Non-isothermal Desorption Research Articles

Thermodynamic and kinetic studies of Mg–Fe–H after mechanical milling followed by sintering

Abstract The transition metal complex hydride Mg2FeH6 has been successfully synthesized utilizing mechanical milling of a 2Mg–Fe mixture followed by heating at 673 K under 6 MPa of hydrogen pressure, without pressing step. The obtained yield of Mg2FeH6 was about 50%. Hydrogen storage properties of the Mg–Fe–H system, i.e. capacities, absorption/desorption kinetics and thermodynamic parameters, were examined. The pressure-composition isotherm (PCI) measurements of the samples at 548–673 K showed that the alloys possessed good cyclic stability and reversibility. Enthalpies and entropies of decomposition of the Mg–Fe–H system were evaluated by van’t Hoff plots. The absorption/desorption rates at 573 K were very fast in comparison with the reported data. The non-isothermal desorption of hydrogen was found greatly dependent on the thermal history of the sample.

Polyimide foams from powder: Experimental analysis of competitive diffusion phenomena

In the present study, various diffusive processes have been investigated during foaming of powdered precursors of polyimide. A detailed analysis of the powdered precursor's characteristics allows for an enhanced morphological understanding of the resulting microstructures and foam unit cell. Parameters that are central to the foaming process such as particle morphology, volatile concentration and sorption–desorption processes are evaluated. Isothermal and non-isothermal desorption experiments have been carried out by thermogravimetric analysis (TGA), and specific diffusive processes have been correlated to thermodynamic and kinetic transitions by means of modulated differential scanning calorimetry (MDSC) of the corresponding materials. It was found that two primary fluxes of volatiles, one out of the external surface of the particles (responsible for volatile desorption) and the other into the growing bubble (responsible for vapor supersaturation inside the bubble) compete against each other creating a competitive scenario that becomes the controlling factor for potential inflation within the precursor particles.

Heat and mass transfer during bubble growth in an alternating electric field

We studied mass and heat transfer during nonisothermal desorption of a solvable dielectric gas to a stagnant bubble from a surrounding gas-saturated dielectric liquid under the influence of the alternating electric field. Mass flux is directed from a host fluid to bubble, and the applied electric field causes a creeping flow around the bubble. The moving boundary problem is solved in the approximations of a thin concentration and temperature boundary layers and finite dilution of an absorbate in the absorbent. The nonlinear partial parabolic differential equations of mass and energy conservation are solved by means of a generalized similarity transformation, and the solution is obtained in a closed analytical form for all frequencies of the applied electric field. Numerical calculations show an essential (by a factor of 2÷3) enhancement of the rate of mass transfer in sulfur dioxide-water system under the influence of alternating electric field for a stagnant bubble.

Study of Fourier transform infrared-temperature-programmed desorption of benzene, toluene and ethylbenzene from H-ZSM-5 and H-Beta zeolites

In the present study, an infrared (IR) high temperature cell was used, in combination with a Fourier transform infrared (FTIR) spectrometer for the development of an alternative temperature-programmed desorption (TPD) procedure. Three different adsorbates, i.e., benzene, toluene and ethylbenzene were non-isothermally desorbed from two zeolites H-ZSM-5 and H-Beta. The FTIR-TPD profiles were fitted with the help of the complementary error function. The fitting process was carried out with the help of a computer program which allows us to calculate two parameters, the temperature, T 0 (K) and the temperature range Δ T (K), which, in conjunction with the complementary error function, characterizes the FTIR-TPD profile. Was found that the parameter T 0 is linked with the adsorption energy of the adsorbate in the zeolite and the parameter Δ T was correlated with the transport process of the desorbed molecules inside the zeolites during the desorption process and with the presence of more than one type of adsorption sites. In conclusion, was confirmed that the FTIR-TPD methodology is appropriate for in situ observation of adsorbed molecules on zeolites, and that this technique makes available information concerning the adsorbed state of guest molecules in non-isothermal desorption.

Investigation of the non-isothermal water desorption on alkali-metal cation-exchanged X-type zeolites: a temperature-programmed diffuse reflection infrared Fourier transform spectroscopic (TP-DRIFTS) study

The non-isothermal desorption of water on alkali-metal cation-exchanged X-type zeolites was investigated by means of temperature-programmed diffuse reflection infrared Fourier transform spectroscopy (TP-DRIFTS). Based on the areas under the DRIFT spectra recorded at temperature intervals of 10 K, the desorption profiles could be calculated. Their shape is principally identical to the desorption curves obtained by conventional TPD (EGD/EGA). Using a numerical regularisation method, desorption energy distribution functions have been calculated. The assignment of several characteristic bands to differently bonded water molecules and explanations for certain observed trends have been given.

Interaction of Water with Alkali-Metal Cation-Exchanged X Type Zeolites: A Temperature-Programmed Desorption (TPD) and X-ray Diffraction Study

We have investigated the interaction of water with alkali-metal cation-exchanged X type zeolites with different Si/Al ratios by means of temperature-programmed desorption (TPD). The nonisothermal desorption of water shows, depending on the type of cation (Li+, Na+, K+, Rb+, and Cs+) and the exchange degree, differently structured desorption profiles. Using a numerical regularization method, desorption energy distribution functions have been calculated. The width of the distributions of about 30−50 kJ mol-1 shows clearly the energetic heterogeneity of the water−zeolite interaction. All desorption energy distributions exhibit two characteristic ranges. Beside a small part at lower energy values the distribution functions have a pronounced main peak between 55 and 65 kJ mol-1 and a second range at higher energy values (65−95 kJ mol-1). The systematic changes with respect to position and intensity of the peaks at higher energy values show clearly the influence of the respective cation on the water desorption ...

Sorption and desorption of gases (CH 4, CO 2) on hard coal and active carbon at elevated pressures

The study of sorption and desorption of carbon dioxide and methane on three samples of hard coal with different C daf content and on the active carbon is presented in this paper. The sorption isotherms were measured by means of the volumetric method at two temperatures, 288 and 298 K, at pressures up to 6 MPa. The porosities of the samples were determined by measuring their apparent (mercury method) and true density (helium method). The obtained experimental data were described using the thermal sorption equation of virial form. The isosteric heats of sorption were calculated as the coverage function, and the non-isothermal desorption equilibrium curves were also calculated. For each system studied a temperature drop during the desorption down to 1 bar pressure was computed as well. The obtained results indicate an adsorption-absorption mechanism of carbon dioxide and methane on hard coals.

Heat Effects in ZLC Experiments

The problem of nonisothermal desorption in a zero length column (ZLC) experiment is considered theoretically. Simple analytical expressions for the ZLC desorption curve are derived for certain limiting situations in which the governing equations reduce to a linear form. More general numerical solutions are calculated for a wide range of experimental conditions assuming both negligible mass transfer resistance and finite mass transfer resistance controlled by intraparticle diffusion. A simple criterion for negligible thermal effects is developed. It is shown that when the ZLC technique is applied to the measurement of diffusion in unaggregated zeolite crystals, as originally intended, heat effects are generally insignificant. However, when applied to the measurement of macropore diffusion in relatively large adsorbent particles heat effects can become important and may cause major modification of both the desorption rate and the shape of the desorption curve. A recent experimental ZLC study carried out with commercial adsorbent particles, under conditions of macropore diffusion control, showed an anomalous dependence of the desorption rate on both temperature and particle size. These effects can be qualitatively explained by the nonisothermal model. A more precise quantitative representation of these experiments will require a more refined model incorporating a nonlinear equilibrium isotherm as well as intraparticle diffusional resistance.

Adsorption of water on zeolites of different types

We have investigated the interaction of water with Na+-ion exchanged zeolites of different structures (LTA, FAU, ERI, MOR and MFI) by means of temperature-programmed desorption (TPD). The non-isothermal desorption of water shows, depending on the zeolite type, differently structured desorption profiles. In every case the profiles have, however, two main ranges. Using a regularization method, desorption energy distribution functions have been calculated. The desorption energy distributions between 42–60 kJ mol−1, which can be attributed to a non-specific interaction of water, show two clearly distinguished energy ranges. The water desorption behaviour of this range correlates with the electronegativity of the zeolites and the average charge of the lattice oxygen atoms calculated by means of the electronegativity equalization method (EEM). The part of the desorption energy distributions in the range of 60–90 kJ mol−1, reflecting interactions of water with Na+ cations, shows two more or less pronounced maxima. In agreement with vibrational spectroscopic studies in the far infrared region, it may be concluded that all samples under study possess at least two different cation sites.

A Method for the Measurement of Carbon Dioxide Desorption from Coal in the Elevated Pressure Range

During the liberation of gas from a coal bed, the temperature of the system is decreased because desorption is an endothermic process and heat exchange with the surroundings is difficult. A method for measuring gas desorption in the elevated pressure range, enabling investigations under isothermal and quasi-adiabatic conditions, was described. The results of carbon dioxide desorption from Polish coal were presented. The study was carried out using different rates of decrease in the external gas pressure for different coal grain sizes. The non-isothermal desorption curves thus obtained were described using empirical equations. Extrapolation of the equation constants obtained enabled the desorption curves to be calculated for the limit of decrease in rate of the external gas pressure and of grain size. It was found experimentally that the dependence of the decrease in coal temperature on the amount of desorbed gas is linear provided that heat exchange with the surroundings is limited.

Adsorption of water on ZSM-5 zeolites

The non-isothermal desorption of water adsorbed on ZSM-5 zeolites at room temperature shows, depending on the cation type (H +, Na +), differently structured desorption curves. A kinetic analysis based on various methods shows that the course of desorption is well described by a first order rate equation considering a distribution function for the desorption energy. In addition, adsorption isotherms were measured at room temperature. The adsorption energy distribution functions calculated with a regularization method can be correlated with the desorption energy distributions. FTIR investigations of water-loaded zeolites enables the assignment of desorption steps to different adsorbate complexes.

95/05592 Non-isothermal desorption of carbon dioxide from Polish coals with different texture

95/05592 Non-isothermal desorption of carbon dioxide from Polish coals with different texture

Non-Isothermal Desorption of Carbon Dioxide from Polish Coals with Different Texture

The process of gas evolution from coal depends to a great extent on coal texture, and specially on the structure of its pores. A transport of sorbate from within a grain to its external surface, and therefore the evolution of gas desorbed from coal depends on the structure of pores. Results of the investigations of nonisothermal desorption of carbon dioxide from several Polish coals were given in the article. A comparison of nonisothermal desorption curves obtained under the same conditions for coals with different pore volumes and pore volume distributions showed that the-process of gas evolution depends on the porous structure of coal. Basing on the analysis of experimental data, a possibility of the differentiation between outburst-prone coals and those which are believed not to lead to outbursts was shown. The properties of capillary structure characteristic for outbursts-prone coals were determined.

Non-Isothermal Desorption of Carbon Dioxide from Polish Coals with Different Texture

Energy & Environment, Vol. 6, No.3, 1995, pp. 237–246 The following corrections are required: (a) Equation 1 should read: [Formula: see text] (b) Heading for Table 2 should read: Coefficients of equation (1) (c) Bottom of page 244, for “Tm” read Tm. (d) Caption to Figure 5. ◯-◯-◯ - W430, ▪-▪-▪ - NR126, •-•-• - Th, Δ-Δ-Δ — Vi.

Convective Heat and Mass Transfer in Nonisothermal Moisture Desorption

The effect of forced convective heat and mass transfer coefficients on the predictive ability of a dynamic nonisothermal set of coupled partial differential equations was investigated. Two forms of the temperature gradient coefficient in the mass conservation equation of the model were used. The predictability of the transient average moisture and temperature of a wood specimen under nonisothermal desorption conditions as evaluated for the two coefficient forms and for various values of the heat and moisture convective coefficients. The analysis showed that the model using the simplest form of the two coefficients, resulted in a very good prediction of the experimental data, and that the moisture-time and temperature-time curves, were strongly affected by the values of the two convective coefficients. Finally, a method to determine the optimum values of the two convective coefficients is suggested

Fundamental Study of the Thermal Desorption of Toluene from Montmorillonite Clay Particles

Nonisothermal desorption of toluene from individual montmorillonite clay particles was measured experimentally and modeled mathematically to elucidate details of the overall thermal desorption process. A single-particle reactor was used. It consisted of a porous, 2-6-mm clay pellet formed around a 0.05-mm diameter thermocouple in a 6- or 9-mm o.d. glass tube. The tube was surrounded by a supplementary heater and placed in a GC oven. Desorption rates were obtained as a function of heating rate, clay type, particle size, and purge gas flow rate. In addition, the adsorption isotherms for two toluene/clay systems and one n-dodecane/clay system were measured and correlated using the Freundlich isotherm. At the conditions examined, the rate-controlling mechanism is associated with intraparticle diffusion. Isothermal desorption experiments using clay pellets of different sizes demonstrate that local desorption kinetics are not rate-controlling. Toluene shows a slower desorption rate than n-dodecane at low concentrations. This is attributed to the hindered removal of toluene from interlamellar regions of the clay. 39 refs., 9 figs., 4 tabs.

Non-isothermal regeneration of molecular sieve adsorbers in a cyclic process of air purification from moisture and carbon dioxide

Abstract Non-isothermal desorption of moisture and carbon dioxide from a layer of molecular sieve NaX has been studied on the basis of the proposed mathematical model. It is shown that it is not expedient to maintain the carbon dioxide desorption temperature above 353 K in the real industrial process while the minimum temperature of the moisture desorption should be within the range 373–423 K. The application of the thermal wave adsorption method to the problem of regeneration from the main impurities of air allows the power consumption to be significantly reduced. A simplified model of a cyclic process of air purification under incomplete desorption of moisture has been suggested. A comparison is made of the results of the calculations and the test data obtained on the commercial adsorbers of a low-pressure air separation plant when the regenerating gas temperature is decreased from 653 to 373 K. Good qualitative and satisfactory quantitative agreement of the calculated and observed results is observed.

Non-equilibrium, non-isothermal desorption of single adsorbate by Purge

Non-equilibrium, non-isothermal desorption of single adsorbate by Purge

Nonequilibrium, nonisothermal desorption of single adsorbate by purge

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTNonequilibrium, nonisothermal desorption of single adsorbate by purgeRavi Kumar and Glenn R. DissingerCite this: Ind. Eng. Chem. Process Des. Dev. 1986, 25, 2, 456–464Publication Date (Print):April 1, 1986Publication History Published online1 May 2002Published inissue 1 April 1986https://doi.org/10.1021/i200033a018RIGHTS & PERMISSIONSArticle Views129Altmetric-Citations35LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (791 KB) Get e-Alerts

An approach to the kinetics of water desorption from A-zeolites. Part I. Isothermal and non-isothermal desorption

Desorption of water from zeolites: NaA, LiA, KA, CdA, ZnA, MgA and CaA were examined. Isothermal and non-isothermal kinetic analyses were carried out. It was found that the process of desorption can be presented as a first-order reaction. Due to the existence of several water—zeolite complexes, neither isothermal nor non-isothermal kinetic methods may be applied directly.