Separation of a gas mixture through a polymer membrane metallized with palladium

Investigations were carried out on the permeation of He, H₂, Ar, O₂ and N₂, in the range 26–60°C, and the separation at 20°C of a H₂ + N₂ gas mixture through a polytetrafluoroethylene (PTFE) membrane metallized in vacuum with a 1000 Å thick layer of palladium. In both experiments it was found that the thin layer of Pd acts as a gas-selective barrier which determines an increase of the H₂ permeation selectivity, respectively H₂-N₂ separation factor with up to 360%. The causes which determine the small decrease, up to 16%, of the H₂ flux which permeates through the metallized membrane are also discussed.     

Calculation of the degree of separation for a membrane module at the low permeability of the purified gas through the membrane

The calculation of the degree of separation for a low-permeable impurity in a membrane module at a low permeability of the purified gas through the membrane was carried out. It was shown that the dependence of the degree of separation on the permeability of the purified gas at its constant sampling rate is extreme. Such a character of the dependence in the membrane module will also influence the value of the degree of separation of hybrid processes, for example, absorbing pervaporation.     

Molecular dynamics simulation of small gas molecule permeation through CAU-1 membrane

CAU-1 is one of aluminum-based amine-functionalized Metal–Organic Frameworks(MOFs). Gas permeation and separation behaviors through CAU-1 membrane were simulated by the dual-control plane nonequilibrium molecular dynamics(DCP-NEMD) method. The thickness of membrane was 3.55 nm.Gases CO_2, N_2, CH_4, H_2, He, Kr and Xe were chosen for the calculation in both single component and binary mixtures. The permeation process was calculated in grand canonical(l VT) ensemble with periodic boundary conditions(PBC) in x-and y-directions at different temperatures. The calculated permeance of H_2, CH_4, N_2, CO_2 and Kr decreased with increasing temperature in both single and binary system, while that of Xe with kinetic molecule of 0.41 nm increased with increasing temperature. It shows Xe permeation is governed by activated diffusion. The simulated separation factors of CO_2/N_2 and CO_2/CH_4 of 4.2 and 1.3 respectively were lower than the experimental ones when only considering van der Waals interaction. Further consideration of electrostatic potential leads to improved calculation CO_2/N_2 and CO_2/CH_4 separation factor of 23.0 and 12.9 respectively that were consistent with the experimental ones of 26.2 and 14.8. It suggests the necessity of considering the Coulomb interactions between CO_2 and NH_2-on the pore wall of CAU-1 for permeation of CO_2. For H_2/N_2 and H_2/CH_4 the ideal selectivities also keep consistent with our experimental results. Interestingly, the simulated separation factor for noble Kr/Xe reaches infinite, predicting that CAU-1 membrane possesses potential separation properties for radioactive Kr/Xe.     

Diffusion of methane through coal

Both methane and helium flow through solid coal by Knudsen diffusion, even at relatively high pressures. Knudsen permeabilities were determined for samples of Pittsburgh, Pocahontas No.3, and Oklahoma Hartshorne coals. The average permeability for dry methane was 1.3 × 10⁻⁶ cm²/s for Pittsburgh coal, 20 × 10⁻⁶ for Pocahontas No.3 coal, and 13.8 × 10⁻⁶ for Oklahoma Hartshorne coal. A molecular-sieve effect exists for methane in all three coals examined and it is very strong in Pittsburgh coal. If the methane is saturated with water vapour the permeability decreases by a factor of 3 to 25, depending on the coal. The Knudsen permeability of solid coal discs seems to bear no relation to coalbed permeability. Lag-time measurements indicate systems of both larger and finer pores and the presence of blind pores. This effect is again more pronounced in the Pittsburgh coal.     

Diffusion from a viscous stream flowing through a rectangular channel

The problem of the diffusional deposition from laminar strems flowing through rectangular channels with various aspect ratios has been studied. Axial diffusion has been neglected and an analytical solution which is valid near the entrance of the channel obtained. The effect of the aspect ratio on the deposition is given and the results compared with the existing numerical solutions.     

Stability enhancement of anaerobic digestion through membrane gas extraction under organic shock loads

A novel submerged silicone membrane extraction technique for removing the dissolved gases, CO₂ and H₂, was explored as a method for enhancing the stability of anaerobic digesters under shock loads. The effectiveness of this strategy was demonstrated during a five-fold step increase in the feed concentration from 4 kg m⁻³ to 20 kg m⁻³ Chemical Oxygen Demand (COD) to two anaerobic chemostats, a control and a membrane reactor, operating under steady state. Data revealed that membrane extraction of CO₂ enabled neutral pH values to be maintained during the shock, in contrast to the control; and this appeared to enhance acetate degradation. In addition, while liquid phase hydrogen removal was only partially successful due to a biofilm building up on the membrane, it also appeared to enhance the rate of volatile fatty acid degradation, and hence improved reactor stability. Floc sizes were monitored during the course of the shock load experiment and found to decrease from 46 μm to 30 μm over 35 days in the control, but from 37 μm to <5 μm in the membrane reactor. This decrease in floc size was postulated to be due to the growth of a relatively pure methanogenic biofilm on the membrane. © 1998 Society of Chemical Industry     

Diffusion through swelling membranes with a Robin boundary condition

A Robin boundary condition at the downstream surface of stretched membrane is used to study diffusion through swollen polymer membranes. Four dimensionless parameters are used to describe the diffusion process. These parameters are associated with the effect of swelling, relaxation, and retardation times of the polymer matrix as well as the normal derivative condition at the downstream side of the membrane. The effect of these four parameters is discussed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Investigation of Diffusion through Packed Beds by a Monte Carlo Method

Abstract

A Monte Carlo method is used to study diffusion through packed beds. It is found that the distribution remains approximately Gaussian and can be well represented by a multiplicative factor γ on the free-space diffusion coefficient. For cubically packed beds, γ is given to good approximation by the Giddings theory.     

The permeation of hydrocarbons through a polyethylene membrane

Permeation data of a number of linear, branched and cyclic hydrocarbons have been determined up to a pressure of 70% of the saturation vapour pressure and a temperature range of 30–50°C. The concentration of solvent in the membrane at the liquid site has been determined by desorption. An average diffusion coefficient has been computed assuming the liquid concentration in the membrane at the permeate side to be zero. The activation energy of the permeation has been correlated with molecular structure.     

Simulation of Solute Diffusion through Porous Media

Abstract

Simulation of solute diffusion through porous media (membrane) was carried out by a random walk procedure. The porous media used were three kinds of two-dimensional square networks of channels with almost the same average pore diameters and porosities, but with different pore size distributions. In the simulation the partition equilibrium of solute between the bulk feed phase and the membrane phase was established, and the apparent permeability in the steady state was evaluated. First, only the steric interaction between the solute and the pore wall was considered. In spite of the similar average pore diameters and porosities of the networks, the apparent permeability and selectivity were dependent on the kinds of networks. The network with more small pores showed the smaller permeability and the higher selectivity. When the diameter of solute which can be actually transported is fairly smaller than the average pore diameter, the network with broad pore distribution and with more small pores is found to be useful for obtaining higher selectivity. Next, the electrostatic and dispersion (van der Waals) interactions between the solute and the pore wall were introduced in this simulation. By such long-range interactions the selectivity increased while the apparent permeability decreased. This result showed that the introduction of the repulsive electrostatic interaction between membrane and solute is one useful method for enhancing the selectivity. The effects of the kinds of networks on transport properties in the presence of long-range interactions were similar to those in the absence of such interactions.     

Diffusion of Amines through Polysulfonated Ion-Exchange Membranes

Abstract

The permeabilities and diffusion behaviors of aniline, p-chloroaniline, and p-nitroaniline were investigated in polysulfonated ICE-450 ion-exchange membranes as a function of pH at 25°C. The permeabilities for these compounds increased with decreasing of receiver side pH. The modes of membrane/solution distribution and diffusion of amines, as a controlling factor to the membrane permselectivity in terms of basicity value, molecular size, and electrophilic groups of amines, were correlated consistently with the flux data as well as with the estimated membrane permselectivity parameters. The transport system depends on the basicity values of amines, and the pH gradients were the main driving force. It is shown that the diffusion parameters measured on single component experiments provide a good prediction of behavior.     

Diffusion of gases through polyurethane block polymers

The diffusivities of simple gases through a series of polyurethane block copolymers of differing aromatic urethane content and type of soft segment were measured using a quadrapole mass spectrometer as a detecting device. Although an Arrhenius expression generally described the temperature dependence of diffusion in this system, a discontinuity was observed in the Arrhenius plots for some materials, and the discontinuity was found to be related to the onset of the glass transition in the hard domains. Increasing the hard segment content of the materials decreased the diffusivity due to the increase in the activation energy of diffusion. Increasing the soft segment length brought about a decrease in the activation energy with an increase in diffusivity. Polyster urethanes had lower activation energies for diffusion than polyether urethanes of similar hard segment composition. Finally, as the penetrant diameter was increased, a decrease in the diffusivity and an increase in the activation energy was noted.     

Diffusion of ions through hardened cement pastes

Diffusion of Na⁺ and Cl⁻ ions through thin hardened paste section was determined as a function of water/cement ratio, curing time, curing temperature, and diffusion temperature. Diffusivities of Na⁺ ions were smaller than for Cl⁻ ions in the temperature range measured up to 60°C. Activation energies for diffusion were calculated, and contrasts were made with diffusion through a low porosity quartzite rock having minimal surface interaction with the diffusing species.     

Diffusion effects in catalytic gas oil cracking

Wojciechowski's four parameter model of catalyst decay is applied to conversion data for gas oil cracking over a diffusion limited catalyst. The parameters thus obtained are compared with those obtained previously for a diffusion free form of the same catalyst, cracking the same feed stock under identical experimental conditions. The comparison shows that the presence of diffusion affects not only the rate of catalytic cracking but also the rate of aging. It is also shown that the introduction of diffusion phenomena has no effect on the mechanism of aging. The presence of diffusion limitations however has the effect of making the feed stock appear more homogeneous in reactivity     

含硅水凝胶膜中Na+的渗透性

采用腔膜法测定Na 在γ-(甲基丙烯酰氧)丙基三甲氧基硅烷(KH570)与甲基丙烯酸羟乙酯(HEMA)及N-乙烯基吡咯烷酮(NVP)共聚物水凝胶角膜接触镜材料中的渗透系数,研究了Na 渗透系数的影响因素。结果表明,Na 在水凝胶膜中的扩散过程为稳态扩散,其渗透系数随温度升高而显著增大,但温度超过60℃,Na 渗透系数的增大幅度显著下降;水凝胶膜中Na 的渗透系数随NVP及KH570质量分数增多而增大,但KH570的质量分数超过15%,Na 的渗透系数反而随KH570含量增多而减小,其原因在于Na 被聚合物中的微孔陷阱捕获而无法跳出。     

Diffusion coefficient of hydrogen in a Pd-Ag membrane: Effect of hydrogen solubility

The permeabilities of hydrogen through a Pd₇₅-Ag₂₅ membrane have been measured at temperatures ranging from 423 to 573 K and under hydrogen pressure differences ranging from 69 to 256 kPa. From the available literature solubility data a neural network model has been developed in order to simulate the variation of the hydrogen content in the alloy as a function of pressure and temperature. Then, from steady state permeability measurements and calculated solubilities, the diffusion coefficients of hydrogen have been computed. At a given temperature, the diffusion coefficient has been found to decrease with the hydrogen content (0.1 ≤ n = H/M ≤ 0.37). A simple relation is then suggested to predict the variation of the diffusion coefficient on both temperature and hydrogen content. To account for the variation of the diffusion coefficient with n, the integration of Fick's law of diffusion has been performed numerically, resulting in non-linear profile of hydrogen content through the membrane under steady state permeation.