Oscar M. Hernández-Calderón

Optimal design of integrated agricultural water networks

Abstract This paper presents a mathematical programming model for the optimal design of water networks in the agriculture. The proposed model is based on a new superstructure that includes all configurations in terms of use, reuse and regeneration of water in a field constituted by a number of croplands. The model also includes the allocation of pipelines, pumps and storage tanks in different irrigation periods. The objective function consists in maximizing the annual profit that is formed by the economic incomes owing to the crop sell minus the costs for fresh water, fertilizer, storage tanks, treatment units, piping and pumping. The proposed multi-period optimization problem is formulated as a mixed integer non-linear programming formulation, which was applied to a case study to demonstrate the economic, environmental and social benefits that can be obtained.

Solving the heat and mass transfer equations for an evaporative cooling tower through an orthogonal collocation method

Abstract In this paper, the orthogonal collocation technique is utilized to solve the Poppe method equations for heat and mass transfer in counter flowing wet-cooling towers. The six differential equations for unsaturated and supersaturated air from the Poppe method are simplified, yielding three differential equations that use the Heaviside function. The humidity ratio is demonstrated to be a finite power series at a normalized water temperature. The air enthalpy is expressed as a function of the normalized water temperature and the unknown coefficients of the expansion from the humidity ratio. The discrete formulation is solved using the Newton–Raphson method using an explicit Jacobian. The proposed methodology is applied to eight examples, and the results are compared to the results obtained when the governing equations are integrated with the Dormand–Prince method. The results indicate that the accuracy is similar between both techniques. However, the orthogonal collocation requires less CPU time.

A possible mechanism of metabolic regulation in Gibberella fujikuroi using a mixed carbon source of glucose and corn oil inferred from analysis of the kinetics data obtained in a stirrer tank bioreactor

A nonstructured model was used to study the dynamics of gibberellic acid production in a stirred tank bioreactor. Experimental data were obtained from submerged batch cultures of Gibberella fujikuroi (CDBB H‐984) grown in varying ratios of glucose‐corn oil as the carbon source. The nitrogen depletion effect was included in mathematical model by considering the specific kinetic constants as a linear function of the normalized nitrogen consumption rate. The kinetics of biomass growth and consumption of phosphate and nitrogen were based on the logistic model. The traditional first‐order kinetic model was used to describe the specific consumption of glucose and corn oil. The nitrogen effect was solely included in the phosphate and corn oil consumption and biomass growth. The model fit was satisfactory, revealing the dependence of the kinetics with respect to the nitrogen assimilation rate. Through simulations, it was possible to make diagrams of specific growth rate and specific rate of substrate consumptions, which was a powerful tool for understanding the metabolic interactions that occurred during the various stages of fermentation process. This kinetic analysis provided the proposal of a possible mechanism of regulation on growth, substrate consumptions, and production of gibberellic acid (GA3) in G. fujikuroi.

Modeling of Effective Moisture Diffusivity in Corn Tortilla Baking: Effective moisture diffusivity…

The objective of this work was to model the mass transfer in corn tortilla baking using different approaches for effective diffusivity based on the Ficks law of diffusion and to evaluate the impact of the process on quality parameters. The 1st one assumes constant effective diffusivity (method of slopes by subperiods and method of successive approximations) and the 2nd one considers variable effective diffusivity (quadratic function of time and Weibull distribution). In addition, the Weibull distribution was applied to fracturability. The effective moisture diffusivity inside the tortilla during baking is not constant and the estimations generated when considering variable diffusivity with quadratic time and Weibull distribution showed better fits (both, R2 = 0.999) to the average moisture content. Quality parameter fracturability was affected by the baking process and the Weibull model adequately described (R2 = 0.996) the fracturability behavior. This work will allow an adequate estimation of the concentration profiles and histories for mass transfer operations in products that can be represented as an infinite plate. The obtained analytical solutions with variable diffusivity will help to estimate the optimal conditions of the baking process to achieve the required final moisture content for baked corn tortilla shells. PRACTICAL APPLICATION The analytical solutions of the Ficks law of diffusion for the moisture content in products that can be represented as an infinite plate, considering variable diffusivity, can be useful in studies when accurate estimations of effective diffusivity and concentration are needed.

Hydrodynamics and Mass Transfer Simulation in Airlift Bioreactor with Settler using Computational Fluid Dynamics

Abstract In this work, the effect of inlet-gas superficial velocity over the circulation liquid velocity, gas holdup and mass transfer, from an airlift bioreactor with settler were studied by Computational Fluid Dynamics (CFD) modeling and contrasted with experimental results. Multiphase mixture model and κ-ε turbulence model were used to describe the two phases gas-liquid flow pattern in airlift bioreactor. The hydrodynamic parameters such as liquid circulation velocity and gas holdup were computed by solving the governing equations of continuity, moment and turbulence transport using the finite volume method. Global mass transfer coefficient was evaluated through the Higbie’s penetration theory and the two-phase fluid dynamic theory. Comparison between our numerical data and experimental data previously reported in the literature was done. Numerical and experimental data were very close, and the differences found were discussed in terms of the limitations of this study.

PHOTOCHEMICAL DEGRADATION OF NITROBENZENE BY S2O8 -2 IONS AND UV RADIATION

In this work nitrobenzene (NB) degradation was studied through an advanced oxidation process (AOP) by using a photochemical reactor and two kinds of oxidizing reagent: potassium persulfate (K 2 S 2 O 8 ) and sodium persulfate (Na 2 S 2 O 8 ), to promote generation of HO • radicals. It was determined the effect of different parameters on nitrobenzene degradation, such as: wavelength of UV lamps, concentration of oxidizing reagent, type of oxidizing reagent and number of UV lamps in the reactor. By using UV lamps with radiation wavelength of 254 nm, NB degradation reached almost 100 % meanwhile, irradiating at 350 nm resulted in < 35 % degradation. For experiments using UV lamps of 254 nm, the reaction rate increased with the radiant energy, being higher by using four UV lamps than two UV lamps for a reaction time of 60 min. The specific energy consumption for two UV lamps showed the lowest value being of 2626.0 KW.h/Kg of NB degraded.