Propane Mixture Separation Characteristics and Stability of

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Propylene/Propane Mixture Separation Characteristics and Stability of Carbon Molecular Sieve Membranes Xiaoli Ma, Suzanne Williams, Xiaotong Wei, Jay Kniep, and Jerry Y.S. Lin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02721 • Publication Date (Web): 16 Sep 2015 Downloaded from http://pubs.acs.org on September 23, 2015

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Industrial & Engineering Chemistry Research

Propylene/Propane Mixture Separation Characteristics and Stability of Carbon Molecular Sieve Membranes Xiaoli Maa, Suzanne Williamsa, Xiaotong Weib, Jay Kniepb, Y.S. Lina,* a-

School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287 b-

Membrane Technology and Research, Inc., Newark, California 94560

Abstract: Propylene/propane mixture separation properties of high quality carbon molecular sieve (CMS) membranes prepared on mesoporous γ-alumina supports were studied at different feed pressures (up to 100 psia), feed compositions and permeation temperatures. The membrane exhibits a mixture propylene/propane selectivity of above 30 under the studied conditions, with no plasticization effect observed at feed pressures up to 100 psia (~700 kPa). Temperature dependences of the permeance and selectivity suggest a diffusion dominated mechanism for the permeation and separation of propylene/propane through the CMS membrane. The propylene/propane separation characteristics tend to stabilize after a decline in gas permeance and an increase in propylene/propane selectivity while on stream of propylene/propane mixture for 10 days. The initial changes in separation properties of the CMS membrane are due to the reduction in the effective pore size caused by oxygen chemisorption and physical aging of the pore structure. CMS membranes can be pre-aged under proper conditions to stabilize the structure and separation performance for practical separation application.

Keywords: CMS membrane, propylene/propane separation, stability, adsorption, physical aging.

* Corresponding author. Tel.: +1 480 965 7769; fax: +1 480 965 0037. E-mail address: [email protected] (Y.S. Lin).

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1. Introduction Propylene/propane separation is traditionally performed by a highly energy-intensive cryogenic distillation process in columns containing over 200 trays,1-3 due to the close boiling points of propylene and propane. Membrane-based technology has emerged as a more energyefficient alternative for propylene/propane separation.4,5 The membrane candidates that have been explored include polymeric,5,6 inorganic7-9 and facilitated transport membranes.10,11 Among them, the performance of polymer membranes is limited by the upper-bound trade-off between permeability and selectivity and plasticization issue,6 while facilitated transport membranes suffer from the long term stability problem due to carrier poisoning. Therefore, recent efforts have been devoted to developing more thermally and chemically stable microporous inorganic membranes including ZIF-8,12,13 hybrid silica14 and carbon molecular sieve (CMS) membranes.15-17 Despite the differences in membrane materials and micropore structure, these inorganic membranes show comparable separation performance in terms of propylene permeance and propylene/propane mixture selectivity.18 However, CMS membranes offer better processability to form defect-free asymmetric hollow fiber or thin-film composite membranes than ZIF-8 and silica membranes. This advantage makes CMS membrane a more cost-effective option for propylene/propane separation than other inorganic membranes. Most studies on CMS membranes have focused on improving the structure of polymer precursors and optimizing the pyrolysis parameters.19-21 Only a limited number of studies have reported propylene/propane permeation and separation properties of CMS membranes. Gas transport through microporous carbon materials can be described by the solution-diffusion mechanism. Adsorption of propylene and propane onto CMS materials shows similar equilibrium properties, but considerably different kinetics.22-24 This indicates a diffusion controlled mechanism for the separation of propylene and propane by CMS sorbents or membranes. For example, CMS membranes derived from a 6FDA-based polyimide precursor prepared by Koros’ group only exhibit a propylene/propane sorption selectivity of around 1.1; however, the diffusivity selectivity is as high as 90, resulting in an ideal membrane selectivity up to 100.15 Hayashi et al.16 studied the temperature dependence of the single-component permeance of propylene and propane through a CMS membrane produced with a BPDA-pp′ODA polyimide 2


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precursor. The obtained activation energy for permeation is positive and comparable to those for the other non-hydrocarbon gases such as oxygen and nitrogen, which is consistent with the activated diffusion dominated mechanism for the permeation of propylene and propane. Similar temperature dependence was reported on a CMS hollow fiber membrane fabricated by Okamoto et al.,17 with the permeance increasing and the selectivity decreasing with temperature. Operational stability is another important characteristic to be considered for the implementation of membrane separation technology. Although thermally and chemically resistant,25,26 some CMS membranes have been reported to suffer sorption-induced instability issues. Jones and Koros27 found that significant membrane performance losses in terms of both permeance and selectivity occurred with a feed stream containing organic contaminants as low as 0.1 ppm. Water adsorption has also been reported to reduce membrane performance;28 however, this effect is reversible and can be avoided through coating a hydrophobic polymer layer on the membrane surface.29 Menendez and Fuertes30 reported a rapid loss of permeance with time for their phenolic resin derived CMS membrane during storage in dry air conditions. Such change in gas permeance was believed to arise from the chemisorption of oxygen from air, because much lower permeance losses were observed for their membranes stored in nitrogen. It was recently discovered by Koros’s group that CMS membranes derived from high fractional free volume polymer precursors experience pore structural change due to physical aging, similar to glassy polymer membranes.32 As the CMS ages, pores shrink over time to the thermodynamically more stable state, causing a reduction in the gas permeance. However, CMS membranes after several months of aging still exhibit stable ethylene/ethane mixture separation performance for at least 115 hours. In another study, Koros and co-workers33 found that a feed of 50% CO2/50% CH4 mixture is able to suppress the physical aging of CMS membranes through an inhibition effect on the structure relaxation brought by the adsorbed highly condensable CO2 in the micropores. Propylene/propane separation is often operated at 30-40 oC on the output of refinery steam crackers or on petrochemical purge streams, both of which are typically at elevated pressures (100-150 psia) with varied propylene/propane compositions. The separation properties of CMS membranes under these critical industrial operating conditions have not been reported in literature. Furthermore, despite some studies on the stability of CMS membranes, the long term 3



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stability of CMS membranes under propylene/propane on-stream separation condition is still unclear. The objectives of this work are to understand the effects of several important operation parameters on the propylene/propane mixture permeation and separation properties of CMS membranes, and to study the on-stream stability including aging mechanism and accelerated preaging treatments on stability improvement of CMS membranes.

2. Experimental Section 2.1 Membrane synthesis and characterization The membrane synthesis procedures followed those reported earlier.7,18 α-alumina supports were first prepared by the pressing/sintering technique, followed with the coating of γalumina layer by a sol-gel method.34,35 The 6FDA-based polyimide polymer films were coated onto the surface of the mesoporous γ-alumina layer by the dip-coating method. After drying, the supported polymer films were pyrolyzed in an ultra high purity argon gas atmosphere using an oxygen removal-assisted pyrolysis setup to form the final CMS membrane. The surface and cross-section morphology of the CMS membranes were characterized by scanning electron microscopy (SEM, Philips, XL 30) at an accelerating voltage of 20 kV. The thicknesses were determined from the cross-section SEM images. FTIR spectra for γ-alumina support and CMS membranes were collected by a Nicolet 4700 FT-IR spectrometer. The spectrum of γ-alumina support was used as the background for the measurement of CMS membranes. Raman spectra were collected using a custom built Raman spectrometer. The CMS membrane samples were excited using a 150 mW Coherent Sapphire SF laser with a 532 nm laser wavelength. The peak area for each band in Raman spectra was calculated using Origin software.

2.2 Gas permeation and separation measurements The performance of the CMS membranes for the propylene/propane mixed-gas separation was measured using a previously reported cross-flow setup36,37 with modifications, as shown in Figure 1. During testing, the feed was a propylene/propane mixture at a total flow rate of 50 mL/min, and the permeate side was swept by nitrogen at a flow rate of 50 mL/min. On the feed side, the gas flow rates for propylene and propane were controlled by two mass flow 4


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controllers (MKS Instruments 1179A series), so as to control the feed composition. The total pressure was controlled by a needle valve on the retentate line. The gas compositions on both feed side and permeate side were measured by gas chromatography (SRI Instruments, SRI 8610C) equipped with a flame ionization detector (FID) and a 6'×1/8" silica gel packed column, and with helium as carrier gas. The temperature of the membrane cell was controlled by a temperature programmed furnace (Lindberg/Blue, Model#TF55030A). For the study on the effect of feed pressure, the total feed pressure was changed from 32 to 100 psia at a feed composition fixed at 50/50 and room temperature. The feed composition was varied by the flow meter, with a total feed pressure controlled at 30 psia to study the effect of feed composition on separation properties. A temperature range from room temperature to 100 °C was used to study the temperature dependence of separation performance. All the gas permeation measurements were conducted two or three times to determine the average values of permeance and selectivity. The measurement errors are within the range of 3–10%.

2.3 Stability test Fresh CMS membranes were used for each stability test. One membrane was used for the on-stream propylene/propane separation performance stability study. The feed is a 50%/50% propylene/propane mixture with a total feed pressure of ~30 psia at a total flow rate of 50 mL/min, and nitrogen at a flow rate of 50 mL/min was used as sweeping gas. Both permeances and selectivity were measured during the studied time period. The stability of 50%/50% helium/nitrogen system was also studied using another fresh membrane. The same setup was used except that argon gas was used as the sweeping gas for the helium/nitrogen system. The gas composition on the permeate side for the helium/nitrogen system was measured by a GC (Agilent 6890N) with a TCD detector and an AllTech HayeSep DB 100/120 column. Two additional membranes were used to study the effect of oxygen concentration on the aging. The first membrane was subjected to industrial grade nitrogen with 500 ppm oxygen, and the nitrogen single gas permeance was continuously measured. The second membrane was exposed to dry air for a certain amount of time, during which the nitrogen single gas permeance was monitored over the aging time.

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2.4 Regeneration and pre-aging of membranes Membrane regeneration was conducted through heat treatment under two different conditions: (1) 120 °C for 24 h in ultra-high purity argon or vacuum; (2) 300 °C for 24 h in a 5% H2/95% Ar atmosphere. The pre-aging of the membrane was conducted in dry air and propylene/propane mixture stream at different temperatures.

3. Results and Discussion 3.1 Propylene/propane mixture separation Figure 2 shows the typical surface (a) and cross-section (b) SEM images of the CMS membranes synthesized in this work. The asymmetric structure of the composite membranes can be clearly seen in the cross-section image. The intermediate mesoporous γ-alumina, with a pore diameter of ~3 nm and a surface roughness of ~4 nm, provides negligible gas transport resistance as compared to that through the top CMS layer. The CMS membrane layer has a pore diameter of ~4.0 Å, as estimated from the gas permeation cutoff method,7 offering strong molecular sieving effect toward propylene/propane separation. All the membranes prepared are of high quality and exhibit a propylene/propane mixture selectivity above 20. It should be noted that the separation performance of the CMS membrane is not stable during long term tests. As will be discussed later, the membrane aged significantly faster in the first few days. In order to minimize the effect of aging on the change of permeance and selectivity, the membrane was stored in pure nitrogen for 3 days prior to the study on separation performance under different conditions and all the measurements were completed within 2 days. Figure 3 shows the effect of feed pressure on propylene and propane permeances and the propylene/propane mixture selectivity of the CMS membrane. Both the permeance and selectivity slightly decrease with increasing feed pressure. Similar pressure dependence was observed for single component propylene and propane permeance. Gas permeation (P) through microporous inorganic membranes is controlled by diffusivity (D) and sorption properties or solubility (S): P=D×S. For permeating gas with non-linear adsorption isotherm for the membrane material, the solubility term can be considered the average slope of the adsorption isotherm between the permeate and feed side partial pressure for the permeating gas25. At a given 6


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temperature, diffusivity mainly depends on the molecular size of the permeating gas relative to the pore size of membrane and thus can be assumed to be independent of the feed pressure. The solubility term (or average slope of the adsorption isotherm) generally decreases with increasing feed pressure due to the fact that the adsorption isotherms of propylene and propane on CMS membranes can be described by Langmuir isotherm equation.15,21 Thus, with the increase of feed pressure, the solubility term for both gases decreases, resulting in a decrease in gas permeance. The decreased selectivity with increasing feed pressure suggests that propylene experiences a larger decrease in solubility than that for propane. It should be noted that polymer membranes usually have a plasticization problem upon exposure to hydrocarbon gases at high pressure.2 In contrast, due to the high rigidity of the inorganic pore structure, the CMS membrane prepared in our work exhibits good separation performance even at feed pressures up to 100 psia (~700 kPa). Figure 4 shows the effect of feed composition on the separation performance. At a given total feed pressure, an increase in feed propylene composition corresponds to an increase in the partial pressure of propylene or a decrease in the partial pressure of propane on the feed side. As shown, with the increase of propylene fraction on the feed side, the propylene permeance slightly decreases due to the reduced sorption coefficient with the increase of propylene partial pressure. At the same time, the propane permeance increases because of the increase of sorption coefficient as a result of decreasing propane partial pressure on the feed side. Consequently, the propylene/propane selectivity decreases with the increase of propylene fraction in the wide feed composition range studied in this work. Figure 5 shows the effect of operation temperature on permeance and selectivity. It can be seen that the separation performance shows a strong dependence on the operation temperature. With the increase of temperature, the permeance for both propylene and propane increases and the selectivity decreases. The propylene permeance at 100 °C is about 4 times that at 25 °C while still maintaining a mixture selectivity of about 14. Activation energy for permeation, estimated from the Arrhenius plots of the permeance data, is 15.3 and 25.4 kJ/mol for propylene and propane, respectively. Increasing the permeation temperature will increase the diffusivity but decrease the sorption, and the combined effects determine the temperature dependence of permeance. The increasing permeance with temperature suggests that the activation energy for diffusion for propylene and propane is larger than their heat of adsorption, i.e., the diffusion is 7



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dominating the permeation and separation of propylene/propane through the CMS membrane prepared in this work. The above temperature dependence for propylene/propane permeation/separation performance of CMS membrane is different from that for ZIF-8 membrane. Lai and co-workers8 found that the propylene permeance of ZIF-8 membranes decreases almost linearly by about 40% as the temperature increases from room temperature to 120 °C, because the heat of adsorption for propylene is larger than the activation energy for diffusion. However, the activation energy for diffusion for propane is larger than the heat of adsorption, resulting in an increasing propane permeance and consequently a decreasing mixture selectivity with temperature. Similar temperature dependence of propylene and propane permeance for ZIF-8 membrane was reported by Lin and co-workers.12 For CMS membranes, the permeance for both propylene and propane increases with increasing temperature, making CMS membranes a more favorable option for high temperature operation.

3.2 On-stream stability of CMS membrane Fresh CMS membranes were used for the stability studies. Figure 6 shows the stability of separation performance of one CMS membrane for a 50%/50% propylene/propane feed with a total pressure of ~30 psia at room temperature. As shown, the permeance for both propylene and propane decreases with on-stream time. The decline in the permeance for propane is faster than that for propylene, resulting in an increase in the mixture selectivity with on-stream time. The permeance for both propylene and propane tends to level off after on-stream for approximately 10 days (240 h). The permeation/separation stability of a CMS membrane exposed to a helium/nitrogen mixture was also tested using another fresh membrane. These results are shown in Figure 7. A similar decrease in the permeance for He and N2 and increase in He/N2 selectivity were observed, further confirming the change of pore structure or pore surface property of the CMS membranes prepared in our work while on stream. There are three possible reasons for the observed changes in permeance and selectivity of CMS membranes while on stream: (1) adsorption of water vapor, (2) oxygen chemisorption, and (3) physical aging. Physical adsorption of water vapor or other larger hydrocarbon species can narrow or block the pores for CMS membranes. As reported in literature,28 adsorption of water 8


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vapor only takes place when the relative humidity reaches a certain value. This is unlikely to happen in our measurement due to the fact that the propylene/propane mixture only contains water and other impurities at ppm levels. Furthermore, adsorption of water vapor usually causes a reduction in both permeance and selectivity as opposed to the increase in selectivity observed in this work. Also, physical adsorption of water and other hydrocarbon species is reversible. In this work, the permeance and selectivity of the aged membranes could not be recovered after a heat treatment at 120 °C for 24 h in ultra high purity argon or vacuum. Thus, reason (1) can be excluded for the permeation/separation property change of the CMS membranes while on-stream of a propylene/propane or helium/nitrogen mixture. It is known that oxygen can be chemisorbed on active carbons.38-40 To examine if this takes place on our CMS membrane, FTIR spectra were collected before and after the 5 days long helium/nitrogen mixture separation test. As shown in Figure 8, the fresh CMS membrane exhibits a typical band at 1605 cm-1 ascribed to C=C bond, and a tiny band at 1723 cm-1 coming from the residual C=O groups on the membrane surface. The relative intensity of C=O to C=C bond increases after the aging, suggesting the formation of additional C=O groups from the chemisorption of oxygen onto the surface of CMS membrane. The effect of oxygen concentration in the gas stream on the on-stream property change of CMS membranes was studied by monitoring the nitrogen permeance of two fresh membranes with oxygen containing N2 streams of ~30 psia with different oxygen concentrations: (1) industrial grade nitrogen gas with an oxygen concentration of ~500 ppm; and (2) dry air with 21% oxygen. Figure 9 shows the normalized nitrogen permeance as a function of on-stream time for the membranes. The nitrogen permeance for the membrane with the 21% oxygen feed declines much faster than the membrane with 500 ppm oxygen feed. The decline in nitrogen permeance and the strong dependence of the decline on oxygen concentration in the permeating stream suggest that oxygen chemisorption on CMS narrows the effective pore size of the CMS membrane. Physical aging of CMS membranes causing the pore structural change was recently discovered by Koros and co-workers.32 Analogous to the unrelaxed free volume in glassy polymers, the micropores in CMS membranes derived from large free volume polymers tends to shrink over time to reach a thermodynamically more stable state, resulting in a decrease in permeance and an increase in selectivity. The 6FDA-based polymer precursor used in our work 9



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also possesses a relatively large free volume. Moreover, the rate of the change in permeances and selectivity shown in Figures 6 and 7 is similar to that reported by Koros and co-workers, suggesting that physical aging is the main reason for the change of permeation and separation properties of our CMS membrane. To further confirm this, Raman spectroscopy was carried out on the CMS membrane before and after aging in pure nitrogen stream for one week. As shown in Figure 10, both spectra show the so-called G and D bands locating at around 1580 and 1350 cm-1, respectively. The graphite-like G band is caused by the bond stretching of all pairs of sp2 atoms in both rings and chains.41 The D band, which is not present in single crystals of graphite, originates from symmetry breaking occurring at the graphene edges in sp2 carbon materials containing porous or impurities defects. The ratio of D band to G band can be used as some measure of the disorder within the carbon structure.41-43 After storage in industry grade nitrogen for one week, the peak area ratio A(D)/A(G) of the CMS membrane decreases from 1.81 to 1.58, suggesting the formation of a more orderly structure driven by the physical aging. Figure 11 shows the time dependence of the normalized nitrogen permeance of the membrane aged in flowing dry air at room temperature for a longer time period, and the permeance after regeneration. The nitrogen permeance tends to level off over time, and eventually stabilize after ~950 h of aging. Propylene/propane separation performance data for the CMS membrane after being aged in dry air for 1200 h is given in Figure 12. As shown, an unchanged propylene/propane separation performance was also observed for at least three days for this pre-aged membrane. It is expected that for sufficiently long pre-aging oxygen chemisorption would stop after all the active sites on the membrane were consumed. Physical aging, as demonstrated in Koros’s work for their 6FDA/BPDA-DAM derived CMS membrane,32 can also reach the stable state after several months. This aged membrane was regenerated through a heat treatment at 300 °C for 24 h in a 5% H2/95% Ar atmosphere. Nitrogen permeance was partly recovered to 83% of the fresh permeance by the heat treatment under reducing environment, which decomposed the surface oxygen containing groups formed from oxygen chemisorption. The remaining 17% unrecovered permeance is mostly likely due to the permanent structure change caused by physical aging of the membrane and/or the incomplete elimination of C-O surface groups.

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The results given in Figure 11 and Figure 12 show that pre-aging in air for 1200 h has largely stabilized the CMS membrane structure. For this particular membrane, however, the final selectivity is low and unsuitable for practical application. The rate of oxygen chemisorption onto activated carbon is dependent on not only the oxygen concentration, but also the temperature. Higher oxygen concentration and higher temperature are expected to accelerate the aging caused by oxygen chemisorption. On the other hand, we believe that a higher temperature will also accelerate the physical aging of the CMS membrane as high temperature facilitates the structural rearrangement of the imperfectly packed graphene sheets in the membrane. To shorten the pre-aging time, a higher oxygen concentration and/or higher temperature were used to pre-age the fresh CMS membranes. Membrane performances after the pre-aging under different conditions are listed in Table 1. Heat treatment in dry air at 50 and 100 °C greatly accelerates the aging but results in an extremely low propylene permeance due to excessive oxygen chemisorption. In contrast, aging a fresh CMS membrane in a propylene/propane stream at 100 °C for 144 h and 120 ºC for 24 h leads to a membrane with reasonably good propylene permeance. The membrane pre-aged at this condition shows relatively stable propylene/propane separation performance for at least 4 days, as shown in Figure 13. In the subsequent on stream test at room temperature, the performance of this membrane did not change within 3 days. Although longer testing time is necessary to evaluate the long term stability of this membrane, these results have indicated that a heat treatment in gas streams with a low oxygen concentration is a desirable approach to accelerate the aging while maintaining a minimum degradation in the separation performance.

4. Conclusions The propylene/propane permeance and selectivity of CMS membranes change slightly with the feed pressure up to 100 psia and mixture composition. CMS membranes do not show plasticization effects with a propylene/propane mixture feed pressure up to 100 psia (~700 kPa). However, permeation/separation properties of CMS membrane depend strongly on temperature, with the propylene permeance increasing by about 3 times as the temperature increases from 25 to 100 °C. The permeance for both propylene and propane declines, and the propylene/propane selectivity slightly increases with time while exposed to a gas mixture within the first 10 days, 11



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after which the separation characteristics tends to stabilize. Similar time dependence of permeance and selectivity are observed while on stream of He/N2 mixture. Such changes in the membrane performance are due to a reduction in the effective pore size of the membrane as a consequence of oxygen chemisorption onto surface carbon active sites and physical aging of the micropore structure. Heat treatment in an atmosphere with a low oxygen concentration can effectively accelerate the aging and largely stabilize the separation performance with minimal reduction in propylene permeance.

Acknowledgement The authors would like to acknowledge the support of the National Science Foundation (IIP-1127395) for this project.

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Page 17 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Schematic diagram of the cross-flow membrane separation setup for studying C3H6/C3H8 mixture gas separation properties of CMS membranes under different operation conditions.

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

(b)

Figure 2. Surface (a) and cross-section (b) SEM images of a γ-alumina supported CMS membrane.


Page 18 of 31

Page 19 of 31

Feed pressure, kPa 200

300

400

500

600

700

-8

10

60 C3H6

2

50 -9

10

40 C3H6/C3H8 30 C3H8

-10

10

Selectivity

Permeance, mol/Pa·s·m

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 10

20

40

60

80

100

Feed pressure, psia

Figure 3. Effect of feed pressure on the C3H6/C3H8 separation performance for CMS membrane at 25 °C with a 50%/50% C3H6/C3H8 feed.



-8

10

60

-9

50 40

10

C3H6/C3H8 30 C3H8

-10

10

Selectivity

2

C3H6


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

20 10

0

20

40

60

80

100

Propylene fraction, %

Figure 4. Effect of the feed side propylene fraction on the C3H6/C3H8 separation performance of a CMS membrane at 25 °C and a feed pressure of 30 psia.


Page 21 of 31

40 C3H6 10

30 C3H8

-9

10

C3H6/C3H8

20

Selectivity

2

-8


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-10

10

10 20

40

60

80

100

Temperature, C

Figure 5. Temperature dependency of gas permeances and C3H6/C3H8 mixture selectivity for a CMS membrane with a 50:50 C3H6/C3H8 feed gas mixture and a feed pressure of 30 psia.



-9

8.0x10

40

6.0x10

C3H6 -9

4.0x10

C3H6/C3H8

30

-10

4.0x10

Selectivity

2

-9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

20 C3H8

-10

2.0x10

10 0

50

100

150

200

250

300

Time, h

Figure 6. Time dependence of the on-stream C3H6/C3H8 separation performance of a CMS membrane for a 50:50 C3H6/C3H8 feed gas mixture with a feed pressure of ~30 psia at room temperature. .


Page 23 of 31

80 -7

2

1.6x10

He

60

He/N2 -7

1.5x10

40 -9

5.0x10

-9

4.0x10

N2

Selectivity


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

-9

3.0x10

0

20

40

60

80

100

120

Time, h

Figure 7. Time dependence of the on-stream He/N2 separation performance of a CMS membrane for a 50:50 He/N2 feed gas mixture with a feed pressure of ~30 psia at room temperature.



Fresh

Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

C=O

C=C

Aged

4000

3000

2000

Wavenumber, cm

1000

-1

Figure 8. FTIR spectra of a CMS membrane before and after aging in a 50:50 He/N 2 gas stream for 5 days.


Page 25 of 31

1.0

Normalized N2 Permeance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


~ 500 ppm O2 0.8

0.6 21% O2 0.4

0.2 0

40

80

120

Time, h

Figure 9. Effect of the oxygen concentration on the stability of the N2 permeance through CMS membranes.



Fresh

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

Aged

800

1200

1600

Raman shift, cm

2000 -1

Figure 10. Raman spectra of a CMS membrane before and after aging in a pure nitrogen stream for one week.


Page 27 of 31

1.2

Normalized N2 Permeance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Regeneration

Aging in 21% O2

1.0 0.8 0.6 5% H2,

0.4

300C/24h

0.2 0.0 0

400

800

1200

1410

Time, h

Figure 11. N2 permeances of a CMS membrane at fresh state, different aging time in dry air, and after regeneration in a reducing atmosphere.



20 -10

6.0x10

C3H6

15

-10

4.0x10

10

-10

C3H6/C3H8

2.0x10

Selectivity

2

Permeance, molPasm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

5

0 0

20

40

60

Time, h

Figure 12. Stability of on-stream C3H6/C3H8 separation performance of a CMS membrane after being aged in dry air at room temperature for 1200 h (50 days).


Page 29 of 31

80 -8

2

1.0x10

C3H6

60

-9

8.0x10

40 -9

6.0x10

20

C3H6/C3H8

-9

4.0x10

Selectivity


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

-9

2.0x10

0

20

40

60

80

100

120

Time, h

Figure 13. Stability of on-stream C3H6/C3H8 separation performance at 120 ºC of a CMS membrane pre-aged in C3H6/C3H8 stream.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 31

Table 1. Performance of CMS membranes after pre-aging at different conditions. Pre-aging conditions

Performance after pre-aging

Dry air

21%

50 °C/24 h

7.15×10-10 at RT

Further decline in C3H6 permeance ~37% in one day

Dry air

21%

100 °C/24 h

5.11×10-10 at RT

~15% in one day

C3H6/C3H8

~500 ppm

100 °C/24 h

2.24×10-8 at 100 °C

~20% in one day

C3H6/C3H8

~500 ppm

100 °C/144 h +120 °C/24 h

8.60×10-9 at 120 °C

~6% in one day

Gas stream

Oxygen Temperature/aging concentation time

C3H6 permeance mol/Pa∙s∙m2


Page 31 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Only 254x90mm (150 x 150 DPI)


Propane Mixture Separation Characteristics and Stability of

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