Effective permeability of binary mixture of carbon dioxide and methane and pre-dried raw biogas in supported ionic liquid membranes

doi:10.1016/j.seppur.2015.08.022

Separation and Purification Technology

Volume 153, 16 October 2015, Pages 14-18

https://doi.org/10.1016/j.seppur.2015.08.022 Get rights and content

Highlights

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The influence of temperature and stage cut on gas permeation through SILM were measured.
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Results obtained with binary mixture and real pre-dried biogas were compared.
•
The SILMs can be used with real pre-dried biogas.

Abstract

The influence of the temperature and stage cut on permeation of CO₂ and CH₄ through two different supported ionic liquid membranes were studied. The measurements were performed with binary mixture of CH₄ and CO₂ and with real pre-dried biogas collected in sewage plant. The influence of temperature on permeability followed the Arrhenius behavior in agreement with solution diffusion model of transport. The influence of stage cut was also very small, what helped to confirm that the membrane was operated under optimal conditions.

The results obtained with real pre-dried biogas were very close to those obtained with the binary mixture. This was caused by the fact that the amount of trace compounds present in real pre-dried biogas was very small (less than 1 vol.%). The results showed that supported ionic liquid membranes (SILMs) can be used for biogas upgrading. It can be also concluded that the use of model mixture instead of real biogas for measurements brings relevant results.

Introduction

The ionic liquids have become widely studied substances in different scientific fields. Their unique chemical and physical properties as negligible vapor pressure and low chemical reactivity predestine them for preparation of supported liquid membranes (SILMs). The supported ionic liquid membranes are widely tested for the separation of CO₂ from gaseous streams. It was found that selectivity of the SILMs is mainly determined by the different solubility of separated gases in ionic liquids [1]. Some models trying to predict the values of solubility and diffusivity of gases in ionic liquids were published [2], [3], [4]. The real effectiveness of the gas separation by ionic liquid membranes was analyzed by means of Robeson plot [5]. Many reviews summarized the state of art of the gas separation by ionic liquid membranes from both theoretical (prediction based on the known solubility) and experimental point of view [6]. The values of permeability presented in literature are often calculated from solubility and diffusivity of gases in ionic liquids. These values thus cannot be directly applied on SILMs because of the influence of the support [7]. Here we present the directly measured values of the permeability of the real “whole” SILMs, which are more important for their practical utilization.

Separation of CO₂ from different gaseous streams is an important industrial issue. Different methods like pressure swing adsorption, water scrubbing, amine absorption and membrane separations are employed [8]. The membrane separation of CO₂ from biogas became very popular in last years. Various membranes have been tested to make the separation effective and feasible. There are some industrial technologies employing membrane modules for CO₂ separation but many researches still try to develop better and more effective membranes. Unfortunately the sophisticated preparation and expensive materials of such new membranes usually cannot complete the already available commercial membranes. The supported ionic liquid membranes were proven to be effective for CO₂ separation (the diffusion coefficient of a gas is significantly higher in “liquid membrane” than that in a solid membrane) [9] but to our best knowledge the published experiments are still strictly limited to model mixtures and no tests with any real gas (natural gas, biogas) have been done. The economic feasibility of industrial application of SILMs is still questionable but the polymeric membranes are often sensitive to the biogas trace compounds. The biogas trace compounds are often denoted to be the reason of membrane degradation. Especially the sulfur compounds (hydrogen sulfide, mercaptanes) are problematic because the pure solid sulfur produced by their oxidation causes the membrane fouling. The ionic liquids though are probably immune to those compounds and even if the trace compounds were absorbed in the membrane, the ionic liquid can be washed out of the support and with a convenient solvent and regenerated by e.g. heating so the ionic liquid can be immobilized repeatedly.

In our previous publication [10] we tried to apply the existing models of diffusivity and solubility of gases in ionic liquids on prediction of permeability (or mass transfer coefficients) in supported ionic liquid membranes. Here we continue in verification of the practical utilization of the ionic liquid membranes by testing their temperature stability, temperature dependence of permeability and the influence of CO₂ stage cut on membrane operation. The binary mixture of 35 vol.% of CO₂ and 65 vol.% of CH₄ and raw biogas taken from sewage plant were used. The theory of transport, way of calculating the permeability and selectivity and the process of support and ionic liquid selection was already described in details in [10]. The Robeson plot, comparison of measured membrane transport properties with literature data and the influence of support is also discussed in [10]. Here we want to extend the state of art of SILM operation by using them with real pre-dried biogas.

Section snippets

Theoretical

The permeability used in this manuscript was calculated as (1) $P = \frac{J_{i} l}{p_{i}^{ret} - p_{i}^{perm}}$ where J_i is a permeation flux of the particular gas through the membrane, l is the thickness of the porous support (assumed to be equal to the membrane thickness [10]) and $p_{i}^{ret} - p_{i}^{perm}$ is the partial pressure difference of the particular difference of the particular gas across the membrane.

The selectivity was calculated as the ratio of permeabilities.

The Arrhenius type of equation was used to describe the temperature

Materials and membrane preparation

Hydrophilic PVDF was used as a support for supported liquid membranes. Pore diameter of the support was 0.1 μm, the porosity was 70% and its thickness was 125 μm. Porous PTFE disc of 10 μm pore diameter, porosity of 80% and 130 μm thickness was used to protect the membrane from leakage of the ionic liquids into the membrane cell. Both types of the porous support were purchased from Millipore Inc.

Pure gases for the experiments (CO₂, CH₄, N₂) were purchased from Linde Gas with a stated purity of at

Temperature dependence of mixed gas permeability and selectivity

The temperature dependence of permeability of both permeating gases and mixed gas selectivity is displayed in Fig. 2a for [emim][Tf₂N] membrane and Fig. 2b for [emim][dca] membrane, respectively. Data points obtained for permeation of model binary mixture and raw biogas are always displayed in one figure to see the differences for both ionic liquid membranes. The error of the measurement is from 5% to 8% thus the error bars would be smaller than the data points.

The linear dependence of

Conclusions

The temperature dependence of permeability, selectivity and influence of CO₂ stage cut on permeability and selectivity were measured and discussed. Binary mixture of CO₂ and CH₄, and pre-dried raw biogas from sewage plant were used for the measurements and their results were compared. Supported ionic liquid membranes consisting of commercially available porous PVDF and [emim][Tf₂N] or [emim][dca] were tested. Both membranes were used continually at least one month as long as the measurements

Acknowledgements

Financial support from the Ministry of Education, Youth and Sports by Project No. LH-14006 is gratefully acknowledged by M. Kárászová, and P. Izák would like to acknowledge financial support received from the Czech Science Foundation by Project No. 14-12695S.

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Effective permeability of binary mixture of carbon dioxide and methane and pre-dried raw biogas in supported ionic liquid membranes

Highlights

Abstract

Introduction

Section snippets

Theoretical

Materials and membrane preparation

Temperature dependence of mixed gas permeability and selectivity

Conclusions

Acknowledgements

J. Membr. Sci.

J. Membr. Sci.

Sep. Purif. Tech.

Sep. Purif. Tech.

J. Membr. Sci.

J. Membr. Sci.

Gas permeation studies in supported ionic liquid membranes

J. Membr. Sci.