Gas Separation & Purification最新文献

This paper presents several reduced models for the intraparticle diffusion/convection mechanisms in spherical homoporous particles under isothermal conditions. These semi-empirical linear driving force approximations were developed for the cyclic steady state that is attained when a cyclic concentration and/or total pressure perturbation is imposed at the particle surface. The models are applicable to linear systems or those obeying bicomponent Langmuir isotherms. The linear driving force approximations have two or three parameters for each component and are presented for the following cases: Knudsen diffusion, molecular diffusion, molecular plus Knudsen diffusion in series, viscous flow, and also for both diffusion mechanisms in series, in parallel with the convective flow, as in the dusty gas model. Comparison between the results obtained with these approximations and with the complete models shows that on average the relative quadratic error is ≈10% for non-smooth and 5% for smoother perturbations.

A 60%-40% and a 92%-8% methane nitrogen mixture were separated in a two-bed pressure swing adsorption (PSA) unit using a carbon molecular sieve (CMS) adsorbent. The CMS adsorbent used in this separation is a dual resistance type comprising a barrier resistance on the crystal surface and diffusion resistance in the crystal interior. This dual resistance is modelled using the linear driving force (LDF) PSA model. The parameter Ω of the LDF model, which relates the theoretical and experimental results, was found to be significantly different from that for conventional diffusion-controlled processes. This difference is attributed to the presence of a barrier resistance in the CMS.

A recently published model for the prediction of flow distributions in annular packed bed systems has been extended to provide pressure maps throughout the system and overall pressure drop. For a particular fixed filter design, the analysis shows that the best flow distribution (0.58% average deviation from uniform flow) and least overall pressure drop (7% less than the maximum) is obtained from a U-type flow arrangement with inward radial flow across the annular bed. A sensitivity analysis has revealed that the lowest overall pressure drop is obtained when flow maldistribution is minimal.

A study was made on the permeation rate of pure gases including helium, hydrogen, oxygen, nitrogen, carbon dioxide and methane, and the separation of CO₂/CH₄ gas mixtures by a silicone-coated aromatic polyamide membrane. The ideal and actual separation factors of the polymer towards these gases were calculated. The membranes were prepared by the phase-inversion technique and coated with silicone rubber after drying. While the permeation rates of He, H₂ and CO₂ were not affected significantly by coating, the permeation rates of N₂ and CH₄ gases decreased to below the detectable limit of the flow meter. The permeation rate of O₂ decreased after coating. The actual separation factor for mixtures of CO₂/CH₄ varied from 64 to 166, one of the highest separation factors reported in the literature.

Adsorption isotherms for the pure gases methane and carbon dioxide and their binary mixtures at constant gas composition have been determined in a high pressure microbalance at 20, 40 and 60°C and equilibrium pressures up to 1.7 MPa on activated carbon. The gravimetric method proposed by Van Ness has been used to obtain the composition of the adsorbed phase for the binary mixtures at 100 and 530 kPa. The ideal adsorbed solution (IAS) theory has been applied to predict the binary adsorption equilibria from the single-component isotherms. The predictions of the IAS theory are only in moderate agreement with experimental data. The activity coefficients calculated for both components in the adsorbed phase are analogous to the respective vapour-liquid equilibria positive deviations from Raoult's law. Finally, the real adsorbed solution (RAS) theory has been used to fit the experimental data.

The intrinsic behaviour of several single-stage and multi-stage permeator systems has been studied using a recently developed algebraic design model. Upper and lower bounds with respect to product purity and recovery in single-stage systems are presented. It is shown that single-stage permeators without recycle can exhibit maxima in permeate purity as a function of the pressure ratio across the membrane. It is illustrated why bypass configurations may be economically profitable in single-stage systems. The effect of product recycle in single-stage systems has been studied. It is shown that permeate recycle can reduce the compressor load in single-stage permeator systems. Similar benefits of retentate recycle have not been identified. The characteristic behaviour of multi-stage systems has been studied. Based on general design criteria, various module configurations have been classified as suitable for recovery of either the slowest or the fastest permeating component. The effect of using different membrane materials at each stage of a multi-stage permeator cascade has been studied. It is shown that improvements with respect to product recovery can be achieved.

Henry's law constants and intracrystalline diffusivities of oxygen and nitrogen in molecular sieve RS-10 have been measured by the pulse chromatographic method. Both moment analysis and time domain fitting of the experimental response curves give consistent results. Heats of adsorption and activation energies have been determined from the equilibrium and kinetic measurements carried out at different temperatures.

The adsorptive and kinetic behaviour patterns of SO₂ and water vapour on mordenites and pentasil zeolites were investigated using the breakthrough curve method. For all the zeolites studied, the breakthrough experimental data show a decrease in the equilibrium capacities for both SO₂ and H₂O with increasing SiO₂/Al₂O₃ ratio. At the lower ratios SO₂ adsorption is believed to be influenced by the basicity of the zeolite. The presence of water in the gas reduces its SO₂ capacity to varying degrees, depending on the SiO₂/Al₂O₃ ratio. In contrast, the presence of CO₂ (in the wet SO₂-containing gas) has very little effect. The hydrophobic indices, which were used to interpret the selectivity of SO₂ adsorption, showed different trends with SiO₂/Al₂O₃ ratios. The Langmuir-Freundlich and extended Langmuir-Freundlich equilibrium models were used to predict equilibrium properties for the single-component and binary systems, respectively. The linear driving force-based non-isothermal model was used to fit experimental breakthrough curves for the single-component systems. Overall mass transfer resistances derived from the model were compared with the values obtained for SO₂ and water vapour adsorption in pelleted samples using a simplified biporous adsorbent model. Breakthrough curves for the binary systems were calculated using kinetic and equilibrium data of the single-component systems.

A hollow-fibre permeator containing silicone rubber membranes has been used for the removal of CO₂ from a breathing gas mixture. The effect of permeate gas absorption with water in the shell side of the permeator has been investigated and compared with conventional systems with and without permeate gas purge. Experimental results compare favourably with theoretical predictions. It has been shown that the use of an aqueous solution as an absorbing medium in the permeate side of the permeator can greatly improve CO₂ separation from the gas mixture and reduce the need to maintain a high pressure ratio across the membrane. The improved CO₂ selectivity is found to be caused mainly by the liquid film which, however, creates film resistance, resulting in much reduced CO₂ permeation flux. The CO₂ permeation flux can be improved using an alkaline solution instead of neutral water.

The US Department of Energy's Morgantown Energy Technology Center (METC) sponsors the research and development of two technologies involving electrochemical membranes: fuel cells and separation devices. METC also works towards developing electric power generation systems which utilize both of these technologies. This paper discusses the general theory and operation of electrochemical membrane separation devices, the advantages of networking them, and some potential applications.