BACKGROUND TO THE INVENTION

THIS invention relates to a supercritical extraction process the extraction of hydrocarbons from a mixture containing hydrocarbons, water and particulate material, such as oil sludges.

Within the petroleum industry, there are a number of mixtures which conventional separation techniques are incapable of separating. The most common cause of this inability to separate the mixtures are the presence of emulsions or mineral particulate matter. In many cases, due to the poor recovery of the hydrocarbons that is possible, these mixtures are seen as waste streams, and as such are disposed of as is, without any attempts to recover any of the valuable hydrocarbons. In other cases, such as with oil and tar sands, specialised techniques have been developed to separate the hydrocarbons from the mixtures and produce a hydrocarbon rich stream that can be further processed. Many of these oil sand extraction processes are either expensive to operate, or else produce large amounts of waste by-products, which can be hazardous to the environment. It is an object of the invention to provide an improved process for the separation of hydrocarbons from a mixture containing hydrocarbons, water and particulate material, such as oil sludges.

SUMMARY OF THE INVENTION

According to the invention there is provided a supercritical extraction process for the separation of hydrocarbons from a mixture containing hydrocarbons, water and particles, wherein:

a. in an extraction stage, the mixture is mixed with a solvent in the form of a supercritical fluid consisting of or containing a fluorinated fluid component, to obtain a solvent-rich phase containing hydrocarbons and an aqueous phase containing water and particles, which the phases are separated in a separation step.

Preferably, the fluorinated fluid is selected from trifluoromethane (CHF ₃ ), hexafluoroethane and mixtures thereof.

The supercritical fluid may also contain carbon dioxide and/or an

alkane fluid. The alkane fluid may be ethane

Typically, the extraction stage a. is followed by the following stages:

b. a solvent recovery stage, in which solvent is recovered from the solvent-rich phase containing hydrocarbons to provide a recovered solvent stream, and a product stream comprising hydrocarbon solutes;

c. a solvent recycling stage, in which the recovered solvent is re- pressurised and returned to the extraction stage;

d. a final product preparation stage, wherein remaining solvents in the product stream is separated from the hydrocarbon solutes; and e. a raffinate treatment stage, in which any residual amounts of solvent are removed from the aqueous phase. The separation step in the extraction stage a. preferably comprises:

i. a liquid-liquid separation vessel for separating the solvent-rich phase from the aqueous phase;

ii. a flash vessel for separating the vapour solvent-rich phase from the aqueous phase;

iii. a flash vessel for separating the vapour solvent-rich phase from the aqueous phase; or

iv. the mixing and separation steps occurs in a single unit.

Preferably, the extraction stage a. operates at a temperature and pressure greater than the pure solvent critical temperature and pressure.

A mass ratio of the oil sludge to the solvent entering the extraction stage a. is typically between about 1 to 1 and about 1 to 20.

The solvent recovery stage b. is preferably:

i. a single flash vessel is used to recover solvent from solutes;

li. a series of flash vessels, each at a lower pressure to the previous, is used to separate solvent from solutes; or

iii. a stripping column is used to separate solvent from solutes, with the solvent exiting as a distillate, and recovered solutes exiting from the bottom of the column.

The stripping column typically contains a packing material or, is fitted with equilibrium trays.

Preferably, a condenser, producing a reflux, is added to the top of the stripping column in order to improve the purity of the recovered solvent. Preferably, a reboiter is added to the bottom of the stripping column in order to improve the solvent recovery.

Typically, the solvent recycling stage c. comprises a series of solvent recovery vessels; and

i. the solvent recovered from each solvent recovery vessel is cooled and re-pressurised separately and thereafter blended; or

ii. the solvent recovered from each solvent recovery vessel is blended at reduced pressures, and thereafter cooled and re-pressurised.

In the solvent recycling stage c:

i. the recovered solvent may be cooled to a sub-cooled liquid at temperatures below the saturation temperature prior to being re- pressurised with a single liquid pump;

ii. the recovered solvent may be cooled to form a sub-cooled liquid at a temperature below the saturation temperature and thereafter, a series of liquid pumps and intercoolers are used to re-pressurise the liquid solvent;

iii. the solvent may be re-pressurised as a superheated vapour, using a compressor, at temperatures greater than the saturation temperature; or

iv. the solvent may be re-pressurised using a combination of compressors for vapours and pumps for liquids, and with condensers and intercoolers supplying the necessary cooling requirements.

Preferably, in the solvent recycling stage c, the re-pressurised solvent is blended with fresh solvent prior to being re-used in the extraction stage.

Preferably, the re-pressurised solvent and the fresh solvent are added to, and stored in an intermediary storage vessel, from which the solvent is withdrawn for use in the extraction. In the final product preparation stage, d.:

i. any solvent remaining in the product stream may be removed by reducing the pressure to about atmospheric pressure; or

ii. any solvent remaining in the product stream may be removed by reducing the pressure to below the atmospheric pressure.

In the final product preparation stage d.:

i. the remaining solvent that may be removed from the final product may be vented to the atmosphere;

ii. the remaining solvent that may be removed from the final product may be disposed of by incineration; or

iii. the remaining solvent that may be removed from the final product may be captured and re-pressurised for re-use in the extraction process.

In the raffinate treatment stage e.:

i. the pressure of the aqueous phase may be reduced such that the residual solvent present in this phase would be vaporised and removed from the stream; and

ii. the liquid and/or solid components in the aqueous phase that are insoluble in the solvent are treated and disposed of.

In the raffinate treatment stage e.:

i. the pressure may be reduced to about atmospheric pressure, and the aqueous phase is separated into a vapour and a liquid phase in a flash vessel; or

ii. the pressure may be reduced below the atmospheric pressure, and the aqueous phase is separated into a solvent-rich vapour phase and a liquid and/or solid mixture comprising of insoluble components.

In the case of mainly water being present in the insoluble liquid and/or solid mixture, the liquid is sent to the wastewater treatment facilities. In the case of some solid mineral particles being present in the insoluble liquid and/or solid mixture, these are mechanically separated from the mixture, with the liquid being sent to the wastewater treatment facilities, and the solids being sent to landfill.

In the case of the insoluble liquid and/or solid mixture consisting mainly of solids, the solids is sent to a landfill.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a flow diagram of a first embodiment of a supercritical extraction process including a series of flash vessels for recovering a solvent and each solvent stream is re- pressurised individually;

Figure 2 is a flow diagram of a second embodiment of a supercritical extraction process including a series of flash vessels for recovering a solvent and each solvent stream is combined and then re-pressurised together; and

Figure 3 is a flow diagram of a third embodiment of a supercritical extraction process including a stripping column for recovering a solvent.

DETAILED DESCRIPTION OF INVENTION

In accordance with the present invention, a supercritical fluid extraction (SCFE) process is used to separate hydrocarbons from a mixture containing hydrocarbons, water and mineral particles, using a supercritical solvent. This is performed by a number of process stages. The first stage entails the contact of the supercritical solvent with the mixture to be separated.

The mixture to be separated describes any combination of hydrocarbons, water and mineral particles that is produced or obtained by the petroleum industry. A non-limiting list of such mixtures includes refinery sludges, oil field sludges, oil storage tank sludges, oil sands, tar sands, contaminated soils and the surface run-off from petroleum processing facilities. The broad term Oil sludge' will be used to describe any of these such systems.

The supercritical solvent comprises a fiuorinated solvent, or a mixture of a fluorinated solvent and carbon dioxide and/or an alkane solvent. The fiuorinated solvent may comprise trifluoromethane hexafluoroethane and/or 1,1-difluoroethene as well as mixtures thereof. The alkane solvent may comprise ethane

The solvents used in this invention must be able to separate the more polar, and longer chain length hydrocarbons from water. Additionally, the mineral particles should not be soluble in the solvent, and thus remain in the raffinate. Solvents with high solubilities of the desired solutes must be utilised to perform the separation. Due to their low polarity (dipole moments), pure carbon dioxide and low molecular weight alkanes are not capable of attaining sufficiently high solubilities of the desired solutes. It was initially supposed that the use of the fiuorinated components, especially those with high dipole moments, would result in the water fractions being selectively extracted from mixtures, rather than the hydrocarbon fractions. Most surprisingly, the inventors have determined that in the process of the present invention the water is almost completely immiscible in the fluorinated components, while there is a substantial solubility of the hydrocarbon fractions in the solvents. In addition to this, the fluorinated components with low dipole moments were assumed to exhibit low solubilities of the longer chain length hydrocarbons, in a similar manner to carbon dioxide and alkane solvents. Surprisingly in the process of the present invention this is not be the case. Without wishing to be bound by theory, the inventors believe that this may be due to quadrupole and octupoie moments present in the fluorinated solvents.

Two advantages exist in the use of supercritical fluorinated solvents, or mixtures of fluorinated solvents with carbon dioxide or ethane, over pure supercritical carbon dioxide or pure supercritical ethane. The first advantage being the reduced flow rates of solvent that are required for the proposed process. These reduced fiowrates are due to the increased solubilities of the solutes in the fluorinated solvents. The second advantage of the alternative solvents are the reduced pressures at which the extraction process is operated. The reduced pressures and reduced fiowrates result in substantially reduced costs for a process.

The mass ratios of the solvent added to the oil sludge feed could vary between about 1 :1 to about 20:1. The solvent to feed ratio that is used, as well as the temperature and pressure at which the extraction is performed, are dependent upon the composition of the oil sludge, and the desired separation. The temperature and pressure that are used are in the pure solvent critical region.

After the supercritical solvent and the oil sludge have been mixed, the phases produced are subsequently separated into a solvent rich phase and a water rich phase, also referred to as a raffinate comprising water and mineral particulate, in a separation stage. The extracted hydrocarbons (product) are soluble in the solvent, and are therefore present in the solvent rich phase. The separation is performed at the same temperature and pressure as the mixing stage. The extraction stage comprises the mixing stage and the separation stage. In one embodiment of the invention the mixing stage and separation stage occurs in a single unit.

Two embodiments of the invention exist with regards to the separation of the solvent from the product. In the first embodiment of the invention, a series of flash vessels, with a pressure reduction between each vessel is used to separate the solvent from the product. The pressure of the fluid entering the first flash vessel is reduced below the critical pressure of the solvent. The recovered solvent is withdrawn from the top of the flash vessel as a vapour. From the bottom of flash vessel, the solvent-product mixture is removed, the pressure is further reduced, and the fluid enters a second flash vessel. The solvent is again recovered as a vapour from the top of this vessel, and if required, the pressure of the product withdrawn from the bottom of the vessel can be further reduced and sent to another flash vessel. This sequence can be repeated until sufficient solvent has been recovered from the product The solvent that is recovered from each flash vessel is sent to the solvent recycling stage.

In the second embodiment of the invention, a stripping column is utilised to recover the solvent from the product. In this embodiment of the invention, the pressure of the fluid exiting from the extraction stage is reduced to below the critical pressure of the pure solvent. This fluid is added near to the bottom of a stripping column, in which the solvent is separated from the product, with the solvent exiting as the distillate from the column, and the product exiting as the bottoms. In this embodiment of the invention, the temperature at the top of the column is controlled such that it is below the critical temperature of the solvent, ensuring that the fluid remains sub- critical. The product that exits from the bottom of the stripping column may require further treatment, by reducing the pressure to atmospheric pressure, or even below, ensuring that only trace concentrations of the solvent remain in the product. In one embodiment of this invention, this very low pressure solvent stream recovered from the bottoms is re- pressurised with a compressor and added to the remaining recovered solvent In the second embodiment of this invention, the low pressure solvent is vented to the atmosphere. In the third embodiment of this invention, the low pressure solvent is disposed of by incineration. The choice of the three embodiments that have been provided for would be based upon the composition of the solvent. The stripping column operates at a higher pressure than the flash vessels, and thus the solvent that is recovered is at a higher pressure.

To recycle the recovered high pressure solvent, it must be compressed to the pressure required in the extraction stage. In the case of the series of flash vessels used for solvent recovery, each individual stream can be re- pressurised separately, or alternatively, the numerous recovered solvent streams can be combined into a single stream and thereafter re- pressurised. In one embodiment of the invention, the re-pressurisation of the recovered solvent, it is first sub-cooled to sufficiently below the saturation temperature to prevent cavitation, and the re-pressurisation is thereafter performed by a high pressure liquid pump. In another embodiment of the invention, the recovered solvent is cooled to form a sub- cooled liquid at a temperature only slightly below the saturation temperature and thereafter, a series of liquid pumps and intercoolers are used to re-pressurise the liquid solvent. The temperature difference between the sub-cooled liquid and the saturation temperature may be smaller than or equal to 5 K or 10 K. In another embodiment of the invention, the solvent is re-pressurised as a superheated vapour, using a compressor, at temperatures greater than the saturation temperature. The temperature difference between the superheated vapour and the saturation temperature may be greater than or equal to 5 K or 10 K. In another embodiment of the invention, the solvent is re-pressurised using a combination of compressors for vapours and pumps for liquids, and with condensers and intercoolers supplying the necessary cooling requirements. The solvent is then returned to the mixing and separation stage. Any small amounts of solvent that are present in the water-rich phase (raffinate) from the extraction stage must be separated from it. in one embodiment of the invention, the separation of the solvent from the raffinate can be performed by reducing the pressure of the raffinate to about atmospheric pressure in a flash vessel and the raffinate stream is separated into a vapour and a liquid phase in a flash vessel. In a second embodiment of the invention, the separation of the solvent from the raffinate can be performed by reducing the pressure of the raffinate to below atmospheric pressure in a flash vessel and the raffinate stream is separated into a solvent-rich vapour phase and a liquid and/or solid mixture comprising of the insoluble components. This would ensure that only trace concentrations of the solvent remain in the product. In one embodiment of this invention, this very low pressure solvent stream can be re-pressurised with a compressor and added to the remaining recovered solvent. In the second embodiment of this invention, the low pressure solvent is vented to the atmosphere. In the third embodiment of this invention, the low pressure solvent is disposed of by incineration. The choice of which of these three embodiments to use would depend upon the amount of solvent present in the raffinate, as well as its composition.

After removal of the solvent from the raffinate, the remainder of the raffinate must be treated and disposed of. In one example of this invention, the remaining raffinate can be separated mechanically to produce a mineral particle fraction and a water fraction. In another example, the raffinate must be sent to further waste water treatment facilities for further treatment. In another example, the remainder of the raffinate contains insoluble liquid and solid components. If the remaining raffinate contains mainly water, then the liquid is sent to a wastewater treatment facilities. If the remaining raffinate contains some solid mineral particle then these are mechanically separated from the mixture, with the liquid being sent to the wastewater treatment facilities, and the solids being sent to landfill. If the remaining raffinate contains insoluble components consisting mainly of solids, then it is sent to landfill.

Referring now to FIGURE 1 , the oil sludge feed to the process (stream 1) is mixed with the solvent (stream 2) coming from the storage vessel (unit XIV), in a mixing vessel (unit I). The mass ratio of stream 1 to stream 2 is between about 1:1 to about 1:20, depending on the composition of stream 1 , and the solvent that is used. The resulting mixture (stream 3) is sent to a separator (unit II) in which the solvent rich phase (stream 4) is separated from the raffinate comprising insoluble components (stream 5). In one embodiment of the invention, this separation vessel is a flash vessel, but in another embodiment of the invention, this separation vessel is a settling unit, in which the solvent rich fluid is allowed to separate from the remaining insoluble components. The two phases are continuously removed from the vessel; one from the top and the other from the bottom. In this non-limiting example, the solvent rich phase is removed from the top of the separation vessel and the insoluble components are removed from the bottom. The invention does, however, also allow for these phases to be inverted, with the solvent rich phase exiting from the bottom, and the insoluble components being removed from the top. The pressure and temperature at which the mixing and separation units operate are above the critical point of the solvent. The conditions that are used are dependent upon the solvent used to perform the extraction.

The pressure of the solvent rich phase exiting unit II is reduced, and the fluid enters the first flash vessel (unit III). In this flash vessel, an almost pure solvent is removed as a vapour from the top of the vessel (stream 6), and the fluid exiting from the bottom (stream 7) is sent to the next flash vessel in the series (unit IV). Again, with this second flash vessel, the separated solvent exits from the top of the vessel as a vapour (stream 8), whilst the fluid exiting from the bottom (stream 9), is sent to the last solvent recovery flash vessel (unit V). From unit V, the solvent is recovered as a vapour and exits from the top of the vessel (stream 10). The fluid exiting from the bottom of this flash vessel (stream 11) contains only small concentrations of the solvent. In this embodiment of the invention, each of the recovered solvent streams (streams 6, 8 and 10) is sent to a separate pre-cooler (units V, VIII and X respectively), in which the stream is cooled to a temperature sufficiently low to prevent cavitation within the liquid pumps. From units V, VIII and X respectively, the sub-cooled liquids (streams 12, 13 and 14) are sent to liquid pumps (units VII, IX, and XI respectively, in which they are compressed to the required pressures, before being returned to the storage vessel (stream 21). A small amount of fresh solvent (stream 22), equal to that lost through the process, is added to the storage vessel (unit XIV), to supplement that which is recovered (stream 21).

The bottoms from the last flash vessel (stream 11) enters a final product preparation vessel (unit XII), in which the pressure is reduced to about atmospheric pressure. In an alternative embodiment of this invention, the pressure in the final preparation vessel is reduced to sub-atmospheric pressures. The low pressures in this vessel removes all but some trace concentrations of the solvent from the product (stream 18). The final low- pressure solvent (stream 17) is captured, and this stream is either re- compressed and recycled, vented to the atmosphere, or incinerated.

The raffinate (stream 5) from the extraction stage (units I and II) is sent to further to a raffinate treatment stage (unit XIII), where any solvent remaining in the stream is separated and captured (stream 19). The remaining raffinate (stream 20), which would consist of mostly water and mineral particulate matter, is sent to waste water treatment facilities for further processing.

Referring to FIGURE 2; which portrays a non-limiting example of the invention, using the first embodiment of the solvent recovery stage, but with a single solvent recycling stage for all of the recovered solvent streams; the oil sludge feed to the process (stream 1) is again mixed with the solvent stream (stream 2) in a mixing vessel (unit I). The mass ratio of the oil sludge feed to the solvent feed is between about 1:1, and about 1:20, depending on the composition of the oil sludge, and the desired recovery and purity of the product.

The oil sludge-solvent mixture (stream 3) from unit I then enters a separator (unit II), where the solvent-rich phase and the remaining insoluble components are separated from each other. As a non-limiting example, this separator could be a settling vessel, with the phases separating out according to their densities. Alternatively, this separator could be in the form of a flash vessel, in which the solvent-rich phase is a vapour or a supercritical fluid, which is removed from the top of the separator. In this case, the remaining, insoluble components (product stream) would be removed from the bottom of the vessel. The pressure and temperature at which the mixer and separator would operate would be dependent upon the composition of the oil sludge, as well as on the supercritical solvent that is being used to perform the separation. The pressure and temperature of operation would be selected such that the process operates above the solvent critical point.

For either of the separator examples, after separation of the phases, the pressure of the solvent-rich phase (stream 4) is reduced to below the solvent critical pressure, and the stream enters a series of flash vessels (units III, IV and V), each operating at a pressure lower than the previous vessel. In these flash vessels, the solvent is recovered from the product, with the recovered solvent exiting from the top of the vessels, and the product, with the dissolved solvent exiting from the bottom. Thus, the solvent-rich from the separation vessel (stream 4), enters the first flash vessel (unit III), and is separated into a recovered solvent stream (stream 6) and the remaining product and solvent mixture (stream 7). This stream then enters into a second flash vessel (unit IV), in which it is separated into a recovered solvent stream (stream 8) and a stream (stream 9) containing the product along with the solvent soluble therein, at the operating pressure of the vessel. In this example, the stream exiting the second flash vessel (stream 9) enters the third and final flash vessel (unit V), from which the recovered solvent exits from the top (stream 10), is blended with stream 8 (stream 12) and with stream 6 (stream 13). This stream is then cooled in a heat exchanger (unit VI). The recovered solvent is cooled to a sub-cooled liquid in unit VI. The temperature of the solvent exiting unit VI (stream 14) is sufficiently low to ensure that no cavitation occurs within the subsequent liquid pump (unit VII).

The fluid exiting from the bottom of unit V (stream 11) is sent to a final product treatment stage (unit VIII), which operates at sub-atmospheric pressures, removing most of the solvent which remains soluble in the product. In this example, the solvent (stream 15) that is removed from the final product (stream 16) could be vented to the atmosphere, or disposed of by incineration.

The raffinate (stream 5) from the separator stage (unit II) must be further treated before disposal (unit IX). This treatment includes the removal of any solvent that could be present and treatment of the water and solids. The solvent present in this stream is separated from the insoluble material by subjecting the mixture to sub-atmospheric pressures, and allowing the solvent to boil off as a vapour (stream 17). The remaining liquid and solid materials (stream 18) can then be sent to further wastewater treatment facilities.

The high pressure solvent stream (stream 19), exiting from the liquid pump (unit VII), is returned to a solvent storage vessel (unit X). Additional fresh solvent is added to this storage levei to compensate for that lost in units VIII and IX. Stream 2 is withdrawn from this unit.

FIGURE 3 gives a third non-limiting example of this invention, using the second embodiment of the solvent recovery section. In this example, the oil sludge feed to the process (stream 1) is mixed with the supercritical solvent (stream 2) in a mixing vessel (unit I). The oil sludge and solvent mixture (stream 3) is then sent to a separation vessel (unit II), where the solvent-rich phase is separated from the insoluble components. This separation vessel could take the form of a liquid-liquid separation vessel such as a liquid-liquid settling tank, in the case that the solvent-solute mixture is a liquid phase, or it could take the form of a flash vessel in the case that the solvent-solute mixture is a vapour or a supercritical fluid. The phases that would be present in the separation vessel would be dependent upon the solvent that was used as well as the composition of the oil sludge. The solvent-rich phase from the separation vessel (stream 4), is expanded slightly, and enters near to the bottom of a stripping column (unit III). The insoluble components (raffinate) from the separator exit in stream 5. in the stripping column, which could be a packed column (containing packing material) or a plated column (which may be fitted with equilibrium trays), the solvent moves upward through the column, exiting at the top (stream 6) and the extracted products move downwards, exiting from the bottom of the column (stream 9). A portion of the vapour-phase stream that exits from the top of the stripping column is condensed in a condenser (unit IV) and returned to the column as a reflux (stream 7). The remainder of the recovered solvent from the stripping column (stream 8) also moves through a condenser (unit VI) wherein it is condensed to below the solvent saturation temperature in preparation for re-compression. A portion of the bottoms stream (stream 9) from the stripping column is returned to the stripping column as a vapour (stream 10), after passing through a reboiler (unit V).

The portion of the bottoms stream that is not vaporised in the reboiler (stream 11) is sent to a final product treatment stage (unit VIII), wherein it is tempered to the temperature that is required by the downstream processes, as well as being subjected to a sub-atmospheric pressure to remove any residual solvent. In this example, the solvent that is separated from the product during this final product treatment stage (stream 13) is either vented to the atmosphere, or disposed of by incineration. The final product produced (stream 14), is further processed elsewhere.

The recovered solvent stream (stream 12) exiting from the condenser unit (unit VI) is re-compressed by a liquid pump (unit VII), and is returned (stream 17) to a solvent storage vessel (unit X). A fresh stream of solvent (stream 18) is blended with the recycled solvent in this vessel, to replace any solvent that is lost from the process in units VIII and IX. The recycled solvent and the fresh solvent is blended prior to being re-used in the extraction stage. The solvent used in the extraction (stream 2) is withdrawn from this storage vessel.

Further treatment of the raffinate from the separator stage (stream 5) is performed by unit IX. In this unit, the raffinate is subjected to sub- atmospheric pressures, causing most of the solvent remaining in the raffinate to vaporise. This solvent vapour is removed (stream 15) and either vented to the atmosphere, or disposed of by incineration. The choice of which of these two options is dependent upon the solvent that is used. The remaining mixture (stream 16), which would comprise of the insoluble components, is then sent to wastewater treatment facilities.

It will be appreciated that the above is only one embodiment of the invention and that there may be many variations without departing from the spirit and/or the scope of the invention.