Structured zeolite NaX coatings on ceramic cordierite monolith supports for PSA applications

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Abstract

Novel structured adsorbents in the form of thin zeolite films grown on substrates designed for low pressure drop have a great potential to improve pressure swing adsorption (PSA) processes. In the present work, template free films of NaX zeolite were grown on the walls of ceramic cordierite supports using a seeding technique. The supports had 400 parallel channels per square inch. Films were grown both from a gel and a clear synthesis solution. The materials were analyzed by scanning electron microscopy, X-ray diffraction, N2 adsorption/desorption measurements, Hg-porosimetry as well as CO2 breakthrough experiments. When a gel was used for film growth, a film consisting of well intergrown crystals with a thickness of about 1 μm was obtained. However, a large amount of sediments were deposited on top of the film, which resulted in a dispersed CO2 adsorption breakthrough front. Zeolite films grown in one longer hydrothermal treatment in a clear solution were less intergrown and consisted of both NaX and hydroxysodalite crystals and, in addition, some sediments were deposited on top of the film, which again resulted in a dispersed breakthrough front. By using a multiple-step synthesis procedure and a clear synthesis solution, well intergrown NaX films, free from sediments and with only a very small fraction of hydroxysodalite crystals could be prepared. The CO2 breakthrough front for the latter adsorbent was sharper than the front for an empty adsorption column and only shifted in time. This indicates that the flow distribution in the adsorbent is even and that the mass transfer resistance in the film is very low due to the small film thickness and high effective diffusivity for CO2 in the NaX film and still, the adsorption capacity is considerable. The even flow distribution, very low mass transfer resistance and low pressure drop in combination with considerable adsorption capacity in this adsorbent indicates that it is a promising adsorbent for PSA applications. The findings from the present work will be important for the development of structured adsorbents to use as a competitive alternative to traditionally used adsorbents in PSA.

Introduction

Pressure swing adsorption (PSA) is a commonly used technology for gas separation which is relying on selective adsorption of one of the components in a gas mixture at elevated pressure on certain materials, such as zeolites [1], [2], [3], [4]. A typical PSA system involves a number of connected columns containing adsorbent material that undergo successive pressurization and depressurization steps in order to produce a continuous stream of purified product gas.

Zeolites are porous crystalline aluminosilicates of SiO44- and AlO45- tetrahedra connected by oxygen bridges [5]. Zeolites are widely used in PSA gas separation processes and are particularly suitable for oxygen enrichment from air by PSA (low silica zeolite X) [6], [10], [54] or H2 recovery from refinery fuel gas or coke oven gases [38], [56], whereas zeolite 5A is used in carbon-zeolite layered beds to obtain a high purity (>99.99%) hydrogen stream.

Zeolite NaX is particularly suitable for carbon dioxide capture from flue gas or oxygen enrichment from air by PSA [6], [7], [8], [9], [10]. CO2 and N2 are adsorbed in the zeolite pores due to the strong quadrupolar interaction between the molecules and the electric field generated by the charge balancing cations in the micropores of the zeolite [5]. The possibility to exchange the cations represents a valuable measure for tuning the volume available for adsorption, as well as the selectivity. Consequently, the adsorption capacity of CO2 in zeolite X increases with decreasing ion size and increasing charge density, in the order Cs+, Rb+, K+, Na+, Li+ [11]. Low silica zeolite X in the Li+ form (Li-LSX) exhibits a higher selectivity with respect to N2 than zeolite NaX, due to the higher polarizing power of the small Li+ cation, and is therefore the most widely used zeolite adsorbent in air separation processes [10], [52], [53]. The adsorption capacity may also be controlled by tuning the Si/Al ratio of the zeolite.

In PSA processes there is a trade-off between pressure drop and mass- and heat-transfer limitations [12]. The adsorbents in industrial PSA processes are usually in the form of beads, pellets or extrudates. Larger adsorbent particles results in lower pressure drop, but increased mass- and heat-transfer limitation due to the long mass- and heat-transfer path along the radius of the particles resulting in a reduced performance of the process [38], [51]. Thus, it is of great importance to develop novel structured adsorbents, which may exhibit higher mass transfer coefficients, higher throughputs, and lower pressure drop, and represent a competitive alternative to adsorbents in form of beads/pellets [13], [14], [15], [48], [49], [50], [51].

High mass transfer coefficients may be achieved by using thin films of an adsorbent material grown on structured supports, with the advantage of having a short diffusion path. Although the adsorbent loading is often small compared to traditional adsorbents, high throughputs may be achieved by increasing the cycle frequency and still maintain a low pressure drop [51]. In this context, monolithic adsorbents are of great interest, and a few reports on the use of monolith structures in adsorption processes have been published [13], [14], [15], [51]. Monoliths are structured materials with parallel channels, available with various cell densities and cell shapes. Low pressure drop, uniform flow distribution, thermal stability and unproblematic scale-up are some of the advantages with these materials [13], [15]. The mineral cordierite, 2MgO × 2Al2O3 × 5SiO2, is often used as the main material in monoliths. Cordierite monoliths with tailored macrostructure, high porosity and low thermal expansion are used as supports in automotive catalytic converters and as diesel particulate filters (DPF) [16], [17]. Monolith substrates may be wash-coated, dip-coated, slip-coated, slurry-coated or extruded directly into catalytic bodies using appropriate materials [18], [19]. Due to the difficulty in washcoating the zeolite, a binder material is needed, which causes a reduction in the accessible surface area of the zeolite coating and increases the diffusion resistance of the adsorbing species. Due to surface tension effects of the washcoat during manufacturing, more material is deposited at the corners of the monolith channels, consequently increasing the diffusion path of the adsorbing species [18]. However, the performance of a zeolite coating is dependent on its morphology. A perfect, smooth film without intercrystalline pores will have other transport properties than a coating comprised of a multilayer of crystals with intercrystalline porosity. For well defined zeolite films, the mass transfer resistance can be controlled by varying the film thickness. This was demonstrated recently by our group [20], where thin ZSM-5 zeolite films without binder material were grown on 400 cells per square inch (cpsi) cordierite monoliths by first depositing a monolayer of colloidal zeolite seed crystals on the monoliths and then growing the crystals to thin films. The catalytic activity of the films was subsequently evaluated by p-xylene isomerisation, and the effect of film thickness on mass transfer was clearly demonstrated. As opposed to wash-coated monoliths, these films had an even thickness, i.e., no effects of surface tension in the corners. Growth of zeolite films without binder material on ceramic cordierite supports was previously also reported by Zamaro et al. [46] (mordenite) and our group [30] (faujasite).

In another work, the adsorption and diffusion of CO2 in a carbon monolith adsorbent were studied with the Zero Length Column (ZLC) method by Brandani et al. [21]. The ZLC data showed that the dispersion in the monolith was controlled by mass transfer resistance rather than axial mixing. Zeolite monoliths consisting of 5A zeolite and Na-bentonite with square lattice channels and a wall thickness of 0.98 mm were prepared by Li et al. [14], [19], [22]. The adsorption performance of the zeolite monolith was compared with that of 5A zeolite pellets (1.5 mm in diameter and 3.6 mm long) used for the production of oxygen enriched air. The main outcomes of the work were that the adsorption performance of the zeolite monolith was of the same magnitude as the pellets whilst the pressure drop through the zeolite monolith was 3–5 times lower than that for the packed bed. The lower pressure drop resulted in a 3–5 times faster pressurization time when the PSA unit was loaded with the zeolite monolith. Shorter pressurization time is an advantage [14], [19], [22], [51] and allows faster PSA cycles which results in a higher productivity and should compensate for the lower adsorbent loading when using monoliths instead of traditional adsorbents [51].

Possible ways of improving the separation performance of the zeolite monoliths would thus be to reduce further the mass- and heat-transfer resistance by reducing the wall thickness and increasing the cell density of the zeolite monolith [12], [51].

In the present work, another approach, i.e., zeolite coated monoliths, was pursued to arrive at adsorbent monoliths with low mass- and heat-transfer resistance. Instead of preparing zeolite monoliths, very thin zeolite NaX films with thicknesses ranging from 0.4 to 1.5 μm were grown on 400 cpsi cordierite monoliths to arrive at zeolite coated monoliths. The zeolite coated monoliths were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), N2 adsorption and desorption at liquid nitrogen temperature, and CO2 adsorption breakthrough profiles.

Section snippets

Synthesis

A seeding method was used for zeolite film growth on monoliths. The method is very flexible and allows for control of the thickness and preferred [23], [24], [25], [47] orientation of the crystals in the film. In addition, the adsorption [26], [27] as well as diffusion [28], [29] properties of the film has been studied and reported. In the present work, cordierite monoliths with a cell density of 400 cpsi (Corning) were used as supports. Twelve monolith samples were cut with a sharp tool to a

Characterization by SEM, weight gain, and zeolite loading

Fig. 1a shows a SEM image of a surface of the wall of an uncoated 400 cpsi cordierite monolith. Cordierite grains form an uneven surface and some macropores are also observed between the grains. A low magnification SEM top view-image of the 400 cpsi cordierite monolith is shown in Fig. 1b, which illustrates macropores with various size between the grains in the cordierite monolith. Fig. 1c shows a monolayer of 80 nm NaX seeds crystals adsorbed onto the surface of the support. The seeds cover the

Conclusions

Well intergrown zeolite films with a limited amount of sediments and HS crystals were grown on 400 cpsi cordierite monoliths by using a clear solution and a multiple-step synthesis. Films grown in the clear solution in one longer hydrothermal treatment were less intergrown and sediments and large HS crystals were observed. Films grown in the gel were well intergrown but a large amount of sediments on top of the film was observed. The different morphologies of the zeolite films resulted in

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