Method and system for purifying contaminated lubricating oil

Abstract

The present invention relates to a method for purifying contaminated lubricating oil, including: (1) pre-treating the contaminated lubricating oil to remove a part of contaminants and water to obtain a pre-treated lubricating oil; (2) subjecting the pre-treated lubricating oil to a membrane separation treatment to obtain a first purified lubricating oil and a lubricating oil rich in contaminants; (3) mixing the lubricating oil rich in contaminants with a flocculant to perform a flocculation treatment to obtain a mixture; and (4) separating a flocculate from the mixture to obtain a second purified lubricating oil. The present invention also relates to a system for purifying contaminated lubricating oil including, in sequence, a pre-treatment unit, a membrane separation unit, a flocculation unit and a separation unit.

Description

[0001] METHOD AND SYSTEM FOR PURIFYING CONTAMINATED LUBRICATING OIL

[0002] FIELD OF THE INVENTION

[0003] The present invention relates overall to oil recovery and purification techniques, and specifically to a method and a system for purifying contaminated lubricating oil.

[0004] BACKGROUND OF THE INVENTION

[0005] Lubricating oil plays an important protective role in lubrication, shock absorption, heat dissipation, rust prevention and mechanical life extension, etc. for traffic vehicles, ships and mechanical processing equipment, etc. Due to the influence of working environment of mechanical equipment and the life of lubricating oil itself, after a period of use of oil products, various pollutants increase and its own aging occurs, resulting in the decline of various performance indicators, and even the situation that the application requirements cannot be met happens.

[0006] These contaminants can affect the performance of lubricating oil. For example, hard particles can cause damage to machine equipment or products made therefrom. In high temperature environments, contaminated lubricating oil can form paint films by polycondensation: these viscous particles can clog the pipe and adhere to a friction pair surface, resulting in increased friction and accelerated wear. Water and metal particulate contaminants can induce oxidation reactions, which ultimately leads to the destruction of lubricating oil. Moreover, when the mechanical equipment is lubricated, lubricating oil films are sometimes very thin, such as elastohydrodynamic lubricating films, usually having a thickness of oil films of less than 1 pm. Theoretically, as long as a diameter of the particles in the oil products is larger than a thickness of the oil film, it may bring about negative effects on the friction pair, which cause the phenomena of surface deterioration to the friction pair. At the same time, since the particles in the oil products are very small but have a very large specific surface area, they can act as catalysis for the aging of the oil products, so that the aging of the oil products is accelerated. Therefore, it is of great significance to develop processes for the effective purification of lubricating oil to remove as much as possible of the contaminants in oil products.

[0007] At present, there are mainly the following methods for the purification and treatment of lubricating oil: on-line filtration and dehydration; waste oil treatment carried out using distillation and hydrogenation techniques; flocculation and centrifugation techniques of Alfa Laval, etc.

[0008] On-line filtration and dehydration, however, consume a large quantity of filter cores and require additional labor costs, and they are inefficient. Distillation and hydrogenation techniques require a large amount of stock, similar to a certain working section in the chemical industry, and moreover, a large investment is required. It is difficult to reach a balance point between profits and costs. Flocculation and centrifugation techniques of Alfa Laval can result in the loss of certain additives, causing impacts on the performance of lubricating oil.

[0009] Therefore, there remains a need for an improved method and system for purifying contaminated lubricating oil.

[0010] SUMMARY

[0011] The present invention was made in view of the above-mentioned problems existing in the prior art.

[0012] In a first aspect, the present invention provides a method for purifying contaminated lubricating oil, including the following steps:

[0013] (1) pre-treatment of the contaminated lubricating oil to remove a part of contaminants and water, resulting in a pre-treated lubricating oil, the pre-treatment preferably including one or more of centrifugation, dehydration, natural sedimentation and filtration;

[0014] (2) membrane separation treatment of the pre-treated lubricating oil obtained in step (1), resulting in a first purified lubricating oil and a lubricating oil rich in contaminants;

[0015] (3) mixing of the lubricating oil rich in contaminants obtained in step (2) with a flocculant for a flocculation treatment, resulting in a mixture; and

[0016] (4) separation of a flocculate from the mixture obtained in step (3), resulting in a second purified lubricating oil.

[0017] In a second aspect, the present invention provides a system for purifying contaminated lubricating oil, including:

[0018] - a pre-treatment unit used for pre-treating a contaminated lubricating oil to remove a part of contaminants and water to obtain a pre-treated lubricating oil, the pre-treatment unit preferably including one or more of a centrifuge, a dehydration device, a natural sedimentation device and a filtration device;

[0019] - a membrane separation unit downstream of the pre-treatment unit, including a membrane separation device used for a membrane separation treatment of the pre-treated lubricating oil from the pre-treatment unit to produce a first purified lubricating oil and a lubricating oil rich in contaminants, a clean oil tank used for receiving the first purified lubricating oil and a concentrated oil tank used for receiving the lubricating oil rich in contaminants; - a flocculation unit downstream of the membrane separation unit including a storage device used for storing a flocculant, a flocculation device used for performing a flocculation treatment, a first pipe used for supplying the lubricating oil rich in contaminants from the concentrated oil tank to the flocculation device, and a second pipe used for supplying the flocculant from the storage device to the flocculation device, the flocculation device being used for mixing the lubricating oil rich in contaminants from the concentrated oil tank and the flocculant from the storage device supplied to the flocculation device and performing the flocculation treatment; and

[0020] - a separation unit downstream of the flocculation unit used for separating flocculate from the mixture from the flocculation unit to obtain a second purified lubricating oil.

[0021] When a separate flocculation technique is used to purify lubricating oil, particularly high viscosity lubricating oil, the efficiency of the purification process is low due to the slow speed of the gravity sedimentation process. However, when a separate membrane separation technology is used for purifying the lubricating oil, an oil product after the membrane separation treatment is divided into two parts, wherein one part is a purified oil product, and the other part is a "concentrated" oil product with a higher concentration degree of dirtier contaminants, and for this part of "concentrated" oil product, it is still difficult to effectively recycle in the prior art, and therefore it is generally discarded, resulting in the loss of oil liquor yield.

[0022] To this end, the present invention proposes the use of a novel process combining flocculation with membrane separation technology to purify lubricating oil. The applicant found that by using firstly a membrane separation to purify lubricating oil and then treating the "concentrated" oil product obtained by the membrane separation with a flocculation process, one can obtain a purified oil product having a higher cleanliness and have a higher treatment efficiency and capacity even for lubricating oil with a high viscosity, and at the same time, almost no waste oil is discharged during the treatment, so that the yield of the purified oil is significantly increased, and therefore the present invention achieves an optimal balance between treatment efficiency and yield. In some industries where the cost of lubricating oil procurement is high, the purification method combining flocculation with membrane separation technology according to the present invention has a very broad application prospect.

[0023] In addition, it is difficult to purify the "concentrated" oil product obtained by membrane separation with common filtration or membrane separation due to the high degree of concentration of contaminants. The applicant has further found that for this part of the "concentrated" oil product obtained by membrane separation, one can obtain an oil with improved cleanliness and yield compared to using other techniques (e.g. filtration or membrane separation techniques) or using other flocculants, when the flocculation treatment is carried out using a flocculant including an alkylene glycol, in particular ethylene glycol.

[0024] BRIEF DESCRIPTION OF THE DRAWING

[0025] In order to more clearly illustrate the technical solutions of the present invention, a brief description will be given below of the drawing required for the description of embodiments. It should be understood that this drawing is only for the purpose of facilitating an easier understanding of the present invention by the skilled person and is not intended to limit the scope of the present invention.

[0026] FIG. 1 schematically illustrates a method and a system for purifying contaminated lubricating oil according to an exemplary implementation of the present invention.

[0027] DETAILED DESCRIPTION

[0028] In order to make the invention objects, technical solutions and advantageous effects of the present application more clear, the detailed description for the present invention will be made below. It should be noted that the various aspects, features, implementations, and advantages described in the present application can be compatible and / or can be combined together.

[0029] Unless otherwise defined, the meaning of technical terms in the present description is same as that commonly understood by one skilled in the art.

[0030] The present invention relates to a method and a system for purifying contaminated lubricating oil.

[0031] A detailed description for the present invention will be made below.

[0032] Method for Purifying Contaminated Lubricating oil

[0033] A method for purifying contaminated lubricating oil according to a first aspect of the present invention includes in sequence, a pre-treatment step, a membrane separation step, a flocculation step and a separation step. In particular, the present invention relates to a method for purifying contaminated lubricating oil, including the following steps :

[0034] (1) pre-treatment of the contaminated lubricating oil to remove a part of contaminants and water, resulting in a pre-treated lubricating oil, the pre-treatment preferably including one or more of centrifugation, dehydration, natural sedimentation and filtration;

[0035] (2) membrane separation treatment of the pre-treated lubricating oil obtained in step (1), resulting in a first purified lubricating oil and a lubricating oil rich in contaminants; (3) mixing of the lubricating oil rich in contaminants obtained in step (2) with a flocculant for a flocculation treatment, resulting in a mixture; and

[0036] (4) separation of a flocculate from the mixture obtained in step (3), resulting in a second purified lubricating oil.

[0037] A detailed description for each step will be made below.

[0038] Step (1)

[0039] In step (1), the contaminated lubricating oil is pre-treated to remove a part of contaminants and water to obtain a pre-treated lubricating oil. This pre-treatment step can remove larger particles and most of the water in the contaminated oil product, reducing the load on subsequent membrane separation and flocculation process.

[0040] In one or more implementations, the pre-treatment includes subjecting the contaminated lubricating oil to one or more treatments selected from: centrifugation, dehydration, natural sedimentation and filtration.

[0041] In a preferable implementation, the pre-treatment includes an implementation by filtration through a metal sintered filter core (coarse filtration) of the contaminated lubricating oil.

[0042] In a preferable implementation, the pre-treatment includes an implementation by centrifugation and dehydration of the contaminated lubricating oil, and preferably the pre-treatment includes an implementation by centrifugation and vacuum dehydration of the contaminated lubricating oil.

[0043] In principle, there is no particular restriction for the method according to the present invention with respect to the type and source of the contaminated lubricating oil, which can be selected by a person skilled in the art according to actual needs. For example, the contaminated lubricating oil may be selected from one or more of super essential oils, hydraulic oils, refrigeration compressor oils, gear oils, lithium battery separator white oils, vacuum pump fluorine oils, quenching oils and metal rolling oils, and in particular it may be super essential oils.

[0044] The method according to the present invention can be used to treat lubricating oil with a wide viscosity range, and in particular can be suitable for treating lubricating oil with high viscosity. In particular, the kinematic viscosity of the contaminated lubricating oil at 40°C may be 2-10 cst, for example 2, 3, 4, 5, 6, 7, 8, 9, 10 cst or a range defined by any two of the aforementioned values.

[0045] In one or more implementations, the lubricating oil in the contaminated lubricating oil is a super essential oil having a kinematic viscosity of 2-10 cst, for example, 4-5 cst at 40°C. The contaminated lubricating oil may comprise various possible contaminant particles. For example, the contaminants in the contaminated lubricating oil may include one or more selected from dust, gravel, metal and rust residue.

[0046] This pre-treatment step may remove a part of the contaminants comprised in the contaminated lubricating oil, and in particular may remove up to 95 wt.%, for example up to 90 wt.%, up to 85 wt.%, up to 80 wt.%, up to 75 wt.%, up to 70 wt.%, up to 65 wt.%, up to 60 wt.%, up to 55 wt.%, up to 50 wt.%, up to 45 wt.%, up to 40 wt.%, up to 35 wt.%, or up to 30 wt.% of the contaminants comprised in the contaminated lubricating oil. This pre-treatment step may remove contaminant particles having a larger size, for example contaminant particles having a diameter greater than 4 pm, particularly contaminant particles having a diameter greater than 6 pm, more particularly contaminant particles having a diameter greater than 14 pm.

[0047] This pre-treatment step may remove a part of the water comprised in the contaminated lubricating oil, and in particular may remove up to 95 wt.%, for example up to 90 wt.%, up to 85 wt.%, up to 80 wt.%, up to 75 wt.%, up to 70 wt.%, up to 65 wt.%, up to 60 wt.%, up to 55 wt.%, up to 50 wt.%, up to 45 wt.%, up to 40 wt.%, up to 35 wt.%, or up to 30 wt.% of the water comprised in the contaminated lubricating oil.

[0048] Step (2)

[0049] In step (2), the pre-treated lubricating oil obtained in step (1) is subjected to a membrane separation treatment to obtain a first purified lubricating oil and a lubricating oil rich in contaminants. This membrane separation step can be performed continuously. Through this membrane separation step, most of the lubricating oil is purified, and only a small part of the lubricating oil rich in contaminants is left, which ensures a high purification efficiency.

[0050] In principle, there is no particular restriction for the method according to the present invention with regard to the membrane used in the membrane separation treatment, and a person skilled in the art can select any membrane known to be useful in the oil product separation treatment according to actual needs. For example, the membrane used for the membrane separation treatment may be selected from one or more of ceramic membrane, silicon carbide membrane and graphene membrane, preferably graphene membrane.

[0051] The membrane separation treatment may be carried out at a temperature of 20°C-80°C, preferably 25°C-50°C. For example, the temperature for membrane separation treatment can be of 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, or a range defined by any two of the aforementioned values. The membrane separation treatment may be carried out at an operating pressure of 0.5 -6.0 kg / cm2, preferably 1.0-3.0 kg / cm2. For example, the operating pressure for membrane separation treatment can be of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0 kg / cm2, or a range defined by any two of the aforementioned values.

[0052] A higher cleanliness level can be achieved for the first purified lubricating oil. In particular, as regards waste oils having a cleanliness level according to ISO 4406 of 27 / 26 / 21, the first purified lubricating oil may achieve a cleanliness level according to ISO 4406 of 15 / 12 / 10, for example.

[0053] Step (3)

[0054] In step (3), the lubricating oil rich in contaminants obtained in step (2) is mixed with a flocculant to perform a flocculation treatment to obtain a mixture.

[0055] The flocculant may include one or more selected from polyalkylene glycols, polyacrylamides and polyacrylic acids. In a preferable implementation, the flocculant may include a polyalkylene glycol such as polyethylene glycol, polypropylene glycol, etc., preferably polyethylene glycol.

[0056] In step (3), a mixing mass ratio of the lubricating oil rich in contaminants to the flocculant may be 500: 1-1000:1, for example, 500:1, 600: 1, 700:1, 800:1, 900:1, 1000: 1 or a range defined by any two of the aforementioned values.

[0057] The flocculation treatment may include a sedimentation after mixing of the lubricating oil rich in contaminants obtained in step (2) with the flocculant. In one or more implementations, the sedimentation time is 1-30 hours, more preferably 3-20 hours. For example, the sedimentation time may be 1, 5, 10, 15, 20, 25, 30 hours or a range defined by any two of the aforementioned values. The applicant found that the method according to the present invention can considerably reduce the treatment time, in particular the sedimentation time required in the flocculation process, by using a combination of membrane separation and flocculation techniques. However, if the contaminated lubricating oil is treated only by a separate flocculation process (i.e. without prior membrane separation treatment), typically several multiples of more time is required as compared with the present invention for the same amount of treatment.

[0058] The present invention first uses pre-treatment and membrane separation techniques to purify lubricating oil, and then performs flocculation treatment on the lubricating oil rich in contaminants obtained from membrane separation. As most of the contaminated lubricating oil has been purified through membrane separation technology, the amount of lubricating oil rich in contaminants that requires flocculation treatment is significantly reduced in the flocculation process in the method according to the present invention, which greatly shortens the required sedimentation time as compared with using a separate flocculation process to purify the lubricating oil. Therefore, even treatment of lubricating oil with a high viscosity can be completed in a shorter time. In addition, use of the flocculation process in the method according to the present invention for the treatment of the concentrated oil obtained from membrane separation can recover the oil liquor in the concentrated oil product, which increases the overall oil liquor yield. In particular a purified oil liquor yield of at least 97% can be achieved by adopting the purification method according to the present invention, and the flocculation process can capture particles of almost all sizes, in particular particles of submicron and nano-scale in the concentrated liquor, thereby resulting in a purified oil product with an excellent cleanliness level.

[0059] Step (4)

[0060] In step (4), a flocculate is separated from the mixture obtained in step (3) to obtain a second purified lubricating oil.

[0061] In principle, there is no particular restriction with regard to the specific separation means, and a person skilled in the art can select any separation means that can be used for separating flocculates.

[0062] In one or more implementations, in step (4), said separation includes a filtration of the mixture obtained in step (3), for example, one or more selected from cellulose filtration, filter core filtration, electrostatic filtration and centrifugal filtration. In one or more implementations, in step (4), said separation includes cellulose filtration.

[0063] Optional Recycling Step

[0064] The second purified lubricating oil obtained in step (4) may also be recycled. In particular, the method for purifying contaminated lubricating oil according to the present invention may also include recycling the second purified lubricating oil into step (1) (i.e. the pre-treatment step) to perform a secondary purification for the second purified lubricating oil. This recycling process can further improve the cleanliness of the resulting purified oil product, and contribute to improved yield and purification efficiency.

[0065] System for Purifying Contaminated Lubricating oil

[0066] A system for purifying contaminated lubricating oil according to the second aspect of the present invention includes, in sequence, a pre-treatment unit, a membrane separation unit, a flocculation unit and a separation unit. Specifically, the system for purifying contaminated lubricating oil according to the present invention includes: - a pre-treatment unit used for pre-treating a contaminated lubricating oil to remove a part of contaminants and water to obtain a pre-treated lubricating oil, the pre-treatment unit preferably including one or more selected from a centrifuge, a dehydration device, a natural sedimentation device and a filtration device;

[0067] - a membrane separation unit downstream of the pre-treatment unit, including a membrane separation device used for subjecting the pre-treated lubricating oil from the pre-treatment unit to a membrane separation treatment to produce a first purified lubricating oil and a lubricating oil rich in contaminants, a clean oil tank used for receiving the first purified lubricating oil and a concentrated oil tank used for receiving the lubricating oil rich in contaminants;

[0068] - a flocculation unit downstream of the membrane separation unit including a storage device used for storing a flocculant, a flocculation device used for performing a flocculation treatment, a first pipe used for supplying the lubricating oil rich in contaminants from the concentrated oil tank to the flocculation device, and a second pipe used for supplying the flocculant from the storage device to the flocculation device, the flocculation device being used for mixing the lubricating oil rich in contaminants from the concentrated oil tank and the flocculant from the storage device supplied to the flocculation device and performing the flocculation treatment; and

[0069] - a separation unit downstream of the flocculation unit used for separating flocculate from the mixture from the flocculation unit to obtain a second purified lubricating oil.

[0070] Each unit will be described in detail below.

[0071] Pre-treatment Unit

[0072] The pre-treatment unit is used for pre-treating a contaminated lubricating oil to remove a part of contaminants and water to obtain a pre-treated lubricating oil. This pre-treatment unit can remove large particles and most of the water in the contaminated oil product, reducing the load on subsequent membrane separation and flocculation process.

[0073] In one or more implementations, the pre-treatment unit includes one or more of the following devices: a centrifuge, a dehydration device, a natural sedimentation device and a filtration device. In a preferable implementation, the pre-treatment unit includes a filtration (coarse filtration) device with a metal sintered filter core. In a preferable implementation, the pre-treatment unit includes a combination of the centrifuge and the dehydration device, preferably a combination of the centrifuge and a vacuum dehydration device.

[0074] Membrane separation Unit The membrane separation unit, configured to perform a membrane separation treatment on the pre-treated lubricating oil, is disposed downstream of the pre-treatment unit.

[0075] Said membrane separation unit may include a membrane separation device used for subjecting the pre-treated lubricating oil from the pre-treatment unit to a membrane separation treatment to produce a first purified lubricating oil and a lubricating oil rich in contaminants, a clean oil tank used for receiving the first purified lubricating oil and a concentrated oil tank used for receiving the lubricating oil rich in contaminants.

[0076] In one or more implementations, the concentrated oil tank may have a stirring device used for stirring the lubricating oil rich in contaminants uniformly so as to facilitate transportation.

[0077] Flocculation Unit

[0078] The flocculation unit, configured to perform a flocculation treatment for the lubricating oil rich in contaminants from the concentrated oil tank, is disposed downstream of the membrane separation unit.

[0079] The flocculation unit may include a storage device used for storing a flocculant, a flocculation device used for performing a flocculation treatment, a first pipe used for supplying the lubricating oil rich in contaminants from the concentrated oil tank to the flocculation device, and a second pipe used for supplying the flocculant from the storage device to the flocculation device. The flocculation device may be used for mixing the lubricating oil rich in contaminants from the concentrated oil tank and the flocculant from the storage device supplied to the flocculation device and performing the flocculation treatment.

[0080] Separation Unit

[0081] The separation unit, configured for separating flocculate from the mixture from the flocculation unit to obtain a second purified lubricating oil, is disposed downstream of the flocculation unit.

[0082] In one or more implementations, the separation unit can include a filtration device. In one or more implementations, the separation unit can include a cellulose filtration device.

[0083] In addition to each unit described above, the system for purifying contaminated lubricating oil according to the present invention may also include a pipe downstream of the separation unit used for recycling the second purified lubricating oil from the separation unit to the pre-treatment unit. A method and a system for purifying contaminated lubricating oil according to an exemplary implementation of the present invention will be described below with reference to FIG. 1.

[0084] In FIG. 1, a contaminated lubricating oil to be treated is first transported from a dirty oil tank 1 to a pre-treatment device 3 by means of a pump 2. The pre-treatment unit 3 may be a combination of a centrifuge and a vacuum dehydration device, which is mainly used to remove large particles and most of the water from the dirty oil. The pre-treated lubricating oil is then supplied to a membrane separation unit 4 for treatment, resulting in two fractions of oil, wherein clean oil liquor is supplied to a clean oil tank 5 and another fraction of oil liquor rich in contaminants is supplied to a concentrated oil tank 6. The concentrated oil tank 6 may have a stirring paddle, which uniformly stirs the oil liquor rich in contaminants for transportation. Next, on the one hand, the oil liquor rich in contaminants is transported from the concentrated oil tank 6 to the flocculation device 10 by means of a pump 7. On the other hand, a flocculant stored in the storage device 8 is transported to the flocculation device 10 by means of the pump 9. A metering pump can be used to accurately transport the flocculant. In the flocculation device 10, a flocculant is dispersed in the oil liquor rich in contaminants to perform flocculation and sedimentation treatment on this oil liquor, resulting in a flocculated mixture. Finally, the resulting mixture is transported into a separation device 11 to separate the flocculate, resulting in a clean oil liquor.

[0085] Examples

[0086] In order to make the objects, technical solutions and advantages of the present invention more clearly understood, the present invention will be described in further detail below in combination with examples. It should be understood that the specific examples described herein are used to only illustrate the present invention but not used to limit the present invention.

[0087] Hereinafter, the contents or concentrations mentioned are both expressed as mass unless indicated otherwise.

[0088] Materials Used

[0089] The materials used in each of the following embodiments are as follows:

[0090] Raw waste oil to be treated: a super essential oil having a kinematic viscosity of 4.2 cst at 40°C and having the following cleanliness:

[0091] Membrane for membrane separation treatment: graphene membrane (purchased from Weihai Suotong Energy Saving Technology Co. Ltd.);

[0092] Flocculant used: the product ORLOl purchased from SKF, including polyethylene glycol.

[0093] Process of Purification Treatment

[0094] A total weight of 9.6 kg of the raw waste oil to be treated was supplied to a coarse filter screen of 50 microns for filtration to perform a pre-treatment. The coarse filtered oil was then supplied to a membrane separation device (operating temperature: 25°C, operating pressure: 1 bar) was subjected to a membrane separation treatment for 0.5 hour, resulting in a purified oil (8.4 kg) and a concentrated liquor (1.2 kg) containing a large amount of impurities. The concentrated liquor was transported to a dirty oil tank, the purified oil obtained from purification by membrane separation was transferred to a clean oil tank, then the concentrated liquor was transported from the dirty oil tank to a flocculation device in a DST experimental apparatus (DST Proof of Concept (POC), serial No. : 1010-1990 obtained from SKF), and the flocculant was transported from the storage tank to the flocculation device so that the concentrated liquor was mixed with the flocculant in a mass ratio of 1000: 1 in the flocculation device, flocculation reaction was carried out for 1 hour and sedimentation was carried out for 4 hours. Then the mixture resulting from the flocculation treatment was transported to the separation device of the above-mentioned DST experimental apparatus to separate the flocculate (operating temperature: 25°C, operating pressure: 0.1 bar). Finally, 0.98 kg of oil was recovered and returned to the pre-treatment process for cyclic purification, and the single yield of the obtained purified oil=l-(1.2-0.98) / 9.6=97.71%. The cleanliness levels according to ISO 4406 of the oil at each stage are shown in Table 1 below:

[0095] Table 1 Cleanliness Level of Oil at Each Stages in the Purification Process As can be seen from the above table, the concentration of contaminants of various particle sizes in the concentrated liquor obtained by the membrane separation treatment increased sharply with respect to the untreated oil and the pre-treated oil. Such a concentrated liquor is considered to be of no recycling value in the prior art, and thus is usually discarded directly. However, as can be seen from the above table, the cleanliness level of the purified oil obtained by subjecting such a concentrated liquor to a flocculation treatment has reached a level which can be relatively close to that of the purified oil obtained by the membrane separation treatment. Its cleanliness level can be further increased by recycling the cleaned oil obtained by the flocculation treatment to the pretreatment step.

[0096] From the above results, it can be seen that the method according to the present invention results in a clean oil product having a higher cleanliness level by using a combination of the membrane separation and flocculation process, and from an overall viewpoint of yield and treatment time, the method according to the present invention exerts the respective advantages of the two processes as compared to a separate membrane separation treatment or a separate flocculation treatment, which achieves an optimal balance of both yield and purification efficiency.

Claims

CLAIMS1. A method for purifying contaminated lubricating oil, including the following steps:(1) pre-treatment of the contaminated lubricating oil to remove a part of contaminants and water, resulting in a pre-treated lubricating oil, the pre-treatment preferably including one or more of centrifugation, dehydration, natural sedimentation and filtration;(2) membrane separation treatment of the pre-treated lubricating oil obtained in step (1), resulting in a first purified lubricating oil and a lubricating oil rich in contaminants;(3) mixing of the lubricating oil rich in contaminants obtained in step (2) with a flocculant for a flocculation treatment, resulting in a mixture; and(4) separation of a flocculate from the mixture obtained in step (3), resulting in a second purified lubricating oil.

2. The method according to claim 1, also including recycling the second purified lubricating oil to the step (1) to perform a secondary purification for the second purified lubricating oil.

3. The method according to claim 1 or 2, wherein the membrane separation treatment is carried out at a temperature of 20°C-80°C, preferably 25°C-50°C, and / or the membrane separation treatment is carried out at an operating pressure of 0.5-6.0 kg / cm2, preferably 1.0-3.0 kg / cm2.

4. The method according to claim 1 or 2, wherein the flocculant includes a polyalkylene glycol, preferably polyethylene glycol.

5. The method according to claim 1 or 2, wherein in the step (3), a mixing mass ratio of the lubricating oil rich in contaminants to the flocculant is 500:1-1000: 1.

6. The method according to claim 1 or 2, wherein the flocculation treatment includes a sedimentation after mixing the lubricating oil rich in contaminants obtained in step (2) with the flocculant, preferably for a sedimentation time of 1-30 hours, more preferably 3-20 hours.

7. The method according to claim 1 or 2, wherein the contaminants in the contaminated lubricating oil include one or more selected from dust, gravel, metal and rust residue.

8. The method according to claim 1 or 2, wherein the lubricating oil in the contaminated lubricating oil is a super essential oil, for example, having a kinematic viscosity at 40°C of 2-10 cst.

9. A system for purifying contaminated lubricating oil, including:- a pre-treatment unit used for pre-treating a contaminated lubricating oil to remove a part of contaminants and water to obtain a pre-treated lubricating oil, the pre-treatment unit preferably including one or more of a centrifuge, a dehydration device, a natural sedimentation device and a filtration device;- a membrane separation unit downstream of the pre-treatment unit, including a membrane separation device (4) used for subjecting the pre-treated lubricating oil from the pre-treatment unit to a membrane separation treatment to produce a first purified lubricating oil and a lubricating oil rich in contaminants, a clean oil tank (5) used for receiving the first purified lubricating oil and a concentrated oil tank (6) used for receiving the lubricating oil rich in contaminants;- a flocculation unit downstream of the membrane separation unit, including a storage device (8) used for storing a flocculant, a flocculation device (10) used for performing a flocculation treatment, a first pipe used for supplying the lubricating oil rich in contaminants from the concentrated oil tank (6) to the flocculation device (10), and a second pipe used for supplying the flocculant from the storage device (8) to the flocculation device (10), the flocculation device (10) being used for mixing the lubricating oil rich in contaminants from the concentrated oil tank (6) and the flocculant from the storage device (8) supplied to the flocculation device (10) and performing the flocculation treatment; and- a separation unit downstream of the flocculation unit, used for separating flocculate from the mixture from the flocculation unit to obtain a second purified lubricating oil.

10. The system according to claim 9, wherein the system also includes a pipe downstream of the separation unit for recycling the second purified lubricating oil from the separation unit to the pre-treatment unit.

Images