Vacuum Desorption Research Articles

Simulation studies of a vacuum and temperature swing adsorption process for the removal of VOC from waste air streams

A theoretical study of the cyclic combined vacuum and temperature swing adsorption (VTSA) process for removing of volatile organic compounds (VOC) from waste air streams is done on the basis of computer simulation. The VTSA system consists of two adsorption columns with a fixed bed of activated carbon. The adsorption cycle for each column is operated in four steps: adsorption, indirect adsorbent bed heating, vacuum desorption and cooling. A nonequilibrium, nonisothermal mathematical model of the VTSA process is developed and solved using the numerical method of lines. Exemplary simulation results are presented for the system: 2-propanol–activated carbon Sorbonorit 4. The effect of desorption temperature, pressure and purge gas use on 2-propanol desorption efficiency is investigated.

Modeling of Vacuum Desorption in Freeze-Drying Process

A mathematical model of multicomponent vacuum desorption, which occurs in vacuum freeze-drying process, was developed. In freeze-drying porous biomaterials and pharmaceuticals are considered and the vacuum freeze-drying process, especially the moisture desorption in its final stage, is investigated. In this article, the drying with conductive heating and constant contact surface temperature was considered. Pressure drop is taken into account in the model formulation but was neglected in process simulation because of thin material layers undergoing freeze-drying. Model equations were solved by numerical method of lines. Moisture content and temperature distributions within the drying material were predicted from the model as a function of drying time.

Intrinsic factors determining the physical behaviour and durability of the Miocene sandstones used to build the Zaghouan-Carthage aqueduct (Tunis)

The present paper addresses the relationship between the intrinsic factors, physical behaviour and durability of Miocene Age sandstone used to build Tunisian Heritage Monuments, and more specifically the Zaghouan-Carthage aqueduct. A petrography study (optical microscopy and mercury intrusion porosi metry) was conducted to characterize the porous system of the rock. Stone hydric behaviour was also determined by finding vacuum saturation, desorption, capillary and water vapor transmission. Finally, mechanical strength (compressive strength, abrasion resistance) and durability (via accelerated sodium sulfate crystallization ageing) were also found. The results obtained were indicative of good hydric performance due to the macroporous nature of the stone and the connectivity of its porous system. This rock was also found to have very low mechanical strength due to its scant lithification, making it particularly susceptible to salt weathering. It was also observed to be highly resistant to chemical alteration, given the absence of chemically unstable minerals in its composition. The durability of the material was consequently found to depend directly on the presence or absence of salts in the monument.

Simulation and analysis of pressure swing adsorption: ethanol drying process by the electrical analogue

Abstract One column pressure swing adsorption (PSA) for dehydration of mixture of ethanol 93.54% and water 6.46% (wt) was simulated numerically by the electrical analogue model at feed pressure 2.0–5.0 bar, operating temperature 120–150 °C, and vacuum desorption. Increasing feed pressure may increase recovery and productivity. An anisotropic valve connecting the column entrance with the product tank may improve the operating results to obtain 99.9% ethanol at feed pressure 3.5 bar. Vacuum desorption may raise product purity and recovery to obtain 99.9 ethanol and 60% recovery at feed pressure 2.0 bar and blowdown to 0.4 bar.

Vacuum desorption of light hydrocarbons adsorbed on soil particles: A new method in geochemical exploration for petroleum

Light hydrocarbons in soils are the most important indicators in geochemical hydrocarbon exploration. However, the methods actually used to detect light hydrocarbons in soil sometimes provide data with a low signal-to-noise ratio and/or high level of interference. This research explores vacuum desorption, a new method for light hydrocarbon analysis. Vacuum desorption is a partial extraction technique designed to identify light hydrocarbons that are tightly adsorbed on soil particles. This method was used in the Huimin sag (basin) of east China. Microseepage anomalies are developed over oil pool S105 of the Shanghe field in this basin and larger seepage anomalies are developed over faults. In contrast to the method of acid-extraction of soil hydrocarbons, the vacuum desorption method provides a low-noise background that enables a more effective identification of anomalies. The new method is also cost and time effective and is capable of covering large areas.

Hydration properties of the molecular chaperone alpha-crystallin in the bovine lens.

Topographic studies of crystalline fractions from different morphological layers of the young adult bovine lens were conducted. Crystallin profiles were obtained for each lens layer, using thin-layer isoelectric focusing in polyacrylamide gel (IEF). Water soluble (WS) crystallins from the lens equator revealed a separation into HM (high molecular weight) alpha(L)-, beta(H)-, beta(L)-, beta(S)-, and gamma-crystallins. The nature of the water insoluble (WI) protein fraction in the separated lens layers reflected the aggregated state of alpha(L)-, beta(L)-, beta(S)-, and gamma-crystallins in different regions of the lens, concealed in the central cavity of the alpha-crystallin chaperone model. The IEF data demonstrate a possible chaperone-like function for alpha-crystallin in the nucleus and inner cortex of the lens, but not in the outer cortex. The water binding properties of bovine lens alpha-crystallin, calf skin collagen, and bovine serum albumin (BSA) were investigated with various techniques. The water adsorptive capacity was obtained in high vacuum desorption experiments volumetrically, and also gravimetrically in controlled atmosphere experiments. The NMR spin-echo technique was used to study the hydration of protein samples and to determine the spin-spin relaxation times (T(2)) from the protons of water adsorbed on the proteins. Isolated bovine lenses were sectioned into 11-12 morphological layers (from anterior cortex through nucleus to posterior cortex). The water content in relation to dry weight of proteins was measured in individual morphological lens layers. During water vapor uptake at relative humidity P/P(0) = 0.75, alpha-crystallin did not adsorb water suggesting that hydrophobic regions of the protein are exposed to the aqueous solvent. At relative humidity P/P(0) = 1.0, the adsorption of water by alpha-crystallin was 17% with a single component decay character of spin echo (T(2) = 3 msec). Addition of water to alpha-crystallin to about 50% of its weight/weight in the protein sample showed T(2) = 8 msec with only one single component decay of the spin-echo signal. The single component decay character of the spin echo indicates water tightly bound by alpha-crystallin. Under a relative humidity P/P(0) = 1.0, collagen and BSA adsorbed, correspondingly, 19.3 and 28% of water and showed a two-component decay curve with T(2) about 5 and 40 msec. The findings demonstrate the presence of two water fractions in collagen and BSA which are separated in space. The IEF data suggest a tight binding of water with alpha-crystallin with similar distribution patterns in the lens layers. To conclude, it was found that alpha-crystallin can immobilize water to a greater extent than other proteins such as collagen and BSA. These results shed new light on structural properties of alpha-crystallin and its superhydration properties and have important implications for understanding the mechanism of the chaperone-like action of this protein in the lens and non-ocular tissues.

Dynamical model of total and partial pressures in a vacuum system due to intermittent desorption

The dynamical characteristics of a vacuum system in respect of pressure are critical for studying the dynamical desorption processes such as those produced by mechanically stimulated desorption in vacuum. In the present work, a mathematical model of the instant pressure in a vacuum system has been developed assuming an intermittent square-wave desorbed gas flow. In this case, the duration of the pressure transient is proposed to be used as a generalized criterion of the rate of the desorbed gas flow and, consequently, of the instant pressure behaviour. The dependences of the transient duration on the pumping speed, system volume, molar mass of the desorbed gas as well as on the period and the on–off ratio of the desorption process have been revealed. The developed model made no assumptions on the physical nature of the gas source, i.e. desorption, leaking, etc., and can be used for different applications.

NMR spin–echo studies of hydration properties of the molecular chaperone α-crystallin in the bovine lens

The water-binding properties of bovine lens α-crystallin, collagen from calf skin and bovine serum albumin (BSA), were investigated with various techniques. The water absorptive capacity was obtained in high vacuum desorption experiments volumetrically, and also gravimetrically in controlled atmosphere experiments. NMR spin–echo technique was used to study the hydration of protein samples and to determine the spin–spin relaxation times ( T 2) from the protons of water, absorbed on the proteins. Isolated bovine lenses were sectioned into 11–12 morphological layers (from anterior cortex through nucleus to posterior cortex). Crystallin profiles were obtained for each lens layer using thin-layer isoelectric focusing in polyacrylamide gel (IEF). The water content in relation to dry weight of proteins was measured in individual morphological lens layers. During the water vapor uptake P/ P 0=0.75, α-crystallin did not absorb water, suggesting that hydrophobic regions of the protein are exposed to the aqueous solvent. At P/ P 0=1.0, the absorption of water by α-crystallin was 17% with a single component decay character of spin–echo ( T 2=3 ms). Addition of water to α-crystallin to about 50% of its w/w in the protein sample showed T 2=8 ms with only one single component decay of the spin–echo signal. The single component decay character of the spin–echo indicates at the tightly bound water by α-crystallin. Under a relative humidity P/ P 0=1.0, collagen and BSA absorbed correspondingly 19.3% and 28% of water and showed a two-component decay curve with T 2 of about 5 and 40 ms. The findings demonstrate the presence of two water fractions in collagen and BSA which are separated in space. The IEF data suggest a tight binding of water with α-crystallin with similar distribution patterns in the lens layers. The IEF data demonstrate a possible chaperone-like function for α-crystallin in the nucleus and inner cortex of the lens, but not in the outer cortex. To conclude, it was found that α-crystallin can immobilize and bind water to a greater extent than other proteins such as collagen and BSA. These results shed new light on structural properties of α-crystallin and have important implications for understanding the mechanism of the chaperone-like action of this protein in the lens and non-ocular tissues.

Effect of Carbon and Boron on Diffusivity of Deuterium in Molybdenum and Tungsten

ABSTRACTThe desorption behavior of deuterium from molybdenum, Mo-0.001 mass %C, Mo-0.06-0.092 mass %B, tungsten and W-0.011 mass %C prepared with arc-melting in Ar-10%D2 was studied using a vacuum desorption method in the temperature range of 533–1152K. The diffusivity of deuterium measured for molybdenum was D=4.97x10−7 exp(−36.7kJ/mol/RT)m2s−1 and D=1.77x10−6 exp(−55.8kJ/mol/RT)m2s−1 for tungsten. The diffusivity of deuterium for both molybdenum and tungsten decreased with an addition of C and B. This decrease was considered due to the trap effect of carbide and boride. The trapping energy estimated was 54 kJ/mol for Mo-C and Mo-B and 82kJ/mol for W-C.

Application of Lichens for Assessment of Atmospheric Pollution by Tritium

ABSTRACTIt is known that lichens are used for assessment of atmosphere pollution by heavy metals, radioniclides, sulfur and nitrogen oxides, etc. However there were published in scientific literature only limited data on usage of lichens as bio-indicators of tritium1,2. There are presented in the paper the results of lichen application study for assessment of atmospheric pollution by tritium. Both tritium in tissue free water (TFWT) and organically bound tritium (OBT) were measured in lichen. Lichen species Hypogimnia physodes was used as a basic bioindicator. Pieces of lichen were sampled within the distance of 30 km of emission source. Established sampling sites were rectangular in shape with linear dimensions 100*100 m. Lichen samples were sampled from various trees: birch tree, aspen tree, pine tree and linden tree at the level of 1.5 m above the ground. Thermal vacuum desorption technique was used to extract TFWT from lichen samples. Pyrolitic oxidation of dried lichen samples by vanadium oxide was used to extract tritium from OBT. Air monitoring stations equipped with active and passive samplers were used to sample HT and HTO from the atmosphere. Liquid scintillation counting was used to measure tritium content in water samples.It was determined that tritium content in lichen samples (both in TFWT and OBT) in vicinity of an emission source is higher than that of tritium content in lichen at distant sampling sites. Variation of tritium activity of TFWT was about 10 times, variation of tritium activity in OBT was about 70 times. It was supposed that tritium content in TFWT was in equilibrium with tritium content in atmosphere at the minute of sampling, while tritium content in OBT was determined by tritium content in atmosphere over longer period of time.

Investigation of phthalocyanine crystals exposed to NO 2 ambient gas

Crystals of free base phthalocyanine (H 2pc) and its unsubstituted metal derivatives Mpc (where M=Ni, Cu, Zn, Pb, and pc=C 32H 16N 8 2− =phthalocyanine ligand) were examined after being exposed to NO 2 ambient gas. Scanning electron microscopy (SEM) supported by X-ray microprobe analysis (EDAX), UV–Vis and FTIR spectroscopy as well as X-ray powder diffractometry (XRD) were applied to study the crystals chemistry and surface morphology. The NO 2 molecules were found to penetrate the crystal unit which led to the breakdown of the phthalocyanine macrocycle. The surface of the doped crystals has been scaled and covered by a layer composed predominantly of phthalimide. Despite vacuum desorption, it has been proved some free NO 2 to be still adsorbed by the studied material.

The effect of density on the corrosion of NdFeB magnets

Abstract The corrosion and degradation of Nd–Fe–B magnets by hydrogen from moisture in the environment has been termed the Negative Harris Effect. In order to provide more information on this phenomenon, different density Nd–Fe–B magnets have been subjected to vacuum desorption, XRD and electrochemical corrosion examination. During the normal vacuum sintering procedure used in producing the magnets, the residual hydrogen coming from the hydrogen decrepitation process is desorbed completely. However, the Nd–Fe–B magnets can absorb hydrogen readily from moisture in the environment. In these studies it has been found that the higher the density of the magnets the less is the hydrogen absorbed. The combined effects of the lattice expansion due to the formation of Nd 2 Fe 14 B–H solutions and NdH 2+ x phases as a result of hydrogen absorption and the volume expansion on formation of Nd(OH) 3 , lead to the eventual disintegration of the magnets. Thus hydrogen plays a vital role in the corrosion of Nd–Fe–B type magnets when they are exposed to a humid environment.

Arsenic induced mass transport of GaAs on V-groove GaAs substrate

We investigate the effect of As 4 over pressure annealing on clean grating profiles aligned along the [0 1 1] and [0 1 1] directions in a molecular beam epitaxy system (MBE). The grating cleaning is performed using hydrogen radicals or thermal desorption in ultra high vacuum. Subsequent annealing with As 4 causes grating deformations with a stronger effect for grating profiles aligned along the [0 1 1] direction. This deformation is enhanced after hydrogen radical cleaning and an almost complete planarization is observed for this case. The planarization conditions are determined and we demonstrate that mass transport driven by the arsenic is the principal mechanism for the [0 1 1] grating planarization. To our knowledge this is the first report on the deformation of V-groove gratings on GaAs substrate induced by mass transport.

Reconstructions upon thermal desorption in ultra high vacuum of InSe covered Si(111) surfaces

At an InSe/Si(111) heterostructure, the transition between the cubic Si substrate and the layered compound InSe remains a problem. The surface structure of InSe films grown by molecular beam epitaxy on Si(111) substrates has been studied under ultra high vacuum by low energy electron diffraction, Auger electron spectroscopy, and helium atom scattering upon sequential thermal erosion of the film until full removal. Between the initial 1 × 1 diagram of InSe(001) and the In-induced √3 × √3 R 30° pattern of Si(111), two intermediate stages were observed. One, above the single layer coverage range, corresponds to the coexistence, over a strained bonding half-layer, of InSe layers strained to the Si parameter and of others which are relaxed to bulk InSe parameter. The second stage, below the half-layer coverage range, is characterized by a nearly 5 × 5 mesh of Si(111) which could arise from two-dimensional islands made of both In and Se.

Kinetic aspects of the solid hydrogenation– disproportionation–desorption–recombination process in Nd13.67Co15.74Al0.77Ga0.27Zr0.03Fe62.2B7.33 alloys

The hydrogenation–disproportionation–desorption–recombination process is known as a novel method for producing permanent magnets. The hydrogen driven reactions involved in this process are carefully studied here using Mössbauer effect, x-ray diffraction and scanning microscopy, in the Nd13.67Co15.74Al0.77Ga0.27Zr0.03Fe62.2B7.33 alloy. The temperature dependence of the hydrogenation disproportionation in the 119 kPa isochore line was obtained. Hydrogen absorption seems to occur in two steps: absorption from Nd at the grain boundaries followed by absorption from the Nd2Fe14B type phase. In order to determine the recombination kinetics, a typical mass of the alloy was submitted to disproportionation reaction at 800 °C under a hydrogen pressure of 119 kPa, followed by recombination, which was achieved by vacuum desorption at 800 °C and subsequent quenching to room temperature after various time intervals. It was found that the relative fraction of the Nd2(Fe,Co)14B phase, as obtained from the Mössbauer effect, increases as f=1−exp(−ktn) with n=0.99±0.05. From this growing law it is inferred that recombination occurs without long range diffusion and starts at the grain boundaries with zero nucleation rate. An activation energy of 0.64±0.13 eV/atom was obtained for this process.

Identification of the Adsorption Mode of Thiophene on Sulfided Mo Catalysts

The adsorption and reactions of thiophene (C4H4S) on sulfided Mo/Al2O3 catalysts have been investigated over wide ranges of pressure (10-9−5 × 102 Torr) and temperature (140−693 K) using infrared (IR) spectroscopy and temperature-programmed desorption (TPD). When dosed at 190 K, thiophene adsorbs molecularly onto sites located on MoS2-like structures and on uncovered alumina regions of sulfided Mo/Al2O3 catalysts as determined by IR spectroscopy. Thiophene is weakly chemisorbed to the catalyst surface and desorbs in a single peak with a maximum rate of desorption at 243 K. Based upon interpretation of IR spectra, the adsorption mode of thiophene on the sulfided Mo portion of the catalyst surface has been determined to be η1(S), with thiophene bonded to coordinately unsaturated (cus) Moδ+ sites located on the edge planes of MoS2-like structures. The saturation coverage of thiophene on MoS2-like structures of a sulfided 17.6 wt % Mo/Al2O3 catalyst is estimated to be C4H4S/Mo = 0.074. Thiophene is observed to become reactive on sulfided Mo/Al2O3 catalysts only in the presence of gas-phase hydrogen and at high temperatures (i.e., 693 K). Temperature-programmed desorption in ultrahigh vacuum following heating of a sulfided 9.4 wt % Mo/Al2O3 catalyst to 693 K in a thiophene/H2 mixture results in desorption of C4 hydrocarbons, H2S, and H2 from the catalyst surface.

Method for rapid tritiated water extraction from environmental samples

We have developed a thermal vacuum desorption process to rapidly extract water from environmental samples for tritium analysis. Thermal vacuum desorption allows for extraction of the moisture from the sample within a few hours in a form and quantity suitable for liquid scintillation counting and allows detection of tritium at the levels of <2 Bq/l of milk, <0.5 Bq/g of vegetation, and <0.5 Bq/g of soil. We developed a prototype unit that can process batches of twenty or more samples within 24 hours. Early data shows that a high percentage of water is extracted reproducibly without enrichment or depletion of the tritium content. The quench coefficient of the extracted water is low allowing for accurate, direct liquid scintillation counting. In most samples, good comparison has been observed with results using freeze-dry lyophilization as the water extraction method

A Fourier transform mass spectrometry study of ethene oligomers in HMFI zeolite

Abstract Fourier transform ion cyclotron resonance mass spectrometry is applied to identify adsorbed species formed from ethene in zeolite HMFI. Long chain alkenes (up to 18 carbon atoms) are detected from the zeolite exposed to ethene at 298 K from vacuum desorption, laser induced desorption and laser ablation–chemical ionization experiments. Reaction with ethene at 420 K forms a mixture of linear alkenes, branched alkanes, and some mononuclear aromatics. After reaction at 560 K, the major adsorbed species are alkylbenzenes and alkylnaphthalenes.

A combined vacuum and temperature swing adsorption process for the recovery of amine from foundry air

The removal and recovery of an amine from waste air of a foundry by adsorption is discussed on the basis of experiments and simulation studies. The results show that activated carbons as well as a hydrophobic Y-zeolite are well suited for the purification. Since parts of the amine are chemically bound to the adsorbents, a decrease in adsorption capacity is found for both adsorbents after the first cycles. For the desorption and recovery, a combined vacuum and temperature swing (VTSA) technique is used. To achieve a sufficient degree of regeneration without using too low pressures, a small amount of purge gas is added during the vacuum desorption. This allows the condensation to be performed at an ambient temperature (10–20°C). In experiments with a single adsorption column as well as in cyclic experiments with a three column setup it was shown that more than 95% of the continuously fed amine can be removed and recovered from the waste air. The further purification down to the required clean gas concentration has to be done in a second purification step, e.g. a bio-filter.

Desorption characteristics of rare earth (R) hydrides (R=Y, Ce, Pr, Nd, Sm, Gd and Tb) in relation to the HDDR behaviour of R–Fe-based-compounds

Vacuum desorption characteristics of binary hydrides of rare earth metals (R=Ce, Pr, Nd, Y, Sm, Gd, Tb) were studied. A complete decomposition of hydrides, confirmed by X-ray diffraction, was achieved at temperatures up to 820 °C. When heating the hydrogenated material in vacuum, two distinct hydrogen desorption events were observed for all the hydrides under investigation. The hydrogen desorption behaviour is different for the two groups of metals, namely group I (Ce, Pr and Nd) and group II (Y, Sm, Gd and Tb). The hydrogenation–disproportionation–desorption–recombination (HDDR) process was shown to take place for the Nd 6Fe 13Ge and Nd 6Fe 12.8Ga 1.2 intermetallic compounds, both with the Nd 6Fe 13Si-type structure. The hydrogen absorption and desorption properties of these compounds were studied in relation to the desorption characteristics of neodymium hydride.