Separation and Capture of CO₂ through A Zeolitic Membrane ^†

Abstract

This paper presents the development and evaluation of a technology for CO₂ separation and capture from a mixture of post-combustion gases through a zeolitic membrane. A silicalite-1 membrane was hydrothermally synthesized to selectively separate CO₂ from a CO₂/N₂ mixture and permeation tests were performed on the mixture and the simple gases. The composition and morphology of the silicalite-1 crystals were confirmed by XRD and SEM/EDS. The recovered CO₂ was used as raw material for its transformation into inorganic media like carbonates.

1. Introduction

One of the actions to achieve a reduction in CO₂ emissions is the introduction of combustion gas separation techniques, through using membrane technology [1]. Zeolites have a significantly large, uniform porosity system, excellent thermal and chemical stability, so they are particularly promising as membranes capable of separating gases at industrial level [2], like CO₂ from combustion gases [3]. The gas separation properties in membranes depend on: the material (permeability, separation factors), the structure of the membrane and the thickness, the membrane wall and the design of the module and the system [4]. After separation and capture, CO₂ can be used partially for commercial purposes or it can be stored, isolating it from the atmosphere thereby reducing its concentration in it [5]. Carbonation involves the formation of solid carbonates, a reaction between carbon dioxide and alkaline/alkaline earth oxides [6]. Considering these aspects, this study presents the development and evaluation of CO₂ separation technology from a mixture of post-combustion gases through a zeolitic membrane and its capture of CO₂ by carbonation.

2. Materials and Methods

A tubular silicalite-1 membrane was hydrothermally synthesized, then subjected to permeation and gas separation studies of a CO₂/N₂ mixture performing in a stainless steel permeation module in the 25 to 200 °C range temperature and 30 psi pressure of the feed gases: the CO₂/N₂ mixture was fed in 1:1 volumetric ratio. The composition of the gas flow was analyzed by means of a gas chromatograph (Agilent 6890) with an HP-PLOTQ column connected in line, with helium as the carrier gas. The synthesis gel was prepared with Degussa 200 aerosil, as a silica source, 1M tetrapropylammonium hydroxide (TPAOH) Aldrich, as structure directing agent and deionized water. The gel was allowed to mature for 72 h with constant agitation. The synthesis of the silicalite-1 layer was carried out on a commercial monotubular ceramic support of γ-alumina (Pall). The 15 cm long tubular support was waterproofed at both ends (1.5 cm each end). The synthesis of the silicalite-1 layer was carried out on a commercial monotubular ceramic support of γ-alumina (Pall). The 15 cm long tubular support was waterproofed at both ends (1.5 cm each end). The synthesis was carried out at 170 °C and autogenous pressure, for 72 h. This procedure was repeated 4 times over to ensure formation of an homogenous, defect-free silicalite-1 layer on the inner wall of the support. The membrane was characterized by elemental analysis through a SEM/EDS equipment (Carl Zeiss, model supra PV55, fitted with an Oxford detector for Energy Dispersive Spectroscopy Analysis (EDS)) and by X-ray diffraction (XRD) (in Philips X’Pert equipment). The recovered CO₂ was captured by inorganic media promoting the formation of carbonates. The CO₂ was fed to a reactor with Sr(OH)₂, Ba(OH)₂ or Mg(NO₃)₂ 0.06M alkaline solutions at room temperature and normal atmospheric pressure (≈25–30 °C and 585 mmHg Mexico City, Mexico), with the CO₂ fed at a flow of 15.17 mL/min. A Na(OH) 0.1 M solution was added to the Mg(NO₃)₂ solution in order to obtain Mg(OH)₂, for further CO₂ carbonation reactions. After reaction the solids were filtered out, then dried at 80 °C, and characterized by elemental analysis, as stated.

3. Results and Discussion

3.1. Characterization of membrane

Figure 1a shows a membrane cross section SEM micrograph where the growth of the zeolite crystals can be observed on the inner support surface used in the synthesis; energy dispersive spectroscopy analysis is also presented (Figure 1b) which yielded a composition: 58 wt% oxygen and 38.01 wt% silicon, as major elements. Figure 1c shows the XRD diffraction pattern of the synthesized zeolite crystals compared to the zeolite MFI reference standard, obtained from the Zeolite Structures Database [7]. In the XRD diffraction pattern, the similarity in the peaks intensity and their location in the 2θ angle can be observed.

3.2. Results of permeation and gases separation

The permeation results of the simple gases N₂ and CO₂ in the membrane show a decreasing trend when temperature increases, being in the 1.6176 × 10⁻⁷ to 2.8354 × 10⁻⁷ mol m⁻²s⁻¹ Pa⁻¹ range for N₂ and 1.7451 × 10⁻⁷ to 2.8954 × 10⁻⁷ mol m⁻²s⁻¹Pa⁻¹ for CO₂. In the case of the CO₂/N₂ mixture, the permeation also decreases as a function of temperature; however, the permeation values are greater than that of the simple gases, namely, from 4.4404 × 10⁻⁷ to 6.7358 × 10⁻⁷ mol m⁻²s⁻¹Pa⁻¹ (Figure 2).

According to Bakker, et al. (1997), the temperature dependence of the flows in steady state, in the range of −83 to 407 °C through a silicalite-1 membrane, can be described using two diffusion mechanisms. Under conditions where a considerable amount of gas is adsorbed, diffusion is carried out by mass transport in an adsorbed site (surface diffusion) while at high temperatures, diffusion is described by gas transport diffusion [8] since there is virtually no adsorption.

The results of the gases separation show a maximum separation factor (SF) of 2.1 at 25 °C, that decreased with increasing temperature, as can be seen in

Table 1. Results of gas separation factor.

Temperature (°C)	SF
25	2.1003
50	1.9162
100	1.6681
150	1.4075
200	1.2273

.

The separation factor is the result of the composition of the permeate flow and not of the amount of flow, therefore, considering a higher molecular adsorption at low temperatures and the size of the molecules of CO₂ (3.3 Å) and N₂ (3.64 Å) it is presumed that the membrane performed a slightly selective separation of CO₂ over the entire temperature range, due to the CO₂ molecule smaller size with respect to that of N₂.

3.3. Solids Recovered from Carbonation Reactions

The dry weight of the solids recovered in the carbonation reactions was recorded in order to perform a mass balance and obtain the conversion percentage of each reaction with respect to the initial concentration of the alkaline solutions and with the CO₂ flow fed. The results on conversion percentage are shown in

Table 2. Conversion percentage in carbonation reactions.

Reaction Time (Minutes)	Conversion Percentage
	Sr(OH)2	Ba(OH)2	Mg(NO3)2
20	40.22	85.09	57.60
30	26.06	69.07	29.07
40	20.11	66.50	17.67

.

3.4. Characterization of Solids Recovered after Carbonation Reactions

The Figure 3 shows the morphology of the solids recovered in the carbonation reactions with the different alkaline solutions as well as the elemental analysis carried out that shows the presence of carbon in the solids.

4. Conclusions

The permeation studies allowed the determination of the surface diffusion mechanism of gases through the membrane. A slight selectivity of CO₂ with respect to N₂ was obtained after gases separation with a factor up to 2.1 at 25 °C. Considering the mechanism of diffusion, the characteristics of the membrane, as well as the separation factor, it is concluded that the synthesis resulted in a membrane capable of separating CO₂ from a CO₂/N₂ mixture under the study conditions, although it is possible to obtain more favorable results by chemically modifying the basic surface properties of the membrane.

The CO₂ capture system shows that it is possible to obtain other compounds starting with CO₂, recovering it as carbonates, and the characterization of these compounds showed that they can be used as raw material in another process.