The Membrane Thickness Effects on Pervaporation

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Table of Contents

The Effects of Membrane Thickness on the Pervaporation of Organic Solvents
A Review of Pervaporation Experiments
Results
Conclusion
References

Industrial undertakings involve a lot of synthetic processes. They also involve mixing, separation, and other procedures (Keyan, Junde & Jungshen, 2013). A number of different methods are used in the separation of fluids. One of the most common techniques is pervaporation. Over the past few years, this method has been widely used in conjunction with another process known as permeation to dehydrate materials. According to Keyan et al. (2013), the process is mostly used in common organic compounds. Such materials include esters, alcohols, and ketones.

In this paper, the author conducts a review of the literature revolving around pervaporation in relation to industrial processes. The review is carried out in the context of the article published by Villaluenga, Khayet, Godino, Seoane, and Mengual (2004). The article focuses on the thickness of the membrane and its effects on the said process. To this end, the author of this paper analyses the findings and arguments made by Villaluenga et al. (2004). An explanation of how the analysis was carried out, and the results obtained is provided. The literature review concludes with the author’s personal opinion on the subject.

The Effects of Membrane Thickness on the Pervaporation of Organic Solvents

As mentioned earlier, pervaporation is a common procedure in the industrial production of materials. It is the preferred method of separating mixtures that are organic in nature. In their study, Villaluenga et al. (2004) sought to determine how the thickness of a membrane affects this procedure. Villaluenga et al. (2004) focused on specific solvents to conduct this study. They concentrated on methanol and a methyl tertiary butyl ether mixture. In the process of carrying out the study, the thinking and rationalization of these researchers were governed by the fact that pervaporation has to take place through a membrane.

Nobert (2003) provides a working definition of the pervaporation process. According to Nobert (2003), it is an avenue through which liquids are sifted by means of partial separation. As indicated earlier in this paper, the process requires a membrane for the actual separation to occur. The principle behind this technique is the difference in the molecular sizes of the liquid mixtures. Villaluenga et al. (2004) argue that the term ‘pervaporation’ has various implications. For example, the name suggests that the ‘permeate’ being sought after undergoes a partial change of state. The material changes its state from liquid to gas.

The whole process is likened to the sieving of a given mixture. Under such circumstances, the desired substance is generated from one side of the sieve, while the mixture is in contact with the wall of the strainer on the opposite end (European Commission, 2010). In the pervaporation process, there are two important elements that come into play. The elements determine the nature of the whole procedure. There are the feed mixture and the ‘permeate.’ The latter is the substance that should be obtained using the procedure. It should be separated from the mixture. Villaluenga et al. (2004) point out that the permeate comes out of the membrane in a gaseous state.

Villaluenga et al. (2004) are of the opinion that the pervaporation process is affected by a number of factors. The said factors may be from within or from without the process itself. Consideration is given to the composition of the feed materials in the mixture. In addition, the researchers argue that the temperature and pressure of the permeate are additional elements that affect the pervaporation process. However, Villaluenga et al. (2004) focused on the thickness of the membrane. The nature of the material used in the separation process also affects the whole procedure. Previous studies by Binning, Lee, Jennings, and Martins (1961) illustrate the selectivity related to the separation of different mixtures. Binning et al. (1961) hold that such selectivity depends on the thickness of the membrane.

In a separate study, Qunhui, Ohya, and Negishi (1995, pp. 223-232) reviewed the effects of membrane thickness in terms of their permselectivity. The three researchers used chitosan as their preferred membrane in the experiment. Their study revolved around the separation of a mixture of water and ethanol. Qunhui et al. (1995) used membranes of different thicknesses in their study. The size of the sifters used ranged from 30µm to 50µm. The scholars observed that selectivity tended to increase when the thickness exceeded 30µm. As such, they concluded that increased thickness leads to high levels of selectivity. However, a unique observation was made when the thickness exceeded 50µm. At these levels, the researchers found out that the selectivity became constant (Qunhui et al., 1995).

In a bid to understand the dynamics associated with membrane thicknesses, Kanti, Srigowri, Madhuri, Smitha, and Sridhar (2004, pp. 259-266) carried out a study along these lines. Their study was aimed at analyzing the size of the strainer in relation to permselectivity. It is noted that the research involved the dehydration of a mixture consisting of water and ethanol. According to Kanti et al. (2004), the components of the mixture weighed 4.6% and 94.4%, respectively. The size of the membranes ranged from 25µm to 190µm. In this scenario, the flux of materials was observed to decrease when the thickness of the membrane was increased.

From the two experiments mentioned above, several conclusions can be made. For example, it is apparent that the impacts of the size of the membrane on the whole process are best understood by observing two important parameters. The elements analyzed here include selectivity and flux as far as pervaporation is concerned. Villaluenga et al. (2004) point out that constant selectivity is realized whenever there is optimal membrane thickness. At the same time, flux is seen to decrease when the thickness of the membrane is increased. In both cases, it is observed that the size of the membrane can be varied or manipulated to obtain the desired results.

A Review of Pervaporation Experiments

The experiment conducted by Villaluenga et al. (2004) was basically an analysis. In light of this, Villaluenga et al. (2004) point out that there were variations in the thickness of these membranes. The materials used in the experiment were polymers. The first was poly(2,6-dimethyl-1,4-phenyl oxide) [herein referred to as PPO]. On its part, the second polymer used in the study as a membrane was cellulose acetate (herein referred to as CA). According to Villaluenga et al. (2004), the CA polymer had a molecular weight of 37,000. In addition, its degree of acetylation was found to be 39.8%. On the other hand, the PPO had a viscosity of 1.57 dL/g. The density of this membrane was set at 1.04 g/cm³(Villaluenga et al., 2004).

A similar study was conducted by Cao, Shi, and Chen (2000, pp. 89-97). In this research, Cao et al. (2000) pointed out that it is important to prepare a casting solution prior to a pervaporation experiment. Villaluenga et al. (2004) appear to hinder this recommendation in their comparative analysis. To this end, Villaluenga et al. (2004) made use of 2 solvents as casting solutions. Where cellulose acetate was used as the membrane, the researchers applied acetone. On the other hand, chloroform was used as the solvent where PPO was the membrane. Each of the polymer solutions was poured over a circular glass surface. They were subsequently dried to obtain the membrane.

The first process of drying is usually slow. In this initial step, the solution on the glass surface is allowed to evaporate. According to Nobert (2003), such a procedure should take place in a fume chamber owing to the toxic nature of the polymers in the solution. As a result, Villaluenga et al. (2004) set the conditions in the fume chamber such that the humidity was 30%. The temperature was set at 23 ⁰C. The conditions were maintained until the membrane was fully formed (Villaluenga et al., 2004). The second phase of drying required the membrane to be heated in an oven that was set at 70 ⁰C over a period of eight hours. The final phase required the material to be kept in a vacuum oven for at least 12 hours.

The membrane from the PPO, as aforementioned, was prepared using chloroform. The percentage weight of this material was 4% (Villaluenga et al., 2004). The solution was also placed on a glass surface and allowed to dry. The mixture dried over a period of 24 hours. During that duration, the temperature was maintained at 25 ⁰C (Villaluenga et al., 2004). Thereafter, the membrane was dried for another 24 hours in a fume chamber. According to the European Commission (2010), drying in a vacuum is important as it removes any traces of the solvent. Villaluenga et al. (2004) placed the membrane in a vacuum for a total of 72 hours to achieve the same objective.

The actual experiment was carried out in a system that comprises of several elements. Tabe-Mohammadi, Villaluenga, Kim, Chan, and Rauw (2001) suggest that such a system should consist of permeate traps. They should also contain a pressure transducer and a separation cell. In addition, there needs to be a total of 2 vacuum pumps and one circulation pump. The total surface area over which the pervaporation would occur was set at 28 cm². Villaluenga et al. (2004) spread out the feed mixture over the membrane to allow pervaporation to take place.

In most cases, the pervaporation process relies on the pressure as a determining factor. Consequently, Villaluenga et al. (2004) maintained the pressure in the system described above at approximately 105 pascals. The vacuum pumps help in the evacuation of the permeate, which is then collected in one of the traps. According to Villaluenga et al. (2004), it is important to place the traps in a liquid nitrogen bath. Upon completion of the experiment, the collected permeate was brought to room temperature.

Lipnizki and Tragardh (2001) argue that the best way to understand the effects of membrane thickness on the process is to have the various experiments conducted simultaneously. The same was observed by Villaluenga et al. (2004). In this study, the two membranes that were formed had their feed solutions placed at the same time. They had three samples for each of the experiments. The objective was to observe the changes in permeate selectivity and flux. The data obtained from each of these experiments were recorded separately to make the relevant comparisons

Results

PPO Membrane

Villaluenga et al. (2004) used the PPO membrane to examine the pervaporation of methanol and a methyl tertiary butyl ethyl (herein referred to as MTBE). The researcher collected data depending on the varying thicknesses of the membranes. It is important to note that the feed mixture in all the three samples was maintained at a constant throughout the experiment. The flux generated varied with the size of the membranes. For example, the thin material produced higher levels of permeate than the thick membrane. According to Villaluenga et al. (2004), the thinnest membrane was recorded at 28µm, whereas the thinnest was 126µm.

Cellulose Acetone Membrane

With regards to the cellulose acetone membranes, the feed mixture was kept at a constant of 21% (Villaluenga et al., 2004). When the thickness of the membranes was reduced, there was a corresponding decrease in flux. In this case, the size of the smallest strainer was set at 20µm. On its part, the thickest was 96µm. The volume of the flux in the thinnest membrane was seen to be twice that in the thickest membrane. The same observations were made in the PPO membrane. The selectivity was independent regardless of the membrane used. Villaluenga et al. (2004) point out that the case was the same with regards to the thickness of the membranes used in the experiment.

Conclusion

From the experiments cited in this paper, it is apparent that pervaporation is directly affected by a number of factors. One of these elements is the thickness of the membrane used. According to Kanti et al. (2004), the process of pervaporation is affected by the size of the membrane applied in separating the mixture. The procedure is similar to what takes place during the separation of a mixture by means of a sieve. In this analogy, the surface of the sieve acts as the membrane. The implication is that substances that are thicker than the strainer’s pores will not be sieved out. The same explains why thickness is an important aspect of pervaporation.

The results obtained in the experiments conducted by Villaluenga et al. (2004) make it clear that the size of membranes plays an important part in determining the flux produced. Thick membranes yielded low volumes of permeate in both experiments. The case was different for thinner membranes. Based on this, one can conclude that the membranes offer some sort of resistance to a feed mixture. Increasing the thickness will lead to a corresponding increase in the resistance posed to the molecules.

The information provided in this review will help in the development of membranes that will be used in industrial separation processes. According to Nobert (2003), pervaporation is increasingly becoming an important technique for separating mixtures. The application of this technology in chemicals of organic nature is very. It is especially important when the intention is to obtain specific volumes of permeates. As such, it is safe to assume that the thickness of a membrane affects the pervaporation process by influencing the volume of flux obtained after the procedure is completed.

References

Binning, R., Lee, J., Jennings, E., & Martins, F. (1961). Separation of mixtures by permeation. Industrial and Engineering Chemistry, 53, 45-50. Web.

Cao, S., Shi, Y., & Chen, G. (2000). Influence of acetylation degree of cellulose acetate on pervaporation properties for MeOH/MTBE mixture. Journal of Membrane Science, 165(1), 89-97. Web.

European Commission. (2010). Membrane technologies for water applications: Highlights from a selection of European research projects. Brussels, Belgium: Directorate-General for Research. Web.

Kanti, P., Srigowri, K., Madhuri, J., Smitha, B., & Sridhar, S. (2004). Dehydration of ethanol through blend membranes of chitosan and sodium alginate by pervaporation. Separation and Purification Technology, 40(3), 259-266. Web.

Keyan, H., Junde, N., & Junsheng, L. (2013). Separation of methanol from methanol/water mixtures with pervaporation hybrid membranes. Journal of Applied Polymer Science, 128(3), 1469-1475. Web.

Lipnizki, F., & Tragardh, G. (2001). Modeling of pervaporation: Models to analyze and predict the mass transport in pervaporation. Separation and Purification Methods, 30, 49-51. Web.

Nobert, M. (2003). Removal of methanol by pervaporation. Sulzer Technical Review, 1, 19-21. Web.

Qunhui, G., Ohya, Y., & Negishi, J. (1995). Investigation of permselectivity of chitosan membrane thickness. Journal of Membrane Science, 98, 223-232. Web.

Tabe-Mohammadi, A., Villaluenga, G., Kim, J., Chan, T., & Rauw, V. (2001). Effects of polymer solvents on the performance of cellulose acetate membranes in methanol/methyl tertiary butyl ether separation. Journal of Applied Polymer Science, 82(12), 2882-2895. Web.

Villaluenga, J., Khayet, M., Godino, P., Seoane, B., & Mengual, J. (2004). Analysis of the membrane thickness effect on the pervaporation separation of methanol/methyl tertiary butyl ether mixtures. Separation and Purification Technology, 47, 80-87. Web.

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