Silica aerogel; synthesis, properties and characterization

Polysaccharide-based aerogels—Promising biodegradable carriers for drug delivery systems

Aerogel technology provides high added-value lightweight materials with outstanding textural properties (i.e., high surface area and open porosity). Aerogels are obtained from wet gels by using a suitable drying technology, usually supercritical drying process, able to avoid the pore collapse phenomenon in order to keep intact the porous texture of the wet material. In this sense, the study of the kinetic profile of the gel supercritical drying is regarded as a key aspect to be considered in the specific case of the design of aerogel-based systems. In this work, the drying profile with supercritical carbon dioxide (scCO 2 ) of alcogels (in ethanol) was determined using a customized supercritical fluid extraction equipment. The drying of alcogels from different precursors (inorganic-silica-, organic-starch-), densities (silica aerogel of densities 0.08 and 0.15 g/cm 3 ) and morphologies (cylindrical monoliths, microspheres) was studied. Depending on the nature of the gel precursor, the extent of drying (i.e., drying time duration) gave significant differences in the end textural properties of the dried gel.

Supercritical drying of aerogels using CO2: Effect of extraction time on the end material textural properties

Spray drying: An overview on wall deposition, process and modeling

Synthesis and physical properties of TEOS-based silica aerogels prepared by two step (acid–base) sol–gel process

This article presents both theoretical and experimental determination of the hydrodynamics of drying air in an industrial countercurrent spray dryer. Air velocity measurements were performed in selected areas of the tower to determine the flow pattern in the dryer and to collect data to validate the computational fluid dynamics (CFD) modeling and verify the results. For no-swirl operation mode, 3D CFD calculations showed high instability of the air flow in the dryer. A bent, pillow-shaped flow above the drying air inlet, which promotes deposition of particles in this area of the wall, was detected. CFD calculations also proved that when drying air was introduced to the tower tangentially; from 20° to 30° in relation to the initial air flow configuration in the spray-drying tower, the air flow was stabilized and air velocity near the wall increased, which might reduce wall deposition of particles. Comparison of experimental and theoretical results showed that the CFD model of countercurrent spray-drying pr...

Modeling of Air Flow in an Industrial Countercurrent Spray-Drying Tower

Extraction of a primary solvent with liquid or supercritical carbon dioxide is the most difficult to control stage in low-temperature silica aerogel production. Diffusion of primary ethanol through alcogel structure to surrounding CO 2 was investigated in carefully controlled experiments with cylindrical alcogel samples. Changes of the alcohol concentration in carbon dioxide leaving the autoclave were followed with on-line chromatograph analysis and experiments were repeated for temperatures around the critical point of carbon dioxide. On the basis of the concentration histories during the drying, the temperature dependence of diffusion coefficient in the ethanol–CO 2 mixture inside the silica gel at constant pressure was identified.

Diffusion of ethanol–carbon dioxide in silica gel

The paper presents a unique methodology of CFD modeling of heat and mass transfer during a counter-current spray-drying process based on the concept of a negative heat source reflecting the energy necessary for evaporation of water. The volumetric internal heat source term in Navier-Stokes equations describing heat transfer was used to determine energy transfer from explicit to latent heat. Total power consumption during the drying process was estimated on the basis of extensive industrial measurements of air temperature distribution along the axis and radius during regular tower operation. The probability function to control the distribution of density of power consumption as a function of the distance from the atomizers was determined by the trial and error method to fit experimental temperature measurements in the industrial tower. This approach allowed us to carry out 3D CFD accurate calculations of the counter-current spray-drying process analyzing continuous phase only. General methodology applied i...

Model of Heat and Mass Transfer in an Industrial Counter-Current Spray-Drying Tower

In low-temperature aerogel production, replacement of an original solvent with liquid or supercritical carbon dioxide is the crucial stage, and it is one fully controlled by a diffusional mechanism. The diffusion of ethanol as the primary pore fluid through an alcogel structure to the surrounding near-critical CO 2 was investigated in carefully controlled experiments with cylindrical alcogel samples. Changes of the alcohol concentration in carbon dioxide leaving the autoclave were followed with an online gas chromatographic analysis. The experiments covered the range of temperatures from 292 to 315 K and pressure from 6.9 to 9.1 MPa. On the basis of the concentration histories during the drying, the diffusion coefficient in the ethanol–CO 2 mixture inside the silica gel was identified for each experiment. A comparison of measured concentrations with values derived from a mass transfer model is presented and discussed. The method for identifying an effective diffusion coefficient is described and the values of ethanol–CO 2 effective diffusivity in a silica network are presented. The correlation of effective diffusion coefficient in the porous structure of silica gel with the Schmidt number ratio covering the entire investigated range of temperature and pressure is proposed.

Effective diffusion coefficient in the low temperature process of silica aerogel production

Gels dried in supercritical conditions give a new class of materials called aerogels. The process of silica aerogel production in which extraction with liquid (or supercritical) carbon dioxide is used, consists of four main stages: elution, diffusion, heating and evacuation. Aerogel production from silica gels prepared from tetraethoxysilane is described. The autoclave process which started with alcohol-liquid carbon dioxide exchange was followed by supercritical evacuation. Some important technical aspects of the process are considered. The effect of elution rate, temperature and duration of diffusion is investigated.

Z. Bartczak

Papers

Modeling of Air Flow in an Industrial Countercurrent Spray-Drying Tower

Diffusion of ethanol–carbon dioxide in silica gel

Model of Heat and Mass Transfer in an Industrial Counter-Current Spray-Drying Tower

Effective diffusion coefficient in the low temperature process of silica aerogel production

Drying of Silica Alcogels by Liquid and Supercritical C02