Assessing membrane performance and modeling transport of solute through membranes: from nanofiltration to membrane chromatography
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Authors
Hao, Zerui
Issue Date
2024-08
Type
Electronic thesis
Thesis
Thesis
Language
en_US
Keywords
Environmental engineering
Alternative Title
Abstract
Membrane technology has played a key role across different fields, facilitating the treatment of essential consumables (e.g., drinking water, therapeutic medicines, dairy products and beverages). Membrane processes can be categorized into process that optimize solvent throughput, such as water (solvent-targeted), or maximize solute recovery, such as therapeutic medicines and vaccines (solute-targeted). Meeting the growing demand for these products requires a better understanding of membrane processes, including strategic membrane selection and modification.Membrane processes involve complex phenomena that are challenging to understand and control. As a result, membrane process development often involves empirical iterative or trial and error approaches, which consume significant time and resources. Mechanistic models provide a route to address these challenges, improve process understanding, and guide optimization. One way to optimize membrane processes for specific applications is to develop new membrane materials, to provide better permeability, better selectivity, or better resistance to changes in productivity during filtration (fouling). Therefore, it is critical to have a framework for evaluating membrane performance that is unbiased.
The work described in this thesis addresses these challenges in three phases. The first phase evaluates transient behavior during filtration, using nanofiltration (NF) as an application domain, and specifically as an example of a “solvent-targeted” application. While membrane processes are often designed for steady-state operation, they often exhibit transient or non-steady behavior caused by solute adsorption, leading to overestimates of selectivity during startup, and posing risks of water quality deterioration due to desorption when feed concentration changes. Dye removal by NF in both organic and aqueous phases is used as a model system to investigate transient selectivity (i.e., anomalously high rejection) during the early stages of filtration, due to molecule adsorption onto membrane surfaces, and steady-state rejection that arises after the membrane's sorption capacity has been saturated.
A comprehensive transport model was developed, including diffusion, convection, physical (steric) partitioning, adsorption, and varying boundary conditions. Static binding experiments were performed to evaluate dye adsorption onto each NF membrane, and the resulting equilibrium liquid and solid phase concentrations were correlated using the Langmuir isotherm. Flux decline because of solute adsorption was observed, which was described well by assuming that the permeability was reduced in proportion to the mass of solute adsorbed (i.e., the Langmuir fouling model). Integrating both the isotherm and fouling model into the transport model enabled the description of transient selectivity in the form of breakthrough curves that were asymptotic to steady state sieving. The transport model was evaluated using both local equilibrium Langmuir isotherm, and Langmuir adsorption kinetics, which account for second order adsorption and first order desorption. A comparison of these two approaches revealed that only the kinetic model could accurately capture the entire breakthrough curve and steady-state rejection values, suggesting a sorption rate limited process.
The second phase of this thesis focus on a “solute-targeted” membrane processes to capture mRNA, used for vaccines. We have developed a mechanistic mathematical model to describe the dynamic purification of mRNA using microporous membranes modified by graft polymerization of polythymidine (Oligo dT) to promote affinity interactions. The model accommodates different membrane configurations (flat sheet and hollow fiber) and was extended to describe other chromatography techniques including monoliths and traditional resin bead columns. To better understand the response of the membrane adsorber, we first measured the overall dispersion and other mixing phenomena in the system, including process tubing and the membrane column, using transport of conservative tracers. Dispersion and transport in the system were modeled using a combination of mixed (CSTR) and plug flow (PFR) domains (so-called ideal reactors).
The elution of captured mRNA using an elution buffer to weaken affinity interactions was described using a convection-dispersion-desorption model with Langmuir desorption kinetics, where the desorption rate constant was modified to account for the changing buffer composition (e.g., salt concentration) as a function of time during elution. The model was calibrated using pure mRNA solution and then validated by predicting capture and elution of mRNA from a complex mixture after in-vitro transcription (IVT) using the flat sheet membrane. The simulation results demonstrate the model’s predictive capability. We found that desorption is the rate-limiting mechanism during elution, providing guidance to improve elution process efficiency. The study demonstrates the usefulness of the developed model to help understand complex transport phenomena, and to help solve industrial challenges by expediting process development and facilitating scale-up studies.
The third and final phase of this thesis introduces a framework to assess membrane performance without bias, using productivity, fouling potential, and energy consumption as metrics. Membrane scientists and engineers invest significant effort in synthesizing new membranes, often involving the optimization of surface chemistries to enhance selectivity and minimize the potential for retained species that could decrease membrane permeability (i.e., induce fouling). Furthermore, they routinely conduct comparisons of membrane performance across various applications, selecting the most suitable membrane for scaling up in a particular application When different membranes under consideration have different permeability values (or resistances, Rm), traditional plots of flux versus time for constant pressure operation, or pressure versus volume in constant flux operation are biased, and often do not produce a valid assessment of performance. In this study, we elucidate how Rm affects fouling kinetics across various fouling mechanisms and experimental protocols. We demonstrate that traditional plots often obscure two important performance criteria: productivity, defined as the accumulated volume throughput, and energy consumption. The practical implications of our findings include the possibility of overestimating the performance of membranes with lower productivity or higher energy consumption. Additionally, screening studies may inadvertently select lower-performing membranes under the mistaken belief of superior performance.
We have introduced two novel methods to assess membrane performance in an unbiased manner. In the first approach, we have developed a graphical approach using normalized coordinates. This approach yields linear plots with slopes solely dependent on fouling parameters, and are unbiased, i.e., independent of membrane resistance Rm. Therefore, these normalized coordinates, customized for each fouling mechanism and operational mode, effectively isolate fouling potential. In a second approach, we developed new graphical approaches to visualize the potential trade-offs between better antifouling performance but lower membrane permeability by examining either productivity (volume throughput) for constant pressure operation, or specific energy for constant flux operation. Our study establishes a comprehensive framework for evaluating membrane performance under fouling conditions, incorporating fouling, energy consumption, and volume throughput as metrics, independent of membrane resistance. This framework offers valuable guidance for process design and the development of antifouling membrane materials.
Overall, the work reported in this thesis presents an investigation into various membrane processes and applications, including both "solvent-targeted" nanofiltration and "solute-targeted" membrane chromatography applications. These two applications were explored and interconnected through the modeling of solute transport behavior within the membrane. Additionally, we present a framework for evaluating membrane performance under fouling conditions, with a focus on optimizing solvent productivity.
Description
August 2024
School of Engineering
School of Engineering
Full Citation
Publisher
Rensselaer Polytechnic Institute, Troy, NY