Desorption Model Research Articles

Experimental and theoretical characterization of methane and CO2 sorption hysteresis in coals based on Langmuir desorption

Sorption hysteresis is important for primary gas production and CO2 sequestration in coalbed methane (CBM) reservoirs. We represent the degree of hysteresis using an areal hysteresis index (AHI) method incorporating a hysteresis parameter β, representing the ratio of available sorption sites for desorption relative to adsorption. This approach was applied to quantify the gas-coal sorption hysteresis to both methane and CO2 on sub-bituminous and bituminous coals and on anthracite. A theoretical desorption model describes hysteresis based on the molecular dynamic equilibrium between gas adsorption and desorption rates. Volumetric excess ad-/desorption isotherms of methane and CO2 show that adsorption capacities for anthracite are higher than those for the sub-bituminous and bituminous coals over the entire experimental pressure range. Hysteresis for CO2 is greater than for methane for all four coal samples investigated. The relationships between β and Langmuir pressure, Langmuir volume and final equilibrium pressure are weakly negative with null relationships between β and coal properties. Sorption hysteresis is caused by Langmuir pressure and gas type based on theoretical and experimental analyses although its physical mechanism remains unclear. A βCH4−βCO2 plot is proposed as a screening tool to determine likely gas substitution contents for enhanced coalbed methane (ECBM) and CO2 sequestration in coals.

Mathematical modeling of gas desorption from a metal–organic supercontainer cavity filled with stored N2gas at critical limits

Gas escape rates from within the cavity of a MOSC were predicted by molecular dynamics and analytical equations.

СОВМЕСТНАЯ ПОВЕРХНОСТНАЯ СЕГРЕГАЦИЯ СЕРЕБРА И ОЛОВА В ТРОЙНЫХ СПЛАВАХ Cu–Ag–Sn

The study of Cu–Ag–Sn ternary alloys by the methods of temperature-programmed desorption (TPD) and statistical modeling is с arried out. The results of the TPD experiment on the combined surface segregation of Ag and Sn atoms in dilute solutions of silver and tin in copper in polycrystalline and liquid states are presented. It was found out that the desorption of particles from the surface of the alloy occurs from regions enriched with this component. In addition, the silver atoms form, in the main, slightly layered islets. Data are also provided for computer simulation of the surface of these alloys by the Monte Carlo method in combination with the embedded atom method (EAM). Using the EAM method, it was possible to avoid a priori assumptions about the composition and structure of the alloy surface. Simulation showed that the binding energy of surface atoms is close to the experimental activation energies of thermal desorption for Cu and Ag atoms. But there is a significant difference for tin atoms. Secondly, in the modeling for the solid state of the Cu 98 Ag 1 Sn 1 alloy, the surface concentrations of Θ Ag ≈ Θ Sn = 30 at.%, which for Ag atoms are close to the results of the TPD experiment. In the melt, the degree of surface segregation is reduced to 8 at.% Ag and 14 at.% Sn. Both the TPD experiment and the simulation give an excess of 10 to 30 times the surface concentrations of Ag and Sn over bulk values, both in the solid and in the liquid state of the alloy. Third, computer models reveal surface structures in the form of zigzag chains of Sn atoms, which alternate with chains of Ag atoms. The profiles of tin concentrations in alloys confirm the hypothesis of a two-layered character of segregation of tin.

A flexible and robust model for low temperature catalytic desorption of CO2 from CO2-loaded amines over solid acid catalysts

A numerical model for catalyst-aided carbon dioxide (CO2) desorption from CO2-loaded aqueous amines solution has been developed. The model includes a hot water-heater and considers phase separation at the top of the desorption column. The model was validated with experimental data obtained from an integrated CO2 capture pilot plant which used 5M monoethanolamine (MEA) solution with two industrial catalysts, namely, HZSM-5 and γ-Al2O3. The model considers the presence of electrolytes and multi-component mass transfer as well as both the physical and chemical contribution of the catalyst in aiding the process. The data obtained from model simulation were in good agreement with the experimental data in terms of CO2 production rates with an absolute average deviation of approximately ±8.9%. The simulation slightly over-predicted the CO2 production rate at the low temperature regime (75°C) and under-predicted the CO2 production rate at the high temperature regime (95°C). The developed model provides a range of capabilities for further studies of the catalytic CO2 desorption process.

Modelling of column lithium desorption from Li+-loaded adsorbent obtained by adsorption from salt brine

Desorption technology is important to fabricate the economical process for Li+ recovery from brine by adsorption method, but there have been no systematic studies on the column Li+ desorption. The purpose of this paper is to find the elution conditions to have the eluate with high Li+ concentration in a short time, from which Li2CO3 can be precipitated without pre-concentration. The promising conditions of column Li+ desorption could be proposed on the bases of column desorption experiments and model calculations.The natural salt brine (pH6.8) containing 0.21M (1M=1moldm−3) Li+, 2.7M Na+, 0.51M K+, 1.3M Mg2+, 5.4M Cl−, and 0.25M SO42− was used for the Li+ adsorption. The granulated adsorbent sieved to 0.42–0.71 or 0.71–1.0mm was prepared with colloidal silica as a binder. The Li+-loaded adsorbents were prepared by treating the granulated adsorbent with the NaHCO3 added brine by a batch method. The column elution experiments were carried out by passing HCl or H2SO4 solutions with different concentrations through the Li+-loaded adsorbents at different flow rates.A model consisting of Li+ migration in the solid phase and of surface H+/Li+ exchange was proposed for the analysis of desorption behavior. The calculated elution curves well approximated the experimental ones when the effective acid concentration and the elution delay were considered. These approximations may be caused by the influence of the other metal ions co-adsorbed from brine. The calculation showed that the elution by passing a 2M acid solution at a space velocity of 10/h up to 2 bed volume produces the eluate of high Li+ concentration (1.15M) with high Li+ recovery (87%). This process has the advantages of eliminating the step of pre-concentration of eluate for precipitation and reducing the acid consumption.

Two-controlling mechanisms model for hydrogen desorption in the Li4(NH2)3BH4 doped Mg(NH2)2–2LiH system

The limiting step of the dehydrogenation process and the desorption kinetic model of the composite 0.7Mg(NH2)2–1.4LiH–0.2Li4(NH2)3BH4 under different hydrogen back pressures at low temperature (200 °C) were studied in this work. It was determined that a single mechanism model was not able to explain the behavior of the reaction at low and high reacted fractions simultaneously. A combination of two controlling mechanisms, which reproduce accurately the behavior of the system, was proposed. The rate equation deduced involves the contribution of a second grade Avrami model and a 3D diffusion model as a function of pressure and conversion. At low conversions, the limiting step of the reaction is the formation of the products. Once a thin layer of product is formed on the surface, the diffusion through it becomes the new limiting step. With pressure increase, the mechanism change occurs later during desorption due to the higher difficulty in creating nucleation points on a surface exposed to a higher concentration of hydrogen.

Effect of Potamogeton crispus L. on bioavailability and biodegradation activity of pyrene in aged and unaged sediments

In order to clarify the effect of Potamogeton crispus L. (P. crispus) on bioavailability and biodegradation activity of pyrene in aged and unaged sediments, model calculation based on experimental results was carried out. During a 36-day experiment, the dissipation ratio of pyrene was increased by planting but decreased by aging. P. crispus improved the dissipation more significantly in aged sediments (45.9%) than in unaged sediments (17.6%). Results derived from a two-compartment desorption model showed that the decrease of rapidly desorbing fraction of pyrene was in the order of aged sediments without plant (A)>unaged sediments without plant (U)>unaged sediments with plant (UP)>aged sediments with plant (AP). Moreover, the results of biodegradation kinetic model showed that the first-order biodegradation coefficient was in the order of AP>UP>U and A, which was consistent with that of sediment redox potential. These modeling results indicated that planting could enhance the bioavailability (73.9%) and biodegradation activity (277%) of pyrene more significantly in aged sediments as compared to unaged sediments (13.1% and 150%, respectively), which should be the key reasons leading to more significant dissipation increment of pyrene in aged sediments by P. crispus.

Practical enhancement factor model based on GM for multiple parallel reactions: Piperazine (PZ) CO2 capture

Reactive absorption is a key process for gas separation and purification and it is the main technology for CO2 capture. Thus, reliable and simple mathematical models for mass transfer rate calculation are essential. Models which apply to parallel interacting and non-interacting reactions, for all industrially relevant reaction regimes must be developed and validated against experimental measurements.In a previous work, we presented the general model (GM) enhancement factor model for (m+n)-th order reversible reactions and validated it against the numerical solution of the two-film model for absorption, desorption and pinch conditions.In this work, we apply the GM model to multiple parallel reactions. We deduce the model for piperazine (PZ) CO2 capture and we validate it against wetted-wall column measurements using 2, 5 and 8 molal PZ for temperatures between 40°C and 100°C and CO2 loadings between 0.23 and 0.41mol CO2/2mol PZ. We show that overall second order kinetics describes well the reaction between CO2 and PZ accounting for the carbamate and bicarbamate reactions. Here we prove the GM model for piperazine and MEA but we expect that this practical approach is applicable for various amines, blends of amines and promoted amines with similar kinetics. We believe that this practical implementation of mass transfer rate calculation will be in the accuracy range of a wetted wall column experiment for other parallel reaction systems. This is in line with our observation from other similar solvents studied, not shown here.Furthermore, we compare the GM model and the numerical solution of the complete two-film model predictions to MEA wetted-wall data and we prove that it is safe to assume that GM and the two-film model give practically identical results. We demonstrate that the expected predictability of CO2 mass transfer rates using off-the-shelf correlations generally is ±20%.

Modelling particle growth under saturated and starved conditions in emulsion polymerization

AbstractModelling of particle growth under saturated and starved conditions in emulsion polymerization was investigated in order to allow describing the broadening of the particle size distribution and account for the effects of restricted diffusion inside the monomer‐swollen polymer particles. In emulsion polymerization, if particle nucleation and coagulation are avoided, the reaction proceeds as a consequence of radical capture, desorption, and termination reactions inside the particles. Therefore, first, investigation of radical entry and desorption models was done under saturation of polymer particles with monomer, where diffusion parameters are constant. The discrimination criterion of the different models was based on fitting the reaction and the total particle size distribution. Indeed, the broadening of the particle size distribution was found to reveal a dependency of radical capture on the particle size; such an effect would be misestimated if only the mean particle size is considered. Second, the models for starved conditions that account for the variation of radical diffusion inside the polymer particles, such as the gel and glass effects, were investigated with comparison to a wide variety of experimental conditions spanning from weak to strong gelation. The dominating mechanisms as well as the interests of both intervals are highlighted.

A novel process for NH3-based CO2 capture by integrating flow-by capacitive ion separation

In order to reduce the regeneration energy, a novel process for NH3-based CO2 capture by integrating a flow-by capacitive ion separation device was proposed. The schematic diagram of the flow-by capacitive ion separation device was designed. The electrolyte ion adsorption and desorption models for a type of flow-by capacitor operated at constant current were developed and validated. The NH3-based CO2 capture process integrated with the flow-by capacitive ion separation device was simulated and its performances under different operating conditions of the capacitive ion separation device were obtained. By choosing the suitable operating parameters of the capacitive ion separation device, when integrating the flow-by capacitive ion separation device, the regeneration energy of the NH3-based CO2 capture process can be significantly reduced. The energy saving when integrating the capacitive ion separation device can reach above 27%.

Abilities of helium immobilization by the UO2 surface using the “ab initio” method

We present density functional theory calculation results concerning the uranium dioxide crystals with a helium atom incorporated in the octahedral sites on a nano superficial layer of UO2 fuel element. In order to quantify the capability of helium immobilization we propose a quantum model of adsorption and desorption which we compare with the classical model of Langmuir. Significant differences between the models are maintained in a wide temperature range including high temperatures of the order of 1000 K. By the proposed method of quantum isotherms it was established that the octahedral positions near the metal surface are good traps for helium atoms. While in a temperature close to 1089 K it predicts an intensive release of helium, which is consistent with the experimental results.

A unified kinetic model for adsorption and desorption – Applied to water on zeolite

A kinetic model for reversible adsorption and desorption processes under non-isothermal conditions based on reaction kinetics is deduced. This is done by explicit temperature dependence of all equations of the model. However in the performed experiments the coupling with the thermal bath was strong enough to neglect thermal conductivity and set the temperature as a timely-varying and spacial-constant parameter. Furthermore adsorption- and desorption-mechanisms of higher order and diffusion are considered. The formulation with partial differential equations is adaptable to the geometry of the adsorbent. To solve the system of coupled differential equations numerical methods of Wolfram Mathematica® are used. This model is applied to water on zeolite 3A. Beside the non-isothermal thermogravimetric analysis and the differential scanning calorimetry an isothermal experiment for the time-dependent measurement of the adsorption is used to evaluate the model. The sorption isotherms of a commercial zeolite manufacturer serve as a further source of comparison. The parameters are fitted numerically with the χ2 -method to the experimental data. As a result of this paper a unified sorption model is introduced.

Electron-stimulated desorption of cesium atoms from adlayers on a gold surface

We have studied the process of electron-stimulated desorption (ESD) of Cs atoms from a Cs/CsAu/Au/W system. It is established that ESD takes place from a Cs adlayer and the adjacent CsAu layer of this system. A model of Cs atom desorption from the Cs/CsAu/Au/W system is proposed. The results confirm the semiconductor nature of CsAu compounds.

Prediction of CO2 loading of amines in carbon capture process using membrane contactors: A molecular modeling

Separation of carbon dioxide from gas streams with respect to CO2 negative environmental effects is one of the most significant parts of gas separation processes. For this purpose, using membrane contactors is a promising technology and finding high performance solvents is crucial to development of this technology. Aqueous solutions of amines are the most employed solvents for this aim. The fact that performance of amines in carbon capture is greatly affected by variation in their molecular structure has been proven. Quantitative structure property relationship (QSPR) is a technique which employs molecular structure related variables known as descriptor for modeling. In this paper, QSPR method is employed for modeling and predicting CO2 loading of different amines after absorption and desorption steps in carbon removal process using membrane contactors. Physico-chemical and theoretical descriptors were calculated for a data set including sixteen amines. Variable selection and model development were performed by genetic algorithm-multiple linear regression (GA-MLR) method. Developed models could predict CO2 loading of amines’ solutions after absorption and desorption by coefficient of determination of 0.986 and 0.989 and standard error of the estimate equal to 0.0572 and 0.0496, respectively. In comparison between desorption and absorption model, desorption model is more accurate and absorption model provides simpler descriptors. Validity of the developed models were confirmed by different statistical tests and models showed high predictability power. Furthermore, mechanistic interpretation was applied in order to explain the relation between CO2 loading and descriptors which appeared in the models. It was found out that steric hindrance effect has a more significant influence on desorption than absorption.

Pseudo-kinetics of batch extraction of Crotalaria juncea (Sunn hemp) seed oil using 2-propanol

Crotalaria juncea (Sunn hemp) is one of the popular fiber crops of India. Its seeds contain appreciable amount of bio-oil which has promising fuel characteristics. Batch kinetics of extraction of C. juncea oil has been carried out and some pseudo-kinetic models (two-site kinetic desorption model (TSKDM), one-site kinetic desorption model (OSKDM) and second order pseudo-kinetics) are used to validate the experimental outcome. It is found that pseudo-second order kinetics fits more suitably in this case of oil extraction which approves a two stage leaching process, starting with an intense dissolution and scrubbing of maximum oil at the beginning followed by a much slower diffusion stage to remove the residual oil.

Microbial bioavailability of 2,2′,4,4′-Tetrabromodiphenyl ether (BDE-47) in natural sediments from major rivers of China

Microbial degradation plays a crucial role in eliminating polybrominated diphenyl ethers (PBDEs) in environments. However, the microbial bioavailability of PBDEs in aquatic sediments is not well understood. In this work, the bioavailability of 2,2′,4,4′-tetrabromodiphenyl ether (BDE-47), a typical PBDE congener, to PBDE-degrading microorganisms in natural sediments from six Chinese rivers under anaerobic conditions was investigated. The contents of black carbon (BC) and total organic carbon (TOC) in the six sediment samples were in the range of 0.025%–0.30% and 0.03%–3.38%, respectively. BDE-47 desorption from various sediments was fitted well with the first-order three-compartment desorption model. The desorbing fraction of sediment-associated BDE-47 at each desorption time interval exhibited a significant negative correlation with the BC content (p < 0.01). In the sediments, the anaerobic debromination of BDE-47 by microorganisms underwent a stepwise debromination pathway generating mainly three lower brominated congeners (BDE-28, -17 and −4). The microbial debromination ratio of BDE-47 ranged from 4.21% to 7.89% in various sediments after 120 d incubation anaerobically, and it negatively correlated with the content of sediment BC significantly (p < 0.01). However, the desorbing fraction and microbial debromination ratio of BDE-47 only showed weak correlations with the TOC content in sediments (p > 0.05). Furthermore, there was a significant positive correlation of desorbing fraction of BDE-47 from sediments with its microbial debromination ratio (p < 0.01) as well as with the level of its three lower brominated products (p < 0.05) after the first 20 d incubation. This study suggests that the BDE-47 bioavailability to microorganisms in anaerobic river sediments is mainly influenced by the content of sediment BC which controls the desorbing fraction of sediment-associated BDE-47.

Controlled Release of Ciprofloxacin from Core-Shell Nanofibers with Monolithic or Blended Core.

Sustained controlled drug release is one of the prominent contributions for more successful treatment outcomes in the case of several diseases. However, the incorporation of hydrophilic drugs into nanofibers, a promising novel delivery system, and achieving a long-term sustained release still pose a challenging task. In this work we demonstrated a robust method of avoiding burst release of drugs and achieving a sustained drug release from 2 to 4 weeks using core-shell nanofibers with poly(methyl methacrylate) (PMMA) shell and monolithic poly(vinyl alcohol) (PVA) core or a novel type of core-shell nanofibers with blended (PVA and PMMA) core loaded with ciprofloxacin hydrochloride (CIP). It is also shown that, for core-shell nanofibers with monolithic core, drug release can be manipulated by varying flow rate of the core PVA solution, whereas for core-shell nanofibers with blended core, drug release can be manipulated by varying the ratios between PMMA and PVA in the core. During coaxial electrospinning, when the solvent from the core evaporates in concert with the solvent from the shell, the interconnected pores spanning the core and the shell are formed. The release process is found to be desorption-limited and agrees with the two-stage desorption model. Ciprofloxacin-loaded nanofiber mats developed in the present work could be potentially used as local drug delivery systems for treatment of several medical conditions, including periodontal disease and skin, bone, and joint infections.

Sorbed Anthracene Degradation by Sophorolipid Producing Yeasts

The organic pollutan adsorption/desorption process by microbial degradation had been less studied than metal ones. The sorption assays alone did not predict desorption, due to hysteresis, irreversibility, fixed compounds in different sites, with diverse desorption rates. Most of the studies dealt with bacteria rather than filamentous fungi and yeasts. So, our aims were to isolate yeasts from polluted sediments, to quantify its potential to uptake anthracene (An) and to evaluate the bioavailability by a desorption model. Yeasts were isolated from hydrocarbon-polluted samples, 40-isolates grew in anthracene-plates. Molecular characterization was achieved by sequence analysis of the ITS1-5.8S rRNA-ITS4 and 26S rRNA regions; morphological and physiological determination were also done. Candida parasilopsis , Pichia anomala and Rhodothorula mucilaginosa were the prevalent yeasts. An-degradation was assessed in soil-systems with 0, 50, 100, 150, 200 and 250 I¼g An/l, 3 differentes sorbens types, organic carbon, organic nitrogen, PAHs, sand:silt:clay, pH and cation exchange capacity. Sophorolipids excretion were confirmed by HPLC, UV-detector with active fraction at 9.669 min (RT 9.646 min = sophorolipid-standard). A desorption model with equilibrium, nonequilibrium and nondesorption areas, was applied to explain the experimental data, An-transformation was greater in the organic liquid-phase than in the soil-sorbed ones; the desorption-coefficients and soil components were negatively correlated with the kinetic parameters. The An-release depended on the sophorolipid excretion, soil matrix and particles sizes. Desorption parameters significantly fitted the yeast uptake, with R 2 = 0.97, R 2 = 0.90 and R 2 = 0.97 for C. parasilopsis , P. anomala and R. mucilaginosa, respectively.

Core-Shell Electrospun Fibers Encapsulating Chromophores or Luminescent Proteins for Microscopically Controlled Molecular Release.

Core-shell fibers are emerging as interesting microstructures for the controlled release of drugs, proteins, and complex biological molecules, enabling the fine control of microreservoirs of encapsulated active agents, of the release kinetics, and of the localized delivery. Here we load luminescent molecules and enhanced green fluorescent proteins into the core of fibers realized by coaxial electrospinning. Photoluminescence spectroscopy evidences unaltered molecular emission following encapsulation and release. Moreover, the release kinetics is microscopically investigated by confocal analysis at individual-fiber scale, unveiling different characteristic time scales for diffusional translocation at the core and at the shell. These results are interpreted by a two stage desorption model for the coaxial microstructure, and they are relevant in the design and development of efficient fibrous systems for the delivery of functional biomolecules.

Hydrogen discharging and thermal behaviors of ZrCo based hydrogen storage beds equipped with heat-fins or metal-foam

A numerical study is conducted to investigate the influence of additional components – a heat-fin and metal-foam – on the hydrogen discharging performance of metal hydride beds. First, a three-dimensional, transient hydrogen desorption model developed in our previous works is applied to two different experimental metal hydride beds (MHBs): one with a heat-fin and the other with metal-foam. Then, the simulation results are compared with the experimental data measured during the heat-up and hydrogen discharging processes. The prediction based on the proposed model was found to be in good agreement with the experimental data in terms of the beds temperature evolution and the hydrogen discharging time. In addition, a parametric study is conducted on various MHBs with heat-fins or metal-foams that were designed to contain the same amount of ZrCo powder for comparison purposes. Our results clearly demonstrate that a superior MHB design should exhibit higher effective conductivity in the metal powder region and a lower overall thermal mass, which can be achieved by using metal-foam with optimum porosity.