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An Extraction - Adsorption Combo-Process for Re-refining of Used Oil Ranvir Singh#, Manoj Srivastava, Manoj Kumar, U.C. Agrawal and M.O.Garg Indian Institute of Petroleum, Dehradun-248005, India Abstract Lubricating oil has wide range of applications. It‟s by far major application is in motor vehicles and engines where it is used to smoothen the functioning of engine parts and thereby improve the performance as well as extend the life of engine. Periodically, it has to be replaced at regular intervals because of contamination of oil caused by inclusion of „wear „and „tear‟ of metal parts in it. Worldwide, the earlier practice of dumping the contaminated oil after use poses many serious environmental problems. The rising crude prices are also adversely affecting the margins of lube refineries. This scenario is leading to thinking in favour of „re-refining‟ and „re-use‟ of used lube oils. Generally, used oil contains wide range of hydrocarbons (C15–C50), but its actual chemical composition depends largely on the „crude source‟, „mother distillates‟ and nature of „virgin base oils‟. In a study carried out at Indian Institute of Petroleum, Dehradun and being reported in the present paper, used oil distillate from a leading re-refiner in USA was selected as feed stock. The oil sample was characterized for ASTM distillation D-1160, specific gravity, sulphur content, color and odor etc. The used oil was subjected to „solvent extraction‟ (where NMP was taken as solvent in „continuous phase‟ and used oil distillate as „dispersed phase‟). Typical process parameters such as temperature of the single stage glass extractor, solvent-to-feed ratio and water content in NMP (to provide more affinity towards the aromatics) were varied. The paper discusses effect of these parameters in terms of quality of oil product (raffinate obtained after extraction). In order to attain the stringent limits for sulfur content in re-refined used oil, solvent free raffinate was percolated through „charcoal bed‟ to further complement „extraction step‟. Significant improvement in the quality of re-refined used oil in terms of sulfur content, color and odor has been achieved through the combo process which has been discussed in the paper in detail.

Keywords: adsorption, charcoal bed, NMP extraction, re-refining, used lube oil. Introduction Lubricating oil serves as a sink to the wear and tear of engine parts and other waste material gases generated in internal combustion processes. This lubricating oil after use for a recommended period is turned into used lubricating oil which contains absorbed condensates and accumulated polyaromatic hydrocarbons and looses several of its desirable properties. Further, used oil also contains degraded additives such as high percentage of detergents/additives/dispersants, synthetic VI improvers, pour point depressant etc. Due to all these, it has to be replaced with fresh oil after certain time to restore/improve the engine performance. Further use of used oil as such - without any re-processing – or dumping it as such is strictly banned worldwide, as it creates serious health hazard and poses other „ecological‟ and „environmental problems‟. Hence, re-refining of used oil is regarded these days as one of the areas of prime importance and further re-refined oils are considered to be equivalent to the virgin base oils. Since olden days, reclamation of used oil by „acid- clay process‟ has been extensively used to recover valuable „base oil‟ present in used oil samples collected from various sources and has also played an important role in terms of recycling of product in the market. Due to stringent environmental conditions, need of time is to shift to some alternative technologies e.g. UOP Hylube process, Meinken/Hybrid Meinken process, IFP- Snamprogratti process, KTI process, CEP-Mohwak process, PROP process, DEA technology etc. Each of these developed technologies has specific „advantages‟ or „disadvantages‟ over other alternatives in terms of cost and availability of ternary components such as hydrogen availability, solvent requirement, or energy inputs in case of energy intensive process (i.e. incineration). In the context of all these, there is no universally accepted well defined technology available for application to all of the used oils of different grades.

*Corresponding author: Tel: +91-135-2660113-116 ; Fax No: +91-135-2660098 /2660202;

Email: rvsingh@iip.res.in

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„Solvent extraction‟ is relatively simple process where solvent and feed are mixed in appropriate quantity and left for sufficient residence time, allowing the feed –solvent mix to settle in two separate „phases‟ viz. raffinate and extract. Most of the carbonaceous impurities present in used oil get concentrated in extract leaving the raffinate relatively enriched with purer base oil. A literature survey shows use of various polar and non-polar solvents for re-refining of used oils. Crowley (US Patent 4,169,044) described technology for used oil re-refining by using „propane‟ as solvent. The patent claims advantage of using propane as solvent to eliminate the carbon, lead, phosphorous, calcium and other metal additives in used oil. Solvent-to-feed ratio in extraction step was reported to be in the range of 0.6 to 1 and recycling of raffinate into previous step to improve the yields of finished product. Top and bottom temperatures of solvent extraction unit were maintained in the range of 170 to 200

0C and

160-1850C respectively.

Fletcher et al (US Patent 4,399,025) described a process for re-refining of used oil by using tetra hydro furfural alcohol as solvent. Further, solvent was recovered from the raffinate and extract stream by distilling under vacuum of (2.5 mm Hg) and at 200

0F and steam stripping for removing trace components An yield

of ~ 82 % was reported at extraction temperature of 1300F and solvent-to-feed ratio of 1. Applicability of

process for light as well heavy fraction of lube oil was also reported. Elbashir et al studied the method to predict effective solvent extraction parameters for recycling of used lubricating oil. For this purpose, three extracting solvents namely 2-propanol, 1-butanol, methyl-ethyl-ketone were tried on the basis of their solubility parameter; MEK was found by them to be better than 2-propanol and 1-butanol. Liquid supercritical ethane was employed as a solvent for the re-refining of used lubricant oils by (Jesusa Rinc´on). They claimed that the waste oil contaminated with heavier compounds present in used oil get dissolved in „ethane‟ under the super critical conditions. The pressure variation in the range of 40–145 Kg/cm

2 and temperature from 20- 95

0C showed improvement in its yield, quality and separation efficiency.

However, they reported that a soft finishing treatment is further required in order to attain the specifications required for base oil. Fontana et al showed the effectiveness of „ultrasonics‟ on removal of lead content in waste oil. The waste oil was mixed with aqueous solution of HNO3 and further sent to „sonicator‟ which drastically reduces time of normal extraction step as high frequency ultrasonic waves break the „chemical bonds‟ by „mechanical‟ means and results in improvement in yield. In the study discussed in this paper, a combo process (solvent extraction and adsorption) was employed to re-refine used oil to meet required leveling desired properties in respect of colour and sulfur and pass the API a Group – II specifications. Experimental Study Materials & Test Methods Employed : Used oil distillate sample received from small a prominent used oil refiner in USA was obtained and phyisio-chemically characterized; the data has been reported in Table 1. Commercial grade NMP supplied from BASF, Germany has been used as solvent along with appropriate quantity of distilled water for extraction studies. The percentage of water in solvent was determined by standard Karl-Fisher method. Various types of hydrocarbons in used oil were determined by using ASTM D 2549 method using AR/LR grade hexane, benzene, toluene, di-ethyl ether, chloroform, methanol and ethanol. Standard ASTM/BIS procedures were used to characterize feed and products. Activated charcoal was used as adsorbent to improve the colour of re-refined oil.

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Table 1: Physico–chemical Characteristics of Used Oil Distillate

Properties

Specific gravity6060

0.8592

Refractive Index (at 20oC) 1.4720

Viscosity index (VI) 87

Pour point (0C) -15

Sulphur (ppm) 1519.5

Hydrocarbon type analysis (ASTM-2549) Saturates Aromatics Sulphur (ppm) in saturates Sulphur (ppm) in aromatics

90.53 9.47 37.1 1.6

LLE Setup & Procedure : The single stage extractor of one liter capacity with multi-pronged stirrer attached with electric motor (50 Hz/220V AC) was used for mixing solvent and feed for extraction. The speed of stirrer in the extractor can be varied 0-1200 rpm. Oil flows through oil circulation bath in the annulus space of around 2 mm to maintain the constant temperature throughout the extractor. Known quantity of solvent and feed were added in the extractor where it was continuously stirred at constant speed at 400rpm for 45-60 minutes to allow continuous molecular interaction between the Hydrocarbon and solvent molecules at selected temperature and normal atmospheric pressure conditions. The well mixed solution was separated into two phases which were then allowed to settle for about two hours at the same temperature. The settled phases namely „raffinate‟ and „extract‟ were withdrawn separately. Both, phases contain solvent but in different proportions due to variation of solubility of solvent with different hydrocarbons. Both, the extract and raffinate phases were made solvent free by atmospheric distillation under nitrogen stripping to avoid oxidation. Solvent free raffinate and extract samples were characterized for their important physico-chemical properties. Recovered solvent from experiment was in admixture reused with fresh solvent in further experiments. RESULTS & DISCUSSION The physico-chemical properties of the used oil distillate are given in Table- 1. The specific gravity of the used oil distillate at 60/60 is 0.8592 and refractive index at 20°C is 1.4720, which reflects that used oil distillate is predominantly rich in paraffins. The viscosity index (VI) of used oil distillate is 87, which falls in the range of group-I base oil. The pour point of distillate is -15°C which is good enough to serve even in extra cold weather conditions. The hydrocarbon type analysis by ASTMD – 2549 also revealed that it contains 90.53wt% saturates (i.e.> 90%), but presence of high sulphur 1519.5 ppm (more than 300 ppm) put this used oil distillate into group-I base oil category. The ASTM distillation data (ASTM D-1160) of the distillate has been plotted in Figure - 2. The graph shows that the initial boiling point (IBP) of distillates is ~250°C and final boiling point (FBP) is ~450°C. The lower IBP indicate that the used oil distillate is contaminated with lower boiling hydrocarbons coming from extraneous sources such as washing solvents, naphtha, furnace oil etc. at garages/collection centers. These light hydrocarbons are not the part of lube oil and can be separated and rejected during fractionation step and can be blended in furnace oil or any other low value products.

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Effect of Temperature: To see the effect of extraction temperature on used oil quality, single stage extraction runs under identified conditions except the operating temperature were given. In two sets of these extraction runs, solvent-to-

feed ratio was kept at 1 and temperature at 50/90C. The average results of these runs are summarized in the Table 2. Table 2: Effect of Temperature on Desulfurization of Used Oil

Run 1 Run 2

Extraction temperature O

C 50 90

Solvent neat NMP neat NMP

Solvent-to-feed ratio 1.0 1.0

Raffinate yield ( % wt) 89.5 76.0

Raffinate density D415

(g/ml) 0.8532 0.8526

Used oil distillate sulfur (ppm) 1519.5 1519.5

Raffinate sulfur (ppm) 678.4 563.4

It is clear from Table-2 that on increasing the extraction temperature from 50 to 90

0C, solubility of

hydrocarbons in solvent gets improved with the result that raffinate yield comes down from 89.9% (at 50

0C) to 76% (at 90

0C). The increase in solubility of solvent led to more removal of sulphur containing

aromatic hydrocarbons from raffinate as shown by more reduction in sulfur (563.4 ppm) at higher extraction temperature i.e. at 90

0C rather than less reduction in sulfur (678.4 ppm) at lower extraction

temperature i.e. at 500C as compared to used oil distillate. Higher extraction temperature leads to greater

improvement in raffinate quality but at the cost of yield. Effect of Solvent Composition: The effect of solvent composition i.e. NMP and water percentage on raffinate yield and sulphur reduction is shown in Figure 3.

R² = 0.9627

200

250

300

350

400

450

500

0 10 20 30 40 50 60 70 80 90 100

Tem

pe

ratu

re(d

eg

C)

% Volume Distilled

Figure - 2 : ASTM D1160 curve for used Oil distillate

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Figure-3 Effect of solvent composition on sulfur reduction at Temperature 500C

Figure-4: Effect of solvent composition on sulfur reduction at Temperature 900C

The results show that addition of water in solvent NMP leads to increase its selectivity toward aromatics, present in feed. The addition of 5% water in NMP leads to reduction in sulfur to 653.8ppm. The most of sulfur molecules attached with aromatic ring structure compounds goes in aromatic rich phase (extract phase) and leaves the raffinate phase lower in sulfur at 563.4ppm, which is still moderate value and fails to meet group-II base oil specifications; at the same time yield of raffinate goes on increasing but quality of raffinate product deteriorates, as density of raffinate goes up (0.8541 g/ml). Similar trend is observed for follows at the addition of 10% water in solvent in which case yield goes up to 95.3% but sulfur in raffinate also goes up a little higher to 700.5ppm and density of product goes to 0.8544 g/ml. Hence, an optimization has to be made in respect of water content in solvent based on trade –off between the yield and product quality. A similar trend was also observed when extraction was carried at higher temperature, 90

0C as shown in Figure 4.

88

90

92

94

96

98

100

102

88

90

92

94

96

98

100

102

53 55 57 59 61 63 65

% N

MP

Co

nte

nt

Sulphur reduction in %

Fig 3: Sulphur Reduction Vs Solvent Composition With Effect on Raffinate Yield

% yield

% NMP Content

60

70

80

90

100

88

90

92

94

96

98

100

102

48 49 50 51 52 53 54 55 56 57

% R

aff

inate

yie

ld

% N

MP

Co

nte

nt

Sulphur-Reduction in %

Fig 4 : Sulphur-reduction Vs Solvent Composition with Effect on Raffinate Yield

% NMP Content

% yield

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Effect of Solvent-to-feed ratio: Solvent treat rate, also commonly known as solvent- to- feed ratio (S/F), plays a significant role in achieving desired product quality. To see the effect of solvent-to-feed ratio on product quality, extraction of used oil was carried out at fixed temperature (90

0C) but varied solvent- to - feed ratio of 1 and 1.4. It has

been observed that sulfur get reduced to 678.4ppm from 1519.5 in used oil distillate at s/f of 1.0 as shown in Figure-5. It has also been shown that on increasing treat ratio (~1.4), there is some further reduction in sulfur level in product. This suggest that to improve product quality or lower sulphur level, higher solvent- to- feed ratio are desirable but this also leads to some reduction in product yield.

Figure-5: : Effect of solvent-to-feed ratio on sulfur reduction at Temperature 900C

Activated Carbon Bed Treatment A 50 gram of raffinate oil sample obtained from one of the extraction run was percolate through an „activated carbon Bed‟. For this, 50 gm of activated carbon was filled in a glass column having length 400 mm and diameter 20 mm. The adsorption process was carried out at ambient temperature. The sample collected after passing through activated carbon bed was characterized for sulfur. Results of this experiment showed that reduction in sulfur in oil is from 392.5 to 308.5 ppm. Although, there is a significant reduction in sulfur, but it could meet the specification of group-II base oil which is 300 ppm.

Conclusion: Based on the analysis of experimental data of the feed and product, including their physico-chemical characterization, following are the important conclusions may be drawn: i. The initial boiling point (IBP) of feed stock of sample was 250.5

0C, which shows presence of lighter

components of gas oil range in the feed stocks. Presence of these lighter components (which may be lost during solvent recovery) is affecting the consistency of viscosity of oil and its raffinate products.

ii. The feedstock contains around 1519 ppm of sulfur, which is more than 5 times of prescribed limit of

API standard Group–II oil (300 ppm). It may be desirable to determine the source/check at collection points, if contamination from furnace oil etc is occurring. The feed stocks have poor color and color stability, whereas raffinate products obtained after NMP extraction have shown improved „color‟ and very good „color stability‟. Similarly, after NMP extraction, odor of product is also much reduced as compared to used oil distillates samples as such.

0

200

400

600

800

1000

1200

1400

1600

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

S-V

alu

e (

pp

m)

S/F ratio

Effect of S/F on S-reduction

S-value(ppm)

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iii. Study of effect of operating temperature NMP Extraction of used oil distillates reveals that increase in temperature beyond a certain range neither improves raffinate yield nor it helps in reduction of sulfur in the raffinate.

iv. Addition of water in NMP (modification in solvent composition) for enhanced selectivity/increased affinity towards aromatics leads to improvement in raffinate yield but at the same time it also reduces desulfurization capacity of solvent.

v. The minimum level of sulfur in refined oil that could be achieved using NMP, extraction was 392.5 ppm with a VI of 95 in the raffinate product. It seems that for meeting group –II quality, mild „hydrogenation step‟ is also needed to further process the product obtained by „solvent extraction‟.

vi. The principal reason for inadequate removal of sulfur to the required level may be due to the fact that it may be present in the form of aliphatic type of sulfur compounds attached to the main chain of saturates which are very difficult to remove by NMP extraction or other physical separation methods.

vii. To cope up with problem of inadequate desulphurization, attempts were made by modification in process scheme i.e. combination of extraction with adsorption with charcoal bed at specified conditions. This combo-processing brought sulfur down to 308ppm but tradeoff between processing cost and other process needs to be carefully evaluated.

References

1. A.Fontana et al “ultrasonic removal of heavy metals from waste oils” Fuel processing Technology 48(1996) 107-113.

2. Selective Extraction using Mixed solvent system US Patent 6,416,655 B1 (July 9, 2002)

3. Process for Re-refining used oil by solvent extraction US Patent 7,226,533 B2 (June 5, 2007)

4. Method of Re-refining waste oil by distillation & Extraction US Patent 6,117,309 B2 (June 12, 2000)

5. Extraction of hydrocarbon oils using combination polar extraction solvent-aliphatic –aromatic or polar extraction solvent-polar substituted naphthenes extraction solvent mixture US Patent 4,909,927 (march20, 1990)

6. Batch process for re-refining used oil US patent publication no US2002/0036158A1 (march28, 2002)

7. Method of solvent recovery in refining hydrocarbon mixture with N-methyl 2-pyrrolidone US Patent 3,476,681 (November4, 1969)

8. Re-refining used lubricating oil US Patent 4,169,044 (September24, 1979)

9. Solvent extraction of hydrocarbon oils producing an increase yield of improved quality raffinate US Patent 5,616,238 (April 1, 1997)

10. Solvent extraction process US Patent 4,311,583 (January19, 1982)

11. Solvent Extraction processes for used oil Re-refining US Patent 4,311,583 (August16, 1983)

12. Jelena Lukic , Aleksandar Orlovic , Michael Spiteller, Jovan Jovanovic, Dejan Skala “Re-refining of waste mineral insulating oil by extraction with N-methyl-2-pyrrolidone Separation and Purification Technology 51 (2006) 150–156

13. Mohammad A. Al-Ghouti, Lina Al-Atoum “Virgin and recycled engine oil differentiation: A spectroscopic study” Journal of Environmental Management xx (2007) 19

14. Jesusa Rinc´on,Pablo Ca˜nizares , Mar´ıa Teresa Garc´ıa “Regeneration of used lubricant oil by ethane extraction” J. of Supercritical Fluids 39 (2007) 315–322