Effect of Operation Symmetry on Pressure Swing Adsorption Process

In a multi-bed pressure swing adsorption (PSA) process, cycle steps with gas flow transferring from one bed to another such as equalization, purge, etc. are generally practiced to enhance the product recovery. However, if the flows for the connected beds in these steps are not balanced, the PSA process may not operate in a symmetrical manner. In the modeling of the PSA process, most of the simulations consider only one bed and assume that the rest of the beds would behave in a same way. In order to assess the impact of bed symmetry on the PSA performance, a new PSA model capable of studying bed symmetry in a two-bed system is developed. Experimental results from this paper show that uneven equalization flow can result in a lower product purity and a peculiar purity curve at different equalization levels. This phenomenon can be successfully predicted by this model. Simulation results also show that in large-scale PSA units, asymmetrical operation can cause drastically different temperature profiles in different adsorbers and hence a much lower performance. This paper demonstrates the importance of maintaining operation symmetry in PSA processes.     

Determination of organochlorine pesticides and their metabolites in soil samples using headspace solid-phase microextraction

An analytical procedure was developed using headspace solid-phase microextraction (HS-SPME) for the determination of organochlorine pesticides (OCPs) and their metabolites in sandy soil samples. The developed procedures involving fiber selection, temperature effect, absorption time, soil matrix and the addition of solvents of different polarity were optimized. Also, the results were compared to those achieved using Soxhlet extraction standard method. The 100-microm polydimethylsiloxane (PDMS) and 65-microm PDMS-divinylbenzene showed good extraction efficiency for 18 organochlorine pesticides. An increase in the extraction efficiency of organochlorine pesticides and the metabolites was observed when the temperature increased, and an optimum temperature of 70 degrees C for extracting OCPs was obtained. The application of other hydrophilic solvents had different effects on the extraction of organochlorine pesticides and the metabolites. Higher responses of OCPs were obtained when 5 ml of water was added to the soil. Good linearity of OCPs between 0.2 and 4 ng/g soil was observed. The relative standard deviation was found to be lower than 25%. Also the limits of detection were between 0.06 and 0.65 ng/g, which were lower than those obtained using Soxhlet extraction. Moreover, the optimized HS-SPME procedure was applied to the analysis of OCPs in certified reference material (CRM) 804-050 soil and compared with Soxhlet extraction procedure. Results obtained in this study were in good agreement with those obtained using Soxhlet extraction. The mean values obtained using HS-SPME technique were in the range of 16.5 to 1459.6 mg/kg, which corresponds to the recoveries of 68% to 127% of the certified values of CRM soil.     

Pressure-Swing Adsorption Using Layered Adsorbent Beds with Different Adsorption Properties: II—Experimental Investigation

An experimental study was conducted on a layered-bed pressure-vacuum-swing adsorption, PVSA, process with adsorbents that differ in their adsorption properties. An oxygen, O₂, PVSA process was employed as an example for investigating how the process performance is affected by bed-layering configuration under different operating conditions for specific purge, product purity, and cycle feature. For two adsorbents with similar nitrogen-to-oxygen, N₂/O₂, selectivity but different N₂ and O₂ capacities, placing the high-capacity adsorbent at the product end and the low-capacity adsorbent at the feed end of the adsorption bed results in a better performance than in the case of reversing the layer positions of those adsorbents. The benefit of placing the adsorbent with higher capacity at the product end becomes more significant at high O₂ product-purity levels. The experimental data obtained in this investigation agree well with simulation results reported earlier.     

Size and morphological effect of Au–Fe3O4 heterostructures on magnetic resonance imaging

In this paper, we analyse the melting of a spherically symmetric nanoparticle, using a continuum model which is valid down to a few nanometres. Melting point depression is accounted for by a generalised Gibbs–Thomson relation. The system of governing equations involves heat equations in the liquid and solid, a Stefan condition to determine the position of the melt boundary and the Gibbs-Thomson equation. This system is simplified systematically to a pair of first-order ordinary differential equations. Comparison with the solution of the full system shows excellent agreement. The reduced system highlights the effects that dominate the melting process and specifically that rapid melting is expected in the final stages, as the radius tends to zero. The results agree qualitatively with limited available experimental data.     

Nanoflower-like composites of ZnO/SiO2 synthesized using bamboo leaves ash as reusable photocatalyst

In this study, the synthesis of ZnO/SiO₂ nanocomposites using bamboo leaf ash (BLA) and tested their photocatalytic activity for rhodamine B decolorization have been conducted. The nanocomposites were prepared by the sol–gel reaction of zinc acetate dihydrate, which was used as a zinc oxide precursor, with silica gel obtained from the caustic extraction of BLA. The effect of the Zn content (5, 10, and 20 wt%) on the physicochemical characteristics and photocatalytic activity of the nanocomposites was investigated. The results of X-ray diffraction, scanning electron microscopy, gas sorption, and transmission electron microscopy characterization confirmed the mesoporous structure of the composites containing nanoflower-like ZnO (wurtzite) nanoparticles of 10–30 nm in size dispersed on the silica support. Further, the nanocomposites were confirmed to be composed of ZnO/SiO₂ by X-ray photoelectron spectroscopy analysis. Meanwhile, diffuse-reflectance UV–visible spectrophotometry analysis of the nanocomposites revealed band gap energies of 3.38–3.39 eV. Of the tested nanocomposites, that containing 10 wt% Zn exhibited the highest decolorization efficiency (99%) and fastest decolorization rate. In addition, the degradation efficiencies were not reduced significantly after five repeated runs, demonstrating the reusability of the nanocomposite catalysts. Therefore, the ZnO/SiO₂ nanocomposite obtained from BLA is a promising reusable photocatalyst for the degradation of dye-polluted water.     

Isometric immersions of tori into R3 with given mean curvature

It is known that a compact Riemannian surface can admit at most two (2) geometrically distinct, i. e., non-congruent isometric immersions into R ³ with given non-constant mean curvature. If the genus is zero, then there is at most one such immersion. Here, we show that there is at most one such immersion in each of the following cases for surfaces of genus one: 1) there exists a point p such that (H ² ? K)(p) = 0, where K is the curvature of the Riemannian metric and H is the given non-constant mean curvature (umbilic point); 2) the surface is a surface of revolution; 3) the surface is a tube formed by moving a circle in such a way that its center describes a smooth plane curve and its plane is constantly perpendicular to this curve. We also indicate the difficulties as to why the so-far existing methodologies cannot give a clear-cut answer to the question if it is possible to reduce the at most two immersions to at most one for any compact Riemannian surface of genus greater than zero.     

Pressure-Swing Adsorption Using Layered Adsorbent Beds with Different Adsorption Properties: I—Results of Process Simulation

A simulation study was conducted on layered-bed pressure-swing adsorption, PSA, processes with adsorbents that differ in their adsorption properties. As an example, an oxygen, O₂, vacuum-swing adsorption, VSA, process was analyzed to investigate relationships between process performance and adsorption properties of the adsorbents used. For two adsorbents with identical nitrogen-to-oxygen, N₂/O₂, selectivity but different N₂ and O₂ capacities, placing the high-capacity adsorbent at the product end and the low-capacity adsorbent at the feed end of the adsorption bed gives a better performance than the case of reversing layering of these adsorbents. However, for two adsorbents with different values of N₂/O₂ selectivity but identical N₂ capacity, changing the bed-layer configuration does not show a significant difference in O₂-VSA performance. The advantages of layering a high-capacity adsorbent on product end of the bed are demonstrated by an examination of the N₂-loading difference in a VSA cycle. The modeling study also reveals an effect of cycle features (e.g., equalization step) on the effectiveness of using layered-bed configurations in VSA/PSA processes. It suggests that layering appropriately two adsorbents with different adsorption properties could result in better VSA/PSA-process performance than using a single-layer bed with either of the two adsorbents.     

Determination of organochlorine pesticide residues in foods using solid-phase extraction clean-up cartridges

Fourteen organochlorine pesticide residues in fatty foods were determined using a simple and rapid procedure based on solid-phase extraction (SPE) clean-up cartridges with octadecyl (C18)-bonded porous silica, a tandem C18 and Florisil column, Alumina-N and Florisil. A Florisil cartridge eluted with 12 ml petroleum ether-ethyl ether (95 + 5) was the most efficient clean-up procedure capable of eliminating the matrix interference and satisfying the agreed acceptable recovery for the large numbers of organochlorine pesticides in nine kinds of foods having different fat contents. Average recoveries of organochlorine pesticides in shellfish, fish and meats ranged from 77 to 105%, 84 to 98% and 85 to 107%, respectively. In addition, analysis of a certified Standard Reference Material (SRM 1945) verified the satisfactory performance of Florisil clean-up cartridge. This SPE method not only yielded comparable results for nonfatty foods, but also provided a reliable separation and quantification of organochlorine pesticides for analyzing a large number of foods with a wide range of fat content.     

Tuning of Pressure Swing Adsorption Systems Based on Differential Pressure Profile

A method for tuning a Pressure Swing Adsorption (PSA) system aimed to achieve symmetrical operating conditions based on pressure differential in the adsorption vessels is developed in this study. Simulation of an oxygen Pressure-Vacuum Swing Adsorption (PVSA) process indicates that the pressure drop inside the adsorption vessel is closely related to the nitrogen concentration and gas velocity. The technique is applied to the tuning of an oxygen PVSA process. Adsorbent vessels of the PSA system are monitored and tuned by making corrective adjustments in each of the steps in a PSA cycle in response to imbalances in the differential pressure profiles in each of the adsorbent vessels. The method developed in this study provides a faster, easier, and more effective way to bring a PSA plant to its symmetrical, optimal state than those based on other parameters such as concentration, temperature, and pressure profile. This paper is dedicated to the memory of Professor Wolfgang Schirmer.