Development of alumina supported ternary mixed matrix membranes for separation of H₂/light-alkane mixtures

Abstract

Ternary component mixed matrix membrane was prepared from PES, SAPO-34 and 2-hydroxy 5-methyl aniline on a macroporous alumina disk by the solvent evaporation method in order to investigate the effect of existence of an inorganic support. The membrane and its pure PES/Alumina counterpart were characterized by single gas permeability measurements of H₂, CH₄, C₂H₆ and C₃H₈. The corresponding H₂/CH₄ selectivities of membranes were 71.3 and 41. The membranes were also used to separate equimolar mixtures of H₂ with CH₄, C₂H₆ and C₃H₈ over a temperature range of 35–90 °C. The separation selectivities of ternary component membrane were 73.4 for H₂/CH₄, 242.9 for H₂/C₂H₆ and >1000 for H₂/C₃H₈ at 35 °C, which are comparable to the separation selectivities of pure PES on alumina. The permeances of all gases through PES/SAPO-34/HMA/Alumina membrane were, however, higher than those through PES/Alumina membrane at 90 °C. Despite its very complex morphology, the PES/SAPO-34/HMA/Alumina membrane preserved its structure and quality during the separation of different gas mixtures over temperature cycles between 35 and 90 °C. The CO₂ and CH₄ adsorption isotherms of PES-SAPO-34-HMA system were also obtained at 25 °C. The adsorption capacity of ternary component system was 1.55 mmol CO₂/g and 0.45 mmol CH₄/g, which is appreciably higher than the adsorption capacity of pure PES.

Introduction

Energy efficient separation of gas mixtures is of enormous industrial and economical importance in refinery and natural gas processing [1]. Although polymeric membranes have been successfully developed for separating gas mixtures [1], [2], [3], the fundamental trade-off between selectivity and permeability limits their large scale usage.

An extensive research has been conducted to develop composite membranes, which combines the separation performance of polymers and microporous fillers to increase both permeability and selectivity over the past two decades. The efforts on these so-called mixed matrix membranes (MMM) have often been concentrated on polymer/filler interface to eliminate the non-selective voids [4], [5], [6]. A number of approaches have been proposed to overcome detrimental impact of those voids on the performance of mixed matrix membranes [6], [7], [8], [9], [10].

The addition of low molecular weight additives as the third component has been successfully adapted to improve the compatibility between polymer and filler [11], [12], [13], [14], [15], [16]. For this purpose Yong et al. [11] added TAP (2,4,6-triaminopyrimidine) to polyimide (PI)-zeolite 4A membrane. TAP is expected to form hydrogen bonding between PI chains and zeolite 4A crystals. The CO₂ permeability of PI/zeolite 4A (43% wt)/TAP(21% wt) fell to 0.19 Barrer accompanying a remarkable increase of CO₂/CH₄ selectivity to 617 in comparison to neat PI membrane. Sen et al. [12] developed ternary component membranes by incorporation of para-nitroaniline (pNA) into the polycarbonate/zeolite 4A membrane. The H₂/CH₄ ideal separation factor for a PC/zeolite 4A (20% wt)/pNA (1% wt) was three times as high as that for pure PC membrane. However, the H₂ and CH₄ permeabilities through ternary component membrane were 71% and 24% of those through pure PC membrane. Karatay et al. [13] and Cakal [14] prepared PES/SAPO-34(20%)/2-hydroxy 5-methyl aniline (HMA, 4%) membranes by annealing above and below glass transition temperature of PES, respectively. The HMA incorporated PES and PES/SAPO-34 membranes had lower CO₂ and CH₄ permeances but much higher CO₂/CH₄ selectivities than their HMA free counterparts. The effect of HMA on PES was similar to the effect of pNA on PC. Low molecular weight additives trigger substantial changes in the membrane characteristics and influence membrane performance by stiffening the polymer matrix [16] and improving the adhesion between zeolite and polymer [12], [13], [14]. Since LMWA exhibited remarkable increase in selectivities with a decrease in permeabilities, thinner membranes are essential to obtain higher fluxes.

Mixed matrix membranes have been formed as self-supporting flat sheets or asymmetric hollow fibers with selective thicknesses between 25 and 125 μm or 0.1and 0.5 μm, respectively [17]. Thinner membranes may not be stable mechanically. Therefore supported mixed matrix membranes should be developed and the effect of existence of inorganic support on the membrane performances should be evaluated. Composite membranes with asymmetric structure consist of several layers with a gradual decrease in pore size and thickness over a macroporous support. The dense-top layer is responsible for separation while the macroporous support provides mechanical stability without significant contribution to the separation [18]. For this purpose, different polymers (polysulfone, polyethersulfone) and ceramics (alumina, mullite) were used as support. The smoothness of support surface, the degree of penetration of the membrane casting solution into the support pores and adhesion of layers are the key factors determining the formation of successful membranes [19], [20].

The polymeric membranes on ceramic supports have been prepared from rubbery polymers such as polydimethylsiloxane, polyvinylalcohol, or from highly crystalline polymers such as cellulose acetate usually for pervaporation applications [19], [20], [21], [22], [23], [24]. Alumina disks and tubes with a pore size of 0.2–0.8 μm were usually used as support in these studies. Several studies were also carried out with glassy polymer–ceramic composite membranes for pervaporation. Kreiter et al. [25] prepared the polyimide membranes on ceramic support by dip-coating and characterized these by dehydration of n-butanol solutions. The membrane had water fluxes in the range of 1–6 kg/m²-h with separation factors greater than 360. On the other hand, glassy polymers were seldom used to prepare polymer–ceramic composite membranes for gas separation. Rezac and Koros [26] synthesized 6FDA-IPDA membranes with a thickness range of 0.15–1 μm on γ-alumina disks by the solution deposition technique. The O₂ flux across the composite membrane was six times as high as that across the neat polymer counterpart, while both membranes had O₂/N₂ selectivity of approximately 5. The authors stated that dense continuous membrane layer was obtained only if diameter of swollen polymer chains was larger than the pore size of the ceramic support. A very limited number of studies have been conducted to prepare mixed matrix membranes of glassy polymers on ceramic supports. Weng et al. [27] prepared SBA-15 incorporated polyphenylene oxide (PPO) membranes on alumina support with carbon molecular sieve (CMS) intermediate layers. H₂ and CO₂ permeabilities through PPO/SBA-15/CMS/Al₂O₃ membrane both increased by about 58.8% and 7.7% but N₂ and CH₄ permeabilities decreased by 25% and 51%, compared to the self-supporting PPO/SBA-15 membrane. The H₂/CH₄ selectivity improvement from 18 to about 50 with the use of CMS/Al₂O₃ support was reported [28].

In this study the ternary mixed matrix membranes were prepared on ceramic supports for the first time in the literature. The mixed matrix membranes were composed of PES, SAPO-34 and HMA. SAPO-34 with a pore size of 0.38 nm is of high potential in the separation light permanent gases like H₂, CO₂ and CH₄ [29], [30]. The membranes were prepared using nanosize SAPO-34, which is anticipated to yield better separation properties as compared to membranes with micron-size crystals [31], [32]. Macroporous alumina disks were used as ceramic supports. The compatibility problem that may arise not only between SAPO-34 and PES but also between PES and alumina should be handled in the preparation of PES/HMA/SAPO-34/alumina membranes. The effect of alumina support on membrane performances of PES and ternary component MMMs on alumina disks were evaluated by separating equimolar mixtures of H₂/CH₄, H₂/C₂H₆ and H₂/C₃H₈ in the temperature range of 35–90 °C. The sorption characteristics of PES, SAPO-34, HMA system were also investigated by determining the CO₂ and CH₄ adsorption isotherms.