STUDY OF AMINOSILICA ADSORBENTS FOR CO2 CAPTURE: EXPERIMENTAL STRUCTURE-FUNCTION RELATIONSHIPS

Open Access
Author:
Kamali Shahri, Seyed Mehdi Mehdi
Graduate Program:
Energy and Mineral Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 06, 2017
Committee Members:
  • Robert Rioux, Dissertation Advisor
  • Caroline Clifford, Committee Chair
  • Chunshan Song, Committee Member
  • Xiaoxing Wang, Committee Member
  • Ramakrishnan Rajagopalan, Outside Member
Keywords:
  • CO2 adsorption
  • Kinetics
  • thermodynamics
  • heat of adsorption
  • efficiency
  • aminosilica material
  • type I adsorbent
  • surface chemistry
  • surface modification
  • CO2 adsorption modeling
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 iv 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 60oC 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 60oC 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 60oC 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. v 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. The kinetic investigation on the BTR/DSC combination showed similar results in terms of capacity, heat of adsorption, temperature variation, and secondary amine addition. High amine- OH interaction and low CO2 diffusivity into multilayer amines were found as the major issues for the reduction in amine capacity and efficiency. For instance, the efficiencies for 10 wt% TETA impregnated on silica and silica-modified surfaces (with octyl groups) at 60oC increased significantly from 0.16 to 0.35, respectively, indicative of reduced amine-OH interaction. In addition, efficiency was enhanced from 0.17 to 0.26 for 40 wt% TETA/SiO2 as the temperature ascended from 25oC to 80oC, revealing the effect of facilitated diffusion. Increasing the number of secondary amines decreased the optimum heat of adsorption for the highest overall rates and also increased the overall rates. For example, as the 2o/1o ratio increased from 1 to 2 for 40 wt% amine-impregnated silica at 40oC, the optimum heat of adsorption was reduced from 85 to 69 kJ/molCO2 and the overall rates were enhanced from 0.013 to 0.015 mmolCO2/g.s. This indicates that an increase in the secondary amine ratio offers several benefits for CO2 adsorption. Surface functionalization toward hydrophobicity could also assist to improve capacity, efficiency, and CO2 adsorption kinetics as exemplified above for efficiency. For example, the overall adsorption rate for 10 wt% TETA/SiO2 at 25oC increased from 0.0075 to 0.0122 mmolCO2/g.s as the hydroxyl groups on the support were replaced with methyl groups. A rigorous spatiotemporal modeling was applied to the BTR/DSC data to estimate the kinetics and thermodynamic parameters at isothermal conditions. This unique mathematical model predicted the adsorption and desorption rate constant as well as the heat of adsorption. The model circumvented unphysical simplifications, such as linear driving force and uniform adsorption rates, by considering dispersion and convection phenomena.