Kinetic approach to modelling CO2 adsorption from humid air using amine-functionalized resin: Equilibrium isotherms and column dynamics

On Comparing Packed Beds and Monoliths for CO2 Capture from Air Through Experiments, Theory, and Modeling

This study compares the performance of amine-functionalized γ-alumina sorbents in the form of 3 mm γ-alumina pellets and of a γ-alumina wash-coated monolith for CO2 capture for direct air capture (DAC). Breakthrough experiments were conducted on the two contactors to analyze the adsorption kinetics and performance for different gas feeds. A constant pattern analysis revealed dominant mass transfer resistances in the gas film and in the pores, with axial dispersion also observed, particularly at higher concentrations. A 1D, physical model was used to fit the experiments and thus to estimate mass transfer and axial dispersion coefficients, which appear to be consistent with the hypotheses derived from constant pattern analysis. A dual kinetic model to describe mass transfer was found to better describe the tail behavior in the monolith, whereas a pseudo-first-order model was sufficient to describe breakthroughs on packed beds. A substantial two-order magnitude decrease in mass transfer coefficients was noted when reducing the feed concentration from 5.6% to 400 ppm CO2, thus underscoring the significant mass transfer limitations observed in DAC. Comparison between the contactors revealed notably higher mass transfer coefficients in the monolith compared to the packed beds, which are attributed to shorter diffusion lengths and lower equilibrium capacity. While the faster mass transfer coefficients observed in the monolith experiments led to reduced specific energy consumption and increased adsorption productivity compared to the packed bed at 400 ppm, no significant improvement was observed for the same process at the higher concentration of 5.6% CO2 in the feed. This finding highlights the need to tailor the contactor design to the specific gas separation requirements. This research contributes to the understanding and quantification of mass transfer kinetics at DAC concentrations in both packed bed and monolith contactors. It demonstrates the crucial role of the contactor in DAC systems and the importance of optimizing the adsorption step to enhance productivity and DAC performance.

Assessing Impacts of Atmospheric Conditions on Efficiency and Siting of Large-Scale Direct Air Capture Facilities

The cost and efficiency of direct air capture (DAC) of carbon dioxide (CO2) will be decisive in determining whether this technology can play a large role in decarbonization. To probe the role of meteorological conditions on DAC we examine, at 1 × 1° resolution for the continental United States (U.S.), the impacts of temperature, humidity, atmospheric pressure, and CO2 concentration for a representative amine-based adsorption process. Spatial and temporal variations in atmospheric pressure and CO2 concentration lead to strong variations in the CO2 available in ambient air across the U.S. The specific DAC process that we examine is described by a process model that accounts for both temperature and humidity. A process that assumes the same operating choices at all locations in the continental U.S. shows strong variations in performance, with the most influential variables being the H2O gas phase volume fraction and temperature, both of which are negatively correlated with DAC productivity for the specific process that we consider. The process also shows a moderate positive correlation of ambient CO2 with productivity and recovery. We show that optimizing the DAC process at seven representative locations to reflect temporal and spatial variations in ambient conditions significantly improves the process performance and, more importantly, would lead to different choices in the sites for the best performance than models based on a single set of process conditions. Our work provides a framework for assessing spatial variations in DAC performance that could be applied to any DAC process and indicates that these variations will have important implications in optimizing and siting DAC facilities.

Minimal Kinetic Model of Direct Air Capture of CO2 by Supported Amine Sorbents in Dry and Humid Conditions

Dilute concentration (∼400 ppm) and humidity are two important factors in the direct air capture (DAC) of CO2 by supported sorbents. In this work, a minimal DAC CO2 adsorption-kinetics model was formulated for supported amine sorbents under dry and humid conditions. Our model fits well with a recent DAC experiment with supported amine sorbent in both dry and humid conditions. Temperature and flow rate effects on breakthrough curves were quantitatively captured, and increasing temperature led to faster CO2 adsorption kinetics. Moisture was shown to broaden the breakthrough curve with slower CO2 adsorption kinetics but significantly improve the uptake capacity. The present minimal model provides a versatile platform for kinetic modeling of the DAC of CO2 on supported amine and other chemisorption systems.

Measurement of Physicochemical Properties and CO2, N2, Ar, O2, and H2O Unary Adsorption Isotherms of Purolite A110 and Lewatit VP OC 1065 for Application in Direct Air Capture

Direct air capture (DAC) using solid adsorbents has gained significant attention as a carbon dioxide removal (CDR) technology to help limit global temperature rise to below 2 °C. One large area of focus is the development of new adsorbent materials for DAC. However, the necessary data needed to employ these materials in process models for adsorbent screening are rarely available. Here, we showcase Purolite A110, a commercially available amine-functionalized polymeric resin, as a new candidate adsorbent for DAC and compare its properties to a current benchmark, Lewatit VP OC 1065. For both materials, we report their chemical features and composition, skeletal, particle, and bed density, total pore volume, particle porosity, BET area, thermal stability, and specific heat capacity. We determine their equilibrium sorption properties by measuring the volumetric CO2 isotherms at 288, 298, 308, 333, 343, 353, and 393 K, N2 and H2O isotherms at 288, 298, and 308 K, and Ar and O2 isotherms at 298 K. For CO2, N2, and H2O, we also present the corresponding isotherm model fitting parameters and heats of adsorption. These data can help facilitate process modeling and optimization studies to properly assess these adsorbents at scale.

Direct Air Capture of CO2 Using Amine/Alumina Sorbents at Cold Temperature

Rising CO2 emissions are responsible for increasing global temperatures causing climate change. Significant efforts are underway to develop amine-based sorbents to directly capture CO2 from air (called direct air capture (DAC)) to combat the effects of climate change. However, the sorbents' performances have usually been evaluated at ambient temperatures (25 °C) or higher, most often under dry conditions. A significant portion of the natural environment where DAC plants can be deployed experiences temperatures below 25 °C, and ambient air always contains some humidity. In this study, we assess the CO2 adsorption behavior of amine (poly(ethyleneimine) (PEI) and tetraethylenepentamine (TEPA)) impregnated into porous alumina at ambient (25 °C) and cold temperatures (-20 °C) under dry and humid conditions. CO2 adsorption capacities at 25 °C and 400 ppm CO2 are highest for 40 wt% TEPA-incorporated γ-Al2O3 samples (1.8 mmol CO2/g sorbent), while 40 wt % PEI-impregnated γ-Al2O3 samples exhibit moderate uptakes (0.9 mmol g-1). CO2 capacities for both PEI- and TEPA-incorporated γ-Al2O3 samples decrease with decreasing amine content and temperatures. The 40 and 20 wt % TEPA sorbents show the best performance at -20 °C under dry conditions (1.6 and 1.1 mmol g-1, respectively). Both the TEPA samples also exhibit stable and high working capacities (0.9 and 1.2 mmol g-1) across 10 cycles of adsorption-desorption (adsorption at -20 °C and desorption conducted at 60 °C). Introducing moisture (70% RH at -20 and 25 °C) improves the CO2 capacity of the amine-impregnated sorbents at both temperatures. The 40 wt% PEI, 40 wt % TEPA, and 20 wt% TEPA samples show good CO2 uptakes at both temperatures. The results presented here indicate that γ-Al2O3 impregnated with PEI and TEPA are potential materials for DAC at ambient and cold conditions, with further opportunities to optimize these materials for the scalable deployment of DAC plants at different environmental conditions.

Adsorption of Pb2+ and Cu2+ in wastewater by lignosulfonate adsorbent prepared from corn straw

The heavy metal ions contained in industrial wastewater are a great threat to human health. Exploring a adsorbent which have low-cost, green environmental friendly, high adsorption capacity, good recycle is key to solve heavy metal ions pollution. Lignin sulfonate was obtained by treating corn stover, and then modified lignin sulfonate was obtained by hydrothermal method. The porous structure makes heavy metal ions occupy more internal adsorption sites. Modified lignosulfonate adsorbent efficiency removes heavy metals in wastewater especially Cu2+ and Pb2+. The adsorption capacity of Cu2+ on modified lignosulfonate is 450.3 mg g-1, Pb2+ is 475.4 mg g-1. In addition, for 40 mg L-1 Cu2+ and Pb2+ using 0.4 g L-1, the adsorption equilibrium is only reached within 60 min. Meanwhile, the removal ratio of Pb is 83 %, Cd is 72 %, Cu is 87 %, Zn is 36 %, Mn is 25 %, Cr is 95 %, and Fe is 99 % in wastewater using 0.4 g L-1 adsorbent in 2 h. This research develops a practical adsorbent to remove heavy metals from actual wastewater.

Bibliometric review of carbon neutrality with CiteSpace: evolution, trends, and framework

The concept of carbon neutrality has been promoted and implemented in increasing countries since the twenty-first century. In-depth research on carbon neutrality has helped improve the environmental conditions and played a particular role in sustaining economic and social development. However, there is a less comprehensive review of the status in this field; therefore, this article uses the information visualization software CiteSpace to thoroughly analyze carbon neutrality research from multiple perspectives. This study aims to reveal the current research evolutions and hotspots in this field, predict future research trends, and construct the framework for better understanding. The results find that the number of papers published on carbon neutrality keeps increasing annually, and carbon neutrality has been the widely participated domain. In addition, publications by organizations and in top journals have aroused wide attention, and the hot spots on carbon neutrality have shifted to policy, recovery, and efficiency. Based on the results, a knowledge framework of this domain is constructed to give readers a clearer understanding of the evolvement and trends, which will also provide targeted references and help for future researchers.

Recent advances in direct air capture by adsorption

Significant progress has been made in direct air capture (DAC) in recent years. Evidence suggests that the large-scale deployment of DAC by adsorption would be technically feasible for gigatons of CO₂ capture annually. However, great efforts in adsorption-based DAC technologies are still required. This review provides an exhaustive description of materials development, adsorbent shaping, in situ characterization, adsorption mechanism simulation, process design, system integration, and techno-economic analysis of adsorption-based DAC over the past five years; and in terms of adsorbent development, affordable DAC adsorbents such as amine-containing porous materials with large CO₂ adsorption capacities, fast kinetics, high selectivity, and long-term stability under ultra-low CO₂ concentration and humid conditions. It is also critically important to develop efficient DAC adsorptive processes. Research and development in structured adsorbents that operate at low-temperature with excellent CO₂ adsorption capacities and kinetics, novel gas-solid contactors with low heat and mass transfer resistances, and energy-efficient regeneration methods using heat, vacuum, and steam purge is needed to commercialize adsorption-based DAC. The synergy between DAC and carbon capture technologies for point sources can help in mitigating climate change effects in the long-term. Further investigations into DAC applications in the aviation, agriculture, energy, and chemical industries are required as well. This work benefits researchers concerned about global energy and environmental issues, and delivers perspective views for further deployment of negative-emission technologies.

A graphical approach for mixed ratio optimisation in the binary mixed amine solution

Chemical absorption method plays an important role in the process of CO2 separation. One major problem for chemical absorption is huge energy consumption, which is affected by the performance of absorbents. Developing a type of absorbent with high absorption capacity and low regenerative energy consumption is a research topic that attracts attention. The combination of two or more amines is one way to develop new solvents. However, the change of amine liquid ratio can cause a series of complex nonlinear changes in absorption capacity, absorption heat, the heat of vaporisation and sensible heat. It is of interest to visualise the amine solution mixing ratio optimisation to help reduce the energy consumption and increase the absorption capacity. Derivative analysis of standardised vs variables diagram (DSVD), a kind of graphical method based on maximum benefit and minimum consumption, is proposed to determine the optimal mixing ratio of binary amine solution. This novel approach helps to visualise what kind of amines are not suitable for compounding, what kind of amines have the best compounding ratio, and how to determine the optimal compounding ratio. The optimal mixing ratio of the Methyldiethanolamine (MDEA) - Piperazine (PZ) system and MDEA - Monoethanolamine (MEA) were optimised by this method. The optimal ratio of MDEA - PZ and MDEA - MEA are 0.6 (PZ: MDEA = 0.6:0.4, wt.%) and 0.8 (MEA: MDEA = 0.8:0.2, wt.%).