José A. Delgado

Adsorption of lindane from water onto GAC: effect of carbon loading on kinetic behavior

Abstract Batch kinetic experiments of adsorption of lindane in dilute aqueous solution onto F-400 granular activated carbon (GAC) with different carbon weight and initial concentration were analyzed. Concentration decay of lindane was measured by means of SPME (solid phase micro extraction). A high level of energetic and structural heterogeneity of the F-400 activated carbon was observed by means of TG analysis. An adsorption model including film-, macropore-, and micropore-diffusion based on the branched pore model was fitted to experimental data. The effect of carbon loading on model parameters can be summarized as follows: (i) the effective macropore diffusivity, including pore and surface diffusivity, and the micropore rate coefficient have little dependence on carbon loading, and (ii) the fraction of adsorptive capacity in macropores depends strongly on carbon loading, which is attributed to the difference in adsorptive affinity in macro- and micropores.

Carbon dioxide/methane separation by adsorption on sepiolite

In this work, the use of sepiolite for the removal of carbon dioxide from a carbon dioxide/methane mixture by a pressure swing adsorption (PSA) process has been researched. Adsorption equilibrium and kinetics have been measured in a fixed-bed, and the adsorption equilibrium parameters of carbon dioxide and methane on sepiolite have been obtained. A model based on the LDF approximation has been employed to simulate the fixed-bed kinetics, using the Langmuir equation to describe the adsorption equilibrium isotherm. The functioning of a PSA cycle for separating carbon dioxide/methane mixtures using sepiolite as adsorbent has also been studied. The experimental results were compared with the ones predicted by the model adapted to a PSA system. Methane with purity higher than 97% can be obtained from feeds containing carbon dioxide with concentrations ranging from 34% to 56% with the proposed PSA cycle. These results suggest that sepiolite is an adsorbent with good properties for its employment in a PSA cycle for carbon dioxide removal from landfill gases.

Adsorption and Diffusion Parameters of Methane and Nitrogen on Microwave-Synthesized ETS-4

ETS-4 is a titanium silicate developed by Engelhard Corporation, which possesses a small pore network, the size of which can be reduced by heat treatment to improve its kinetic selectivity in nitrogen/methane separation. Most of the reported studies about ETS-4 employ crystals synthesized with conventional heating. Furthermore, information available on the adsorption properties of ETS-4, especially the diffusion properties, is scarce. In this work, Na-ETS-4 crystals have been synthesized by microwave heating and have been exchanged with strontium to obtain Sr-ETS-4 using also microwave heating. This method for obtaining the strontium form of ETS-4 has not been reported before. Both materials have been dehydrated to reduce their pore size. The adsorption and diffusion parameters of nitrogen and methane on these materials, which have not been measured for microwave-synthesized ETS-4 up to the present date, have been estimated by modeling the desorption breakthrough curves of both gases using a fixed bed of ETS-4 crystals. The kinetic selectivity of nitrogen over methane at 298 K of microwave-synthesized Sr-ETS-4 is 26. This value is higher than the maxima reported in the literature for this material.

Microwave synthesis of Sr-ETS-4 and evaluation of its molecular sieving properties in the nitrogen/methane separation

This work addresses the synthesis of the Na-ETS-4 titanosilicate, and the subsequent transformation into Sr-ETS-4 by ion exchange, using microwave radiation as the heating source for both processes, instead of the conventional method. The crystallization kinetics and the ion exchange degree obtained with microwave and conventional synthesis have been compared. The molecular sieving properties of the microwave synthesized Sr-ETS-4 in the nitrogen/methane separation have been evaluated by studying the adsorption and desorption of nitrogen and methane in a fixed bed of Sr-ETS-4 crystals. The nitrogen/methane equilibrium selectivity was 6 approximately, confirming the molecular sieving effect of the studied material.

Modeling the Regeneration of a Polymeric Resin Column Saturated with Ethanol by Air Purge and External Heating

In this work, the regeneration of a polymeric resin column saturated with ethanol with air purge and external heating has been studied. A theoretical model is proposed for describing the mass and heat transfers in the system, and it has been validated with experimental data. This model can be used for simulating the regeneration step in a concentration-thermal swing adsorption process for fuel-grade ethanol production. The model has been used to analyze the process dynamic behavior, and to compare the performance of the regeneration step using external heating and hot air purge.

A Stefan-Maxwell Model of Single Pore Pressurization for Langmuir Adsorption of Gas Mixtures

A model based on the application of the Maxwell-Stefan approach has been used to describe the dynamics of intraparticle transport (pore diffusion, surface diffusion and convection) in a single pore during and after a pressurization process. The model was first compared with the model proposed by Taqvi and Levan (Adsorption, 2, 299–309 (1996)) for a linear adsorption isotherm. The effect of several parameters (pressurization rate, adsorption capacity, bulk gas-phase mole fraction, adsorption affinity and pore radius) was studied, evaluating the relative importance of each mass-transport mechanism in different conditions. A binary mixture of an inert and an adsorbable component was considered first, extending the analysis of the pore radius effect to a ternary mixture. In general, surface diffusion is dominant with very low pore radius, whereas gas-phase fluxes dominate in a large pore. However, depending on the value of the bulk gas-phase mole fraction (which is related to the surface coverage level through the adsorption equilibrium isotherm), the equilibrium and rate parameters, and the surface to volume ratio, surface diffusion cannot be always neglected for large pores. More generally, system non-linearity can switch the dominant mechanism and create fronts.

Air Regeneration of a Silicalite Column Loaded with Ethanol: Modelling and Energy Estimation

Concentration-thermal swing adsorption (CTSA) using an ethanol-selective adsorbent can be an attractive option to reduce the energy costs associated with the separation of ethanol from water. In this process, ethanol regeneration is the only step with significant energy consumption, in which the adsorbent column saturated with liquid ethanol is purged with a hot inert gas to recover ethanol by condensation. Despite many showing interest in applying CTSA, the method has received little attention in the open literature, and consequently little information is available about its energy requirement. In this work, we experimentally measured the regeneration dynamics of a heated silicalite column saturated with liquid ethanol by air purge using different column wall temperatures and purge gas flow rates. Using a theoretical model based on conservation equations, the mass and heat transfer kinetics in this process are adequately reproduced. The proposed model has been used to estimate the overall energy requirement of the ethanol vapourization/desorption and the subsequent condensation processes.

Modelling the Desorption of Ethanol from an Externally Heated Saturated Activated Carbon Column by Purging with Air

The desorption of ethanol from an externally heated column loaded with activated carbon and purged with air has been studied. The column was initially saturated with liquid ethanol. This process simulates the regeneration step in a concentration-swing/thermal-swing adsorption process for the production of fuel-grade ethanol. A theoretical model was developed for describing the mass- and heat-transfer kinetics of the process. Validation of the model was achieved by comparing its predictions with experimental data, employing initial column temperatures between 298 K and 395 K and air flow rates between 1.6 × 10−6 m3/s and 11.3 × 10−6 m3/s. Experimentally measured adsorption isotherms of ethanol from ethanol/air gas mixtures onto activated carbon, as well as the regeneration column dynamics, are reported.

Estimation of Adsorption Parameters from Temperature-programmed Desorption Thermograms: Application to the Adsorption of Carbon Dioxide onto Alumina

In this work, a model is proposed for estimating the desorption activation energy from a TPD thermogram. The model considers the effects of diffusional resistance, the reactor cell volume, the sample weight and the carrier gas flow rate. The adsorption and desorption rate constants are estimated using theoretical expressions deduced from statistical thermodynamics, assuming first-order desorption kinetics. This model has been applied to the CO2/activated alumina system. The TPD thermogram has been obtained with a commercial apparatus, and the distribution of the desorption activation energy has been analyzed. A sensitivity analysis has been performed to determine the importance of different phenomena which can alter the position of the TPD peaks (re-adsorption, diffusional resistance, volume of sample cell). An experimental study of the effect of diffusional resistance has also been performed, comparing the TPD thermograms obtained with powdered and pelletized alumina. The experimental results have been compared with the model predictions.

Modelling of the separation of normal paraffins from kerosene fractions by a simulated moving bed process

Linear paraffins are widely used in the manufacturing of industrial and domestic detergents. Some adsorbents selectively separate these linear hydrocarbons by adsorption from petroleum feedstocks. LTA molecular sieves (5A zeolite) adsorb linear paraffins while excluding the rest of the components of kerosene (branched hydrocarbons and aromatics). Equilibrium and kinetic parameters are available in the literature for light paraffins in the vapour phase, however, there is scarce information concerning high molecular weight paraffins in liquid phase, especially at the operating conditions of commercial processes. In a previous work, the equilibrium and kinetics of high molecular weight n-paraffins C5, C10, C14 and C18 were studied for the adsorption in liquid phase on 5A zeolite. The aim of this work is to study the equilibrium and kinetics of n-paraffins C12 and C16, as well as mixtures of n-paraffins C10, C12 and C14 in the same conditions. n-pentane has been included in the study as it is mainly used as desorbent in the cyclic simulated moving bed (SMB) commercial process. Pure component isotherms were obtained, as well as a multicomponent isotherm. By comparing them, it was observed that selectivities are significantly lower in mixtures (for example, selectivity towards C14 with respect to C12 is lowered from 2.84 for pure paraffins to 1.05 for mixtures). A theoretical model has been developed to describe the column adsorption dynamics of the studied systems. The model has been included in an SMB simulation program (SMBSIM), and the model prediction has been validated by comparison with the separation performance data reported for a commercial SMB unit that separates normal paraffins from a hydrotreated kerosene fraction The model predicts the separation of linear paraffins with 99.2% purity and 96.3% recovery (5% error obtained for n-paraffin concentration in the extract and non-adsorptives in the raffinate).