Encapsulation of highly viscous CO₂ capture solvents for enhanced capture kinetics: Modeling investigation of mass transfer mechanisms

Highlights

• Encapsulation of water lean solvent in CO₂-permeable polymer increases flux by 50×.

• CO₂ transfer into hybrid ‘Solvent Impregnated Polymer’ strongly diffusion limited.

• Shrinking core model in good agreement with gravimetric and fixed bed data.

• Absorption rate into SIPs containing NOHMs comparable to similar CCS chemisorbents.

Abstract

The encapsulation of highly viscous liquid-like Nanoparticle Organic Hybrid Materials (NOHMs) inside a gas permeable polymer to form SIPs (Solvent Impregnated Polymers) significantly enhanced the CO₂ capture kinetics of NOHMs, leading to a remarkable 50-fold increase in CO₂ flux compared to the neat NOHMs. To understand the mechanism for enhanced CO₂ mass transfer within these hybrid materials, kinetic modeling of CO₂ uptake into SIPs containing polyethylenimine functionalized NOHMs, denoted NPEI-SIP, was conducted. CO₂ mass transfer into NPEI-SIP films was found to conform, both qualitatively and quantitatively, with a diffusion-controlled moving front model. The diffusion-controlled model was also used to simulate CO₂ uptake within a fixed bed containing polydisperse NPEI-SIP particles, and this model accurately predicted experimentally measured breakthrough curves at 25 °C and 50 °C. The 50-fold increase in gas flux was shown to be a consequence of the very large CO₂ permeability within the polymer–solvent composite. The increase in gas flux is also dependent on the diffusion–reaction regime in which the chemical solvent operates, and the largest improvement will occur when immobilizing solvents, such as NOHMs, which operate in the instantaneous-reaction regime. The CO₂ capacity (~3 mol CO₂/kg) and saturation time (~5 min) of 430 μm SIP particles were comparable to popular CO₂ chemisorption materials such as amine grafted silicates, in spite of the slow kinetics of NOHM-I-PEI in liquid form (CO₂ saturation time ~24 h for a 1 mm thin film).

Introduction

Anthropogenic greenhouse gas emissions have risen sharply over the last few decades [1], [2]. Unless significant action is taken to reduce emissions, and possibly to remove CO₂ from the atmosphere, significant and disruptive changes in global mean temperature will occur [3]. A prominent approach for both reducing CO₂ emissions and removing CO₂ from the atmosphere is Carbon Capture and Storage (CCS), in which dilute CO₂ is isolated and geologically stored [4]. A large number of technologies have been investigated for carbon capture, including liquid solvents [5], solid sorbents [6], membranes [7] and various composite materials [8], [9].

Several next-generation, water-lean solvents for CCS are under active development, including reactive ionic liquids [10], CO₂-Binding Organic Liquids [11], and liquid-like Nanoparticle organic hybrid materials (NOHMs) [12]. It is possible that the relatively low water content of these solvents could reduce the energy required for carbon capture, by reducing evaporative losses in the stripper, and also by reducing losses around the main cross heat exchanger associated with the large specific heat capacity of water [13] (note that these claims have been debated within the literature [14]). Unfortunately, these anhydrous solvents are often more viscous than aqueous solvents, causing handling issues and, in some cases, reduced gas flux [13].

To overcome these issues, several authors have created high-surface area composite materials, in which liquid solvent droplets are immobilized within a solid capsule or gel. Vericella et al. [15] developed microencapsulated solvents (MECS), in which 100$\mu \mathrm{m}$ − 600$\mu \mathrm{m}$ droplets of solvent are encased in a thin, CO₂-permeable polymer shell. The very large surface area of these capsules led to 1 to 2 orders of magnitude increase in volumetric gas absorption rates [16]. To further improve the scalability of the manufacture of these materials, Moore et al. [17] developed Solvent Impregnated Polymers (SIPs), in which liquid solvents are encapsulated inside a gas permeable polymer, creating a hybrid material which can be shaped into thin strands or small particles with very large interfacial areas for CO₂ capture (Fig. 1). These materials may be created via a scalable, one-pot method, and the large CO₂ permeability of the polymer matrix may enhance the gas flux into the material. A similar concept was explored by Nguyen et al. [9], who manufactured composites using a solid colloidal precursor rather than the emulsion precursor used by Moore et al. [17].

In our recent study [18], the manufacture of SIP particles containing polyethylene-functionalized NOHMs (named ‘NOHM-I-PEI’) encapsulated in a CO₂-permeable silicone acrylate (TEGO Rad 2650, EVONIK) matrix was reported. NOHM-I-PEI is an ideal candidate for the SIP motif. This NOHM has a low water content, is highly stable at a wide range of temperatures, has negligible vapor pressure, has an exceptionally large equilibrium CO₂ capacity (up to 7 mol CO₂/kg NOHMs), and its properties may be tunable for both traditional point source carbon capture, and also Direct Air Capture (DAC). However, this material is very viscous, leading to strong diffusion-limitations and slow CO₂ uptake. When NOHM-I-PEI was encapsulated inside silicone acrylate, the CO₂ flux into the material increased by a factor of 50, relative to a neat NOHM-I-PEI liquid with the same surface area [18]. By cryogenically grinding the NPEI-SIP film into small particles (~0.5 mm average diameter), the rate of CO₂ absorption was increased by a further order of magnitude.

The 50-fold increase in gas flux observed by Rim et al. [18] was significantly larger than the 2 to 4-fold increase in flux predicted by the model of Moore et al. [17], who considered encapsulation of chemical solvents operating within the pseudo-first order regime. In order to understand this discrepancy, in this study CO₂ mass transfer mechanisms within NPEI-SIP are investigated. Diffusion-limited models are developed for CO₂ uptake into thin films of NPEI-SIP, and also for CO₂ absorption within a packed bed of polydisperse NPEI-SIP particles. These models are validated against experimentally measured CO₂ uptake and breakthrough curves, and are used to analyze the diffusion–reaction regimes in which the SIP motif is likely to lead to the greatest increase in absorption rates. Finally, mass transfer rates within SIPs containing NOHMs-PEI are compared with other chemisorbents such as amine grafted silicates, and the implications for material development and process design are discussed.