Highlights of non-equilibrium,non-isobaric,non-isothermal desorption of nitrogen from a LiX zeolite column by rapid pressure reduction and rapid purge by oxygen

The effects of adsorption kinetics, column pressure drop, gas phase mass and heat dispersions, gas–solid heat transfer resistance, and adsorber adiabaticity on desorption of N₂ from a LiX zeolite column by O₂ purge as well as pressurization–depressurization of the column using pure N₂ were recently studied using a numerical model of these processes [Chai et al. in Ind Eng Chem Res 50:8703, 2011, Chai et al. in Adsorption 18:87, 2012, Chai et al. in AIChE J 59:365 2013; Rama Rao et al. in Adsorption 2013]. The role of adsorbent particle size and column length to diameter ratio in determining the durations and efficiency of these processes were also investigated. These studies revealed several important limiting and optimum conditions for optimum operation of these processes which can be useful in design of a practical rapid pressure swing adsorption (RPSA) process for medical oxygen concentrator (MOC) application. The purpose of this short review article is to consolidate and re-emphasize these important results in a single article to be used as a guideline for design of a RPSA-MOC unit.     

CO_2 residual concentration of potassium-promoted hydrotalcite for deep CO/CO_2 purification in H_2-rich gas

Elevated-temperature pressure swing adsorption is a promising technique for producing high purity hydrogen and controlling greenhouse gas emissions. Thermodynamic analysis indicated that the CO in H_2-rich gas could be controlled to trace levels of below 10 ppm by in situ reduction of the CO_2 concentration to less than 100 ppm via the aforementioned process. The CO_2 adsorption capacity of potassiumpromoted hydrotalcite at elevated temperatures under different adsorption(mole fraction, working pressure) and desorption(flow rate, desorption time, steam effects) conditions was systematically investigated using a fixed bed reactor. It was found that the CO_2 residual concentration before the breakthrough of CO_2 mainly depended on the total amount of purge gas and the CO_2 mole fraction in the inlet syngas.The residual CO_2 concentration and uptake achieved for the inlet gas comprising CO_2(9.7 mL/min) and He(277.6 mL/min) at a working pressure of 3 MPa after 1 h of Ar purging at 300 mL/min were 12.3 ppm and0.341 mmol/g, respectively. Steam purge could greatly improve the cyclic adsorption working capacity, but had no obvious benefit for the recovery of the residual CO_2 concentration compared to purging with an inert gas. The residual CO_2 concentration obtained with the adsorbent could be reduced to 3.2 ppm after 12 h of temperature swing at 450 °C. A new concept based on an adsorption/desorption process, comprising adsorption, steam rinse, depressurization, steam purge, pressurization, and high-temperature steam purge, was proposed for reducing the steam consumption during CO/CO_2 purification.     

Numerical simulation of rapid pressurization and depressurization of a zeolite column using nitrogen

The required durations of pressurization and depressurization steps of a rapid pressure swing adsorption process are primarily governed by adsorbent particle size, adsorption kinetics, column pressure drop, column length to diameter ratio, and the valve constant of the gas inlet and outlet control valve attached to the adsorbent column. A numerical model study of the influence of these variables for an adiabatic LiX zeolite column is presented using pure N₂ as an adsorbate gas. An adsorbent particle size range of 200–350 μm was found to minimize (<1 s) the times required for the pressurization and depressurization steps.     

The Analysis of volatile compounds by purge and trap with whole column cryotrapping (WCC) on a fused silica capillary column

The purge and trap (P&T) method of analysis has been interfaced with fused silica capillary column gas chromatography. This interfacing has been accomplished without splitting the P&T trap desorption carrier gas. Thus, 100% of the purged compounds are transferred to the column. The analytes are cryofocussed on the column using whole column cryotrapping (WCC) at ?80°C. The resulting P&T/WCC procedure is extremely well-suited to the analysis of trace purgeable aqueous organic compounds. Samples and standards containing a variety of aromatic standard compounds were analyzed. The standards included benzene, toluene, ethylbenzene, xylenes, C₃-C₄-benzenes, and naphthalene, as well as three P&T internal standard compounds. Chromatographic peak widths were uniformly less than 6 s at the base and excellent precision was obtained in the relative retention time data for all compounds. The chromatogram of a groundwater sample contaminated with aromatic gasoline compounds is also presented. Since P&T/WCC works well with fused silica capillary columns, the full sensitivity and chromatographic efficiency of capillary gas chromatography is made available to P&T analyses.     

Capture of CO<Subscript>2</Subscript> from flue gas by vacuum pressure swing adsorption using activated carbon beads

Vacuum pressure swing adsorption (VPSA) for CO₂ capture has attracted much research effort with the development of the novel CO₂ adsorbent materials. In this work, a new adsorbent, that is, pitch-based activated carbon bead (AC bead), was used to capture CO₂ by VPSA process from flue gas. Adsorption equilibrium and kinetics data had been reported in a previous work. Fixed-bed breakthrough experiments were carried out in order to evaluate the effect of feed flowrate, composition as well as the operating pressure and temperature in the adsorption process. A four-step Skarstrom-type cycle, including co-current pressurization with feed stream, feed, counter-current blowdown, and counter-current purge with N₂ was employed for CO₂ capture to evaluate the performance of AC beads for CO₂ capture with the feed compositions from 15–50% CO₂ balanced with N₂. Various operating conditions such as total feed flowrate, feed composition, feed pressure, temperature and vacuum pressure were studied experimentally. The simulation of the VPSA unit taking into account mass balance, Ergun relation for pressure drop and energy balance was performed in the gPROMS using a bi-LDF approximation for mass transfer and Virial equation for equilibrium. The simulation and experimental results were in good agreement. Furthermore, two-stage VPSA process was adopted and high CO₂ purity and recovery were obtained for post-combustion CO₂ capture using AC beads.     

Mechanisms of oxygen adsorption and desorption on polycrystalline palladium

The adsorption and desorption of oxygen on a polycrystalline palladium (Pd(poly)) surface (10-to 100-μm crystallites; ～32% (100), ～18% (111), ～34% (311), and ～15% (331)) at P _O2 ≤ 1.3 × 10^?5 Pa and T = 500–1300 K have been studied by TPD and mathematical modeling. The kinetics of O₂ adsorption and desorption on Pd(poly) are primarily governed by the formation and decomposition of oxygen adsorption structures on the (100) and (111) crystallite faces. The O₂ adsorption rate is constant at ? ≤ 0.15–0.25 owing to the formation of the p(2 × 2) structure with an O_ads-surface bonding energy of D(Pd-O) = 364 kJ/mol on the (100) and (111) faces. The adsorption rate decreases with increasing coverage at ? ≥ 0.15–0.25 because of the growth, on the (100) face, of the c(2 × 2) structure, in which D(Pd-O) is reduced to 324 kJ/mol by lateral interactions in the adsorption layer. A high-temperature (～800 K) O₂ desorption peak is observed for ? ≤ 0.25, which is due to O₂ desorption from a disordered adsorption layer according to a second-order rate law with an activation energy of E _des = 230 kJ/mol. A lower temperature (～700 K) O₂ desorption peak is observed for ? ≥ 0.25, which is due to O₂ released by the c(2 × 2) structure according to a first-order rate law with E _des = 150 kJ/mol. At ? ≥ 0.25, there are repulsive interactions between O_ads atoms on Pd(poly) (ε_aa = 5–10 kJ/mol).     

A new type of two-dimensional packed + capillary column GC system. I. Theory

Summary To match the need of high efficiency capillary column (plate number N₂, Plate height H₂ and carrier gas linear velocity U₂), a new high efficiency packed column (N₁, H₁ and U₁) at high carrier gas velocity with small capacity factor has been developed in the light of theoretical discussion of factors effecting the column efficiency utilization ratio.     

Capture of CO2 from flue gas streams with zeolite 13X by vacuum-pressure swing adsorption

Vacuum swing adsorption (VSA) capture of CO₂ from flue gas streams is a promising technology for greenhouse gas mitigation. In this study we use a detailed, validated numerical model of the CO2VSA process to study the effect of a range of operating and design parameters on the system performance. The adsorbent used is 13X and a feed stream of 12% CO₂ and dry air is used to mimic flue gas. Feed pressures of 1.2 bar are used to minimize flue gas compression. A 9-step cycle with two equalisations and a 12-step cycle including product purge were both used to understand the impact of several cycle changes on performance. The ultimate vacuum level used is one of the most important parameters in dictating CO₂ purity, recovery and power consumption. For vacuum levels of 4 kPa and lower, CO₂ purities of >90% are achievable with a recovery of greater than 70%. Both purity and recovery drop quickly as the vacuum level is raised to 10 kPa. Total power consumption decreases as the vacuum pressure is raised, as expected, but the recovery decreases even quicker leading to a net increase in the specific power. The specific power appears to minimize at a vacuum pressure of approximately 4 kPa for the operating conditions used in our study. In addition to the ultimate vacuum level, vacuum time and feed time are found to impact the results for differing reasons. Longer evacuation times (to the same pressure level) imply lower flow rates and less pressure drop providing improved performance. Longer feed times led to partial breakthrough of the CO₂ front and reduced recovery but improved purity. The starting pressure of evacuation (which is not necessarily equal to the feed pressure) was also found to be important since the gas phase was enriched in CO₂ prior to removal by vacuum leading to improved CO₂ purity. A 12-step cycle including product purge was able to produce high purity CO₂ (>95%) with minimal impact on recovery. Finally, it was found that for 13X, the optimal feed temperature was around 67°C to maximize system purity. This is a consequence of the temperature dependence of the working selectivity and working capacity of 13X. In summary, our numerical model indicates that there is considerable scope for improvement and use of the VSA process for CO₂ capture from flue gas streams.     

Determination of rates of collision-induced vibrational and intramultiplet transitions for YO(A2II) molecules in Ar- and N2-diluted flames

Rate contants for single-step collisional transitions between specified vibrational and electronic-doublet levels of YO(A²II) molecules were determined under multiple-collision conditions at constant total gas pressure. We recorded simultaneously the thermal emission spectrum and the fluorescence spectra induced by a cw dye laser tuned successively at various vibronic transitions. We used premixed N₂-diluted or Ar-diluted H₂-O₂ flames at 1 atm and = 2350 K, containing YO vapour. Relative rate constants (normalized to the quenching plus radiative de-excitation rate constants) were obtained by solving a set of simultaneous master equations involving the stationary laser-induced population increments. Absolute rate constants were derived therefrom by determining indirectly the fluorescence efficiency. Exoergic doublet-mixing collisions appeared to have greater probabilities than Δv = ?1 or ?2 transitions and were about equally probable as quenching collisions. Ar and N₂ were about equally efficient as collision partner. The rate constants of each process and of its reverse were compared with detailed-balance. The rotational levels in the pumped band appeared to preserve the e/f symmetry index partially.     

Kinetics of interaction of gas phase oxygen with cerium-gadolinium oxide

The method of isotopic exchange with gas phase analysis was used to study the kinetics of oxygen interaction with the Ce_0.80Gd_0.20O_{1.90 ? ??} oxide at the oxygen pressure of 0.27?C1.33 kPa in the temperature range of 700?C800°C. The values of oxygen interphase exchange rate and diffusion coefficient, and also effective activation energies of the processes of oxygen exchange and diffusion were determined. The contributions of three exchange types and amounts of equivalent exchangeable oxygen were calculated. It was shown that the limiting exchange stage is the process of dissociative oxygen adsorption/desorption on the surface of the Ce_0.80Gd_0.20O_{1.90 - ??} oxide.     

Excimer formation mechanism in gaseous krypton and Kr/N2 mixtures

A new approach for the understanding of the energy relaxation dynamics of excited atoms involving a long-lived molecular precursor is presented here for krypton. Excitation of the gas close to the 5s[3/2]₂ metastable atomic level (E _at. ?E _exc.<kT) is achieved with an intense VUV laser source (I ≈ 10¹² photon/pulse) realized by resonantly enhanced 4-wave mixing (2ω₁ + ω₂) in room temperature mercury vapor (N _Hg ≈ 10¹³ at./cm³). The decay of the II. continuum luminescence (145 nm) is studied. In the pressure range 200–500 mbar, decay rates depend linearly on pressure but have a negative zero-pressure intersect. We show here that this result can be understood as an effect of the exchange of energy between two different “reservoirs” of atomic (5s[3/2]₂) and molecular (1_g) nature, and can be an inherent peculiarity of the recombination kinetics of excited atoms with several product channels. The efficiency of the model is checked for the Kr/N₂ system. Rate constants for relaxation processes are determined in pure krypton and in Kr/N₂ mixtures.     

Adsorption of carbon disulfide on activated carbon modified by Cu and cobalt sulfonated phthalocyanine

A series of adsorbents were studied for removal efficiency of carbon disulfide (CS₂) under micro-oxygen conditions. It was found that activated carbon modified by Cu and cobalt sulfonated phthalocyanine (CoSPc) denoted as ACCu–CoSPc showed significantly enhanced adsorption ability. Reaction temperature was found to be a key factor for adsorption, and 20 °C seems to be optimal for CS₂ removal. Samples were analyzed by N₂-BET, XRD, XPS, SEM–EDS and CO₂-TPD. The characterization results demonstrated that large quantities of SO₄ ^2? anions were formed and adsorbed in the reaction process. SO₂, CS₂ and COS were detected in the effluent gas generated from the temperature programmed desorption of ACCu–CoSPc–CS₂. Therefore, it can be concluded that ACCu–CoSPc most likely acted as a catalyst in the adsorption/oxidation process on the surface of the impregnated sample. The generated sulfide and sulfur oxide can cover the active sites of adsorbents, resulting in pronounced reduction of adsorbent activity. Finally, the exhausted ACCu–CoSPc can be regenerated by thermal desorption.     

HPLC-ELSD Determination of Kryptofix 2.2.2 in the Preparations of 2-Deoxy-2-[18F]fluoro-d-glucose and other Radiopharmaceuticals

A new and simple high-performance liquid chromatography with evaporative light scattering detector method for the determination of Kryptofix 2.2.2 (K-222) in the radiopharmaceuticals of 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG) and 3′-deoxy-3′-[¹⁸F]fluorothymidine ([¹⁸F]FLT) was developed. A C₁₈ column was used and the mobile phase was 10?% (v/v) methanol and 90?% (v/v) water (0.1?% trifluoroacetic acid, v/v) at a flow rate of 0.2?mL?min^?1. The drift tube temperature was 40?°C. The pressure of nebulizing gas (N₂) was 3.0?bar. The gain was 10. Good separation of K-222 from main related substances could be achieved. Excellent linearity (r ²?=?0.9995) was obtained over the range of 5–100?μg?mL^?1. The precision ranged from 0.68 to 5.16?% (RSD) and the accuracy ranged from ?3.05 to 2.62?% (RE). The limit of detection was 2?μg?mL^?1. This method offers simple, rapid and quantitative detection of K-222, thus making it acceptable for routine determination.     

Nonequilibrium vibrational populations and dissociation rates of oxygen in electrical discharges

Nonequilibrium vibrational distributions and dissociation rates of molecular oxygen in both electrical and thermal conditions have been calculated by solving a system of master equations including V-V (vibration-vibration), V-T (vibration-translation) and e-V (electron-vibration) energy exchanges. The dissociation constant under thermal conditions (i.e. without electrons) follows an Arrhenius law with an activation energy of 120 kcal/mole, while the corresponding rates under electrical conditions (5000 ? T_e ? 15000 K, 300 ? T_g ? 1000 K, 10¹¹ ? n_e ? 10¹² cm^?3,5 ? p ? 20 torr) increase with decreasing gas (T_g) and electron (T_e) temperatures and pressure (p) and with increasing electron density (n_e). These results are explained on the basis of the different interplay of V-V and V-T energy exchanges and are rationalized by means of simplified models proposed in the literature. The accuracy of the present results is discussed paying particular attention to the dependence of V-V and V-T rate coefficients on the vibrational quantum number. A comparison of the calculated dissociation rates with the corresponding ones obtained by the direct electron impact mechanism shows that the present mechanism prevails at low electron and gas temperatures. Finally a comparison is shown between theoretical and experimental dissociation rates under electrical and thermal conditions.     

Flux response technology applied in zero length column diffusivity measurements

Flux Response Technology (FRT) has been successfully adapted as an in situ perturbation technique in dynamic gas sorption measurements to extract hydrocarbon diffusion coefficients in alumina/CeZrO _x washcoats of cordierite monoliths. FRT works by measuring minuscule changes in flowrate between two gas streams for any gaseous process involving a change in volume (δV/δt). Ad- and desorption transients, which can be collected during the same experimental run have been analysed using the zero length column (ZLC) method to study propane diffusivity within an alumina/CeZrO _x washcoat as a function of temperature. Extracted diffusivities and activation energies compare favourably with literature data.     

Efficient Solar‐Driven Nitrogen Fixation over Carbon–Tungstic‐Acid Hybrids

Ammonia synthesis under mild conditions is of supreme interest. Photocatalytic nitrogen fixation with water at room temperature and atmospheric pressure is an intriguing strategy. However, the efficiency of this method has been far from satisfied for industrialization, mainly due to the sluggish cleavage of the N≡N bond. Herein, we report a carbon–tungstic‐acid (WO₃ ? H₂O) hybrid for the co‐optimization of N₂ activation as well as subsequent photoinduced protonation. Efficient ammonia evolution reached 205 μmol g^?1 h^?1 over this hybrid under simulated sunlight. Nitrogen temperature‐programmed desorption revealed the decisive role of carbon in N₂ adsorption. Photoactive WO₃ ? H₂O guaranteed the supply of electrons and protons for subsequent protonation. The universality of carbon modification for enhancing the N₂ reduction was further verified over various photocatalysts, shedding light on future materials design for ideal solar energy utilization.     

A 3D Ba-MOF for selective adsorption of CO_2/CH_4 and CO_2/N_2

An unexpected in-situ hydrolysis reaction occurred during the solvothermal reaction of N,N'-bis(4-carboxy-2-methylphenyl)pyromellitic di-imide) and Ba(NO_3)_2,and a novel porous Ba-MOF,[H_2 N(CH_3)_2]_(0.5) [Ba_(1.5)(L)(DMA)]·1.5 DMA·1.5 H_2 O(UPC-70,H3 L=2-(4-ca rboxy-2-methylphenyl)-1,3-dioxoisoindoline-5,6-dicarboxylic acid,DMA=N,N-dimethylacetamide),was obtained on the basis of the partial hydrolysate.The as-synthesized 3 D network with 1 D open channels of different sizes(24 A and 10 A)contains abundant open metal sites after removal of solvents,which is conducive to the preferential adsorption of CO_2.The subsequent gas sorption measurement reveals the high separation selectivity of UPC-70 for CO_2/CH_4(15) and CO_2/N_2(32) at ambient conditions,and GCMC theoretical simulation provides good verification of the experimental results,indicating that UPC-70 is a potential candidate for CO_2 capture from flue gas and natural gas.     

The gas transport properties of amine-containing polyurethane and poly(urethane-urea) membranes

A series of amine-containing polyurethanes and poly(urethane-urea)s based on 4,4′-diphenylmethane diisocyanate and either poly(ethylene glycol) of molecular weights 400 or 600 were prepared as gas separation membranes. The amine functional groups of N-methyldiethanolamine (MDEA) and/or tetraethylenepentamine (TEPA) were introduced into the hard segment as a chain extender. The gas transport data of He, H₂, O₂, N₂, CH₄ and CO₂ in these polymer membranes were determined by using the Barrer's high-vacuum technique and the time-lag method. The restriction of chain mobility has been shown by the formation of hydrogen bonding in the soft segment and hard-segment domains, resulting in the increase in the density, glass transition temperature of soft segments (T_gs). The separation mechanism of various gas pairs used in industrial processes is also discussed. Effect of pressure on permeability of the gases above and below T_gs was studied. It was found that the gas permeability increased or decreased with upstream pressure above T_gs, and should be described by a modified free-volume model. On the other hand, the condensable CO₂ exhibits a minimum permeability at a certain upstream pressure below T_gs. The permeability of He and H₂ were pressure independent above and below the T_gs.     

Mechanism of inhibition of hydrogen/oxygen flames of various compositions by trimethyl phosphate

The effect of the catalytic recombination reactions of H and OH^? involving phosphorus-containing products of trimethyl phosphate (TMP) combustion on the burning velocity and the structure of H₂/O₂/N₂ flames at atmospheric pressure has been investigated. An earlier mechanism for inhibition of rich hydrogen/oxygen flames by organophosphorus compounds has been tested and modified by comparing experimental data with the results of simulation. The sensitivity analysis of the calculated flame speed to the rate constants of chain branching reactions and chain termination reactions involving phosphorus-containing compounds has revealed significant specific features of the inhibition mechanism of hydrogen flames with various stoichiometries and dilution ratios. Unlike the inhibition efficiency of hydrocarbon flames, in which the reactions of H and OH^? radicals with PO, PO₂, HOPO, and HOPO₂ play the key role, the inhibition efficiency of hydrogen flames at atmospheric pressure is determined by the interaction of hydrogen and oxygen atoms with TMP and with organophosphorus products of its decomposition in the low-temperature zone of the flame. The sensitivity analysis has demonstrated that, as the equivalence ratio (?) or the dilution ratio is increased, the ratio of the chain branching rate to the rate of chain termination via reactions involving phosphorus compounds decreases. As a consequence, the efficiency of inhibition of H₂/O₂/N₂ flames, as distinct from that of hydrocarbon flames, increases as ? is raised from 1.1 to 3.0 and as the mixture is progressively diluted with nitrogen.