Sublimation Rate Research Articles

CFD Model of Apple Atmospheric Freeze Drying at Low Temperature

An Internet-controlled atmospheric freeze dryer was designed, built, and tested in the Department of Heat and Mass Transfer, Technical University of Łódź. A film sublimation-based CFD model was developed and verified using Fluent 6.1 commercial CFD software. The model enables the simulation of phase change and water vapor diffusion process within porous media. Transport of non-condensable species can be calculated with species transport inbuilt model of Fluent 6.1 and used to predict sublimation rate under given conditions. Results were compared with AFD experimental data of 10-mm Idared apple cubes. The viability of applying the film sublimation model to atmospheric freeze-drying process was demonstrated. Higher mass flux was found on the leading edge, relatively uniform mass flux within the porous zone illustrates that vapor diffusion dominates atmospheric freeze drying process at low temperature (below 0°C). CFD results for apple cubes show a predomination of diffusional resistance of porous tissue.

Hydrogen reduction kinetics of electrometallurgical slime

The interaction of hydrogen with the zinc-containing electrometallurgical slime of the Severstal’ metallurgical works has been studied. The sequence of transformations in the slime heated to 1100°C in hydrogen or air has been established. The experimental and calculated weight losses coincide. Some of the carbonates are shown to decompose in the temperature range 300–700°C, and most iron and zinc oxides are reduced to a metal. In the temperature range 650–850°C, zinc is almost completely sublimated. At temperatures above 800°C, complex oxides are reduced and calcium and magnesium carbonates and sulfates are likely to decompose. Experimental digital data on the zinc sublimation rate are processed by the least squares method with approximating equations used in thermal analysis. The kinetics of nonisothermal zinc sublimation is comprehensively analyzed using a unique procedure developed for taking into account the background of a peak and the effect of accompanying processes. An equation for the calculation of the zinc sublimation rate under experimental conditions (fluidized bed) is given and tested.

Spatial and temporal variability of surface mass balance near Talos Dome, East Antarctica

Predictions concerning Antarctica's contribution to sea level change have been hampered by poor knowledge of surface mass balance. Snow accumulation is the most direct climate indicator and has important implications for paleoclimatic reconstruction from ice cores. Snow accumulation measurements (stake, core, snow radar) taken along a 500‐km transect crossing Talos Dome (East Antarctica) have been used to assess accumulation signals and the representativeness of ice core records. Stake readings show that accumulation hiatuses can occur at sites with accumulation rates below 120 kg m−2 yr−1. Differences between cores and stakes can lead to statistical misidentification of annual layers determined from seasonal signals at sites with accumulation rates below 200 kg m−2 yr−1 because of nondetection of higher and lower values. Achieving ±10% accuracy in the reconstruction of snow accumulation from single cores requires high accumulation (750 kg m−2 yr−1). Low‐accumulation sites are representative if cumulative rates computed over several years are used to reach the 750 kg m−2 yr−1 threshold. Temporal variability of accumulation over the last two centuries shows no significant increase in accumulation. Wind‐driven processes are a fundamental component of surface mass balance. Spatial variations in accumulation are well correlated with surface slope changes along the wind direction and may exceed 200 kg m−2 yr−1 within 1 km. Wind‐driven sublimation rates are less than 50 kg m−2 yr−1 in plateau areas and up to 260 kg m−2 yr−1 in slope areas and account for 20–75% of precipitation, whereas depositional features are negligible in surface mass balance.

Sublimation Kinetics during Freeze-Drying of Pharmaceutical Protein Formulation

The influence of total gas pressure and shelf temperature on sublimation kinetics of BSA-based formulation in glass vial as geometrical configuration was determined with a pilot freeze-dryer in standard operating freeze-drying conditions. The sublimation rate curves showed three different periods with a plateau corresponding to a stationary regime. These kinetics data were mainly dependent on the shelf temperature and slightly influenced by the total gas pressure. Thus, the sublimation process in our conditions was mainly governed by overall heat transfer rate from the plate and from the surroundings to the sublimation front. Moreover, it proved that the water vapor mass transfer mechanism through the dried layer occurs by molecular diffusion in Knudsen regime. Finally, these experimental sublimation kinetics data were found in a quite fair agreement with the set up results. They confirm the validation of previous modeling of mean product temperature profiles during the freeze-drying by using the finite element code FEMLAB in real vial geometry (2-D).

Estimating sublimation of intercepted and sub‐canopy snow using eddy covariance systems

AbstractDirect measurements of winter water loss due to sublimation were made in a sub‐alpine forest in the Rocky Mountains of Colorado. Above‐and below‐canopy eddy covariance systems indicated substantial losses of winter‐season snow accumulation in the form of snowpack (0·41 mm d−1) and intercepted snow (0·71 mm d−1) sublimation. The partitioning between these over and under story components of water loss was highly dependent on atmospheric conditions and near‐surface conditions at and below the snow/atmosphere interface. High above‐canopy sensible heat fluxes lead to strong temperature gradients between vegetation and the snow‐surface, driving substantial specific humidity gradients at the snow surface and high sublimation rates. Intercepted snowfall resulted in rapid response of above‐canopy latent heat fluxes, high within‐canopy sublimation rates (maximum = 3·7 mm d−1), and diminished sub‐canopy snowpack sublimation. These results indicate that sublimation losses from the sub‐canopy snowpack are strongly dependent on the partitioning of sensible and latent heat fluxes in the canopy. This compels comprehensive studies of snow sublimation in forested regions that integrate sub‐canopy and over‐story processes. Copyright © 2007 John Wiley & Sons, Ltd.

Field observations on spatial variability of surface hoar at the basin scale

Surface hoar deposited on the snow surface represents, once buried by subsequent snowfall, one of the principal weak layers on which dry snow slab avalanches release. To predict instabilities caused by a buried surface hoar layer, its spatial extent needs to be known. Avalanche forecasting relies, among other things, on meteorological data from automatic stations. In principle, surface hoar formation can be predicted from these data. In order to study the spatial variation in surface hoar formation and destruction, daily observations were made during one winter at 23 locations of different aspect, slope inclination, and wind exposure within an area of about 3 km2. Four automatic weather stations were located within the study area: one on level terrain and three across a ridge. Despite the good instrumentation the correlation between surface hoar growth and calculated sublimation rate was poor. Distinct spatial patterns of surface hoar growth were found. Surface hoar crystals were frequently larger at the ridge site than in the surroundings of the automatic weather station on level terrain. The variation in surface hoar formation was mainly due to different prevailing wind regimes during the formation periods. The surroundings of the automatic weather station on level terrain were under the influence of local katabatic winds that dried up the air so that growth conditions were locally less favorable. Our observations suggest that predicting surface hoar formation for complex alpine terrain on the basis of data from an automatic weather station, the standard procedure in avalanche forecasting, seems nearly impossible unless at least the local wind regime is known at high resolution (≤10 m). For both surface hoar formation and surface hoar destruction observations suggest wind conditions to be most crucial for spatial variation.

Increase of SiC sublimation growth rate by optimizing of powder packaging

In this paper, a comprehensive model for silicon carbide (SiC) sublimation crystal growth considering powder packaging is presented. This model is based on heat and mass transfer of porous powder charge with different sizes of the particle and accounts for induction heating, powder charge sublimation, vapor transport, and porosity evolution in a SiC sublimation crystal growth system. The mechanism of vapor transport is proposed by introducing a driving force arising from temperature difference between powder charge and seed in the growth system. Powder porosity evolution and sublimation rate variations are predicted based on vapor transport mechanism. Effects of powder geometry, such as particle sizes, volume and size ratios of different particles, and driving forces on the sublimation rate are studied. In addition, simulation results for powder sublimation with and without a central hole are presented. The results indicate that the sublimation rate can be increased by optimizing the powder packaging, or by creating a hole in the center of packed powder.

Interface reaction and wetting in the CaF 2/Me systems

Wetting experiments using the sessile drop method were performed in the CaF 2–Me systems (Me = Cu, Ge, Al, In and Ga). Liquid Cu, Ge, In and Ga, do not wet the CaF 2 substrate and contact angles are significantly higher than 90°. No condensed products were found at the metal/ceramic interface. In the CaF 2/Al system, evidence of an interface interaction was detected and the contact angle is about 92°. It was established that the spreading behavior and the values of the apparent contact angle depend on the rate of evaporation of the molten metals, the rate of sublimation of the substrate and on the value of the work of adhesion at the interface. The experimental observations are well accounted for by a thermodynamic analysis of the ternary Ca–Me–F systems.

Sublimation rate of ice under simulated Mars conditions and the effect of layers of mock regolith JSC Mars‐1

We have studied the sublimation of ice buried beneath ≤200 mm of JSC Mars‐1 model regolith under simulated Mars conditions. As expected, even thin layers of regolith cause large decreases in sublimation rate, up to one order of magnitude at 50 mm. When the depth of the regolith was 50 to 200 mm we detected water desorbing from the overlying layers for which we were able to determine a desorption coefficient of 1.45 ± 0.5 × 10−3 h−1. After allowing for the effect of desorption, we found that the diffusion coefficient for water vapor through our regolith is 1.74 ± 0.70 × 10−4 m2 s−1, in excellent agreement with theoretical values. We thus find that a 1‐m thick layer of ice buried below a meter of regolith resembling JSC Mars‐1 on Mars at 235 K would last ∼800 years.

Some theoretical considerations in modeling laser-induced incandescence at low-pressures

Laser-induced incandescence (LII) of nanoparticles at low pressures has received some attention in recent years as a particle sizing technique or a technique for inferring the mean value of the absorption function of the particle material. In this paper, we are concerned with some fundamental issues in the theory of LII with particular attention paid to those encountered at very low pressures. The commonly adopted Rayleigh approximation for particle laser energy absorption and subsequent thermal emission is critically evaluated against the Mie solution in the range of size parameter relevant to LII. The Rayleigh approximation can cause significant error in particle laser energy absorption rate, especially when shorter wavelengths are used, and potentially in the particle temperature inferred from the two-color LII. We also demonstrate that claims that low-pressure LII can be used for particle sizing are flawed, due to the use of an incorrect expression for radiation heat loss rate from the particles in this regime, and unjustified neglect of particle sublimation heat loss. Using the currently best available carbon sublimation rate expression and physical parameters, the relative importance of heat conduction, thermal radiation, and sublimation heat loss from an isolated carbon particle was investigated for different ambient pressures, particle temperatures and particle diameters. To ensure particle radiation heat loss is dominant over conduction and sublimation the ambient pressure and the particle temperature should be kept respectively lower than 10-4 atm and below about 2800 K. Under these conditions the effective temperature of a particle ensemble containing non-aggregated polydisperse primary particles to the power of -4 is proportional to the mean value of the particle absorption function, provided the particles are in the Rayleigh regime in the near infrared. The effect of aggregation on particle absorption and emission is briefly discussed.

Evaluation of manometric temperature measurement, a process analytical technology tool for freeze-drying: Part II measurement of dry-layer resistance

The purpose of this work was to study the factors that may cause systematic errors in the manometric temperature measurement (MTM) procedure used to determine product dry-layer resistance to vapor flow. Product temperature and dry-layer resistance were obtained using MTM software installed on a laboratory freeze-dryer. The MTM resistance values were compared with the resistance values obtained using the "vial method." The product dry-layer resistances obtained by MTM, assuming fixed temperature difference (DeltaT; 2 degrees C), were lower than the actual values, especially when the product temperatures and sublimation rates were low, but with DeltaT determined from the pressure rise data, more accurate results were obtained. MTM resistance values were generally lower than the values obtained with the vial method, particularly whenever freeze-drying was conducted under conditions that produced large variations in product temperature (ie, low shelf temperature, low chamber pressure, and without thermal shields). In an experiment designed to magnify temperature heterogeneity, MTM resistance values were much lower than the simple average of the product resistances. However, in experiments where product temperatures were homogenous, good agreement between MTM and "vial-method" resistances was obtained. The reason for the low MTM resistance problem is the fast vapor pressure rise from a few "warm" edge vials or vials with low resistance. With proper use of thermal shields, and the evaluation of DeltaT from the data, MTM resistance data are accurate. Thus, the MTM method for determining dry-layer resistance is a useful tool for freeze-drying process analytical technology.

A novel method to increase the growth rate in sublimation crystal growth of advanced materials

Sublimation crystal growth technique is widely used for the growth of optoelectronic materials, such as aluminum nitride (AlN). In this paper, an integrated model is developed to study the effects of powder geometry on crystal growth rate whereas induction heating, powder charge sublimation, vapor transport, and porosity evolution are considered. The mechanism of vapor transport is proposed by introducing a driving force arising from the temperature difference in AlN sublimation growth system. Powder porosity evolution and sublimation rate variation are predicted based on the vapor transport mechanism. In addition, a new method by optimizing the initial powder porosity and creating holes in the packed powder is proposed to increase the crystal growth rate. Simulation results for the sublimation of powder with and without central hole are presented. It is found that the powder sublimation rate is higher when hole is present. This is also validated experimentally. Effects of initial porosity, particle size and driving force on the sublimation rate are also studied. Finally, the powder geometry is optimized based on numerical simulations. The findings from this investigation can also be applied to SiC since SiC sublimation growth is similar to AlN.

New estimates for the sublimation rate for ice on the Moon

The strong hydrogen signal that the Lunar Prospector saw at the Moon's poles suggests that water ice may be present near the surface of the lunar regolith. A robotic mission to obtain in situ samples and to quantify the amount of this valuable resource must be designed carefully to avoid dissipating too much heat in the regolith during coring or drilling and, thus, causing the ice to sublimate before it is processed. Here I use new results for the saturation vapor pressure of water ice to extend previous estimates of its sublimation rate down to a temperature of 40 K, typical of the permanently shaded craters near the lunar poles where the water ice is presumed to be trapped. I find that, for temperatures below 70 K, the sublimation rate of an exposed ice surface is much less than one molecule of water vapor lost per square centimeter of surface per hour. But even if a small ice sample (∼4 ng) were heated to 150 K, it could exist for over two hours without sublimating a significant fraction of its mass. Hence, carefully designed sampling and sample handling should be able to preserve water ice obtained near the lunar poles for an accurate measurement of its in situ concentration.

The dust coma of the active Centaur P/2004 A1 (LONEOS): a CO-driven environment?

Context. Centaurs reside in a region of the Solar System where many volatile species begin to have significant sublimation rates, and several examples of cometary activity have been observed in this group. The source of activity is not well known for most of the active Centaurs. If the sublimation rate of water is low, the sublimation of other volatiles, such as CO, could drive the presence of a coma.Aims. The aim of this paper is to study the dust environment of the active Centaur P/2004 A1 (LONEOS), which was observed to have a significant coma and a very long perspective anti-tail (neck-line) at a heliocentric distance of 5.5 AU.Methods. R -band images taken at the TNG telescope were used as input for an inverse numerical model, meant to derive the physical parameters of dust grains in the coma.Results. The Centaur showed an asymmetric coma and a neck-line extending at least to 1.5 105 km in the solar direction, emitted half an orbit before the observation. The Af ρ value measured in a 5'' aperture is 334 ± 15 cm, indicating a significant dust production rate, comparable to that of several short-period comets at much lower heliocentric distances. The emitted grains are larger than 1 cm, and the dust production rate has been around 100 kg s-1 during the last ten years. For R n = 10 km and a maximun size of the uplifted grain >3 cm, a CO molecular abundance Q > 1030 mol s-1 is required if it is uniform over all the surface.

Numerical Study of Behavior of Outgas from Heat Shield of Solar Probe

When a solar probe penetrates into the solar corona for in-situ observation of the solar wind, the graphitic heat shield temperature is expected to increase to over 2000 K and surface sublimation will occur. The sublimation gas is numerically studied by using the direct simulation Monte Carlo method. The length scale and the Knudsen number are evaluated with respect to the interaction phenomena between the solar wind and the outgas from the heat shield. When the total sublimation rate and the number density of the solar wind are large, the velocity distribution function of the solar wind around the probe is expected to become quite different from that of the undisturbed freestream due to molecular collisions. Numerical results indicate that the total sublimation rate must be 10 -6 kg/s or lower to avoid serious errors in the solar wind measurements. Thermal analyses show that the inclined graphitic disk heat shield can satisfy this requirement. Detailed rarefied flow simulation is also made around the spacecraft consisting of the heat shield and the main body. Favorable location for the sensor is discussed from a viewpoint of the solar wind observation.

Evaporation of substances under kinetic or diffusion control in sublimation in a vacuum

A diffusion-kinetic model of sublimation of substances in a vacuum is presented. The model defines two regions of the evaporation process depending on temperature: evaporation in the kinetic and diffusion control. The model was verified in experiments with benzoic acid and naphthalene by calculation of the sublimation rate of Zn, Co, V, and W, and by the known values of α.

Freeze-drying of nanocapsules: Impact of annealing on the drying process

Freeze-drying process was recently applied to improve the long-term storage stability of nanocapsules. Thermal treatment by annealing is an interesting process to optimize a freeze-drying cycle of these colloidal vectors. The objective of this paper is to investigate the impact of annealing on primary and secondary drying characteristics and on nanocapsules (NC) properties. Nanocapsules were prepared from poly-ɛ-caprolactone (PCL) biodegradable polymer and stabilized by polyvinyl alcohol (PVA), and then freeze-dried with two cryoprotectants: sucrose and poly vinyl pyrrolidone (PVP). Freeze-dried nanocapsules were characterized by size measurement and transmission electron microscopy after reconstitution. The effect of annealing on the kinetics of sublimation, on the mass transfer resistance and on the porosity of the freeze-dried product has been studied in the case of PVP. Finally, the effect of annealing on the kinetic of secondary drying was studied and the results were coupled with the isotherm of sorption. Results showed that PCL nanocapsules could be freeze-dried without any modification of their properties in presence of the two cryoprotectants used. Annealing of nanocapsules suspensions could accelerate the sublimation rate without any modification of nanocapsules size in the case of the two studied cryoprotectants. Such improvement could be explained by the increase of ice crystals size after annealing and by the diminution of mass transfer resistance by the dried layer. The acceleration of sublimation rate seems to depend on the temperature of annealing. The annealing of sucrose solution slows down the secondary drying kinetic whereas no effect is observed in the case of PVP.

Synthesis, Properties, and Applications of Ferromagnetic‐Filled Carbon Nanotubes

AbstractFerromagnetic‐filled carbon nanotubes are new nanostructured materials with many possible applications. They can be synthesized using the thermal decomposition of metallocenes of the iron triad. Two different methods (solid and liquid source CVD) are suitable for producing, at very high filling rates, filled nanotubes on precoated Si substrates. The diameters of deposited filled nanotubes are particularly dependent on the size of catalyst particles on the substrate, while the lengths depend more on the sublimation and decomposition rate of metallocene. The growth mechanism of filled carbon nanotubes is based on the root growth mode. Multiwalled carbon nanotubes, filled with body‐centered cubic Fe, show unusual magnetic properties. Aligned‐growth nanotube ensembles can reach coercivities up to 130 mT (bulk iron 0.09 mT). Ferromagnetic‐filled carbon nanotubes can be successfully used both as cantilever tips in magnetic force microscopy and as a nanocontainer for new therapies in medicine.

Ion cluster desorption from frozen NH 3 induced by impact of fast multi-charged ions

Ion cluster desorption yields due to the impact of 252Cf fission fragments on condensed ammonia are determined. Identification and abundance of the ion desorbed species were provided by a time-of-flight mass spectrometer. Ammonia ice thin layers deposited on Au film were heated from 25 K to complete sublimation. The typical pressure during heating process was 10 −8 mbar. Measurements in the 25–150 K temperature range show emission of (NH 3) n NH m ± ion clusters, where n varies from 0 to ∼30 and m varies from 0 to 3 (for negative ions) or to 5 (for positive ions). The distribution is described by the sum of two decaying exponentials when n increases, which is interpreted by the simultaneous occurrence of two cluster formation regimes: (i) recondensation of sublimated molecules from the projectile track and (ii) emission of pre-formed bulk fragments from the periphery of the impact site. Moreover, the cluster yields of positive and negative ion series are approximately proportional to each other, suggesting that the ion cluster distribution shape is dictated by the neutral molecules but absolute yield is controlled by the emission of the ion species. In the 125–140 K range, sublimation rate increases, the ice thickness decreases rapidly and all the mentioned cluster series tend to disappear. The series (HCN) n CN − then appears, originating in the clustering of residual hydrocarbon contaminants on the metal substrate surface. The residual gases are analyzed as a function of the NH 3 ice target temperature and their partial pressure compared to the ion desorption yield.

Electron reflectivity measurements of Ag adatom concentrations on W(1 1 0)

The density of two-dimensional Ag adatom gases on W(1 1 0) is determined by monitoring local electron reflectivity using low-energy electron microscopy (LEEM). This method of adatom concentration measurement can detect changes in adatom density at least as small as 10 −3 ML for a μm size region of the surface. Using this technique at high temperatures we measure the sublimation rates of Ag adatoms on W(1 1 0). At lower temperatures, where Ag adatoms condense into monolayer islands, we determine the temperature dependence of the density of adatoms coexisting with this condensed phase and compare it with previous estimates.