FIELD OF INVENTION

The invention relates to a method and a system for purifying used oil, and in particular, to such method and system using nanofiltration and oil polishing steps.

BACKGROUND TO THE INVENTION

The following discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was published, known or part of the common general knowledge in any jurisdiction as at the priority date of the application.

Used oil generally includes all kinds of synthetic and mineral oil such as crude oil, spent engine oil, hydraulic oil, and cooking oil. Used oil is often a mixture of oil products collected from various sources and it may contain contaminants such as carbonaceous deposits and particles resulting from, say, bad combustion of motor fuels; metal contaminants brought in by, say, wear and corrosion and external dust; water from engines and storage; and spent additives that are added to, say, improve the anti-knock properties or improve the viscosity of the oil. Lubricating oil (or lube oil, for short) typically contains approximately 90% base oil and 10% additives. The base oil is mainly mineral oil derived from petroleum fractions or crude oil. The additives are mainly added to the lube oil to impart desirable characteristics.

If the contaminants present in the used oil are separated from the used oil, the resultant oil is lube base oil, which is the original base component of lube oil.

Used oil may pose a threat to the environment and may have negative impacts on the eco-system and our health if it is not handled or disposed of properly. For example, during the regular maintenance of industrial machinery or regular servicing of automobiles where an oil-change is performed, the used lube oil, when improperly disposed of, can adversely impact fish and plant life, and polluting the air that we breathe. It is estimated that hundreds of millions of gallons of waste oil enter the ocean annually.

The direct burning of used oil without any tight emission control releases toxic chemicals and metals content present in the used lube oil into the atmosphere. Although acid clay treatment of used oil had earlier been used to provide emission control, in the more recent years, its use has been restricted in many countries as it is a cause of secondary pollution to underground water and generation of acid sludge. On the other hand, used oil may become a valuable resource if it is recovered and purified or refined properly. Average crude oil has about 3 to 8% lube content and about 60 to 80% lube content can be recovered from used automotive oil. If the used automotive oil is burned off or discharged into the ocean without adequate recovery of the lube content, this would translate to a hefty loss of a valuable natural resource. Worldwide lubricant demand is expected to reach 41.7 million metric tons by 2010 (according to a report by Freedonia Group Inc. which does market research and trends). The demand growth is expected to be driven by the rapidly increasing rate of motor vehicle ownership and rapid growth in worldwide manufacturing, which lead to a sharp boost in demand for industrial lubricants such as process oils and hydraulic fluids. Based on an average content of 5% lube content in crude oil and further based on this reported worldwide demand, 834 million metric tons of crude oil will have to be recovered and refined in 2010 in order to meet the worldwide demand for lubricant. If the lube content could be successfully recovered from used lubricant oil, and with a possible 60% recovery in spent oil, the world would have saved 480 million metric tons of crude oil per year.

The scarcity of fossil fuel resources is alarming the world as the demand increases. Due to the depletion of the natural resources, the international crude oil price climbs steadily upwards. Recycling of used oil into usable lube base oil in an environmental friendly and cost effective manner has become a sensible and economical option. Many of the reported existing used oil treatment techniques do not purify or treat the used oil back to its original base oil condition. Such reported techniques include centrifuge, electro-static pre-treatment, vacuum evaporation, mechanical filtration, decantation, adding absorbents, adding additives, adding catalyst, and solvent extractions. For example, existing filtration techniques are used to filter some contaminants in the oil, mostly only solid particle. When the oil filter gets clogged, the filter media is losing its integrity which allows dirt on the filter surface to migrate through the filter. Oil purification, on the other hand, is designed to completely remove all contamination and bring the dirty oil back to new oil condition. Oil purification will alter the physical and chemical changes in the oil without depleting the original oil additives. The process which involves extensive refinery distillation could possibly extract base oil components from used oil but this process is too expensive and cumbersome to make it commercially attractive.

Therefore, it is desirable to provide a method and a system for purifying used oil that overcomes, or at least alleviates, the above problems.

SUMMARY OF THE INVENTION

Throughout this document, unless otherwise indicated to the contrary, the terms "comprising", "consisting of, and the like, are to be construed as non-exhaustive, or in other words, as meaning "including, but not limited to".

In a first aspect of the present invention, there is provided a method for purifying used oil. The method comprises passing a feed containing used oil through an organophilic nanofiltration membrane thereby allowing molecules having molecular weight lower than 1 ,000g/mol present in the used oil to filter through, and polishing the filtered used oil to remove colouring of the used oil to thereby obtain purified used oil. High molecular weight impurities are separated from the used oil with the use of nanofiltration membrane technology. Transmembrane pressure is applied across the membrane to allow the separation.

Advantageously, the organophilic nanofiltration membrane allows molecules having molecular weight lower than 800g/mol, and preferably, 600g/moi to filter through. Advantageously, water and/or solid particles having size larger than 600nm, preferably 1 ,000nm, and preferably 10pm, and preferably 30pm, and preferably Ι ΟΟμm, present in the feed are removed prior to the passing step.

Preferably, the removing step comprises centrifuging. Preferably, the organophilic nanofiltration membrane comprises a composite membrane composed of a dense material and a porous support material.

Preferably, the thickness of the dense material is between 1 and 30pm. More preferably, the thickness of the dense material is between 1 and 5pm.

Preferably, the dense material is a polysiloxane. More preferably, the dense material is polydimethylsiloxane.

Preferably, the porous support material is selected from the group consisting of polyacrylonitrile, polyamideimide and titania, polyetherimide,

polyvinylidenedifluoride, and polytetrafluoroethylene.

Preferably, the organophilic nanofiltration membrane has a molecular weight cut-off of between 200 and 2,000Dalton.

Preferably, the organophilic nanofiltration membrane has pore size between 0.5 and 2nm.

Preferably, the organophilic nanofiltration membrane is heated up to below 90°C, and more preferably, 80°C. Preferably, the organophilic nanofiltration membrane is subjected to a pressure difference of 25 to 40bars, and more preferably, 25 to 30bars. Upstream of the organophilic nanofiltration membrane is subjected to atmospheric pressure.

The polishing step comprises exposing the filtered used oil to absorption medium to remove colouring of the used oil. Advantageously, the absorption medium stabilises oxidation of the used oil.

Preferably, the absorption medium comprises fullers earth column.

Preferably, the fullers earth column is heated up to 80°C. Preferably, the fullers earth column is heated by hot air drawn into the fullers earth column.

Preferably, the fullers earth column is subjected to a pressure difference of 10 to 15psi. In another aspect of the present invention, there is provided system for purifying used oil. The system comprises a membrane separation unit comprising a organophilic nanofiltration membrane capable of allowing molecules having molecular weight lower than 1 ,000g/mol present in a feed containing used oil to filter through and an oil polishing unit to remove colouring of the filtered used oil to thereby obtain purified used oil.

Advantageously, the system further comprises a centrifuge position upstream of the membrane separation unit to remove water and solid particles having size larger than 600nm.

Preferably, the oil polishing unit comprises fullers earth columns to absorb and remove the colouring of used oil.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures, which illustrate, by way of example only, embodiments of the present invention, FIG. 1 is a schematic setup of the purification system in accordance with one aspect of the present invention;

FIG. 2 shows samples of the feed source, the permeate before oil polishing and the polished oil obtained from the system of FIG. 1 ;

FIG. 3 is a flow diagram of the purification system, detailing the overall pre- treatment, nanofiltration and oil polishing steps of FIG. 1 ;

FIG. 4 is a flow diagram of the purification system, detailing the pre-treatment and nanofiltration steps of FIG. 1 ; and FIG. 5 is a flow diagram of the purification system, detailing the oil polishing step of FIG. 1.

DETAILED DESCRIPTION

The invention relates to a method and a system for purifying used oil, and in particular, to such method and system using nanofiltration and oil polishing steps. By purification, it is meant to treat and restore used oil back to its original lube oil condition.

There are several processes available for treating used oil. Table 1 below compares the general characteristics of various types of processes. Clearly, the membrane separation process possesses numerous strengths over other processes.

In accordance with a first aspect of the invention, there is provided a method for purifying used oil. The method comprises passing a feed containing used oil through an organophilic nanofiltration membrane thereby allowing molecules having molecular weight lower than 1 ,000g/mol present in the used oil to filter through, and polishing the filtered used oil to obtain purified used oil. High molecular weight impurities are separated from the used oil with the use of nanofiltration membrane technology. Transmembrane pressure is applied across the membrane to allow the separation. With this method, the impurities content is greatly reduced in the permeate. Advantageously, prior to passing the feed through the membrane, the feed is pre-treated to remove or separate solid particles present in the used oil.

Preferably, a high-speed centrifuge is used in the pre-treatment step. This reduces the viscosity as well as the content of the impurities, thereby achieving higher permeate flux and better separation of the used oil from the contaminants present in the feed. It will be apparent to a person skilled in the art that the size of the solid particles removed or separated depends on the strength of the centrifugal force applied. Typically, macromolecules and nanoparticles are not removed. Other means for separating solid particles may also be used in the pre-treatment step.

Preferably, the organophilic nanofiltration membrane is a composite membrane that allows organic molecules having molecular weight of approximately 600 to 1 ,000g/mol to pass through. Molecules having molecular weight greater than 1.OOOg/mol are preferentially retained and trapped by the membrane. The membranes preferably possess a molecular weight cut-off of between 200 and 2,000DaIton. More preferably, the membrane comprises a support material which is usually porous, and a separating layer which is dense. The membrane is positioned in a membrane separation unit such that the feed first permeates through the dense separating layer and subsequently through the porous support material. The dense separating layer is the actual separating layer that retains the undesirable high molecular weight molecules of the feed. The organophilic nanofiltration membrane typically has a membrane pore size between 0.5 and 2nm. The separation process is a combination of size exclusion and diffusion. Due to their size and complex structure, the high molecular weight molecules in the feed are not able to dissolve in the dense separating layer and diffuse through the membrane, i.e. trapped or retained by the dense separating layer. The dense separating layer is preferentially composed of a cross-linked polymeric structure. The free volume of the polymer defines the transport of compounds through the separating layer and directly corresponds with the molecular cut-off of the membrane.

Additionally, the membrane has to withstand pressures up to 40 bars and temperatures up to 80°C.

The dense separating layer is organophilic. In other words, the dense

separating layer preferentially allows organic molecules to diffuse through it.

Preferably, the dense separating layer comprises a polysiloxane or a

polydimethylsiloxane (PDMS for short). Other organophilic material may also be used as the dense separating layer. The porous support may be any suitable material that provides sufficient mechanical strength to the overall membrane structure. For example, the porous support material may be selected from the group consisting of polyacrylonitrile, polyamideimide and titania, polyetherimide, polyvinylidenedifluoride, and polytetrafluoroethylene.

To ensure a high flux of the permeate, the thickness of the dense separating layer should be as thin as possible. Preferably, the thickness is in the range between 1 and 30pm. More preferably, the thickness of the dense separating layer is in the range between 1 and 5pm.

The oil polishing step serves to remove the colouring of the filtered used oil.

Advantageously, this step helps to stabilise oxidation of the used oil and lighten the colour of the used oil. Oil oxidation is undesirable and results in catastrophic and permanent chemical changes to the base oil molecules. Briefly, the oxidation reaction results in the sequential addition of oxygen to the base oil molecules to form a number of different chemicals species, including aldehydes, ketones, hydroperoxides and carboxylic acids. The rate at which base oil molecules react with oxygen depends on a number of factors. The most critical factor is

temperature. Like many chemical reactions, oxidation rates increase exponentially with increasing temperature due to the Arrehenius rate rule. For most mineral oils, a general rule of thumb is that the rate of oxidation doubles for every 10°C rise in temperature above 75°C. Temperature control is therefore important during the purification process. An oil polishing unit is provided to remove the colouring of used oil. Preferably, the oil polishing unit comprises absorption medium capable of removing the colouring and at the same time stabilising oxidation of the used oil. More preferably, the absorption medium is capable of being regenerated, thereby reducing the load and need for replacement. In one embodiment, the oil polishing unit comprises fullers earth column. Other absorption means and medium are also useful.

FIG. 1 is a schematic setup of the purification system in accordance with one aspect of the present invention. Briefly, a membrane separation unit 32 comprising an organophiiic nanofiltration membrane is provided. A settling tank 1 and a centrifuge 15 are provided upstream to the membrane separation unit 32. The settling tank 1 contains a feed composed of used oil. The dark coloured feed sample (labeled "FEED") is shown in FIG. 2. Water, sand and larger solids suspending in the feed are settled and removed from the settling tank 1. The feed is then centrifuged to further remove suspended and medium-sized particles before being fed to the membrane separation unit 32. In the membrane separation unit 32, additives present in the feed are removed via operation of the organophiiic nanofiltration membrane. The feed is separated into permeate and retentate streams. The permeate sample (labeled "PERMEATE" in FIG. 2) is collected in a permeate tank where the permeate is subsequently treated in an oil polishing unit 100 to stabilise and prevent oxidation of the permeate. After passing through the membrane separation unit 32, the permeate now displays a lighter colour. Through the oil polishing unit 100, a clear and light colouring of the base oil sample (labeled "POLISHED" in FIG. 2) is obtained and stored in a base oil tank 49. The retentate is collected in a residual tank 110 for side use as asphalt flux.

[Examplel Overall Process for Purifying Used Oil

FIG. 3 is a flow diagram of the purification system, detailing the overall pre- treatment, nanofiltration and oil polishing steps of FIG. 1. The overall process comprises a pre-treatment, membrane separation unit (FIG. 4), and oil polishing unit (FIG. 5).

Process for Removing Contaminants Present In Used Lube Oil Using Organophilic Nanofiltration Membrane

FIG. 4 is a flow diagram of the purification system, detailing the pre-treatment and nanofiltration steps of FIG. 1.

Used oil is initially stored in specially designed settling tanks 1 and 2. Used oil is drawn from the buffer compartment in these tanks through valves 52 and 53. Valves 8 and 9 provide the drawn-down from the bottom of the buffer

compartments.

Circulation pump 11 circulates the used oil through the heat exchanger 4 where oil is heated up to 80°C by a boiler 5. A first stage filter system 3 provides filtering from 100 to 30μm. The filtered used oil stored in settling tanks 1 and 2 is then drawn through valve 12 via feed pump 13. The filtered used oil is passed through the heat exchanger 4 again to bring the temperature to 80°C before feeding into the centrifuge 15. Water and particles greater than 20pm is separated from the centrifuge 15.

The pre-treated used oil is stored in an intermediate tank 16. A feed pump 17 pumps the pre-treated used oil through a filter system 18 filtering down to 10pm. The final pre-treated oil is pumped into holding tanks 25 and 26, ready for organophilic nanofiltration membrane separation.

A circulation pump 31 draws the final pre-treated used oil from valves 27 and 28 through a heat exchanger 29 heating the oil to 80°C before circulating into the membrane separation unit 32. The final pre-treated used oil with a viscosity at 40°C of 71 mm ² /s is fed into the membrane separation unit 32 at a rate of 135kg/h (1.17m/s cross-flow velocity). As the organophilic nanofiltration membrane operates in a cross-flow manner, the permeate, which is the product oil from the organophilic nanofiltration membrane, is drawn off from the membrane separation unit 32 at valve 34 (FIG. 5). The rest of the final pre-treated oil is continuously circulated back to the holding tanks 25 and 26 through valves 23 and 24. A differential pressure of up to 15bars is maintained across the membrane separation unit 32 resulting in cross-flow velocities of 1 m/s across the organophilic nanofiltration membranes.

The membrane separation unit 32 is equipped with 00085m ² PDMS

(polydimethylsiloxane) membranes and is operated at pressure of 25 to 30bars with the oil temperature maintaining at 80°C. The pressure at the permeate side (i.e. the exit of the membrane separation unit 32) is kept nearly atmospheric. The viscosity of the permeate at 40°C is measured to be 26mm ² /s.

The permeate is then fed to the oil polishing system from valve 34 and drawn in by feed pump 35.

Process for Polishing Filtered Used Oil

FIG. 5 is a flow diagram of the purification system, detailing the oil polishing step of FIG. 1. Permeate passing through the membrane separation unit 32 is used for the oil polishing in FIG. 5. The oil polishing unit 100 uses regenerative fullers earth for decolorisation of the permeate. The oil polishing unit 100 operates in dual modes: oil polishing and column regeneration.

In the oil polishing mode, permeate from the membrane separation unit 32 is fed into the oil polishing unit 100 through feed pump 35. A heat exchanger 36 provides the heat exchange to bring the permeate temperature to 95°C before feeding the permeate to fullers earth columns 39. A pre-filter 37 provides filtering up to 5pm. The polished oil is drawn from the bottom of the fullers earth columns 39 through a final polishing filter 48 of 3pm to a clean base oil tank 49. After processing a certain capacity of permeate, the fullers earth gradually becomes saturated and its absorptive efficiency is gradually reduced as a consequence.

The fullers earth columns 39 are required to be regenerated to restore the fullers earth columns 39 back to its original state in order to perform its absorption function. At the fullers earth columns 39 regeneration stage, the permeate is first purged from the fullers earth columns 39 to a holding tank 46. This is achieved by a blower where air is taken in from blower 50 through valve 51 into the fullers earth columns 39. The air is hot and serves to dry and heat up the fullers earth columns 39. The heating temperature of the fullers earth columns 39 is maintained at about 80°C before the permeate is introduced into the fullers earth columns 39 via vacuum suction. The vacuum pressure is maintained at 10 to 5psi to slowly draw the permeate into the fullers earth columns 39. Typically, at the end of fullers earth column regeneration, the amount of permeate left in the fullers earth columns 39 is only about 0.01 %. The rest of the permeate is captured in holding tank 46.

Regeneration of the fullers earth columns is achieved by a combination of heat application and hot gas. The contaminants are extracted from the fullers earth columns 39 through a combustor 44 where they are flared off. A demister 43 extracts the moisture and remainder of permeate from the heated air stream.

Control tank 41 provides control medium to direct oil or switch to air flow for the two different modes of operation. After regeneration, the fullers earth columns 39 are renewed and able to start the oil polishing operation mode again.

Results Comparative Data

The specifications of industrial grade (SN150) new lube base oil for motor vehicles are given in Table 3 below.

The specifications of new lube base oil for motor vehicles obtained from crude oil by proprietary processes of various refining companies are given in Table 4A- 4D. Each highlighted column indicates the equivalent SN150 grade specification of new lube base oil of the respective refining companies.

Used oil from motor vehicles are heavily contaminated by the following contents:

- Heavy metal contaminants from spent additives and engine wear metals;

- Micro carbon residuals resultant from carbon sludge in engines;

- Water; and - Oxidation of oil resulting in color change.

In order to treat used oil back to its original base oil condition, there is a need to remove the above contaminants and to significantly improve the colour of the used oil. In addition, the purified base oil must meet the following specifications of new base oil: o Viscosity @40°C and 100°C

o Viscosity Index

o Colour • Flash Point

• Sulfur Content

• Micro Carbon Residue

• Pour Point

Present Data

Used oil from motor vehicles are treated and purified in accordance with the present invention to meet the requisite specifications. Tests are conducted with SGS (Societe Generate de Surveillance) for the quality of the base oil thus purified in accordance with the present invention. Table 5 below tabulates the test results of the quality of the purified base oil. The test results clearly demonstrate that the quality of the polished oil is comparable to the SN150 grade of new lube base oil.

Although the above example provides for used oil from motor vehicles, it is to be understood and appreciated that if used oil from other applications such as heavy industries or marine industries is treated and purified in accordance with the teaching of the present invention, the respective industrial grade (e.g. SN250, SN400 or SN500) new lube base oil may be obtained. The primary objective is to restore the used oil to its original base oil condition.

The present process is able to achieve the following results: - a) Removal of over 90% of the total heavy metal contaminants present in the used oil feed source; b) Reduction by 80% of the engine "wear metals" such as Cu, Pb, Fe, and Cr; c) Reduction by 90% of micro-residues; d) Removal of over 95% of water; e) Removal of most additives; and f) Significant improvement in colouring. The afore-described method provides a simple and economical way to resolve the inherent problems of reprocessing and recycling used oil. With a combination of pre-treatment and post treatment processes in combination with the use of an organophilic nanofiltration membrane to separate undesired molecules bigger than 1 ,000g/mol, permeate of the organophilic nanofiltration could be recovered and reused as lube oil whose quality is comparable to that obtained directly from crude oil refining. Further, the retentate may be used as asphalt extender.

In restoring the treated lube oil back to its original condition, the purified lube base oil could subsequently be blended with new additives to produce new lubricant oil, thereby reducing the need to rely on the fast depleting pool of fresh lube oil obtained from crude oil refining.

There are several advantages in employing organophilic nanofiltration membranes in the lube oil recovery process: a) Membrane Stability The organophilic nanofiltration membranes are solvent stable and it can withstand operations under high pressures (up to 40bars) and high temperatures (up to 90°C). The composite membrane does not foul easily. Accordingly, the life span of the organophilic membranes is expected to last longer. Prior art

membranes swell in solvent/organic medium under such operating environments and precise control has to be exercised to prevent excessive swelling so that the membranes are able to perform the separating function. Otherwise, frequent replacement of the prior art membranes is needed. Moreover, additives present in the used oil feed source may be detrimental to the prior art membranes such as those used in mechanical filtration and ultra-filtration, and entrainers or extraction media are often needed to remove such additives prior to separation. With the present method, removal of additives prior to separation is no longer needed. b) Meets Quality Requirement

In order to separate all carbon deposits, ash, wear metals, water, spent additives from used oil, the separation range has to be specific and tight. The organophilic nanofiltration membranes target separation of molecules having molecular weights of between 600 and 1 ,000g/mol, which is the specific range where lube base oil can be extracted from the feed source. With a combination of size exclusion and diffusion from the dense pores of the organophilic nanofiltration membranes, lube base oil is easily separated from the used oil feed source. c) Small Footprint and Energy Saving

The entire recovery and refining process requires minimal heat and pumps to move the fluid. Compared to distillation system and thermal cracking facilities, the present method does not require extensive steam boilers or furnaces for operation. The present process thus achieves significantly lower energy costs compared to conventional thermal processes. Also, the overall process is much simplified and involves a smaller number of process steps. The overall footprint of a recovery plant will therefore be much smaller and the operational costs will also be much lower, making this method economically feasible and commercially attractive.

Organophilic nanofiltration is an energetically advantageous process since no phase change occurs and no corresponding evaporation enthalpy is required. For compounds with high boiling points like the lube oil, this results in significant energy cost savings. d) Minimal Emission and Waste Production

Unlike acid/clay treatment or chemical cracking where acid sludge is generated, the present method does not produce such waste. Evaporation occurs for high temperature processes such as distillation and thermal cracking, and in the process ash and other contaminants are released into the atmosphere as harmful vapours. The present method operates at mild temperatures of 90°C and below. There is no flue gas or smog emission, and therefore the present method does not pollute the environment. Both retentate and permeate can be used in commercially viable applications. Waste disposal and the associated environmental problems are avoided.

Although the foregoing invention has been described in some detail by way of illustration and example, and with regard to one or more embodiments, for the purposes of clarity of understanding, it is readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes, variations and modifications may be made thereto without departing from the spirit or scope of the invention as described in the appended claims.