Poly(octylmethylsiloxane)/oleyl alcohol supported liquid membrane for the pervaporative recovery of 1-butanol from aqueous and ABE model solutions
Highlights
► Blended supported liquid membranes (SLMs) were developed for butanol recovery. ► Oleyl alcohol (OA) was used as the extractant and POMS as the stabilizing agent. ► Optimum pervaporation performance was observed at 30 wt% OA content SLM. ► The SLM showed high selectivity toward 1-butanol from fermentation broth. ► The above SLM exhibited excellent stability with minimal liquid membrane loss.
Introduction
Environmental concerns and the continuous depletion of fossil fuels have stimulated the development of technologies for the conversion of renewable resources into alternative transportation fuels, such as bio-butanol. 1-Butanol has a large potential as a fuel because it has superior properties than ethanol; it has lower Reid vapor pressure, higher energy content, higher blending capacity and higher hydrophobicity [1]. As a result, acetone–butanol–ethanol (ABE) fermentation has regained much attention in recent years for the industrial production of 1-butanol and other high value liquid fuels. But the low solvent concentration of ABE in fermentation broths (1–2%, w/v) makes the conventional solvent recovery methods (i.e. distillation) highly uneconomical, rendering the overall ABE process infeasible [2], [3].
Among the advanced solvent recovery technologies, pervaporation (PV) is considered as a cost-effective alternative as it is highly energy-efficient and conveniently generates concentrated organics from dilute solutions [4]. Several studies on the PV separation of 1-butanol have been reported using various types of membranes, such as: (1) porous polymeric membranes [5], [6], (2) dense polymeric membranes [7], [8], [9], [10], [11] (3) polymeric mixed matrix membranes (MMMs) with inorganic filler [12], [13], [14], [15], [16], and (4) supported liquid membranes (SLMs) [17], [18], [19].
Among these types of membranes, SLMs have shown exceptionally high selectivities and promising flux values in pervaporative recovery of alcohols. Aside from the generally lower mass transfer resistance offered by the liquid membrane (LM), the overall separation performance of SLMs is primarily governed by its affinity to the target solutes. Such LM–solute interactions are typically determined through the partition coefficient (Kp) parameter. For instance, it has been shown that oleyl alcohol (OA) with a moderately high 1-butanol Kp = 3 exhibited high 1-butanol flux (390 g/(m2 h)) and separation factor (180) as an SLM system for the pervaporation (30 °C) of 1–3.75 wt% 1-butanol feed [18]. Similarly, trioctyl amine-based SLM yielded 11 g/(m2 h) of 1-butanol flux and 275 separation factor for the pervaporation (54 °C) of 1.5 wt% of 1-butanol feed [19]. Both LMs (OA and TOA) showed exceptional performances but OA has the additional advantage over TOA and other organic extractants due to its biocompatibility with microorganisms (i.e. non-toxicity) and its relatively higher 1-butanol Kp [19], [20], [21]. Such features are important especially if the SLM-based PV is to be integrated with fermentation systems for enhanced 1-butanol yields. However, the main limitation of SLM relies on its unreliable mechanical stability due to the eventual detachment and loss of LM from the support matrix [22]. One way to minimize the physical deterioration of SLM is by blending the LM extractant with another component.
Although poly(dimethylsiloxane) (PDMS) is the most commonly preferred polymeric material for pervaporative organic separation, several studies have shown that poly(octylmethylsiloxane) POMS exhibits a slightly better separation performance than PDMS [22], [23], [24]. These results were corroborated by Bennett et al. [25] wherein the presence of longer chain alkyl groups on the siloxane backbone significantly enhances its affinity toward the organic compounds. It is clear from these studies that the potential of POMS for 1-butanol separation can also be exploited. The availability of POMS in liquid form as a silicone fluid presents its suitability for SLM applications. Moreover, the inherent hydrophobicity, viscosity of POMS and its miscibility with long-chain linear alcohols make it possible to obtain a potentially selective and more stable LM with simple blending of the liquid polymer with oleyl alcohol [22].
The present study focuses on the development and application of a highly selective SLM for the pervaporative recovery of 1-butanol from aqueous solutions and model ABE fermentation broths. The SLM is composed of blended OA and POMS as LM impregnated in a porous flat sheet support matrix (Celgard® 2400). The PV performances of SLMs with varied LM blend compositions were evaluated in terms of permeate mass flux, permeate concentration, separation factor and pervaporation separation index (PSI). Furthermore, the partitioning of 1-butanol in the LM phase and the hydrophobicity of the SLM were investigated at various OA contents in POMS through equilibrium liquid–liquid extraction experiments and contact angle measurements, respectively. LM losses incurred in the SLMs after PV operations were also measured as basis for SLM stability.
Section snippets
Reagents and materials
Reagents used were purchased from various companies: OA (60%) was from Acros-Organics (NJ, USA), POMS (viscosity = 600–1000 cSt; density@25 °C = 0.91 g/cm3) was procured from ABCR GmbH & Co. (KG, Karlsruhe, Germany), 1-butanol (>99.0%) was supplied by Showa Chemical Co. Ltd. (Japan), HPLC grade acetone was from J. T. Baker (NJ, USA), HPLC grade absolute ethanol was obtained from Fisher Scientific (USA), methanol was from Merck KGaA (Darmstadt, Germany) whereas butyric acid (>99%) and phosphoric acid
Effect of OA content on 1-butanol Kp in LM/water system
The 1-butanol Kp with different OA/POMS compositions tested at 0.5%, 1.5% and 2.5% (w/v) initial 1-butanol concentrations are shown in Fig. 2. Results reveal that 1-butanol Kp increased linearly concurrent with an increase of OA content at 25 °C while minimal differences were observed at different initial 1-butanol concentrations.
To explain these results, Hansen solubility parameter (HSP) distance (Ra) was calculated between different species in the system using Eq. (8) [31]:
Conclusions
Blended OA/POMS SLMs were developed and tested for the pervaporative recovery of 1-butanol from model solutions. Among the SLMs with varied OA/POMS compositions, the 30 wt% OA/POMS SLM exhibited the best performance for the pervaporation of aqueous 1-butanol solution. The PV performance of the SLM was improved at higher operating temperature but more remarkably, when feed concentration was increased. Highest 1-butanol flux and separation factor were obtained when the SLM was operated at 60 °C.
Acknowledgment
This research was supported by the Priority Research Centers Program (Project No. 2012-0006693) and by the National Research Foundation of Korea (NRF) (Project No. 2012R1A2A1A01009683) funded by the Korea government Ministry of Education, Science and Technology (MEST).
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