Charting the Path to Decarbonize Crude Oil Refineries with Membrane-Based Fractionation

The demand for energy-efficient separation methods in industrial chemistry remains paramount, urging exploration beyond traditional processes like distillation, which accounts for a substantial portion of global energy consumption. The critical separation processes, such as hydrocarbon extraction from crude oil, Uranium extraction from seawater, separation of alkanes and alkenes, sequestration of greenhouse gases from flue gases, extraction of rare-earth metals from ores, separation of benzene derivatives from the mixture and removal of trace contaminants from water present substantial challenges and opportunities for advancement. The currently used methods for these separations are highly energy-intensive. Even the curriculum of budding engineers and chemists’ places heavy emphasis on the ages-old methods of separation. In parallel, the global decarbonization requirements continue to accelerate due to climate change and sustainable development perspectives. There is a clear need to explore alternative energy- efficient membrane or adsorbent-based technologies to overcome the challenges ranging from complex compound compositions to achieving the required scale and purity levels for industrial applications for these separation requirements. Resolving these challenges necessitates enhanced education in separation technologies for future chemists and engineers, scalable deployment strategies, realistic mixture considerations in experiments and multidisciplinary collaboration.

The process of crude oil fractionation, which involves the separation of crude oil into various petroleum products, is crucial for the global supply chain of fuels and commodities [1]. The current global demand for crude oil across various sectors (Figure 1) is around 101.4 million Barrels Per Day (mbd). The demand for 2030 is forecasted to increase further by 5.5 mbd [2]. With the current global production of petroleum and liquid fuels forecasted to continue increasing, the hydrocarbon industry must aggressively reduce its carbon footprint. Traditional crude oil refineries consume a significant amount of energy accounting for nearly 1% of the global energy use [3], out of which around 80% of the energy is used by the distillation columns to power the separation process. The practical minimum energy consumption estimated for refineries by the Department of Energy, USA stands at 52.7MJ/barrel [4]. Scaling this figure with the global demand of 101.4 mbd indicates a daily global energy consumption for refining of approximately 52.4 × 1014J. This energy consumption is equivalent to 1.6% of the crude oil getting consumed to refine itself (considering the calorific value of furnace oil). A mere 5% reduction in this colossal amount of daily energy usage could illuminate about 1 million household LED bulbs for a full year. The boiling points of hydrocarbons directly correspond to their carbon numbers, essentially their molecular size. Separating molecules based on their boiling points necessitates energy for vaporization, whereas molecular size-driven separation, achieved through sizeselective membranes like molecular sieves, demands less energy. For crude oil, the energy required for boiling significantly surpasses that needed for pressurizing the liquid, presenting a substantial opportunity for energy conservation. Additionally, the membranes seamlessly integrate into existing infrastructures, paving the way for hybrid processes (Figure 2). Energy savings linked to membrane use in crude oil refining hinge on the API gravity of the crude oil. Generally, lower API gravity aligns with a higher potential for energy savings, while higher API gravity correlates with reduced savings. For instance, refining Bombay High crude oil, with an API gravity typically ranging between 32 to 38°, indicates a potential for around 5% energy savings. This analysis makes it imperative to explore alternative methods such as membrane-based crude oil fractionation as this technology can drastically cut down on the energy requirement, compared to more traditional methods, and reduce the carbon footprint of refineries.

Figure 1:Sector-wise global consumption of crude oil [1].

Figure 2:Conceptual layout of an innovative hybrid membrane-distillation column setup for crude oil refining. The choice of membranes is based on molecular weight cutoffs, with operating pressures and temperatures adjusted according to feed compositions. Hydraulic energy from the membrane reject stream is recycled via an energy recovery system to pressurize a portion of the feed. The residual volume of the crude oil may be processed through a smaller distillation column for enhanced resource recovery. (BP= Boiling point).

The advancement of membrane-based technology in crude oil refining necessitates a comprehensive understanding of two key aspects: the intricate composition of crude oil and the compatibility between the membrane material and crude oil, alongside a thorough understanding of the separation mechanisms facilitated by these membranes. Simulation studies can play a critical role in understanding the separation mechanisms and mass transportation through the membranes. The sequence of studies has presented a progressive and comprehensive approach to modeling complex liquid mixtures via polymer membranes. Commencing with the establishment of a predictive framework that showcased remarkable accuracy in predicting permeate compositions and fluxes [5], subsequent research brought forth advancements in thermodynamic modeling [6] and the development of a data-driven predictive model [7]. These strides encompassed considerations ranging from guest solubility and membrane swelling to the integration of machine learning algorithms, offering a multifaceted understanding of separation processes. Furthermore, the integration of innovative membrane diffusion models and chemical structure-based predictive models signifies a promising trajectory toward more precise and scalable predictive tools tailored for membrane- based crude oil fractionation.

The recent advancements in membrane materials have shown promising potential for crude oil fractionation, exemplified by the research conducted by different groups. One such study on the novel Spiro-Bifluorene Aryl Diamine (SBAD) membrane, unveiled high mechanical robustness and molecular separation performance in toluene [3]. However, the permeance range was very low ranging from 0.1 to 0.7 LMH/bar. This membrane demonstrated better separation of small solutes and hydrocarbons than the Polymer of Intrinsic Microporosity (PIM-1). However, it is crucial to acknowledge that PIM-1, originally developed for gas separation [8], might not serve as a perfect benchmark for crude oil refining, making direct comparisons complex [3]. Furthermore, membrane fabrication from polymers with high thermal stability produced via a copper-catalyzed chemical reaction using solutionprocessable polytriazoles from spirocyclic monomers, resulted in transient interconnections between micropores and significant hydrocarbon separation abilities [9]. The absence of typical rigid ladder structure in these polymers, which is typically observed in PIM-1, resulted in better chain mobility to allow for the dynamic connection of large microporous void spaces leading to higher permeance. The superior performance of the membrane in permeance, molecular-weight cutoff and selectivity positions it as a promising candidate for membrane- based crude oil separations, showcasing its potential to enhance efficiency and selectivity in this crucial industrial process.

Additionally, membrane having a 10-nanometer-thin selective layer of Polytriazole with Pendant Hydroxyl (OH) groups (PTA-OH) showed its aptitude in handling intricate liquid feeds containing high-boiling polar aprotic solvents typical in aromatic fractions extraction from refinery streams and diverse mixtures in crude oil [10]. Additionally, the presence of PTA-OH favours the crosslinking of the polymer under milder conditions. The crosslinked membranes exhibited notably high permeance, surpassing state-of-the-art integrally asymmetric membranes at elevated temperatures and showing comparative or superior values to Thin- Film Composite (TFC) membranes with organic solvent stability [10]. These membranes displayed separation capabilities among various molecule classes and sizes, demonstrated by the separation of a hydrocarbon mixture dissolved in toluene. The most recent TFC membranes employed Multiblock Oligomer Amines (MOA) that enable rapid transport of nonpolar solvents, presenting a promising avenue for small hydrocarbon liquid molecule fractionation [11]. The research highlighted the superior size- and class-based selectivity of MOA membranes compared to the current commercial benchmark for hydrocarbon liquid processing. Particularly, the MOA membranes displayed a high permeance (14.8±1.2LMH/ bar) of toluene. Moreover, compared to the SBAD membrane in ref. [3] requiring high temperatures for similar selectivity, the MOA membrane demonstrated over five times higher permeance at room temperature (30 °C). While the developed membrane materials exhibit promising results, the field of membrane-based fractionation remains in its nascent phase, lacking a robust product ready for market deployment. Nevertheless, considering the escalating concerns regarding climate change, the rapid expansion of the global economy, and the worldwide push for decarbonization, this field holds immense potential to enact significant change on a global scale.

Membranes hold immense importance in petroleum refining, presenting a crucial route to curtail energy usage and carbon emissions during petroleum product manufacturing. Their integration into current refineries suggests a promising hybrid approach, combining membrane and distillation-based fractionation methods for enhanced efficiency. However, the progression of this field remains in its initial stages primarily due to the challenge of ensuring the compatibility of membrane materials with crude oil compositions. To overcome this challenge, we need comprehensive research to refine the promising materials and push them towards deploying ability. This involves rigorous testing, the development of scalable production methods and establishing the viability of these membranes for large-scale industrial applications. Additionally, collaboration between academia, industry, and policymakers is crucial to drive innovation, secure investments and establish regulatory frameworks that support the development and adoption of these technologies.

The authors (BS and SK) acknowledge the funding (MLP0075) from the Council of Scientific and Industrial Research, India, to carry out this work. CSIR-CSMCRI PRIS number 174-2023 has been allotted to this manuscript.

The authors declare no conflict of interest.

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