Elsevier

Chemical Engineering Journal

Volume 361, 1 April 2019, Pages 736-750
Chemical Engineering Journal

Synthesis of novel regenerable 13X zeolite-polyimide adsorbent foams

https://doi.org/10.1016/j.cej.2018.12.096Get rights and content

Highlights

  • High performance polyimide foams containing 80% 13X were successfully fabricated.

  • Adsorbent foams have excellent regenerable properties.

  • 10k polyvinylpyrrolidone pore-former improved accessibility to zeolites.

  • CFD models allowed molecular behaviour in complex foam structures to be understood.

Abstract

A new generic synthesis method is presented for the production of a polyimide (PI)/adsorbent (80 wt% 13X zeolite) regenerable foam filter. The method uses a dual parallel reaction foaming process comprising CO2 generation (blowing) and polymerisation reactions. The paper describes the development of the foam structure and its characterisation in the context of removing CO2 from air. Polyvinylpyrrolidone (PVP) of different molecular weights (10k, 40k and 58k) was used as a pore former to allow more adsorption sites to be exposed to CO2. In dynamic adsorption breakthrough experiments at 101.325 kPa and 293 K, 10k PVP foams demonstrated an equilibrium loading of 0.039 g g−1 for CO2 (at 40,000 ppmv in air), showing the best equilibrium time and adsorption capacity. The foams and equivalent commercial 13X beads were able to achieve loadings of 0.094 g g−1 and 0.097 g g−1 (at 40 mbar), respectively, when tested using pure CO2 in an Intelligent Gravimetric Analyser. At pressures beyond 100 mbar, a weighted average isotherm shows only a 1.3 wt% reduction in adsorption capacity due to the polymer binder. The foams showed superior CO2/N2 selectivity compared to other adsorbents in literature. The thermal analysis of pure PI and 13X powder showed that the foams can be regenerated at 300 °C. Computational Fluid Dynamics simulation was successfully implemented in order to understand the CO2 adsorption behaviour on the new foam filter. Such modelling proved to be invaluable in understanding adsorptive behaviour through the complex foam structures as this is difficult to achieve experimentally.

Graphical abstract

Visualisation of the movement of the mass transfer zone throughout the foam with the aid of modelling.

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Introduction

Zeolites and many other adsorbents are produced in powder form and engineered in the form of beads, granules or extrudates for conventional gas adsorption systems. These adsorbents require a binder to provide mechanical strength and macroporous structure for access to their active sites [1]. The particles, consisting of 15–20 w/w% binder, are commonly contained in packed beds, which are cost effective and versatile. According to many experts, the adsorption performance of packed beds, however, is far from optimised and often incurs significant pressure drop as well as high mass transfer resistances [2], [3]. Attrition of particles could also occur due to the movement of the particles in the packed beds or during pressurisation-depressurisation steps. Uneven gas flow distribution through the bed can also contribute to poor performance of a packed bed [4], [5]. It is very clear that there are many opportunities for the improvement and optimisation of adsorptive gas separation processes which should include the development of improved structured adsorbents [6].

To address the above issues, adsorbent structures such as nanotubes, fibres, laminates and monoliths have gained considerable interest as they enable immobilisation of particles to polymeric binding materials. The addition of an adsorptive filler to a polymeric membrane has been demonstrated to be an effective way of improving performance by enhancing sorption capacity for one or more of the compounds to be separated [7]. Parallel channel monolithic structures with controllable shape, cell density and wall thickness offer low pressure drop operation and higher mass transfer rates [8]. Pressure drop through monoliths is typically three to five times lower than that in a pellet system [9], although the mass transfer characteristics of monoliths have been reported to be inferior compared to packed beds [10]. Monoliths with a high cell density provide a better separation performance but such a design becomes complex and thus difficult and expensive to manufacture [6].

Use of adsorbents in foam form is a relatively novel development [6]. Unlike monoliths, adsorbent foam structures can be manufactured and tailored into a wide variety of shapes, as they are not limited by the geometry of the extrusion die [11]. Although research has been carried out on adsorbent polyimide (PI) membranes, limited research has been reported on adsorbent polymeric foams [12]. Although foams are sponge-like structures with high porosity, the amount of adsorbent material in them may not be sufficient to exhibit the same volume activity as packed beds [6]. To overcome this issue, Yoon et al [13], reported a hydrothermal synthesis method of growing zeolite crystals within a macroporous structure, and thereby made the adsorbent surface accessible.

Ceramic foams are used for catalytic applications. Generally, these are either alumina or metallic and hence they are suitable for high temperature, abrasive and severely corrosive applications as a result of their high thermal and chemical stability [14]. Nonetheless, ceramic foams are disadvantageous as the manufacturing process is time-consuming and requires substantial heating at high temperatures for the calcining-sintering which can reach temperatures up to 1000 °C and the need for impregnation of chemical precursors for adsorbent encapsulation [15]. Polymeric/adsorbent composite foams, on the other hand, can be synthesized into any geometrical shape at room temperature via a simple process comprising of two parallel reactions [11], [16]. Therefore, this paper concentrates on the development of a highly loaded adsorbent foam structure by manipulating the adsorbent content, the pore-former such as polyvinylpyrrolidone (PVP), the composition and the reaction conditions thereby creating the potential to increase the adsorbent content and the mass transfer in the foams compared to other adsorbent structures. An example model adsorbent selected for this research was commercially available 13X zeolite.

The criteria for selecting a suitable polymer is that it should be thermally stable at temperatures higher than 300 °C (used for regeneration), have good mechanical strength and have low foam manufacturing cost. PI and polybenzimidazole (PBI) are typical examples of commercial polymers which can be used as a potential binder for foam filters because of their high mechanical strength and thermal stability [17]. A pure PI foam has been described by Liu et al [18]. The preparation of PBI foams would involve the addition of sulfuric acid to a PBI solution and heating up to temperatures of about 350–600 °C [19]. More importantly, PI is reported to have excellent CO2 solution-diffusion properties and permeation transport [20], and provides a class of amorphous high performance polymers characterised by excellent thermal properties and resistance to inorganic acids and bases [21]. Hence, PI was selected for producing the adsorbent foams in this work. A main challenge of this research is to ensure that embedded zeolite crystals provide the access to adsorb molecules such as CO2, while retaining thermal stability for regeneration.

This paper presents the research and development of regenerable 13X zeolite/PI foam structures which have high adsorption capacity. The CO2 adsorption performance of the foams and the effect of PI on the adsorption performance have been quantified using dynamic adsorption flow breakthrough curves and intelligent gravimetric analyser (IGA) adsorption isotherms. The thermal stability of the foam structures has been analysed using a thermogravimetric analyser (TGA), in order to select a suitable regeneration temperature. Alongside the experimental work, numerical modelling has been used to elucidate the mass transfer characteristics of CO2 through the foams including the adsorption of CO2 onto the surface of the immobilised 13X zeolite.

Section snippets

Materials

Pyromellitic dianhydride (97%, PMDA), 1-methyl-2-pyrrolidone (Reagent grade, 99%, NMP), silicon oil (viscosity 350 cSt), ethanolamine (Amine Catalyst), dibutyltin dilaurate (Tin Catalyst), poly [(phenyl isocyanate)-co-formaldehyde] (Isocyanate) and polyvinylpyrrolidone (PVP) with molar mass of 10000 Da (10k), 40000 Da (40k) and 58000 Da (58k) were all supplied by Sigma-Aldrich. Adsorbent 13X powder was supplied by Honeywell UOP (HU), Air Products (AP) and Zeochem (ZC). The adsorptive gas for

Fabrication of PI/13X zeolite foams

Table 1 and Fig. 3 shows the importance of saturating the zeolite prior to the foaming process. The different batches of 13X zeolite supplied were dried in the furnace at 300 °C for 24 h as shown in Table 1, to quantify the amount of water required for a generalised adsorbent foam formulation. Since 13X from AP and ZC were supplied saturated with moisture, significant weight losses of 24.8% and 19% respectively, were observed. Therefore, Fig. 3 (a) and (b) shows that 0.5–1 mL of water was

Numerical modelling

A 2-dimensional axisymmetric model was developed and solved using a commercial package COMSOL Multi-Physics V5.3. Species and momentum conservation equations were coupled and solved to describe the transport of CO2 through the foam as well as CO2 adsorption throughout the foam.

Conclusions

Foams containing 80 wt% 13X/20 wt% Polyimide (PI) were successfully fabricated using a new generic synthesis method comprising of a dual parallel reaction foaming process, which consists of a CO2 generation (blowing) reaction and a polymerisation reaction. Three molecular weights of polyvinylpyrrolidone (PVP) namely, 10k, 40k and 58k, were used as pore-formers to enable more exposure of adsorption sites to contaminants. The adsorbent foam produced with 10k PVP was found to be superior in

Acknowledgements

Ramya G would like to thank University of Bath for the University Research Studentship. The authors gratefully acknowledge the technical assistance provided by Fernando Acosta, Cassie Reis and Clare Bell in the Department of Chemical Engineering at the University of Bath. Also, for the assistance provided by Mr Philip Fletcher for using the scanning electron microscopy (SEM) at the Microscopy and Analysis Suite (MAS) was acknowledged.

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