(2an) Designing Membrane Systems for the Direct Separation of Multicomponent Organic Solvent Mixtures

The convergence of advanced separation technology with meticulously designed membranes holds great promise in enabling decarbonization in the field of industrial separations. These futuristic membrane systems emerge as sustainable and energy-efficient solutions capable of effectively facilitating the entire range of separation processes, encompassing the intricate fractionation of complex multicomponent mixtures to the precise discrimination of individual molecules.

The vital role of separation processes in addressing essential societal needs, such as the provision of clean water and air, the establishment of sustainable energy sources, and the facilitation of effective healthcare solutions, cannot be overstated. However, it is imperative to recognize that these processes, while indispensable, also contribute significantly to high energy consumption and carbon emissions. Conventional thermal separation methodologies, such as distillation and evaporation, are responsible for a substantial portion of the overall energy consumption, accounting for approximately 10–15%, along with considerable CO₂ emissions.

Membrane separation has emerged as a prominent approach to enhance energy efficiency and decrease carbon emissions. Instead of relying on latent heat, membrane-based separation techniques leverage the permeation rate ratios determined by the molecular size, shape, and affinity of the penetrant to the membrane material. This shift towards membrane-based processes is exemplified by the successful transition from thermal desalination to room temperature reverse osmosis process, resulting in significant energy savings of approximately 80%. As a result, various membrane-based methods for organic solvent separation, such as organic solvent nanofiltration (OSN), organic solvent reverse osmosis (OSRO), and organic solvent forward osmosis (OSFO), have gained prominence. Despite advancements in separation technology research, traditional chemical plants have been slow to transition to newer processes. Challenges include the trade-off between throughput and selectivity in organic separations, and the lack of comprehension and design of separation systems with complex raw mixture. State-of-the-art membranes face difficulties in achieving both high throughput and selectivity simultaneously at an economically viable cost. Moreover, membrane systems designed for isolating individual components in binary mixtures may struggle with multi-component mixtures or highly diluted/concentrated species. Overcoming these challenges and engineering separation systems for complex mixtures under diverse conditions are pivotal for the advancement of separations science.

The primary objective of the research was to showcase the viability of a membrane cascade, encompassing the whole separation process from bulk feed to the final product. Specifically, the study focused on investigating forward osmosis molecular differentiation and direct complex mixture separation at room temperature, utilizing the hydrocarbon processing industry as a model system.

Sub‐0.1 nm Organic Liquid Separation in Ultramicroporous Carbon Membrane

The ultramicroporous carbon hollow fiber membranes synthesized with 6FDA-polyimides can directly separate liquid-phase hexane isomers (i.e., n-hexane, 2-methylpentane, and 2,3-dimethylbutane) based on molecule size and shape differences, via the “new” OSFO process utilizing “draw solvent”. Unlike membranes used in organic solvent nanofiltration (OSN), the carbon molecular sieve (CMS) membranes with ultramicropores offer molecular specificity, allowing for "solvent"-"solvent" separations. Previous studies on OSFO have primarily focused on molecular weight cut-off regimes used in OSN processes. Typically, OSFO relies on the use of a "draw solute" to induce osmotic pressure, but this approach can lead to reverse solute flux, and membrane fouling. In contrast, OSFO in our work demonstrates molecular specificity and sub-0.1 nm resolution without the need for a "draw solute." The use of a large molecular weight "draw solvent" generates sufficient osmotic pressure gradient for successful permeation without back-diffusion (i.e., reverse solute flux in previous OSFO). While additional separation from the draw solvent is required downstream, the recovery of light species from the heavy draw requires less energy, as observed in prior studies on conventional forward osmosis processes.

The 6FDA–DAM CMS membrane possesses rigid ultramicropores that closely match the size of hexane isomers, allowing for shape-selective kinetic separation of molecules. Such a discrimination mechanism has been successfully translated into the liquid-phase separation of hexane isomers. OSFO process with hollow fiber membrane module proves the potential for scale-up and has successfully separated 2,3-dimethylbutane from the complex ternary mixture, proving the potential of the new OSFO process for challenging separation targets. The chemically-stable carbon molecular sieve OSFO membrane platform has the potential to be applied in a wide range of challenging organic liquid separations, not only for the hexane isomers. Synergistically combining the benefits of different materials and processes will lead to more energy-efficient large-scale separation processes.

Rational Design of Organic Solvent Osmosis for Throughput Enhancement

The development of organic solvent separation methods that offer high throughput with a low carbon footprint is of significant industrial importance. One promising approach is the utilization of a molecularly selective membrane in conjunction with hydraulic pressure and a chemical potential gradient. This innovative method enhances solvent transport across the membrane compared to conventional techniques that rely on a single driving force, while maintaining molecular selectivity. The effectiveness of organic solvent pressure-assisted osmosis (OSPAO) for discriminating challenging alcohol molecules with sub-angstrom size differences has been demonstrated.

Separating different alcohols is typically energy-intensive, even with state-of-the-art membrane processes like OSRO. However, the OSPAO process using the thin 6FDA–DAM CMS layer (i.e., ~ 2µm) synthesized on the outer surface of α-alumina substrate hollow fiber membrane achieved liquid-phase molecular differentiation of ternary alcohol mixtures with nominal energy cost and moderate separation factors. By utilizing ultramicropores in a 6FDA–DAM CMS, sharp discrimination of alcohols (i.e., methanol, ethanol, and isopropanol) based on their diffusion selectivities has been achieved. Optimizing the various osmosis process conditions, including the judicious selection of draw solvent (i.e., 1,3,5-triisopropylbenzene and polyethylene glycol 200) and modification of operation conditions, enables OSPAO separations of ternary alcohol mixtures with various compositions to achieve higher selectivity than the OSN method and significantly enhanced throughput compared to the OSFO modality. This provides a feasible strategy for separating challenging pairs by designing both the membrane material and the process, and extends the potential application of OSPAO to industrial liquid separation processes with a low carbon footprint.

Investigating the Membrane-based Crude Oil Fractionation of Various Materials

Crude oil separation plays a crucial role in the global production of fuels and commodities, which is predicted to increase significantly by 2050. However, the hydrocarbon industry faces the "grand challenge" of reducing carbon emissions while maximizing hydrocarbon production in retrofitted refineries. Advanced membrane separation strategies offer a solution by minimizing energy consumption and integrating seamlessly into existing infrastructures. In non-aqueous conditions for separating small liquid organic molecules, the membrane's solvent resistance and the balance between flux and selectivity are vital considerations. Understanding the intrinsic properties of membrane materials that influence permeation behavior, such as diffusion and sorption, is crucial for achieving effective size sieving and class selectivity. Scalability is also a key criterion for membrane manufacturing in this field.

The field of bulk crude oil separation is relatively unexplored, lacking definitive benchmarks for predicting separation efficacy. Previous studies have often relied on heated or diluted crude oil samples, creating a gap between laboratory findings and practical applications. The advanced materials such as glassy polymers, polymers with intrinsic microporosity, and carbon molecular sieves (CMSs) were employed to unravel the complexities of bulk crude oil separation. Commercial polymeric membranes including Puramem Selective, Puramem Flux and onf–2 were used as benchmarks to assess separation performance. Advanced polyimide membranes, such as spirobifluorene-based polyimides (e.g., SBF–6FDA), were utilized to enhance the separation system throughput while maintaining the appropriate selectivity. To ensure scalability, hollow fibers were fabricated through dip-coating, and thin-film composite (TFC) membranes were created by knife-casting or spin-coating on substrate membranes. The modular membranes were evaluated in a realistic cross-flow separation system using bulk crude oil as the feed directly, providing a practical depiction of the separation process and highlighting remarkable performance achievements in this unique context.