A novel time lag method for the analysis of mixed gas diffusion in polymeric membranes by on-line mass spectrometry: Method development and validation (original) (raw)
Related papers
A Novel Method of Characterizing Polymer Membranes Using Upstream Gas Permeation Tests
2011
Characterization of semi-permeable films promotes the systematic selection of membranes and process design. When acquiring the diffusive and sorption properties of gas transport in non-porous membranes, the time lag method is considered the conventional method of characterization. The time lag method involves monitoring the transient accumulation of species due to permeation on a fixed volume present in a downstream reservoir. In the thesis at hand, an alternative approach to the time lag technique is proposed, termed as the short cut method. The short cut method appoints the use of a two reservoir system, where the species decay in the upstream face of the membrane is monitored, in combination with the accumulation on the downstream end. The early and short time determination of membrane properties is done by monitoring the inflow and outflow flux profiles, including their respective analytical formulas. The newly proposed method was revealed to have estimated the properties at 1/10 the required time it takes for the classical time lag method, which also includes a better abidance to the required boundary conditions. A novel design of the upstream reservoir, consisting of a reference and working volume, is revealed, which includes instructional use, and the mechanics involved with its operation. Transient pressure decay profiles are successfully obtained when the reference and working volumes consisted of only tubing. However when tanks were included in the volumes, large errors in the decay were observed, in particular due to a non-instantaneous equilibration of the pressure during the start up. This hypothesis was further re-enforced by examining different upstream tank-based configurations.
Mixed gas transport study through polymeric membranes
Journal of Membrane Science, 1998
Membrane permeation and separation characteristics of mixed gas/membrane systems are typically calculated from singlecomponent transport parameters, namely, diffusion coef®cients and solubility constants. In certain gas systems involving gaseous or vapor mixtures, where mass transport is affected by coupling effects or the competition of penetrants for unrelaxed free volume, such calculations can lead to erroneous estimates of the membrane separation ef®ciency. In this paper, we discuss the design and development of an experimental setup for observing mixed gas permeation through non-porous membranes. The novelty of the setup is the unique inline sampling interface which allows injection of permeate mixed gas samples for concentration analysis without introducing any leaks into the permeate volume. Also, a data cropping technique is introduced to elucidate the transport properties of gases through membranes under mixed gas permeation conditions. The method is employed in mixed gas permeation studies of a rubbery polymer, (PDMS), and a glassy polymer, (NEW-TPI), membranes, using different feed compositions of a CO 2 /CH 4 binary gas system. The results show an absence of synergistic effects in gas permeation through the rubbery polymer, however, in the glassy polymer the transport parameters of each of the gases are highly affected by the presence of the other gas. Also, a change in the feed gas composition alters the separation properties of the glassy polymer.
Gas Transport in Mixed Matrix Membranes: Two Methods for Time Lag Determination
The most widely used method to measure the transport properties of dense polymeric membranes is the time lag method in a constant volume/pressure increase instrument. Although simple and quick, this method provides only relatively superficial, averaged data of the permeability, diffusivity, and solubility of gas or vapor species in the membrane. The present manuscript discusses a more sophisticated computational method to determine the transport properties on the basis of a fit of the entire permeation curve, including the transient period. The traditional tangent method and the fitting procedure were compared for the transport of six light gases (H 2 , He, O 2 , N 2 , CH 4 , and CO 2 ) and ethane and ethylene in mixed matrix membranes (MMM) based on Pebax ® 1657 and the metal-organic framework (MOF) Cu II 2 (S,S)-hismox·5H 2 O. Deviations of the experimental data from the theoretical curve could be attributed to the particular MOF structure, with cavities of different sizes. The fitting procedure revealed two different effective diffusion coefficients for the same gas in the case of methane and ethylene, due to the unusual void morphology in the MOFs. The method was furthermore applied to mixed gas permeation in an innovative constant-pressure/variable-volume setup with continuous analysis of the permeate composition by an on-line mass-spectrometric residual gas analyzer. This method can provide the diffusion coefficient of individual gas species in a mixture, during mixed gas permeation experiments. Such information was previously inaccessible, and it will greatly enhance insight into the mixed gas transport in polymeric or mixed matrix membranes.
Multifaceted Convergence Study for Evaluating Gas Diffusion Parameters of Polymeric Membranes
International Journal of Engineering and Applied Science , 2019
In this paper, gas transport properties of PTFE (polytetrafluorethylene) polymeric membranes are studied. We focused our work on three methods based on, Darcy's pore flow model, Fick's diffusion and numerical analysis. Effective diffusion coefficients obtained by Darcy's and Fick's laws are compared. In numerical analysis, simplified computational fluid dynamics model was created using virtual porous medium concept. Virtual porous media is generated by adding a momentum sink term in the governing equations of fluid flow. Diffusion process described by Darcy's method is performed using transfer of clean air through the membrane under induced pressure gradients. Fick's diffusion parameters are obtained based on trace compound (C 3 H 8 O) transport through the membrane under induced concentration gradients. Relatively high inertial permeability compared to viscous permeability (10 2) are recorded in Darcy's analysis. Also, effective diffusion coefficient obtained by Fick's law shows 10-2 decrement compared to the same obtained by Darcy's pore flow model. From computational fluid dynamics (CFD) results, virtual pore flow model successfully demonstrated its soundness on analyzing membrane transport properties.
Separation and Purification Technology, 2011
In addition to gas permeation simulation in polymeric membranes, much progress has been attained on the theories whereby the mechanisms of transport are described. In this way, numerous studies are assigned to achieve methods for determination of diffusion coefficient. Consequently, some correlated equations are obtained which consider diffusivity dependency on concentration, position or time, or any conjunction of them. Compared to the traditional time lag, a comprehensive approach developed by Frisch gives a general mathematical procedure for achieving the concentration dependent diffusion coefficient, disregarding the effect of membrane physical properties. In this study, a comprehensive algorithm is presented for direct determination of diffusion coefficient. It determines diffusion coefficient by two approaches: first, through the traditional time lag method, and second, considering the concentration dependent system. A comprehensive mathematical model was developed and solved for CO 2 gas permeation through a nonporous polymeric membrane. The results showed that considering the concentration dependent system (CDS) for diffusion coefficient led to the small deviation of about 13%, while the deviation of 360% by the concentration independent system (CIS) was acquired. Finally, a reasonable conformity between the predicted values based on concentration dependant method and experimental data was perceived.
Journal of Industrial and Engineering Chemistry, 2013
This study presents a new mathematical model to investigate the ternary gas mixture permeation across a synthesized composite PDMS/PA membrane. A novel algorithm is introduced for direct determination of diffusion coefficients. It pertains to study gas permeation through concentration dependent systems and comparing with traditional time lag method confirms the precision of this approach. Feature is that this method does not require physical properties of the membrane. Accordingly, it can be used as a general comprehensive model. In addition, molecular pair and molecular trio interactions were taken into account and in order to investigate the deviation of gas mixture from ideality, fugacities were calculated. The results showed that permeabilites of H 2 and CH 4 increase with increasing feed temperature and fugacity, while that of C 3 H 8 decreases. Moreover, increasing C 3 H 8 concentration improved permeation properties of all components. The results demonstrated that considering the concentration dependent system (CDS) leads to the small deviation of about less than 10%, while the deviation of 50-100% by the concentration independent system (CIS) was acquired. Additionally the results indicated that permeability of the lighter gases is specially affected by diffusivity, while solubility is dominant on permeability of the heavier gases.
Membrane Characterization Based on the Upstream Pressure Decay in a Dynamic Gas Permeation Test
Journal of Fluid Flow, Heat and Mass Transfer, 2014
The time-lag method is believed to be the most usable method for extracting the membrane properties via a simple dynamic permeation experiment; however this method suffers from major drawbacks that limit its use for some materials and under certain conditions. One of the major drawbacks of the time-lag method is that it relies on monitoring the pressure rise downstream from the membrane due to gas permeation, whereas the method is derived by assuming that the pressure downstream from the membrane is maintained at zero during the entire permeation experiment. To rectify this problem, it is proposed to characterize the membrane based on the pressure decay upstream from the membrane during a conventional time-lag experiment, while continuously evacuating the downstream side of the membrane. This modification allows for a better adherence to the boundary conditions on which the time-lag method relies. Right after initiation of the gas permeation experiment the membrane behaves as a semi-infinite solid, and the rate of pressure decay is directly proportional to the square root of time. Once the permeating gases emerge downstream from the membrane, the membrane no longer behaves as a semi-infinite solid and the pressure decay becomes a non-linear function of the square root of time. In the proposed new method the membrane properties are extracted based on the deviation of the recorded pressure decay from the semi-infinite behavior in the square root of time domain. In this paper we present the mathematical bases of the new method along with preliminary experimental results. The latter indicate that diffusivity, solubility and permeability of nitrogen in polyphenylene oxide (PPO) membrane used in this study are very close to the literature values.
The Role of Surfaces in Gas Transport Through Polymer Membranes
Polymers
This paper describes a procedure to measure the permeability P, diffusivity D, and rate of adsorption k1, thus determining the solubility S and rate of desorption k2 of He, N2, O2, CH4, and CO2 on a polydimethylsiloxane (PDMS) membrane. The described procedure is able to determine experimentally all the physical quantities that characterize the gas transport process through a thin rubber polymer membrane. The experiments were carried out at room temperature and at a transmembrane pressure of 1 atm. The results are in good agreement with the available data in the literature and offer an evaluation of k1 and k2.
Journal of Membrane Science, 1997
Permeability functions are developed for pure gases and mixtures of N2, CH4, CO2, and 02 as a function of driving pressure for polysulfone and silicone rubber membranes. The gas mixtures used in the measurements include N2-O2, CO2-N2-O2, and CH4-N2--CO2. The functions are expressed in a linear form in terms of species partial pressures. The functions are in good agreement with the measured data. Positive and large weights for the fastest permeating species, CO2, are obtained in the permeability functions for all species in polysulfone. The opposite is obtained for the slower permeating species in the polysulfone permeability functions. This result is consistent with the membrane behaviour, where permeability of faster species is reduced by presence of slower species. The species weights in the silicone rubber permeability functions are 1-5 orders of magnitudes smaller than the function constant. This behaviour is dictated by limited variations in species permeability in silicone rubber.