The Membrane Thickness Effect on the Pervaporation: Analysis

Introduction

Membrane technology has developed over the years to include several processes, which are commercially viable (Baker, 2004). The pervaporation is a process of separating liquid mixtures. It involves contacts of “a feed liquid mixture with one side of a membrane while permeate is separated as vapour from the other side” (Baker, 2004). Pervaporation technology is commercially viable in two areas. First, the most important application of pervaporation is the separation of water from concentrated alcohol solutions (Blume, Wijmans and Baker, 1990; Wynn, 2001). Second, pervaporation is also useful in the removal of small quantities of volatile organic compounds (VOCs) from contaminated water (Cox and Baker, 1998).

This is a critical analysis of an article, Analysis of the membrane thickness effect on the pervaporation separation of methanol/methyl tertiary butyl ether mixtures by Villaluenga, Khayet, Godino, Seoane, and Mengual (2005). In this study, the results indicated that the “permeate changed through different types of membranes but with a significant decrease as the thickness of the membrane increased while there was no variation in the separation factor as it stayed virtually unchanged” (Villaluenga et al., 2005). The researchers based the study on a resistance-in-series model.

Purpose

The purpose of the study was to investigate “the effect of the thickness membrane on the pervaporation separation of methanol/methyl tertiary butyl ether mixtures” (Villaluenga et al., 2005). The polymers were cellulose acetate and poly (2, 6-dimethyl-1, 4-phenylene oxide). The interest was to understand the pervaporation behaviour in separating under the mixture under the influences of the polymers. Separation factors and the rate of permeation are critical in understanding the relationship among polymers, membrane thickness, and the liquid mixture. The study used a resistance-in series model to analyse the effect of the membrane thickness on the pervaporation outcomes.

Many studies have demonstrated the importance of membrane thickness in the pervaporation process. Consequently, they have concluded that thin membranes allow fast flux in liquid mixtures than thick membranes. The study main objective was important because many methods of developing membranes for different purposes have emerged. Thus, it was imperative to understand how polymers used in these methods affected the thickness of the membrane and pervaporation activities.

Literature Review

The researchers conducted a thorough review of past studies on membrane thickness, polymers, and pervaporation. Consequently, they noted that several models were available for explaining membrane and liquid flux, but they choose the resistance-in series model due to its popularity in pervaporation. The model accounts for mass transfer of the fluid to the permeate vapour in different four stages, which involve mass transfer from feed bulk to the feed membrane, membrane sorption, membrane matrix, and desorption to the permeate.

These authors based their study on a sound theoretical model after a thorough literature review in the field in order to ensure that the paper relied on a solid foundation and results could be scientifically verified.

Experiment

Under the experimental stage, the paper covered two important aspects of the research, which included materials for the study and preparation of the membrane by the use of polymers. Cellulose acetate (CA) polymer and Poly (2, 6-dimethyl-1, 4-phenylene oxide) (PPO) were used in the study.

The researchers also identified all processes involved in the preparation of the membrane. For instance, they showed that casting solutions were obtained from dissolving “3.3 wt. % of CA polymer in DMF and the PPO polymer solution was prepared by using 4 wt. % in chloroform” (Villaluenga et al., 2005). These are specific details, which future researchers could follow to obtain similar outcomes. They also indicated sources of their materials.

In addition, the paper also noted that the pervaporation process relied on an approach from different sources (Godino, Villaluenga, Khayet, Seoane and Mengual, 2004; Tabe-Mohammadi, Villaluenga, Kim, Chan and Rauw, 2001). It showed that there were “separation cell, a circulation pump, two permeate traps, two vacuum pumps and a pressure transducer with membrane pervaporation surface area of 28 cm2 to enhance efficacy” (Villaluenga et al., 2005).

Theory

The study theory was a resistance-in-series model. The model was appropriate for the study because it considered “the mass transfer of fluid mixture from the feed bulk to permeate” (Villaluenga et al., 2005). The researchers made some assumptions about the model, which include a lack of flow coupling influences, diffusivity constancy, membrane equilibrium, and constant conditions. However, from previous studies (Godino et al., 2004; Tabe-Mohammadi et al., 2001), the first two assumption seemed odd in the case of “pervaporation of methanol and MTBE mixtures when PPO and CA membranes” (Villaluenga et al., 2005). However, the consistency of results and the model predictions were used to provide rationale for the model.

Results and discussion

The researchers provided detailed explanations on how results were obtained. Consequently, they concluded that membrane resistance or fluid mixture boundary layer were comparable for the pervaporation systems. That is, the selectivity was almost free from effects of the membrane thickness while the boundary layer resistance influenced the overall mass transfer. Thus, selectivity depended on the thickness of the membrane. This study concurred with other previous studies (Raghunath and Hwang, 1992; Nijhuis, Mulder and Smolders, 1991; Ji, Sikdar and Hwang, 1994).

Conclusion

The study concluded that the membrane thickness led to decrease in the permeate flux while separation factor did not change. This study reflects a well designed scientific study with clear purpose, theoretical model, methodology, results and discussion and conclusion. However, the researchers did not give the study implications.

References

Baker, R. W. (2004). Membrane Technology and Applications (2nd ed.). West Sussex: John Wiley & Sons. Web.

Blume, I., Wijmans, J. G., and Baker, R.W. (1990). The Separation of Dissolved Organics from. Journal of Membrane Science, 49, 253. Web.

Cox, G., and Baker, R.W. (1998). Pervaporation for the Treatment of Small Volume VOC contaminated. Industrial Wastewater, 6, 35. Web.

Godino, P., Villaluenga, J., Khayet, M., Seoane, B., and Mengual, J. (2004). Pervaporation of Alcohols and Methyl t ert Butyl Ether through a Dense Poly(2,6-dimethyl-1,4-phenylene oxide) Membrane. Industrial & Engineering Chemistry Research, 43(10), 2548-2555. Web.

Ji, W., Sikdar, S. and Hwang, S.-T. (1994). Modeling of Multicomponent Pervaporation for Removal of Volatile Organic Compounds from Water. Journal of Membrane Science, 93, 1–19. Web.

Nijhuis, H.H., Mulder, V., and Smolders, C.A. (1991). Removal of trace organics from aqueous solutions. Effect of membrane thickness. Journal of Membrane Science, 61, 99-111. Web.

Raghunath, B., and Hwang, S.T. (1992). Effect of boundary layer mass transfer resistance in the pervaporation of dilute organics. Journal of Membrane Science, 65(1-2), 147-161. Web.

Tabe-Mohammadi, A., Villaluenga, J., Kim, H., Chan, T., and 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., and Mengual, J. (2005). Analysis of the membrane thickness effect on the pervaporation separation of methanol/methyl tertiary butyl ether mixtures. Separation and Purification Technology, 47(2005), 80–87. Web.

Wynn, N. (2001). Pervaporation comes of age. Chem. Eng. Prog, 97(10), 66. Web.