Adsorption

The principles of simulated moving bed (SMB) and thermal swing adsorption (TSA) are combined to develop a traveling wave mode, thermally assisted SMB. The four-zone, thermal SMB and the corresponding one-column, thermal Analog to the SMB are studied for separating binary systems with linear isotherms. Design parameters and operating conditions are determined using the local equilibrium theory and detailed simulations are done with the commercially available chromatography/SMB software package Aspen Chromatography v12.1. Simulations were performed for the separation of toluene/xylene with silica gel as the adsorbent and n-heptane as the desorbent. The SMB and Analog are operated with a heat exchanger heating or cooling the fluid before it enters each adiabatic column. The advantage of the traveling wave mode compared to the direct mode of heat transfer is since heat transfer rates are not limiting, the SMB and Analog systems can be scaled up easily.

The adsorption of naphthalene and pyrene on two different types of commercial activated carbons was studied by batch and column experiments. Adsorption equilibrium was measured at three different temperatures. Heats of adsorption were estimated from the equilibrium results and compared to other previous reports. From the column experiments, using parameters obtained from the batch experiments and literature correlations, effective surface diffusivities were estimated for naphthalene and pyrene on both adsorbents in different feed concentrations. The corrected diffusivities, using Darken equation, appear to be almost constant for naphthalene (ca. 1.3⋅10−8 cm2/min), and for pyrene (ca. 2.3⋅10−10 cm2/min), in both activated carbons.

The adsorption of SO2 from pseudo binary mixtures with water and CO2 on hydrophobic zeolites (MFI and MOR type) was investigated using the breakthrough curve method. The SO2 and water breakthrough curves were compared with theoretical ones based on an axially dispersed plug flow through the column and the linear driving force rate equation. In addition, different semi-predictive multi-component equilibrium equations were used for the breakthrough modeling: Langmuir 1, Langmuir 2 and Langmuir-Freundlich extended models. The overall mass transfer coefficients were derived by matching theoretical with experimental breakthrough curves for single component systems, i.e., water vapor or SO2 in a carrier gas. They were also predicted from a simplified bi-porous adsorbent model and compared with experimentally derived values. The presence of CO2 species in ternary mixtures with water vapor and SO2, even at relatively high concentrations of 9 vol%, had no significant effect on the breakthrough behavior of the other two species. For that reason the CO2 species was ignored in the analysis of the resulting pseudo binary mixtures. The breakthrough model was solved by finite element orthogonal collocation method using the commercial software gPROMS. Both extended Langmuir 1 and Langmuir 2 based models gave reasonable predictions of the water and SO2 breakthrough curves for pseudo binary mixtures involving a mordenite sample for all water concentration levels used in this study (up to 3.5 vol%). However, the same models were successfully used to predict SO2 breakthrough curves for a MFI sample only at low water concentrations, i.e., 1.5 vol%. At the higher water levels both models failed to describe equilibrium behavior in the MFI sample due to the introduction of multi-layer adsorption in the interstices between small MFI-26 crystals.

Tin and titanium ferrocyanides were studied as adsorbents for alkali metal ions, viz., 134Cs and 22Na, which represent radioactive wastes. The ferrocyanides were prepared in granular form. The tin version contained 11.2% water, while the titanium version contained 17.7% water. The exchange capacities for Cs+ and Na+ in the hydrated tin version were about 1.5 and 0.7 meq/g, respectively, while those in the titanium version were 2.2 and 1.2 meq/g, respectively. Drying at 250°C decimated those capacities. The diffusional time constant of Cs+ at 25°C, determined via Fick's second law, was of order of magnitude 1 × 10−3 s−1, though there were minor differences due to particle size and the form of ferrocyanide. Similarly, the effective diffusivity was of order of magnitude 1 × 10−8 cm2/s. The titanium version responded slightly faster than the tin version. Likewise, equilibrium measurements in mixtures with sodium nitrate, potassium nitrate, or uranium oxide, showed that the titanium version exhibited significantly greater selectivity for Cs+ than did the tin version. Unfortunately, tests of complete elution of the Cs+ from the ferrocyanides were mostly disappointing. Work continues on that subject.

In the developing countries where the cost is often a decisive factor, extensive studies were undertaken to test the most effective factors on the preparation, optimization and validation of the magnetic particles (or, more accurately, magnetizable particles) for removal of heavy metals from wastewaters. The objective of the proposed work was focused to provide promising solid-phase materials, which, are relatively in expensive and combine high surface capacity with fast efficient treatment. Four various metal oxides including hydrous ferric oxide (HFO), hydrous stannic oxide (HSO) and mixed ferric/stannic oxide (HMO), were prepared by precipitation with ammonia from metal chloride solutions. Two mixed oxides were prepared with different Sn/Fe ratios of 50% and 20%. Optimal conditions for the activation of these particles and the subsequent mixing of various metals oxides are tested together with the utility of the method to get a new composite material with developed chemical characteristics over their individual metal oxides. Factors affecting the sorption behavior of the prepared samples in basic and acid media were elucidated. The magnetic treatment procedure using the mixed oxide (50%) enables the equilibration step to be carried out rapidly mainly due to ferric oxide during the magnetization process and efficiently due to high capacity of the stannic oxide. A key factor in achieving very high uptake percentage is the reduction of non-specific binding of various heavy metals to the solid phase support. This is usually achieved by increasing the ion exchange capability, in addition to their adsorption process.

A novel nanoporous C/SiO2 composite was synthesized using graphite precursor by a soft chemical method and its adsorption properties were characterized by nonane, water, and nitrogen adsorption. It was found that only one part of micropores can strongly confine nonane molecules, indicating a wide micropore size distribution. Water adsorption leads to disappearance of one part of micropores, possibly due to closing of pores by dissociative adsorption of water on defective sites. Fractal analysis of the original and the differential nitrogen adsorption isotherms indicate that micropores have a rougher surface and mesopore a flatter surface, suggesting that micropores are surrounded by silica particles and mesopores involve the flat carbon layers.

ZSM-5 zeolites with SiO2/Al2O3 molar ratio of 24 were treated in 0.05 M aqueous sodium hydroxide solution at 325 K in different periods. The samples were characterized by means of nitrogen adsorption at 77 K, field emission scanning electron microscopy, X-ray diffractometry, and Fourier transform infrared spectroscopy. Analysis of the experimental results showed that the alkaline treatment periods have influence on the developments and structures of mesopores in the alkaline-treated ZSM-5 zeolites. Alkaline treatment initially develops mesopores mainly from the boundary portion of MFI zeolites to the bulk, while prolonged treatment destroys the mesopores, and an optimum mesoporosity is obtained by the treatment for 1.5 h. On the other hand, crystallinities and short-range order in alkaline treated zeolites have remained virtually unchanged according to the examination from X-ray diffractometry and Fourier transform infrared spectroscopy.

Four nanoporous carbons obtained from different polymers: polypyrrole, polyvinylidene fluoride, sulfonated styrene–divinylbenzene resin, and phenol–formaldehyde resin, were investigated as potential adsorbents for carbon dioxide. CO2 adsorption isotherms measured at eight temperatures between 0 and 60 °C were used to study adsorption properties of these polymer-derived carbons, especially CO2 uptakes at ambient pressure and different temperatures, working capacity, and isosteric heat of adsorption. The specific surface areas and the volumes of micropores and ultramicropores estimated for these materials by using the density functional theory-based software for pore size analysis ranged from 840 to 1990 m2 g−1, from 0.22 to 1.47 cm3 g−1, and from 0.18 to 0.64 cm3 g−1, respectively. The observed differences in the nanoporosity of these carbons had a pronounced effect on the CO2 adsorption properties. The highest CO2 uptakes, 6.92 mmol g−1 (0 °C, 1 atm) and 1.89 mmol g−1 (60 °C, 1 atm), were obtained for the polypyrrole-derived activated carbon prepared through a single carbonization-KOH activation step. The working capacity for this adsorbent was estimated to be 3.70 mmol g−1. Depending on the adsorbent, the CO2 isosteric heats of adsorption varied from 32.9 to 16.3 kJ mol−1 in 0–2.5 mmol g−1 range. Overall, the carbons studied showed well-developed microporosity and exceptional CO2 adsorption, which make them viable candidates for CO2 capture, and for other adsorption and environmental-related applications.