Separation technique for oil ontaminated sludge

Abstract: The present invention relates to a method for separating water and oil from a sludge which contains water, oil and solids which method com­ prises 1) contacting the sludge with an effective amount of 1 or more surfactants chosen and balanced to form certain types of microemulsions with equal volumes of decane and 3% aqueous NaCl, 2) mixing said surfactants and sludge with sufficient agita­ tion to obtain a uniform mixture, 3) allowing said mixture to stand under gravitational settling or centrifugation to obtain the separation from the bottom sediment of an aqueous phase which may contain solubilized oil with or without an addi­ tional floating oil phase, 4) decanting separately the oil and aqueous phases and 5) further treating the aqueous phase with heat and/or NaCl to separate the solubilized oil or further treating the bottom sediment with selected surfactants to release adhering oil.

Description: The present invention relates to the separation of water and/or oil from a sludge mixture of water, oil and solids by the use of surfactants. The disposal of large volumes of refinery-generated oily sludge is a costly operation. The destabilization of this sludge to separate the oil, water, and solid phases would greatly reduce the volume for disposal and would represent significant oil recovery. In US-A-3 756 959 there is disclosed a method of breaking of emulsion-suspensions containing mineral oil, water and finely divided solids. The material is contacted with surfactant, mixed therewith and then separation is effected by settling under gravity or by centrifugation. An aqueous phase is thereafter separated from the remainder. The object of the present invention is to destabilize sludges with varying oil contents by the application of improved microemulsion technology. This object is attained by the present invention which provides a method for separating water and/or oil from a sludge which contains water, solids and oil, which method comprises: a) contacting the sludge with a surfactant b) mixing the surfactant and the sludge with sufficient agitation to obtain a uniform mixture; c) allowing the mixture to stand under gravitational settling or centrifugation to obtain separation from the bottom sediment of an aqueous phase; and d) separating the aqueous phase from the remainder, characterised by e) firstly determining the amount of oil in the sludge; then f) employing one or more surfactants in step (a) according to option i), ii) or iii) below: (i) if the sludge has an oil content of more than 10wt%, contacting the sludge with one or more hydrophilic surfactants wherein a hydrophilic surfactant is defined as a surfactant having the properties of providing, at 1.5wt% concentration in equal volumes of decane and 3wt% aqueous NaCl, a lower phase microemulsion such that the volume ratio of oil to surfactant in the microemulsion phase is at least 0.5; or (ii) if the sludge has an oil content of 6wt% or less, contacting the sludge with one or more lipophilic surfactants wherein a lipophilic surfactant is defined as a surfactant having the properties of providing, at 1.5wt% concentration in equal volumes of decane and 3 wt% aqueous NaCL, an upper phase microemulsion such that the volume ratio of water to surfactant in the microemulsion phase is at least 0.5; or (iii) if the sludge contains from 2 to 10wt% oil, contacting the sludge with a combination of one or more hydrophilic surfactants and one or more lipophilic surfactants as defined above; and g) separating the aqueous phase from the bottom sediment, and from an additional floating oil phase, if present. The bottom sediment resulting from options (ii) or (iii) may be further treated with one or more hydrophilic surfactants, and the resulting aqueous phase is decanted after standing or centrifugation and further treated with heat and/or NaCl to separate any solubilized oil contained therein. For sludges in the range of about 2 to about 10% oil content, both lipophilic and hydrophilic surfactants are effective though their mode of action differs. Both cause the sludge to shed an aqueous supernatant phase. However, with hydrophilic surfactants the supernatant phase is rich in solubilized oil while the bottom sediment is lean in oil. With lipophilic surfactants the supernatant phase is lean in oil while oil concentrates in the bottom sediment. In preferred embodiments of the present invention the surfactants are selected from one or both of: (a) an alkali metal, ammonium, alkyl ammonium, alkanol ammonium or ethoxylated alkyl or alkanol ammonium salt of an alkyl or alkylaryl sulfonic acid of the generic formula: R-S0₃H wherein R is an alkyl or alkylaryl group containing 8 to 30 carbon atoms in the alkyl chain and the aryl group may be additionally substituted with one or two alkyl groups containing 1-3 carbon atoms each and (b) an ethoxylated surfactant of the generic formula: R₁X(CH₂CH₂0) n Y wherein R₁ is an alkyl or mono- or di-alkyl aryl group containing 8 to 30 carbon atoms, X is -0-, -S-, or -SO₂NH- Y is -H, -SO₃⁻M⁺ or -(PO₃H)⁻M⁺ wherein M⁺ is an inorganic on ammonium cation including alkyl substituted ammonium cations; R₂ is an alkyl group containing 1 to 20 carbon atoms or a polyethoxy ether group containing from 1 to 30 (CH₂CH₂0) groups; R₃ is H or an alkyl group containing 1 to 3 carbon atoms; R₄ is a poly hydroxy group derived from glycerol, glycols, sorbitol, or various sugars; and n is an integer from 1 to 30. In other preferred embodiments of the present invention the surfactant(s) is dissolved in a hydrocarbon solvent which may contain a cosolvent selected from the group consisting of alkylene glycol monoalkyl ethers and C₂ to C₅ alkanols. In yet other preferred embodiments of the present invention the surfactant blend contains one or more of (i) the sodium or monoethanol amine salt of C₁₂-0-xylene sulfonic acid, and (ii) an ethoxylated dinonyl phenol containing 9 moles of ethylene oxide per mole of surfactant. In a most preferred embodiment of the present invention the surfactant blend contains one or more of the ethoxylated alkyl ammonium salts of alkylaryl sulfonic acids containing two to twenty ethylene oxide groups per molecule. For the process of separating oil from a relatively high oil content sludge an effective amount of a hydrophilic surfactant(s) is mixed with the sludge using sufficient agitation to form a uniform mixture. Hydrophilic surfactants can be employed when the oil content in the sludge is at least 2 wt%, although they are especially beneficial when the oil content is 10 wt% or more. The effective amount will vary with the composition of the sludge to be treated and is within the skill of one having ordinary skill in the art to determine. The determination of an effective amount is illustrated in the examples cited below. The surfactant-sludge mixture is allowed to stand until separate layers of oil, water and solids are formed wherein the uppermost layer is oil, the middle layer is water which may contain solubilized oil and the bottom layer is solids. (In some cases the water and oil or the water and solids combine to form a single layer. The oil, water and solids layers may then be separated by decantation. The oil is recycled as a refinery stream, the water is discharged with or without additional waste water treatment and the solids are used as land fill. The hydrophilic surfactant(s) for purposes of this invention is defined as one or more surfactants in combination having the properties of providing, at 1.5 wt% concentration in equal volumes of decane and 3 wt% aqueous NaCl, a lower phase microemulsion at 20°C such that the volume ratio of oil to surfactant (Vo/Vs) in the microemulsion phase is at least 0.5, preferably greater than 1 and most preferably greater than 2. The term "lower phase" microemulsion is descriptive in context since it means that the aforementioned system consisting of the hydrophilic surfactant and equal volumes of oil and water separates into an aqueous lower phase containing most of the surfactant in equilibrium with an excess oil phase which is essentially surfactant-free. The hydrophilic surfactant which is defined by the above properties includes, but is not limited to the alkyl and alkylaryl sulfonic acid salts wherein the alkyl group is a C₉ to C₂₄, preferably C₉ to C₁₈ linear, branched, or bilinear structure; the aryl group is selected from benzene, toluene, orthoxylene, and naphthalene, preferably benzene; and the salt is a salt of an alkali metal or alkanol amine. Also included, and preferred are the ethoxylated alkylphenols. Most preferred are the ethoxylated C₁₂-C₁₈ alkyl ammonium salts of C₉-C₂₄ alkyl and alkylaryl sulfonic acids containing 6 or more ethylene oxide (hereinafter EO) groups, wherein the alkyl and aryl groups are as previously defined above. Representative examples of hydrophilic alkyl and alkylaryl sulfonic acid salts include monoethanol ammonium nonyl o-xylene sulfonate, sodium dodecyl benzene sulfonate, ammonium tetradecyl benzene sulfonate, diethanol ammonium hexadecyl benzene sulfonate, and sodium dodecyl naphthalene sulfonate. Preferred hydrophilic sulfonic acid salts include heptaethoxy octadecyl ammonium dodecyl o-xylene sulfonate (designated C₁₂XS-Et7) and decaethoxy octadecyl ammonium dodecyl ortho xylene sulfonate (designated C₁₂XS-Et10). The ethoxylated alkyl amines used in preparing the ethoxylated alkyl ammonium salts of alkyl aryl sulfonic acid can be obtained from Akzo Chemie America under the trade name Ethomeens® or from Exxon Chemical-Tomah Products. Representative hydrophilic ethoxylated alkyl phenols include Igepal® DM 710, Igepal ® DM 730 and Igepal ® DM880 available from GAF which are chemically di-nonyl phenols ethoxylated with 15, 24 and 49 moles of EO, respectively. Preferred is Igepal ® DM 530 which is dinonyl phenol ethoxylated with 9 moles of ethylene oxide. Other suitable ethoxylated alkyl phenols include Tritons® X100, X102 and X114 available from Rohm and Haas of Philadelphia, PA and Igepals CO 610, 630, 660, 710, 720, 730, 850 and 880 which are chemically mono-octyl or nonyl phenols ethoxylated with from 8 to 30 moles of EO. The hydrophilic surfactant(s) is delivered to the sludge neat or dissolved in a suitable solvent to reduce viscosity and facilitate mixing. A hydrocarbon liquid (oil) acts as a solvent for the blend of surfactants. The hydrocarbon liquid, which may be comprised of one or more hydrocarbon oils, is selected such that (1) they are miscible with the sludge oil and (2) the surfactants are soluble in, or miscible with the hydrocarbon liquid, which hydrocarbon liquid may also contain water. For example, it may be beneficial to add up to 10 wt% water to improve the solubility of the surfactants in the hydrocarbon oil. Non-limiting examples of hydrocarbons suitable for use include Isopar ® M, No. 2 diesel fuel oil, kerosene, naphthas, white oil, and the like. The hydrocarbon oil is used in an effective amount, that is, an amount such that the viscosity of the blend of surfactants and hydrocarbon oil is such that it may be easily mixed with the sludge. The effective amount of course will vary depending upon the technique used to apply the surfactant blend to the sludge and is within the skill of one having ordinary skill in the art to determine. Under certain circumstances, up to 50, generally 10 to 25, weight percent of a cosolvent is included in the solvent to improve the solubility and reduce the viscosity of the surfactant in the hydrocarbon medium. The cosolvents are of the class of alkylene glycol monoalkyl ethers, C₂ to C₅ alkanols and mixtures thereof. Representative cosolvents include ethers such as ethylene glycol monopropyl ether, ethylene glycol monoethyl ether, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monomethyl ether, diethylene glycol n-butyl ether, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether and tripropylene glycol monomethyl ether, and alkanols which include straight and branched chain members such as ethanol, propanol, butanol, and pentanol. Of the alkanols, tertiary amyl alcohol (TAA) is preferred. Of the ethers, ethylene glycol monobutyl ether is preferred. It is understood, where appropriate some of the above cosolvents can be used in place of the hydrocarbon solvent. Non-limiting examples of the more preferred cosolvents which can be used in place of the hydocarbon solvent include: ethylene glycol mono butyl ether, ethylene glycol mono ethyl ether, diethylene glycol mono ethyl ether and diethylene glycol mono butyl ether. It is understood that when using a cosolvent, the ratio of the surfactants may have to be readjusted for changes in phase behavior brought about by the addition of the cosolvent. For the process of separating water from a relatively low oil content sludge an effective amount of a lipophilic surfactant(s) is mixed with the sludge using sufficient agitation to form a uniform mixture. Lipophilic surfactants may be employed when the oil content in the sludge is 10 wt% or less, although they are especially beneficial when the oil content is 6 wt% or less, particularly 4 wt% or less. The surfactant-sludge mixture is allowed to stand until separate layers of water which may contain some solubilized oil and sediment are formed wherein the upper layer is water and the bottom sediment is a mixture of oil and solids. Water is decanted and discharged with or without additional waste water treatment. The bottom sediment is used as land fill if the oil content is sufficiently low. If appreciable oil is present in the sediment (> 1%), the solids may be contacted with the hydrophilic surfactant(s) used for deoiling as described above or an aqueous solution or lower phase microemulsion made with the aforementioned hydrophilic surfactants. The separated oil is then recycled as a refinery stream. The lipophilic surfactant(s) is delivered to the sludge neat or dissolved in a suitable solvent as described above. The lipophilic surfactant for purposes of this invention is a surfactant having the properties of providing, at 1.5 wt% concentration in equal volumes of decane and 3% aqueous NaCl, an upper phase microemulsion at 20°C such that the volume ratio of water to surfactant (Vw/Vs) in the microemulsion phase is at least 0.5, preferably greater than 1 and most preferably greater than 2. The term "upper phase" microemulsion as used in defining the lipophilic surfactant ingredient means that the system consisting of the surfactant in equal volumes of oil and water separates into a surfactant-containing oil upper phase in equilibrium with an excess aqueous phase which is essentially surfactant-free. The lipophile having been defined by the above properties includes, but is not limited to, the ethoxylated alkyl phenols. Also included, and preferred, are the alkyl and alkylaryl sulfonic acid salts wherein the alkyl group is a C₉ to C₂₄, preferably C₁₂ to C₂₄ linear, branched, or bilinear structure; the aryl group is selected from benzene, toluene, orthoxylene, and naphthalene, preferably benzene; and the salt is a salt of an alkali metal or alkanol amine. More preferred are the ethoxylated C₁₂-C₁₈ alkyl ammonium salts of C₉-C₂₄ alkyl and alkylaryl sulfonic acids containing less than six EO groups wherein the alkyl and aryl groups are as previously defined above. Representative examples of lipophilic alkyl aryl sulfonates include monoethanol ammonium dodecyl o-xylene sulfonate, sodium tetradecyl o-xylene sulfonate, sodium hexadecyl o-xylene sulfonate, diethanol ammonium pentadecyl o-xylene sulfonate, triethanol ammonium octadecyl o-xylene sulfonate (prepared from penta and hexa propylene), sodium octapropylene benzene sulfonate, sodium tetracosyl toluene sulfonate, and various high molecular weight petroleum sulfonates. Preferred are the sodium and monoethanol ammonium salts of dodecyl o-xylene sulfonic acid. Most preferred are di-and penta ethoxy octadecyl ammonium dodecyl o-xylene sulfonate designated C₁₂XS-Et2 and C₁₂XS-Et5 respectively. Representative lipophilic ethoxylated alkyl phenols include Igepals® CO 210 and CO 430 which are nonyl phenols containing 1.5 and 4 moles of EO respectively, and Tritons® X15 and X35 which are octyl phenols containing 1 and 3 moles of EO, respectively. The surfactant may also comprise a blend of an ethoxylated surfactant and a salt of an alkyl or alkylaryl sulfonic acid. Preferred blends of ethoxylated alkyl phenols with alkylaryl sulfonates include combinations of Igepal ® DM 530 or Igepal ® DM 710 with the sodium or monoethanol amine salt of C₁₂ o-xylene sulfonic acid. Alternatively, ethoxylated alkyl ammonium (Ethomeen)® salts of the above alkylaryl sulfonic acids containing varying degrees of ethoxylation are blended. Preferred blends of ethoxylated alkyl ammonium salts of alkylaryl sulfonic acids include pentaethoxy octadecyl ammonium dodecyl benzene sulfonate combined with heptaethoxy octadecyl ammonium dodecyl benzene sulfonate and diethoxy cocoa ammonium dodecyl o-xylene sulfonate with decaethoxy octadecyl ammonium dodecyl o-xylene sulfonate. Most preferred is the blend of pentaethoxy octadecyl ammonium dodecyl o-xylene sulfonate with hepta or deca ethoxy octadecyl ammonium dodecyl o-xylene sulfonate, i.e., a blend of C₁₂XS-Et5 with C₁₂XS-Et10. For sludges in the range of 2 to 10% oil content, both lipophilic and hydrophilic surfactants are effective though their mode of action differs. Both cause the sludge to shed an aqueous supernatant phase. However, with hydrophilic surfactants the supernatant phase is rich in solubilized oil while the bottom sediment is lean in oil. In this case with proper choice of surfactants the supernatant phase after separation can be heated or contacted with NaCl to drive the system to an upper phase microemulsion so that water separates out and oil and surfactants concentrate in the upper phase. This upper phase is then separated and recycled as a refinery stream while the water is discharged. Ethoxylated surfactants are suitable for this purpose. With lipophilic surfactants, the supernatant aqueous phase obtained after contacting the sludge is lean in oil while oil concentrates in the bottom sediment. Under these circumstances, after decanting the supernatant, the sediment may be contacted with the hydrophilic surfactant(s) used for deoiling as described above or an aqueous solution or water-continuous microemulsion made with the aforementioned hydrophilic surfactants. The separated oil is then recycled as a refinery stream. Under circumstances as required by variation in sludge oil composition and content or aqueous phase composition, it is often beneficial to blend the aforementioned hydrophilic and lipophilic surfactants. The wt. ratio of hydrophile/lipophile (H/L ratio) can thus be adjusted (balanced) to accommodate variations in sludge composition. The following examples will demonstrate the invention and show the critical nature of adjusting H/L ratio to accommodate sludge oil and water content and composition. It is understood that these examples in no way serve to limit the true scope of this invention, but rather, are presented for illustrative purposes. Example 1 : Dewatering Low Oil-Content BW Refinery Separator Bottoms Sludge Analysis: water (Dean Stark) = 84 wt% Solids (Evaporation) = 12 Oil (Mass Balance) = 4 Aqueous Salinity = ∼0.8% NaCl The surfactant system selected was based on the surfactants obtained by neutralizing C₁₂ o-xylene sulfonic acid with a C₁₈ ethoxylated amine supplied by Akzo Chemie America under the trade name Ethomeen ®. This system was found to have high uptake (large amounts of water and oil solubilized) in previous studies. These surfactants will be referred to as C₁₂XS-Etn where n denoted the number of ethylene oxide (EO) groups per molecule. As discussed previously, C₁₂XS-Etn surfactants with n < 6 are considered lipophilic surfactants while those with n ≧ 6 are considered hydrophilic surfactants. Blends of lipophilic with hydrophilic C₁₂XS-Etn are treated as having an intermediate EO content. The surfactants used in this example are blends of C₁₂XS-Et5 with C₁₂XS-Et10. These surfactants were delivered neat or dissolved in toluene or isopropanol as 50% solutions for ease of mixing. The surfactant solutions were added at the desired surfactant concentration to 50 ml of BW Refinery sludge in a 100 ml graduate. The mixture was shaken 30 times and allowed to stand 23 hrs at room temperature. The volume and color of water which separated was noted and recorded in Table I. These data show that 1) Low oil content sludge containing oil bound to solids can be dewatered using critically balanced blends of lipophilic and hydrophilic surfactants. For the preferred surfactants, C₁₂XS-Et5/C₁₂XS-Et10, delivered in toluene solutions the best ratio is 33/67. The data demonstrate the critical nature of surfactant selection for a particular sludge. 2) The critical dependence of water separation on H/L ratio is reduced when the surfactants are delivered neat. The range of effective H/L ratios is broadened to include ratios from 50/50 to 0/100. 3) A surfactant concn. of ∼3% is optimum for this sludge. Using 5% surfactant has little added effect while 1% has virtually no effect. Thus an effective amount of surfactant to treat this sludge would be in the range 2-4% 4) Surfactant delivery in isopropanol solution gave results comparable to delivery in toluene solution. 5) The surfactants were moderately effective in removing ∼30% of the water from this particular sludge. Example 2 : Deoiling High Oil Content BR Refinery API Separater Bottoms Estimate: water = 48 vol % solids = 40 Oil 12 C₁₂XS-Etn surfactants were contacted neat at the desired concentration with BR refinery API Separator sludge using a procedure similar to that of Example 1. The volume percent of separated phases was measured after 1-3 weeks storage at room temperature and recorded in Table II. These data show that 1) High oil content sludge containing free oil can be dewatered and deoiled using a hydrophilic surfactant. The hydrophilic surfactant, C₁₂XS-Et10, is best in the illustrated application. The data demonstrate the critical nature of surfactant selection for a particular sludge. 2) C₁₂XS-Et10 was highly effective in separating both oil and water from this particular sludge. Phase separation was estimated to be complete. 3) A surfactant concentration of -1% is optimum for this sludge. Using 3% surfactant had no advantage and possibly a debit. A concentration of 0.5% was not as effective in deoiling the sludge. An effective amount of surfactant for this sludge is from 0.5 to 3%. Example 3 : Dewatering Santa Ynez Treater Bottoms Analysis: 30% Treater Bottoms, 70% Produced Water Water 82% Solids 12% Oil 6% The C₁₂XS-Etn surfactants were delivered neat at the desired surfactant concentration to a 50 ml sample of sludge in a 100 ml graduate. The sludge consisted of 30% Santa Yuez Treater Bottoms which analyzed: 40% water, 40% solids and 20% oil. The Treater Bottoms were cut back with 70% produced water which was essentially free of oil and solids. The surfactant sludge mixture was shaken and allowed to stand at room temperature. The volume of water which separated after 2 hrs was noted and recorded in Table III. These data show that 1) Low oil content sludge can be rapidly dewatered (< 2hrs) by a lipophilic surfactant with critically adjusted H/L properties. For the preferred surfactants, C₁₂XS-Et5 provides the best balance of H/L properties. 2) Comparison with Table I shows that choice of the best surfactant depends on the specific sludge composition. Santa Yuez sludge requires a more lipophilic surfactant than BW refinery sludge. 3) There is no performance advantage in using higher than 1% surfactant on this sludge. Obviously, the lower the surfactant concentration the better for economic reasons. Example 4 : Dewatering BR Refinery Dirty Master Separator (DMS) Sludge The C₁₂XS-Etn surfactants were contacted at the desired concentration with BR refinery Dirty Master Separator Sludge using a procedure similar to that of Example 1. The volume percent of separated water was measured after approximately 18 hours storage at room temperature and recorded in Table IV. These data show that 1) The composition of this sludge is highly variable depending on when the sample was taken. This makes surfactant selection very critical. 2) Moderately low oil content sludge (∼4-5% oil) can be dewatered by both lipophilic surfactants and combinations thereof with hydrophilic surfactants. In excess of 75% of the water present can be separated. However the amount of solubilized oil present in the water phase depends strongly on the hydrophilic vs lipophilic properties of the surfactant. With C₁₂XS-Et10, more oil is released from the sludge and solubilized in the aqueous phase than with C₁₂XS-Et5. This is evidenced by the blackness of the aqueous phase with C₁₂XS-Et10. This observation is confirmed by the analyses data in Example 5. 3) With a low oil content sample (10/31/88), C₁₂XS-Et5 provides effective dewatering at concentrations as low as 0.2 wt% Example 5 : Analysis of Surfactant-Treated BR Refinery DMS Sludge Test and Analytical Procedures Larger samples of sludge were treated with surfactant in 250 ml and 500 ml graduates so that the sediment and supernatant phases of the treated sludge could be accurately analyzed to quantify the performance of the microemulsion surfactants. For these larger samples the surfactant was mixed with the sludge by use of a Waring Blender for 30 seconds. The water content of the total sludge before treatment and water in the sediment phase of the treated sludge were determined by distillation and collection and measurement of the distilled water. The oil content of the total sludge, treated sediment and supernatant samples was determined by extracting the oil from the test samples by a Freon solvent. The Freon was then evaporated and the remaining oil was weighed. The method is referred to as "Gravimetric". The solids content is determined by washing/dissolving the oil from a water-free sludge sample by a toluene solvent. The remaining solids are then weighed and referred to as "Toluene Insoluble Solids". These data, as set forth in Tables V and VA, (with samples 10/21/88 and 5/5/88 being comparative samples), confirm that 1) Moderately low oil content sludge (10/21/88 sample) is dewatered by both hydrophilic (C₁₂XS-Et10) and lipophilic (C₁₂XS-Et5) surfactants, but more effectively by the latter. 2) With C₁₂XS-Et5 virtually all the oil concentrates in the sediment layer as evidenced by the extremely low average oil content (∼0.05%) in the supernatant aqueous phase and high average oil concentration (∼16%) in the sediment. 3) This mode of action is confirmed even with very low oil content sludge (11/3/88 sample).

Claims: A method for separating water and/or oil from a sludge which contains water, solids and oil, which method comprises: a) contacting the sludge with a surfactant b) mixing the surfactant and the sludge with sufficient agitation to obtain a uniform mixture; c) allowing the mixture to stand under gravitational settling or centrifugation to obtain separation from the bottom sediment of an aqueous phase; and d) separating the aqueous phase from the remainder, characterised by e) firstly determining the amount of oil in the sludge; then f) employing one or more surfactants in step (a) according to option i), ii) or iii) below: (i) if the sludge has an oil content of more than 10 wt.%, contacting the sludge with one or more hydrophilic surfactants wherein a hydrophilic surfactant is defined as a surfactant having the properties of providing, at 1.5 wt.% concentration in equal volumes of decane and 3 wt.% aqueous NaCl, a lower phase microemulsion such that the volume ratio of oil to surfactant in the microemulsion phase is at least 0.5; or (ii) if the sludge has an oil content of 6 wt.% or less, contacting the sludge with one or more lipophilic surfactants wherein a lipophilic surfactant is defined as a surfactant having the properties of providing, at 1.5 wt.% concentration in equal volumes of decane and 3 wt.% aqueous NaCl, an upper phase microemulsion such that the volume ratio of water to surfactant in the microemulsion phase is at least 0.5; or (iii) if the sludge contains from 2 to 10 wt.% oil, contacting the sludge with a combination of one or more hydrophilic surfactants and one or more lipophilic surfactants as defined above; and g) separating the aqueous phase from the bottom sediment, and from an additional floating oil phase, if present. A method according to claim 1 wherein option f (ii) is carried out if the oil content of the sludge is less than 4 wt%. A method according to claim 1 or 2, wherein the separated aqueous phase resulting from option f (i) or f (iii) is further treated with heat and/or a salt such as NaCl, to separate any solubilized oil contained therein. A method according to claim 1 or 2, wherein the bottom sediment resulting from option f (ii) or f (iii) is further treated with one or more hydrophilic surfactants, and the resulting aqueous phase is decanted after standing or centrifugation and further treated with heat and/or NaCl to separate any solubilized oil contained therein. A method according to any preceding claim wherein the surfactant(s) is selected from one or more of: (a) an alkali metal, ammonium, alkyl ammonium, alkanol ammonium or ethoxylated alkyl or alkanol ammonium salt of an alkyl or alkylaryl sulfonic acid of the generic formula: R-SO₃H wherein R is an alkyl or alkylaryl group containing 8 to 30 carbon atoms in the alkyl chain and the aryl group may be additionally substituted with one or two alkyl groups containing 1 - 3 carbon atoms each; or (b) an ethoxylated surfactant of the generic formula: R₁X(CH₂CH₂0) n Y wherein R₁ is an alkyl or mono- or di-alkylaryl group containing 8 to 30 carbon atoms, X is -0-, -S-, or -SO₂NH-; Y is -H, -SO₃⁻M⁺ or - (PO₃H)⁻M⁺ wherein M⁺ is an inorganic on ammonium cation including alkyl substituted ammonium cations; R₂ is an alkyl group containing 1 to 20 carbon atoms or a polyethoxy ether group containing from 1 to 30 (CH₂CH₂0) groups; R₃ is H or an alkyl group containing 1 to 3 carbon atoms; R₄ is a poly hydroxy group derived from glycerol, glycols, sorbitol, or various sugars; and n is an integer from 1 to 30, or a blend of (a) and (b) A method according to claim 5, wherein the hydrophilic surfactant, or at least one of the hydrophilic surfactants, is an ethoxylated C₁₂-C₁₈ alkyl ammonium salt of a C₉ - C₂₄ alkyl or alkylaryl sulfonic acid containing 6 or more ethylene oxide groups. A method according to claim 5, wherein the lipophilic surfactant, or at least one of the lipophilic surfactants, is an ethoxylated C₁₂ - C₁₈ alkyl ammonium salt of a C₉ - C₂₄ alkyl or alkylaryl sulfonic acid containing less than 6 ethylene oxide groups. A method according to any preceding claim wherein the surfactant(s) are dissolved in up to 50 wt% of a solvent comprised of one or more of a hydrocarbon liquid in which the surfactants are soluble; an oxygenated cosolvent; and less than 10 wt% water. A method according to claim 8, wherein the solvent is a hydrocarbon solvent selected from No. 2 diesel fuel oil, kerosene, and white oil. A method according to claim 8 or 9, wherein a cosolvent is present and the cosolvent is selected from one or more alkylene glycol monoalkyl ethers and C₂ to C₅ alcohols. A method according to claim 10, wherein the cosolvent is a tertiary amyl alcohol and is present in the range of 10 to 50 wt% based on the total weight of the solvents employed.

Inventors: BOCK JAN, CANEVARI GERARD PAUL, ROBBINS MAX LEO, BOCK, JAN, CANEVARI, GERARD PAUL, ROBBINS, MAX LEO
Applicants: EXXON RESEARCH ENGINEERING CO, EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1990
Main classification: B01D17
IPC codes: B01D17, C02F11, C02F1