Extracting Oil and Tar from Sand

diagram of ionic liquid, sand, and bitumen separation

The Separation of Oil or Tar from Sand Using Ionic Liquids

Dr. Paul Painter, Penn State University

The separation and recovery of oil or tar from sand is a critical problem in a number of industries and crucial to the mitigation of environmental disasters, such as those associated with oil spills. For example, oil or tar sands compose a significant proportion of the world’s known oil reserves. The largest deposits are found in Canada and Venezuela, which have combined oil sand reserves estimated to be equal to the world’s total reserves of conventional crude oil. Canada’s tar sands now provide the U.S. with more than one million barrels of oil a day. Significant quantities of tar sands (estimated to contain 32 billion barrels of oil) can also be found in Eastern Utah in the US. These deposits are complex mixtures of sand, clays, water and bitumen, a “heavy” or highly viscous oil. Extraction and separation of bitumen from surface-mined oil sands for the purpose of processing to fuels is much more expensive than extracting conventional oil by drilling and involves the use of significant amounts of energy and water. The water used in the process is ultimately stored in vast tailing ponds. It is a complex mixture of water, dissolved salts, minerals, residual bitumen, surfactants released from the bitumen and other materials used in processing and is acutely toxic to aquatic life. The environmental problems associated with extracting oil from tar sands are now a source of considerable concern. Some of these problems and the need for large quantities of water have prevented the exploitation of the Utah deposits (along with the higher viscosity of the bitumen and consolidated nature of the tar sands).

Oil contaminated sand and soil is also a major problem after oil spills, either accidental as in the Exxon Valdez or Deepwater Horizon incidents, or as a result of a deliberate act of war or terrorism, as in Kuwait, where large areas of the desert remain contaminated with residual oil that has now aged and become difficult to remove. Various methods for separating oil from sand or other mineral containing materials such as drill cuttings or oil sludge have been described over the years. These include incineration, distillation, washing with detergents or extraction using organic solvents. Some of these methods have proved to be uneconomic because of their energy requirements, others do not completely remove the oil from the sand, or the chemicals used may also pose unacceptable environmental concerns. As a result, oil field or refinery wastes are often stored in pits or landfills. Removal to landfills also appears to be the fate of most of the contaminated sand removed from beaches in the Gulf of Mexico after the recent Deepwater Horizon oil spill. Long-term storage of such material is a major concern.

In work completed in the last 18 months at Penn State, it has been shown that certain ionic liquids, ILs, can be used to separate bitumen from tar or oil sands and oil from beach contaminated sand. The separation is usually conducted in conjunction with a non-polar solvent to lower the viscosity of the tar or bitumen and facilitate separation. The separation occurs at room temperature and does not result in the generation of waste process water. Essentially all of the bitumen is recovered in a very clean form, with no detectable mineral fines, which interact preferentially with the IL, and no contamination from the IL. The minerals (sand) are also recovered in an uncontaminated form after removing residual IL with small amounts of (cold) water. Because of the unique properties of ILs, the water and IL used in this process can be readily separated, recovered and recycled through the system.

ILs have been widely studied in the last few years and there is now a vast literature concerning the structure and properties of these materials. The properties that most relevant to our recent work are their outstanding chemical and thermal stability, very low degree of flammability and almost negligible vapor pressure. This makes them attractive solvents for this and other chemical processes. In our work we have used a particularly versatile group of ILs is based on 1-alkyl-3-methylimidazolium cations (illustrated below). The properties of these materials can be “tailored” by varying the nature of the substituents (R) on the ring and the nature of the anion.

chemical diagram

 

As mentioned above, we have found that bitumen can be cleanly separated from medium- and low-grade Canadian oil sands and also Utah tar sands using certain ionic liquids. The pictures below show what can be observed after simply stirring an oil sand sample with an imidazolium IL and toluene at ambient temperatures (~25˚C) in a glass tube. Three layers are formed. The bottom layer consists of sand and clays suspended in the IL, the middle layer contains the ionic liquid with a small amount of entrained bitumen and some mineral fines, while the top layer is a toluene/bitumen mixture. This was easily removed from the other layers using a pipette. Essentially all of the bitumen was released from the sand and no fine particles could be detected in the bitumen phase. Water was not used in this stage of the separation, but relatively small amounts were used to separate residual IL from the sand and clays in the bottom layer. Because both the IL and water can be easily separated and recycled through the system, this process has the potential for ameliorating many of the environmental problems associated with current extraction methods.

diagram of ionic liquid, sand, and bitumen separation

Figure 1: Separation of bitumen from a Canadian oil sand sample using an ionic liquid.

 

We have found that aqueous solutions of certain ILs work as well as more concentrated solutions or pure ILs (still at room temperature). Using this approach we have built a bench-scale unit where kilogram quantities of tar sands or oil-contaminated sand can be separated and scale-up can be demonstrated. A picture of the unit in its present form is shown in figure 2, and a video of the unit in operation can be seen here.

First, although ILs are powerful solvents for a number of materials, non-polar polymers appear to be unaffected by the highly hydrophilic IL used in this initial work. We have also “soaked” various gaskets (polyethylene, fluoroelastomers, etc.) in the ILs that are of interest to us for 4 months (and counting) with no apparent dissolution. Accordingly, for our main mixing and separation vessel, we simply used a large (high density) polyethylene container, drilled some holes and installed inlet and outlet ports. The larger of these allows the oil sand/IL-water mixture and organic solvent (usually kerosene or naphtha) to be pumped around a closed loop to “condition” the oil/sand mixtures and detach tar or bitumen. Some stirring and shear is necessary to mix and promote oil/sand detachment.

Picture of the initial set up for the bench scale unit

Figure 2: Picture of the initial set up for the bench scale unit

 

Phase separation of bitumen/solvent mixture from IL/water mixture and sand upon standing.

Figure 3: Phase separation of bitumen/solvent mixture from IL/water mixture and
sand upon standing.

We have installed “Y” connections in the closed loop to allow the removal of the bitumen/solvent layer from the top and the sand from the bottom. In one experiment, for example, an oil sand/IL/kerosene mixture was pumped around the loop for 5 minutes (while the bulk of the material in the main vessel was being stirred). The contents were then allowed to settle. Upon standing we got a beautiful phase separation, similar to what was obtained in simple laboratory experiments, as can be seen in figure 3 (above). Experiments are under way to determine how much “conditioning” or shear is necessary to achieve a fast separation. Also, cyclones are being installed to separate “cleaned” sand from residual IL and oil/solvent mixtures from IL/water.

The separation unit we have built is crude, but cheap, robust and works well! Work is underway to reduce this process to practice.