Method for regenerating waste lubricant oils using supercritical co2

Abstract

The present application relates to a method for regenerating waste lubricant oils (WLO), particularly those that coagulate, using supercritical carbon dioxide (scCO2) extraction. The method uses a one-step high-pressure extraction system where waste oils are treated with scCO2 at controlled temperature and pressure conditions. The scCO2 dissolves the oils, leaving behind contaminants and additives. The method is effective for any WLO, and mixtures thereof, regardless of composition, effectively recovering oils even for coagulating waste oil, which conventional methods are not able to regenerate.

Description

METHOD FOR REGENERATING WASTE LUBRICANT OILS USING SUPERCRITICAL CO2

[0001] This application relates to a method for regenerating waste lubricant oils using supercritical CO2.

[0002] Waste lubricant oils (WLO) are classified as hazardous waste under European legislation [1]. The EU has promoted recycling and regeneration of WLO since 2008[2], as it is the most sustainable option for reducing environmental impact, conserving resources, and supporting the circular economy [3–6]. In response to rising environmental concerns, resource scarcity, and legislative pressure, European countries have been shifting from the polluting acid-clay method for waste lubricating oil regeneration to cleaner technologies like distillation, solvent extraction, pyrolysis, and membrane processes. Although modern regenerated products can meet high-quality standards comparable to commercial base oils, current methods hinder wider adoption. Solvent extraction, while avoiding problems associated with acid treatments, still raises safety concerns due to hazardous, energy-intensive solvents and waste management difficulties.

[0003] Additionally, all distillation-based methods, or other operating at high temperature, face operational issues due to the chemical pre-treatment of WLO with potassium hydroxide, which destabilizes colloidal systems especially in synthetic and semi-synthetic oils and can lead to coagulation [7], reducing fluidity and causing plant shutdowns. Thus, there is a pressing need for efficient, environmentally friendly, composition-independent, and high-yield regeneration methods.

[0004] Supercritical fluid extraction (SFE) using carbon dioxide (scCO2) presents a promising solution, namely as a replacement of conventional cleaning solvents, reducing the pollution associated with the operations that would otherwise require significant amounts of volatile organic compounds. Supercritical CO2is an inert, non-toxic, and environmentally friendly solvent that can achieve effective separation at low temperatures. It overcomes challenges like coagulation and solvent residue, allowing WLO regeneration where conventional technologies fail. This study uses scCO2to define a new, efficient methodology for regenerating WLO in alignment with circular economy principles.

[0005] A method for treating an oil using a fluid in a supercritical state has been described in 1980, application FR7928173A, then in 2000, application EP1165728, and later in 2023, application EP4183463.The first document describes contacting dried used oils with an inert gas or gaseous mixture above its critical pressure and temperature and separating products (oils, additives) by lowering the pressure and or temperature. The method described is very generic while the presently disclosed method employs supercritical carbon dioxide as the extraction solvent, without the mandatory drying pretreatment of the waste oil.Second document discloses the treatment of a used lubricating oil under supercritical conditions in a column-based system that separates the feed into light, heavy and intermediate fractions, with the intermediate undergoing further processing. The general range disclosed is 50–500 bar and 40–200 °C, but the process examples focus on ~250 bar and 60–80 °C.The presently disclosed method differs because it does not rely on a column-based fractionation architecture nor on a mandatory three-cut split, but instead it operates with a supercritical CO₂ extractor in batch or semi-batch mode, with only one separation step and at lower temperatures. This document claims regeneration of used oils in a countercurrent packed distillation column, where oil flows downward and supercritical CO₂ flows upward, with a mandatory solvent-to-oil flow-rate ratio of 15–50, operation in the 50–150 °C range and a consecutive fractionation step. The examples use a specific oil feed to demonstrate selective phosphorus and silicon removal, but do not extend to blends of different origins. The presently disclosed method differs fundamentally by employing a top-charged reactor with upward CO₂ flow, operating at 40 °C, below the patent’s temperature range, and is done in one-step only. Moreover, feasibility of regeneration is demonstrated for heterogeneous oil blends, including those that would be segregated due to coagulation phenomena.

[0006] Document CN110484346A describes a batch process in a stirred high-pressure kettle, where a pretreated used oil is contacted with liquid or supercritical CO₂ (60–180 bar and 32 to 77º C window), with added non-polar co-solvents up to 15%, such as n-hexane or cyclohexane, and the extract is collected after depressurization into an ice-cooled cylinder. It describes pretreatment by natural settling, filtration and vacuum distillation to remove water, with example outcomes of about 19.8 % regenerated oil and metal reductions to one sixth to one half of the original levels. By contrast, the presently disclosed method uses a top charged reactor with upward flowing CO2rather than a stirred kettle, operates always in the supercritical range (not with liquid CO2), but without added organic co solvents, and the equipment has a separator instead of ice bath collection.

[0007] In summary, the cited patents describe multi-stage processes with at least two fractionation or collection steps, where different fractions of oil are recovered and, in the case of EP1165728, further post-treated. Both EP1165728 and EP 4183463A1 operate at minimum 50 °C and were validated only on one or two specific used engine oils, not on representative blends. CN 110484346B also relies on a batchwise process, but includes subcritical CO2(liquid)conditions, with depressurization into an ice-cooled cylinder and the possible addition of organic co-solvents, again demonstrated on a single pretreated oil. By contrast, the presently disclosed method enables regeneration through a single-step extraction process, with supercritical CO2, without co-solvents, at low temperatures (as low as 40 °C), and validated on real multi-source waste oil mixtures, including partially synthetic blends that typically coagulate and would otherwise be segregated rather than regenerated. Moreover, the effectiveness of EP4183463 A1 was assessed only in terms of heavy-metal removal, without evaluating base-oil quality, removal of other contaminants, or hydrocarbon composition, that is provided in the method herein. Given that lubricant formulations are increasingly synthetic, it is crucial to validate regeneration methods against diverse oil compositions and blends, which our invention explicitly addresses and demonstrates.

[0008] Herein is disclosed a method for regenerating waste lubricant oils, particularly those that coagulate, using supercritical carbon dioxide extraction. At least one waste oil is treated with scCO2at controlled temperature and pressure conditions.General Description

[0009] The method here detailed uses scCO2as green solvent for the regeneration of waste lubricant oils and defines a strategy for extracting the oils and reintegrating them into their life cycle, in line to the principles of circular economy, maintaining the value of the products with minimum waste.

[0010] This application provides a method for regenerating a waste lubricant oil, or mixtures thereof, including those that coagulate, using supercritical carbon dioxide extraction.

[0011] The method is operable with or without pre-treatment steps such as dehydration, filtration, distillation, thereby reducing operational constraints while remaining compatible with industrial practice.

[0012] In the context of the present application, the term “regenerating” a waste oil is understood as the process of restoring used or contaminated oil to a condition where it can be reused, meaning restoring it to the properties of the virgin oil / base oil.

[0013] The method can be carried out in a customized high-pressure extraction device where waste oil is treated with scCO2under controlled temperature and pressure conditions in a single step operation. The method operates at low temperature and effectively separates the oil, or mixtures thereof, from contaminants, oxidation products, and additives, overcoming coagulation issues and providing regenerated oils with high yield and purity. The method is equally effective whether applied directly to untreated waste oil or to pre-filtered / dehydrated feedstocks, offering industrial flexibility while minimizing energy consumption.

[0014] The method includes the following key aspects:

[0015] Regeneration of waste lubricant oils using supercritical carbon dioxide extraction, where the WLO is introduced into a high-pressure extraction vessel and exposed to scCO2at temperatures and pressures above its critical point.

[0016] Treatment of oil happens in one single extraction stage.

[0017] Effectively regenerates WLO mixtures with different composition, including oil that coagulates due to saponification reactions during the alkali pretreatment, a problem commonly encountered in conventional regeneration processes involving high temperatures and / or distillation steps.

[0018] The method can be applied directly to untreated oils, without requiring pre-treatment such as dehydration or filtration, although it remains compatible with oils that have undergone such steps.

[0019] It is optimized by controlling key variables such as pressure, flow rate, and extraction time, which are adjusted to maximize yield while minimizing environmental impact.

[0020] The oils are easily recovered in the separator after depressurization.

[0021] There are no hazardous solvents added, no high temperature employed, it is eco-friendly, low-cost and presents relatively high yield.

[0022] Results in regenerated oils with improved physicochemical properties, meeting the technical specifications for re-use in the production of new lubricating oils.

[0023] For easier understanding of this application, figures are attached in the annex that represent the preferred forms of implementation which nevertheless are not intended to limit the technique disclosed herein.Fig.1

[0024] shows a schematic representation of an embodiment of the experimental setup / high-pressure extraction device: 1- CO2cylinder; 2- Thermostatic bath with cooling coil; 3- Cooling Pump; 4- Heat exchanger; 5- High-pressure Extraction Vessel; 6- First Back Pressure regulator; 7- Safety valve; 8- Second Back Pressure regulator; 9- Separator.Fig.2

[0025] shows the response surface plots for WLO regeneration yield.Fig.3

[0026] shows the pareto chart of effects in variable Yield (A) and Experimental vs Predicted Values for Yield (B).Fig.4

[0027] shows the profile for predicted values and desirability.Fig.5

[0028] shows the profile for predicted values and desirability.

[0029] Now, preferred embodiments of the present application will be described in detail with reference to the annexed drawings. However, they are not intended to limit the scope of this application.

[0030] The present application relates to a method for regenerating a waste lubricant oil, or mixtures thereof, using supercritical carbon dioxide.

[0031] The method for regenerating waste lubricant oils comprises the following steps:

[0032] Supplying at least one waste lubricant oil into a high-pressure extraction vessel;

[0033] Providing carbon dioxide at a temperature between -5 and 0ºC and a pressure between 4.5 and 6.0 Pa;

[0034] Pressurizing the carbon dioxide at a pressure between 8 and 20 MPa and heating it at a temperature between 31 and 60ºC to obtain supercritical carbon dioxide;

[0035] Feeding the supercritical carbon dioxide to the high-pressure extraction vessel, wherein the supercritical carbon dioxide is fed via a supply line arranged at the bottom of the vessel;

[0036] Contacting the at least one waste lubricant oil with the supercritical carbon dioxide at a temperature between 35ºC and 60ºC, at a pressure between 8 and 20 MPa, for a time between 100 and 200 min, to extract at least one regenerated oil;

[0037] Recovering the at least one regenerated oil and supercritical CO2to a separator;

[0038] Depressurizing the separator to separate the supercritical carbon dioxide and obtain at least one regenerated oil.

[0039] The regenerated oil is also understood as the oil without contaminants obtained at the end of the method, said regenerated oil comprises the characteristics of the oil that can be used in the production of new lubricants.

[0040] In one embodiment, the at least one waste lubricant oil is selected from, but not limited to, waste engine oils, automotive crankcase oils, transmission oils, gear oils, industrial hydraulic fluids, turbine oils, compressor oils, transformer insulating oils, marine lubricants, metalworking fluids, and mixtures thereof, including multi-source blends of mineral, semi-synthetic, or synthetic lubricants, as well as oils containing ester-based additives or components prone to coagulation under alkaline treatment, or mixtures thereof.

[0041] In one embodiment, the extraction flow rate is between 6 to 20 mL / min.

[0042] In one embodiment, the method further comprises a pre-treatment step of the at least one waste lubricant oil, selected from dehydration, filtration, or distillation. In one embodiment, the method is carried out without such pre-treatment.

[0043] In one embodiment, the carbon dioxide has a purity higher than 99%.

[0044] In one embodiment, the method yields up to 70% of at least one regenerated oil.

[0045] In one embodiment, the high-pressure extraction vessel suitable to carry out the method comprises a packing material. In one embodiment, the packing material is in the form of beads. In one embodiment, the packing material is made of glass, or ceramic, or stainless-steel, but not limited to. In one embodiment, the packing material comprises an average diameter between 3 and 10 mm. In a particular embodiment, glass beads of 5 mm in diameter are used.

[0046] In the embodiment shown in, a high-pressure extraction device suitable to carry out the presently disclosed method comprises:

[0047] A cooling coil immersed in a thermostatic water bath (2), suitable for condensing carbon dioxide to its liquid state prior to pressurization;

[0048] A cooling pump (3), suitable for delivering liquid carbon dioxide at the required extraction pressure;

[0049] A heat exchanger (4) suitable for heating the compressed carbon dioxide to supercritical conditions;

[0050] A high-pressure extraction vessel (5) into which at least one waste lubricant oil is introduced and contacted with supercritical carbon dioxide, the vessel being provided with a first back pressure regulator (6) to maintain the desired operating pressure;

[0051] A safety valve (7) provided in the high-pressure extraction vessel (5);

[0052] A second back pressure regulator (8) arranged between the high-pressure extraction vessel (5) and a separator (9), suitable to control the depressurization of the effluent stream;

[0053] A separator (9) suitable for receiving the mixture of at least one regenerated oil and supercritical carbon dioxide, effecting the phase separation by pressure reduction, thereby recovering the at least one regenerated oil while enabling the carbon dioxide to be vented or recycled.

[0054] In one embodiment, the carbon dioxide is provided in a CO2cylinder (1).

[0055] In one embodiment, the cooled carbon dioxide can be heated to the supercritical temperature with a heat exchanger (4) arranged after the pump.

[0056] In one embodiment, the high-pressure extraction vessel (5) in which the contact between the at least one waste lubricant oil and the supercritical carbon dioxide occurs, comprises a packing material. In one embodiment, the packing material is in the form of beads.

[0057] In one embodiment, the packing material, for example the beads, increases the interfacial area and promote mass transfer during extraction.

[0058] In one embodiment, the packing material is made of glass, or ceramic, or stainless-steel, but not limited to. In one embodiment, the packing material comprises an average diameter between 3 and 10 mm. In a particular embodiment, glass beads of 5 mm in diameter are used.

[0059] In one embodiment, the extraction pressure is controlled with at least two back pressure regulators. In one embodiment, a first back pressure regulator (6) is arranged in the high-pressure extraction vessel and a second back pressure regulator (8) is arranged between the high-pressure extraction vessel (5) and the separator (9) and is meant to control the pressure release over the latter.

[0060] In one embodiment, the separator (9) is a second vessel connected downstream of the high-pressure extraction vessel, configured to receive the effluent stream comprising solubilized oil in supercritical carbon dioxide. Within the separator, controlled decompression of the CO2takes place, thereby allowing the recovery of the at least one regenerated oil, while allowing the CO2to be vented or recycled back to the extraction loop. In one embodiment, the separator is made of stainless steel.

[0061] The results obtained from the examples below showed that supercritical extraction with CO2is an effective method for the regeneration of waste lubricant oils, including oils that would be segregated due to coagulation phenomena. Performance of supercritical extraction with CO2slightly varies depending on the oil sample, showing a superior yield for noncoagulating samples (above 80%), yet with a very reasonable yield for samples that exhibit coagulation (up to 75%). This regeneration method has shown to be able to remove most oxidation products and additives from the oil samples, reducing viscosity and viscosity index. The regenerated samples complied with the technical specifications for base oils (SN-100 or SN-150) resulting from the regeneration method, as defined by the Portuguese law in force in 2025, with color being the only parameter that requires and additional correcting step.

[0062] The method was modelled using response surface methodology and optimal extraction conditions were determined using a profiler and desirability approach. As a result, an operating pressure of 140 bar (14 MPa) and a flow rate of at least 14mL / min were chosen as the optimal conditions for the experiments.Examples

[0063] The method is illustrated by the following examples, each based on a waste oil mixture with different properties. General method conditions, optimization procedures, and analytical methods are common to all examples unless otherwise specified.

[0064] General process conditions and analytical methods

[0065] The method involves precise control of pressure and temperature to ensure the carbon dioxide remains in its supercritical state, above 31°C and 7.38 MPa. The method conditions for extraction range between 8 and 20 MPa, temperature ranges between 31 and 60ºC. The flow rate can range between 6 and 20 mL / min.

[0066] Using response surface methodology (RSM) and statistical analysis, the extraction method is optimized for maximum yield. The method achieves a yield of over 80% for non-coagulating oils and up to 75% for coagulating oils.

[0067] Materials and methods

[0068] Waste lubricant oil samples were collected by SOGILUB S.A, the national organization responsible for the Portuguese waste lubricant oil management system and recovered from waste treatment facilities. Two representative sets of samples were taken, all consisting of mixtures of waste lubricant oil originated from several types of producers, garages, industry and other producers, e.g. transportation, construction, agriculture or public sectors, among others, and were selected considering the existence or absence of pre-treatment and their response stability to alkaline treatment [7]. All samples were kept in plastic containers and stored at a temperature between 20 and 25ºC, in dark conditions to maintain their integrity until further use.

[0069] CO2with purity higher than 99.995% (water content < 40 ppm) was supplied by Messer.

[0070] Regeneration experiments of samples with supercritical carbon dioxide

[0071] Regeneration of waste lubricant oil was done in a customized supercritical fluid extraction (SFE) device, showed in, equipped with a 300 mL stainless-steel extraction vessel designed to work at high pressure and followed by a 20 mL stainless-steel separator to recuperate regenerated oils, built by Paralab. CO2exiting from the cylinder was cooled down, to ensure its liquid state, both by a stainless-steel coil submerged into a thermostatic circulating water bath set to -2ºC (Lauda Eco RE415G) and a heat exchanger, mounted in an HPLC analytical pump (AZURA P 4.1S from Knauer) that supplied CO2into the device. CO2was then heated with a heat exchanger to the operation / extraction temperature, to attain supercritical conditions. ScCO2then entered, from bellow, the extraction vessel, packed with 5mm glass beads and loaded with the sample of WLO to regenerate. Pressure was controlled with two back pressure regulators. A first back pressure regulator, Equilibar U6L was arranged over the extraction vessel, controlling the pressure over extraction. BPR pressure was operated using a N2cylinder with manual regulation. A second back pressure regulator was mounted between the extraction vessel and the separator and was suitable to control the pressure release over the latter.

[0072] During the experiment the contaminants were retained in the extraction vessel while CO2and the oils move to the separator. CO2dissolving capacity was modulated by altering experimental conditions (p and T). Regenerated oil samples were recovered in the separator, by depressurization of CO2.

[0073] Physicochemical characterization

[0074] Samples were characterized regarding the density at 40 ºC, kinematic viscosity at 40 ºC and 100ºC, viscosity index, total acid number (TAN), saponification number (SN), water content, chlorine and sulfur content. Properties were determined using reference ASTM methods. Kinematic viscosity was obtained at 40 ºC and 100ºC according to ASTM D7042, using an Anton Parr-SVM 3000 Stabinger Viscosimeter. Density at 15 ºC and 40ºC was determined according to ASTM D4052, using a digital densimeter, Mettler Toledo DM40.

[0075] Total acid number (TAN) was determined by potentiometric titration according to ASTM D664. Titrations were carried out using a solution of alcoholic potassium hydroxide 0.1 M to neutralize the lubricant acidic components. A mixture of toluene, isopropanol and water, 50, 49.5 and 0.5% v / v, respectively, was used as titration solvent. TAN was expressed in milligrams of KOH required to neutralize the acidic constituents per gram of oil.

[0076] Saponification number (SN) was determined by potentiometric titration following ASTM D94. Oil samples were dissolved in a mixture of a KOH alcoholic solution 0.5 M, and butanone 50 mL each. Considering the difficulty of dissolving organic samples such as lubricant oil and some additives in these solvents, it was necessary to add 25 ml of White Spirit solution. The mixture was heated in total reflux for 30 min, and the condenser washed with 50 mL of naphtha to remove any remaining sample. Finally, the titration was performed with HCl 0.5 M. The SN corresponded to the milligrams of KOH required to saponify fatty material present in 1 g of oil.

[0077] Water percentage was determined by coulometric Karl Fischer titration, according to ASTM D6304, using an Metrohm automatic titrator 870 KF Titrino Plus. A sample of oil was dissolved in Aquametric Solvent CM for Karl Fisher and titrated with Hydranal Composite 5. When all the water was titrated, the excess iodine (generated from the Karl Fischer reaction at the anode) was detected by an electrometric end point detector and the titration was terminated. The quantity of water was proportional to the total integrated current according to Faraday’s Law, considering that 1 mole of iodine reacts with 1 mole of water.

[0078] Composition analysis:

[0079] FTIR spectroscopy

[0080] The vibrational analysis of the sample was conducted using Fourier Transform Infrared Spectroscopy (FTIR), by reflectance, employing the non-destructive sampling technique of Attenuated Total Reflectance (ATR). The infrared spectrum of the solid sample at a temperature between 20 and 25ºC was recorded, in the range of 4000-550 cm-1, using a Perkin Elmer Frontier spectrometer (FT NIR / MIR), equipped with an FR-DTGS detector and a KBr beam splitter. The spectra were recorded with a resolution of 4.0 cm-1 with 32 accumulations. A Perkin Elmer sampling accessory, the universal module for ATR (Attenuated Total Reflectance - UATR) with a diamond / ZnSe crystal, was used, applying a constant force of 10 N in all recordings.

[0081] Elemental analysis

[0082] Determination of chlorine and sulfur content was made by wavelength-dispersive X-ray fluorescence spectrometry, according to ISO 15597, using an X-Supreme 8000 EDXRF spectrometer from Oxford Instruments. Determination of other elements (Al, Si, P, Fe, Zn, Pb, etc.) was made by energy-dispersive X-ray fluorescence spectrometry, using a Nex CG Rigaku spectrometer.

[0083] Quantification of total carbon, hydrogen and nitrogen was done using a ECS 8040 CHNSO elemental analyzer from NC Technologies according to the standard test procedures. The technique used for the determination of CHNS / O was based on the quantitative “dynamic flash combustion” method.

[0084] Results and discussion:

[0085] Example Set 1 – Process feasibility and optimization of extraction parameters

[0086] In a first set of experiments, two representative waste lubricant oils were selected to assess feasibility and optimize the extraction conditions. Sample 1 is for an oil mixture unaffected by the addition of KOH while Sample 2 is an oil mixture that has tested positive for saponification (it coagulates) preventing it to be treated by conventional regeneration methods that involve distillation. Both samples were used as received, after steps of dehydration and filtration treatment in the waste treatment facility. Both samples were subjected to supercritical CO₂ extraction in the apparatus described above, with systematic variation of pressure, temperature, flow rate, and contact time. Physicochemical and elemental characterizations were performed prior and after extraction.

[0087] Preliminary screening of experimental conditions

[0088] Four variables were screened for impact on yield, namely temperature, pressure, CO₂ flow rate and extraction time, to determine the most significant input factors in SFE, and the representative screening matrix and yields are presented in Table 1.

[0089] Table 1: Preliminary study of the influence of SFE parameters in extraction yield for Sample 1 and 2

[0090] RunTemperature / ºCPressure / barCO2density / kg / m3Flow rate CO2 / mL / minResidence time / minYield / %14082303.051302.324083317.9033010.434082303.05319414.3440104654.66320323.8540123726.04321228.5640123726.04524223.4740122723.36335530.5840123726.04318419.5950123602.44318021.01040122726.041018061.41140122723.361218068.312*40122723.361218161.1

[0091] Notes: Runs 1 to 11 were performed with Sample 1, Run 12 was performed with Sample 2.

[0092] At 40 °C, increased pressure and flow improved yield, consistent with higher CO₂ density and enhanced solvent renewal. Short residence times reduced yield. A temperature increase from 40 to 50 °C had a minor effect under the tested conditions, therefore 40 °C was retained for sustainability and to limit thermal history. Effectiveness was proved across different composition (Run 12).

[0093] Design of Experiments for SFE

[0094] A 3² full factorial design (Box–Behnken design) was applied with pressure (X1) and CO₂ flow rate (X2) as factors at three levels each, using yield as the response, since those were the main factors identified in preliminary screening tests. The optimization strategy of the extraction process and the statistical analysis was carried out using STATISTICA7 software. The experimental matrix, that included nine runs, each performed in duplicate, and yields for Sample 1 are shown in Table 2.

[0095] Table 2: Experimental points of the Box Behnken design and the experimental yield for Sample 1

[0096] RunFlow rate X1Pressure X2Yield %168028.42611042.03614047.14108041.051011057.061014065.17148048.981411066.491414073.2

[0097] Optimization design:

[0098] Response surface

[0099] The experimental data for the yield of the supercritical fluid extraction for Sample 1 is presented in Table 3. Statistical analysis and response surface regression procedure were applied. The effects of pressure and flow rate on the yield of extraction for WLO are shown on. An increasing pressure and flow rate resulted in a higher extraction yield, thus attaining a higher amount of regenerated oil.

[0100] The mathematical model representing the yield of the regeneration of WLO as a function of the independent variables within the region under investigation was expressed by the following second order polynomial Eq.1.

[0101]

[0102] whereYis the yield of base oils regenerated, andX1,X2are the coded variables for pressure and flow rate, respectively. Regression coefficients were determined by the standard least squares technique

[0103] The effect of treatment variables as the linear, quadratic and interaction terms were tested for adequacy and fitness by analysis of variance (ANOVA). The statistical analysis, based on a 95% confidence level, indicated that the linear and quadratic effects of the two factors were statistically significant (p-values < 0.05) (Table 4). In contrast, interactions between the flow rate (X1) and pressure (X2) did not produce a significant effect on this response (p > 0.05). Pareto chart of effects () also reinforces the observations that interactions between variables do not impact in yield. Still, there is a very good agreement between experimental and predicted values for yield with this model.

[0104] Table 3: Regression coefficients of the predictive model.

[0105] Regression CoefficientStd.Errort Ratiop Valueβ0-82.674610.90526-7.581160.004761β15.91100.989705.972540.009393β11-0.21060.04370-4.819660.017018β21.36720.176337.753840.004462β22-0.00500.00078-6.485810.007441β120.01140.004122.775970.069223

[0106] Note: coefficients in bold are statistically significant.

[0107] Desirability analysis

[0108] To find optimal process conditions of extraction for different types of WLO, the above regression models were used to generate a prediction profiler and maximize the overall desirability function. The response surface plots shown inindicated that optimal conditions are found near the maximum pressure and flow rate conditions.

[0109] anddisplays the prediction profile and desirability function for WLO Sample 1 extraction. The dashed lines show the predicted responses with 95% confidence intervals. The rightmost column presents the desirability function, along with the specified control points. The bottom row illustrates the desirability for each factor. The model predicted optimal operation near the upper tested levels of pressure and flow. The chosen operating window for subsequent work was 14 MPa, ≥14 mL·min⁻¹, and 40 °C, with extraction times of 100 to 200 minutes.

[0110] Validation of predictive models

[0111] To ensure the predicted model was not biased, experimental verification was performed by testing the ability of reproducing some of the experimental conditions obtained in the preliminary tests, within the experimental range. As it is observed in Table 4, the model was able to predict yield quite accurately, with less than 2% deviation values. Additionally, it gives a reasonable prediction of the yield of extraction for Sample 2.

[0112] Table 4: Experimental validation of predictive model.

[0113] RunPressure / barFlow rate CO2 / mL / minPredicted Yield / %ExperimentalYield / %REa)(%)101221061.761.40.4111221267.068.3-1.912*1221267.061.19.7

[0114]

[0115] Sample characterization

[0116] The physicochemical properties and molecular composition of analyzed waste lubricant oil (Sample 1 and 2) and regenerated base oil (Sample 1R and 2R) were determined according to standard ASTM methods. The density, viscosity and viscosity index, total acid number, saponification index and water content of the samples were measured, together with elemental content, and relative composition obtained from IR spectra, being the results presented in Table 5. Table 6 presents the content of elements obtained from EDXRF measurements.

[0117] Physicochemical properties and composition analysis of used and regenerated base oils samples, including technical specifications defined by Portuguese legislation in 2025 [8].

[0118] SampleUnitsTest MethodWLO parametersSamplesSpecifications for base oils obtained from regeneration operation a)for regeneration a)121R2R Density at 15ºCkg / m3ASTM D1298800-10008718778548650.85-0.88Kinematic viscosity at 40ºCcStASTM D44510-100555821.53115-28Kinematic viscosity at 100ºCcStASTM D445 119.59.54.5 Viscosity Index ASTM D2270 197148123126>90Water wt %wt %ASTM D6304<10.410.300.20.2 Total acidmgKOH / gASTM D6640.43 -4.482.262.861.261.11 numberSaponification numbermgKOH / gASTM D940.8 -6421.1044.178.5713.40 Coagulation ASTM D94NegativeNegativePositiveNegativePositive Color ASTM D1500   03 / abr03 / abr<2.5Elemental content        Clppm <200056.0080.0019.00ND S%  0.400.400.30<100 Nppm  0.30.30.30.3 Cppm  74737172 Hppm  15.615.415.515.5 Composition        Parafinicswt %IR Spectra 76787874>60Naphtenicswt %IR Spectra 23222223<30Aromaticswt %IR Spectra 1003<10

[0119] Notes: a) from Portuguese law in force [8]

[0120] Table 6: Concentrations of major and trace elements (ppm) in waste lubricant oil samples before (sample 1 and 2) and after regeneration (sample 1R and 2R), as determined by EDXRF

[0121] ElementUnitsSample 1Sample 2Sample 1RSample 2RMgppm13022316.1NDAlppm11516282.290.7Sippm51.4412033.91540Pppm515503234252Sppm4530452035203320Clppm71.584.640.546.4Kppm44.359.9NDNDCappm14501510NDNDCrppm2.962.861.28NDFeppm89.61095.472.98Cuppm21.52229.117.3Znppm6166761.061.71Moppm62.552.1NDNDSnppm21.72216.915.1Pbppm4.885.181.85ND

[0122] Note: ND - bellow instrument detection limit

[0123] The regenerated oil extract appeared as a brownish-red translucent liquid with a notably lower viscosity compared to the original oil sample. This change in both appearance and viscosity suggests that the regeneration method using supercritical carbon dioxide successfully removed contaminants and high molecular weight compounds, leading to altered physicochemical properties of the oil. The results obtained for Samples 1 and 2 confirm that supercritical CO₂ extraction achieves a broad regeneration effect on waste lubricating oils. Physicochemical properties (Table 5) showed consistent improvements: the total acid number (TAN) was reduced by 30–60%, viscosity was adjusted to values compatible with base oil ranges, and density shifted closer to that of virgin base oils. These changes indicate the effective removal of acidic degradation products and other species formed during oil use.

[0124] Elemental analysis by EDXRF (Table 6) supported this conclusion, demonstrating a sharp decrease in metals and other elements. Sulfur and chlorine levels were reduced by more than 40%, while phosphorus and zinc—typical from anti-wear additives—dropped by over 50%. Transition metals such as iron and copper, which catalyse further oxidation, were almost completely eliminated, >80–90% reduction. These results highlight the ability of the method to selectively remove both additive residues and wear metals in a single step, without chemical pre-treatment.

[0125] Spectroscopic analysis by FTIR confirmed the chemical composition of the samples but also the transformation of the oils with this process. FTIR analysis of hydrocarbon composition (Table 5) showed that both Samples 1 and 2 were predominantly paraffinic, with a significant naphthenic contribution and only a minor aromatic fraction, as evidenced by the weak band near 1600 cm⁻¹. The main features in the 2954–2856 cm⁻¹ and 1460–1376 cm⁻¹ regions confirmed the prevalence of paraffinic structures with CH₂ / CH₃ vibrations, together with a band at 722 cm⁻¹ associated with short-chain paraffins. After regeneration, the overall hydrocarbon distribution remained essentially unchanged, confirming that the process preserves the molecular backbone of the oils. The main compositional change was a slight reduction in the aromatic contribution, consistent with the preferential extraction of polar and aromatic species by supercritical CO₂. Characteristic absorption bands of esters and oxidation products (around 1740 cm⁻¹) decreased markedly after regeneration, as well as signals at 1155 cm-1and 1160 cm-1, from viscosity improvers, such as ZDDP and polymethacrylates, reflecting depletion of additive-derived functionalities. At the same time, the band near 720 cm⁻¹, assigned to paraffinic hydrocarbons, was relatively enriched, indicating that the regenerated oils are chemically closer to virgin mineral base oils. Thus, regeneration led to cleaner oils with lower aromatic and additive content, while maintaining their paraffinic / naphthenic base-oil character.

[0126] The convergence of these three datasets—physicochemical, elemental, and spectroscopic—provides consistent evidence that the process removes polar oxidation products, additives, and metal contaminants, while enriching the hydrocarbon backbone.

[0127] Taken together, the regenerated oils from both non-coagulating and coagulating samples presented yields of 60–70% and properties compatible with reuse as base oils. Compliance with technical specifications as defined by the Portuguese Environment Agency and based on Portuguese legislation in force in 2025[8] was achieved in all key technical parameters except color, which could be corrected by a mild polishing step of clay adsorption followed by filtration. Overall, regeneration efficiencies above 60 % were consistently achieved at 40 °C and 14 MPa, confirming that a single-step supercritical extraction suffices for both non-coagulating and coagulating WLO mixtures.

[0128] Example Set 2 – Validation on multi-source blends (Samples 3–6)

[0129] To validate the robustness of the optimized process, four additional waste lubricant oils (Samples 3–6) of different origin and composition were studied. These included mixtures from automotive and industrial sources, including partially synthetic formulations, high additive loads and moderate water content. This set consists in 4 samples, where Sample 3 and 4 are oil mixtures unaffected by the addition of KOH, and Sample 5 and 6 are oil mixtures that tested positive for saponification. Samples 3 and 4 had been subject to steps of dehydration and filtration treatment while Sample 5 and 6 were used straight from discharge. Such diversity reflects the real feedstocks typically encountered in industrial regeneration, including oils that would normally be excluded from conventional processes due to instability, coagulation, or high impurity levels.

[0130] Regeneration under optimized conditions

[0131] All samples were processed under the optimized conditions defined in Example Set 1, namely 14 MPa, 40 °C, CO₂ flow of 14 mL·min⁻¹, extraction time 180 minutes, without the addition of organic co-solvents. Extraction yields are presented in table 7 and range between 50 and 80%, with higher values observed for non-coagulating oil mixtures and just slightly lower yields for coagulating oil mixtures.

[0132] Table 7: Regeneration yields for supercritical extraction of waste lubricant oil mixtures (samples 3-6)

[0133] CoagulationPre-treatmentYieldSample 3Negativeyes80Sample 4Negativeno64Sample 5Positiveyes50Sample 6Positiveno75

[0134] The method was applied directly to both untreated oils and oils subjected to simple pre-treatment operations, namely filtration and dehydration. The yield differences observed for ester-rich, coagulating oils can be explained by their chemical behaviour during conventional pre-treatments. These samples are more prone to partial hydrolysis of esters during dehydration steps, where the combination of elevated temperature and residual water can lead to the formation of more polar compounds (alcohols and acids). Such products have lower solubility in supercritical CO₂ and are therefore not efficiently recovered during extraction. By avoiding mandatory filtration and dehydration, the present method circumvents this limitation, resulting in higher regeneration yields for ester-rich oils compared to what would be expected after conventional pre-treatment. In addition, the omission of these steps simplifies the process and reduces energy and operational costs, which is particularly advantageous for large-scale industrial deployment.

[0135] Characterization of regenerated oils

[0136] Table 8: Physicochemical properties and composition analysis of used lubricant oil (Samples 3-6) and regenerated base oil (Samples 3R-6R), including technical specifications defined by Portuguese legislation in 2025 [8]

[0137] UnitsMethodSamplesTechnical specificationsa)34563R4R5R6RDensity at 15ºCkg / m3ASTM D12988768768758858578598658630.85-0.87Kinematic viscosity at 40ºCcStASTM D44556.260.336.858.822.423.518.823.515-28Kinematic viscosity at 100ºCcStASTM D44525.811.47.016.24.64.64.04.8Viscosity IndexASTM D2270457.0185.8156.3293.1124.3109.4113.2126.2> 90Water wt %wt %ASTM D63040.81.660.44.230.50.2TANmgKOH / gASTM D6643.903.972.904.080.250.580.510.45CoagulationASTMD94negativenegativepositivepositivenegativenegativepositivepositiveColorASTM D15003.5 -4.53.5-4.53.5 -4.53.5-4.5<2.5Elemental analysisClppm< 500< 500< 500< 325< 325< 325CompositionParaffinicswt %IR Spectra70.869.771.471.975.975.576.276.8> 60Naphthenicswt %IR Spectra19.320.219.719.217.217.316.916.4< 30Aromaticswt %IR Spectra9.910.18.98.96.97.26.96.8< 10

[0138] Table 9: Concentrations of major and trace elements (ppm) in waste lubricant oil samples before (Samples 3-6) and after regeneration (Samples 3R-6R), as determined by EDXRF

[0139] Elementunits33R44R5C5R66RAlppm12971.7116691347526887.1Sippm53.430.0162100.170.04549129.6Pppm487201413160.3569230487178Sppm42702920404028005220350039403530Clppm11343.088.84584.540219178Cappm15005.961000<LOQ1930<LOQ922<LOQFeppm1113.8290.15.684.0<1098.52.77Cuppm21.36.5022.715.321.5189.6313.4Znppm6742.624546.3797105352.70Pbppm3.92—10.822.76-2.41—

[0140] Physicochemical characterization of regenerated samples obtained from the supercritical extraction process showed consistent improvements in acid value, viscosity, and density. TAN decreased significantly, from 3–5 mgKOH / g to below 1 mgKOH / g, confirming removal of oxidation products. Viscosity and density stabilized within SN-100 base oil specifications, regardless of starting condition, and water content was reduced to residual levels, even for the samples that contained water.

[0141] FTIR spectra of the regenerated oils were similar in overall hydrocarbon profile to their contaminated feeds, dominated by paraffinic and naphthenic bands, 2954–2856 cm⁻¹, 1460–1376 cm⁻¹, and 722 cm⁻¹. A weak aromatic band near 1600 cm⁻¹ was observed in the used oils, which diminished slightly after regeneration, indicating partial removal of aromatic and polar compounds. The hydrocarbon backbone in the regenerated samples remained intact, with paraffinic fractions exceeding 75% and aromatics reduced to <10%. Signals attributed to oxidation products 1716 cm⁻¹ decreased substantially, consistent with depletion of additives and degradation products. Importantly, the regeneration process preserved the paraffinic / naphthenic backbone of the oils while selectively removing the more unstable fractions. Spectroscopic analysis also confirmed the removal of oxidation products and additives, especially sulfonates, ZDDP derivatives and succinimides. Elemental analysis by EDXRF corroborated these findings, showing a marked decrease in elements and metals across all samples. Transition metals (Fe, Cu, Pb) were almost eliminated, detergents (Ca, Mg) and Zn / P from ZDDP were drastically reduced, and Mo was fully removed. Sulfur, while only partially reduced ≈40%, decreased consistently, aligning with the scCO₂ selectivity for organosulfur species.

[0142] Taken together, these results confirm that the method provides robust regeneration across diverse oil mixtures, restoring oils to specifications comparable to SN-100 / SN-150 base oils [8] with the sole requirement of colour correction, which remained in the ASTM D-1500 3–4 range and can be corrected by mild post-treatment by clay adsorption. Most importantly, the results highlight the unique advantage of this process for coagulating, ester-rich oils, which conventional methods must segregate. In contrast, direct treatment with scCO₂ not only regenerates these oils effectively but achieves better yields precisely by omitting the filtration and dehydration steps, thereby simplifying operations and lowering industrial energy costs.

[0143] [1]European Parliament, DIRECTIVE 2008 / 98 / EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 19 November 2008 on waste and repealing certain Directives, 2018. http: / data.europa.eu / eli / dir / 2008 / 98 / oj.

[0144] [2]GEIR (Groupement Européen de l’Industrie de la Régénération), An environmental review of waste oils regeneration, 2014. http: / www.geir-rerefining. org / documents / PositionpaperGEIR161104.pdf (accessed May 5, 2023).

[0145] [3]A. Kupareva, P. Mäki-Arvela, D.Y. Murzin, Technology for rerefining used lube oils applied in Europe: A review, Journal of Chemical Technology and Biotechnology 88 (2013). https: / doi.org / 10.1002 / jctb.4137.

[0146] [4]V. Kanokkantapong, W. Kiatkittipong, B. Panyapinyopol, P. Wongsuchoto, P. Pavasant, Used lubricating oil management options based on life cycle thinking, Resour Conserv Recycl 53 (2009). https: / doi.org / 10.1016 / j.resconrec.2009.01.002.

[0147] [5]N. Abdalla, H. Fehrenbach Heidelberg, H. Fehrenbach, Updating the study Ecological and energetic assessment of re-refining waste oils to base oils Substitution of primarily produced base oils including semi-synthetic and synthetic compounds, 2018. www.ifeu.de.

[0148] [6]G. -Gutierrez, P. Klenert, R. Tonini, EUR 31575 EN Environmental and socio-economic sustainability of waste lubricant oil management in the EU, JRC SCIENCE FOR POLICY REPORT - European Comission, 2023. https: / doi.org / 10.2760 / 7597.

[0149] [7]C.T. Pinheiro, V.R. Ascensão, M.S. Reis, M.J. Quina, L.M. Gando-Ferreira, A data-driven approach for the study of coagulation phenomena in waste lubricant oils and its relevance in alkaline regeneration treatments, Science of the Total Environment 599–600 (2017). https: / doi.org / 10.1016 / j.scitotenv.2017.05.124.

[0150] [8]Agência Portuguesa do Ambiente, ESPECIFICAÇÕES TÉCNICAS DOS ÓLEOS USADOS, 2021. (accessed September 21, 2024).

Claims

A method for regenerating waste lubricant oils comprising the following steps:Supplying at least one waste lubricant oil into a high-pressure extraction vessel;Providing carbon dioxide at a temperature between -5 and 0ºC and a pressure between 4.5 and 6.0 Pa;Pressurizing the carbon dioxide at a pressure between 8 and 20 MPa and heating it at a temperature between 31 and 60ºC to obtain supercritical carbon dioxide;Feeding the supercritical carbon dioxide to the high-pressure extraction vessel, wherein the supercritical carbon dioxide is fed via a supply line arranged at the bottom of the vessel;Contacting the at least one waste lubricant oil with the supercritical carbon dioxide at a temperature between 35ºC and 60ºC, at a pressure between 8 and 20 MPa, for a time between 100 and 200 min, to extract at least one regenerated oil;Recovering the at least one regenerated oil and supercritical CO2 to a separator;Depressurizing the separator to separate the supercritical carbon dioxide and obtain at least one regenerated oil.Method according to the previous claims, wherein the extraction flow rate is between 6 to 20 mL / min.Method according to any of the previous claims, wherein the at least one oil is selected from waste engine oils, automotive crankcase oils, transmission oils, gear oils, industrial hydraulic fluids, turbine oils, compressor oils, transformer insulating oils, marine lubricants, metalworking fluids, and mixtures thereof, including multi-source blends of mineral, semi-synthetic, or synthetic lubricants, as well as oils containing ester-based additives or components prone to coagulation under alkaline treatment, or mixtures thereof.Method according to any of the previous claims, wherein the method further comprises a pre-treatment step to the at least one waste lubricant oil selected from dehydration, filtration, distillation.Method according to any of the previous claims, wherein the carbon dioxide has a purity higher than 99%.A high-pressure extraction device suitable to carry out the method disclosed in any of the claims 1 to 4, comprising:A cooling coil immersed in a thermostatic water bath (2), suitable for condensing carbon dioxide to its liquid state prior to pressurization;A cooling pump (3), suitable for delivering liquid carbon dioxide at the required extraction pressure;A heat exchanger (4) suitable for heating the compressed carbon dioxide to supercritical conditions;A high-pressure extraction vessel (5) into which at least one waste lubricant oil is introduced and contacted with supercritical carbon dioxide, the vessel being provided with a first back pressure regulator (6) to maintain the desired operating pressure;A safety valve (7) provided in the high-pressure extraction vessel (5);A second back pressure regulator (8) arranged between the high-pressure extraction vessel (5) and a separator (9), suitable to control the depressurization of an effluent stream;A separator (9) suitable for receiving the mixture of at least one regenerated oil and supercritical carbon dioxide, effecting the phase separation by pressure reduction, thereby recovering the at least one regenerated oil while enabling the carbon dioxide to be vented or recycled.Device according to the previous claim, wherein the high-pressure extraction vessel (5) comprises a packing material.Device according to any of the claims 6 to 7, wherein the packing material is made of glass, or ceramic, or stainless-steel.Device according to any of the claims 6 to 8, wherein the packing material comprises an average diameter between 3 and 10 mm.Device according to any of the claims 6 to 9, wherein the packing material is in the form of beads.

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