Matrimid®5218 dense membrane for the separation of azeotropic MeOH-MTBE mixtures by pervaporation

https://doi.org/10.1016/j.seppur.2018.01.045Get rights and content

Highlights

  • Matrimid®5218 is proposed, for the first time, for organic-organic separations by pervaporation.

  • Matrimid®5218 is capable to separate MeOH from MTBE particularly at azeotropic concentrations.

  • The swelling degree of the polyimide increased as a function of MeOH concentration.

  • Increasing the temperature has a positive effect on MeOH flux and separation factor.

Abstract

Matrimid®5218 dense membranes were produced using NMP by solvent evaporation. The membranes have been used, for the first time, in pervaporation (PV) separation of azeotropic methanol (MeOH)- methyl tert-butyl ether (MTBE) mixtures (14.3 and 85.7 wt%, respectively). The membranes were characterized by TGA, SEM, DSC, contact angle and swelling tests. The PV experiments were carried out at different feed temperatures (25–45 °C) and vacuum pressures (0.0538, 0.2400, 2.1000 mbar). Moreover, an analysis of the PV process through the Arrhenius relationship has been provided. The feed temperature (in the range of 25–45 °C) affected mainly the MeOH permeation producing an increasing on its partial permeate flux and separation factor as well. Indeed, the best performances of Matrimid® were found at 45 °C and 0.054 mbar, where a permeate flux and a separation factor of about 0.073 kg m−2 h−1 and 21.16, respectively, were obtained. Finally, the Matrimid PV performance was compared with other polymeric membranes at the azeotropic conditions.

Introduction

Pervaporation (PV) technology, as the combination of permeation and evaporation processes, is efficiently able to separate organic-organic mixtures formed by close-boiling compounds [1], [2], [3], [4]. Methanol (MeOH)-methyl tert-butyl ether (MTBE) is one of the most studied organic/organic mixture. The importance of this separation lies in the fact that MTBE is an octane enhancer which is used to produce lead-free gasoline aiming the reduction of air pollution. Even though its use has been limited in some countries (e.g. USA) due to the groundwater contamination, it is still used in European and Asian countries in order to satisfy the growing demand of fuel worldwide production [5]. On the other hand, the primary applications for MeOH are the production of chemicals and the use as a fuel. The reacting MeOH with isobutylene produces MTBE forming a minimum boiling azeotrope at a composition of 14.3 wt% MeOH and 85.7 wt% MTBE [6]. In MTBE production, the excess of MeOH is commonly removed from the final product by using distillation; however, this process is not energy efficient due to the formation of the azeotropic organic-organic mixture [7]. Today, PV technology represents a valid candidate for the replacement of conventional separation processes, such as distillation, for the separation of azeotropic mixtures [8]. Taking into account the similar nature of MeOH and water, many hydrophilic polymers have been proposed in the preparation of polymeric membranes for MeOH-MTBE separation, including cellulose acetate (CA) [9], CA-poly(N-vinyl-2-pyrrolidone) (PVP) blend [10], poly(vinyl alcohol) (PVA) [7], [11], [12], poly(lactic acid) (PLA) [13], polyarylethersulfone with cardo (PES-C) [14], poly(ethylene-co-vinyl acetate) (EVAc) [15], modified poly(ether ether ketone) (PEEKWC) [8], PVA-CA blend [16], acrylic acid plasma polymerized poly(3-hydroxybutyrate) [17], cross-linked 2-hydroxyethyl methacrylate (PAMHEMA) [18]. To the best of our knowledge, there is no report on the use of Matrimid® 5218 membranes for the separation of MeOH-MTBE azeotropic mixtures. Matrimid® 5218 has been used just for the dehydration of alcohols (ethanol,1-butanol, t-butanol, isopropanol), MTBE, and acetic acid [19], [20], [21], [22] and never for the separation of organic-organic mixtures. Generally, this polymer has been widely studied for gas separation [23], and minimally for other membrane processes e.g. hyperfiltration of methyl ethyl ketone-toluene mixture [24] and organic solvent nanofiltration [25]. Matrimid® 5218 is a commercial polyimide which presents excellent thermal stability and mechanical performances [26], [27], [28]. As most of the polyimides, is stable in most organic solvents (hexane, MeOH, benzene, toluene, MTBE, acetone, to mention just a few) and weak acids [29]. It has also high affinity to water molecules based on its hydrophilic nature. The chemical structure of this polymer is reported in Fig. 1.

Based on the evidence that hydrophilic polymeric membranes are potential candidates for MeOH-organic separation by means of PV, in the present study, we propose Matrimid® 5218 dense membranes, thanks to its chemical resistance and hydrophilic nature, for the separation of the MeOH-MTBE azeotropic mixture. The effect of operating conditions, such as feed temperature and vacuum pressure in permeate side, on total permeate flux and separation factor is investigated. In addition, the Matrimid® membranes were characterized by thermo-gravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), degree of swelling (DS), and water contact angle.

Section snippets

Materials

Matrimid® 5218 (3,3′,4,4′ -benzophenone tetracarboxylic dianhydride (BTDA) and 5(6)-amino-1-(4′-aminophenyl)-1,3,3-trimethylindane (DAPI)) (molecular weight, Mw = 80,000) was kindly provided by Huntsman (Everberg, Belgium). The solvents, N-methyl-2-pyrrolidone (NMP) (C5H9NO, b.p. 204 °C, >99.9%), methanol (99.8%), methyl tert-butyl ether (MTBE) (99.7%), were purchased from Sigma-Aldrich (St. Louis, USA) and used without further purification.

Membrane preparation

Matrimid® 5218 was dried over night in oven at 120 °C

DSC (Tg), TGA, SEM and water contact angle determination

The glass transition (Tg) temperature for Matrimid® membranes was around 310.14 ± 0.22 °C, which is in agreement with previous reports [31], [32]. Concerning the TGA curve for Matrimid® which can be seen in Fig. 3, a ∼10% weight loss starting from 50 up to 300 °C was revealed. This typical thermal behavior of Matrimid® membranes has been documented before [33], where the weight loss is generally attributed to the presence of residual solvent or guest molecules [27]. Lately, Matrimid® membrane

Conclusions

In this study, Matrimid® membranes have been successfully tested for the PV separation of MeOH-MTBE azeotropic mixture. The effect of some process parameters, such as feed temperature and vacuum pressure, has been evaluated. The best performance of Matrimid® membrane in terms of separation factor (α = 21.2) for such azeotropic separation was found at 45 °C and 0.054 mbar where the permeation of MeOH was favored. Through the analysis of the PV process by Arrhenius relationship, it was found that

Acknowledgments

R. Castro-Muñoz acknowledges the European Commission – Education, Audiovisual and Culture Executive Agency (EACEA) for his PhD scholarship under the program: Erasmus Mundus Doctorate in Membrane Engineering – EUDIME (FPA No 2011-0014, Edition V, http://eudime.unical.it).

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