Development of Mixed Matrix Membranes Containing Zeolites for Post ...

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ScienceDirect Energy Procedia 63 (2014) 160 – 166

GHGT-12

Development of mixed matrix membranes containing zeolites for post-combustion carbon capture. Nicholas Bryana, Elsa Lasseuguettea, Marion van Dalenb, Nadia Permogorovb, Alvaro Amieirob, Stefano Brandania, Maria-Chiara Ferraria* a

University of Edinburgh, School of Engineering, Edinburgh, EH9 1JL, UK b JMTC, Sonning Common, Reading, RG4 9NH, UK

Abstract Mixed matrix membranes were synthesized from poly(amide-b-ethylene oxide) (PEBAX MH1657) and zeolite 13X by a solvent casting method for CO2/N2 separation. The gas permeation properties of neat PEBAX membranes and 5, 10 and 15%(wt.) 13X loadings were determined for pure CO2 and N2 via constant volume – variable pressure method. An increase in CO2 permeability was observed with increasing loading of 13X. The greatest CO2/N2 selectivity of 47 was observed at the maximum loading. Results possibly suggest an effect on the FFV by the inclusion of the 13X.

© 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2013 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of GHGT. Peer-review under responsibility of the Organizing Committee of GHGT-12 Keywords: Carbon capture; membrane; mixed matrix membane; post-combustion carbon capture; zeolite 13X; time-lag ; gas separation; PEBAX; poly(amide-b-ethylene oxide);

1. Introduction Anthropogenic climate change is resulting in a drive towards low carbon energy sources. One option for low carbon energy is fossil fuels using post-combustion carbon capture. This option shows particular suitability in the UK due to the number of coal and gas power plants currently operating and the modular nature of post-combustion capture allowing for retrofit.

* Corresponding author. Tel.:.(+44) 131 650 5689 E-mail address: [email protected]

1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of GHGT-12 doi:10.1016/j.egypro.2014.11.016

Nicholas Bryan et al. / Energy Procedia 63 (2014) 160 – 166

Membranes have been proven as a commercially viable gas separation technology [1,2]. Separation of carbon dioxide from combustion flue gases using selective membranes shows promise to be a low energy capture option potentially offering significant energy savings over the currently more developed absorption technologies [3,4]. Mixed matrix membranes (MMMs) are membranes that are composed of polymers embedded with inorganic particles. By combining the polymers with the inorganic fillers, improvements can be made to the selectivity compared to the pure polymer membranes as well as offering a lower cost alternative and improved handling properties relative to inorganic membranes [5]. This improvement over pure polymeric membranes is significant because polymeric membranes used for gas separation exhibit a trade-off between selectivity and permeability. Polymeric membranes are limited by this tradeoff and very few have been found to exceed what is known as the Robeson bound [6], an empirical upper limit on the selectivity for a given permeability specified for gas pairs, in this case carbon dioxide and nitrogen. Figure 1 shows this limit along with a collaborative database of polymer permeation data.

Figure 1. Robeson plot showing available literature data on polymer selectivity and permeability (blue circles)[7], the current upper bound (red line) and a prediction of the properties of a mixed matrix membrane produced from a hypothetical polymer/filler combination (green triangles, loading of filler increasing from 0-40% (vol.)).

Several bases on which gas transport in polymeric membranes is altered by the addition of inorganic fillers exist. The fillers can alter the packing structure and free volume of the polymer, hence altering permeation properties; the filler is usually chosen for its selectivity towards one species. Whilst mixed matrix membranes show potential to be novel materials for energy efficient carbon capture there are several challenges to overcome in the design and synthesis before their wider use can become a reality. One of the largest challenges in designing MMMs is material selection. Given the vast number of polymers and fillers choosing compatible materials that will combine advantageously is no easy feat. Numerous models to predict the resultant MMM properties based on that of their constituent materials exist and continue to be developed. Highlighted in figure 1 is the prediction of the Maxwell model for the combination of a high selectivity polymer with a hypothetical filler, from no loading (neat polymer) up to 40% (vol.) loading of filler, demonstrating the possible benefits of such a composite material. The Maxwell equation (eq. 1) [8] was initially developed to predict the permittivity in a heterogeneous dielectric, however, this problem is analogous to gas permeation through heterogeneous membranes and as such the Maxwell equation can be applied to gas by replacing the dielectric properties with that of the permeability of the continuous (polymer) and discontinuous (filler) phases [9].

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(Eq. 1)

The nondimensionalised solution to the Maxwell equation is shown in figure 2. From the non-dimensional solution, it can be seen that as the permeability of the discontinuous phase (the filler) increases relative to that of the continuous phase (the polymer) (Pd/Pc = Ȝd), an asymptote exists where at a given loading, ‫׋‬, the effective permeability, Peff, of the composite phase shows little increase. This implies that no matter how permeable a filler may be, there is a limit to how permeable the overall composite membrane can be made compared with the neat polymer (Pr = Peff/Pc). This is significant as it restricts the choice of polymers that may make successful mixed matrix membranes in applications where high throughput are required to that which already have a high permeability. Models such as the Maxwell model are useful, however are often inaccurate for a wide range of material combinations and can only be applied to low filler loading.

3

P r = P eff/P c

2.5

2

1.5

1 30 0.4

20

0.3 0.2

10 0.1 Pd/Pc

0

0

load

Figure 2. Solution to the non-dimensionalised Maxwell equation.

The key factors that dictate any membrane gas separation process are the membrane area, pressure ratio, permeance (which is directly linked to permeability and thickness) and selectivity [10,11]. Hence the main properties that are significant factors in designing MMMs are permeability and selectivity. Properties such as resistance to impurities and water, ageing and mechanical properties which relate to how thin the membrane can be made are also important as they can significantly affect the transport properties and the productivity of the system. Rough targets for the permeability and selectivity to facilitate economically feasible carbon capture are usually considered in the range of 1000 to 10,000 Barrer and 50-100 respectively. Arguably counter-intuitively it is not desirable for the selectivity to be too high (approximately greater than 100) [3,12]. A membrane of too high selectivity can require a greater membrane area to achieve the same degree of separation due to the high concentration of CO2 on the permeate side reducing the driving force for CO2 transport. The interface between the polymer and inorganic layer is also the source of a number of technical challenges in fabricating membranes. Defects at the interface such as voids, polymer rigidification and blockage of the filler pores are common problems cited in literature [13] and it is essential these problems are not overlooked when developing a membrane. Zeolite 13X has been studied extensively for carbon capture as a solid adsorbent [14,15]. It shows significantly favorable adsorption characteristics for the separation of CO2 from N2. This selective sorption implies 13X may be promising material for use in a mixed matrix membrane to increase both permeability and selectivity of the polymer.


PEBAX has previously been studied as a membrane for gas separation both with and without fillers or additives. [16–22]. Previous work has highlighted the high selectivity (around 50 at 35 ºC [21]) for polar/non-polar gas separations such as CO2/N2 due to the affinity between polar gases and polyether segments [21]. Typical CO2 permeabilities of neat PEBAX membranes are in the region of 70-90 Barrer [17,18,22]. While this could potentially present some process configuration challenges to make its use economically feasible for carbon capture due to the high volume of the flue gas to be treated, this study focuses on the effects of the addition of a highly selective filler to a selective polymer. 2. Experimental 2.1 Materials PEBAX MH1657, poly(amide-b-ethylene oxide) (Pebax, Arkema) was kindly provided by Arkema. PEBAX is a block copolymer comprising of polyamide blocks and polyether blocks as seen in figure 3. The crystalline amide block acts as a dense phase while the ether block acts as amorphous permeable regions due to its high chain flexibility [18].

Figure 3. Chemical structure of PEBAX where PA is an aliphatic block and PE is a polyether block [16].

Zeolite 13X was obtained from Sigma Aldrich. The particle size distribution of the 13X was determined using a Mastersizer 2000 at Johnson Matthey Technology Centre and are found in Table 1. Table 1. Particle size distribution of zeolite 13X Percentage mass with a particle size less than x (%)

x (ȝm)

10

1.778

50

3.973

90

11.461

2.2 Membrane preparation Prior to use, the 13X was dried under vacuum at 120 °C for 8 hours to remove any moisture present in the crystals. Meanwhile a 10% (wt.) PEBAX polymer solution in a 70% ethanol: water mixture was prepared at 70-80 ºC. Once the polymer was fully dissolved, the 13X was added and stirred for a further 4 hours. A waring blender was subsequently used immediately prior to casting to ensure a homogeneous mixture and to remove any agglomerations of 13X. The membranes were then cast onto level glass plates using a casting knife and left to dry for 24 hours under ambient conditions. The membranes were then conditioned under vacuum at 120 ºC for 16 hours to remove any remaining solvent.

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2.3 Permeation experiment The permeation properties of the PEBAX-13X MMMs were tested using the constant volume - variable pressure method in an in-house built time-lag apparatus of which a schematic can be seen in figure 4a.

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Figure 4. (a) Schematic of the constant volume – variable pressure apparatus, and (b) an example result showing the experimental results (green solid line) and fit of the linear section (red dashed line).

The thickness of the membranes was determined with a digital micrometer (Mitutoyo). The downstream pressure was measured using a MKS Baratron pressure transducer. A typical experimental curve can be seen in Figure 4b. The permeability can be determined from the linear section of the plot: the gradient at this point can be related to the flux through the membrane. The time lag, ߠ, is the x-intercept of the fit to the linear section and is used to determine the diffusivity. 3. Results and Discussion Repeat measurements were made for each sample to ensure reproducibility. The pure gas permeabilities and diffusivities are shown in tables 2a and 2b respectively. The CO2 permeability increased with the loading of 13X in the PEBAX as can be seen in figure 5 while the trend in N2 permeability is less obvious leading to small variations in selectivity. Relative to the variation in CO2 permeabilities, there is little change in N2 permeability and the greater change in CO2 permeability with loading agrees with the stronger interaction of CO2 with 13X compared to N2. Table 2a. Pure gas permeability and ideal selectivity of CO2 and N2 13X Loading (% wt.) 0 10 15

PCO2 (Barrer) 1 2 81.4 104 114

82.2 103 112

PN2 (Barrer) 1 2 2.01 3.01 2.43

1.98 3.10 2.40

Table 2b. Pure gas diffusivities of CO2 and N2

ĮCO2/N2 (-)

13X Loading (% wt.)

41 39 47

0 10 15

DCO2 (x10-8 cm2 s1) 1 2 82 485 168

76 100 173

DN2 (x10-8 cm2 s1) 1 2 62 50

381 91 53

An additional sample with 5% loading was prepared that showed greater permeabilities than the other samples and a large decrease in selectivity. This was attributed to the presence of larger defects at the interface between the 2 phases; even if the preparation procedure was the same, this membrane was considerably thinner than that of the other loadings (35ȝm compared with 58, 82 and 75ȝm for 0, 10 and 15% loadings respectively). Further analysis with SEM will be able to confirm this hypothesis. It was also noted that after exposure to high temperature (135°C under vacuum for 10 hours) there was a significant drop in both CO2 and N2 permeability and an increase of


Ϯ

Figure 5. Pure gas permeabilities of CO2 and selectivity of membranes

selectivity as can be seen in table 3. It is suspected that the exposure to high temperature accelerated the aging process of the polymer and a collapse of the free volume was observed. The neat PEBAX showed no such change in permeability further suggesting an alteration of the polymer chain packing and free volume of the membrane caused by the inclusion of 13X, however further work is required to confirm this. 13X is highly hydrophilic and is usually regenerated at high temperatures and as such the effects observed here may hold strong implications for the use of 13X in mixed matrix membranes. Table 3. Permeabilities of CO2 and N2 before and after exposure to high temperature

As prepared Post thermal treatment

PCO2 (Barrer) 1 2

PN2 (Barrer) 1 2

455 140

249 8.2

448 130

225 8.0

ĮCO2/N2 (-) 2 17

4. Conclusions Mixed matrix membranes with different loadings of Zeolite 13X in PEBAX MH1657 were successfully prepared. Single gas permeation experiments showed an increase in the CO2 permeability with increasing loads of particles in the film while the selectivity seems not so greatly affected. This could also infer the introduction of some voids at the interface between particles and continuous phase. These defects were observed in the case of a thinner sample that also showed great sensitivity to temperature but further research is needed to fully investigate this effect. Acknowledgements We thank Johnson Matthey and the EPSRC for the CASE award that supports NB and the UKCCSRC for the funding of the permeation cell apparatus. References [1] [2] [3]

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