Process Analysis of Asymmetric Hollow Fiber Permeators, Unsteady State Permeation and Membrane-Amine Hybrid Systems for Gas Separations
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The global market for membrane separation technologies is forecast to reach $16 billion by the year 2017 due to wide adoption of the membrane technology across various end-use markets. With the growth in demand for high quality products, stringent regulations, environmental concerns, and exhausting natural resources, membrane separation technologies are forecast to witness significant growth over the long term (Global Industry Analysts Inc., 2011). The future of membrane technology promises to be equally exciting as new membrane materials, processes and innovations make their way to the marketplace. The current trend in membrane gas separation industry is, however, to develop robust membranes, which exhibit superior separation performance, and are reliable and durable for particular applications. Process simulation allows the investigation of operating and design variables in the process, and in new process configurations. An optimal operating condition and/or process configuration could possibly yield a better separation performance as well as cost savings. Moreover, with the development of new process concepts, new membrane applications will emerge. The thesis addresses developing models that can be used to help in the design and operation of CO2 capture processes. A mathematical model for the dynamic performance of gas separation with high flux, asymmetric hollow fiber membranes was developed considering the permeate pressure build-up inside the fiber bore and cross flow pattern with respect to the membrane skin. The solution technique is advantageous since it requires minimal computational effort and provides improved solution stability. The model predictions and the robustness of the numerical technique were validated with experimental data for several membrane systems with different flow configurations. The model and solution technique were applied to investigate the performance of several membrane module configurations for air separation and methane recovery from biogas (landfill gas or digester gas). Recycle ratio plays a crucial role, and optimum recycle ratios vital for the retentate recycle to permeate and permeate recycle to feed operation were found. From the concept of two recycle operations, complexities involved in the design and operation of continuous membrane column were simplified. Membrane permselectivity required for a targeted separation to produce pipeline quality natural gas by methane-selective or nitrogen-selective membranes was calculated. The study demonstrates that the new solution technique can conveniently handle the high-flux hollow fiber membrane problems with different module configurations. A section of the study was aimed at rectifying some commonly believed perceptions about pressure build-up in hollow fiber membranes. It is a general intuition that operating at higher pressures permeates more gases, and therefore sometimes the membrane module is tested or characterized at lower pressures to save gas consumption. It is also perceived that higher pressure build-up occurs at higher feed pressures, and membrane performance deteriorates at higher feed pressures. The apparent and intrinsic permeances of H2 and N2 for asymmetric cellulose acetate-based hollow fiber membranes were evaluated from pure gas permeation experiments and numerical analysis, respectively. It was shown that though the pressure build-up increases as feed pressure increases, the effect of pressure build-up on membrane performance is actually minimized at higher feed pressures. Membrane performs close to its actual separation properties if it is operated at high feed pressures, under which conditions the effect of pressure build-up on the membrane performance is minimized. The pressure build-up effect was further investigated by calculating the average loss and percentage loss in the driving force due to pressure build-up, and it was found that percentage loss in driving force is less at high feed pressures than that at low feed pressures. It is true that unsteady state cyclic permeation process can potentially compete with the most selective polymers available to date, both in terms selectivity and productivity. A novel process mode of gas separation by means of cyclic pressure-vacuum swings for feed pressurization and permeate evacuation using a single pump was evaluated for CO2 separation from flue gas. Unlike transient permeation processes reported in the literature which were based on the differences in sorption uptake rates or desorption falloff rates, this process was based on the selective permeability of the membrane for separations. The process was analyzed to elucidate the working principle, and a parametric study was carried out to evaluate the effects of design and operating parameters on the separation performance. It was shown that improved separation efficiency (i.e., product purity and throughput) better than that of conventional steady-state permeation could be obtained by means of pressure-vacuum swing permeation. The effectiveness of membrane processes and feasibility of hybrid processes combining membrane permeation and conventional amine absorption process were investigated for post-combustion CO2 capture. Traditional MEA process uses a substantial amount of energy at the stripper reboiler when CO2 concentration increases. Several single stage and multi-stage membrane process configurations were simulated for a target design specification aiming at possible application in enhanced oil recovery. It was shown that membrane processes offer the lowest energy penalty for post-combustion CO2 capture and likely to expand as more and more CO2 selective membranes are developed. Membrane processes can save up to 20~45% energy compared to the stand-alone MEA capture processes. A comparison of energy perspective for the CO2 capture processes studied was drawn, and it was shown that the energy requirements of the hybrid processes are less than conventional MEA processes. The total energy penalty of the hybrid processes decreases as more and more CO2 is removed by the membranes.