2.2 Water Molecular Clusters and "Cage Effect"

SCW can promote the process of heavy oil cracking. Chen et al. found through DFT calculations that the cracking energy of polyvinylidene fluoride in SCW decreased from 455.84 kJ/mol to 423.06 kJ/mol, indicating that SCW has a promoting effect on C-C bond cleavage. This is because water molecules can form a certain scale of solvent cluster structure around hydrocarbon radicals, and the water molecule clusters surrounding the C-C bond increase the distance between carbon atoms during the process of approaching the C-C bond, thereby weakening the strength of the C-C bond. In addition, water molecule clusters contain a large amount of ·OH, which can take hydrogen atoms from the carbon chain. The carbon that loses hydrogen atoms forms a double bond with adjacent carbon, causing the adjacent carbon to supersaturate and undergo bond breaking (Figure 3a).

Similar to the C-C bond in hydrocarbons, the presence of water molecular clusters reduces the cleavage energy of the C-C bond in the aromatic ring. Zhang et al. combined reaction force field and DFT to conduct molecular dynamics simulations on the cracking of coal to hydrogen in SCW. Compared with traditional thermal cracking, the cracking energy of aromatic C-C bonds in SCW was reduced by 287.3 kJ/mol. The decrease in cracking energy leads to PAHs being prone to ring opening reactions in SCW. As shown in Figure 3b, when the carbon atom of PAHs undergoes dehydrogenation to form a free radical, ·OH is more likely to bind to the carbon atom, and subsequently, the carbon atom is prone to thermal decomposition and bond breaking with neighboring carbon atoms. Water molecule clusters that lose ·OHbecome hydrogen rich free radical (·H) water molecule clusters. ·H can directly bind to PAHs to saturate them, or combine with chain hydrocarbon radicals to avoid dehydrogenation and cyclization of PAHs. It can also extract hydrogen from water molecules to generate hydrogen and new ·OH, which continue to attack PAHs and cause further ring opening. This process greatly suppresses the generation of PAHs and reduces the number of aromatic rings in PAHs. In this process, ·H and ·OH promote each other to form a virtuous cycle. The raw material molecules are continuously decomposed into smaller products in this cycle, and the aromatic ring continues to open. While degrading large molecules into small molecules, coke formation is avoided as much as possible. It is worth noting that when the temperature is very high (greater than 2000 K), PAHs tend to undergo direct thermal cracking rather than ·OH reaction for ring opening.

Through first principles molecular dynamics simulations, Liu Ying found that when water molecules come into contact with hydrocarbon radicals at a specific angle, the water molecular orbitals have a certain impact on the molecular orbitals of hydrocarbon radicals. Due to the disordered thermal motion of both water molecules and hydrocarbon radicals, the influence of water molecules on hydrocarbon radicals is in a fluctuating state and randomly fluctuates to a certain extent, ultimately affecting the reaction of hydrocarbon radicals. The solvent cluster surrounding hydrocarbon radicals creates a "cage effect", which limits the range of motion of hydrocarbon radicals to water molecule clusters, resulting in reactions that can only occur within the "cage" and are difficult to occur between the "cages". The existence of the cage effect leads to a significant decrease in the diffusion coefficient of hydrocarbon radicals in the aqueous phase, ultimately resulting in a rapid decrease in the rate of bimolecular reactions. The existence of "cage effect" is an important reason for the influence of water oil ratio on the SCW modified heavy oil system.

In summary, the presence of water molecular clusters suppresses the interactions between carbon atoms, reducing the cleavage energy of C-C bonds and making them prone to chain breakage or ring opening. More importantly, the ·OH generated by water molecule clusters can attack the C-C bond and disrupt the interactions between carbon atoms. The reduction of cracking energy can promote the ring opening of PAHs while inhibiting the conversion of chain hydrocarbons to PAHs. In addition, the "cage effect" caused by the formation of water molecule clusters inhibits the mass transfer between the "cages" in the aqueous phase, and inhibits the reaction rate of PAHs generation and further coking.

2.3 Hydrogen Supply by Water Molecules

At present, there is controversy among researchers about whether water molecules are chemically inert during the process of lightweighting, that is, whether water molecules provide hydrogen atoms for hydrocarbon free radical reactions in heavy oil upgrading. Ma Caixia et al. treated kerosene with SCW for 20 minutes at 390-480℃ and water density of 0.36-0.44 g/cm³, and found that the content of asphaltene decreased from 54.08% (w) to 16.04% (w). After SCW modification, the composition of PAHs decreased, and water molecules may provide hydrogen atoms for the cleavage and ring opening of PAHs branches. Dutta et al. used heavy water as a tracer to track the fate of hydrogen atoms in water molecules during the reaction process. They treated asphalt at 350-480℃ and found that deuterium atoms transferred from water molecules to coke and liquid product molecules. It indicates that water molecules have chemical activity and undergo addition reactions with PAHs, therefore deuterium atoms originating from water molecules will be found in the product. Takehiko Shoutani et al. used 18O as a tracer and found through mass spectrometry analysis that there were CO peaks with m/z=30 and CO₂ peaks with m/z=48 in the gas phase, but there were no O₂ molecular peaks with m/z=34 or 36, indicating that oxygen and hydrogen atoms were directly added to PAHs molecules simultaneously.

By studying SCW cracking vacuum residue under conditions of 420~460℃ and water density of 0.104~0.132 g/cm³, Zhao et al. believe that water molecules do not provide hydrogen to the raw materials. The main contribution of water molecules in the reaction is the generation of ·OH. ·OH competes with hydrocarbon radicals, especially PAHs radicals, during the reaction process, reducing the condensation probability. The deuterium atoms present in the product are caused by the transfer of hydrogen atoms during the reaction process, rather than the direct hydrogenation of heavy oil by water molecules. The specific reaction process is as follows: during the chain growth process, free radicals take hydrogen atoms from water molecules to form ·OH, which then takes hydrogen atoms from hydrocarbons to form water. The total amount of water does not change before and after the reaction. But this mechanism cannot explain the phenomenon of oxygen atoms originating from water molecules in the products discovered by Takehiko Shoutani and others. Radfarnia et al. used SCW to crack olefins in anaerobic conditions and estimated the amount of hydrogen produced by SCW by calculating the oxygen balance of CO and CO₂ in the gas-phase products. They inferred that the amount of hydrogen produced by hydrolysis was negligible relative to the total amount of hydrogen in the gas-phase products. Therefore, it is believed that SCW is not the main hydrogen donor. The hydrogen donor is an intermediate product generated during the reaction process, such as tetrahydronaphthalene (a type of PAHs). Tetrahydronaphthalene can be converted into dihydronaphthalene in SCW, which is then converted into naphthalene (a type of PAHs). During this process, a large number of hydrogen atoms are removed, thereby providing hydrogen. During the free radical reaction process, PAHs have unstable branched chains and are prone to breakage to form free radicals. At this point, the active hydrogen atoms provided by the hydrogen donor combine with the free radicals to terminate the reaction and form light hydrocarbons. The solubility of PAHs that have lost their branched chains decreases, leading to condensation and coking in the water phase and oil phase.

The current controversy over the hydrogen supply capacity of SCW in research mainly lies in whether water molecules are the source of hydrogen supply during the heavy oil upgrading process. Specifically, when water molecules decompose into ·OH and ·H, do they directly add to hydrocarbon radicals, or do hydrogen atoms in water only undergo substitution with hydrogen atoms in hydrocarbon radicals? Through lateral analysis of oxygen atoms, researchers have made some speculations about the hydrogen supply capacity of SCW, but the conclusions drawn are inconsistent. This is because there is a significant difference in the SCW density used in the study. The differences in physical properties such as dielectric constant, hydrogen bonding, and ion product of SCW with different densities lead to differences in hydrogen supply capacity, resulting in different reaction mechanisms. In addition, some studies suggest that the direct addition of water molecules to hydrocarbon radicals is rare, accounting for only 0.5% of the total reaction. However, there are many components in heavy oil that can provide hydrogen, and it is difficult to observe the hydrogen supply phenomenon of water molecules when they coexist with other hydrogen donors. Therefore, if we want to clarify whether SCW has hydrogen supply capacity during the reaction process, whether using experimental or molecular simulation methods, we should start from the hydrogen atom and track the process of hydrogen atom transfer from water molecules to heavy oil molecules, in order to improve the reaction mechanism and determine whether water plays a role in hydrogen supply.

3.Other Fluids used for Supercritical Upgrading of Heavy Oil

The study of SCW modified heavy oil reveals that the main role of water molecules is to inhibit coking reactions in the free radical reaction network, promote cracking reactions, and possibly contribute to hydrogen supply. These properties are not unique to water molecules. Therefore, some scholars have attempted to use other supercritical fluids for heavy oil upgrading.

One method is to mix other fluids into the SCW system to promote the hydrogenation process, such as enhancing the water gas reaction in the reaction system. The specific process involves the dissociation of water molecules into ·H and ·OH, where ·OH reacts with CO to generate ·COOH, followed by the dissociation of ·COOH to generate CO₂ and activated ·H. Activated ·H can provide hydrogen for heavy oil and promote the generation of ·OH, thereby promoting the opening of PAHs. The hydrogen supply capacity of this system is much stronger than that of pure SCW system. Therefore, adding components that promote ·H activation to the reaction system is beneficial for heavy oil upgrading. Hydrogen rich additives can also be added for hydrogen supply, Bai et al. added polyethylene to the reaction system, and a large amount of ·H was generated during the pyrolysis of polyethylene. The PAHs free radicals generated by heavy oil molecules can obtain ·H, which plays a role in the lightweighting of heavy oil.

Another method is to replace SCW with other fluids. He Xuanming et al. found that supercritical methanol can achieve the lightweighting of heavy oil under relatively mild conditions (260-290℃, 9-13 MPa). Meanwhile, light components are prone to enrichment in methanol. Compared to SCW modification, supercritical methanol modification greatly reduces reaction temperature and pressure, and has industrial potential. The light components of heavy oil modified by supercritical methanol contain higher naphtha/diesel ratio and less asphaltene and heteroatom content. Methanol not only acts as a reaction solvent, but also as a hydrogen donor. But the heteroatom O will be introduced into the product maltene, and some asphaltene will be generated, causing waste of raw materials and energy.

The use of supercritical organic solvents to modify heavy oil demonstrates a better product distribution and milder operating process than using SCW to modify heavy oil. More importantly, it avoids the step of removing water after lightweighting and allows for direct processing of the lightweighting mixture, reducing costs. However, there is currently a lack of research on the use of supercritical organic solvents for heavy oil upgrading, and even limited understanding of the physical and chemical properties of other supercritical fluids except for water and CO₂. Therefore, more systematic research is needed on the upgrading of heavy oil with other supercritical fluids.

4. Conclusion

SCW cracking is an emerging technology for the processing and utilization of fossil fuels, which demonstrates advantages that traditional thermal cracking does not have in terms of product distribution, effectiveness, and environmental friendliness. It provides an effective way for the effective utilization of heavy oil mainly composed of PAHs. After SCW modification, the heavy oil molecules become smaller, the number of heterocyclic atoms decreases, and the viscosity significantly decreases, indicating industrial potential. However, the high temperature and pressure required for SCW (374℃, 22.1 MPa), as well as the presence of sulfur, nitrogen and other atoms in heavy oil, pose high requirements for the equipment's pressure resistance and corrosion resistance, resulting in high equipment investment, energy consumption, and operating costs. Therefore, the economic viability of this technology needs to comprehensively consider crude oil prices, process costs, and policy environmental requirements.

Starting from the generation and transformation of PAHs, combined with experimental and molecular dynamics simulations of SCW modified heavy oil, it can be concluded that SCW molecules have a diluting and dispersing effect on heavy oil molecules, timely protecting the generated light components and avoiding further dehydrogenation condensation. In addition, the "cage effect" formed by clusters of water molecules around heavy oil molecules not only inhibits the generation of PAHs but also weakens the C-C bond cleavage energy, promoting the conversion of PAHs to small molecules. Inhibiting coking while increasing the yield of light components. The hydrogen atoms in water molecules can be transferred to the modified products, but due to the complexity of the raw materials and reaction network, further research is needed to clarify the mechanism of water molecules participating in heavy oil upgrading and provide a theoretical basis for the industrial application of SCW modified heavy oil. In addition, other supercritical fluids have also shown potential for heavy oil upgrading, but currently there is still a lack of theoretical and applied research in this area, and more exploration is needed.