Ángel Martín

Encapsulation and Co-Precipitation Processes with Supercritical Fluids: Applications with Essential Oils

Essential oils have important commercial applications as preservatives and flavours, and more recently as natu- ral antimicrobial agents. These applications require a suitable formulation constituted by biodegradable compounds that protect the essential oil from degradation and evaporation at the same time that allows for a sustained release. Microcap- sules of biopolymers loaded with essential oils meet these requirements. Such microcapsules can be prepared with differ- ent processes such as spray-drying, freeze-drying and coacervation, and supercritical fluids are an advantageous medium for this purpose. Some supercritical fluid-based precipitation processes have already been applied to produce these micro- capsules. Amongst them, the results obtained with Particles from Gas Saturated Solutions (PGSS), PGSS-drying and Con- centrated Powder Form (CPF) processes are particularly promising. Recent developments in the preparation of formula- tions with supercritical fluids include the preparation of liposomes and micelles, which can be suitable carriers for essen- tial oils.

Precipitation Processes with Supercritical Fluids : Patents Review

Supercritical fluid techniques for materials precipitation have been proposed as an alternative to conventional precipitation processes as they can improve the performance of these processes in terms of reduction of particle size and control of morphology and particle size distribution, without degradation or contamination of the product. These techniques have received much attention during the last years, and their feasibility and performance have been proved for many substances. Several precipitation technologies, in which the supercritical fluid plays different roles (solvent, anti solvent, co solvent, solute, atomization agent...) have been developed. This article presents a review of the patents related to supercritical precipitation technologies, with emphasis on the description of the different precipitation methods and mechanisms exploited by these technologies, and on the technical solutions given for the practical problems of the technologies.

Hydrogen Storage in sH Clathrate Hydrates: Thermodynamic Model

A thermodynamic model of equilibrium states of structure sH clathrate hydrates of hydrogen with methyl tert-butyl ether, methyl cylohexane, or 1,1-dimethyl cyclohexane is presented. The model uses the van der Waals-Platteeuw statistical-thermodynamical model to represent the hydrates and the cubic-plus-association equation of state to represent the fluid phases. Good agreement between experimental and calculated phase equilibrium data has been obtained, with average absolute pressure deviations between experiments and calculations ranging from 1.2 to 2.1% depending on the promoter. The model has also been used to estimate the occupancy of the cavities of the hydrate by hydrogen and the promoter, as well as the hydrogen storage capacity of the hydrate. This capacity has been found to vary between 0.85 and 1.05% of hydrogen by weight at the conditions of formation of the sH hydrates (270-280 K and 60-100 MPa).

Thermodynamic Modeling of Promoted Structure II Clathrate Hydrates of Hydrogen

This work presents a thermodynamic model of equilibrium states of binary structure II gas hydrates of hydrogen with different promoters based on the van der Waals-Platteeuw statistical thermodynamic model. Since the determination of the occupancy of the small cavities by hydrogen is still an unresolved question, the model allows for multiple occupancy of these cavities. The agreement of the predictions of the model with experimental data is satisfactory, with average absolute pressure deviations below 10%. Model results indicate that at the pressures of hydrogen hydrate formation, the small cavities of the hydrate are singly occupied by hydrogen and that at moderate pressures (P<30 MPa), hydrogen hardly displaces the promoter from the large cavities, even when the concentration of the promoter is reduced below the stoichiometric concentration. Model results are also used to optimize the conditions for hydrogen storage in the gas hydrates.

Experimental and Computational Investigation of the sII Binary He−THF Hydrate

The objective of this work is to study the binary He-THF hydrate with both experimental and theoretical approaches. Experimental data for the hydrate equilibrium at pressures up to 12.6 MPa are reported for the binary He-THF hydrate with stoichiometric THF composition (i.e., 5.56 mol % THF). These data are used to calibrate a thermodynamic model [J. Phys. Chem. C2009, 113, 422] for the prediction of hydrate equilibrium that is based on the van der Waals-Platteeuw statistical thermodynamic theory. Then this model is used to extrapolate the obtained experimental data to much higher pressures, and good agreement is observed with other available experimental data at pressures up to 150 MPa. This model is also capable of estimating the cavity occupancies for He and THF. The results show that the large cavities are completely occupied by THF molecules, whereas the small ones are partially occupied by He atoms. The He occupancy of the small cavities is less than 60%, even at high pressures (100 MPa). The occupancies predicted from this model are in close agreement with similar results from molecular simulations and a previously reported thermodynamic approach.

A Simplified van der Waals-Platteeuw Model of Clathrate Hydrates with Multiple Occupancy of Cavities

In clathrate hydrates formed by small guest molecules such as H(2) or He, hydrate cavities may be occupied by clusters of several guest molecules. Multiple occupancy of cavities is important for applications of clathrate hydrates as gas storage and transportation media due to the increase of storage capacity of the material associated with multiple occupancy. Computational approaches for clathrate hydrates with multiple occupancy such as Grand Canonical Monte Carlo (GCMC) simulations or van der Waals-Platteeuw (vdW-P) models with rigorous calculations of Langmuir adsorption constants are complex and require considerable computational effort. In this work, a simplified vdW-P model for clathrate hydrates with multiple occupancy is presented. In this model, it is assumed that guest molecules inside cavities form clusters in which molecules occupy fixed positions with respect to each other. For validation of this supposition, results obtained with this model have been compared with GCMC simulations of sII He and H(2) hydrates with multiple occupancy. Results of the simplified procedure presented in this work regarding the fractional occupancy of cavities by molecular clusters agree well with GCMC simulations. The simplified vdW-P model presented in this work requires a small computational effort, equivalent to calculations with the standard vdW-P model for hydrates with single occupancy.

Effect of the spraying conditions and nozzle design on the shape and size distribution of particles obtained with supercritical fluid drying

In the perspective of production of dry therapeutic protein formulations, spray drying of lysozyme (as a model protein) into supercritical carbon dioxide was studied. The effects of the nozzle (i.e., co-current coaxial converging and converging-diverging, and T-mixer impinging) and process conditions (i.e., flow rates, pressure) on the drying of the lysozyme prepared in aqueous solution dried with supercritical carbon dioxide enriched with ethanol were investigated. The particle size distribution, width of particle size distribution and morphology were used to determine the effect of the various parameters assessed. Particles with a median size of approximately 1.5, approximately 5 or approximately 25 microns were produced depending of the nozzle selected. A basic comparative study of the nozzle was done by computational fluid dynamics, but the differences in particle size could not be depicted by these computations. The proportional increase of the flow rates (up to fivefold) caused a decrease in particle size (7- to 12-fold), and doubling the pressure caused a moderate decrease of the size (5-20%). The individual effect of the supercritical carbon dioxide, ethanol and solution streams was explained with a mass transfer model. Changing the ratio between flow rates slightly affected the particle size in various ways because of the swelling and shrinking stages of the drying droplet in supercritical carbon dioxide enriched with ethanol.

Compact Integration of a GSM-19 Magnetic Sensor with High-Precision Positioning using VRS GNSS Technology

Magnetic data consists of a sequence of collected points with spatial coordinates and magnetic information. The spatial location of these points needs to be as exact as possible in order to develop a precise interpretation of magnetic anomalies. GPS is a valuable tool for accomplishing this objective, especially if the RTK approach is used. In this paper the VRS (Virtual Reference Station) technique is introduced as a new approach for real-time positioning of magnetic sensors. The main advantages of the VRS approach are, firstly, that only a single GPS receiver is needed (no base station is necessary), reducing field work and equipment costs. Secondly, VRS can operate at distances separated 50–70 km from the reference stations without degrading accuracy. A compact integration of a GSM-19 magnetometer sensor with a geodetic GPS antenna is presented; this integration does not diminish the operational flexibility of the original magnetometer and can work with the VRS approach. The coupled devices were tested in marshlands around Gandia, a city located approximately 100 km South of Valencia (Spain), thought to be the site of a Roman cemetery. The results obtained show adequate geometry and high-precision positioning for the structures to be studied (a comparison with the original low precision GPS of the magnetometer is presented). Finally, the results of the magnetic survey are of great interest for archaeological purposes.

Supercritical Water Oxidation (SCWO) of Solid, Liquid and Gaseous Fuels for Energy Generation

The SCWO (Supercritical Water Oxidation) process is well known for being able to destroy any kind of compound without producing prejudicial byproducts. This fact together with the high potential for energy production (because the high pressure high temperature effluent generated in the process) makes it a good candidate for generating energy from bio-fuels, especially those which valorization by conventional combustion can be problematic. In this work, different literature energetic studies of the SCWO process are analyzed. When comparing the heat produced by direct expansion of the effluent and by indirect heating steam generation it is observed that when direct expansion is used, the energetic efficiency is much higher than when the effluent is used to heat an auxiliary fluid of a Rankine or Brayton cycle. Nevertheless, the production of energy by direct expansion of the SCWO is not technically available in the short term. In any case, obtaining a high temperature effluent it is a key point for optimizing energy utilization. To do so, reactors in which the effluent is not diluted or reactor working at hydrothermal flame regime are desirable. Also the lay-out of the plant is important for energy utilization and factors as preheating scheme must be thoroughly studied. In addition to all of this, SCWO process has the additional advantage of the possibility of CO2 sequestration.

On Standard Reductions to Relative Gravity Measurements. A Case Study Through the Establishment of the New Local Gravity Net in the Province of Valencia (Spain)

Abstract Standard reductions to gravity readings due to Earth tides, ocean loading and attraction, polar motion, instrumental height and air pressure variations and loading of atmospheric masses are studied in this paper from a practical point of view, that is, taking into account their numerical values and their influence on gravimetric readings and relative gravimetric observations. The study was carried out using the observations and definition of a new local gravimetric net. This new local gravimetric net has been established in the province of Valencia (Eastern Spain) to meet the increasing requirements of geophysics, geology, geodesy and geodynamics. The net comprises 21 sites, which are an average of 45 km apart and was measured using Lacoste & Romberg D203 and G301 gravimeters. Gravity values were determined using one fixed station in relation to an absolute one and 202 relative gravimetric observables. Reductions are applied for Earth tides (with real accurate amplitude and phase-difference for the principal tidal waves analysed from 301 digitally recorded days of gravity readings) where oceanic attraction and loading has been considered. In addition, reductions for polar motion, vertical gradient to instrument height and air pressure and loading of atmospheric masses have been applied. The net was established using least square adjustment where the weights of each relative gravimetric observable were determined by iterative estimation in accordance with the Huber robust estimation procedure. Obtained standard deviations of the final gravity values have an average value of 18x10-8 ms-2 (18 μGal), minimum value of 10x10-8 ms-2 and maximum value of 26x10-8 ms-2. The statistical analysis of the results concludes with a precision and reliability determination. Discussion of the numerical values obtained in the standard gravimetric reductions shows the importance of each one in the final solution, bearing in mind that the relative gravimetric observables have been obtained using Lacoste & Romberg instruments and the geographical location of the net. The main conclusion is that only Earth tides reduction (with approximate amplitude and phase-difference numbers for the principal tidal waves) have to be taken into account.