3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Integration and synthesis of heat and mass exchanger networks for CO2 capture in power plants(2020-03) Yoro, Kelvin OdafeCO2 capture and storage (CCS) have been identified as a promising technology that could allow for the continual use of coal in power generation, and yet maintain a near-zero emission of CO2 into the atmosphere. However, the major challenge facing the retrofitting of CO2 capture systems to existing power plants is that most CCS techniques are material and energy-intensive. This subsequently reduces the operational efficiency of the power plants and also increases the energy penalty, as well as operational cost. To date, the blending of sorbents with inhibitors such as piperazine (PZ) solvents, amine-2-amino-2-methyl-1-propanol (AMP), the use of phase change materials, amine scrubbing and the use of external utilities such as steam and cooling water have been the common strategy to minimize energy consumption during CO2 capture in power plants. In this thesis, a technique to minimize the high energy and material requirement during absorptive and adsorptive CO2 capture in coal-fired power plants was proposed, developed and tested. The principles of heat and mass integration were employed in this study to minimize energy consumption and resource usage during CO2 capture in power plants. Heat exchanger networks (HENs) were synthesized in this study to address energy minimization while the application of mass exchanger networks (MENs) were introduced to address the excessive consumption of resources and utilities such as cooling water, steam, sorbents etc during CO2 capture. Furthermore, a systematic method for the synthesis of combined heat and mass exchanger networks (CHAMENs) with regeneration was developed and tested in this thesis to simultaneously minimize the use of energy and mass during CO2 capture considering sorbent regeneration. Also, to ensure the cost-effectiveness of the MENs, a technique was presented to target the minimum mass separating agents (sorbent) as well as the capital costs for the heat and mass exchanger networks. The method used was simple and based on insight, rather than relying on a mathematical 'black-box' approach. Graphical tools new to CCS studies, such as the y-x and the y-y* composite curve plots were introduced to allow the minimum exchanger sizes to be determined before network design. Capital cost targets were traded off against the established operating cost targets to optimize the total cost first with no design being necessary. The total annualized cost (TAC) for the network which includes the cost of mass separating agents, hot v and cold utilities, was minimized using an objective function to reduce process cost. To account for fluctuations in operating parameters during absorptive CO2 capture, a simultaneous (mathematical programming) approach was used to synthesize optimal heat exchanger networks (HEN) with non-isothermal mixing and fluctuating parameters, while minimum utility cost for the CO2 absorption process was determined at selected parameter points. The HENs synthesis procedure presented in this thesis took into consideration quantified uncertainties in inlet temperatures and flow rates to address the major shortcomings observed in previously reported methodologies. Area targeting of heat exchangers was investigated in this study to determine the capital cost of the synthesized multi-period network. Where a simultaneous synthesis technique was used, a multi-period MINLP model was developed to generate a HEN with optimized heat exchanger areas and total annualized costs while the concept of pinch analysis was used for the sequential approach. A linear programming technique was then used to synthesize an effective transport network for captured CO2 from power plants in different locations using a hypothetical case study. The synthesized CHAMENs network confirmed $ 0.1998 million/yr as the estimated total annualized cost accruable if combined network with regeneration is integrated for CCS as suggested in this research. The outcome of this study further revealed that about 19.1 % of sorbent used for adsorptive CO2 capture could be saved by integrating mass exchanger networks in CCS studies. Besides, this study also discovered that about 25-30 % of the energy used during CO2 separation could be saved by integrating the heat exchanger networks developed and described in this thesis. Conclusively, this study established that the application of heat exchanger network (HEN) and mass exchanger network (MEN) to minimize energy and material consumption during CO2 capture is unique and effective in CO2 capture systems where solid and liquid sorbents are used, compared to existing methods. Interestingly, the techniques proposed in this study which are relatively new to CCS can be fully extended to other CO2 capture techniques after minor modifications. The major shortcoming identified in this study is the generation of some process data, hence scientific assumptions had to be made in such cases.Item Numerical simulation of CO2 adsorption behaviour of polyaspartamide adsorbent for post-combustion CO2 capture(2017) Yoro, Kelvin OdafeClimate change due to the ever-increasing emission of anthropogenic greenhouse gases arising from the use of fossil fuels for power generation and most industrial processes is now a global challenge. It is therefore imperative to develop strategies or modern technologies that could mitigate the effect of global warming due to the emission of CO2. Carbon capture and storage (CCS) is a viable option that could ensure the sustainable use of cheap fossil fuels for energy generation with less CO2 emission. Amongst existing CCS technologies, absorption technology using monoethanolamine (MEA) is very mature and widely embraced globally. However, the absorption technology has a lot of challenges such as, low CO2 loading, high energy requirement for solvent regeneration, corrosive nature etc. On this note, the adsorption technology using solid sorbents is being considered for CO2 capture due to its competitive advantages such as flexibility, low energy requirement for sorbent regeneration, non-corrosive nature etc. On the other hand, adsorbents have a very vital role to play in adsorption technology and there is need to understand the behaviour of adsorbents for CO2 capture under different operating conditions in order to adapt them for wider applications. On this note, the study contained in this dissertation investigated the adsorption behaviour of a novel polymer-based adsorbent (polyaspartamide) during post-combustion CO2 capture using experimental study and mathematical modelling approach. Polyaspartamide is an amine-rich polymer widely used in drug delivery. In addition, its rich amine content increases its affinity for CO2. Its porosity, thermal stability and large surface area make it a promising material for CO2 capture. In view of this, polyaspartamide was used as the adsorbent for post-combustion CO2 capture in this study. This dissertation investigated the kinetic behaviour, the diffusion mechanism and rate limiting steps (mass transfer limitation) controlling the CO2 adsorption behaviour of this adsorbent. Furthermore, effect of impurities such as moisture and other operating variables such as temperature, pressure, inlet gas flow rate etc. on the CO2 adsorption behaviour of polyaspartamide was also investigated. Existing mathematical models were used to understand the kinetics and diffusion limitation of this adsorbent during CO2 capture. Popularly used gas-solid adsorption models namely; Bohart- Adams and Thomas model were applied in describing the breakthrough curves in order to ascertain the equilibrium concentration and breakthrough time for CO2 to be adsorbed onto polyaspartamide. Lagergren’s pseudo 1st and 2nd order models as well as the Avrami kinetic models were used to describe the kinetic behaviour of polyaspartamide during post-combustion CO2 capture. Parameter estimations needed for the design and optimization of a CO2 adsorption system using polyaspartamide were obtained and presented in this study. The Boyd’s film diffusion model comprising of the interparticle and intra-particle diffusion models were used to investigate the effect of mass transfer limitations during the adsorption of CO2 onto polyaspartamide. Data obtained from continuous CO2 adsorption experiments were used to validate the models in this study. The experiments were conducted using a laboratory-sized packed-bed adsorption column at isothermal conditions. The packed bed was attached to an ABB CO2 analyser (model: ABB-AO2020) where concentrations of CO2 at various operating conditions were obtained. The results obtained in this study show that temperature, pressure and gas flow rate had an effect on the adsorption behaviour of polyaspartamide (PAA) during CO2 capture. Polyaspartamide exhibited a CO2 capture efficiency of 97.62 % at the lowest temperature of 303 K and pressure of 2 bar. The amount of CO2 adsorbed on polyaspartamide increased as the operating pressure increased and a decrease in the adsorption temperature resulted in increased amount of CO2 adsorbed by polyaspartamide. The amounts of CO2 adsorbed on polyaspartamide were 5.9, 4.8 and 4.1 mol CO2/kg adsorbent for adsorption temperatures of 303, 318 and 333 K, respectively. The maximum amount of CO2 adsorbed by polyaspartamide at different flow rates of 1.0, 1.5 and 2.5 ml/s of the feed gas were 7.84, 6.5 and 5.9 mmol CO2/g of adsorbent. This shows that higher flow rates resulted in decreased amount of CO2 adsorbed by polyaspartamide because of low residence time which eventually resulted in poor mass transfer between the adsorbent and adsorbate. Under dry conditions, the adsorption capacity of polyaspartamide was 365.4 mg CO2/g adsorbent and 354.1 mgCO2/g adsorbent under wet conditions. Therefore, the presence of moisture had a negligible effect on the adsorption behaviour of polyaspartamide. This is very common with most amine-rich polymer-based adsorbents. This could be attributed to the fact that CO2 reacts with moisture to form carbonic acid, thereby enhancing the CO2 adsorption capacity of the material. In conclusion, this study confirmed that the adsorption of CO2 onto polyaspartamide is favoured at low temperatures and high operating pressures. The adsorption of CO2 onto polyaspartamide was governed by film diffusion according to the outcome of the Boyd’s film diffusion model. It was also confirmed that intra-particle diffusion was the rate-limiting step controlling the adsorption of CO2 onto polyaspartamide. According to the results from the kinetic study, it can be inferred that lower temperatures had an incremental effect on the kinetic behaviour of polyaspartamide, external mass transfer governed the CO2 adsorption process and the adsorption of CO2 onto polyaspartamide was confirmed to be a physicochemical process (both physisorption and chemisorption).