Near and supercritical water (SCW), collectively termed hydrothermal water (HTW) in this review, have been successfully used for carbonization, gasification, liquefaction and upgrading of hydrocarbon resources including crude oils (with a specific emphasis on heavy oils and heavy fractions), microalgae, lignocellulosic biomasses, and wastes. Although HTW requires elevated pressures compared to some competitive technologies, it has potential advantages in terms of process intensity, process energy balance, water recovery and product distribution.

In terms of process intensity, HTW is performed at higher densities than gas-phase pyrolysis reactions, thereby affording potential throughput benefits as well as enhanced heat and mass transport. For process energy balance, performing reactions in a dense water phase means that feedstocks, many of which are naturally available in a wet form, need not be dried prior to processing. Moreover, heat recovery can be performed in compact and efficient liquid-liquid heat exchangers, thereby further improving system energy balance.

… In terms of product distribution, high-temperature processing in a water-rich environment reduces carbon lost to coking—and a growing body of work suggests that water may instead donate hydrogen to the carbon resource, further improving the quality and yield of liquid products. Despite these advantages, HTW technologies have yet to achieve sustained commercial success.

—Timko et al.

More than a third of the world’s energy needs are met using oil; reliance on oil will likely continue for decades to come, especially in the transportation sector. Increasingly, however, crude oil tends to be heavier, and higher in sulfur, than the lightweight, clean, and easily refined crudes produced in the past. When refined, these heavier, more sour crudes yield a higher fraction of lower-value, heavier products such as asphalt along with residual coke.

Processes now used to upgrade and desulfurize heavy crude oil are expensive, energy-intensive, and require hydrogen, which companies typically produce from natural gas—a high-cost process that consumes valuable gas resources and releases high levels of CO₂.

So there’s a lot of interest in finding alternative processes for converting low-quality crude oil into valuable fuels with less residual coke and for removing the sulfur efficiently and economically without using hydrogen.

—Ahmed Ghoniem, the Ronald C. Crane ('72) Professor of Mechanical Engineering at MIT

SCWU uses water rather than natural gas as the source of the hydrogen molecules needed for the key chemical reactions in the refining process. Although oil and water don’t normally mix, using supercritical water solves that problem.

Under extreme conditions—specifically, at pressures and temperatures above 220 atm and 375 degrees Celsius—water goes into a supercritical state in which it is as dense as a liquid but spreads out to fill a confined space as a gas does. Add oil to supercritical water (SCW) and stir, and the two will mix together perfectly, setting the stage for the desired chemical reactions without any added hydrogen from natural gas.

Industrial and academic researchers have demonstrated that mixing heavy oils with SCW produces lighter hydrocarbons containing less sulfur and forming less waste coke. But no one has understood exactly how it happens or how to optimize the process.

For the past five years, Ghoniem and William Green, the Hoyt C. Hottel Professor of Chemical Engineering at MIT, have been working to close gaps in the fundamental knowledge about the chemistry involved as SCW and oil molecules react and about the flows and mixing behaviors that will produce the desired reactions and reaction products.

Combining those new insights, the researchers are developing new computational tools to help guide energy companies that want to implement the new process.

Testing designs and operating conditions at large scale and extreme pressures is almost impossible. Our goal is to provide computer models that companies can use to predict performance before they start building new equipment

—Ahmed Ghoniem

When crude oil is mixed with SCW, the hundreds of chemical compounds present can react together in different combinations and at different rates, in some cases producing intermediate compounds that are then involved in further reactions.

The challenge with SCW processing is that you have to let the oil and SCW mix together long enough for the reactions that remove sulfur and break down large hydrocarbon molecules to happen—and then stop the process to prevent further reactions that form products you don’t want.

—William Green

Based on a series of experiments, Green and his team have defined key chemical reactions that take place, how quickly they occur, and the intermediate products that are formed. As a sample sulfur-containing compound, they used hexyl sulfide, a large molecule made up of 12 carbon atoms, 26 hydrogen atoms, and one atom of sulfur. To test the impact of the SCW, they performed two parallel experiments.

In one, they heated hexyl sulfide without adding water; in the other, they mixed the hexyl sulfide with SCW. In both cases, they removed samples from their reactor vessel at regular intervals up to 30 minutes.

What happens when hexyl sulfide is processed in a reactor vessel by raising the temperature without adding water (left) and by mixing it with supercritical water (SCW, right). In each experiment, samples were removed at regular intervals up to 30 minutes. All samples taken at or after 10 minutes include smaller hydrocarbons, some with bound sulfur. But only the SCW samples include pentane, carbon monoxide (CO), and carbon dioxide (CO2). The presence of the last two products indicates that water (H2O)—the only source of oxygen—must be reacting and providing the hydrogen atoms needed to remove the sulfur as hydrogen sulfide (H2S). Click to enlarge.

As expected, both sets of samples include a variety of smaller hydrocarbon compounds, some with bound sulfur. But the SCW products (at right in the figure above) included pentane—a smaller hydrocarbon not seen in the absence of SCW—and carbon monoxide and CO₂. Since water is the only source of oxygen in the mixture, it must be reacting with the carbon compounds. In addition, in the SCW experiments, less of the carbon goes into heavy compounds that are likely to lead to coke formation.

The appearance of the samples supports that conclusion: The non-SCW product is dark brownish-black in color, while the SCW product is a light yellow, clear liquid consistent with coke suppression by the SCW.

To further clarify the chemical reactions and how they are affected by temperature, pressure, and SCW concentration, the researchers combined their experimental work with theoretical modeling and analysis.

Based on those studies, they identified the whole series of chemical reactions by which hexyl sulfide breaks down and releases its sulfur in the presence of SCW. According to that reaction mechanism, the sulfur-bearing hexyl sulfide is first broken apart, forming a smaller molecule with the sulfur atom in a very reactive form. In the absence of water, that highly reactive sulfur-bearing molecule would join with others like itself to form a long chain and eventually become coke. But in the presence of water, it reacts with the water, and the products ultimately include lighter hydrocarbons that are readily converted into valuable light fuels. The sulfur combines with hydrogen atoms to form hydrogen sulfide, a gas that can easily be removed and dealt with using existing technology.

Those results define—for the first time—the key roles played by water in the SCW system.

We confirmed that the hydrogen atoms needed to convert the sulfur to hydrogen sulfide can be provided by water rather than by hydrogen gas, as in the conventional process. And our empirical data show that the new SCW method does make less coke than the conventional process, for reasons that we’re now trying to clarify.

—William Green

The results described thus far elucidate reactions and reaction rates under different conditions. Knowing what those conditions are inside a practical reactor is a parallel challenge.

When oil is injected into flowing SCW, interactions between the two flows determine how mixing and heating proceed, first at the macroscale and then down to the microscale at which chemical reactions occur. The trick is to encourage and control optimal mixing. Using a stirring device is impractical, given the extreme supercritical conditions; the researchers must generate such mixing naturally.

To understand the details of flows and mixing, the researchers are using three-dimensional computational fluid dynamics (CFD), a method of simulating fluid flows within a well-defined region. Such modeling involves equations that describe the flow, mixing, and energy transfer between streams of fluids.

But with supercritical fluids, key parameters such as viscosity and density are in ranges not seen under normal (non-supercritical) conditions. Nevertheless, the researchers were able to use powerful computers to accurately solve their CFD model, accounting for the complex changes that occur as fluids move from normal to supercritical conditions. To their surprise, they found that supercritical flows do indeed behave differently. For example, they become turbulent earlier than do comparable flows under normal conditions.

In one practical implementation of their model, they simulated mixing between SCW and oil near a T junction consisting of a horizontal pipe with a smaller pipe coming into it from the top. SCW flows through the horizontal pipe, and cold oil—here a sample hydrocarbon—is injected into it through a vertical pipe.

Results from a computational fluid dynamics model show flows and mixing as oil is injected into supercritical water (SCW) flowing down a pipe from left to right. Interaction between the two fluids causes the formation of two vortices — coherent swirls that spin in opposite directions, mixing the streams together. Shown here as gray tubes, these vortices are initially separate structures, but soon break down as the streams mix. Colored cross sections show concentrations of oil and SCW at five locations along the pipe. Blue indicates regions of cold oil, while red indicates regions of hot SCW. Intermediate colors are mixed layers with varying concentrations of oil and SCW. Click to enlarge.

Initial interaction between the two streams causes the formation of two coherent vortices—rotating structures in the fluids shown in the figure as gray tubes. At first, the vortices are separate swirls that spin in opposite directions, mixing the oil and SCW together. Moving along the pipe, the vortices break down, and mixing rates decay.

The colored circles in the figure above show mixing between the two fluids at five cross sections located along the pipe. Blue regions are rich in cold oil; red regions are rich in hot SCW; and regions shown in intermediate colors have varying concentrations of the two fluids.

The oil enters the cross section at the top and water at the bottom. As the spinning vortices form, the oil is driven downward near the center of the pipe, and the water is driven upward along the walls. In the first cross section, the interface layer between the oil and SWC is thin and sharp. In subsequent cross sections, that layer expands and diffuses, showing the extent of the mixing.

The researchers conclude that most of the fluid mixing and associated heat transfer is due to the swirling action of the vortices. However, they note that the mixing rate and heating rate differ—and that both influence the chemistry that occurs in regions where the fluids are mixed. Given design and operating details—the kind of oil; pressures, speeds, and temperatures of the incoming flows; shape and size of the pipes; and so on—the CFD simulation can predict how this natural mixing process will progress and how temperatures will change at different locations over time.

The researchers are continuing to generate new knowledge that will help SCW processing become an economically viable commercial option. For example, they are clarifying the reactions whereby carbon-carbon bonds are broken in the heaviest fractions, including asphalt. They are quantifying the different rates at which various oils will diffuse and mix in SCW—an effect first discovered in their modeling analyses. They are taking a closer look at inexpensive catalysts that can help encourage the breakdown of large hydrocarbons and are stable enough to be regenerated and reused. And they are exploring the possibility of linking SCW processing with other environmentally friendly desulfurization and upgrading technologies to create a combined system that will make it practical to continue producing high-value fuels from all kinds of oil for decades to come.

This research was supported by Saudi Aramco, a founding member of the MIT Energy Initiative.

Resources

• Michael T. Timko, Ahmed F. Ghoniem, William H. Green (2015) “Upgrading and desulfurization of heavy oils by supercritical water,” , Volume 96, Pages 114-123 doi: 10.1016/j.supflu.2014.09.015

• Ashwin Raghavan, Ahmed F. Ghoniem (2014) “Simulation of supercritical water–hydrocarbon mixing in a cylindrical tee at intermediate Reynolds number: Impact of temperature difference between streams,” The Journal of Supercritical Fluids, Volume 95, Pages 325-338 doi: 10.1016/j.supflu.2014.09.030

• Yuko Kida, Caleb A. Class, Anthony J. Concepcion, Michael T. Timko and William H. Green (2014) “Combining experiment and theory to elucidate the role of supercritical water in sulfide decomposition” Phys. Chem. Chem. Phys., 16, 9220-9228 doi: 10.1039/C4CP00711E