Title of Invention: System and method of separating bitumen from tar sands Abstract: Various methods and systems for obtaining bitumen from tar sands are disclosed. The disclosed methods and/or systems can be used to economically achieve a high degree of bitumen recovery from tar sands. The method may include a primary leaching or extraction process that separates most of the bitumen from the tar sands and results in a bitumen-enriched component and a bitumen-depleted component. The bitumen-enriched component includes mainly solvent and bitumen. The bitumen-depleted component includes mainly water and mineral solids with some residual bitumen and solvent. The bitumen-depleted component may be mixed with liquefied petroleum gas, e.g., propane and/or butane, to further separate the residual solvent and bitumen. The disclosed system can include separators configured to separate the various components at the various stages in the process. Detailed Description: Part general_description_paragraphs: Part CROSS REFERENCE TO RELATED PATENT APPLICATIONS: This patent application is a continuation in part of U.S. patent application Ser. No. 11/249,234, pending, entitled “Method for Obtaining Bitumen from Tar Sands,” filed on 12 Oct. 2005, published as U.S. Patent Application Publication No. 2006/0076274, which claims priority to U.S. Provisional Patent Application Ser. No. 60/617,739, filed on 13 Oct. 2004, both of which are incorporated herein by reference in their entireties. In the event of a conflict, the subject matter explicitly recited or shown herein controls over any subject matter incorporated by reference. All definitions of a term (express or implied) contained in any of the subject matter incorporated by reference herein are hereby disclaimed. Part BACKGROUND: Tar sands, which are also referred to as oil sands or bituminous sands, is a common name of geological formations that contain bitumen, an extremely heavy type of crude oil. Tar sands can have a variety of compositions but typically include, in addition to bitumen, water and mineral solids. The mineral solids can include inorganic solids such as coal, sand, and clay. Tar sand deposits can be found in many parts of the world, including North America. One of the largest tar sands deposits is in the Athabasca region of Alberta, Canada. In the Athabasca region, the tar sands formation can be found at the surface, although it may be buried as deep as two thousand feet below the surface overburden. Tar sands deposits are measured in barrels equivalent of oil. It is estimated that the Athabasca tar sands deposit contains the equivalent of about 1.7 to 2.3 trillion barrels of oil. Global tar sands deposits have been estimated to contain up to 4 trillion barrels of oil. By way of comparison, the proven worldwide oil reserves is estimated to be about 1.3 trillion barrels. The bitumen content of tar sands varies from approximately 5 wt % to 21 wt %, with a typical content of approximately 12 wt %. Tar sands also include approximately 1 wt % to 10 wt % water. The remainder is mineral matter such as coal, sand, and clay. Bitumen is best described as a thick, sticky form of crude oil that is so heavy and viscous that it will not flow unless heated or diluted with lighter hydrocarbons. At room temperature, the flowability of bitumen is much like cold molasses. Extracting usable oil from tar sands typically begins by separating the bitumen from the mineral solids. One conventional process includes mixing the ore with hot water to form a bitumen enriched froth. The froth is separated and further processed to isolate the bitumen product. Conventional water based extraction technologies are capable of separating bitumen from higher grade ore but are unable to economically separate bitumen from lower grade ore. Unfortunately, this means that a significant amount of tar sand ore is not capable of being processed to recover the otherwise valuable bitumen. Another problem with conventional water based extraction technologies is the low overall recovery rate of bitumen. Unfortunately, conventional extraction processes discharge much of the bitumen in the ore with the tailings. Not only does this reduce the efficiency of the extraction process due to lower recoveries, but it also presents potential environmental problems that must be addressed. The residual bitumen is not the only problem associated with the tailings from conventional processes. The amount and physical characteristics of the tailings also present significant problems. For example, the tailings from conventional processes typically include two components: (a) wet tailings (commonly referred to as “coarse sand”) that contain about 15 wt % water and (b) a stream of a clay/sand mixture (commonly referred to as “fine tailings”) that contains 70 wt % water. In some circumstances, the total amount of the tailings may be more than the amount of the mined ore, which means that the tailings cannot be returned to the mined area. Some of the tailings must be discharged elsewhere. This creates numerous environmental problems associated with the disposal of the tailings. Furthermore, the relatively high water content of the tailings may require the tailings to be stored in a pond instead of stacked or piled thereby further increasing the problems associated with disposal of the tailings. Fine tailings produced by conventional processes may be especially problematic to dispose. Fine tailings are typically made up of clay, sand, water, and residual bitumen. Due to the water entrained in the clays, fine tailings can have a sludge-like consistency that lasts indefinitely. Byproducts such as fine tailings can be stored in ponds, but these ponds are costly to build and maintain and can be damaging to the local environment, including the local water supply. Many conventional methods for obtaining bitumen from tar sands also have serious technical limitations. For example, many conventional methods use water, which can cause clays in the tar sands to swell and interfere with processing equipment. In addition, some conventional methods result in the undesirable precipitation of soluble asphaltenes. One example of a conventional method is described in U.S. Pat. No. 4,046,668 (the '668 patent). The '668 patent discloses the extraction of hydrocarbons from tar sands with a mixture of light naphtha having from 5 to 9 carbon atoms per molecule and methanol. The method disclosed in the '668 patent is limited, in part, because it requires the simultaneous use of two solvents, which increases processing costs. U.S. Pat. No. 4,347,118 (the '118 patent) discloses a method in which pentane is used to extract bitumen from tar sands. The method disclosed in the '118 patent requires the use of two fluidized bed drying zones. Operation of these fluidized bed drying zones requires a large amount of energy, limiting the efficiency of the overall method. Furthermore, the pentane solvent does not solubilize the asphaltene fraction of the bitumen that is not pentane soluble. Thus, this fraction of the bitumen is discharged with the tailings. For Athabasca type bitumen this can amount to 20 wt % to 40 wt % of the total initial hydrocarbon content of the tar sands. U.S. Pat. No. 5,143,598 (the '598 patent) discloses a method that includes adding heptane to tar sands to form a bitumen-rich heptane phase and then displacing the bitumen-rich heptane phase with water. This method utilizes steam vaporization and condensation, which are low-efficiency processes. Also, the use of heptane, a non-aromatic solvent, in this method can result in the precipitation of the heptane insoluble asphaltene fraction present in the bitumen phase. The heptane insoluble asphaltene fraction is discharged with the tailings. In addition, using water not only generates large amounts of aqueous waste but also creates oil-water emulsions that are very difficult to breakdown. The use of water can also introduce undesirable impurities into bitumen, such as chlorine, and can result in undesirable swelling of clays in the tar sands. Furthermore, the bitumen recovered by this method typically has a low purity and requires additional processing, such as by centrifugation. This further increases the cost of the overall recovery process. Part SUMMARY: Disclosed are embodiments of a method and system for obtaining bitumen from bitumen comprising materials such as tar sands. The disclosed method and system may include multiple solvent extraction or leaching steps to separate the bitumen from the tar sands. The method may include subjecting the tar sands to a first solvent extraction or leaching step in which tar sand ore is mixed with a first hydrocarbon solvent to form a first mixture. The first mixture is separated into a first bitumen-enriched component and a first bitumen-depleted component. The first bitumen-enriched component primarily includes the solvent and the soluble bitumen fraction. The first bitumen-depleted component primarily includes water, mineral solids, and the insoluble bitumen fraction. In the second extraction step, the first bitumen-depleted component is mixed with a second hydrocarbon solvent to form a second mixture. The second hydrocarbon solvent is used to separate additional hydrocarbons (e.g., residual amounts of the first hydrocarbon solvent and additional bitumen) from the first bitumen-depleted component. The second mixture may be separated to produce tailings that have a low hydrocarbon content. The tailings may be subjected to further processing or may be disposed. Alternatively, the second hydrocarbon solvent may be used in place of the first hydrocarbon solvent to form the first mixture. The first mixture may be separated and the first bitumen-enriched component may be mixed again with the same hydrocarbon solvent. In another embodiment, the first mixture may be mixed with a combination of the first hydrocarbon solvent and the second hydrocarbon solvent. The resulting mixture can be separated into a bitumen-enriched component and a bitumen-depleted component. The bitumen-enriched component may be mixed with another hydrocarbon solvent and separated to produce tailings that have a low hydrocarbon content. Embodiments of the disclosed method also can include transporting the tar sands and the first hydrocarbon solvent through a pipeline or mixing the tar sands and the first hydrocarbon solvent in a pipeline. In certain embodiments, tar sands and the first hydrocarbon solvent may be mixed together in any of a number of suitable ways. For example, in one embodiment, the tar sands and the first hydrocarbon solvent may be mixed in a vessel. Sufficient mixing may occur simply by virtue of the manner in which the tar sands and the first hydrocarbon solvent are introduced into the container—e.g., the first hydrocarbon solvent may be introduced at high velocity thereby effectively agitating the contents of the vessel. In other embodiments, a powered mixing device may be provided to mix the contents of the vessel. In further embodiments, the tar sands may be mixed with the first hydrocarbon solvent in a countercurrent process. In yet another embodiment, the tar sands and the first hydrocarbon solvent may be mixed in a pipe. The second hydrocarbon solvent may be mixed with the first bitumen-depleted component in any of the same ways. The first mixture can be separated into the first bitumen-enriched component and the first bitumen-depleted component using any suitable method. The first mixture may be separated in a filtration operation, gravity separation operation, or the like. In one embodiment, the liquids and the solids may be separated initially using a settling operation. In another embodiment, the first mixture may be separated using vacuum filtration or pressure filtration. The first bitumen-depleted component is taken from the bottom of the vessel. The vessel used to separate the liquids and solids may be the same vessel that was used to mix the tar sands and the first hydrocarbon solvent, or it may be a separate separation apparatus specifically designed to separate the components. In certain embodiments, the first bitumen-depleted component is mixed with the second hydrocarbon solvent at an elevated pressure sufficient to maintain the second hydrocarbon solvent in liquid form. This may be desirable when the boiling point of the second hydrocarbon solvent at atmospheric pressure is lower than the process temperature. Suitable pressures may range from approximately 1.1 atmospheres to 10 atmospheres. In other embodiments, the pressure may be approximately 110 kPa to 2000 kPa. The process may be operated as a continuous, batch, or semi-batch process depending on the circumstances. Continuous processes may be used for large scale implementations of the process while batch may be used in smaller scale implementations. It should be understood that regardless of the size of the process, any suitable configuration of the process may be used. The first hydrocarbon solvent may be any suitable solvent such as a light aromatic solvent. Exemplary solvents may include toluene, xylene, kerosene, diesel (including biodiesel), gas oil, light distillate, commercially available aromatic solvents such as Solvesso 100, 150, and 200 and/or naphtha. The first hydrocarbon solvent also can include compounds such as benzene and/or aromatic alcohols. In some embodiments, the first hydrocarbon solvent has a boiling point of approximately 75° C. to 375° C. The second hydrocarbon solvent can include any suitable material or combination of materials that is capable of further separating the first hydrocarbon solvent and/or additional bitumen from the inorganic solids and water. In one embodiment, the second hydrocarbon solvent may include one or more aliphatic compounds having 3 to 9 carbon atoms. In another embodiment, the second hydrocarbon solvent may be cyclo- or iso-paraffin having 3 to 9 carbons or a combination thereof. In yet another embodiment, the second hydrocarbon solvent may include a mixture of any of these hydrocarbons. In another embodiment, the second hydrocarbon solvent may include liquefied petroleum gas (LPG). The term “liquefied petroleum gas” is used broadly herein to refer to any hydrocarbon gas (hydrocarbons that are gases at ambient temperatures and pressures −25° C. and 1 atm) that has been compressed to form a liquid. Thus, the term LPG encompasses any of the widely available commercial LPG formulations as well as other any other mixture of liquefied hydrocarbon gases. In one embodiment, the LPG may have a boiling point (all boiling points are given at atmospheric pressure unless specified otherwise) of approximately −80° C. to 10° C. In a preferred embodiment, the second hydrocarbon solvent includes LPG that comprises propane. The LPG includes at least approximately 80 wt % propane, or, desirably, at least approximately 90 wt % propane. In some embodiments, the LPG may be entirely propane or a mixture or propane and propylene. The second hydrocarbon solvent may include commercially available LPG. Commercial LPG is a natural derivative of both natural gas and crude oil. Commercial LPG is predominantly a mixture of propane and butane (n-butane and/or i-butane) with a small percentage of propylene and/or butylene (any one or combination of the four isomers). In some formulation, commercial LPG may be predominantly or even entirely propane or predominantly or entirely butane. Commercial LPG often contains very small amounts of lighter hydrocarbons such as ethane and ethylene and heavier hydrocarbons such as pentane. Three examples of commercial LPG are shown below in Table 1 TABLE 1 Examples of Commercially Available LPG Commercial Commercial Butane/Propane Component HD-5 Propane Propane Mixture Lighter Min 90% (liq vol.) propane Mixture of propane Mixture of Butane Hydrocarbons Max 5% (liq. vol.) propylene and/or propylene and/or butylenes and propane and/or propylenes Butane and 2.5% (liq. vol.) 2.5% (liq. vol.) — heavier hydrocarbons Pentane and — — Max 2% (liq. vol.) heavier hydrocarbons Residual matter 0.05 ml 0.05 ml — Total Sulfur 123 PPMW 185 PPMW 140 PPMW The composition of commercial LPG may vary depending on the season. For example, in the summer, the LPG may include primarily or even entirely butane due to volatilization issues presented by hot summer temperatures. However, in the winter, the LPG may include a 70:30 mixture of butane to propane since the outdoor temperatures are much colder. The tar sand ore can have a bitumen concentration of approximately 3 wt % to 20 wt %. The process is capable of recovering at least approximately 93 wt %, at least approximately 95 wt %, or at least approximately 97 wt % of the bitumen in the tar sand ore. Most of the bitumen is separated in the first solvent extraction step. In one embodiment, the bitumen concentration of the first bitumen-depleted component is approximately 0.5 wt % to 5 wt %. The second solvent extraction step may be used to separate residual amounts of the first hydrocarbon solvent and additional amounts of bitumen. The tailings component that is discharged from the process may have a hydrocarbon concentration (bitumen+added solvents) of approximately 0.01 wt % to 5 wt %, or, desirably, a hydrocarbon concentration of approximately 0.5 wt % to 2 wt %. The tailings component may have a hydrocarbon concentration of no more than approximately 5 wt %, no more than approximately 3 wt %, alternatively, no more than approximately 2 wt %, or, in some embodiments, no more than approximately 1 wt %. Both the first hydrocarbon solvent and the second hydrocarbon solvent can be separated and recycled back to the process. Accordingly, in some embodiments, the first bitumen-enriched component is separated into a bitumen product and a recovered first hydrocarbon solvent. This separation can be performed, for example, with a distillation process where the first bitumen-enriched component is heated to a temperature greater than or equal to the boiling point of the first hydrocarbon solvent, such as approximately 70° C. to 170° C. These and other embodiments can also include separating the second bitumen-enriched component to recover the second hydrocarbon solvent. This separation can be performed, for example, by flashing the bitumen-enriched component to vaporize the second hydrocarbon solvent. The remaining liquids include bitumen and the first hydrocarbon solvent. The remaining liquids can be separated further to isolate the bitumen and the first hydrocarbon solvent. The system used to separate the various mixtures in the process can include a variety of separators some of which may also be used as mixers, etc. For example, the first mixture may be separated in a first separator, such as a settling vessel or filter. The first separator may also function as the mixing vessel where the tar sand ore is mixed with the first hydrocarbon solvent. A second separator may be used to separate the second mixture to isolate the hydrocarbons from inorganic solids. The second separator may be an expansion vessel, pipe vessel, autoclave, or the like. A third separator may be used to separate the first bitumen-enriched component from the first mixture to isolate the bitumen product from the first hydrocarbon solvent. The third separator may be a multi-hearth solvent recovery furnace, distillation apparatus, or the like. A fourth separator may be used to separate agglomerations of the bitumen and inorganic solids in the tailings. The fourth separator may be a mechanical screening device that separates the agglomerations based on their size. The system may also include pipelines that are used to route the various components to the separators, vessels, and other components. It is to be understood that the foregoing is a brief summary of various aspects of some disclosed embodiments. The scope of the disclosure need not therefore include all such aspects or address or solve all issues noted in the background above. In addition, there are other aspects of the disclosed embodiments that will become apparent as the specification proceeds. The foregoing and other features, utilities, and advantages of the subject matter described herein will be apparent from the following more particular description of certain embodiments as illustrated in the accompanying drawings. Part DETAILED DESCRIPTION: Before describing the details of the various embodiments herein, it should be appreciated that the terms “solvent,” “a solvent” and “the solvent” include one or more than one individual solvent compound unless expressly indicated otherwise. Mixing solvents that include more than one individual solvent compound with other materials can include mixing the individual solvent compounds simultaneously or serially unless indicated otherwise. The separations described herein can be partial, substantial or complete separations unless indicated otherwise. All percentages recited herein are weight percentages unless indicated otherwise. Various embodiments of a method and system for obtaining bitumen from tar sands are described herein. Tar sands are used throughout this disclosure as a representative bitumen comprising material. As used herein, the phrase “tar sands” includes a variety of compositions that include both bitumen and mineral components. Tar sands typically include sand, clay, bitumen, and water. The bitumen in tar sands typically includes oil, resins, and asphaltenes. Depending on the composition, tar sands can have varying levels of hardness. Some tar sands are found in the form of a rock-like ore. Other tar sands have a granular texture similar to that of sand. Still other tar sands can have a different texture between that of sand and that of rock. The bitumen, once separated from the remainder of the tar sands, can be used in a number of ways. For example, the bitumen may be refined into valuable commodities such as oil and gasoline. The disclosed embodiments for obtaining bitumen from tar sands can include two or more solvent extraction or solvent leaching operations, and associated apparatus. In general, solvent extraction (also commonly referred to as leaching) is a process of extracting a substance from a solid by dissolving the substance in a liquid. In this situation, the bitumen is dissolved in one or more solvents to extract the bitumen from the other materials in the tar sands, especially the mineral solids. In some embodiments, the first solvent extraction step includes mixing tar sand ore with a first hydrocarbon solvent. This step separates most of the bitumen into a first bitumen-enriched component. A bitumen product can be obtained by subsequently separating the first bitumen-enriched component to isolate the first hydrocarbon solvent and the bitumen product. The recovered first hydrocarbon solvent can be recycled back to the process. The initial solvent extraction operation also produces a first bitumen-depleted component. The second solvent extraction step includes mixing the first bitumen-depleted component with a second hydrocarbon solvent. The second hydrocarbon solvent is selected to remove most, if not substantially all, of the first hydrocarbon solvent and remaining bitumen from the first bitumen-depleted component. The inorganic solids can be disposed as tailings and the hydrocarbons can be separated to isolate the first hydrocarbon solvent, the second hydrocarbon solvent, and the bitumen product. The first and second hydrocarbon solvents can be recycled back to the process. A variety of techniques and devices can be used to perform each operation in the various embodiments of the process. For example, in some embodiments, the tar sands and the first hydrocarbon solvent are mixed in a vessel to form a first mixture. The first mixture may be separated into a first bitumen-enriched component and a first bitumen-depleted component by filtering or settling and decanting. In one embodiment, the mixing and filtering/settling can occur in the same vessel or in separate vessels. The resulting components may be separated further using any of a variety of suitable processes and apparatuses. The first bitumen-enriched component may be separated to isolate the first hydrocarbon solvent and the bitumen product. In one embodiment, this may be done using a distillation process (e.g., steam stripping process, multi-hearth solvent recovery furnace, and the like). The first bitumen-depleted component may be mixed with the second hydrocarbon solvent in a pressurized vessel, such as an autoclave or pipe vessel. A pressurized vessel is used to keep the second hydrocarbon solvent in liquid form. This mixture may be separated into a second bitumen-enriched component and a second bitumen-depleted component. The second hydrocarbon solvent is recovered from the second bitumen-enriched component by flashing the liquids. The rapid decrease in pressure causes the second hydrocarbon solvent to vaporize and thereby separate from the other hydrocarbons, i.e., the first hydrocarbon solvent and bitumen. Other methods may also be used to recover the second hydrocarbon solvent. Each of the mixing and separation steps can be performed as a continuous, batch, or semi-batch process. Also, certain process steps might be performed as batch processes and others as continuous processes. Continuous processing is typically used in larger scale implementations. However, batch processing may result in more complete separations than continuous processing. The first hydrocarbon solvent may include any suitable solvent or mixture of solvents that is capable of solubilizing bitumen. It is also desirable to use a solvent or mixture of solvents that is economical and relatively easy to handle and store. It may also be desirable for the first hydrocarbon solvent to be generally compatible with refinery operations. In one embodiment, the first hydrocarbon solvent may be a light aromatic solvent. It should be appreciated, however, that a light aromatic solvent is not necessarily 100% aromatic. Instead, a light aromatic solvent may include a mixture of aromatic and non-aromatic compounds. For example, the first hydrocarbon solvent can include greater than zero to about 100 wt % aromatic compounds, such as approximately 10 wt % to 100 wt % aromatic compounds, or approximately 20 wt % to 100 wt % aromatic compounds. Any of a number of suitable aromatic compounds may be used as the first hydrocarbon solvent. Examples of aromatic compounds that can be used as the first hydrocarbon solvent include benzene, toluene, xylene, aromatic alcohols and combinations and derivatives thereof. The first hydrocarbon solvent can also include compositions, such as kerosene, diesel (including biodiesel), gas oil (e.g., light or medium light gas oil), light distillate, commercial aromatic solvents such as Solvesso 100, Solvesso 150, and Solvesso 200 (also known in the U.S.A. as Aromatic 100, 150, and 200, including mainly C10-C11 aromatics, and produced by ExxonMobil), and/or naphtha. In some embodiments, the first hydrocarbon solvent has a boiling temperature of approximately 75° C. to 375° C. The light distillate may be obtained by cracking a mixture of heavier hydrocarbons such as bitumen or crude oil. For example, the light distillate may be obtained by cracking bitumen using the systems and methods described in U.S. Patent Application Publication No. 2006/0144760, entitled “Nozzle Reactor and Method of Use,” filed on 21 Sep. 2005, which is incorporated herein by reference in its entirety. The light distillate may be the light fraction of hydrocarbons separated during distillation of the first bitumen-enriched component as shown in FIG. 7. One potential advantage of using the light distillate is that it is generated on site at the processing location so that it does not have to be shipped from another location. The other solvents listed are also equally suitable to be used in the process. Naphtha, for example, is particularly effective at dissolving bitumen and is generally compatible with refinery operations. Some examples of kerosene include hydrocarbons having between 9 and 15 carbons per molecule. Some examples of diesel include hydrocarbons having between 15 and 25 carbons per molecule. Some examples of light or medium light gas oil include hydrocarbons having between 13 and 20 carbons per molecule. Some examples of naphtha include hydrocarbons having between 4 and 12 carbons per molecule. These examples are not intended to limit the general meanings of the respective terms. The second hydrocarbon solvent can be any suitable hydrocarbon solvent that is useful for extracting bitumen and/or any remaining amounts of the first hydrocarbon solvent. In general, the second hydrocarbon solvent is selected so that it effectively solubilizes the bitumen and the first hydrocarbon solvent and so that it can be readily separated from the other hydrocarbons and mineral solids. In one embodiment, the second hydrocarbon solvent can include one or more aliphatic compounds that are capable of solvating bitumen and/or the first hydrocarbon solvent. Suitable aliphatic compounds can include compounds such as alkanes or alkenes. Any of these aliphatic compounds can be functionalized or non-functionalized. In one embodiment, the second hydrocarbon solvent includes one or more aliphatic hydrocarbons having 3 to 5 carbon atoms. In another embodiment, the second hydrocarbon solvent includes aliphatic hydrocarbons having no more than 5 carbon atoms. The second hydrocarbon solvent may also include lower carbon paraffins, such as cyclo- and iso-paraffins having 3 to 5 carbon atoms. The second hydrocarbon solvent may include one or more of any of the following compounds: methane, ethane, propane, butane, and/or pentane, alkene equivalents of these compounds and/or combinations and derivatives thereof. In one embodiment, the second hydrocarbon solvent includes LPG. Preferably, the LPG is primarily or even entirely propane. However, other LPG formulations are contemplated including commercially available formulations. The composition of common commercial LPG can vary depending on the time of the year, geographical location, etc. Often, LPG is a mixture of propane and butane with small amounts of propylene and butylene. A powerful odorant such as ethanethiol is typically added to make it easy to detect leaks. It should be appreciated that LPG may have a wide variety of compositions. For example, LPG may be primarily or entirely propane, or primarily or entirely butane or may not include either of these materials. LPG is stored and transported under pressure to maintain the hydrocarbons as liquids. In one embodiment, LPG has a boiling point at atmospheric pressure of approximately −80° C. to 10° C., desirably, approximately −55° C. to 5° C., or, suitably, approximately −35° C. to −5° C. The process and system used to separate bitumen from tar sands may have one or more advantages over conventional methods. For example, the use of hydrocarbon solvents to extract the bitumen prevents swelling of clays in the tar sands that may otherwise occur in a conventional water based process. Clays are generally present as large chunks of consolidated material. Conventional methods that include hot water processing typically require mechanical agitation, which breaks down the chunks into small and swollen clay particles. Retaining the clays in a consolidated form helps to minimize interference with operation of the processing machinery. In addition, bitumen-barren clays that have not been exposed to aqueous solutions typically do not significantly disperse, which enables the clays to be separated by settling and/or filtration processes. As another example, the process and system may hold certain advantages over conventional one-step processes involving separation of bitumen from tar sands in a pressurized autoclave with an alkane solvent. These conventional processes typically require careful monitoring of the ratio of bitumen to alkane solvent to minimize precipitation of asphaltenes. If the asphaltenes precipitate, they tend to clog various pieces of equipment, such as filters and centrifuges, and the like. This increases the overall cost and complexity of the separation process. In contrast to these conventional methods, the disclosed method and system avoids many of the problems associated with the precipitation of asphaltenes. The first hydrocarbon solvent is selected so that it solubilizes most, if not substantially all, of the asphaltenes in the tar sands. Since the asphaltenes are solubilized, most are separated in the first extraction step. The second hydrocarbon solvent would normally cause some fraction of the asphaltenes to precipitate. However, since a large majority of the asphaltenes have already been solubilized and dissolved in the first extraction step (approximately 70 wt % to 90 wt %), the second hydrocarbon solvent does not cause significant precipitation of asphaltenes. The use of LPG as the second hydrocarbon solvent may also provide a number of advantages. For example, commercial LPG is widely available and relatively inexpensive compared to some other organic solvents. Also, the volatility of LPG makes it relatively easy to separate from the other hydrocarbons, e.g., bitumen and/or the first hydrocarbon solvent. For example, the LPG may be separated using something as simple as a flash chamber. The gaseous LPG may be collected from the top of the flash chamber and the remaining liquid comprising bitumen and/or the first hydrocarbon solvent may be collected from the bottom of the flash chamber. The gaseous LPG may be compressed until it is in a liquid state and reused in the process. The bitumen and the first hydrocarbon solvent may be separated from each other by distillation or with some other suitable device or technique. In one embodiment, the first bitumen-depleted component may be blasted with a gas stream before the first bitumen-depleted component is mixed with the second hydrocarbon solvent. The gas stream is used to blow the liquids off of the solids. The liquids are primarily the first hydrocarbon solvent and any bitumen that is solubilized by the first hydrocarbon solvent. The gas stream dries the wet mineral solids by impacting the lighter liquids and pushing them away from the heavier solids. The liquids may be combined with the first bitumen-enriched component and subject to further processing to separate the first hydrocarbon solvent and the bitumen. Exposing the first bitumen-depleted component to the gas stream increases the overall recovery of bitumen from the tar sands (see the examples section herein for data to show the increased recovery rate). Of course, other techniques such as filtering or settling may also be used to separate the first bitumen-depleted component before it is mixed with the second hydrocarbon solvent. It should be appreciated that any suitable gas may be used to blow the liquid off the mineral solids. In one embodiment, the LPG in gaseous form may be used. This minimizes the number of different components that are used in the process. Since both the gas purge and the liquid wash use the same material just in a different form, there is no need to stock other materials. In other embodiments, gases such as nitrogen or air may also be used to blow the liquid off the mineral solids. In one embodiment, the first bitumen-depleted component is exposed to the gas in the same vessel where it is mixed with the second hydrocarbon solvent. Not only does this eliminate the need to filter or settle the first mixture, but it cuts down on the number of times the tar sands must be moved as it is extracted. With reference to FIGS. 1-7, a number of embodiments are described of suitable processes and systems for extracting bitumen from tar sands. It should be appreciated that the details of the processes and systems shown in these figures can be combined, changed, or otherwise modified to provide additional embodiments. Accordingly, the specific embodiments shown in the Figures and described in the following should not be used to limit the scope of the claims. In one embodiment, depicted in FIG. 1, tar sands 10 and a first hydrocarbon solvent are routed into a vessel 20 and mixed to form a first mixture. In this embodiment, the first hydrocarbon solvent includes a make-up source 15 and a recycle source 45 of hydrocarbon solvent. The combination of the make-up source 15 and the recycle source 45 form the first hydrocarbon solvent 15, 45 that is mixed with the tar sands 10. The amount of the first hydrocarbon solvent 15, 45 added to the tar sand ore is sufficient to effectively solubilized at least a portion, or desirably all, of the bitumen in the tar sands 10. In one embodiment, the amount of the first hydrocarbon solvent 15, 45 is approximately 0.05 to 2.0 times the amount of the tar sands 10 by weight, approximately 0.05 to 1.0 times the amount of the tar sands 10 by weight, or approximately 0.1 to 0.5 times the amount of the tar sands 10 by weight. The weight ratio of the first hydrocarbon solvent 15, 45 to bitumen may be approximately 1.5:1 to 20:1, desirably, approximately 2.0:1 to 10:1, or, suitably, approximately 2.5:1 to 5:1. The weight ratio of the first hydrocarbon solvent 15, 45 to bitumen may be no more than approximately 10:1, no more than approximately 8:1, desirably, no more than approximately 5:1, or, suitably, no more than approximately 3:1. It should be noted that the ratio of the first hydrocarbon solvent 15, 45 to bitumen is affected by the amount of bitumen in the ore. For example, high grade ore (e.g., >12 wt % bitumen) can be processed with a solvent to bitumen ratio as low as 2:1. However lower grade ore (e.g., 6 wt % bitumen) may need to be processed with a solvent to bitumen ratio greater than 3:1 to provide sufficient liquid to fill up the open space between the particles. The vessel 20 can be selectively opened or closed. In some embodiments, the tar sands 10 are routed into the vessel 20 using a conveyor belt. Conveyor belts are particularly well suited for transporting materials that cannot be transported easily through pipelines. For example, conveyor belts can be useful for transporting tar sands 10 in the form of a brittle ore. The tar sand ore 10 typically has a bitumen concentration of approximately 3 wt % to 20 wt %, approximately 5 wt % to 18 wt %, desirably, approximately 5 wt % to 17 wt % or, suitably, approximately 8 wt % to 14 wt %. It should be appreciated that the amount of bitumen in the tar sand ore 10 can vary significantly depending on its grade. The vessel 20 can include a mixer for mixing the tar sands 10 and the first hydrocarbon solvent 15, 45. In one embodiment, a motorized mixer may be provided to mix the contents of the vessel 20. In another embodiment, a countercurrent process can be used to facilitate mixing of the tar sands 10 and the first hydrocarbon solvent 15, 45. For example, the tar sands 10 can be introduced near the top of the vessel 20, while the first hydrocarbon solvent 15, 45 is introduced near the bottom of the vessel. The mineral solids in the tar sands 10, which typically are heavier than the first hydrocarbon solvent 15, 45, sink while the first hydrocarbon solvent 15, 45 rises. Mixing occurs as the first hydrocarbon solvent 15, 45 flows upwardly against the downwardly flowing tar sand ore 10. In another embodiment, the tar sands 10 and the first hydrocarbon solvent 15, 45 can be mixed prior to entering the vessel 20. For example, the tar sands 10 and the first hydrocarbon solvent 15, 45 can be mixed at or near the location at which the tar sands 10 is mined. In this way, the tar sands 10 can be converted into a slurry that can be transported to the vessel 20 through a pipeline. Additional mixing can occur as the slurry flows through the pipeline. The vessel 20 in this embodiment may function primarily as a hopper, holding tank, or separation vessel. The vessel 20 may function as both a mixer and a separator for separating the first mixture into a first bitumen-enriched component 25 and a first bitumen-depleted component (alternatively referred to as a bitumen-depleted sand component or phase, underflow, or bitumen-depleted tailings component) 30 as shown in FIG. 1. Alternatively, separate vessels can be used for mixing and separating. The first mixture may be separated by, for example, settling the first bitumen-depleted component 30 in the vessel 20. In one embodiment, the vessel 20 may be divided into sections. One section may be used to mix the tar sand ore 10 and the first hydrocarbon solvent 15, 45 and another section may be used to After settling, the first bitumen-depleted component 30 can be extracted from near the bottom of the vessel 20, while the first bitumen-enriched component 25 can be extracted from near the top of the vessel. Upon exiting the vessel 20, the first bitumen-enriched component 25 includes primarily bitumen and the first hydrocarbon solvent 45. The first bitumen-enriched component 25 may also contain some mineral solids and water. In one embodiment, the amount of the mineral solids may be approximately 0.01 wt % to 5 wt % or approximately 0.1 wt % to 2 wt % of the first bitumen-enriched component 25. The first bitumen-enriched component 25 exits the vessel 20 and is routed to a separator 35 to recover the first hydrocarbon solvent 45. The process flow of the bitumen-enriched component 25 is shown in FIG. 1. The separator 35 separates the first hydrocarbon solvent 45 from bitumen product 40. The first hydrocarbon solvent 45 is recycled back to the process and mixed with the tar sand ore 10. The separator 35 may also be configured to separate water and mineral solids from the first bitumen-enriched component 25 if necessary. The separator 35 may be any suitable separator 35 that is capable of separating the first hydrocarbon solvent 45 from the bitumen product 40. In one embodiment, the separator 35 may heat the first bitumen-enriched component 25 and separate the various liquids based on differences in their boiling points, e.g., distillation, etc. The heat can be provided, for example, by a heating unit, such as a heat exchanger. Heating can be done substantially at ambient pressure, at a pressure less than ambient, or at a pressure greater than ambient. The separator 35 also can include a pressure stabilizer to maintain a positive pressure (e.g., an inert gas may be pumped into the separator 35) or a vacuum pump to maintain a negative pressure. In one embodiment, the separator 35 includes a distillation tower (e.g., a vacuum distillation tower). Table 2 shows the boiling points of some of the components that may be used as or included in the first hydrocarbon solvent 45. In one embodiment, the first bitumen-enriched component 25 may be heated to a temperature of approximately 70° C. to 350° C., such as approximately 70° C. to 170° C., approximately 100° C. to 350° C., approximately 125° C. to 200° C., or desirably, approximately 140° C. to 180° C. TABLE 2 Solvent Boiling Points Compound Boiling Point ° C. Fatty Acid Methyl Esters C8 187 C10 224 C12 262 C14 295 C16 338 C18 352 Aromatic Hydrocarbons Toluene 111 Xylene 140 Coal Tar Naphtha 150-220 Petroleum Naphtha 172-215 Light distillate 204-260 In another embodiment, the separator 35 may include a multi-hearth solvent recovery furnace. Multi-hearth solvent recovery furnaces typically include alternating arrangements of centrally located hearths and peripherally located hearths. The hearths can be heated, for example, with oil fired muffles and/or high pressure steam coils. In some embodiments, hearths near the top of the furnace are heated to higher temperatures than hearths closer to the bottom of the furnace. As mentioned above, the first bitumen-enriched component 25 may contain some fine solid material that was not completely separated initially at the vessel 20. The separator 35 may be configured to function despite the presence of this fine solid material. For example, the separator 35 can include a suitable packing material, such as vertical slats, to provide increased surface area for condensation and evaporation. This packing material can be resistant to clogging by the fine solid material. In those embodiments where the separator 35 includes a distillation tower, the fine solid materials may fall to the bottom and be cleaned out periodically. The first hydrocarbon solvent 45 separated with the separator 35 may be recycled back and mixed with the tar sand ore 10. In one embodiment, the amount of recycled solvent 45 may be sufficient to make it unnecessary to continuously provide make-up solvent 15. Instead, make-up solvent may be added only when the total amount of the first hydrocarbon solvent drops below a threshold level. The ratio of recycled first hydrocarbon solvent 45 to fresh first hydrocarbon solvent 15 may be approximately 2:1 to 200:1, approximately 10:1 to 100:1, or approximately 15:1 to 25:1. In other embodiments, the first hydrocarbon solvent 45 may not be fully recycled back to the process. In these embodiments, the first hydrocarbon solvent 45 may be disposed of or used as a diluent for the transportation of bitumen 40 to the bitumen upgrading and/or refining facility. After being separated at the vessel 20, the first bitumen-depleted component 30 primarily includes mineral solids and water. The first bitumen-depleted component 30 may also include residual bitumen and the first hydrocarbon solvent 15, 45. In one embodiment, the first bitumen-depleted component 30 includes approximately 0.5 wt % to 5 wt % residual bitumen. In another embodiment, the first bitumen-depleted component 30 includes no more than 5 wt % residual bitumen, no more than 3 wt % residual bitumen, or, desirably, no more than 1 wt % residual bitumen. The first bitumen-depleted component 30 exits the vessel 20 and is routed to the vessel 75 where the second hydrocarbon solvent 60, 90 is added to form a second mixture. The second hydrocarbon solvent 60, 90 may be a volatile hydrocarbon solvent. A countercurrent process may be used to facilitate mixing of the first bitumen-depleted component 30 and the second hydrocarbon solvent 60, 90 in a similar manner to that described above in connection with mixing the first mixture. In one embodiment, the second hydrocarbon solvent 60, 90 includes LPG. A sufficient amount of the second hydrocarbon solvent 60, 90 is added to the first bitumen-depleted component 30 to at least partially remove or substantially completely remove any remaining bitumen and/or first hydrocarbon solvent 15, 45. In one embodiment, the ratio of the second hydrocarbon solvent 60, 90 to the first bitumen-depleted component, by weight, is approximately 0.01:1 to 1:1, approximately 0.05:1 to 0.5:1, or approximately 0.05:1 to 0.3:1. In one embodiment, the vessel 75 can be, for example, a pressurized vessel, such as an autoclave. The second hydrocarbon solvent 60, 90 is added to the vessel 75 in liquid form. The vessel 75 can operate at a pressure and temperature sufficient to maintain the second hydrocarbon solvent 60 in liquid form. The pressure (at 25° C.) can be, for example, between about 110 kPa and about 2000 kPa, such as between about 150 kPa and about 1500 kPa, between about 180 kPa and about 1200 kPa or between about 210 kPa and about 1050 kPa. The temperature in the vessel 75 may be, for example, between about 5° C. and about 150° C., such as between about 10° C. and about 60° C. or, desirably, ambient temperature. In another embodiment, the second hydrocarbon solvent may be added to the bitumen-depleted component 30 and processed in the manner described in U.S. Patent Application Publication Nos. 2005/0070218 and 2005/0092682, both of which are incorporated herein by reference in their entireties. After sufficient mixing time, the second mixture is separated into a second bitumen-enriched component 65 (alternatively referred to as a bitumen-enriched volatile hydrocarbon solvent phase) and a tailings component 70 (alternatively referred to as a residual sand component, residual sand phase, or second bitumen-depleted sand component). In one embodiment, the second mixture may be both mixed and separated in the vessel 75, as shown in FIG. 1. However, the second mixture may also be mixed and separated in separate vessels or processes. The second mixture may be separated using any suitable process or technique that is capable of separating the primarily hydrocarbon liquid from the mineral solids. In one embodiment, the second mixture may be separated using a gravity separation process such as settling. In another embodiment, the second mixture may be separated by filtering. The tailings component 70 output from the vessel 75 may have a relatively low hydrocarbon content (bitumen+added solvents). In one embodiment, the tailings component 70 has a hydrocarbon concentration of approximately 0.01 wt % and about 15 wt %, such as between about 0.1 wt % and about 10 wt % or between about 0.5 wt % and about 5 wt %. In some embodiments, the tailings component 70 is clean enough to be deposited back into the environment without further processing. In one embodiment, the tailings component 70 may be deposited back in the environment as dry stackable talings. Preferably, the tailings component 70 is deposited back in a location where tar sands ore 10 was previously mined. In one embodiment, additional bitumen may be separated from the tailings component 70 before it is disposed of. A large amount of the bitumen (along with other hydrocarbons such as the added solvents) in the tailings component 70 is present in the form of agglomerations of mineral particles and bitumen, sometimes referred to as “tar balls.” These agglomerations tend to be significantly larger than the mineral particles. Therefore, a screening operation (e.g., dry screening or otherwise) may be used to further separate the tar balls and recycle them back to the process with the tar sand ore 10. This may allow the process to recover approximately 25 wt % to 90 wt % of the contained hydrocarbon content of the tailings component 70. After being separated at the vessel 75, the second bitumen-enriched component 65 primarily includes the first hydrocarbon solvent 15, 45, the second hydrocarbon solvent 60, and bitumen. The second bitumen-enriched component 65 may also include some small amounts of mineral solids and water. The second bitumen-enriched component 65 is routed to a separator 80 after exiting the vessel 75, as shown in FIG. 1. In some embodiments, the second bitumen-enriched component 65 is under pressure as it is introduced into the separator 80 to keep the second hydrocarbon solvent 60 in the liquid phase. The separator 80 is used to separate the second hydrocarbon solvent in the second bitumen-enriched component 65 from a bitumen product 85. It should be appreciated that almost any remaining amounts of the first hydrocarbon solvent 15 are separated into the bitumen product 85. In one embodiment, the second hydrocarbon solvent 60 is separated from the second bitumen-enriched component 65 by rapidly reducing the pressure in the separator 80, which causes the second hydrocarbon solvent 60 to (partially) vaporize. The second hydrocarbon solvent vapor 90 is removed from the separator 80,
{'bibliographic_information': "{'country': 'US', 'doc-number': '12041554', 'kind': 'B2', 'date': '20080303', 'extension': '637', 'invention_title': 'System and method of separating bitumen from tar sands'}", 'source_file': './grant/2011/ipg110726.xml', 'source_data': 'USPTO-Grant'}