Study of Aminosilica Adsorbents for CO2 Capture PDF Download

Author: Seyed Mehdi Kamali Shahri
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Languages : en
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Book Description
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. Asa result, the irrefutable influence of anthropogenic CO2 released into the environment hasconsiderably intensified global warming. Coal- and gas-fired power plants are considered one ofthe 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 theinterim an actual solution is essential to remove the CO2 produced before its emission into theatmosphere. Among various capturing processes, post-combustion capture is highly promisingdue to the flexibility of CO2 removal via liquid or solid materials. The captured CO2 is thensequestered or converted into new chemical compounds. The capturing process is the mostimportant and energy-intensive step. A major advantage of liquid phase adsorbents is their highcapacity; 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. Inthe operating conditions of power plants, amine-impregnated support (Type I) sorbents are themost promising among various solid sorbents, due to the high density of nitrogen-active sites, butsuffer from low capacity and efficiency compared to liquid phase absorption process. In order toapproach the problem and understand the origin of this low efficiency, a scientific understandingof the interaction between CO2 and amine-impregnated supports and the influential parametersinvolved is necessary to further develop new and high-efficiency amine-based adsorbents.Novel experimental techniques have been utilized in this research to assess the kineticsand thermodynamics of CO2 adsorption. The influence of structure (linear vs. branch), aminedensity, amine type (primary, secondary, and tertiary), support surface functionalization, andoperating conditions on the thermodynamics and kinetics of CO2 adsorption have been studied. Aivcombination of volumetric adsorption (VA) and differential scanning calorimetry (DSC) havebeen used to study the equilibrium capacity and thermodynamic parameters. The kinetic study hasbeen conducted through a breakthrough reactor (BTR) coupled with a DSC to evaluate CO2adsorption kinetics.At the equilibrium, linear amines, compared to branched amines, indicate a larger CO2adsorption capacity and lower apparent heat of adsorption. For example, the capacity and heat ofadsorption for 40 wt% linear and branch polyethylenimine (PEI) measured to be 3.68 and 2.36mmolCO2/g, along with 68 and 71 kJ/molCO2 at 60oC and 1 bar CO2, respectively. The apparentheat of CO2 adsorption on amine sorbents consists of the intrinsic heat of adsorption, the energyrequirement for diffusion, and amine reorganization, which then approached the intrinsic heat ofadsorption 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. Forinstance, TETA/SiO2 samples showed adsorption capacity enhancements from 0.34 to 1.87mmolCO2/g and heat of adsorption from 45 to 77 kJ/molCO2 as the weight loading increasedfrom 5 to 40 wt% at 60oC and 1 bar CO2. Increasing the secondary amine in the linear structurealso assisted in enhancing capacity and decreasing heat of adsorption. For example, the CO2uptake for TETA and PEI423 increased from 1.87 to 3.68 mmolCO2/g and the heat of adsorptiondeclined from 77 to 68 kJ/molCO2 at 60oC and 1 bar CO2. Polyethylenimine therefore presenteda better performance than molecular amines, which makes PEI more suitable for industrialapplications. The criteria defined by the National Energy and Technology (NETL) for industrialutilization requires 3-6 mmolCO2/g adsorbent capacity to compete with current for carbon captureand sequestration (CCS) technologies. As yet, the criteria have been met; nevertheless, theadsorption efficiency displayed much lower values compared to the theoretical expectationsbased on the proposed mechanism. For example, in theory, the efficiency for dry conditions isexpected to be 0.5, while reports in the literature revealed values of less than 0.3 in experiments.vEfficiency increases directly enhance on total capacity. Moreover, a decrease in heat ofadsorption also provides a more appealing situation for real application in view of the fact that theenergy penalty for regeneration is reduced.The kinetic investigation on the BTR/DSC combination showed similar results in termsof 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 forthe reduction in amine capacity and efficiency. For instance, the efficiencies for 10 wt% TETAimpregnated on silica and silica-modified surfaces (with octyl groups) at 60oC increasedsignificantly from 0.16 to 0.35, respectively, indicative of reduced amine-OH interaction. Inaddition, efficiency was enhanced from 0.17 to 0.26 for 40 wt% TETA/SiO2 as the temperatureascended from 25oC to 80oC, revealing the effect of facilitated diffusion. Increasing the numberof secondary amines decreased the optimum heat of adsorption for the highest overall rates andalso 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 69kJ/molCO2 and the overall rates were enhanced from 0.013 to 0.015 mmolCO2/g.s. This indicatesthat an increase in the secondary amine ratio offers several benefits for CO2 adsorption. Surfacefunctionalization toward hydrophobicity could also assist to improve capacity, efficiency, andCO2 adsorption kinetics as exemplified above for efficiency. For example, the overall adsorptionrate for 10 wt% TETA/SiO2 at 25oC increased from 0.0075 to 0.0122 mmolCO2/g.s as thehydroxyl groups on the support were replaced with methyl groups. A rigorous spatiotemporalmodeling was applied to the BTR/DSC data to estimate the kinetics and thermodynamicparameters at isothermal conditions. This unique mathematical model predicted the adsorptionand desorption rate constant as well as the heat of adsorption. The model circumventedunphysical simplifications, such as linear driving force and uniform adsorption rates, byconsidering dispersion and convection phenomena.