International Research Journal on Engineering Vol. 3(1), pp. 008-024, April, 2016 Available online at http://www.apexjournal.org

©2016 Apex Journal International

Full Length Research

A comparative study of the solvent powers of phenol, furfural and NMP in improving the viscosity index of

spent lubricating oil

E. Epelle, Y. Lukman and

A. J. Otaru

Department of Chemical Engineering, Federal University of Technology Minna, Niger State, Nigeria.

Accepted 29 January, 2016; Published 26 April, 2016

In response to waste oil’s economic and environmental problems, there has been a growing trend to recycle and reuse waste lubricant oil. This has however been achieved using the most important used oil recycling technology, Re-refining. Although, many methods of used oil re-refining exist, solvent extraction has proven to be of better technical advantage compared to others on the basis of its environmental friendly nature as well as its economic viability. For the purpose of this research work, the solvent extraction process was employed using the solvents (Phenol, NMP and furfural) for the selective absorption of the residual aromatics as well as the contaminants present so as to improve principally the oil’s viscosity index (VI). The performances of these solvents were however accessed based on VI improvement and yield. The solvents were systematically varied alongside temperatures of 40, 60, 80 and 100

oC and solvent to oil ratios of 1:1, 2:1, 3:1 and 4:1 respectively. NMP proved to be the

best solvent over furfural and phenol in terms of VI improvement giving highest value of 105, whereas, furfural gave the best performance over NMP and phenol in terms of raffinate yield with highest yield obtained as 90%. Key words: Extraction, re-refining, furfural, NMP.

INTRODUCTION Lubricants are substances applied to close fitting (guiding) or contact surfaces of machinery principally to reduce friction. They could exist in form of gas, liquid or solid, ranging from the common air to grease and to most popular oils. Onyeji and Aboje (2011) stated that liquid lubricants are the most commonly employed due to their wide range of potential applications whereas gaseous and solid lubricants are recommended in special applications. Lubricating oils which fall under the category of liquid lubricants possess great abilities in performing secondary functions such as reducing wear, overheating, and seizure of rubbing surfaces that increases the expenditure of indicated power on overcoming mechanical losses in the engine and to clean and remove *Correspondent author. E-mail: yusuflukman55@yahoo.com. Tel: 08035873224.

products of wear from machine parts (Ogbeide, 2010). Lube oils are one of the most valuable components in a barrel of crude oil; while the other components such as gasoline, jet and diesel fuel are lost after combustion, lube base oils can be recovered and regenerated to a quality equal or even better than its original virgin form by using various re-refining processes. Used oils are derivatives of oil usage in vehicles and machinery and its generation is closely connected with the increase in population of automobiles and industries as well as mechanization of agriculture (Eman and Shoaib, 2012). Thermal degradation, contamination and oxidation constitute the main mechanisms through which lube oil‟s effectiveness is grossly reduced in internal combustion engines. Some of these contaminations arise from the presence of water, salts, dirt, metal scrapings, and incomplete products of combustion and even breakdown of lube oil additives under use. The resulting effect of the existence of these contaminants is accelerated ageing

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and degraded performance of these oils. MATERIALS AND METHODS Materials and chemicals needed The used oil which served as the major material was supplied by near-by garages and mechanic workshops within the city of Minna. A blend of equal proportions of used oil from 4 different workshops was used. The furfural solvent was manufactured by Henan Allrich Chemical Co. Ltd, China and supplied by the Kaduna Refining and Petrochemical Company (KRPC) with 98% purity. Phenol of 99.2% purity was manufactured by Orchid Chemical Limited, China and supplied by Panlac Chemical Companies, Minna. Certified NMP of 99% purity was manufactured by Sigma-Aldrich Chemicals, China and supplied by Fidson Chemical Company Port-Harcourt. Equipment used

The experiment was conducted using a conical flask (in which oil and solvent were mixed), a beaker (for collection of extract and raffinate phases respectively), measuring cylinders of different volumes, thermometer, a magnetic stirrer equipped with a temperature and rotation regulator that regulated the temperature and speed of stirring in the beaker. A separating funnel clamped to a ring stand was used to effect separation of the 2 phases (extract and raffinate) after the extraction. Table 1 shows the list of equipment used, their manufacturers and their source. Characterization of used oil

The following physical property determination procedures were carried out on the used oil as a means of evaluating the oil‟s properties before extraction and for comparison after extraction. Specific gravity

Density is defined as the mass per unit volume and specific gravity is the ratio of mass of a volume of a specified material at a specified temperature to the mass of the same volume of distilled water at a standard temperature. The specific gravity and density of the oil were determined with the aid of a 25 ml specific gravity bottle and a weighing balance. The bottle was thoroughly washed and dried after which its mass when empty was measured. Thereafter, the bottle was filled with the used oil and the resulting mass was measured. The difference

Epelle et al 009 between these two masses gives the mass of the oil. Similarly, the bottle was filled with distilled water and weighed. The density and specific gravity were determined using the formulas:

Dendsity =𝑀𝑎𝑠𝑠 𝑜𝑓 𝑂𝑖𝑙

𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑂𝑖𝑙…………… . . ………1

Specific Gravity =𝑀𝑎𝑠𝑠 𝑜𝑓 25𝑚𝑙 𝑂𝑖𝑙

𝑀𝑎𝑠𝑠 𝑜𝑓 25𝑚𝑙 𝑊𝑎𝑡𝑒𝑟……… 2

Kinematic viscosity Viscosity is a measure of the internal friction of a fluid and numerous methods have been formulated for its determination (Chiyoda, 1980). Usually these methods involve the measurement of the flow of fluid through an orifice or capillary tube at a carefully controlled temperature. These methods are usually dependent on the specific gravity of the fluid under test in which the kinematic viscosity is obtained (measured in centistoke “cSt”). For the purpose of this research work, the viscosity of the lubricating oil was measured according to the ASTM D-445 in which a sufficiently low capillary having a flow time above 200 seconds was selected to ensure the oil gravitates through sufficiently slowly for accurate timing to be achieved. This test method specifies a procedure for the determination of the kinematic viscosity, “v” of liquid petroleum products, both transparent and opaque, by measuring the time for the volume of liquid to flow under gravity through a calibrated glass capillary U-tube viscometer. The used and solvent treated oil samples were charged into the viscometer, after which the viscometer was immersed in an oil bath which was maintained at a constant temperature of 100°C; (in the measurement of viscosity at 40°C, the viscometer was immersed in a water bath which was maintained at a constant temperature of 40°C with the aid of a thermostat). After thermal equilibrium was attained between the sample and bath temperature within 10-15 minutes, the oil sample was dragged and the time in seconds was recorded for flow of oil sample between the two levels in the capillary tube. Generally, viscosity is related to time of flow of a fixed volume of the test sample through a given capillary viscometer. The kinematic viscosity was then calculated by applying the formula;

𝑣 = 𝑘𝑥𝑡…………………………………………… . .3 Where: “k” is a constant factor for the calibrated viscometer. “t” is the time taken by the oil to flow through the viscometer in seconds. “v” is the kinematic viscosity measured in mm

2/s

(centistoke cSt) (ASTM, 2004).

(3.3)

010 Int. Res. J. Eng

Table 1. List of equipment used.

Reagent and apparatus Manufacturer Source

Digital weighing balance

Magnetic stirrer

Viscometer

Measuring cylinders

Beakers

Flash point tester

Cloud and pour point bath

Citizen (MP 300)

Gallenkamp (SWT320.OIN)

Gallenkamp, England

Pyrex, England

Pyrex, England

Cleaveland

Koehler

Chemical Eng. Lab.

Chemical Eng. Lab.

NCRI Bida

Chemical Eng. Lab.

Chemical Eng. Lab.

NCRI Bida

NCRI Bida

Viscosity index Viscosity index is a measure of the effect of temperature on the viscosity of a lubricant. Ideally the viscosity should vary as little as possible with temperature so that the lubricant can perform adequately over a wide range of engine operating conditions. VI is used as a convenient measure of the degree of aromatic removal during base oil manufacturing process and also a useful tool in comparing base oils only if they are derived from the same distillate feedstock (Mortier et al., 2010).

The viscosity indexes of the used and regenerated oils were calculated according to ASTM-2270 by the application of the procedure for oils of 0-100 VI. In this procedure, the kinematic viscosity of the lube oil at 40 and 100°C were measured and the equation 4 below was used accordingly.

VI =𝐿 − 𝑈

𝐿 − 𝐻𝑋 100 ………………………………… .4

Where: U is the kinematic viscosity at 40°C of the oil whose viscosity index is to be calculated (cSt). L is the kinematic viscosity at 40°C of an oil of zero viscosity index having the same kinematic viscosity at 100°C as the oil whose VI is to be calculated (cSt).

H is the kinematic viscosity of an oil of 100 viscosity index having the same kinematic viscosity at 100°C as the oil whose VI is to be calculated (cSt). The values of L and H are obtained from tables.

For oils having viscosity index greater than 100, another viscosity index formular exists. For this index, the oil is only compared with a reference oil of the H family having the same viscosity at 100°C, and by applying Equation 5,

VI = 10𝑁 − 1

0.00715 + 100 …………………………… 4

N = 1 +𝑙𝑜𝑔𝐻 − 𝑙𝑜𝑔𝑈

𝑙𝑜𝑔𝐵………………………… . .5

B is the kinematic viscosity of the oil at 100C. The accuracy of the calculated viscosity index is dependent only on the accuracy of the original viscosity determination. Flash point The flash point of engine oil is the lowest temperature to which the oil must be heated under specified conditions to give off sufficient vapour to form a mixture with air that can be ignited spontaneously by a specific test flame (ASTM, 2004). Flash point measures the tendency of the specimen to form a flammable mixture with air under controlled laboratory conditions. It is only one of a number of properties used to evaluate the overall flammability hazard of a substance. It gives a rough indication of the lube‟s oil boiling range and is significant in high temperature duties where vapourization of lubricants is unacceptable. Flash point can indicate the possible presence of highly volatile and inflammable materials in a relatively non-volatile or non-flammable material. Also, the degree of contamination of lube oil can be evaluated using this parameter. A substantially low flash point is an indicator that the oil has become contaminated with volatile products such as gasoline.

This implies that if a lubricant has already been in use, a low flash point will indicate dilution by a fuel. A low flash point of lube oil from a plant can also be traced to insufficient stripping out of solvent used in the stripping process (Gary and Handwerk, 2001; Hamawand et al., 2013).

A clean Cleaveland open cup tester was fitted with the used oil samples to the meniscus line with removal of excess oil before proceeding with the test. The cup was properly positioned on the Bunsen burner with a thermometer inserted vertically into the sample via suspension by a holder. The thermometer was suspended at the center of the beaker ensuring that the bulb just dips inside the oil without touching the bottom of the beaker. Bubbles generated as a result of the heating were eliminated via the stirring of the oil with the aid of a

stirrer. The test flame was lighted and the burner switched on and the heat intensity was set to a minimum value for about 30 s. The test flame applicator was passed across the center of the cup at right angles to the diameter in a smooth continuous motion. The time taken in passing the test flame across the cup in each cast was approximately 1 s. The temperature at which a flash appeared at any point on the oil‟s surface (started igniting) on increasing the burner temperature without supporting combustion of the oil was recorded as the flash point. The sample was deemed to have flashed when a large flame appeared which spontaneously propagated itself over the entire surface of the used oil.

Pour point

The pour point of petroleum is an index of the lowest temperature at which oil will flow under specified conditions (Mortier et al., 2010). The maximum and minimum pour point temperatures provide a temperature window where petroleum depending in its thermal history might appear in the liquid as well as in the solid (lowest temperature at which movement of the test specimen is observed under the conditions of test) (Speight, 2002). The pour point data can be used to supplement other measurements of cold flow behaviour and the data are particularly useful for the screening of the effect of wax interaction modifiers on the flow behaviour of petroleum. Most engine oils contain waxes and paraffins that solidify at cold temperatures. Engine oils with high waxes and paraffins will have a higher pour point. Pour point is highly affected by an oils viscosity and engine oils with high viscosity are characterized by having high pour points. The oil must have the ability to flow into the oil pump and then be pumped to the various parts of the engine, even at low temperatures. The pour point of a crude oil is an index of the lowest temperature and handle ability of certain applications (Speight, 2002).

For the purpose of this research work, the pour point was measured according to ASTM D-5853. The used oil samples were poured into a test jar to the level marked and heated to a temperature of at least 20°C above the expected oil‟s pour point but not higher than 60°C (due to potential high vapour pressure). The test jar was immediately closed with a cork carrying the high cloud and pour thermometer (this was adjusted to tightly fit the jar so that the jar and the cork are in a co-axial position). As soon as the test specimen reached the required temperature, the cork carrying thermometer was removed and the specimen was gently stirred using a spatula, after the cork carrying thermometer was put back in place. The test jar was then cooled by using a succession of baths at lower temperatures. Cooling media of possible use in these baths include, (ice and water down to 10°C, crushed ice and salt down to -12°C, ice calcium chloride down to -23°C, solid carbon-dioxide and petrol down to –

Epelle et al 011 57°C). However, the ice and water bath was used for this research work. After the insertion of the test jar into the water bath, pour point observation commenced. Care was taken to avoid shifting of the thermometer in the test specimen or any other form of disturbances as this could lead to erroneous results (ASTM, 2004).

The appearances of the test specimen were examined at different temperature intervals expressed in positive or negative multiples of 3°C when movement was still observed, the test jar was immediately returned to the water bath within the time of 3 s. The removal and replacement procedure was continued until no movement in the test jar was observed when held in a horizontal position for 5 s. This value of temperature was then recorded as the pour point temperature.

Refractive index

Refractive index is the ratio of the velocity of light (of specific wavelength) in air to its velocity in the substance under examination. It may also be defined as the Sin of the angle of refraction, as light passes from air into the substance (ASTM, 2004). This is the relative index of refraction. It is a fundamental physical property that can be used in conjunction with other properties to characterize pure hydrocarbons and their mixtures.

The refractive index was measured according to ASTM-D1218. The refractive index was measured with the aid of a Bausch and Lomb precision refractometer using monochromatic light. It is a high resolution refractometer of an optical mechanical type with the prism temperature accurately controlled. The instruments principle is based on the concept of critical angle. The instrument is limited between 1.3300 and 1.5000 for temperatures between 20 and 30°C. It was ensured that the prism faces were cleaned dry. The prism temperature was checked and was ensured that it was within 0.1°C of the desired temperature. The prism assembly was unlocked and 2 drops of the used oil sample was placed on the prism face after which it was locked. The light source was then turned on and a temperature equilibration time of 3 min was allowed. A close observation of the field consisting of light and dark positions was then made with the help of the eye piece. The instrument was adjusted so that the boundary between the lines and the position of the feed was as sharp as possible. Additional adjustments were made until the sharp boundary line intersected the mid-point of the cross hairs superimposed on the field. The refractive index was then read by a scale. The process was repeated at least 3 times approaching from either side of the sharp boundary line and the various readings averaged to yield the average refractive index of the oil (ASTM, 2004).

012 Int. Res. J. Eng Colour Determination of colour of petroleum products is used mainly for manufacturing control purposes and is an important quality characteristic since colour is readily observed by the product user. In some cases colour may serve as the degree to which a material has been refined (Speight, 2002).

When the colour range of a particular product is known, a variation outside the established range may indicate possible contamination with other products. These contaminants are mainly those generated as a result of use of the oil. However, colour is not always a reliable guide to product quality and hence should not always be used indiscriminately in product specification. The colour was determined according to the ASTM D-1500, in which the used oil sample was placed in a test container and compared with the coloured glass disks of a colorimeter ranging from 0.5 to 8.0.

However, a combination of the above procedure and ordinary visual observation and inspection of oil samples was performed as a reliable method of determining oils colour (Speight, 2002).

Boiling point

The boiling point of a liquid is defined as the temperature at which the vapour escaping from the surface of the liquid has a pressure equal to the pressure existing above the liquid (Scodellaro, 2008). The boiling point of the used oil was determined with the help of an adequately calibrated thermometer by taking the temperature of the oil on heating, immediately physical boiling was observed.

Characterisation of extraction solvents

The solvents were tested for different properties to ensure originality and high purity by comparing results of characterisation with standard results. Boiling point A more accurate method was used for the determination of the boiling points of the solvents. A clean and empty test tube was attached to the thermometer with a sewing thread after which an empty capillary tube was put into the test tube in such a way that the open end of the capillary tube was down. About 5 ml of the solvent was placed in the test tube whereas; a 500 ml beaker was filled with paraffin oil which served as the oil bath. Since estimates of the boiling point of the solvents were known, the oil bath was heated to about 15°C above the expected boiling point of the liquid.

The test tube containing the sample solvent was then put into the oil bath supported by a ring stand and a clamp. The beaker containing the test tube was placed on a hot plate and stirring of the paraffin oil was done with a clean glass rod to ensure uniform heat distribution. The temperature was recorded when rapid bubbles proceeded out of the capillary tube; this signified a higher vapour pressure in the capillary tube than the atmosphere. The hot plate was immediately turned off and the beaker was placed on another hot plate that had not been used to cause a decrease in temperature. As the temperature decreased, the air bubbling gradually slowed down and the temperature was recorded when the last bubble was noticed and some liquid entered the capillary tube. At this point, the vapour pressure of the solvents inside the capillary tube equalled the atmospheric pressure (Scodellaro, 2008) Freezing point

The freezing point is the temperature at which the liquid and solid states of a substance are in equilibrium at a given pressure (usually atmospheric). For pure substances, it is identical with the melting point of the solid form. It is also the temperature at which crystallization begins in the absence of super-cooling (ASTM, 2004). The freezing points for the three solvents employed in the extraction were determined using the following procedures.

A cold ice bath was made by mixing and stirring with a spatula about 60 ml of NaCl crystals in crushed ice (400 ml). The temperature of the ice-salt mixture was noted (about -10°C). A wide mouth test tube, one third full with the solvent whose freezing point was to be determined was placed in the ice-salt mixture.

The cold bath was insulated with a crude but effective method from the surrounding air by wrapping the 500 ml beaker in layers of paper towels enclosed in aluminium foil and held by means of tape. The mouth of the test tube was tightly fitted with a neoprene stopper with 2 holes for the insertion of a thermometer and a stirrer. The liquid was stirred slowly but continuously to minimize super-cooling and to maintain a uniform temperature. Immediate recording of the solvent temperature was performed at about 10°C every 20 s. As the temperature approached the freezing point of the mixture, crystals began to form and a constant temperature was noticed. An average of these nearly constant values of temperature (nearest 1°C) was calculated and recorded as the freezing point of the solvent (ASTM, 2004). Specific gravity The specific gravities of the three extraction solvents were determined using the same procedure described for the used oil‟s density determination.

Dynamic viscosity The kinematic viscosities of the three extraction solvents were first determined using the same procedure described for the used oil‟s kinematic viscosity determination, after which the following formula was applied.

𝑣 = 𝑘𝑥𝑡…………………………… .6 Where: „v‟ is the Kinematic viscosity „ρ‟ is the liquid density. Flash point A clean Cleaveland open cup tester was used using the same procedure described for the used oil‟s flash point determination to determine the flash points of the three solvents. Refractive index The refractive index was measured according to ASTM-D1218 for each of the extraction solvents. Same procedure was used as that for the used oil‟s determination. Extraction methodology

The extraction was carried out successively using the following steps: Dehydration

The location of automobile usage is major determinant of the water found in lubricating oil in service (used lube oil). These traces of water are in most cases inevitable under normal operating conditions arising mainly from air entry into the engine. The waste oil was allowed to settle down for about 24 h and all the free water and sediments were separated by decantation. For further removal after decantation, the oil was heated to a temperature of about 150°C for a period of about 30 min after which the oil was cooled before using it for the next steps. Solvent extraction

A specific volume of the dehydrated oil was poured into a 250 ml beaker after which an appropriate volume of solvent was measured with the aid of a measuring cylinder and added to the oil in the beaker (Equal

Epelle et al 013 volumes of oil and solvent were used initially; a solvent to oil ratio of 1:1, and subsequently, other solvent to oil ratios and different temperatures of extraction were investigated). The solvent-oil mixture was heated to an initial desired temperature of 40°C with the aid of a magnetic stirrer with hot plate after which the magnetic bar was placed in the mixture and stirring began immediately. Temperature control was achieved with the help of an attached thermometer which was dipped into the mixture and also a temperature regulator on the magnetic stirrer. Mixing of the materials at a constant rate at the specified temperature was continued for a period of 30 min with the aid of a rotation regulator on the magnetic stirrer. During the chemical extraction/mixing process the solvent selectively absorbed contaminants and residual aromatic compounds in the oil (Figure 1). Separation

The mixture of the oil and solvent was poured into a separating funnel for the separation of the two phases which was observed as a clear interfacial difference by means of density variation. A precautionary action was taken to ensure bubbles generated on mixing the two liquids were expelled by slightly opening the cork before clamping to the ring stand. This was left undisturbed for a period of 1 h for adequate separation to take effect into the 2 respective phases. The upper liquid phase being the raffinate solution and the bottom liquid phase the heavier extract solution. Material balance enclosure was ensured by weighing the different phases with the aid of a digital weighing balance. The raffinate phase was collected and analyzed for different physical properties whereas; the extract was weighed and thereafter discarded. The overall procedure of extraction and separation was performed in turns for the three solvents; furfural, phenol and NMP respectively (Figures 2 and 3). RESULTS AND DISCUSSION OF RESULTS Characterization of used engine oil Results obtained from the analysis carried out on used oil showed acceptable quality as compared with results from other researchers on Nigerian used oil such as (Olugbodi and Ogunwole 2013; Owolabiet al., 2013). In their work, experimental determination of the physical properties of typical Nigerian fresh lube oil was performed and the results are shown in Table 2 for the purpose of comparing used oil properties with that of Fresh lube oil. Specific gravity The chemical composition of used oil greatly affects its specific gravity with aromatic compounds being the major

014 Int. Res. J. Eng

Figure 1. Magnetic stirring of used oil under temperature observation. 1-Thermometer, 2- Ring Stand, 3-Beaker (with oil and solvent), 4-Display, 5-Temperature and Rotation regulators.

Figure 2. Expelling air bubbles before clamping.

Epelle et al 015

Figure 3. Phase separation using a separating funnel.

Table 2. Comparison of physical properties of used and fresh lube oil.

Physical Properties Used lube oil Fresh lube oil

Viscosity at 40°C (cSt)

Viscosity at 100°C (cSt)

Viscosity Index

Specific Gravity @ 25(°C)

Refractive Index

Pour Point (°C)

Boiling Point (°C)

Flash Point (°C)

Colour

163.2

14.67

88

0.915

1.432

-3

310-350

210

>5

22-248

18.5-22

95

0.899

1.4886

-12

-

246

>5

determinants of the final specific gravity of the oil. An increase in aromatic constituents of the oil yields an increase in specific gravity compared to an oil with a higher composition of saturated compounds (Hamawand et al., 2013).

Another major determinant of the specific gravity of used oil asides aromatic composition is the amount of fine particles of suspended solids in the oil. According to Forsthoffer (2011), one percent weight of solids present in the used oil could increase the specific gravity by 0.007. According to the characterization of the used oil

sample shown in Table 2, fresh lubricating oil has a specific gravity of 0.899 compared to the spent oil with a specific gravity of 0.915. The high value of specific gravity of the used oil could also be attributed to the presence of contaminants, metals and degraded products. Refractive index This characterization parameter is a measure of the bending of light from one medium to another. The

016 Int. Res. J. Eng refractive index of an engine oil can also be regarded as a measure of the engine‟s oil composition. A highly contaminated oil would tend to bend a light ray more than fresh lubricating oil. Hamawand et al. (2013) proposed that low values of refractive index indicate the presence of paraffin material whereas high values indicate the presence of aromatic compounds (Riazi and Roomi, 2001). Table 2 shows the refractive index of used oil sample as 1.432. This value is as a result of the presence of contaminants which also cause an increase in the molecular weight of the oil. Flash point The flash point of the used oil was determined experimentally as 210°C. Since flash points are good indications of the boiling point of an oil, coupled with the fact that the presences of impurities in a solvent greatly affect the boiling point, this high value of flash point shows that the spent oil would have a high boiling point range (310-350°C). A drop in flash point reflects contamination due to dilution of lubricants with unburned fuel during engine operations. If oil contains 3.5% or greater of fuel, the flashpoint will potentially reduce by 55°C (Hamawand et al., 2013).

This could serve as a qualitative test for identifying the components of contamination. The chemical oxidation of these oils results in the formation of volatile compounds which in turn causes a decrease in flash point (Lenois, 1975).

Viscosity Viscosity is considered to be the most important consideration in the choosing of lubricating oils. The oil film‟s strength is an appropriate measure of its viscosity. According to Diaz et al. (1996), base oil‟s production during engine use causes the production of oxidized products such as varnishes and deposits which lead to increased viscosity. Decrease in the viscosity of engine oils is a clear indication of possible fuel contamination. The kinematic viscosities of the used oil at 2 different temperatures of 40 and 100°C are shown in Table 2 which is 163.2cSt and 14.67cSt respectively. These values later dropped drastically in the refined oils to 132.06 at 40°C for NMP extraction at a solvent to oil ratio of 1:1. This signifies that most of the contaminants were selectively removed by the solvents which yielded lower viscosities in all cases of operating conditions of this investigation. Viscosity index High values of viscosity index could be due to the absence of aromatic compounds, volatile compounds as

well as contaminants. High viscosity indexes are indication of excellent thermal stability and low temperature flow behaviours (Singh and Gulati, 1987). From Table 2, the calculated viscosity index of used oil which was 88, which was later improved to different degrees by the selective action of the three extraction solvents employed. Pour point From the results of the characterization of the used oil sample shown in Table 2, the pour point of the used oil sample was -3°C. The pour point is an indication of the waxy content of the used oil and its ability to flow at low conditions of temperature within the engine via the oil pump. The value of pour point obtained for this oil is quite satisfactory which is expected to improve on treatment using the solvent extraction process. This value of pour point indicates that adequate dewaxing was done in preliminary stages of its production and also that the oil will flow at low conditions of low temperature even in its untreated state. The extent of contamination by impurities on the pour point of this oil could be regarded negligible. Characterization of extraction solvents The results of the characterisation of the three solvents are shown in Table 3. The characterizations of the solvents were carried out using standard ASTM procedures. Some of the physical properties determined include; dynamic viscosity (cP) at 25°C, specific gravity at 25°C, refractive index, boiling point and flash point. The values of these parameters for these solvents were in good conformity with standards from literature indicating that these solvents were of a high degree of purity as earlier mentioned in the previous chapter.

Effect of process variables on extraction Application of automobile engine oils as lubricants on motor engines necessitates specific and optimized chemical composition which can satisfy the required chemical lubricating oil properties. A as a result of the continuous use in engines, the chemical composition of the oil is changed with time mainly due to oxidation, oil deterioration and contamination by air, moisture, dust and dirt; whose overall effect is a decrease in effectiveness of the oil‟s performance. Re-refining of spent engine oil using solvent extraction should therefore re-establish the optimum chemical composition of the oil as well as improve the oil‟s viscosity index. The extraction parameters- temperature, solvent to oil ratio and solvent type should be fine-tuned in order to ensure optimum chemical composition of the re-refined oil. In this study, the aforementioned parameters of solvent extraction

Epelle et al 017 Table 3. Physical properties of furfural, phenol and NMP solvents.

Physical properties Furfural Phenol NMP

Dynamic Viscosity (cP)

Refractive Index

Specific Gravity

Boiling Point (oC)

Freezing Point (oC)

COC Flash Point (oC)

1.46

1.5235

1.153

161

-36

66

1.57

1.5424

1.06

181

-

79

1.66

1.4688

1.028

202

-23

91

were varied systemically in order to obtain optimum viscosity index. For the purpose of this research work, the operating conditions of study were chosen based on the best results obtained in the works of various researchers earlier cited on the use of solvent extraction for light lubricating stock treatment which showed some degree of correspondence. Effect of temperature, SOR and solvent type on raffinate viscosity Temperature ought to be given due considerations, whenever a fluid is to be used for any application, hence, only fluids that can sustain operating temperature of such operations should be engaged for such applications else, the very purpose of using the fluid would be lost. The combustion process of fuels in engines indicate that lubricating oils would be invariably heated to very high temperatures, hence the knowledge of the lubricating oil ability to be viscous enough to carry out lubrication of moving parts in engines is very vital.

There are possibilities that some oils may lose their viscosity and even start evaporating under these harsh conditions of operation. Effective operation of motor engines therefore largely depends on the viscosity of the lubricating oil being used; and as a result of this, the viscosity of the lubricating oil is considered a very important factor in lube oil manufacture. In this work, the effects of solvent to oil ratio 1:1, 2:1, 3:1, and 4:1 at 100°C as well as temperatures (40, 60, 80 and 100°C) at 1:1 SOR on kinematic viscosities of the raffinate phases at 100°C using the three solvents were studied. Figures 4 and 5 had shown the effect of temperature and solvent to oil ratio on the kinematic viscosities at 100°C for furfural, phenol and NMP respectively.

It was generally observed that the viscosities of the raffinate phases from the extraction decreased with increasing temperature as well as solvent to oil ratio. The decrease in viscosity was due to increased temperature and this gives credence to the molecular theory that the time of interaction between neighbouring molecules of the liquid decreases because of the increased velocities of the individual molecules.

The decrease in viscosity due to increased solvent to oil ratio could be attributed to the fact that high molecular

weight residual aromatic compounds are removed leaving behind more paraffinic content which have a relatively lower viscosity than those of aromatics. The solvent‟s capacity to selectively absorb contaminants and deposits in the spent oil is a possible reason for decrease in viscosity because accumulation of these contaminants over time could form sludge thereby causing an increased viscosity.

There is also a possibility that although with good extraction efficiency, high solvent to oil ratios have tendencies for more difficult separation using only a simple separating funnel. The resultant effect of this is that some small percentage of solvent present in the raffinate phase which contributes to the decrease in the apparent viscosity of the raffinate mixture.

There is also a clear indication from Figures 2 and 3 that the extraction temperature has a milder effect on the raffinate viscosity compared to the effect of the solvent to oil ratio. The raffinate viscosity produced from furfural was slightly lower than that of phenol and NMP using the same operating variables. This could be attributed to the high solvent power of furfural compared to NMP and phenol in treating used oil.

As earlier established, possible chances of solvent as impurity in the raffinate phase increase with increasing solvent to oil ratio. The physical presence of these low viscosity solvents compared to oil‟s viscosity have more impact on the reduction in the treated oil‟s viscosity than temperature operated within a narrow varying range (Figure 6). One of the main factors affecting the overall performance of the solvent extraction process is the raffinate yield (lubricating oil yield). The effects of solvent to oil ratio and temperature on the yield of raffinate using the three solvents are shown in Figures 6 and 7 respectively.

The probability of dissolving the target unwanted compounds in any of the three solvents at low temperatures is quite low because these contaminants, unwanted hydrocarbons and heavy residual aromatics do not fully separate out.

The solvent power is increased by increasing the temperature but on the other hand, the solvent selectivity is reduced (Mohammed et al., 2007b). Therefore a deeper and more efficient extraction is obtained but at a lower yield. This implies that the solubility of target and

018 Int. Res. J. Eng

Figure 4. Effect of solvent to oil ratio on raffinate viscosity at 100°C for NMP, furfural and phenol

extraction.

Figure 5. Effect of temperature on raffinate viscosity at 100°C for NMP, furfural and phenol extraction.

unwanted components of the oil is enhanced with increase in temperature; however it is rather unfortunate that simultaneously, the solubility of the required product fractions also increase with respect to the solvent. It is depicted clearly from Figures 6 and 7 that the percentage yield decreases with increase in temperature; but a far greater decrease is observed with the effect of solvent to oil ratio. This trend was observed for the three solvents with lowest yields obtained at highest solvent to oil ratios. Again the solvent impurity in the raffinate phase has a greater effect on yield compared to temperature.

Effect of temperature, SOR and solvent type on viscosity index The variation of the viscosity of a lube oil with temperature is reflected by the viscosity index. In this work the study of the effect of the temperature of the extraction and solvent to oil ratio on the raffinates kinematic viscosities at 40 and 100°C was conducted for the purpose of calculating the viscosity index and hence evaluating the performance of the three solvents.

Figures 8 reveals the relationship between solvent to oil

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Epelle et al 019

Figure 6. Effect of solvent to oil ratio on yield of raffinate in NMP, furfural and phenol extraction.

Figure 7. Effect of temperature on yield of raffinate in NMP, furfural and phenol extraction.

ratio and viscosity index for the three solvents whereas, Figure 9 reveals the effect of temperature on viscosity index. The increase in extraction temperature will cause an increased solubility of the target undesired compounds and contaminants in the extraction solvent which increases the viscosity index of the lubricating oil. All viscosity indexes obtained after treatment with each solvent reflected an increase in viscosity index with respect to the viscosity index of the spent oil prior to solvent treatment. It is also indicated from Figure 9 that for all 3 solvents; the effect of solvent to oil ratio had more impact on increasing the oils viscosity index than temperature.

This implies for economic viability of lube oil re-refining plants, a compromise has to be made, taking into consideration; high operating costs of operating at high solvent to oil ratios and best temperature of extraction with also desired selectivity of lube oil. NMP showed a greater ability to increase the oil‟s viscosity index than furfural which in turn gave better results than phenol. Performance comparison of the three solvents For the purpose of selecting the most efficient solvent for proper extraction of unwanted materials and

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020 Int. Res. J. Eng

Figure 8. Effect of solvent to oil ratio on viscosity index for NMP, furfural and phenol extraction.

Figure 9. Effect temperatures on viscosity index for NMP, furfural and phenol extraction.

contaminants present in the oil used in this study, a comparison between the viscosity index and yield of raffinates obtained from the three solvents extraction processes were compared in this investigation.

The comparison was done at an extraction temperature of 100°C and solvent to oil ratio of 4:1 for furfural, phenol and NMP respectively. Figures 10 and 11 shows the effect of solvent type against raffinate viscosity index and yield. It is clearly indicated that the VI of NMP is higher than VIs gotten from furfural and phenol extraction with

phenol having the least VI of 94. A solvent to oil ratio of 1:1 for the NMP extraction as seen from the Figure 8 would produce the same VI of 94 as phenol did at a solvent to oil ratio of 4:1. Low solvent to oil ratios of NMP extraction can be used to produce the same raffinate as 4:1 solvent to oil ratio of phenol. Furfural and NMP had close performance characteristics; however, NMP displayed slightly higher extraction efficiency than furfural with the highest VI of 105 compared to furfural with a value of VI at 103.

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Epelle et al 021

Figure 10. Comparison of viscosity index of raffinates from extraction using NMP, phenol and furfural at 100°C and 4:1 SOR.

Figure 11. Comparisons of the raffinate yields from extraction using NMP, phenol and furfural at

100°C and 4:1 SOR.

Judging in terms of the yield of extraction, raffinate yield obtained by furfural was highest compared to NMP and phenol. This could however be attributed to the wider range of density difference between furfural and used oil as compared to other two solvents.

Also, the lower viscosity of furfural as seen from Table 2 gave it a technical advantage in proper separation of

the two phases using the separating funnel compared to phenol and NMP which were more vulnerable to separation difficulties due to moderately high viscosities. Furfural can be said to be a better solvent in terms of the yield of extraction obtained compared to NMP, However NMP gave the best viscosity index which is the major property of concern and therefore would be taken as the

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022 Int. Res. J. Eng solvent with the best performance. Conclusion Used lubricating oil gotten from automobiles is a very important resource that should not be wasted by being burned as fuel. Using the best available techniques such as solvent extraction, it can be regenerated to the quality that is equal to or even better than the virgin base oil. The ability of the solvent extraction technique in separating the general group of offensive materials and hydrocarbons from used oil was demonstrated in this research work by improving the viscosity/temperature characteristics of the used lubricating oil. The three solvents employed (phenol, furfural and NMP), also showed to different degrees their capacities not only in dissolving the residual aromatic content but also the contaminants present. Best results of performance (at solvent to oil ratio of 4:1 and 100°C) during the extraction process were 94, 103 and 105 for phenol, furfural and NMP respectively.

From the results obtained, the parameter of solvent to oil ratio had more effect on the yield and viscosity indexes of the raffinate phase compared to temperature. Although, the highest solvent to oil ratio used in this work (4:1) is within limits of economic feasibility as reported from diverse literature, a compromise would have to be made in operating the extraction process at a higher temperature and at some lower solvent to oil ratio to save cost if the results from this work would be extrapolated to an industrial scale for a wider range of application.

Furfural showed better ability than NMP in terms of yield of raffinate obtained but at the expense of lower values of viscosity index. Phenol although improved the Viscosity index of the used lube oil, but in all cases gave the worst performance and was rather too problematic in its physical handling due to its level of toxicity; hence it may not be considered for further applications in this area of used oil re-refining except where the cost of furfural and NMP cannot be afforded. NMP gave the best performance results in terms of VI improvement and may be regarded as the best among the three selected solvents for used oil treatment as far as this work is concerned.

Finally, waste oil represents an important energy resource that if properly handled and reused would reduce the level of environmental pollution and also dependence on imported fuels. RECOMMENDATIONS 1. Further study on a wider range of operating extraction conditions of temperature and solvent to oil ratio should be carried out to determine optimum operating conditions using these solvents.

2. Blending treated oil using these solvents with other additives such as VI improvers, Pour point improvers, and anti-oxidants should be done in order to compare the performance with freshly produced lubricant oil. 3. Detailed economic analysis and plant simulations of re-refining used oil using this technique should be carried out in order to compare the production processes of used lube oil re-refining and refining crude oil into lube oils. 4. It has been proposed from diverse literature by many researchers that the presence of a co-solvent such as water in little amounts particularly with NMP and phenol could boost its solvent power. A detailed study on this finding should be carried out using Nigerian lube oil. 5. Further analysis on the extent of metallic content removal by these solvents from the used motor oil should be performed and should be compared with other conventional re-refining techniques such as vacuum distillation, clay refining and adsorption. 6. With the current efforts worldwide, especially in Africa, to create wealth from waste, any efforts made at whatever level towards achieving this, should be appreciated because the benefits on the environment and the economy are enormous. 7. The construction of lube-refining plants should be embarked on by both the government and the private sector, as this would encourage more research in this area using other solvents and also create more jobs and constitute a lucrative business venture. Furthermore, the higher the capacity of the plant, the less the capital cost is required per metric ton of used oil processed. It truly shows “Bigger is better”! REFERENCES Abdel-Jabbar, M. N., Al-Zubaidy, A. E., and Mehrvar, M.

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