Study of Aminosilica Adsorbents for CO2 Capture: Experimental Structure-Function Relationships
Abstract
The global demand for energy has increased continuously since the industrial revolution. Fossil fuels such as coal, natural gas, and oil are the primary sources that satisfy this demand. As a result, the irrefutable influence of anthropogenic CO2 released into the environment has considerably intensified global warming. Coal- and gas-fired power plants are considered one of the major source points of fossil fuel consumption. Although renewable energy (i.e., solar, wind, and others) is considered as the ideal alternative to satisfy the future energy demand, in the interim an actual solution is essential to remove the CO2 produced before its emission into the atmosphere. Among various capturing processes, post-combustion capture is highly promising due to the flexibility of CO2 removal via liquid or solid materials. The captured CO2 is then sequestered or converted into new chemical compounds. The capturing process is the most important and energy-intensive step. A major advantage of liquid phase adsorbents is their high capacity; however, they suffer significantly from a high energy penalty. Solid phase adsorption, which has a lower energy requirement for regeneration, has therefore attracted much attention. In the operating conditions of power plants, amine-impregnated support (Type I) sorbents are the most promising among various solid sorbents, due to the high density of nitrogen-active sites, but suffer from low capacity and efficiency compared to liquid phase absorption process. In order to approach the problem and understand the origin of this low efficiency, a scientific understanding of the interaction between CO2 and amine-impregnated supports and the influential parameters involved is necessary to further develop new and high-efficiency amine-based adsorbents.Novel experimental techniques have been utilized in this research to assess the kinetics and thermodynamics of CO2 adsorption. The influence of structure (linear vs. branch), amine density, amine type (primary, secondary, and tertiary), support surface functionalization, and operating conditions on the thermodynamics and kinetics of CO2 adsorption have been studied. A combination of volumetric adsorption (VA) and differential scanning calorimetry (DSC) have been used to study the equilibrium capacity and thermodynamic parameters. The kinetic study has been conducted through a breakthrough reactor (BTR) coupled with a DSC to evaluate CO2 adsorption kinetics.At the equilibrium, linear amines, compared to branched amines, indicate a larger CO2 adsorption capacity and lower apparent heat of adsorption. For example, the capacity and heat of adsorption for 40 wt% linear and branch polyethylenimine (PEI) measured to be 3.68 and 2.36 mmolCO2/g, along with 68 and 71 kJ/molCO2 at 60°C and 1 bar CO2, respectively. The apparent heat of CO2 adsorption on amine sorbents consists of the intrinsic heat of adsorption, the energy requirement for diffusion, and amine reorganization, which then approached the intrinsic heat of adsorption when the necessary energy was provided for CO2 diffusion and amine conformation. Augmenting the amine weight loading also increased the capacity and heat of adsorption. For instance, TETA/SiO2 samples showed adsorption capacity enhancements from 0.34 to 1.87 mmolCO2/g and heat of adsorption from 45 to 77 kJ/molCO2 as the weight loading increased from 5 to 40 wt% at 60°C and 1 bar CO2. Increasing the secondary amine in the linear structure also assisted in enhancing capacity and decreasing heat of adsorption. For example, the CO2 uptake for TETA and PEI423 increased from 1.87 to 3.68 mmolCO2/g and the heat of adsorption declined from 77 to 68 kJ/molCO2 at 60°C and 1 bar CO2. Polyethylenimine therefore presented a better performance than molecular amines, which makes PEI more suitable for industrial applications. The criteria defined by the National Energy and Technology (NETL) for industrial utilization requires 3-6 mmolCO2/g adsorbent capacity to compete with current for carbon capture and sequestration (CCS) technologies. As yet, the criteria have been met; nevertheless, the adsorption efficiency displayed much lower values compared to the theoretical expectations based on the proposed mechanism. For example, in theory, the efficiency for dry conditions is expected to be 0.5, while reports in the literature revealed values of less than 0.3 in experiments. Efficiency increases directly enhance on total capacity. Moreover, a decrease in heat of adsorption also provides a more appealing situation for real application in view of the fact that the energy penalty for regeneration is reduced.(Abstract shortened by ProQuest.).
- Publication:
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Ph.D. Thesis
- Pub Date:
- 2017
- Bibcode:
- 2017PhDT.......402S
- Keywords:
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- Chemical engineering;Nanotechnology;Materials science