A screening study of alcohol solvents for alkanolamine-based CO₂ capture

Highlights

• Eighteen alcohol solvents were investigated for alkanloamine-based CO₂ capture.

• Alkanolamine absorbents carbonate ratios linearly increase when CO₂ loading increases; the carbamate showed the opposite trend.

• Viscosity of non-aqueous solvent was highly affected by CO₂ loading.

• All (EAE + TEG) solutions showed homogeneous mixture within the whole range of CO₂ loading.

• Developed a high performance non-aqueous solvent system that does not form any precipitate/solid upon exposure to CO₂.

Abstract

The present study was aimed at developing a non-aqueous absorbent system that does not form any precipitate/solid upon exposure to CO₂. More specifically, the non-aqueous absorbent system must be a homogeneous mixture (single phase) within the whole range of CO₂ loading. At the same time, sufficient amine regeneration must occur at a low temperature of 80–90 °C compared to reference non-aqueous monoethanolamine (MEA)-based absorbents. To achieve this target, 18 alcohols were investigated as solvents and 4 alkyl-linked alkanolamines were evaluated as absorbents. Several experiments mixing alkanolamines with alcohols were performed to evaluate their CO₂ capture performance. High-performance non-aqueous liquid absorbents (without solid/precipitate formation) with the advantages of higher absorption rates, higher cyclic capacities, higher regeneration efficiencies, lower specific heats, and lower heats of reaction compared to reference MEA-based non-aqueous absorbents were successfully developed herein. The favorable features of these absorbents will provide the basis for the discovery of potential non-aqueous amine-based absorbents.

Introduction

The carbon dioxide (CO₂) concentration in the atmosphere is increasing annually with the combustion of fossil fuels for power generation, which increases the emission of greenhouse gases that contribute to global warming and climate change (IPCC, 2005). CO₂ emissions resulting from the combustion of fossil fuels and industrial processes were the largest contributors (65%) to global greenhouse gas emissions in 2010 (IPCC, 2014). A drastic reduction in CO₂ emissions is required for the mitigation of global warming. In this context, carbon dioxide capture and storage (CCS) has been gaining attention as an innovative mitigation measure (Boot-Handford et al., 2014; Leung et al., 2014; Rochelle, 2009).

Many technologies are currently employed for the separation and capture of CO₂ from flue gas streams. These techniques are based on different processes, including absorption, adsorption, and membrane separation (MacDowell et al., 2010; Bernardo et al., 2009). Post-combustion CO₂ capture using an aqueous amine solution is the most mature and widely employed of the processes currently employed (Rochelle, 2009; Wang et al., 2017). There are commercial aqueous amine technologies, such as, KS-1, Econamine FG+, Oase Blue, and CANSOLV for CO₂ separation from flue gas sources that hold considerable promise, with low heat energy consumptions of 2.3–2.4 GJ/t-CO₂. Previously, RITE also developed highly efficient single and mixed aqueous amine-based absorbents for targeting CO₂ generated from steel-making industries (Chowdhury et al., 2013; Onoda et al., 2016; Yamada et al., 2013). All of these CO₂ absorbents are capable of reducing the energy consumption associated with CO₂ separation by approximately 2.0 GJ/t-CO₂ (Onoda et al., 2016). However, the total separation cost still remains high.

Usually, during a chemical absorption process, CO₂ is absorbed into the amine solution at low temperatures (approximately 40 °C) and desorbed from the solution after heating (approximately 120 °C). The chemistry of this process is complex, but two main reactions take place, depending on the type of amine (Eqs. (1) and (2)) (Aboudheir et al., 2004; Barth et al., 1984; Vaidya and Kenig, 2007). The reaction between CO₂ and the unhindered (primary or secondary) amines forms a fairly stable carbamate, R¹R²NCOO⁻ (Eq. (1)). On the other hand, the hindered amines, which form an unstable carbamate, and tertiary amines, go through an alternate reaction to form a bicarbonate ion, HCO₃– (Eq. (2)).

2R¹R²NH + CO₂ ↔ R¹R²NCOO⁻ + R¹R²NH₂⁺

R¹R²R³N + CO₂ + H₂O ↔ R¹R²R³NH⁺ + HCO₃⁻

The regeneration of amine from stable carbamate or bicarbonate is done by stripping with water vapor at 100–120 °C. Although aqueous amine solutions are promising to remove CO₂ from flue gas point sources, aqueous amine process often suffer from environment- and health-related concerns due to volatile-solvent losses, thermal and oxidative degradation of amines, corrosion problems, and high energy consumption for solvent regeneration (Reynolds et al., 2012; Gouedarda et al., 2012; Rao and Rubin, 2002). Therefore, there is a high demand for the development of a new alternative solvent that can overcome the aforementioned drawbacks and process larger quantities of CO₂ with a lower energy demand.

To identify cost-effective approaches for CO₂ capture, the past decade has witnessed the development of various new solvents, including concentrated non-aqueous/water-lean solvents. Non-aqueous amine solvents have potential advantages over aqueous amines, specifically lower heat capacity (approximately one-half), lower heat of vaporization of organic solvents, and higher boiling temperature compared to that of water. Many formulations of non-aqueous CO₂ selective solvents have been tested, including amine-based non-aqueous solvents (Aschenbrenner and Styring, 2010; Francesco et al., 2012, 2013, 2014; Lail et al., 2014; Phan et al., 2008; Vincenzo et al., 2013), CO₂-binding organic liquids (Feng et al., 2016; Heldebrant et al., 2008; Jian et al., 2013; Liu et al., 2006; Mathias et al., 2013; Privalova et al., 2012), aminosilicones (Perry and O’Brein, 2011), alkylimidazole blended with amine (Bara and Shannon, 2011), room temperature ionic liquids (RTILs) (Bara et al., 2010; Cadena et al., 2004; Dai et al., 2017; Shiflett and Yokozeki, 2007; Soriano et al., 2008; Yingying et al., 2016; Yujiao et al., 2014), amino-functionalized task-specific ionic liquids (TSILs) (Bates et al., 2002; Sánchez et al., 2011; Sharma et al., 2012; Zhang et al., 2009), and mixed RTILs with alkanolamines (Camper et al., 2008; Feng et al., 2013; Hasib-ur-Rahman et al., 2012; Shannon and Bara, 2011). All these non-aqueous absorbents possess several advantages over the aqueous absorbents namely, high-boiling-point, low vapor pressure, thermally stability with a lower heat capacity than that of water. Despite their potential for CO₂ capture, non-aqueous solvents possess some drawbacks: the non-linear increase in viscosity once CO₂ is absorbed, precipitate formation, or solvent gelation leading to a highly viscous gel or waxy solid.

To overcome the aforementioned drawbacks, this work focuses on the development of a non-aqueous absorbent system that will reduce the viscosity of the CO₂ loaded solution and will not form any precipitate, viscous gel, or waxy solid upon exposure to CO₂. More specifically, the non-aqueous absorbent system must be a homogeneous mixture (single phase) within the whole CO₂ loading range. At the same time, sufficient amine regeneration at a low temperature range of 80–90 °C is desired. This makes it possible to use waste low temperature heat for regeneration at a low cost, resulting in a more cost-effective CO₂ absorption process. Another critical issue in the use of non-aqueous solvents is how to control the water, because water is omnipresent in the process, as it is introduced via the flue gas. The water tolerance of the non-aqueous solvent system is also briefly discussed.