at particle Reynolds numbers (Re p ) of 630, 767, and 1000. The conventional flow model uses effective heat transport parameters: wall heat transfer coefficient ( h w ) and thermal conductivity ( k eff ), whereas the turbulent flow model includes a turbulent thermal conductivity ( k t ), estimating h w via least-squares with Levenberg-Marquardt method. Although predictions of axial and radial measured temperature profiles with both models show small differences, the calculated radial profiles of the axial velocity component are very different. We demonstrate that the model that includes turbulence compares well with mass flux measurements at the packed-bed inlet, yielding an error of 0.77 % in mass flux balance at Re p = 630. We suggest that this approach can be used efficiently for the hydrodynamics characterization and design and scale-up of packed beds with low tube to particle diameter ratio in several industrial applications.