Capture of carbon dioxide from flue or fuel gas mixtures by clathrate crystallization in a silica gel column

Abstract

A column of silica gel was employed to contact water with flue gas (CO₂/N₂) mixture to assess if CO₂ can be separated by hydrate crystallization. Three different silica gels were used. One with a pore size of 30 nm (particle size 40–75 μm) and two with a pore size of 100 nm and particle sizes of 40–75 and 75–200 μm respectively. The observed trends indicate that larger pores and particle size increase the gas consumption, CO₂ recovery, separation factor and water conversion to hydrate. Thus, the gel (gel #3) with the larger particle size and larger pore size was chosen to carry out experiments with concentrated CO₂ mixtures and for experiments in the presence of tetrahydrofuran (THF), which itself is a hydrate forming substance. Addition of THF reduces the operating pressure in the crystallizer but it also reduces the gas uptake. Gel #3 was also used in experiments with a fuel gas (CO₂/H₂) mixture in order to recover CO₂ and H₂. It was found that the gel column performs as well as a stirred reactor in separating the gas components from both flue gas and fuel gas mixtures. However, the crystallization rate and hydrate yield are considerably enhanced in the former. Finally the need for stirring is eliminated with the gel column which is enormously beneficial economically.

Introduction

Capture of CO₂ from power plant flue gases from conventional power plants or from fuel gases from gasification plants is a technological application of gas hydrates under consideration (Aaron and Tsouris, 2005, Kang and Lee, 2000, Klara and Srivastava, 2002, Kumar et al., 2009, Linga et al., 2008, Linga et al., 2007a, Seo et al., 2005). Post-combustion CO₂ capture involves separation of CO₂ from the flue gas mixture emitted from a power plant. A typical pretreated flue gas contains 15–20 mol% CO₂, 5–9% O₂, and the rest N₂. A process combining three hydrate crystallization stages and a membrane separation stage has been proposed for the capture of carbon dioxide from such mixtures (Linga et al., 2008, Linga et al., 2007a). On the other hand pre-combustion capture involves separation of carbon dioxide from a carbon dioxide/hydrogen mixture (Herzog and Drake, 1996, Klara and Srivastava, 2002). A typical fuel gas mixture is a mixture of predominantly H₂ (∼60 mol%) and CO₂ (∼40 mol%) (Booras and Smelser, 1991, Hendriks et al., 1991) coming out from an integrated gasification combined cycle (IGCC) at a pressure of 2.5–7 MPa (IPCC, 2005). This fuel gas mixture is pretreated for removal of particulate matter and H₂S.

The above proposed processes were based on laboratory-scale data employing stirred vessels as crystallizers. Agglomeration of hydrate crystals creates barriers to efficient gas/water contacting in such crystallizers and as a result the rate of crystallization decreases and the conversion of water and gas to hydrate is limited (Englezos, 1996, Lee et al., 2005, Linga et al., 2007b). For example, a 4% conversion at the onset of agglomeration has been reported (Englezos, 1996). There is an ongoing effort to improve the performance of these crystallizers that will enable the continuous and efficient hydrate crystallization for CO₂ capture, natural gas storage and transport and other gas separation applications like separation of HFC-134a (Nagata et al., 2009). Mori (2003) reviewed and discussed the various hydrate formation vessels and their limitations. For example, other arrangements such as bubbles dispersed in water or water droplets injected into a gas atmosphere have been proposed (Gudmundsson et al., 2000, McCallum et al., 2007, Ohmura et al., 2002, Tsuji et al., 2004). Tsouris and co-workers have presented novel and efficient crystallizers in the context of CO₂ sequestration work (Lee et al., 2003, Tsouris et al., 2004, Tsouris et al., 2007, West et al., 2001). In addition, it has also been reported that when hydrate is formed from ice, temperature ramping enhances the conversion (Susilo et al., 2007b, Wang et al., 2002).

Another method to overcome the gas/water contact limitation is to contact the gas phase with water dispersed in the pores of silica gel and have the hydrate formed within the pores (Adeyemo, 2008, Kang et al., 2008, Park et al., 2006, Seo et al., 2005). Porous materials such as silica gel possess high internal surface area per volume. An additional benefit of using porous materials compared to stirred vessels is that there is no need for power consumption due to stirring. Thus, the objective of the present study is to investigate the effectiveness of the silica gel bed for the separation of CO₂ from CO₂/N₂ (flue gas) and CO₂/H₂ (fuel gas) mixtures through hydrate crystallization.