Pervaporation of Ethanol/Water Mixtures Through a High-Silica MFI Membrane: Comparison of Different Semi-Empirical Mass Transfer Models

Pervaporation of binary ethanol/water solutions of 5–10 wt.% ethanol was studied experimentally through a thin supported high-silica MFI zeolite membrane of hydrophobic character in the temperature range of 30–70 °C. The fluxes obtained were very high, 2–14 kg m-2h-1 with ethanol/water separation factors of 4–7. The loss of effective driving force was significant in the supporting layers, which limited the membrane performance. The correlation between the experimental data and three different semi-empirical mass-transfer models was examined. The correlation was good especially when the driving force for mass-transfer was determined based solely on bulk feed, or the bulk feed and permeate conditions together. Somewhat lower correlation was observed when the driving force was corrected with the effect of support resistance. This was most likely due to the inaccuracies of the used mass transfer parameters in the support. The investigated semi-empirical models can be applied for initial stage process design purposes

Development of permporometry for analysis of MFI membranes

Zeolite membranes exhibiting high flux and high selectivity are of major interest for potential future applications. In order to achieve high flux and high selectivity, the zeolite film must be thin (< 1 µm) and free from flow-through defects. The development of thin defect free zeolite membranes requires powerful tools for characterization of flow-through defects in the membranes. Permporometry is one of the most straightforward and powerful techniques for characterization of flow-through pores in ceramic membranes. In permporometry, the flow of a non-condensable gas, e.g., helium, through the membrane is monitored as a function of the activity of a strongly adsorbing compound, e.g., hydrocarbon.In the present work, MFI membranes prepared by a seeding method were characterized by permporometry using helium as the non-condensable gas and n-hexane or benzene as the adsorbing compound. In order to appreciate permporometry data, the membranes were also characterized by scanning electron microscopy (SEM), single gas permeation and separation experiments. The permporometry data were then compared to the SEM morphology of the membranes, permeances of different probe molecules and membrane separation performance.In order to determine the conditions of the permporometry experiment leading to blocking of zeolite pores, a model describing helium transport in the zeolite pores in the presence of n-hexane or benzene was developed. The model is based on percolation theory and knowledge of the adsorption isotherms and adsorption sites for n-hexane and benzene in the zeolite pores. Parameters needed in the model were estimated by Density Functional Theory (DFT) using a Local-Density Approximation (LDA), the most sophisticated theory yet applied to this system. Based on the permporometry data, it was demonstrated that the model could adequately describe helium transport in zeolite pores in the presence of the hydrocarbons.The sensitivity of the permporometry technique towards the defect size has been improved considerably. It was revealed that high quality MFI membranes prepared in the present work contained mainly micropore defects which are most like the defects in the zeolite crystal lattice (intracrystalline defects).The work has shown how permporometry data could be used to estimate the area distribution of the flow-through defects in the membranes. The results on the defect distribution were corroborated by the SEM observations and the separation experiments. The width of cracks, including support cracks, and open grain boundaries observed by SEM was in excellent agreement with the defect width estimated from permporometry data. A straightforward correlation was observed between separation data and permporometry data, i.e. membranes of higher quality according to permporometry analysis exhibited greater separation performance. Also, the permeance of molecules diffusing through defects in the membrane in the separation experiment was found to scale with the permeance of helium through the defects measured in the permporometry experiment. In addition, this work showed that single gas permeance ratios could not detect slight variations in the membrane quality. For membranes with similar however slightly different amount of defects, the ratios are mainly affected by the membrane thickness and support morphology.To summarise, the present work demonstrates that permporometry data adequately reflect membrane quality and that permporometry is a very powerful technique for MFI membrane characterization.Godkänd; 2011; 20110512 (dankor); LICENTIATSEMINARIUM Ämnesområde: Kemisk teknologi/Chemical Technology Examinator: Professor Jonas Hedlund, Institutionen för samhällsbyggnad och naturresurser, Luleå tekniska universitet Diskutant: PhD, senior researcher Olov Öhrman, ETC, Piteå Tid: Fredag den 17 juni 2011 kl 10.00 Plats: C305, Luleå tekniska universite

Quality and performance of zeolite membranes

Zeolite membranes displaying high flux and high selectivity are of major interest for potential industrial applications, such as gas and liquid separations. In order to achieve high flux and high selectivity, the zeolite membrane must be thin (< 1 µm) and free from flow-through defects. The development of thin defect-free zeolite membranes requires powerful tools for characterisation of the defects in the membranes. Permporometry is one of the most straightforward, non-destructive and powerful techniques for characterisation of flow-through pores in inorganic membranes. In permporometry, the flow of a non-adsorbing gas, e.g., helium, through the membrane is monitored as a function of the activity of a strongly adsorbing compound, e.g., hydrocarbon.The work showed how permporometry can be used to quantify defects in the mesopore range in MFI membranes. The results were in excellent agreement with SEM observations and separation experiments. For the first time, it was also shown that flow-through defects down to 0.7 nm in size in MFI membranes could be detected by permporometry and quantified using the permporometry data. The total amount of defects in high quality MFI membranes was found to be small accounting for less than 1% of the total membrane area. In turn, defects with a width below 1 nm constituted as much as 97% of the total area of defects for the best membranes. The permporometry results were consistent with SEM observations and separation experiments, demonstrating that permporometry data adequately reflect membrane quality and that the technique is a very powerful and reliable characterisation tool.This work also illustrated that single gas permeance ratios could not detect slight variations in the membrane quality. For membranes with similar however slightly different amount of defects, the ratios are mainly affected by the membrane thickness and support morphology.The MFI membranes were also evaluated for separation of dilute aqueous solutions of n-butanol and ethanol, and 1 µm zeolite X membranes were evaluated for separation of water from ethanol using pervaporation. The MFI membranes were selective to n-butanol and ethanol, whereas zeolite X membranes were selective to water. The flux observed for the MFI membranes was about 100 times higher than those previously reported for n-butanol/water and about 5 times higher than the highest reported for ethanol/water separation by pervaporation. The zeolite X membranes showed good pervaporation performance in terms of both flux and selectivity. However, both flux and selectivity were found to be reduced by a significant mass transfer resistance in the support in all the pervaporation experiments. At the same time, heat transfer limitations were found to be negligible.Godkänd; 2012; 20121005 (dankor); DISPUTATION Ämne: Kemisk teknologi/Chemical Technology Opponent: Associate professor Derek Creaser, Dept. of Chemical and Biological Engineering, Chalmers University of Technology Ordförande: Professor Jonas Hedlund, Institutionen för samhällsbyggnad och naturresurser, Luleå tekniska universitet Tid: Fredag den 9 november 2012, kl 10.00 Plats: F531, Luleå tekniska universite

Correlation between n-hexane/helium permporometry, single gas and separation data for MFI membranes

Contemporary separation technology of great interest is high quality membranes, possessing high flux and selectivity together with high durability. Supported membranes based on thin MFI zeolite films and graded supports response to the mentioned requirements to a higher extent. To maintain separation efficiency in a high level, powerful fabrication and characterization techniques of membranes are essential. For the last decade, several techniques for characterization of defects have emerged. Of special interest is adsorption-branch permporometry based on measuring the flow of noncondensable gas, e.g. helium, through membrane as a function of the activity of condensable vapour, normally n-hexane or p-xylene. In the present work, silicalite-1 membranes with a thickness of 500 nm were prepared by using seeding method. After the preparation, membranes were characterized by single gas permeation and n-hexane/helium permporometry and then, tested for separation of a mixture of hexane isomers: n-hexane and 2,2-dimethylbutane (DMB). The results of the work showed that single gas permeation data cannot be used for membrane quality estimation. No correlation between single gas permeation data and n-hexane/helium permporometry data has been discovered as well as between single gas permeation data and separation data. The separation experiments showed that the dependence of the separation factor on temperature is quite spontaneous and unpredictable. On the other hand, it was shown that the DMB permeance at room temperature in a n- hexane/DMB mixture can be predicted by permporometry data. Also, an empirical correlation between n-hexane permporometry and the separation data has been found. Finally, the present work also demonstrated the strength of adsorption branch permporometry for zeolite membrane quality characterization.Validerat; 20101217 (root

Characterization of small defects in MFI membranes by permporometry, separation experiments and XHR-SEM

Godkänd; 2010; 20091214 (linste

Ultra-thin hydrophobic MFI membranes

Godkänd; 2013; 20131214 (hanzho

An experimental study of micropore defects in MFI membranes

Godkänd; 2013; 20131129 (dankor

Overview of alternative ethanol removal techniques for enhancing bioethanol recovery from fermentation broth

This study aims at reviewing the alternative techniques for bioethanol recovery, highlighting its advantages and disadvantages, and to investigate the technical challenges facing these alternatives to be widely used. The findings showed that the integration of these techniques with the fermentation process did not meet a large acceptance in the industrial sector. The majority of conducted studies were mainly focusing on ethanol recovery from aqueous standard solution rather than the investigation of these techniques performance in fermentation-separation coupled system. In this context, pervaporation has received more attention as a promising alternative to distillation. However, some challenges are facing the integration of these techniques in the industrial scale as the fouling problem in pervaporation, the toxicity of solvent in liquid extraction, energy consumption in vacuum fermentation. It was also found that there is a lack of the technical economic analysis for these techniques which may limit the spread of its application in the large scale. Currently, hybrid systems integrating distillation with other alternative techniques are considered as an innovative solution to reduce the high cost of the distillation process and the low separation efficiency of the alternatives techniques

Efficient ceramic zeolite membranes for CO2/H2 separation

Membranes are considered one of the most promising technologies for CO2 separation from industrially important gas mixtures like synthesis gas or natural gas. In order for the membrane separation process to be efficient, membranes, in addition to being cost-effective, should be durable and possess high flux and sufficient selectivity. Current CO2-selective membranes are low flux polymeric membranes with limited chemical and thermal stability. In the present work, robust and high flux ceramic MFI zeolite membranes were prepared and evaluated for separation of CO2 from H2, a process of great importance to synthesis gas processing, in a broad temperature range of 235–310 K and at an industrially relevant feed pressure of 9 bar. The observed membrane separation performance in terms both selectivity and flux was superior to that previously reported for the state-of-the-art CO2-selective zeolite and polymeric membranes. Our initial cost estimate of the membrane modules showed that the present membranes were economically viable. We also showed that the ceramic zeolite membrane separation system would be much more compact than a system relying on polymeric membranes. Our findings therefore suggest that the developed high flux ceramic zeolite membranes have great potential for selective, cost-effective and sustainable removal of CO2 from synthesis gas.Validerad; 2015; Nivå 2; 20150514 (dankor