Desorption-controlled separation of natural gas alkanes by zeolite membranes †

The performance of porous membranes is tremendously in ﬂ uenced by desorption, as alkane separations by a pressure stable MFI membrane revealed. High membrane selectivities as well as permeation ﬂ uxes are to be traced back to the fact that a reduced permeate pressure signi ﬁ cantly decreases the loading gradient of the adsorbed molecules in the membrane.

The performance of porous membranes is tremendously influenced by desorption, as alkane separations by a pressure stable MFI membrane revealed.High membrane selectivities as well as permeation fluxes are to be traced back to the fact that a reduced permeate pressure significantly decreases the loading gradient of the adsorbed molecules in the membrane.
][3][4][5][6] In addition to catalytic alkane conversions, the substitution of present separation technologies for natural gas alkanes by low energetic alternatives [7][8][9] would be the essential economic progress for their increased use in the chemical industry.In this context, membranes based on silicon rubber have currently found practical application. 7However, with typical mixed-gas propane/methane selectivities of 3-5 and butane/methane selectivities of 5-10 they are preferably used for natural gas processing.Higher selectivities are necessary for targeted alkane isolation and were reported for more advanced polymer membranes. 10Even so, heavy hydrocarbons cause swelling of polymer membranes, leading to increased permeation for all components and a decrease of the separation ability over time. 11o far, non-swellable zeolite membranes were not applicable since they lost in direct competition with polymer membranes in terms of selectivity.However, due to their specic properties, such as a well-dened pore system, pressure and thermal stability as well as their non-swellability zeolite membranes should be an alternative in the application of selective membrane separation processes. 12,13he understanding of the underlying interactions between zeolite membrane and adsorbed molecules is the key for a successful application.A ve-step step transport model has been proposed for adsorption driven zeolite membrane separation including: (1) adsorption at the external surface; (2) transport from the external surface into the pores; (3) intracrystalline transport; (4) transport from the pores to the external surface and (5) desorption from the external surface. 14,15Xiao and Wei differentiate between activated gas transport and surface diffusion during intracrystalline diffusion in microporous materials. 16According to van de Graaf the permeation ux through the zeolite membrane by surface diffusion can be expressed by eqn (1). 17 N s i is the ux of the component i in [mol m À2 s À1 ] and D s i is the diffusivity of component i in [m 2 s À1 ]; q sat,i and q i represent the saturation concentration in the zeolite in [mol g À1 ] and the occupancy in the membrane, respectively; r is the material density in [g m À3 ].
Additional to differences in occupancy/loading of components, a preferential adsorption of one component hinders the permeation of the other one leading to higher separation selectivities. 18For example, the preferential enrichment of a higher alkane from mixtures consisting of methane/ethane or ethane/isobutene was reported for silicalite-1 and composite alumina-MFI zeolite membranes, respectively. 19,20Thus, the operating parameterspressure and temperatureare considered to be the important control variables inuencing the membrane performance. 15,19The temperature essential to achieve the maximum permeation ux of a single component through the membrane increases with the adsorption strength of the molecule. 21,22The positive inuence of an increased pressure difference on the permeation ux through MFI membranes of n-butane single gas was already described by Gump et al. 23 With regard to the separation of alkanes, the principal applicability of MFI membranes could already have been demonstrated. 24,256][27] While the impact of diffusion and adsorption on zeolite membranes became clear, it was still not effective enough to reach polymer membrane performances.
For this work we used pressure stable nearly defect-free MFImembranes (Fig. S1, ESI †) with a Si/Al ratio of 270 and layer thicknesses of approximately 40 mm on inert a-Al 2 O 3 supports prepared by a two-step synthesis process. 25The customized MFI membranes (effective surface area ¼ 22 Â 10 À4 m 2 ) equipped with a MFI sublayer acting as mechanical stabilizer were used in single and mixed gas permeation experiments with methane and n-butane.The separation ability of the membrane is characterized by the so called separation factorthe molar ratio of n-butane over methane in the permeate divided by the molar ratio of n-butane over methane in the retentate.From congurational biased Monte Carlo (CBMC) simulations realistic adsorption isotherms for single as well mixed gases were computed as previously reported. 27alculated single gas adsorption isotherms of 100 vol% methane and 100 vol% n-butane, respectively, locate the loading of both components and the mechanism of the intracrystalline diffusion in such a membrane therewith (Fig. S2, ESI †).The low loading of methane and high loading of n-butane at the feed and the permeate side of a MFI membrane under operating conditions are illustrated in Fig. 1a.At increasing temperaturesfrom 293 to 348 Ka more pronounced decline in the loading of methane at the feed side is obvious, whereas temperature variation changes the feed loading of n-butane less.At lower permeate pressure and increased temperature the loading of n-butane at the permeate side is signicantly decreased, while nearly no methane molecule is adsorbed below 0.05 bar.The signicance of the loading on the permeation ux across the MFI membrane can be conjectured from a series of single gas permeation measurements accomplished at different temperatures with both, methane and n-butane (Fig. 1b and c circles), and the underlying surface diffusion model (1) (Fig. 1b  and clines).
In the case of methane (Fig. 1b), the loading slope and therewith the ux decreases with increasing temperature, whereas for the case of n-butane (Fig. 1c) the ux increases.From Fig. 1b it can be seen that reducing the permeate pressure results in an almost linear increase of the permeation ux for methane.Obviously, the process is quite temperature dependent, where an increase in temperature leads to reduction in permeation ux.At a permeate pressure of 0.01 bar the permeation uxes are 2437 L h À1 m À2 and 1850 L h À1 m À2 at 298 and 348 K, respectively.Similar temperature dependence of the permeation uxes of methane as a function of feed pressure was reported by Burggraaf et al. 22 The observed attening of the experimental permeation ux of methane (Fig. 1b, circles) at low permeate pressures is due to the feed streams used.At permeate pressures of approximately 0.2 bar nearly the whole feed stream of methane permeates through the membrane.An increase of the feed ux from 6 L h À1 to 12 L h À1 results in a nearly linear increase in permeation (Fig. S3, ESI †).In case of nbutane, due to the stronger molecular interactions with the MFI structure smaller overall permeation uxes were detected (Fig. 1c, circles).However, the reduced pressure on the permeate side improves apparently the permeation as a consequence of the enhanced surface diffusion caused by the decreased loading of n-butane molecules at the permeate side.The increase in the concentration gradient of n-butane at lower permeate pressures due to the decreased coverages at the permeate side was described as well by Gump et al. 23 In contrast to sweep gas, vacuum reduces the diffusion resistance of the permeating species.Furthermore, a higher mobility of the adsorbed molecules is favoured by the moderate temperature increase and contributes to a higher ux.Additionally, as further experiments (Fig. S4, ESI †) show, increase in the feed pressure to 2 bar gives rise to higher permeation uxes for nbutane which are still exponentially depended on the reduced permeate pressure, whereas the whole feed of methane permeates through the membrane at permeate pressure below 0.8 bar due to the higher pressure difference and the smaller molecule size.
The different permeation characteristics of both alkanes, methane and n-butane, at reduced permeate pressures have a signicant inuence on the separation of both molecules.The calculated mixed gas-adsorption isotherms for a model mixture consisting of 92 vol% methane and 8 vol% n-butane reveals the preferable adsorption of n-butane in the operating regime (see Fig. S5, ESI †).Schematic representation of the loading at the feed and permeate side is depicted in Fig. 2a.Here, the loading slope of n-butane in mixture is more pronounced in comparison to the loading slope of the respective single gas adsorption isotherms.Moreover, an increase in processing temperature leads to even higher change in loading of n-butane.For methane a practically negligible small loading in the operating pressure range is obvious.
Arruebo et al. had given an indirect evidence on the signicance of desorption of the permeating species 24 but experiments failed since pressure stable membranes were not available.Since we use pressure stable membranes 25 we clearly can demonstrate the tremendous inuence of desorption on the separation performance.Experiments with the model mixture were conducted at 298 K, 323 K and 348 K, respectively.In this case, the feed pressure was adjusted to 2 bar and a feed ow of 6 L h À1 was applied.Additionally, separation experiments with feed pressure of 1 bar were conducted (Fig. S6, ESI †).In Fig. 2b  and c the permeation uxes and the development of the separation factors are displayed.By reducing the permeate pressure and thus decreasing the loading of the preferably adsorbed nbutane molecules an exponentially improved separation is observed.
The enhanced selectivity could be attributed to the relatively higher permeation ux of n-butane as a direct result of the increased mobility due to higher loading gradients between feed and reduced permeate pressure.Moreover, a comparable exponential dependence of the permeation ux as the one observed in the single gas measurements for n-butane was found.Furthermore, moderate increase of the temperature intensies the process and leads to even higher permeation uxes above 350 L h À1 m À2 and excellent separation factors above 60 which are in the range of advanced polymer membranes. 10It is obvious that the increase in desorption is directly correlated with the enhanced permeation ux.The increased permeation ux itself is the inuencing parameter for the selectivity since more preferably adsorbed n-butane permeates across the membrane at lower permeate-pressures.
In conclusion, the impact of desorption on the separation performance of MFI-membranes is evidenced.The enhanced separation is governed to a great extent by the improved desorption of the mainly adsorbed species, representing a key aspect in the adsorptive separation of natural gas alkanes.Thus, under permeate vacuum inorganic zeolite membranes could be an alternative to polymer membranes for the separation of natural gas alkanes.

Fig. 1
Fig. 1 Single gas permeation of methane and n-butane in MFImembranes, (a) schematic representation of the loading of pure methane and pure n-butane at the feed and permeate side, and as guide for the eyes the loading across the membrane calculated from the adsorption isotherms.(b) Experimental permeation fluxes of methane (circles) and predictions of the permeation flux according to the surface diffusion model (lines) at stepwise reduced permeate pressure at feed pressure of 1 bar.(c) See (b) but for n-butane.

Fig. 2
Fig. 2 Mixed gas permeation experiments of mixture comprising 92 vol% methane and 8 vol% n-butane, (a) schematic representation of the loading on the feed and permeate side, and as guide for the eyes the loading across the membrane calculated from the adsorption isotherms; (b) permeation fluxes and (c) separation factor a C4/C1 at constant feed pressure of 2 bar at stepwise reduced permeate pressure.