Development of Criteria for Selection of Components for Formulation of Amine Blends Based on Structure and Activity Relationships of Amines, and Validation of Formulated Blends in a Bench Scale CO2 Capture Pilot Plant

Abstract

Due to modernisation and industrialisation, an increase in the global energy demand is inevitable. Nuclear, fossil fuels, renewables, hydro and biomass are the major sources of energy. However, considering the current energy framework, fossil fuels appear to be the most reliable and stable energy source. As a result, emphasis on the reduction of emissions of carbon dioxide (CO2, a major type of greenhouse gas (GHG)) is very crucial because almost all fossil fuel activities lead to generation of this environmentally harmful GHG. Scientists have shown that the average global temperature has increased by up to about 1 degree over the last century. Thus, if this issue is left unabated, it will have long lasting consequences both on human lives and the environment. Extreme weather conditions, heat waves, sea level rise, wild fires, health problems are glaring repercussions of global warming and climate change. Various strategies such as use of alternative energy, energy conservation or fossil fuel-energy coupled with carbon capture and sequestration (CCS) are all attempts to mitigate this problem. However, CCS stands out to be the anchor technique due to its compatibility with existing energy infrastructure in conjunction with the reliability of fossil fuel-based energy itself. Post combustion capture which uses a regenerable liquid sorbent, appears to be the predominant technology used and has proven to be successful in most industrial applications. However, this technology is still far from being perfect. My thesis research addresses the imperfections and challenges identified with this technique from the context of sorbent formulation. Optimum sorbent performance cannot be achieved with one single amine sorbent. It is therefore essential to develop new amine sorbent systems by blending and combining their individual strengths to achieve an optimum performance sorbent. Most approaches used to solve this problem use indirect means whereas we need the type of studies that will directly link the chemical structure of the amine sorbent to its performance since this will provide the key in unlocking the rationale in selecting the blend components. For this reason, my research objective focuses on coming up with a rational way to use a fundamental chemical structure – activity relationship study to develop and formulate an optimum sorbent blend. This novel blend is validated in a bench scale pilot plant to ensure that the developed criterion leads to a blend that is practical and implementable. The effects of the chemical structure, namely, side chain structure and number of hydroxyl groups in primary, secondary and tertiary amines as well as the alkanol chain length in primary alkanolamines and the alkyl chain length in secondary and tertiary alkanolamines on amin activities such as CO2 absorption and desorption kinetics, equilibrium loading, heat duty, cyclic capacity, heat of absorption and pKa were studied and used to develop rational criteria for selecting components to formulate an optimum amine blend. Based on the criteria, amines that had a combination of high CO2 absorption parameter and high CO2 desorption parameter were selected. Their mixing ratios and concentrations were varied to obtain the best overall performance. The optimum blend was then validated in a bench scale pilot plant and compared with the benchmark 7M MEA-MDEA solvent blend. The role of a solid acid catalyst in aiding CO2 desorption and further enhancing the performance of the developed novel blend was tested and, again, compared to the benchmark 7M MEA-MDEA blend. The results of this study showed that, in comparison with their straight-chain analogues, steric hindrance present in branched-chain alkanolamines resulted in much faster desorption rate, higher equilibrium CO2 loading and cyclic capacity, much lower heat duty for solvent regeneration, but just a slight decrease in CO2 absorption rate. The effect of chain length studies also showed that, longer alkanol chain lengths of primary alkanolamines and longer alkyl chain lengths of secondary and tertiary alkanolamines led to higher equilibrium CO2 loading and pKa. However, the influence of mass transfer limitations on these positive effects resulted in a maximum trend for initial rate of CO2 absorption for secondary and tertiary alkanolamines. On the other hand, the increase in the chain lengths also caused the generation of larger amounts of bicarbonate ions which resulted in higher CO2 desorption rates and cyclic capacity, but lower heat duty. However, the longer chain alkanolamines also had high viscosities which adversely modified their performance by also introducing mass transfer limitations. The developed criteria, in terms of absorption parameter and desorption parameter, resulted in formulating an excellent bi-solvent aqueous amine blend (comprising 2M BEA + 2M AMP), which had an outstanding desorption characteristics/heat duty as well as very good absorption characteristics. In addition, this work developed a new non-trial-and-error procedure to determine the heat of CO2 absorption based on Gibbs-Helmholtz equation. Also, the pilot plant studies showed that the novel blend, 4M BEA-AMP blend showed outstanding performance in absorber efficiency, heat duty and cyclic capacity over the 7M MEA-MDEA blend, implying that it is a good potential solvent for post combustion CO2 capture thereby validating the developed selection criteria that yielded the 2M BEA + 2M AMP blend. The addition of catalyst in the process led to tremendous improvements in all the performance indicators of the two solvent blend systems.