Theoretical Analysis of the Pressure Regions Where Adsorption Azeotropes Exist in Binary Gas Mixtures.

This work confirmed theoretically whether adsorption azeotropes can form in a binary gas mixture at a pressure P below the intersection pressure of the corresponding single-gas isotherms. The thermodynamically consistent dual-process Langmuir (DPL) model with equal component i saturation capacities q i, j s on site j and the general DPL model with nonequal q i, j s on site j were used for this purpose. Relationships derived from both DPL models, in terms of the single-gas isotherm DPL model parameters, were used to answer this question. When the P range where adsorption azeotropes always exist is infinite beyond the onset P of adsorption azeotropic formation, both DPL models and experimental data showed that it is possible to form adsorption azeotropes in the corresponding binary gas mixture at pressures not only above but even below the single-gas isotherm intersection P. When the P range where adsorption azeotropes always exist is finite beyond the onset P of adsorption azeotropic formation, only the general DPL model predicts the onset P of this finite P range can be below the intersection P of the corresponding single-gas isotherms. Without theoretical proof, the thermodynamically consistent DPL model seemingly restricts this P range to be equal to or greater than the intersection P of the corresponding single-gas isotherms. For a finite P region where adsorption azeotropes always exist in a binary gas mixture, the binary selectivity inverts when traversing from below the lower onset P to the higher cessation P. Both models also showed, counterintuitively, that perfect positive energetic site matching can result in the formation of adsorption azeotropes in binary gas mixtures, not just perfect negative energetic site matching. Overall, this work provides some confirmation that it is indeed possible to form adsorption azeotropes in a binary gas mixture at pressures below the intersection P of the corresponding single-gas isotherms based on two physically sound formulations of the DPL model.

Theoretical Analysis of the Necessary and Sufficient Conditions for the Formation of Adsorption Azeotropes in Binary Gas Mixtures.

This work theoretically assessed the necessary and sufficient conditions for the formation of an adsorption azeotrope in a binary gas mixture when this mixture exhibits either intersecting or nonintersecting single gas isotherms. The thermodynamically consistent dual process Langmuir (DPL) model with equal component i saturation capacities qi,js on site j and the general DPL model with nonequal qi,js on site j were used for this purpose. Analytical expressions derived for both DPL models, in terms of the single gas isotherm DPL model parameters, were used to find examples or to determine theoretically when an adsorption azeotrope forms in a binary gas mixture for both intersecting and nonintersecting single gas isotherms. For the general DPL model, it was determined that neither necessary nor sufficient conditions exist for the formation of an adsorption azeotrope in a binary gas mixture. This means that an adsorption azeotrope can form irrespective of whether the corresponding single gas isotherms intersect or not. For the thermodynamically consistent DPL model, it was determined that the intersection of the single gas isotherms is a sufficient condition for the formation of an adsorption azeotrope in a binary gas mixture, but it is not a necessary condition. This means that intersecting single gas isotherms guarantee the formation of an adsorption azeotrope in the corresponding binary gas mixture, while nonintersecting single gas isotherms can also result in the formation of an adsorption azeotrope in the corresponding binary gas mixture. Overall, this analysis provides a well-posed resolution to the question of necessity and sufficiency for the formation of adsorption azeotropes in binary gas mixtures and the intersection of their corresponding single gas isotherms based on two physically sound formulations of the very popular DPL model.

Limitations of Breakthrough Curve Analysis in Fixed-Bed Adsorption.

This work examined in detail the a priori prediction of the axial dispersion coefficient from available correlations versus obtaining both it and mass transfer information from experimental breakthrough data and the consequences that may arise when doing so based on using a 1-D axially dispersed plug flow model and its associated Danckwerts outlet boundary condition. These consequences mainly included determining the potential for erroneous extraction of the axial dispersion coefficient and/or the LDF mass transfer coefficient from experimental data, especially when nonplug flow conditions prevailed in the bed. Two adsorbent/adsorbate cases were considered, i.e., CO2 and H2O vapor in zeolite 5A, because they both experimentally exhibited significant nonplug flow behavior, and the H2O-zeolite 5A system exhibited unusual concentration front sharpening that destroyed the expected constant pattern behavior (CPB) when modeled with the 1-D axially dispersed plug flow model. Overall, this work showed that it was possible to extract accurate mass transfer and dispersion information from experimental breakthrough curves using a 1-D axial dispersed plug flow model when they were measured both inside and outside the bed. To ensure the extracted information was accurate, the inside the bed breakthrough curves and their derivatives from the model were plotted to confirm whether or not the adsorbate/adsorbent system was exhibiting CPB or any concentration front sharpening near the bed exit. Even when concentration front sharpening was occurring with the H2O-zeolite 5A system, it was still possible to use the experimental inside and outside the bed breakthrough curves to extract fundamental mass transfer and dispersion information from the 1-D axial dispersed plug flow model based on the systematic methodology developed in this work.

On the use of the dual process Langmuir model for predicting unary and binary isosteric heats of adsorption.

Analytic expressions for unary and binary isosteric heats of adsorption as a function of the adsorbed phase loading were derived from the dual process Langmuir (DPL) model using the Clausius-Clapeyron equation. Unary isosteric heats of adsorption predicted from these expressions for several adsorbate-adsorbent systems were compared to values in the literature predicted from the well-accepted graphical approach using Toth and unilan models (Adsorption Equilibrium Data Handbook; Prentice Hall: NJ, 1989). Predictions from the DPL model were also compared to rare experimental unary and binary isosteric heats of adsorption in the literature for another adsorbate-adsorbent system. In all cases, very good agreement was obtained, showing that the DPL model can be used in adsorption process modeling for accurately predicting not only ideal and nonideal mixed-gas adsorption equilibria (Langmuir 2011, 27, 4700), but also unary and even binary isosteric heats of adsorption.

On the use of the dual-process Langmuir model for correlating unary equilibria and predicting mixed-gas adsorption equilibria.

A new model has been developed for predicting mixed-gas adsorption equilibria from multicomponent gas mixtures based on the dual-process Langmuir (DPL) formulation. It predicts ideal, nonideal, and azeotropic adsorbed solution behavior from a knowledge of only single-component adsorption isotherms and the assertion that each binary pair in the gas mixture correlates in either a perfect positive (PP) or perfect negative (PN) fashion on each of the two Langmuir sites. The strictly PP and strictly PN formulations thus provide a simple means for determining distinct and absolute bounds of the behavior of each binary pair, and the PP or PN behavior can be confirmed by comparing predictions to binary experimental adsorption equilibria or from intuitive knowledge of binary pairwise adsorbate-adsorbent interactions. The extension to ternary and higher-order systems is straightforward on the basis of the pairwise additivity of the binary adsorbent-adsorbate interactions and two rules that logically restrict the combinations of PP and PN behaviors between binary pairs in a multicomponent system. Many ideal and nonideal binary systems and two ternary systems were tested against the DPL model. Each binary adsorbate-adsorbent pair exhibited either PP or PN behavior but nothing in between. This binary information was used successfully to predict ternary adsorption equilibria based on binary pairwise additivity. Overall, predictions from the DPL model were comparable to or significantly better than those from other models in the literature, revealing that its correlative and predictive powers are universally applicable. Because it is loading-explicit, simple to use, and also accurate, the DPL model may be one of the best equilibrium models to use in gas-phase adsorption process simulation.

Isolated swine heart ventricle perfusion model for implant assisted-magnetic drug targeting.

An isolated swine heart ventricle perfusion model was developed and used under physiologically relevant conditions to study implant assisted-magnetic drug targeting (IA-MDT). A stent coil was fabricated from a ferromagnetic SS 430 wire and used to capture 100-nm diameter magnetite particles that mimicked magnetic drug carrier particles (MDCPs). Four key cases were studied: (1) no stent and no magnet (control), (2) no magnet but with a stent, (3) no stent but with a magnet (traditional MDT), and (4) with a stent and a magnet (IA-MDT). When applied, the magnetic field was fixed at 0.125T. The performance of the system was based on the capture efficiency (CE) of the magnetite nanoparticles. The experiments done in the absence of the magnetic field showed minimal retention of any nanoparticles whether the stent was present or not. The experiments done in the presence of the magnetic field showed a statistically significant increase in the retention of the nanoparticles, with a marked difference between the traditional and IA-MDT cases. Compared to the control case, in one case there was nearly an 11-fold increase in CE for the IA-MDT case compared to only a threefold increase in CE for the traditional MDT case. This enhanced performance by the IA-MDT case was typical of all the experiments. Histology images of the cross-section of the coronary artery revealed that the nanoparticles were captured mainly in the vicinity of the stent. Overall, the IA-MDT results from this work with actual tissue were very encouraging and similar to those obtained from other non-tissue and theoretical studies; but, they did point to the need for further studies of IA-MDT.

A comprehensive in vitro investigation of a portable magnetic separator device for human blood detoxification.

A portable magnetic separator device is being developed for a proposed magnetically based detoxification system. In this paper, the performance of this device was evaluated via preliminary in vitro flow experiments using simple fluids and a separator unit consisting of one tube and two metal wires, each at the top and bottom of the tube. The effects of the following factors were observed: mean flow velocity U(o) (0.14-45 cm s(-1)), magnetic field strength micro(o)H(o) (0.125-0.50 T), wire size R(w) (0.125, 0.250 and 0.500 mm), wire length L(w) (2, 5 and 10 cm), wire materials (nickel, stainless steel 304 and 430) and tube size (outer radius R(o) = 0.30 mm and inner radius R(i) = 0.25 mm; R(o) = 0.50 mm and R(i) = 0.375 mm; and R(o) = 2.0 mm and R(i) = 1.0 mm). Our observations showed that the experimental results fit well with the corresponding theoretical results from the model we previously developed at a low flow velocity area (for example, U(o) < or = 20 cm s(-1)), strong external magnetic field (for example, > or = 0.30 T) and long wire length (for example, L(w) = 10 cm). The experimental results also showed that more than 90% capture efficiency is indeed achievable under moderate systemic and operational conditions. Pressure drop measurements revealed that the device could work well under human physiological and clinical conditions, and sphere buildup would not have any considerable effect on the pressure drop of the device. The breakthrough experiments demonstrated that a lower flow rate V, higher applied magnetic field micro(o)H(o) and diluted sphere suspension, i.e. lower C(o), would delay the breakthrough. All the results indicate the promise of this portable magnetic separator device to efficiently in vivo sequestrate nano-/micro-spheres from blood flow in the future magnetically based detoxification system.

Kinetic behavior of Ti-doped NaAlH4 when cocatalyzed with carbon nanostructures.

The effects of SWNTs, MWNTs, AC, C(60), and G when used as a cocatalyst with Ti on the dehydrogenation and hydrogenation kinetics of NaAlH(4) were investigated for the first time in the important temperature range of 90 to 250 degrees C. All five carbons exhibited significant, sustaining, and synergistic cocatalytic effects on the dehydrogenation and hydrogenation kinetics of Ti-doped NaAlH(4) that persisted through charge and discharge cycling. SWNTs were the best cocatalyst, G was the worst cocatalyst, and all five carbons were inactive as a catalyst unless Ti was present. The carbon most likely was imparting an electronic contribution through the interaction of its facile pi-electrons with Ti through a hydrogen spillover mechanism, which explained why one carbon was better than another one in terms of optimal aromatic character, out-of-plane exposure of pi-electrons, and interaction of pi-bonds with neighboring sheets.

Physiochemical pathway for cyclic dehydrogenation and rehydrogenation of LiAlH4.

A five-step physiochemical pathway for the cyclic dehydrogenation and rehydrogenation of LiAlH4 from Li3AlH6, LiH, and Al was developed. The LiAlH4 produced by this physiochemical route exhibited excellent dehydrogenation kinetics in the 80-100 degrees C range, providing about 4 wt % hydrogen. The decomposed LiAlH4 was also fully rehydrogenated through the physiochemical pathway using tetrahydrofuran (THF). The enthalpy change associated with the formation of a LiAlH4.4THF adduct in THF played the essential role in fostering this rehydrogenation from the Li3AlH6, LiH, and Al dehydrogenation products. The kinetics of rehydrogenation was also significantly improved by adding Ti as a catalyst and by mechanochemical treatment, with the decomposition products readily converting into LiAlH4 at ambient temperature and pressures of 4.5-97.5 bar.