Patent Publication Number: US-2023143845-A1

Title: A solvent extraction system

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a solvent extraction system and to a method for performing solvent extraction. It is furthermore related to an oil purification system comprising such a solvent extraction system 
     BACKGROUND 
     Solvent extraction is often performed in batches where the phases of a two phase system are brought in good contact with each other through mixing creating a dispersed phase and a continuous phase, whereupon the phases are allowed to separate through sedimentation of the heavy phase due to gravitational force. Another way of performing solvent extraction is using a continuous column where a continuous flow of dispersed phase is brought in contact with a continuous flow of continuous phase in a countercurrent or crosscurrent fashion. The flow of heavy phase down through the column is due to gravitational force while the flow of light phase is achieved by pumping. 
     Contaminated oil can be cleaned by solvent extraction by use of a separation aid, where the separation aid is the dispersed phase. This has been described in for example WO2018/199839. 
     SUMMARY 
     An object of the present invention is to provide an improved solvent extraction system and method. 
     A further object of the invention is to provide a system and a method which are suitable for cleaning of oil. 
     This is achieved by a solvent extraction system and by a method according to the independent claims. 
     According to one aspect of the invention a solvent extraction system is provided comprising:
         an elongated solvent extraction chamber having a first end and an opposite second end and a length, L, between said first and second ends, wherein said solvent extraction chamber comprises at least one port for providing a continuous phase into the solvent extraction chamber and for removing content out from the solvent extraction chamber;   a membrane comprising pores having substantially the same diameter, not differing more than 20%, and center-to-center distances between said pores being substantially the same, not differing more than 20%, wherein said membrane is positioned in connection with the solvent extraction chamber at its first end; and   a dispersed phase inlet which is positioned to be in fluid connection with the first end of the solvent extraction chamber and such that a liquid provided into the solvent extraction chamber through the dispersed phase inlet has to pass through the membrane.       

     According to another aspect of the invention a method for solvent extraction in a solvent extraction system according to the invention is provided. Said method comprises the steps of:
         providing a first liquid into the solvent extraction chamber through one of the at least one port;   providing a second liquid into the solvent extraction chamber from the dispersed phase inlet and via the membrane;   collecting dispersed phase from one of the at least one port; and   collecting continuous phase from one of the at least one port.       

     According to still another aspect of the invention an oil purification system comprising a solvent extraction system according to the invention is provided. Said oil purification system is configured for being connected to a technical equipment which is using oil such that the oil is continuously circulated through the solvent extraction system as the continuous phase for cleaning of the oil. 
     Hereby a solvent extraction system is provided in which a dispersed phase will have uniformly sized droplets whereby solvent extraction will be more effective. A liquid to be a dispersed phase is provided into the solvent extraction chamber through a membrane which will emulsify said liquid into a dispersed phase. The membrane comprises pores which are equal in size and which are distributed evenly over the membrane which will ensure a good emulsification. Hereby the dispersed phase will comprise droplets being very equal in size and hereby a more efficient solvent extraction can be performed than with conventional mixing and dispersing systems. A continuous solvent extraction can furthermore effectively be performed with this solvent extraction system. 
     In some embodiments of the invention the solvent extraction system further comprises a mixer which is provided inside the solvent extraction chamber, wherein said mixer comprises a mixing blade which is positioned closer to the first end of the solvent extraction chamber than ⅛ of the total length, L, of the solvent extraction chamber, said mixing blade being in close relation but not directly contacting an outlet side of the membrane, wherein the dispersed phase will be provided into the solvent extraction chamber from said outlet side of the membrane. Hereby a size of the droplets of the dispersed phase can be further controlled. By controlling the rotation speed of the mixing blade the size of the droplets when leaving the membrane can be controlled. 
     In some embodiments of the invention said pores of the membrane can have a pore diameter between 1-100 μm and the distance between the pores, center-to-center, can be between 10-1000 μm. The membrane can be configured such that a liquid passing said membrane will be emulsified. 
     In some embodiments of the invention the solvent extraction chamber is vertically oriented and gravity is used for separating phases. 
     In some embodiments of the invention the solvent extraction chamber comprises at least two ports comprising a first port for removing content out from the solvent extraction chamber and a second port for providing a continuous phase into the solvent extraction chamber. 
     In some embodiments of the invention the solvent extraction chamber comprises at least three ports comprising a first port for removing dispersed phase out from the solvent extraction chamber, a second port for providing a continuous phase into the solvent extraction chamber and a third port for removing a continuous phase out from the solvent extraction chamber, wherein the first port and the second port are provided at a distance from the second end of the solvent extraction chamber which is smaller than ⅕ of the total length, L, of the elongated solvent extraction chamber and wherein the third port is provided at a distance from the first end of the solvent extraction chamber which is smaller than ⅕ of the total length, L, of the elongated solvent extraction chamber. 
     In some embodiments of the invention a distance between the second end of the solvent extraction chamber and said first port is smaller than a distance between the second end of the solvent extraction chamber and said second port. 
     In some embodiments of the invention the third port comprises a filter for preventing dispersed phase from being transferred out from the solvent extraction system through the third port. 
     In some embodiments of the invention the solvent extraction system further comprises at least two pumps wherein a first pump is connected to the second port and to a first liquid source and is configured for pumping a first liquid from the first liquid source into the solvent extraction chamber, which first liquid will be a continuous phase in the solvent extraction chamber and wherein a second pump is connected to the dispersed phase inlet and to a second liquid source and is configured for pumping a second liquid from the second liquid source into the solvent extraction chamber, which second liquid will be a dispersed phase in the solvent extraction chamber. 
     In some embodiments of the invention said system is configured for being run continuously, wherein the first and second pumps are configured for pumping continuously. 
     In some embodiments of the invention said solvent extraction system is configured for cleaning contaminated oil, wherein the solvent extraction system comprises a first liquid source comprising contaminated oil to be cleaned, which first liquid source is connected to the second port and wherein the solvent extraction system further comprises a second liquid source comprising a liquid separation aid, which second liquid source is connected to the dispersed phase inlet, wherein the separation aid will attract contaminants in the contaminated oil during a solvent extraction performed in the solvent extraction chamber. 
     In some embodiments of the invention the separation aid is liquid at the temperature at which the process is carried out and will by chemical interactions adsorb/absorb contaminating solids or dissolved impurities in the oil to be purified and wherein the separation aid composition is substantially insoluble in the oil to be purified because of its polar properties, forming a two-phase mixture upon mixing with the oil to be purified and wherein the separation aid has a density different from that of the oil to be purified. 
     In some embodiments of the invention the solvent extraction system comprises a membrane emulsification unit comprising said membrane. 
     In some embodiments of the invention a rate of fluid flow of a first liquid provided into the solvent extraction chamber is controlled such that a flow rate through the solvent extraction chamber for the continuous phase is kept lower than a sedimentation/rising rate of a dispersed phase through the solvent extraction chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows schematically a solvent extraction system according to one embodiment of the invention. 
         FIG.  2    shows schematically a solvent extraction system according to another embodiment of the invention. 
         FIG.  3    shows schematically a solvent extraction system according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The method and system of the invention is used for solvent extraction, also known as liquid-liquid extraction. Two liquids which are not soluble in each other, i.e. having different polarities are used and an interchange of components between the liquids is facilitated by the solvent extraction system according to the invention. The two liquids also have different densities whereby the two liquids after mixing will separate. The terms continuous phase and dispersed phase which are commonly used in the technical area of solvent extraction (also called liquid-liquid extraction) are also used throughout this description. A first liquid is called the continuous phase and will be the phase which during the solvent extraction is continuous, i.e. not in droplets, and a second liquid is called the dispersed phase and will be the phase which during solvent extraction is dispersed, i.e. dispersed into droplets which are surrounded by the continuous phase. During solvent extraction some components will pass between these two phases and hereby the dispersed phase taken out from the solvent extraction system after solvent extraction will not contain exactly the same components as the second liquid, also called the dispersed phase, when entering the solvent extraction system and the continuous phase taken out after solvent extraction is accordingly not exactly the same as the first liquid, also called the continuous phase, when provided into the solvent extraction system. By providing the liquid which will be a dispersed phase into a solvent extraction chamber of the solvent extraction system through a membrane a size of the droplets can be controlled and also be very uniform. A membrane can be provided having pores of a suitable size and interspacing. Hereby the dispersed phase can be emulsified when entered into the solvent extraction chamber. A pump forcing the liquid through the membrane and a membrane which is designed for emulsification can be provided according to the invention. Furthermore, a mixing blade of a mixer can in some embodiments be provided in the solvent extraction chamber close to an outlet side of the membrane. Hereby the loosening of the droplets from the outlet side of the membrane can be controlled by controlling the rotation speed of the mixer and hereby the droplet sizes can be further controlled. According to the invention a method for solvent extraction is provided comprising emulsification of the dispersed phase when entered into the solvent extraction chamber. Hereby solvent extraction can be improved. 
     The invention will now be described with reference to the drawings. Many of the details are the same in the different described embodiments and are given the same reference numbers. The solvent extraction according to the invention can be performed in batches or in a continuous flow. A solvent extraction system  1  suitable for a batch process is schematically shown in  FIG.  1    and two slightly different solvent extraction systems  1 ′;  1 ″ suitable for a continuous process are schematically shown in  FIGS.  2  and  3   . Some details which are common for these three different embodiments of the invention will first be described with reference to all of  FIGS.  1 - 3   . 
     A solvent extraction system  1 ;  1 ′;  1 ″ is provided comprising an elongated solvent extraction chamber  3  having a first end  5   a  and an opposite second end  5   b  and a length, L, between said first and second ends  5   a,    5   b.  The solvent extraction chamber  3 , also called chamber  3 , comprises at least one port  10 ,  11 ;  11 ′,  12  for providing a continuous phase into the solvent extraction chamber  3  and for removing content out from the solvent extraction chamber. The at least one port  10 ,  11 ;  11 ′,  12  comprises at least one of a first port  10 , a second port  11 ;  11 ′ and a third port  12 . In the embodiment as shown in  FIG.  1    the solvent extraction chamber  3  comprises a first port  10  and a second port  11  and in the embodiments as shown in  FIGS.  2  and  3    the solvent extraction chamber  3  comprises a first port  10 , a second port  11 ′ and a third port  12 . 
     According to the invention the solvent extraction system  1 ;  1 ′;  1 ″ comprises further a membrane  7  comprising pores which are well defined and substantially equal in diameter and interspacing, wherein said membrane  7  is positioned in connection with the solvent extraction chamber  3  at its first end  5   a.  Said membrane  7  is configured such that a liquid passing through said membrane  7  will be emulsified. The pores of the membrane can have substantially the same diameter, not differing more than 20%, and center-to-center distances between said pores can be substantially the same, not differing more than 20%. 
     The pores of the membrane  7  can for example have a pore diameter between 1-100 μm and the distance between the pores, center-to-center (also called pitch), can for example be between 10-1000 μm. In some embodiments the pores of the membrane  7  can have a pore diameter between 2-10 μm and the distance between the pores, center-to-center, can be between 30-300 μm. In one embodiment of the invention a membrane emulsification unit comprising said membrane  7  is provided to the solvent extraction chamber  3  at its first end  5   a.  The membrane  7  can in some embodiments be hydrophobic or hydrophilic. 
     The solvent extraction system  1 ;  1 ′;  1 ″ comprises furthermore a dispersed phase inlet  9  which is positioned to be in fluid connection with the first end  5   a  of the solvent extraction chamber  3  and such that a liquid provided into the solvent extraction chamber  3  through the dispersed phase inlet  9  has to pass through the membrane  7 . Hereby a liquid which is provided through the dispersed phase inlet  9  into the solvent extraction chamber  3  will be emulsified into a dispersed phase when passing through the membrane  7 . Thanks to the pores of the membrane  7  being equal in diameter and interspacing the dispersed phase will comprise equally sized droplets which will improve the solvent extraction performed in the solvent extraction system  1 ;  1 ′;  1 ″. 
     The solvent extraction chamber  3  is vertically oriented and gravity is used for separating phases. The dispersed phase may either have a larger density than the continuous phase and sink to a bottom of the solvent extraction chamber (the second end  5   b ) or have a smaller density than the continuous phase and rise to a top of the solvent extraction chamber. In that case the solvent extraction systems  1 ;  1 ′;  1 ″ as shown in  FIGS.  1 - 3    are only flipped upside down, i.e. a top of the solvent extraction chamber is in this case the second end  5   b  and a bottom is the first end  5   a.    
     In some embodiments of the invention the solvent extraction system  1 ;  1 ″ further comprises a mixer  15  provided inside the solvent extraction chamber  3 . The mixer  15  comprises a mixing blade  17  which is positioned closer to the first end  5   a  of the solvent extraction chamber  3  than ⅛ of the total length, L, of the solvent extraction chamber  3 . The mixing blade  17  is positioned in close relation but not directly contacting an outlet side  7   a  of the membrane  7 , wherein the dispersed phase will be provided into the solvent extraction chamber  3  from said outlet side  7   a  of the membrane  7 . The rotation of the mixing blade  17  of the mixer  15  can improve the efficiency of emulsification of the dispersed phase when passing through the membrane  7 . The loosening of the droplets from the pores of the membrane can be controlled by controlling the rotation speed of the mixing blade  17 . The sizes of the droplets can be controlled by controlling the rotation speed of the mixing blade  17 . Furthermore, the mixer  15  will improve the mixing of the dispersed phase with the continuous phase inside the solvent extraction chamber  3 . In the embodiments as shown in  FIGS.  1  and  3    the mixer  15  comprises a shaft  31  protruding through the solvent extraction chamber  3  in a centre of the solvent extraction chamber and along its length, L. A motor  33  can then be provided outside the solvent extraction chamber  3  at its second end  5   b  connected to the shaft  31  through a mechanical seal in an end unit  35  of the solvent extraction chamber  3 . However, in another embodiment the mixing blade  17  of the mixer  15  could be connected to a motor in another way, for example the mixer could be magnetically driven. The rotation of the shaft  31  in the embodiments as shown in  FIGS.  1  and  3    may however contribute to a further mixing of the dispersed phase and the continuous phase throughout the solvent extraction chamber  3  which may be positive for the solvent extraction. 
     In the embodiment as shown in  FIG.  1    the solvent extraction chamber  3  comprises a first port  10  for removing content out from the solvent extraction chamber  3  and a second port  11  for providing a continuous phase into the solvent extraction chamber  3 . This can be used for a batch process. A continuous phase is provided into the solvent extraction chamber  3  through the second port  11  and a dispersed phase is provided through the dispersed phase inlet  9  and through the membrane  7 . The dispersed and continuous phase are mixed and solvent extraction can occur. The dispersed phase will move through the chamber  3  to the second end  5   b  of the chamber thanks to gravitation. Depending on the orientation of the solvent extraction chamber (which will depend on density difference of the dispersed and continuous phase) the dispersed phase will either sink to a bottom or rise to a top of the solvent extraction chamber  3 . However, the dispersed phase will always separate to the second end  5   b  of the solvent extraction chamber  3 , which is the top or the bottom of the chamber depending on the orientation of the chamber. After the dispersed phase has been separated to the second end  5   b  of the chamber  3  the dispersed phase can be removed from the chamber  3  through the first port  10 . The first port  10  can be provided in the second end  5   b  of the chamber  3  or close to the second end  5   b  of the chamber  3 . Close to the second end  5   b  can for example be that the first port  10  is provided at a distance from the second end  5   b  of the solvent extraction chamber  3  which is smaller than ⅕ of the total length, L, of the elongated solvent extraction chamber or smaller than ⅛ of the total length, L, of the solvent extraction chamber  3 . After the dispersed phase has been removed the continuous phase can be removed through the first port  10 . In another embodiment a third port  12  can be provided for removing the continuous phase. A third port  12  can in this embodiment be provided close to the second end  5   b  but at a larger distance from the second end  5   b  than the first port  10 , for example at a distance which is smaller than ⅕ of the total length, L, of the solvent extraction chamber  3  or smaller than ⅛ of the total length, L, of the chamber  3 . In this embodiment the second port  11  can be provided close to the first end  5   a  of the chamber  3 , which could be for example at a distance from the first end  5   a  of the solvent extraction chamber  3  which is smaller than ⅕ of the total length, L, of the elongated solvent extraction chamber  3 . 
     In the embodiments as shown in  FIGS.  2  and  3    the solvent extraction chamber  3  comprises three ports  10 ,  11 ′,  12 , comprising a first port  10  for removing dispersed phase out from the solvent extraction chamber  3 , a second port  11 ′ for providing a continuous phase into the solvent extraction chamber  3  and a third port  12  for removing a continuous phase out from the solvent extraction chamber  3 , wherein the first port  10  and the second port  11 ′ are provided at a distance from the second end  5   b  of the solvent extraction chamber  3  which is smaller than ⅕ of the total length, L, of the elongated solvent extraction chamber  3  and wherein the third port  12  is provided at a distance from the first end  5   a  of the solvent extraction chamber  3  which is smaller than ⅕ of the total length, L, of the elongated solvent extraction chamber  3 . These embodiments are suitable for a continuous process where a continuous phase is continuously pumped into the chamber  3  through the second port  11 ′ and a dispersed phase is continuously pumped in to the chamber  3  through the dispersed phase inlet  9 . A solvent extraction system  1 ″ comprising pumps and liquid sources is shown in  FIG.  3   . This solvent extraction system  1 ″ comprises at least two pumps  21   a ,  21   b,  wherein a first pump  21   a  is connected to the second port  11 ′ and to a first liquid source  23   a  and is configured for pumping a first liquid from the first liquid source into the solvent extraction chamber  3 , which first liquid will be a continuous phase in the solvent extraction chamber  3  and wherein a second pump  21   b  is connected to the dispersed phase inlet  9  and to a second liquid source  23   b  and is configured for pumping a second liquid from the second liquid source  23   b  into the solvent extraction chamber  3 , which second liquid will be a dispersed phase in the solvent extraction chamber  3 . This system is suitably run continuously, wherein the first and second pumps  21   a,    21   b  are configured for pumping continuously. Suitably a rate of fluid flow of the first liquid pumped into the solvent extraction chamber  3  by the first pump  21   a  is controlled such that a flow rate through the chamber  3  for the continuous phase is kept lower than a sedimentation/rising rate of a dispersed phase through the solvent extraction chamber  3 . Hereby solvent extraction and separation of dispersed phase through the chamber  3  will be effectively performed in a continuous process. 
     A continuous phase will be continuously collected through the third port  13  which may comprise a filter  13  for preventing dispersed phase from being transferred out from the solvent extraction system  3  through the third port  12 . If the dispersed phase is hydrophilic said filter  13  can for example be a hydrophobic filter for preventing the dispersed phase to interact with the filter  13 . If the dispersed phase instead is hydrophobic the filter  13  can instead be hydrophilic. 
     As shown in both  FIGS.  2  and  3   , a distance between the second end  5   b  of the solvent extraction chamber  3  and said first port  10  can be smaller than a distance between the second end  5   b  of the solvent extraction chamber  3  and said second port  11 ′. Hereby it can be avoided that the continuous phase provided into the chamber  3  is directly transferred to the first port  10 . The second port  11 ′ can also in some embodiments be directed slightly towards the first end  5   a  of the chamber  3  such that the flow of continuous phase provided into the chamber  3  is directed towards the first end  5   a.    
     In some embodiments of the invention the solvent extraction system  1 ;  1 ′;  1 ″ according to the invention is configured for cleaning contaminated oil, such as for example industrial oil, lubrication oil, motor oil, hydraulic oil or processing oil for example used in gear box, hydraulic systems, engines, automotive equipment, construction equipment. In such embodiments a separation aid for cleaning of the oil is provided as the dispersed phase through the dispersed phase inlet  9  and through the membrane  7  such that it is emulsified when entered into the solvent extraction chamber  3 . Contaminated oil is provided through the second port  11 ,  11 ′ of the chamber  3 . The separation aid will attract contaminants in the contaminated oil whereby separation aid with attracted contaminations will be removed through the first port  10  of the chamber  3 . In the embodiment as shown in  FIG.  3    a first liquid source  23   a  which is connected to the second port  11 ′ comprises contaminated oil to be cleaned and a second liquid source  23   b  which is connected to the dispersed phase inlet  9  comprises a liquid separation aid, wherein the separation aid will attract contaminants in the contaminated oil during a solvent extraction performed in the solvent extraction chamber  3 . The membrane  7  is then adapted for this specific separation aid, i.e. such that a suitable droplet size of the separation aid is provided when the separation aid is passing through the membrane  7 . The separation aid may have a viscosity between 30-   400   cSt and the oil may have a viscosity between 1-400 cSt. In that case a suitable droplet size of the separation aid for achieving a good solvent extraction effect may be between 10-30 μm. To achieve this droplet size the pores of the membrane  7  can in some embodiments of the invention have a pore diameter between 2-10 μm and the distance between the pores, center-to-center, can be between 30-300 μm. In some embodiments of the invention the pores of the membrane  7  can have a pore diameter between 2-5 μm and the distance between the pores, center-to-center, can be between 30-60 μm. Hereby a droplet size of the separation aid is provided which is suitable for solvent extraction and hereby cleaning of contaminated oil. 
     The solvent extraction and separation of dispersed phase through the chamber  3  can as described above be effectively performed in a continuous process. Hereby the solvent extraction system  1 ″ according to the invention can be connected directly to a technical equipment which is using oil for continuous cleaning of the oil. A technical equipment which is using oil can for example be an industrial equipment, an automotive equipment, a construction equipment or metalworking machines using industrial oil, hydraulic oil, motor oil and/or lubrication oil, such as for example gear boxes, engines, hydraulic systems etc. An oil purification system  101  comprising a solvent extraction system according to the invention is also provided according to the invention. This is schematically shown in  FIG.  3    by a dotted line  51  from the third port  12  to the first liquid source  23   a.  In this embodiment the first liquid source  23   a  is the technical equipment, also denoted  23   a,  which in this example is directly connected to the solvent extraction system  1 ″ according to the invention for continuously cleaning of the oil which is used in the technical equipment. The second port  11 ′ of the solvent extraction chamber  3  is in this embodiment configured for being connected to the technical equipment  23   a  via the pump  21   a  for receiving contaminated oil to be cleaned and the third port  12  of the solvent extraction chamber  3  is configured for being connected to the technical equipment  23   a  for transferring cleaned oil from the solvent extraction system to the technical equipment. 
     The separation aid is liquid at the temperature at which the process is carried out and will by chemical interactions adsorb/absorb contaminating solids or dissolved impurities in the oil to be purified and wherein the separation aid composition is substantially insoluble in the oil to be purified because of its polar properties, forming a two-phase mixture upon mixing with the oil to be purified and wherein the separation aid has a density different from that of the oil to be purified. 
     The use of a separation aid, also called a chemical booster, for capturing contaminations/impurities in contaminated oil has been described before. A liquid separation aid is added to the oil and mixed therewith and impurities in the oil will be captured by the separation aid. The separation aid is substantially insoluble in the oil, forming a two phase mixture upon mixing and the separation aid attracts impurities in the oil during mixing of oil and separation aid. The separation aid will by chemical interactions adsorb/absorb contaminating solids, or dissolved impurities in the contaminated target oil. The separation aid should be liquid at the temperature at which the process is carried out. The separation aid composition should be substantially insoluble in the contaminated target oil, forming a two-phase mixture upon mixing with the contaminated oil. The liquid separation aid can also have a density different from that of the contaminated oil to be purified. 
     The separation aid is not soluble in the contaminated target oil because of its polar properties and thus colloids consisting of small droplets of the liquid separation aid composition are formed by the stirring, which through chemical interactions (hydrophilic, hydrophobic, and charge interactions) may absorb unwanted solid or the dissolved impurities in the contaminated target oil. In instances where the separation aid has a higher density than the oil the separation aid will at a gravity separation form a lower phase together with the solid and/or dissolved impurities. In instances where the separation aid has a lower density than the contaminated target oil, it will form an upper phase on gravity separation. 
     The liquid separation aid for use in the invention can be made up based on the following components: a) a polar polymer; b) a hydrotrope/solubilizer; and, c) a co-tenside. 
     Suitable separation aids with the properties described above, that can be used in the inventive process, may e.g. constitute a composition comprising a mixture of polar polymers such as polyethylene glycols, polypropylene glycols or similar polyalkylene glycols, organic surface active components with nonionic, anionic, cationic and amphoteric properties with the ability to enhance the solubility of solid or dissolved impurities in to the separation aid. 
     One example of a separation aid which can be used in this invention comprise: a) at least one polar polymer not soluble in oil and with a higher density than the oil, such as polyethylene glycol with an average molecular weight of 190-210 g/mole, e.g. Carbowax PEG 200 (Dow Chemical Company); b) at least one surface active hydrotrope/solubilizer, such as anionic sulfonic acids, phosphate ester-based substances or non-ionic surfactants from the poly-glycoside family, such as Simulsol SL 4, Simulsol SL 7 G and Simulsol AS 48 (Seppic, Air Liquide group); c) at least one amphoteric Co-surfactant, such as an propionate type e.g. Ampholak YJH-40 (Akzo Nobel) which is a sodium caprylimino dipropionate. 
     The separation aid may also have a sufficiently big difference in polarity compared to a polarity of at least one specific additive in the oil to be purified such that the at least one specific additive is not soluble in the separation aid. Hereby valuable additives in the oil can be kept in the oil during the solvent extraction, i.e. during the cleaning of the oil. Thanks to the different polarities the additives, or at least some of the additives, will not be attracted by the separation aid and will stay in the oil. The oil can furthermore from the start be provided with suitable additives having suitable polarities in relation to the used separation aid. Hereby it can be assured that additives are not removed during cleaning of the oil. 
     A number of solvent extraction systems  1 ;  1 ′;  1 ″ according to the invention can be connected in series in order to improve solvent extraction efficiency or in the example of cleaning of contaminated oil in order to improve cleaning efficiency. 
     A method for solvent extraction in a solvent extraction system according to the invention is also provided according to the invention. The steps of the method are described without any specific order below. The method can as described above be a continuous process whereby the order of the method steps will be of no importance and are actually performed simultaneously.
         Providing a first liquid into the solvent extraction chamber  3  through one of the at least one port  10 ,  11 ;  11 ′,  12 . A rate of fluid flow of the first liquid provided into the solvent extraction chamber  3  can suitably as described above be controlled such that a flow rate through the chamber  3  for the continuous phase is kept lower than a sedimentation/rising rate of a dispersed phase through the solvent extraction chamber  3 . In the example where the solvent extraction system according to the invention is used for cleaning a contaminated oil the first liquid is the contaminated oil.   Providing a second liquid into the solvent extraction chamber  3  from the dispersed phase inlet  9  and via the membrane  7 . In the example where the solvent extraction system according to the invention is used for cleaning a contaminated oil the second liquid is the separation aid.   Collecting dispersed phase from one of the at least one port  10 ,  11 ;  11 ′,  12 . In the example where the solvent extraction system according to the invention is used for cleaning a contaminated oil the collected dispersed phase is separation aid with attracted contaminants. The contaminants have been moved from the oil to the separation aid by solvent extraction.   Collecting continuous phase from one of the at least one port  10 ,  11 ;  11 ′,  12 . In the example where the solvent extraction system according to the invention is used for cleaning a contaminated oil the collected continuous phase is cleaned oil.