Model-based Design, Operation and Control of Pressure Swing Adsorption Systems

This thesis is concerned with the design, operation and control of pressure swing adsorption (PSA) systems, employing state of the art system engineering tools. A detailed mathematical model is developed which captures the hydrodynamic, mass transfer and equilibrium effects in detail to represent the real PSA operation. The first detailed case study presented in this work deals with the design of an explicit/multi-parametric model predictive controller for the operation of a PSA system comprising four adsorbent beds undergoing nine process steps, separating 70 % H2, 30 % CH4 mixture into high purity hydrogen. The key controller objective is to fast track H2 purity to a set point value of 99.99 %, manipulating time duration of the adsorption step, under the effect of process disturbances. To perform the task, a rigorous and systematic framework is employed comprising four main steps of model development, system identification, the mp-MPC formulation, and in-silico closed loop validation, respectively. Detailed comparison studies of the derived explicit MPC controller are also performed with the conventional PID controllers, for a multitude of disturbance scenarios. Following the controller design, a detailed design and control optimization study is presented which incorporates the design, operational and control aspects of PSA operation simultaneously, with the objective of improving real time operability. This is in complete contrast to the traditional approach for the design of process systems, which employs a two step sequential method of first design and then control. A systematic and rigorous methodology is employed towards this purpose and is applied to a two-bed, six-step PSA system represented by a rigorous mathematical model, where the key optimization objective is to maximize the expected H2 recovery while achieving a closed loop product H2 purity of 99.99 %, for separating 70 % H2, 30 % CH4 feed. Furthermore, two detailed comparative studies are also conducted. In the first study, the optimal design and control configuration obtained from the simultaneous and sequential approaches are compared in detail. In the second study, an mp-MPC controller is designed to investigate any further improvements in the closed loop response of the optimal PSA system. The final area of research work is related to the development of an industrial scale, integrated PSA-membrane separation system. Here, the key objective is to enhance the overall recovery of "fuel cell ready" 99.99 % pure hydrogen, produced from the steam methane reforming route, where PSA is usually employed as the purification system. In the first stage, the stand-alone PSA and membrane configurations are optimized performing dynamic simulations on the mathematical model. During this procedure, both upstream and downstream membrane configuration are investigated in detail. For the hybrid configuration, membrane area and PSA cycle time are chosen as the key design parameters. Furthermore, life cycle analysis studies are performed on the hybrid system to evaluate its environmental impact in comparison to the stand-alone PSA system

Experimental studies, molecular simulation and process modelling\simulation of adsorption-based post-combustion carbon capture for power plants: a state-of-the-art review

Adsorption-based post-combustion carbon capture is a promising emerging technology for capturing CO2 emissions from fossil-fueled power plants due to the ease of adsorbent regeneration in comparison with solvent-based technologies. To increase its competitiveness, research efforts have focused on the development of new adsorbent materials and processes. This paper presents a state-of-the-art review of such efforts, focusing on lab synthesis and characterization of adsorbent materials, (carbon capture) experimental studies, molecular simulation, process modelling\simulation and techno-economic analysis. Most experimental studies on adsorption-based post-combustion capture are at bench scale. Just a few experimental studies are at pilot scale. There are currently no commercial deployment of adsorption-based post-combustion capture technology. This review paper points out challenges encountered in these experimental investigations utilizing different adsorbent materials, limiting its commercial deployment. These gaps in experimental investigations need further research especially in the chemical modification of the adsorbent materials to increase the adsorption capacity. Molecular simulation of adsorbents and process modelling\simulation of carbon capture processes are cost-effective and time efficient approaches for the assessment of adsorbents’ CO2 capture performance. The review also highlighted the need for more research in the model development of adsorbent materials at molecular scale and the model development of adsorption-based post-combustion process adopting new reactor configurations to further reduce the cost of CO2 capture

PSA system design for separation of ethylene from light hydrocarbon gas streams

The primary goal of this study is the synthesis, design, modeling and simulation of gas adsorption separation processes. In particular, those where differences in the transport or reaction of gas species in materials are important for separation as opposed to equilibrium adsorption properties. Two applications are used for illustration, purification of ethylene from a mixture of light hydrocarbon gases, and the capture of CO2 from air. Chapter 2 is primarily focused on quantifying the differences in optimal performance of a traditional packed bed to that of a novel hollow fiber bed. The hollow fiber bed showed a 5 times higher productivity (for similar product purity and recovery). Chapter 3 (equal contribution from Dr. L. A. Darunte - experiments) is concerned with understanding the impact of mass transfer on separation performance of MMEN-Mg2(dopbpdc) for CO2 capture. We showed that the co-operative insertion mechanism which provides thermodynamic advantages to this material, significantly hampers its separation process kinetics. Chapter 4 (equal contribution from W. You – molecular simulations) is concerned with understanding the impact of binding energy of M-BTC MOFs for ethylene-ethane separation. Temperature was shown to have a significant non-monotonic impact on process performance. We also found mixed metal MM’-BTCs that can outperform the constituent pure metal M-BTCs. Chapter 5 is concerned with understanding the impact of adsorbent property parameters on kinetic separation at a PSA scale (packed bed), therefore bridging the gap between lab scale experiments and PSA design. An illumination algorithm (SAIL) was able to efficiently predict similar results with greater computational efficiency. Overall, my thesis advances the understanding of (a) the impact of bed configuration on PSA performance (b) how inherent material property parameters translate to a process scale performance.Ph.D

Thermally Modulated Fiber Sorbents for Rapidly Cycled Vacuum-Pressure Swing Adsorption of Post-Combustion Flue Gas

Thermally Modulated Fiber Sorbents for Rapidly Cycled Vacuum Pressure Swing Adsorption of Post Combustion Flue Gas Stephen J.A. DeWitt 339 Pages Directed by Dr. Ryan P. Lively The continued rise in the concentration of CO2 in the Earth’s atmosphere driven by society’s rising standard of living and continued reliance on carbon-containing fossil fuels has led to several significant environmental challenges, which will continue to face humanity in the coming decades and centuries. Developing technological solutions to accelerate the reduction of carbon dioxide emissions and curtail the effects of climate change will continue to serve one of the critical directions society will pursue to combat these effects. The capture of CO2 from point sources like coal-fired power plants will likely be a direction for the short and long-term reduction in emissions needed to stabilize the environment. With this in mind, developing technologies to serve this purpose with the least impact on people’s way of life serves as a critical challenge over the coming decades. The removal of CO2 from point sources has been an area of interest for a number of years now, with absorption technology appears best suited to make an immediate impact with pilot and full-scale plants currently being built. In the future, methods of separating and capturing CO2 will look to technologies with the potential to be considerably lower energy requirements, like adsorption and membranes, and these are still in the early stages of development. While these approaches lag behind absorption in technology readiness for CO2 capture today, the potential impact of their adoption in terms of cost reduction has led to considerable research investment in developing new materials and processes to enable their application. In this dissertation, a novel strategy is proposed, and materials are developed to enable the proposed process for post-combustion CO2 capture from coal. As part of this dissertation, four objectives were pursued to understand and enable this process. xxvii i) A process model was created and studied to develop a deeper understanding of the approach’s potential and the necessary materials developments to enable it. ii) Promising materials were synthesized and manufactured into ready-made devices for bench-scale testing. iii) Fundamental challenges of adsorptive separations related to heat management were considered in detail, and a novel manufacturing approach was developed to enable improvement in materials performance. iv) The potential of fiber sorbents for sub-ambient CO2 capture was examined through the operation of single bed PSA cycles using the manufactured materials from previous objectives. The first objective showed that a sub-ambient pressure-driven separation process, coupled with downstream liquefaction, could be used without the need for external refrigeration. Excess low-quality cooling could also be used for the dehydration of the flue gas, a key challenge facing adsorptive CO2 capture. This process would be significantly limited by its capital costs, consistent with expectation. Regardless of the separation considered, the capital and energy costs of pre- and post-treatment of CO2 lead to costs of CO2 capture exceeding $47/tonneCO2, and the best case study of sub-ambient pressure swing adsorption led to costs of capture around$60/tonneCO2. Preliminary analysis shows a more complicated process, where the post-treatment liquefaction is removed or replaced with a low-cost membrane or adsorbent system that could allow for additional cost reductions. The second objective focused on the production and spinning of MOF fiber sorbents, which showed the potential, in simulation, when combined with the work from objective 3 to reach 8-10x the performance of traditional pellet packed bed systems. Work here focused on the development of scale-up and manufacture of multiple MOF fiber xxviii sorbents, overcoming challenges in dope composition formulation and particle size resulting in sorbent leaching not previously reported in fiber sorbent spinning. The third objective focused on the development of a passive internal heat management strategy for pressure swing adsorption in the fiber sorbent morphology. Microencapsulated phase change materials were, for the first time, incorporated into fiber sorbents in the spinning step, allowing for a reduction in the manufacturing complexity of heat management in PSA systems. A 20-25% improvement in the breakthrough capacity of the sorbent and 30-40% reduction in amplitude of the thermal front prove the manufacturing process works and will enable more efficient sorbent performance. The final objective looked at the operation of a pressure swing adsorption unit for the removal of CO2 from simulated flue gas within the sub-ambient process framework. MOF fiber sorbents with and without phase change materials were compared in terms of the tradeoffs between purity, recovery, and CO2 productivity. This preliminary analysis showed there was much potential for sub-ambient CO2 capture, with productivities as high as 0.01 mol kg-1 sec-1 achieved using MOF fibers. Due to the single bed cycles used in these experiments, the recovery of the system suffered (never exceeding 45%), but future work focused on optimizing more complex cycles should allow for improvements in this area. Thermally modulated fiber adsorbents were also considered in the sub-ambient PSA system, showing higher purities and productivities than fibers without heat management at comparable recovery levels.Ph.D

Methane-nitrogen separation by pressure swing adsorption

Master'sMASTER OF ENGINEERIN