Drying Of Silica Gel Research Articles

Plastic deformation and heat-enabled structural recovery of monolithic silica aerogels

Drying shrinkage during ambient pressure drying of silica gels is made reversible by preventing condensation reactions of surface silanol groups via surface modification. This partial recovery of the gel volume and structure is referred to as the spring-back effect (SBE) and enables the production of monolithic silica aerogels by evaporative drying. The SBE is sometimes completed by annealing at mild temperatures. Similarities between drying-related deformations and deformations induced by mechanical stimuli suggest analogous underlying mechanisms. While the causes of drying shrinkage are relatively well-known, it remains unclear how the relaxation of the structure by drying and annealing occurs across the different length scales. Here we show a complete structural recovery of silica aerogels at the macro- and nano-scale enabled by annealing. We propose that residual deformations after drying and mechanical compression are caused by the entanglement of silica clusters that can be unraveled by annealing at 230 °C. The deformation under loading is interpreted as two different re-arrangement mechanisms for dry and annealed gels, by the sliding of the silica clusters along the loading direction and by the compression of large pores beyond the fractal structure, respectively. Our results demonstrate how the shape and structure of silica aerogels can be restored and controlled by thermal activation, broadening the various applications of these materials. We also emphasize how tuning silica gels to promote a two-step SBE by annealing can pave the way toward the production of larger monolithic aerogels by APD.Graphical

Solvent Cavitation during Ambient Pressure Drying of Silica Aerogels.

Ambient-pressure drying of silica gels stands out as an economical and accessible process for producing monolithic silica aerogels. Gels experience significant deformations during drying due to the capillary pressure generated at the liquid-vapor interface in submicron pores. Proper control of the gel properties and the drying rate is essential to enable reversible drying shrinkage without mechanical failure. Recent in operando microcomputed X-ray tomography (μCT) imaging revealed the kinetics of the phase composition during drying and spring-back. However, to fully explain the underlying mechanisms, spatial resolution is required. Here we show evidence of evaporation by hexane cavitation during the ambient-pressure drying of silylated silica gels by spatially resolved quantitative analysis of μCT data supported by wide-angle X-ray scattering measurements. Cavitation consists of the rupture of the pore liquid put under tension by capillary pressure, creating vapor bubbles within the gels. We found the presence of a homogeneously distributed vapor-air phase in the gels well ahead of the maximum shrinkage. The onset of this vapor/air phase corresponded to a pore volume shrinkage of ca. 50 vol % that was attributed to a critical stiffening of the silica skeleton enabling cavitation. Our results provide new aspects of the relation between the shape changes of silica gels during drying and the evaporation mechanisms. We conclude that stress release by cavitation may be at the origin of the resistance of the silica skeleton to drying stresses. This opens the path toward producing larger monolithic silica aerogels by fine-tuning the drying conditions to exploit cavitation.

Experimental study of heat transfer, hydrodynamics and drying kinetics in a centrifugal fluidized bed apparatus

This paper presents the test results of a prototype installation for drying dispersed materials in a centrifugal fluidized bed. The prototype is a batch apparatus with a vertical supply of a drying agent. A diagram of a test bench designed to study the hydrodynamics of the apparatus and the heat and mass transfer process during drying in a centrifugal fluidized bed is given and described. Silica gel with a particle diameter of 3.8 mm was used as a dispersed material. Hydrodynamic experiments were carried out on a “cold” model. Based on their results, the relationship of the hydraulic resistance of the apparatus at various values of the air velocity and mass of material in the working chamber is constructed. Experimental data have been obtained on low-temperature drying of silica gel in a centrifugal fluidized bed for various modes. The data are presented as time relationships of the temperature and humidity of the material and the drying agent. Analysis of the data obtained showed that the heat and mass transfer process in a centrifugal fluidized bed has a high intensity. A positive conclusion is made about the possibility of using this apparatus for low-temperature drying of capillary-porous bodies.

Experimental study of the silica gel drying in a fluidized bed apparatus

In this paper, the design and principle of operation of the dryer with a centrifugal fluidized bed is considered. It is a batch apparatus with a vertical drying agent supply and is intended for drying dispersed materials with a particle diameter from 1 to 5 mm, having a high initial moisture content and a strong moisture and solid dry frame bond. Silica gel with a particle diameter from 2.2 to 2.5 mm, used for drying air and industrial gases, was used as a drying object. The scheme of the test bench is given and described. The results of an experimental study of the process of low-temperature drying of silica gel (the temperature of the drying agent ranged from 35 to 65 °C) are presented. The obtained experimental data is presented as a dependence of the thermophysical parameters of the material and the drying agent on the drying time. The comparison of drying efficiency at different modes is given. On the basis of the obtained data, a positive conclusion about the possibility of using this apparatus for low-temperature drying of capillary-porous bodies is made.

Organically modified silica aerogel with different functional silylating agents and effect on their physico-chemical properties

In this paper, we have successfully focused on a hydrophobic silica aerogels synthesis by the ambient drying of silica gels and the silylation with mono-, di-, and tri-functional agents was studied to improve physico-chemical properties of the aerogels. Silylated silica aerogels with water contact angles 161±2°, 152±3°, 147±3° were obtained using trimethylchlorosilane, dimethyldichlorosilane, and methyltrimethoxysilane as a mono-, di- and tri-functional silylation reagents, respectively. The prepared mono-functional silylated silica aerogels exhibited relatively high thermal stability, low thermal conductivity, transparency, high porosity, and enhanced superhydrophobicity with low surface free energy making them suitable as multifunctional for many domestic and industrial applications. Our mono-functional silylated silica aerogels showed superior physico-chemical properties compared with di- and tri-functional silylated silica aerogels.

Non-uniformity investigation in a combined thermal and microwave drying of silica gel

Local temperature and moisture were determined to analyze the uniformity in vertical and horizontal planes in a combined thermal and microwave (CTMW) drying chamber. Furthermore, the effects of microwave power and temperature of hot air on drying uniformity were also investigated. It could be concluded that the dried silica gel in the center area of the drying chamber obtained the highest drying rate and temperature, followed by the silica in the outermost and the middle portions. The dehydration difference of the three layers decreased with the drying time, whereas the drying uniformity increased. Overall, the change in uniformity in the horizontal direction of the drying cavity was not obvious, and the temperature and moisture content gradients mainly occurred in the vertical direction. The temperature gradient between the layers reached 5 °C. As the hot air temperature and microwave power increased, the drying rate improved and the drying uniformity decreased.

Permittivities of Ultra-Low Dielectric Silica Beads and Aerogels

Materials with relative permittivity or dielectric constant near to that of air (εr~1) are known as ‘ultra-low k’ materials. They find a number of applications in inter-connects of micro-electronic circuits, antennae, high-speed communication substrates etc. Among the inorganic materials, porous silica is the widely studied candidate. Porous silica can be of many types depending upon the extent of porosity and size and connectivity of pores. This paper presents the details of measurement of permittivities and the results of silica beads and silica aerogels. Silica beads, prepared by microwave heating of silica gels, are spherical beads of average 1mm size. Hydrophobic silica aerogels, prepared by ambient pressure drying of silica gels, are irregular chunks of 5-10 mm size. Both are potential bulk fill insulation materials and hence the permittivity can be measured as an aggregate filling a definite volume. The permittivities of these have been measured upto 1 MHz by 3-terminal method using a precision LCR meter and a powder-paste cell as per ASTM-D150-11. The εr values of silica aerogels and silica beads in 20 Hz-1 MHz range could be measured and are less than 1.6 at 1 MHz.

Drying of Silica Gel in a Microwave Electromagnetic Field

This article presents results of an experimental study of the drying kinetics of silica gel in a microwave electromagnetic field. The influence exerted by various process parameters on the drying kinetics under a microwave energy supply is ascertained on the basis of the experimental data. The substantial impact of the initial moisture content of the silica gel and the power supplied on the time required for moisture desorption is demonstrated. Use of a proposed energy criterion has made it possible to generalize experimental results obtained for different regime parameters.

Investigation of the processes of impregnation and drying of granular silica gel

The process of capillary impregnation and drying of silica gel grains in the acousto-convective drier of the ITPM of the Siberian branch of the Russian Academy of Sciences has been investigated experimentally. Two methods for humidifying a material with developed surface and internal structures have been considered. A comparison of these methods has been made and the influence of the impregnation rate on the geometry of silica gel grains has been analyzed. Silica gel grains were dried by three methods: microwave, convective, and acousto-convective. The dependence of the drying rate and the quality of the dried material on the chosen drying method has been shown. To describe the moisture extraction, we propose a mathematical model based on a two-dimensional diffusion equation written in the cylindrical system of coordinates. The moisture distribution in cylindrical samples consisting of silica gel grains has been obtained numerically for various values of the initial moisture content with the use of certain diffusion coefficients and the dependence of the moisture transfer coefficient on the frequency of acousto-convective action.

Diffusion During the Supercritical Drying of Silica Gels

Drying is the most critical elaboration step of large monolithic and crack-free silica aerogel plates. In the present work, we are studying the supercritical CO2 drying and more precisely the first step, here called the supercritical washing step. This phase consists of replacing the liquid phase contained in the nanopores with supercritical CO2. Within this study, this step is governed by molecular diffusion through the gels. These phenomena were investigated experimentally in order to estimate the duration of the washing step. The experimental results were then fitted with an analytical mass transfer model to identify the effective diffusion coefficient.

Drying of Silica Gels to Obtain Aerogels:Phenomenology and Basic Techniques

Click to increase image sizeClick to decrease image sizeKeywords: Silica gelsSilica aerogelsEvaporative dryingSupercritical drying

Shrinkage during drying of silica gel

A model is provided for predicting gel shrinkage during drying by combining the empirical observations that: (1) the bulk modulus of a gel increases with density, ρ, according to K p = K 0 ( ρ ρ y ) m (2.5 ≤ m ≤ 4) ; and (2) the variation in pore radius, r, is approximately proportional to pore volume (contrary to the dependence, r α ρ −1 3 , conventionally assumed). No allowance is made for viscoelastic relaxation, so the model applies only for drying from an inert solvent. The model may be used to guide processing efforts to yield either aerogel-like materials with high porosity or xerogels with high density and small pore size for adsorbents, membranes, etc. The extent of shrinkage is governed by two dimensionless parameters, P and m. The quantity P = A s γ cos(θ)mρ y K 0 (where A s is specific surface area, γ is surface tension, and θ is the contact angle) represents the relative magnitudes of capillary pressure and gel stiffness, and m describes the variation of stiffness with density. For P values less than 1, the shrinkage upon drying is less than 10%, and is reversible. For shrinkages greater than ⋍ 50%, the density increase is irreversible, and is proportional to P 1/( m - 1). Predicted shrinkage is compared with experimental results for silica gels dried from different surface tension pore fluids (2 < γ < 68 dyn/cm), initial wet gel density (0.06 < ρ 0 < 0.15 g/cm 3), and gel stiffness (0.3 < K 0 < 1 MPa).

Control of the porous structure of silica gel by the preparation pH and drying

The decrease of pore dimensions with drying of silica gels as a function of the preparation pH is investigated. The gels of 8.2 wt% SiO 2 in water are precipitated between pH 1 and 10 in a computerized static pH equipment. The shrinkage is monitored by thermoporometry, which is applied to the wet as well to the dried gels. The properties of the dried gels are measured with four traditional techniques: transmission electron microscopy, nitrogen sorption at 77 K, mercury penetration, and a Sears titration. The fixed pH influences the polymerization rate of silicate anions, and therefore the size and branching of the colloidal silica particles in the gel. At a low static pH of 1–2, very small but highly branched particles, ∼ 3 nm diameter, are developed in the gel with an extreme microporosity. With thermoporometry on hydrogels and N 2 sorption on xerogels, it is established that each sol particle is surrounded by approximately eleven others. With thermoporometry, it is determined that the particles are already clustered in the hydrogel. Shrinkage proceeds with drying, and no pores can be measured by thermoporometry on xerogels. The pore radius, R p, of 3.1 ± 0.5 nm decreases, and the specific pore volume decreases from 0.4 ± 0.1 ml g −1 to 0.13 ± 0.05 ml g −1. At a high static pH above 4, less branched and platey sol particles are formed with sizes ⩽26 nm. Before drying, the aggregates are highly porous and the pores are too large to be determined by thermoporometry. At pH 4 and 5, shrinkage proceeds, and the pore radii, R p, decrease to 2.8 ± 0.6 and 6.8 ± 3.0 nm, and the pore volumes to 0.8 ± 0.1 and 0.9 ± 0.1 ml g −1, respectively. The mercury pore volume of the dried gels with a maximum of 2.8 ± 0.2 ml g −1 at pH 5 is larger than both the thermoporometry and nitrogen pore volume, which is explained by macropores with radii of 1–2 μm. The pore radii and pore volumes of the macropores cannot be measured with thermoporometry and nitrogen sorption. The gels measured with thermoporometry, mercury penetration and nitrogen adsorption exhibit a maximum in pore volume between pH 4 and 8. The colloidal silica particles in the gel grow larger and repel each other at high pH. Finally, at very high pH above 8, the gel weakens and suffers more from the capillary forces with drying, due to the decreasing amount interparticle bonds.

Drying of silica gels with supercritical carbon dioxide

Results of drying experiments of aerogels with supercritical carbon dioxide are reported. In addition to the results of experiments with a pilot extracting apparatus, a preliminary design is also given of a large-scale supercritical carbon dioxide extraction plant to be used for drying of aerogels. From the experiments it was found that crack-free aerogels could be obtained when drying with carbon dioxide under supercritical conditions. The lowest temperature and pressure at which crack-free aerogel samples were obtained was at 35 °C and 85 bar, respectively. The temperature had a minor influence on the drying time. It was also found that the diffusion of ethanol into the aerogel pores limits the drying time. This limitation implies that the thickness of the aerogel tiles will have a large influence on the cost of drying of an aerogel.

Instabilities in self-fluidized beds—II experiments

Experimental studies of the behavior of self-fluidized beds in both 2-D and 3-D are reported. The experimental system used was the drying of silica gel under vacuum, and wide ranges of particle sizes and drying rates were covered. Both flow visualization and dynamical measurements of flow rates were utilized in the study. The observations indicate that self-fluidization is always unstable above the source strength required for minimum fluidization. The instability takes one of two forms, depending upon the degree of supercriticality. The primary instability consists of a quasi-steady array of spatially periodic spouts. A secondary instability leads to the emergence and dominance of larger spouts, which are observed to appear and disappear in a cyclic fashion. Measurements are made of wavelengths of the primary instability and the frequency of the secondary instability as functions of the operating variables, and the secondary instability is seen to occur at the same normalized flow rate for all the particles studied. Plausible mechanisms for these instabilities are proposed and discussed.

Optimization of multistage crosscurrent fluidized drying by a special algorithm of the discrete maximum principle with a constant hamiltonian

Optimization of multistage crosscurrent fluidized drying by a special algorithm of the discrete maximum principle with a constant hamiltonian

Reactive polymers. XLI. Preparation of poly(2,3‐epoxypropyl methacrylate) on silica gel

Abstract2,3‐Epoxypropyl methacrylate sorbed on silica gel was radically polymerized after addition of 1,4‐dioxan and initiator. A polymeric layer was formed which was firmly fixed on the silica gel surface. The amount of the polymer on silica gel can be controlled within a wide range by diluting the polymerization mixture. The specific surface area and the specific pore volume in the system decrease with increasing polymer content. The content of the polymer formed increases with increasing specific surface area of the initial silica gel. The amount of the polymer formed remains unaffected by a change in sorption time of the monomer on silica gel and by additional drying of silica gel at 100°C before the sorption of the monomer. Heating of the initial silica gel to 200–800°C has a basic influence on the content and chemical composition of the polymeric layer.

UNSTEADY, TWO-DIMENSIONAL HEAT AND MASS TRANSFER IN A PACKED BED OF FINE PARTICLES WITH AN ENDOTHERMIC PROCESS

The basic differential equations controlling the temperature and concentration field in a single packed bed of fine particles were derived and solved for the general case in which unsteady, two-dimensional heat and mass transfer lakes place with an endothermic process. The time-change of particle- and fluid-temperature and concentration of water vapor (humidity) were calculated by a numerical method which assumed that the rate of the endothermic process can be expressed by a first-order rate equation and that the fluid flowing through the bed is of the piston flow type. The experiments were conducted for the drying of silica-gel and the two-stage dehydration reaction of natural gypsum to demonstrate the applicability of the present theoretical analysis. It has been found that the calculated results show satisfactory agreement with the measured data within the range of the experimental conditions employed.

The kinetics and the mechanism of acoustic drying of capillaryporous materials

We present the results of an experimental investigation into the kinetics of acoustic drying of silica gel and felt at a frequency of 500 cps; we examine the mechanism of the intensifying effect of acoustic energy on the drying process.