Water and CO adsorptions were studied over the stoichiometric and the oxygen-deficient Cu₂O(111) surfaces, using thermal desorption spectroscopy (TDS), ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS). Water is the only desorbing species detected in TDS and the extent of dissociation is unaffected by the surface condition: ≃ 0.25 monolayers of water dissociate on Cu₂O(111) regardless of surface condition. The local defect environment around oxygen vacancies does not play a significant role in the activity of the Cu₂O(111) surface for the dissociation of water. CO is found to bind molecularly to the surface through the carbon atom and with a heat of adsorption of 22 kcal/mol, higher value than that of CO on Cu₂O(100) (16.7 kcal/mol). This suggests that the local geometry of adsorption sites may play an important role in the way CO binds to Cu₂O surfaces. Electronic changes upon CO adsorption and the higher heat of adsorption indicate an increased σ-donor character for CO, with some π-backbonding interactions. The local defect environment around oxygen vacancies does not appear to affect CO adsorption on Cu₂O(111) surfaces. / Master of Science

Porous inorganic solids including mesoporous silicas, zeolites and silicoalumnio-phosphates have been investigated as adsorbents for carbon dioxide, particularly in relation to uptake from flue gases at 0.1 bar and ca. 298 K, but also at higher pressures. The mesoporous silicas SBA-1 and SBA-2, with mesocages separated by narrower windows, have been prepared, calcined at various temperatures and also nitrided with ammonia at high temperature. Nitridation has resulted in framework nitrogen incorporation, but this gave only small increases in the uptake of CO₂ of these mesoporous silicas, which are very low (< 0.2 mmol g⁻¹) at flue gas conditions (0.1 bar, 298 K). A series of cationic forms of the small pore zeolites, chabazite, ZK-5 and Rho, have been prepared by exhaustive cation exchange (and pre-calcination of the as-prepared form of Rho). In addition, a series of ultrastabilised zeolite Rho samples has been prepared to investigate the influence of extra-framework aluminium species on CO₂ uptake. For comparison, the silicoaluminophosphate versions of ZK-5 (SAPO STA-14) and Rho (SAPO(RHO)) have been prepared. Adsorption on Li-, Na-, K- and Ca-forms of chabazite (Si/Al = 3.0) has been related to the crystal structures of their dehydrated forms, as determined by Rietveld refinement against powder X-ray diffraction data (PXRD). For Na- and K-chabazite the structure has been measured in situ by PXRD during CO₂ adsorption. Li-chabazite has the highest uptake from all chabazite cationic forms (4.3 mmol g⁻¹). PXRD of K-chabazite reveals cation migration from eight-membered ring sites to six-membered ring sites upon CO₂ adsorption. Na-chabazite shows partial transformation from rhombohedral to monoclinic symmetry upon prolonged evacuation at high temperature, with resultant non-Type I CO₂ adsorption behaviour. Li-, Na- and K-forms of ZK-5 (Si/Al = 4.16) show high CO₂ uptakes at 0.1 bar and 298 K (Li-ZK-5, 4.7 mmol g⁻¹, which is the highest of the solids measured here). Like all H-forms, H-ZK-5 shows weaker uptake. None of the ZK-5 forms show high selectivity for CO₂ over small hydrocarbons, because cations do not block eight-membered ring windows and the structures do not distort upon dehydration. Uptake of CO₂ on univalent cation forms of zeolite Rho has been studied at low (up to 1 bar) and high (up to 10 bar) pressures. All cationic forms (but not H-Rho) show distortion (Im3̅m to I4̅3m) upon dehydration. Forms of zeolite Rho in which cations occupy window sites in the eight-membered rings between α-cages show hysteresis in their CO₂ isotherms, the magnitude of which (Na⁺,NH₄⁺ < K⁺ < Cs⁺) correlates with the tendency of cations to occupy double eight-membered ring sites rather than single eight-membered ring sites. Additionally, reversible CO₂ uptake using the Zero Length Column method on fully and partially cation exchanged samples has been measured. In situ synchrotron PXRD of CO₂ adsorption on Na-Rho indicates Na cations remain in window sites on the time average, indicating CO₂ uptake must occur by a 'trapdoor mechanism' by which Na cations move away from the windows to allow CO₂ to adsorb. In addition, in situ PXRD reveals the adsorption sites of CO₂ bound cations. Adsorption of small hydrocarbons does not occur on Rho, even at high pressure, indicating that adsorption is selective, and depends on the degree of interaction with the adsorbate rather than simply on the molecular size. Na-Rho is therefore a selective adsorbent for CO₂ over CH₄ with selectivities of 150–25 at 1–9 bar and 298 K, predicted from the single component isotherms, and an uptake of 3.07 mmol g⁻¹ at 0.1 bar. High ‘selectivities' are also observed over K-, Cs- and Ca-forms, examples of a novel type of adsorption selectivity.

This work evaluates stripper performance for CO₂ capture using seven potential solvent formulations and seven stripper configurations. Equilibrium and rate models were developed in Aspen Custom Modeler (ACM). The temperature approach on the hot side of the cross exchanger was varied between 5 - 10°C. The results show that operating the cross exchanger at a 5°C approach results in 12% energy savings for a 7m MEA rich solution of 0.563 mol/mol Alk and 90% CO₂ removal. For solvents with [Delta]H[subscript abs] < 60 kJ/gmol CO₂, stripping at 30 kPa is more attractive than stripping at 160 kPa. Normal pressure (160 kPa) favors solvents with high heats of desorption. The best solvent and process configuration, matrix with MDEA/PZ, offers 22% and 15% energy savings over the baseline and improved baseline, respectively, with stripping and compression to 10 MPa. The energy requirement for stripping and compression to 10 MPa is about 20 % of the power output from a 500 MW power plant with 90% CO2 removal. Rate model results show that a 'short and fat' stripper requires 7 to 15% less equivalent work than a 'tall and skinny' one. The optimum stripper design could be one that operates between 50% and 80% flood at the bottom. Stripping at 30 kPa and 160 kPa require 230 s and 115 s of effective packing volume to get an equivalent work 4% greater than the minimum. Stripping at 30 kPa with [Delta]T = 5°C was controlled by mass transfer with reaction in the boundary layer and diffusion (88% resistance at the rich end and 71% resistance at the lean end) and mass transfer with equilibrium reactions (84% resistance at the rich end and 74% resistance at the lean end) at 160 kPa. The model was validated with data obtained from pilot plant experiments at the University of Texas with 5m K⁺/2.5m PZ and 6.4m K⁺/1.6m PZ under normal pressure and vacuum conditions using Flexipac AQ Style 20 structured packing. Foaming was experienced during tests. The effective packing height was 5.09m for 5m K⁺/2.5m PZ and 6.47m for 6.4m K⁺/1.6m PZ. / text

Thesis (PhD)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The reactive absorption rate of CO2 into non-aqueous solvents containing the primary amine, mono-ethanolamine (MEA) is recognised as a suitable method for measuring the effective interfacial mass transfer area of separation column internals such as random and structured packing. Currently, this method is used under conditions where the concentration of MEA in the liquid film is unaffected by the reaction and the liquid phase reaction is, therefore, assumed to obey pseudo first order kinetics with respect to CO2. Under pseudo first order conditions, the effect of surface depletion and renewal rates are not accounted for. Previous research indicated that the effective area available for mass transfer is also dependent upon the rate of surface renewal achieved within the liquid film. In order to study the effect of surface depletion and renewal rates on the effective area, a method utilising a fast reaction with appreciable depletion of the liquid phase reagent is required. The homogeneous liquid phase reaction kinetics of CO2 with MEA n-Propanol as alcoholic solvent was investigated in this study. A novel, in-situ Fourier Transform Infra-Red (FTIR) method of analysis was developed to collect real time concentration data from reaction initiation to equilibrium. The reaction was studied in a semi-batch reactor set-up at ambient conditions (T = 25°C, 30°C and 35°C, P = 1 atm (abs)). The concentration ranges investigated were [MEA]:[CO2] = 5:1 and 10:1. The concentration range investigated represents conditions of significant MEA conversion. The reaction kinetic study confirmed the findings of previous research that the reaction of CO2 with MEA is best described by the zwitterion reactive intermediate reaction mechanism. Power rate law and pseudo steady state hypothesis kinetic models (proposed in literature) were found to be insufficient at describing the reaction kinetics accurately. Two fundamentally derived rate expressions (based on the zwitterion reaction mechanism) provided a good quality model fit of the experimental data for the conditions investigated. The rate constants of the full fundamental model were independent of concentration and showed an Arrhenius temperature dependence. The shortened fundamental model rate constants showed a possible concentration dependence, which raises doubt about its applicability. The specific absorption rates (mol/m2.s) of CO2 into solutions of MEA/n-Propanol (0.2 M and 0.08 M, T = 25°C and 30°C, P = ±103 kPa) were investigated on a wetted wall experimental setup. The experimental conditions were designed for a fast reaction in the liquid film to occur with a degree of depletion of MEA in the liquid film. Both interfacial depletion and renewal of MEA may be considered to occur. The gas phase resistance to mass transfer was determined to be negligible. An increase in liquid turbulence caused an increase in the specific absorption rate of CO2 which indicated that an increase in liquid turbulence causes an increase in effective mass transfer area. Image analysis of the wetted wall gas-liquid interface confirmed the increase in wave motion on the surface with an increase in liquid turbulence. The increase in wave motion causes an increase in both interfacial and effective area. A numerical solution strategy based on a concentration diffusion equation incorporating the fundamentally derived rate expressions of this study is proposed for calculating the effective area under conditions where surface depletion and renewal rates are significant. It is recommended that the reaction kinetics of CO2 with MEA in solvents of varying liquid properties is determined and the numerical technique proposed in this study used to calculate effective area from absorption rates into these liquids. From the absorption data an effective area correlation as a function of liquid properties may be derived in future. / AFRIKAANSE OPSOMMING: Die reaktiewe absorpsie van CO2 in nie-waterige oplossings van die primêre amien, monoetanolamien (MEA) word erken as ‘n geskikte metode om die effektiewe massaoordragsarea van gepakte skeidingskolomme te bepaal. Tans word die metode gebruik onder vinnige pseudo eerste orde reaksietoestande met betrekking tot CO2. Die pseudo eersteorde aanname beteken dat die konsentrasie van MEA in die vloeistoffilm onbeduidend beïnvloed word deur die reaksie en effektief konstant bly. Onder pseudo eerste orde toestande word oppervlakverarming- en oppervlakvernuwingseffekte nie in ag geneem nie, juis as gevolg van die konstante konsentrasie van MEA in die vloeistoffilm. Daar is voorheen bevind dat oppervlakverarming en oppervlakvernuwing ‘n beduidende invloed het op die beskikbare effektiewe massaoordragsarea. Hierdie invloed kan slegs bestudeer word met ‘n vinnige reaksie in die vloeistoffilm wat gepaard gaan met beduidende oppervlakverarming van die vloeistoffase reagens. Die homogene vloeistoffase reaksiekinetika van CO2 met MEA in die alkohol oplosmiddel, n- Propanol, is in hierdie studie ondersoek. ‘n Nuwe, in-situ Fourier Transform Infra-Rooi (FTIR) metode van analiese is ontwikkel in hierdie ondersoek. Die reaksie is ondersoek in ‘n semienkelladings reaktor met MEA wat gevoer is tot die reaktor om met die opgeloste CO2 te reageer. Die FTIR metode meet spesiekonsentrasie as ‘n funksie van tyd sodat die konsentrasieprofiele van CO2, MEA en een van die soutprodukte van die reaksie gebruik kan word om verskillende reaksiesnelheidsvergelykings te modelleer. Die reaksie is ondersoek onder matige toestande (T = 25°C, 30°C and 35°C, P = 1 atm (abs)). Die konsentrasiebereik van die ondersoek was [MEA]:[CO2] = 5:1 en 10:1. Hierdie bereik is spesifiek gebruik sodat daar beduidende omsetting van MEA kon plaasvind. Die reaksiekinetieka studie het, ter ondersteuning van bestaande teorie, bevind dat die reaksie van CO2 met MEA in nie-waterige oplosmiddels soos alkohole, beskyf word deur ‘n zwitterioon reaksiemeganisme. Die bestaande reaksiesnelheids modelle (eksponensiële wet en pseudo gestadigde toestand hipotese) kon nie die eksperimentele data met genoegsame akuraatheid beskryf nie. Twee nuwe reaksiesnelheidsvergelykings, afgelei vanaf eerste beginsels en gebaseer op die zwitterioon meganisme, word voorgestel. Hierdie volle fundamentele model het goeie passings op die eksperimentele data getoon oor die volledige temperatuur en konsentrasiebereik van hierdie studie. Die reaksiekonstantes van die fundamentele model was onafhanklik van konsentrasie en tipe oplosmiddel en het ‘n Arrhenius temperatuurafhanklikheid. Die verkorte fundamentele model se reaksiekonstantes het ‘n moontlike konsentrasieafhanlikheid gewys. Dit plaas onsekerheid op die fundamentele basis van hierdie model en kan dus slegs as ‘n eerste benadering beskou word. Die spesifieke absorpsietempos (mol/m2.s) van CO2 in MEA/n-Propanol oplossings (0.2 M en 0.08 M MEA, T = 25°C and 30°C, P = ±103 kPa) is ondersoek met ‘n benatte wand (‘wetted wall’) eksperimentele opstelling. Die eksperimentele toestande is gekies sodat daar ‘n vinnige reaksie in die vloeistoffilm plaasgevind het, met beide beduidende en nie-beduidende MEA omsetting. Die doel met hierdie eksperimentele ontwerp was om die invloed van intervlakverarming en intervlakvernuwing op die spesifieke absorpsietempo te ondersoek. Gas fase weerstand was nie-beduidend onder die eksperimentele toestande nie. Beide intervlakverarming en intervlakvernuwing gebeur gelyktydig en is waargeneem vanuit die eksperimentele data. ‘n Beeldverwerkingstudie van die gas-vloeistof intervlak van die benatte wand het bevind dat daar ‘n toename in golfaksie op die vloeistof oppervlak is vir ‘n toename in vloeistof turbulensie. Hierdie golfaksie dra by tot oppervlakvernuwing en ‘n toename in effektiewe massaoordragsarea. ‘n Numeriese metode word voorgestel om die effektiewe area van beide die benatte wand en gepakte kolomme te bepaal vanaf reaktiewe absorpsietempos. Die metode gebruik die fundamentele reaksiesnelheidsvergelykings, bepaal in hierdie studie, in a konsentrasie diffusievergelyking sodat oppervlakverarming en vernuwing in ag geneem kan word. Daar word voorgestel dat die reaksiekinetika van CO2 met MEA in oplossings met verskillende fisiese eienskappe (digtheid, oppervlakspanning en viskositeit) bepaal word sodat die numeriese metode gebruik kan word om ‘n effektiewe area korrelasie as ‘n funksie van hierdie eienskappe te bepaal.

M. Tech. Engineering Chemical. / Aims to investigate how the addition of impurities in a CO2 stream affects the adsorption capacity of CO2 on South African coals. To achieve this aim, the following objectives were carried out. 1. To measure the adsorption isotherms and adsorption capacities of pure CO2 and flue gas mixtures on various South African coals under in-seam conditions including pressures up to 88 bar and isothermal temperature of 35 &#x00BA%x;C; 2. To evaluate the effects of coal rank on the adsorption isotherms and adsorption capacities of pure CO2 and flue gas mixtures; 3. To do a comparative study to evaluate the effects of CO2 impurities on the adsorption capacity of pure CO2 on coal; 4. To study the degree of preferential sorption of the individual flue gas mixtures components on coal; 5. To determine the suitability of the Langmuir, Freundlich, and Temkin adsorption isotherm models in representing pure CO2 adsorption onto coal; and 6. To determine the suitability of Extended Langmuir (EL) adsorption models in representing the flue gas mixture adsorption onto coal.

The rise of anthropogenic CO₂ emissions and the associated increasing levels of CO₂ in the atmosphere are expected to bring uninhabitable conditions on earth due to global climate change and numerous associated environmental crises. To reduce these impacts, warming must be kept to 1.5°C above pre-industrial levels. With the slow transition to more favorable energy generation methods with lower carbon emissions, it is clear that power plants utilizing fossil fuel combustion for electricity will not be reduced to an acceptable level. Thus, the deployment of negative emission and carbon capture, utilization, and storage (CCUS) technologies will be crucial to meet the 1.5°C target. The current state-of-the-art carbon capture technology is point source amine scrubbing, in which diluted aqueous amine solutions are used to absorb CO₂ from power plant flue gases. The CO₂ is captured at ~40°C by the formation of an amine/H₂O—CO₂ complexes. The absorbent solution, now containing CO₂, must be heated to release the CO₂ for further processing and to regenerate the amine for recycling. Though this technology is well-engineered and commercially available, there are some major drawbacks such as energy intensity mainly due to vaporization of the water during CO₂ separation from the amine, corrosivity of the amine material, and the need to transport the released CO₂ for further utilization or sequestration. To this end, dual function materials (DFM) were developed to address these issues. DFMs eliminate the need for the energy intensive regeneration of liquid amine solutions and transportation of CO₂. Comprised of both a capture and catalytic component co-dispersed on the same high surface area carrier, the DFM is able to selectively capture CO₂ from the effluent flue gas and catalytically convert it to methane (or renewable natural gas) with the introduction of preferably renewable H₂ in the same reactor. The DFM can operate isothermally at around 320°C by harnessing the sensible heat of typical power plant effluent flue gases. 5% Ru, 6.1% “Na₂O”/Al₂O₃ was shown to be a very robust, demonstrating stable performance after 50 cycles of capture and catalytic conversion with simulated flue gas. In addition to point-source capture, negative emission technologies like direct air capture (DAC) are required to mitigate climate change. Thus, we investigate the use of DFM for a new application – the direct air capture of CO₂ and subsequent catalytic methanation. Furthermore, for such applications, the loading of Ru was dramatically decreased to alleviate the economic burden for commercialization and wide-scale deployment. This thesis demonstrates the flexibility of the DFM as a carbon capture technology for direct air capture of CO₂ at ambient air temperatures and subsequent methanation (DAC-M) at temperatures in excess of 200°C. Recognizing the energy intensity of isothermal DAC-M operation, the capture and conversion cycles were modified for temperature-swing operation, with adsorption occurring at 25°C, followed by heating up to 300°C in H₂ for methanation. Short-term aging was conducted on 1% Ru, 10% Na₂O/Al₂O₃ in a packed bed configuration for 10 cycles of adsorption in dry air conditions (400ppm CO₂ in air) and methanation. The sample was also tested in humid adsorption conditions (400ppm CO₂, ~2% H₂O/air) to better simulate ambient environments. These tests showed that the DFM is able to operate in a temperature swing mode and exhibits a higher, stable CO₂ adsorption capacity in humid conditions unlike other capture technologies using amines and physical adsorption methods, which show a significant decline in capture capacity. We were able to establish that the DFM has great potential for DAC-M. Consequently, these materials are moving towards advanced process development with our engineering partners under the sponsorship of DOE. Critical parameters such as pressure drop, heating rate, and methanation temperature are primary parameters that must be optimized. New low Ru loading DFMs, 0.5% Ru and 1% Ru DFM, were aged with simulated power plant flue gas (7.5% CO₂, 4.5% O₂, 15% steam, balance N2) for over 50 cycles of capture and catalytic conversion to CH4 (and water) in a packed bed configuration. These conditions of continuous operation at 320°C with 15% steam and 4.5% O₂ are far more severe than for DAC which adsorbs low levels of CO₂ from air at ambient air conditions (0-40°C with 2-5% moisture). Therefore, these power plant effluent test conditions can be considered accelerated aging for DAC-M. A reduced level of 0.5-1% Ru DFM was tested under simulated power plant effluent conditions on several Al₂O₃ structures, particularly tablets and ring tablets for scale-up of the technology. These tests showed a subtle but gradual deactivation of the material. Characterization with CO chemisorption and in-situ FT-IR indicated that the Ru component is deactivated – most likely by sintering – due to the presence of O₂ and H₂O in the flue gas. Microreactor studies show that in the presence of O₂ and H₂O, adsorption capacity is reduced and the rate of methanation is decreased. Upon removing O₂ and H₂O from the adsorption step, the adsorption capacity is restored but the rate and selectivity of methanation declines steadily, indicating that the deactivation is irreversible. Interestingly, the DFM with higher Ru loading showed more stable performance suggesting that higher catalytic content is required for more improved stability. Fortunately, Ru can be leased and recycled, reducing the capital economic burden of higher Ru loadings. Additionally, we expect that given the milder conditions for capture in DAC scenarios, low Ru loaded DFMs will be more stable. Initial DAC-M data substantiates this stability, but longer aging times are required for confirmation. Furthermore, stability may be favored with the use of higher concentration H₂. Finally, this thesis also investigates the use of other Ru+sorbent/carrier combinations for DAC and the apparent enhancement of adsorption arising from the use of a reactive sorbent (e.g., addition of Ru). After screening Al₂O₃-supported Na₂O, CaO, MgO, and BaO in combination with 1% Ru, we are able to show that Ru+Na₂O/Al₂O3 has the best adsorption capacity. This material, relative to the other alkaline oxides studied, shows a unique enhancement in the CO₂ adsorption capacity compared to the bare supported sorbent (Na₂O/Al₂O₃). The enhancement effect is shown to be an asymptotic function of an increasing Ru loading, plateauing after 3% Ru. ZrO₂-supported Ru+Na₂O is also tested but does not show favorable adsorption capacity, indicating that Al₂O₃ is also a crucial component of the DFM formulation. This technology is the subject of a provisional patent application.