Below the room temperature measurements of solubilities in ester absorbents for CO₂ capture

Highlights

• Measured CO₂ solubilities in six physical absorbents with ester groups below the room temperature.

• Calculated Henry’s constants and thermodynamic properties of selected systems.

• Concluded that tributyl phosphate and triethyl phosphate have potential research value for CO₂ capture.

Abstract

Six ester absorbents were selected for CO₂ capture, such as methyl benzoate, methyl heptanoate, ethyl hexanoate, butyl butyrate, triethyl phosphate and tributyl phosphate. CO₂ solubilities in these absorbents were determined under temperatures of 273.15–283.15 K, and pressures up to 1.2 MPa. Henry’s constants of CO₂ + the selected absorbent systems at 273.15 K and 283.15 K were calculated and compared with those at higher temperature. It seemed that decreasing the absorption temperature is obviously beneficial for enhancing absorption performance. In order to assess the absorption capacity for different physical absorbents, Henry’s constants and volumetric solubilities of the selected absorbents were compared with ionic liquids, common solvents and the selected absorbents in our previous work. The result showed that tributyl phosphate and triethyl phosphate were found to be relatively good absorbents by mole and volumetric fraction respectively, and they have potential value for CO₂ capture. Moreover, thermodynamic properties such as entropy of solution, enthalpy of solution and Gibbs free energy of solution for the selected systems were calculated and assessed to study the absorption behavior.

Introduction

In recent years, carbon capture and storage (CCS) has been the effective technology for decreasing the release of excessive carbon dioxide into the atmosphere [1]. Currently, there are several methods as physical/chemical/hybrid absorption method [2], adsorption method [3], [4], cryogenic method [5], membrane method [6] that are used to capture CO₂. Physical absorption method, which is mature and common in industry installations, is widely used under the condition of large amount of feed gas and extremely high CO₂ concentration. It is indicated that the development of new physical absorbent is one of the core parts for carbon capture technology [7].

Many researchers have concluded that different functional groups may lead to different CO₂ capacities for various physical absorbents. Gui [8] concluded that hydroxyl group may be considered as phobic-CO₂ group because strong hydrogen-bond interaction among the molecules of the absorbents may cause low solubilities of CO₂. In our previous work [9], carbonyl group is considered as philic-CO₂ group due to their good solvency for CO₂, which may be attributed to electronic theory of acids and bases. Strong interaction between carbonyl group (Lewis acid) and CO₂ (Lewis base) happens during the CO₂ absorption process. In addition, Perisanu [10] claimed that carbon dioxide shows very high solubilities in solvents containing carbon-oxygen bonds (esters, ethers, some ketones), which follows the principle “like dissolves like”. Vapor-liquid equilibrium data of CO₂-various ester absorbents were determined, which showed the absorbents with ester groups showed excellent absorption behaviour. The solubility of carbon dioxide in six absorbents containing propylene carbonate (PC), ethyl carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixtures of these components were performed at temperatures from 275 to 333 K at atmospheric pressure by Blanchard [11]. It seemed that DEC was the best of the whole selected absorbents. Fandiño [12], [13] reported CO₂ solubilities in four pentaerythritol esters as tetrapentanoate, tetra(2-ethylhexanoate), tetraheptanoate, tetranonanoate from (283 to 348) K and pressures up to 7.5 MPa. The result showed that in the present analyzed range CO₂ is highly soluble in these ester oils. Gui measured CO₂ solubilities in some esters as DMC [14], diethyl succinate [15], DMC + DEC [16], DMC + PC [16] and DMC + EC [16] at the temperature variations from 280 K to 313 K and pressures up to 6 MPa. Miller [17] assessed fifteen different low molar mass compounds including methyl acetate, 2-methoxy ethyl acetate, 2-(2-butoxyethoxy)ethyl acetate, PC, 2-butoxyethyl acetate as CO₂ solvents based on the CO₂-solvent pressure-composition diagram at 298.15 K. When compared on a molar basis, 2-(2-butoxyethoxy)ethyl acetate rich with carbonate ester and ether groups was the best absorbent of the fifteen. Howlader [18] determined solubility of CO₂ in triglyaceride at different temperatures (283.2–303.2 K) and pressures (600–2450 kPa). Tian [19] and Cheng [20] measured CO₂ solubilities in ethyl propanoate, ethyl acetate, diethyl oxalate, ethyl laurate, and dibutyl phthalate at supercritical pressures. Moreover, Lenoir [21] determined CO₂ solubilities in the nineteen absorbents as alcohols, hydrocarbons, ketones, carbonate esters and phosphate esters at 298.15–343.15 K. Henry’s constants of these solvents were reported and compared with each other, which indicated that phosphate esters showed relatively high absorption capacities of CO₂. In our previous work [9], [22], [23], the absorbents with carbonate ester groups such as ethylene glycol methyl ether acetate (EGMEA), propylene glycol methyl ether acetate (PGMEA), 3-methoxy butyl acetate (MBA), ethylene glycol butyl ether acetate (EGBEA) and carbitol acetate (CA) were selected, CO₂ solubilities in these absorbents were determined at the temperatures of 273.15–333.15 K and under pressures up to 1.2 MPa. The result showed that absorption capacities of these absorbents improve by 139–160% and 38–45%, with the condition of absorption temperature decreasing from 333.15 K to 273.15 K and from 293.15 K to 273.15 K respectively. Additionally, it is advantageous for decreasing circulation flow of the absorbent and minimizing the loss of absorbent. In this paper, the relatively low temperatures of 273.15 K and 283.15 K were chose to investigate CO₂ absorption properties of the physical absorbents.

Ester absorbents such as methyl benzoate, methyl heptanoate, ethyl hexanoate, butyl butyrate, triethyl phosphate, tributyl phosphate were selected to study the absorption behaviour at 273.15–283.15 K. Bamberger [24], Weng [25], and Chen [26] determined vapor–liquid equilibrium data of the CO₂-methyl benzoate system at supercritical pressure of 6.10–14.11 MPa, 3.0–14.5 MPa and 2.46–7.36 MPa, and under temperatures of 313.1–393.2 K, 313.15–348.15 K and 308.2–318.2 K respectively. Lenoir [21] measured CO₂ solubilities in triethyl phosphate and tributyl phosphate under the temperature of 325.14 K, and reported the results in the form of Henry’s constants. In addition, Sweeney [27] reported Henry’s constant of CO₂ for tributyl phosphate at 298.14–323.14 K. Vapor-liquid equilibrium data of the CO₂-ethyl hexanoate [28] system and the CO₂-tributyl phosphate [29] system at the temperatures of 308.2–328.2 K and 298.65–435.55 K were determined at supercritical pressure of 1.699–9.218 MPa and 8.42–27.51 MPa respectively. CO₂ solubilities in methyl benzoate, methyl heptanoate, ethyl hexanoate were determined at pressures up to 1.2 MPa and temperatures ranged from 293.15 K to 333.15 K, in our previous work that recently submitted to the journal of chemical thermodynamics [30]. There are no data on CO₂ solubilities in six absorbents under low pressures and 273.15–283.15 K that reported in the literature.

In this work, below the room temperature measurements of CO₂ solubilities in the six absorbents were made at 273.15 K and 283.15 K, and under pressures up to 1.2 MPa. Henry’s constants and thermodynamic properties of the CO₂ + selected solvent systems were calculated and discussed.