Patent Application
20120048781
The present invention relates to a solvent extraction process for extracting bitumen from mined oil sand.
Most commercial bitumen extraction processes for mined oil sand are water-based processes that consume large amounts of water and generate a great quantity of wet tailings.
An alternative to water extraction is solvent extraction of bitumen from mined oil sand, which uses little or no water, generates no wet tailings and can potentially achieve higher bitumen recovery than the existing Clark hot water extraction process or its variants. Solvent extraction is potentially more robust and more environmentally friendly than water extraction.
The majority of solvent extraction processes taught in the prior art use a single solvent or a solvent mixture having a fixed composition throughout the process. For example, solvent could be a light solvent with a typical boiling range of 36-110° C., e.g. C5-C6 (U.S. Pat. No. 4,347,118 and U.S. Pat. No. 4,752,358), cyclohexane (U.S. Pat. No. 4,189,376), toluene (U.S. Pat. No. 4,416,764), heptane/toluene mix (U.S. Pat. No. 4,448,667), or chlorinated C1-C2 (U.S. Pat. No. 4,532,024 and U.S. Pat. No. 6,207,044). However, the main problem with the use of any hydrocarbon light solvent is the fire hazard it poses in a mining environment. An additional problem with using light solvents such as pure toluene is that they are usually not available in large quantities to oil sand bitumen producers.
The most readily available solvent to oil sand bitumen producers is a mixed aliphatic C5-C7 solvent (light naphtha). However, light naphtha is a poor solvent for bitumen and asphaltenes tend to precipitate out after bitumen dilution, which would trap bitumen and solvent in the spent solids, thereby making the recoveries of both components low. Chlorinated light solvents are even less likely to be used in bitumen extraction. Although they are reportedly non-flammable and safe to operators and the environment, they are detrimental to downstream upgrading and refining processes and the cost due to solvent loss is prohibitively high.
Another single solvent which could be used for bitumen extraction would be a heavy solvent with a typical boiling range of 177-343° C., e.g. kerosene (U.S. Pat. No. 4,094,781) or diesel (Canadian Patent No. 1,048,432). However, the main problem with the heavy solvents is the poor solvent recovery. To fully recover the heavy solvents, high-energy input operations such as retorting or coking the spent solids are required. Energy used to heat the spent solids in these operations is usually unrecoverable.
Alternatively, an intermediate solvent such as naphtha with a typical boiling range of 66-205° C. could be used in the extraction (Canadian Patent No. 1,190,877 and U.S. Pat. No. 5,534,136). Naphtha is generally a better solvent for bitumen compared to most light solvents. However, the heavy fractions in naphtha that contribute to good bitumen solubility are also the cause of difficulty in solvent recovery. The energy requirement (temperatures near or over 200° C.) needed for the solvent recovery from spent solids usually makes the process uneconomical. In addition, the light fractions in naphtha would create the same safety issue in a mining environment as the light solvents.
It has been suggested that using two solvents sequentially may overcome some of the problems encountered with the use of single solvents (U.S. Pat. No. 3,131,141, U.S. Patent Application No. 2006/0076274 and U.S. Pat. No. 3,117,922). For example, a heavy, aromatics-rich and non-flammable solvent may be used for bitumen extraction, which would ensure safe operations such as ore wet crushing in a mining environment and no asphaltene precipitation. Subsequently, a light solvent (usually aliphatic) is used for the extraction of the heavy solvent from the spent solids. Solvent recovery from spent solids would be relatively easy after the light solvent replacement.
The solvent switch (from heavy to light) generally occurs after the near complete extraction of bitumen. Hence, the light solvent could be a very poor bitumen solvent, e.g. liquefied propane/butane. However, since heavy solvents usually have high viscosities by themselves, separating heavy solvent-diluted bitumen from a sand matrix requires high heavy solvent-to-bitumen (HS/B) ratio to lower the diluted bitumen viscosity to an acceptable level. In addition, oil sand is preferably transported to extraction plants in a form of solvent-based slurry through pipelines. This requires even higher HS/B ratio, e.g., about 5:1. Further, heavy solvents require higher temperature (over 300° C.) to distill and recycle. Thus, a high HS/B ratio would likely make the process uneconomical. Furthermore, these processes also require almost the same amount of light solvent as the heavy solvent, which greatly increases the cost of solvent storage, handling and recycle.
U.S. Pat. No. 4,389,300 teaches feeding oil sand, presumably dry-crushed, into a single vertical column containing both countercurrent heavy solvent wash and light solvent wash at different depths. Light solvent after countercurrent wash was not completely withdrawn from the column and was allowed to mix with heavy solvent at the point of initial mixing with oil sands. However, in a commercial-scale operation, it is difficult to crush dry oil sands to a lump size amenable to extraction without the aid of solvent or hot water. In addition, the ratio of the two solvents cannot be precisely controlled or varied in various locations of a column without discrete stages. Thus, the proportion of light solvent might be either too small, thereby failing to lower the HS/B ratio significantly, or could be too large, thereby causing asphaltene precipitation.
In summary, it is likely that the prior art solvent extraction processes failed to become commercially viable due to one or more of the following unresolved issues:
1. Hazard of handling flammable solvents in mining environment where sealing and operating equipment under inert atmosphere are difficult to implement.
2. Poor solvent recovery from spent oil sand generates high volatile organic compound (VOC) emission levels.
3. Light solvents that are easy to recover from solids are usually poor solvents for bitumen and cause asphaltene precipitation.
4. Attempts to solve the above issues by sequentially using a heavy solvent and a light solvent greatly increase the operating cost.
5. Being inherently more complicated in bitumen-sand separation and solvent recovery, any solvent extraction process appears uneconomical compared with the existing water-based extraction process.
There is a need for a solvent based extraction process that is both safe, economical and yields both high bitumen and solvent recoveries.
In accordance with a broad aspect of the invention, there is provided a solvent extraction process which uses at least two different solvents and controlled solvent mix ratios during solvent extraction.
In one embodiment, a non-flammable heavy solvent (HS) may be used for initial oil sands mixing and crushing in mining operations. A heavy/light solvent mixture with significant proportion of light solvent (LS) may be used for the oil sands slurry transportation and the first stage of solid-liquid separation, at which time the bitumen concentration is sufficiently high that the presence of light (poor) solvent would not induce asphaltene precipitation. A heavy/light solvent mixture with relatively more HS may be used for the second stage of separation to prevent asphaltene precipitation. Finally, a LS-dominant mixture may be used for the subsequent stages of separation, at which point most of the bitumen has been removed and the amounts of precipitated asphaltene are minimal. Hence, the spent solids would subsequently become almost HS-free. The light solvent would be readily recovered from the spent solids using a thermal/stripping method.
“Heavy solvent” or “HS” as used herein means a solvent with a typical boiling range of 177-343° C. and generally include hydrocarbon liquids in the C10 to C20 range such as kerosene and diesel.
“Light solvent” or “LS” as used herein means a solvent with a typical boiling range of 36-110° C. and generally include hydrocarbon liquids in the C5 to C7 range such as pentane, hexane, cyclohexane and toluene.
In another broad aspect of the invention, a process for extracting bitumen from oil sand using a combination of heavy solvent and light solvent is provided, comprising:
The present invention attempts to exploit the different properties of various solvents to allow for good bitumen recovery (reduced asphaltene precipitation), good solvent recovery and improved safety. Without being bound to theory, the principle behind using a flexible combination of a heavy solvent (HS) and a light solvent (LS) is illustrated in
Each filled circle represents a stage of mixing and/or separation, as discussed in more detail below. The first circle represents the initial mixing of dry oil sand and heavy solvent. The second circle represents the addition of light solvent to the heavy solvent/oil sand slurry to produce a mixture of the HS/LS around 60/40, which facilitates slurry transport through a pipeline, conditioning of the oil sand slurry therein to release bitumen, digest lumps of oil sand, etc. The same circle also represents the conditions in the first stage of the first solid-liquid separator. The third circle represents the conditions in the latter stage of the first separator where the HS/LS ratio is slightly increased to about 70/30. At this solvent mix ratio, very little asphaltene will precipitate out.
The solids produced in the first separator will have a low bitumen concentration and can be further treated with light solvent to reduce the heavy solvent present in the solids in a second separator to produce tailings having little or no bitumen and little or no heavy solvent. In the second separator, the amount of bitumen is low enough that the addition of light solvent will not result in a significant amount of asphaltene precipitation.
The heavy solvent used in the following embodiment is a virgin light gas oil, i.e. a distillation fraction of oil sand bitumen, C12-C32 with a boiling range within about 220-480° C., which would not fall under the Volatile Organic Compounds (VOCs) regulations with respect to air quality in Canada. The preferred boiling range is about 220-330° C. The HS contains approximately 30-50% aromatic compounds and is able to dissolve bitumen asphaltene. It has a flash point more than 10° C. above the process temperature, which is within the range of 20-80° C., preferably around 50° C.
The light solvent in the present embodiment could be mixed C6-C7 with a boiling range of 69-101° C., which light solvent is available from bitumen upgrading units. The preferred LS is aliphatic C7 with a boiling range of 85-101° C.
The tumbler/crusher/pumpbox circuit may also include an integral rotary screen (not shown) for screening the oil sand/water/HS slurry prior to its passage into the pumpbox. Screened oversize may be crushed to pumpable size and also passed into the pumpbox. In one embodiment, the slurry preparation unit 30 operates at a temperature of about 50° C., the source of heat being primarily from the hot HS from conduit 12.
The dense slurry in the pumpbox may be further mixed with a LS stream, which may contain a small amount of HS, from conduit 17 to make the slurry pumpable, e.g., having solids content of 55-65 wt %. In one embodiment, the mass ratio of HS/LS in the slurry is controlled to be in the range of 70/30 to 50/50, preferably about 60/40, by adjusting the flow rate in conduit 17 to ensure no asphaltene precipitation and to facilitate the subsequent solid-liquid separation.
The slurry is then pumped out via conduit 13 to a slurry pipeline 31, which may connect the mine and the extraction plant. Apart from transportation, the slurry pipeline 31 may also serve as a slurry mixer, lump digester and conditioner, thereby aiding the bitumen extraction from the interstices of the sand matrix to the liquid hydrocarbon phase.
The slurry from pipeline 31 is fed into a first multi-stage separator 32, which also receives an LS stream containing small amount of HS from conduit 23 and pure HS from conduit 3 for countercurrent washing. Two liquid streams and one solid stream are produced in the first separator 32. The mass ratio of HS/LS in the washing liquid, i.e. the combined stream from conduits 3 and 23, is maintained in the range of 75/25 to 55/45 by adjusting the flow rate in conduit 3 to make the mass ratio of HS/LS in the wash product in conduit 19 about 70/30. At this solvent mix ratio, there is little or no asphaltene precipitation.
The first product stream of first separator 32 is sent to a distillation unit 40 via conduit 18 to recover LS and HS, removed via conduits 25 and 26, respectively, and to produce bitumen, which is removed via conduit 1. Recovered HS and LS flow into tank 42 and tank 43, respectively. The second (or wash) product stream from the first separator 32 is withdrawn via conduit 19 and is sent to a flash drum 41 to remove LS, which is cooled and recycled through conduit 24 into tank 43, and produce hot HS, which is removed via conduit 12 and used in the slurry preparation step (slurry preparation unit 30).
The solid stream flows out of first separator 32 via conduit 14 to a second multi-stage separator 33, which also receives pure LS from conduit 2 for countercurrent washing. In separator 33, the mass ratio of HS/LS in the hydrocarbons drops from about 70/30 to almost pure LS. Because most of the bitumen has been removed from the solids, the amount of precipitated asphaltene at this stage is minimal. The addition of LS at this stage results in spent solids that are almost HS-free and the light solvent can be readily recovered from the spent solids using a thermal/stripping method.
The product stream from second separator 33, which comprises primarily light solvent, is removed via conduit 20 to splitter 36, where the product is split at a ratio in the range of 50/50 to 90/10 into streams 17 and 23 for reuse in the slurry pipeline 31 and the first separator 32, respectively.
The first and second separators (32 and 33) are preferably, although not limited to, vacuum belt filters with multi-stage countercurrent wash capability and gas-tight enclosure, likely filled with an inert gas, e.g., CO2. The spent solids (filter cakes) from the second separator 33 are removed via conduit 15 into a dryer 34, preferably a rotary indirect dryer, with an inert stripping gas operating at a solids temperature around 100° C., where the spent solids are dried to the LS content of less than 160 mg/kg of solids. This usually requires a low moisture content of less than 0.5 wt % in the solids. The recovered vapors (LS and H2O) and the inert stripping gas, e.g., CO2, flow to a condenser/separator 35. The cooling medium used in condenser/separator 35 may be cold oil sand process water.
The hot process water produced after heat exchange in condenser/separator 35 may be used in water-based bitumen extraction process, which may be running in parallel with the solvent-based process, as described in more detail below. Condensed LS flows out via conduit 22 to the LS tank 43, which also receives a LS makeup via conduit 27. Condensed water flows out via conduit 21 and could be recycled for steam generation if needed. The inert gas is recycled after water and solvent condensation.
The dry tailings are removed via conduit 16 and may be further mixed with mature fine tailings (MFT) that are produced in water-based processes and typically contain 33 wt % solids, at a mass ratio of 1:0.28 to make a trafficable solids mixture containing 85 wt % solids. This mixture, which is more consolidated and less dusty than loose dry sand, is transported to a land reclamation site for disposal. Alternately, the MFT proportion may be significantly higher to make a non-segregated composite tailings, containing 55-65 wt % solids, to be pumped to a field for drying in ambient air. The non-segregating nature of the composite tailings generally makes it dry within a short period of time. The dry tailings may also be sprayed with water and disposed as trafficable solids if MFT is not available.
As previously mentioned, the preferred mass ratio of HS to bitumen is, although no limited to, around 1-1.5 based on the mass flow rate of solvent in conduit 3 and the mass flow rate of bitumen in conduit 1. The preferred mass ratio of LS to bitumen is, although no limited to, 2-4 based on the total mass flow rate of solvent in conduit 2 and the mass flow rate of bitumen in conduit 1. The resulting bitumen recovery is about 95% or greater for Athabasca oil sands containing 40% fines (less than 44 μm) in solids. The recoveries of heavy solvent and light solvent are about 97% and 99.9% or greater, respectively.
It should be noted that the commercial water-based extraction process is generally not capable of processing oil sands with 40% fines without blending with low-fines oil sands. Thus, the present invention also comprises a method of integrating the aforementioned solvent extraction process into the existing water-based extraction process to substantially improve the economic return and reduce wet tailings production. The integration includes the following three aspects: ore segregation, energy sharing, and wet tailings reduction and sequestration.
With reference now to
Furthermore, the bitumen recovery for the normal oil sands portion is improved by approximately 6% from the base case since the feed to water-based extraction is not contaminated with the problem oil sands. The economical benefit from the latter is about 5 times of the benefit from the former. Therefore, the ore segregation method increases the economic return by a factor of 5. This would make the solvent extraction process economically viable despite its large capital investment. This ore segregation can be achieved in the truck-and-shovel mining, since problem oil sands are present in certain ore facies previously characterized by mine geologists.
The largest operating cost for solvent extraction is in the solvent recovery from spent solids. Recovery of LS to the point that is in compliance with VOC emission regulations usually requires evaporation of almost all naturally present and added water from the tailings in the process. Therefore, large energy input is needed to supply the latent heat for water and solvent vapors. However, the hot vapors subsequently need to be condensed using cooling water. With an integrated system, the resulting hot water can then be used in the parallel water-based extraction process, which requires heated or hot water. Thus, through such energy sharing, the operating cost for solvent extraction can be reduced.
Problem oil sands are usually high-fines oil sands. Depending on the compositions of ore bodies, processing ⅛ (12.5%) of the oil sands in a mine through solvent extraction can reduce the amount of mature fine tailings (MFT) generation by about 18-30% (100% being the total amounts of MFT generated in the same mine if all oil sands are processed with water-based extraction). Further, sequestration of the existing MFT from water-based extraction with dry tailings from solvent extraction will make aforementioned trafficable solids or quick-drying composite tailings, thereby further reducing the amounts of MFT in inventory.
A vacuum filtration test was performed using an oil sand sample containing 8.5% bitumen, 4.6% water and 86.6% solids. The fines (less than 44 μm) content was 40% in solids. This oil sand sample had been previously tested in a water-based extraction pilot and yielded 0% bitumen recovery. The filter opening was 180 μm and the vacuum was around 0.7 bar. The filtration temperature was 50° C. The boiling range of the virgin light gas oil (HS) used was 177-424° C. The light solvent (LS) was n-heptane. The filtration rates are shown in Table 1.
Table 1 shows an example of the filtration performance in the first stage separation. When no light solvent was used (test no. 1), the filtration rate was slow even at somewhat lower bitumen concentration. When the HS/LS ratio was 3 (test no. 2), the filtration rate was slow as well. However, when the HS/LS ratio reached 1.5, i.e. 60/40, the filtration rate was significantly improved. Therefore, lowering the HS/LS ratio to 1.5 as shown in test no. 3 will likely result in a faster separation process than some of the prior art where no LS was involved in the first-stage separation as shown in test no. 1. No asphaltene precipitation occurred during the test.
100 g of the oil sand sample of Example 1 was rinsed with five solvent mixtures: (1) 2.4 g bitumen+15 g HS+10 g LS; (2) 12 g HS+4 g LS; (3) 2.5 g HS+10 g LS; (4) 10 g LS; and (5) 10 g LS and then filtered under the same conditions mentioned above. The spent filter cake was heated to 95° C. and stripped with argon at 500 ml/min for 20 min. The recoveries of all hydrocarbons are shown in Table 2.
This example simulated two stages of washing/filtration at different HS/LS ratios in a first hypothetical separator, followed by three stages of countercurrent washing/filtration with a light solvent in a second hypothetical separator, and followed by solids drying to recover almost all residual light solvent.
Spent filter cakes of 5 cm in thickness containing approximately 7 wt % heptane and 4 wt % water were stripped with argon at 95° C. Stripping was stopped at various moisture contents in solids. The residual heptane concentrations in solids are shown in Table 3.
This example showed that the moisture content in packed spent solids must be below 0.5 wt % to achieve the light solvent concentration lower than 160 mg/kg based on data interpolation.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to those embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein, but is to be accorded the full scope consistent with the claims, wherein reference to an element in the singular, such as by use of the article “a” or “an” is not intended to mean “one and only one” unless specifically so stated, but rather “one or more”. All structural and functional equivalents to the elements of the various embodiments described throughout the disclosure that are known or later come to be known to those of ordinary skill in the art are intended to be encompassed by the elements of the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims.
