(395n) Gas Separation By Adsorption On MIL-53(Al): Natural Gas and Biogas Application

Gas separation by adsorption on MIL-53(Al): natural gas and biogas application.

A.F.P. Ferreira¹, A.M. Ribeiro¹, S. Kulaç¹, A.E. Rodrigues¹

¹ Laboratory of Separation and Reaction Engineering, Associate Laboratory LSRE/LCM, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, Porto, arodrig@fe.up.pt

Keywords: MIL-53(Al), biogas, natural gas

Introduction

CO₂ and H₂S are two of the main polluting compounds in natural and bio gas. Therefore, the separation of CO₂ from CO₂ / CH₄ mixtures is a major process in the industry for upgrading the energy content of natural gas. [1] Adsorption, membrane and absorption processes are the three main technologies for CO₂/CH₄ separation [2]. Adsorption based separation is a key industrial technology due to its high energy efficiency, simplicity, low capital investment cost and ease of control. [1] Recently, several authors purposed the separation of CO₂ from CH₄ by adsorptive processes, using MIL-53(Al). In a recent work by Boutin and co-workers [3], the osmotic thermodynamic model was used in order to describe the flexible behavior of MIL-53(Al) upon adsorption of CO₂ and CH₄. Finsy and co-workers evaluated the effects of pressure and gas composition on the efficiency of the CO₂/CH₄ mixtures separation. [4] Stavitski et al. [5] studied the adsorption mechanism of CO₂ and CH₄ in MIL-53(Al), in order to explain its high efficiency for CO₂ capture. Shaped and powder forms of this material were compared by Heymans et al., they obtained lower adsorption capacities for pellets than for powder, however similar separation selectivity was observed for both forms. [6] In this work, isotherms of CO₂ and CH₄ were obtained, based on single component breakthrough data. Adsorption data was fitted using Langmuir and Toth models. Single component breakthrough curves were simulated using gPROMS Software, allowing the global model verification. Furthermore, binary breakthroughs of CO₂/CH₄ at different feed pressures and concentrations were performed.

Experimental

The methane and carbon dioxide adsorption isotherms were determined using the breakthrough curves. A one column PSA set-up was used for the breakthrough experiments for a pressure range from 1.1 to 4.4 bar (total pressure). The same unit was used to perform the one column PSA cycle. This unit is described elsewhere. A back pressure regulator is used to control the pressure inside the set-up. Outlet gas composition was monitored using a Gas Data LMSxi Type G4.18 infrared gas analyzer (Gas Data Ltd, UK). A column with a length of 0.284 m and a diameter of 0.0212m was packed with 52.3 g of MIL-53(Al) pellets. Bed porosity is 0.36 and feed temperature is 303 K. The positions of the thermocouples are: 0.05 m, 0.118 m and 0.226 m. The MIL-53 (Al) adsorbent (BasoliteTM A100) was provided by BASF in the tablet form (2 x 2 mm), with a pellet density of 826 kg/m³.

Results and discussion

Carbon dioxide and methane adsorption isotherms on MIL-53(Al) were obtained by breakthrough experiments, at the pressures of (0.11, 0.29, 0.6, 1.0, 1.6, 2.0, and 3.52) at 303 K. The adsorbed amounts were determined by integration of the experimental breakthrough curves. It was observed that the adsorption is reversible since the amounts of adsorption and desorption branches are in agreement. The adsorbed amounts are about 4 mmol g^-1 and 2 mmol g^-1 for CO₂ and CH₄, respectively, at 303 K and 3.4 bar.  The isotherms assessed in this work are in good agreement the ones found in the literature, such as the results obtained by gravimetric technique in the work of Heymans et al. [6] The dynamic model describes adequately the adsorption/desorption breakthroughs, validating the model for its use in PSA design and simulation.

Conclusions

Methane and carbon dioxide isotherms were determined using the single component breakthrough curve experiments. Langmuir and Toth isotherm models were used to fit the adsorption data and it was observed that both models represent well the adsorption experimental data. Langmuir isotherm model was chosen to simulate CH₄ breakthrough curves. MIL-53(Al) is CO₂ selective, showing therefore a great potential for its application in the biogas and natural gas upgrade field.

Acknowledgements

The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/ 2007-2013) under Grant Agreement No. 228862. MACADEMIA is a Large-scale Integrating Project under the Nanosciences, Nanotechnologies, Materials and New Production Technologies Theme in FP7.

This work is partially supported by project PEst-C/EQB/LA0020/ 2011, financed by FEDER through COMPETE – Programa Operacional Factores de Competitividade and by FCT – Fundação para a Ciência e a Tecnologia.

The authors thank BASF for providing Basolite A100 samples.

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