Article pubs.acs.org/IECR
Regeneration of Waste Lubricant Oil by Extraction−Flocculation Composite Refining Xin Yang, Ligong Chen,* Shuo Xiang, Liang Li, and Di Xia Department of Military Oil Application & Management Engineering, Logistical Engineering University of PLA, Chongqing 401311, China ABSTRACT: Waste lubricant oil can be regenerated by solvent extraction and flocculating agent to get rid of impurities. In this paper, butanol was selected as an extractant, and monoethanolamine (MEA) as a flocculating agent. The aim of the work was to identify the best refining conditions to separate base oil suitable for the formulation of new lubricants and avoid the coextraction of impurities. The effects of time, temperature, solvent/waste oil mass ratios, and flocculant concentration on the separation efficiency and yields were investigated. Optimization conditions of the refining were obtained by experiments: refining time = 20 min, temperature = 30 °C, solvent/waste oil mass ratios = 5g/g, flocculant concentration = 2g/kg solvent. When comparing ibutanol/MEA and n-butanol/MEA, it was found that yields obtained with composites of i-butanol/MEA were slightly higher than those obtained with n-butanol/MEA. On the other hand, because MEA not only agglomerates waste oil contaminants but also reduces the dosage of solvent, the solvent/waste oil mass ratios at which stabilization occurs with composite butanol/MEA were smaller than that obtained with single pure solvent (n-butanol, i-butanol). Under composite refining, metallic and oxidation compound removal was very efficient, and the properties of the recovered oil were notably improved than that in the original waste oil. The recovered oil was almost similar to a HVI150 virgin oil and, therefore, suitable for the formulation of new lubricant oil.
1. INTRODUCTION Waste lubricating oils are byproducts of oil used in vehicles and machinery. Oil must be replaced because of the degradation of the fresh lubricant components and contamination from dirt, water, salts, metals, incomplete products of combustion, additives, and other asphaltic compounds coming from overlay on bearing surfaces. Once replaced by new lubricants, the used oil is a matter of greater environmental concern: polluting water and earth and causing air pollution if burnt. Therefore, to prevent environmental pollution and to conserve petroleum sources, waste oil should be regenerated. Several technological processes have been proposed to recycle waste oil. The most familiar technology is the acidclay process, but the process was prohibited because the acid sludge is a source of pollution that causes serious environmental problems.1 In the acid-free clay process the waste oil is pretreated using atmospheric distillation to remove water and light hydrocarbons followed by vacuum distillation and hydrogenation treatment.2 However, this process is not highly efficient in removal of contaminants, and low quality recovered oil is obtained with unpleasant smells and poor stability.2−4 As mentioned above, persistent efforts should be made to improve waste oil regeneration technologies. Using a single solvent or composite solvents to extract base oil from waste oil was a feasible alternative before vacuum distillation, which is economical, efficient, and safe. Unfortunately waste oil constituents are kept in a stable dispersion as a result of electric repulsion between heteroatoms and asphaltic compounds in the solvent system, a result of which is that the impurities cannot be separated from the base oil. 5,6 Accordingly, the addition of a flocculating agent exerts an action to break this stability and promotes a fast flocculation of impurity particles. © 2013 American Chemical Society
In this study, alcohols with four carbon atoms were selected as extractants that dissolve the base oil, and monoethanolamine (MEA) was chosen as a flocculating agent that removes the contaminants from waste oil for the purpose of reusing it as the lubricant base oil. First, this process seems to overcome the major problem of the sulfuric acid treatment: the production of an acid sludge, which is a source of pollution and causes very difficult disposal problems. Then, for the process solvent can be recovered and recycled, since the organic sludge produced may be used as an asphalt component or may be mixed with liquid fuels and burned. The best use for this sludge is, according to Alves dos Reis et al., an offset ink component.7,8 More specifically, the effect of the refining time, refining temperature, solvent/waste oil mass ratios, flocculant concentration on both the yield and quality of the recovered oil were studied. The quality was evaluated through the determination of physicochemical properties, metallic compounds, and oxidation products. The major purpose of this paper is to offer a feasible choice for waste oil recycling by extraction−flocculation, and select the best process conditions leading to both high yield and quality of the recovered oil.
2. EXPERIMENTAL SECTION 2.1. Materials. Organic solvents (n-butanol, i-butanol) were supplied by Chongqing Reagent Company. Waste lubricant oil was supplied by Swift Horse Engine Repair Factory. First, the waste oil was treated in a rotary evaporator at 70 °C under Received: Revised: Accepted: Published: 12763
May 12, 2013 July 31, 2013 August 19, 2013 September 3, 2013 dx.doi.org/10.1021/ie4015099 | Ind. Eng. Chem. Res. 2013, 52, 12763−12770
Industrial & Engineering Chemistry Research
Article
As noted before, a large number of pure organic liquids, such as hydrocarbons, alcohols, and ketones exhibited the properties mentioned above and were applied to the extract waste oil.2−6,9−12 The following points were considered in the selection of an optimal solvent: Hildebrand solubility theory of the solvents, the existing background on the regeneration of waste oil by solvent extraction, and the selectivity or capacity of the solvent to selectively extract base oil from waste oil. First of all, it should be noted that impurity particles usually have a high molecular weight. According to the Hildebrand solubility theory,13 they should dissolve more easily in the heavier solvents because the solubility parameters of the heavier solvents are close to those of the impurity particles. Therefore, in this study, low-molecular weight solvent was used, which inhibits the dissolution of the impurity particles. Then by previous works, when mixed with waste oil, liquid hydrocarbons not only extract the base oil fractions, but also the solution of macromolecular and other additives as well as dispersion of carbonaceous and other particles.5,6 Consequently, the liquid hydrocarbons were excluded in this research. According to Rincon et al.2−4 and Alves dos Reis et al.,5,6 the ketones and alcohols, between three and five carbon atoms, have been studied to recycle used oil. As a result of the molecular weight, alcohols with three carbon atoms can dissolve little base oil, while those with five carbon atoms not only extract base oil but also dissolved impurities.2−6 In regards to the ketones, the only ketone with less than four carbon atoms, propanone, was almost immiscible with base oil at room temperature. On the other hand, both base oil and impurities were dissolved in ketones with five carbon atoms, which act the same as the alcohols with five carbon atoms. Besides, ketones were strictly controlled in China and availability was difficult. Therefore, it was found that alcohols with four carbon atoms performed as the efficient extraction solvent. Finally, the dipole of butanol was close to that of the lubricating oil (naphthenic distillate) (see Table 2), which is important for support of the solvent to selectively extract base oil from waste oil.
vacuum (500 mmHg) to eliminate water and light hydrocarbons. Both types of compounds are undesirable for the formulation of new lubricants and may modify the property of base oil components in the solvent. Waste oil properties after this treatment are shown in Table 1. At the industrial scale this step usually performed at atmospheric pressure. We made it under vacuum to accelerate the separation process. Table 1. Properties of Waste Lubricating Oil waste oil appearance w (water) (%) v40 (mm2.s 1) v100 (mm2.s 1) viscosity index flash point (°C) pour point (°C) w(sulphur) (%) w(sulfate ash) (%) oxidation stability 150 °C/min acid number (mgKOH·g−1) w (metallic content) (mg/kg) Zn Ca Mg Fe
black opaque trace 104.65 11.58 92.5 184 −7 0.29 0.77 148 1.53 713 1090 332 238
2.2. Extraction Procedure. Mixtures of about 20 g of pretreatment used oil and solvent in solvent/waste oil mass ratios ranging from 1/1 to 11/1, and MEA in weight proportions ranging from 1g/kg solvent to 3g/kg solvent were agitated to ensure adequate mixing (see section 4.1). Then, the mixtures were poured into centrifuge tubes that were later introduced into the support of a centrifuge. After centrifugation at 600 rpm for 8 min, a sludge phase (additives, metallic and oxidation compounds, and carbonaceous particles) was segregated from the mixture of solvent and oil. The solvent was separated from the mixture solution by distillation in a rotary evaporator and the recovered oil was weighed. The extraction yield was calculated as the mass of oil, expressed in grams, separated from 100 g of waste oil. The sludge phase was also weighed in all experiments. 2.3. Analysis of the Metallic Content. The oil samples were heated at 200 °C for 4 h and calcined at 650 °C for 8 h prior to the analyses. The noncombustible residue obtained (ashes) was further treated with hydrochloric acid, filtered, and diluted with deionized water to determine the metallic content. These analyses were performed by atomic emission spectroscopy using an inductively coupled plasma Spectra HK-9600 spectrometer. 2.4. Other Analyses. Sulfur content was measured with a fluorescence measuring apparatus of ZDS-2000. Acid number, pour point, flash point, sulfate ash, oxidation stability, and viscosities at 40 and 100 °C were determined according to GB/ T264, GB/T3535, GB/T3536, GB/T2433, SH/T0193, GB/ T265 procedures, respectively.
Table 2. Dipole of the Solvent and Lubricating Results solvent property μ, D a
nistbutanolb butanolb butanolb butanolb 1.60
1.64
1.66
1.44a
lubricatingb (naphthenic distillate) 1.23−1.64
b
3.26 cm wavelength. Reference 14.
In conclusion, solvent properties suggest that alcohols with four carbon atoms were the best candidates to become the basic component of the extract solvent. In the study, s-butanol and tbutanol were excluded because s-butanol yields two liquid phases at room temperature, and the solid of t-butanol forms below 25.5 °C. 3.2. Choice of Flocculant. The impurities of waste oil are mostly composed of (a) oxidation products, (b) metallic compound, (c) degenerative additives, and (d) carbonaceous particles. The bulk of these particles are kept in stable dispersion by the dispersant additives. Author: Please verify that the changes made to improve the English still retain your original meaning.At room temperature, when mixed with waste oil, the alcohol with four carbon atoms dissolved the desired base oil. But because of the electric
3. EXTRACTANT AND FLOCCULANT 3.1. Selection of Extractant. The extractant that is selected must have two important properties: it should be miscible with the base oil contained in the waste oil being processed and it should reject from a solution with impurities. 12764
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one-factor experimental design was adopted in order to select the best process conditions leading to both high yield and quality of the base oil obtained. Effect of Refining Time. To determine the mixing time for systems to attain equilibrium, a group of experiments was made. This group of experiments was performed at temperature of 30 °C, solvent/waste oil mass ratios of 5g/g, and flocculate concentration of 2g/kg solvent. Results were obtained as follows (Figure 1). It can be observed that the extraction yields increase with time until a
repulsions, it could only segregate a small part of the undesirable materials which formed flakes that settled by gravity.3,5 To avoid the formation of stable dispersions, flocculating agents can be used and added into the solution. By the process of flocculation, particles in suspension form larger agglomerates, or small agglomerates already formed grow as a result of coagulation through high molecular weight polymeric materials.15,16 Attention should be paid to an important finding of a suitable substance as a flocculating agent for recycling waste oil. In this paper, the flocculating agent was chosen by two steps. The first step was the existing background on the regeneration of waste oil by flocculation, and the second step was the difference in the solubility parameters of the solvent and of a typical polyolefin, polyisobutylene. Polyisobutylene is generally introduced in lubricant oils as a viscosity index improver, where it acts to remove the macromolecules of polymeric additives. Moreover, according to Alves dos Reis et al,5,6 when flocculation occurs, macromolecules and carbonaceous particles coflocculate and settle together. When the solubility parameter difference is high, one should expect to obtain high sludge removal values.5,6 Some materials were proposed for this operation, namely, lbutanol by Brownawell and Renard,17 and 2-propanol by Rincon et al.,3 Alves dos Reis et al.5 and Whisman et al.,18 a substance with special amidogen groups by Zhang,19 and a sorbitol oleic acid ester with special amidogen groups by Xiong.20 All of the substances mentioned above exhibit flocculant properties and have potential utility to coagulate the additives and other undesirable impurities in waste oil. According to Alves dos Reis et al.,5,6 the flocculating action for the segregation of sludge from waste oil by polar solvents is basically an antisolvent effect exerted on some macromolecules. In this study, propanol, butanol, and MEA were chosen, and the individual component was guided by the difference |δ1 − δ2|, where δ1 and δ2 are the solubility parameters of the flocculating agent and polyisobutylene, respectively. When |δ1 − δ2| was high, high sludge removal results are expected. Table 3 suggests
Figure 1. Effect of refining time on yield. Composite, i-butanol/MEA, n-butanol/MEA; solvent/waste oil mass ratios, 5g/g; MEA concentration, 2g MEA/kg solvent; T = 30 °C.
constant value is reached. With butanol/MEA, the constant yield was approximately 81%−83%, which was achieved approximately after 20 min of refining. This was because the contact time of the solvent, flocculant, and the waste oil played an important role for systems to attain equilibrium. On one hand, it should be long enough to allow the solvent to act on the base oil contained in the waste oil being processed. On the other hand, it should allow flocculant neutralized charges of particulate matter to break stabilization by electric repulsion.11 Furthermore, flocculant (MEA) maybe adsorb and “bridge” colloidal particles by permitting their aggregation to particle sizes big enough to separate from the liquid phase.15,16 The slightly higher yield and lighter extract coloration attained with i-butanol may be explained by considering the polarity and molecular structure. It is well-known that components exhibit better solubility as their polarities get closer to each other. Since base oil is a mixture of nonpolar or slightly polar molecular components, its polarity is closer to that of i-butanol (55.2) with lower polarity than n-butanol (60.2).22 This difference is probably due to the molecular structure, as i-butanol has a branch chain the same as the base oil molecules (ring hydrocarbons and alkane hydrocarbons with branch chain), and therefore according to the law of affinity has a larger capacity to dissolve base oil components. When the mixing time is compared with that obtained by Rincon et al.,2 it can be observed that the equilibrium for 2butanol is attained at a shorter mixing time, less than 10 min. It is probably for that reason that flocculant (MEA) was added in this researchit increases the time of flocculation and sedimentation of the impurities to be removed. Finally, according to Rincon et al.2 and Alves dos Reis et al.,5,6 as regards to the longer equilibrium time of the 2-pentanol,
Table 3. Difference between the Solubility Parameters of Solvent δ1, and Polyisobutylene, δ2
a
flocculant
δ1, δ2(MPa1/2)
δ1-δ2 (MPa1/2)
polyisobutylenea 1-propanola 2-propanola 1-butanola 2-butanola MEAa
15.5 23.5 24.5 23.1 22.2 31.5
8.0 9.0 7.6 6.7 16.0
Reference 21.
that the solubility parameter of MEA was far different from that of the polyisobutylene, which would be a better flocculating agent than butanol and propanol. In summary, to improve the quality of the base oil extracted from the used lubricant oil without renouncing high extraction yields, MEA was deemed to be a good flocculating agent.
4. RESULTS AND DISCUSSION 4.1. Effect of Factors. On the basis of the statement of the basic problem, several factors involving base oil yield and quality were investigated: the effect of refining time, the effect of refining temperature, the effect of solvent/waste oil mass ratios, and the dosage of flocculating agent. In this research, 12765
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approximately 25 min, it may be because the higher viscosity of 2-pentanol increases the time necessary for the contact and dissolution of base oil in this solvent. To ensure that equilibrium is reached with the whole system, the refining time of 20 min was chosen for performing regeneration of waste oil. According to Figure 1, the recovered oil attained high yield at shorter contact time. Maybe some impurities did not have enough time to aggregate and, as a consequence, they were coextracted together with the base oil. To test this hypothesis, the metallic content of the recovered oil with i-butanol/MEA versus the evolution with time was determined. As expected, it can be observed in Figure 2 that for the shortest mixing time
Figure 3. Effect of refining temperature on yield.
Figure 2. Effect of time on metallic concentration of the recovered oil. Composite, i-butanol/MEA; solvent/waste oil mass ratios, 5g/g; MEA concentration, 2g MEA/kg solvent; T = 30 °C. Figure 4. Effect of temperature on metallic concentration of the recovered oil. Composite, i-butanol/MEA; solvent/waste oil mass ratios, 5g/g; MEA concentration, 2g MEA/kg solvent; t = 20 min.
the metallic content of the recovered oil was very similar to that of the untreated waste oil (Table 1) and the concentrations of these metallic impurities decreased as mixing times increased from 5 to 20 min, finally, being approximately constant at 20 min onward. Effect of Refining Temperature. Studies were carried out at different refining temperatures performed with i-butanol/MEA from 10 to 50 °C, refining time of 20 min, solvent/waste oil mass ratios of 5g/g, and flocculant concentration of 2g/kg solvent. Of note higher temperatures result in solvent vaporization, and consequently, temperatures beyond 50 °C were not tested. As shown in Figures 3 and 4, when the extraction temperature increased, the amount of recovered oil increased, and the percentage of base oil dissolved in i-butanol increased from 78.3% to 83.6%. However, the concentration of impurities first decreased and then increased with the increasing reaction temperature. The reason why recovered oil yields constantly increased might be due to two factors: on one side, with refining temperature increasing, the viscosity of organic solvent mixtures be reduced, which results in an increase in the amount of oil dissolved in i-butanol.2,3 On the other side, the temperature was higher and the solubility of the base oil components in the extraction solvent increased more. Meanwhile, as regards to the content of impurities with increasing temperature of the variable, it may be attributed to the increased solubility of solvent, which results in a large number of impurity groups dissolved in solvent. On the whole, the optimum temperature should result in, simultaneously, maximum sludge removal and minimum oil losses. Therefore,
the optimum reaction temperature was considered to be around 30 °C. Effect of Solvent/Waste Oil Mass Ratios. These experiments were performed by keeping the time and temperature at the best refining conditions selected for recovering base oil from waste lubricant oil with i-butanol/MEA and n-butanol/ MEA, at 20 min and 30 °C, respectively. The flocculant concentration was 2g/kg solvent, and solvent/waste oil mass ratios varied from 1 to 11 g/g. Figure 5 shows that, for both solvents, the refining yields increased with increasing solvent/waste oil mass ratios up to a point at which they stabilize, and it can also be observed that, once yields stabilize, those obtained with n-butanol/MEA and ibutanol/MEA occur at a similar solvent/oil mass ratios of 5g/g. According to Rincon et al.,2−4 the reason for yields of recovered oil to increase with increasing solvent/waste oil mass ratios and then to stabilize is probably because of the combined effect of the following factors: at the smaller ratios, the solvents saturate and do not dissolve all base oil in the waste oil. Then, the base oil dissolved increases with increasing solvent/waste oil mass ratios, but only up to a ratio at which the base oil fraction that the solvent may dissolve is exhausted; thus, from this moment, the yield will not increase anymore. It can also be indicated that the solvent/waste oil mass ratios at which stabilization occurs with butanol/MEA is smaller than those obtained with single butanol as shown by Rincon et al.2 and Alves dos Reis et al.5,6 In these cases, we concluded that MEA could not only 12766
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oil. Figures 7 and 8 show the metallic content of the recovered oil at different solvent/oil mass ratios with composition of i-
Figure 5. Effect of solvent/waste oil mass ratios on yield. Composite, i-butanol/MEA, n-butanol/MEA; MEA concentration, 2g MEA/kg solvent; t = 20 min; T = 30 °C.
Figure 7. Effect of solvent/waste oil mass ratios on metallic concentration of the recovered oil. Composite, i-butanol/MEA; MEA concentration, 2g MEA/kg solvent; t = 20 min.
agglomerate waste oil contaminants but also reduce the dosage of solvent, to accelerate equilibrium for the system. To test this hypothesis, the stabilization of solvent/waste oil mass ratios obtained with a single solvent (n-butanol, i-butanol) was determined. As expected, it can be observed in Figure 6
Figure 8. Effect of solvent/waste oil mass ratios on metallic concentration of the recovered oil. Single solvent, i-butanol; t = 20 min; T = 30 °C.
butanol/MEA and i-butanol. These results supported the explication mentioned above. Lastly, the darker coloration of the base oil with single i-butanol recycling was more evidence to indicate a higher level of impurities in recovered oil. According to the previously mentioned results, a solvent/oil mass ratio equal to 5 g/g was selected for performing experiments. Effect of Flocculant Concentration. This group of experiments was performed at a refining time of 20 min, refining temperature of 30 °C, and solvent/waste oil mass ratios of 5g/ g. It can be seen in Figure 9 that, refining yields at first decreased as the concentration of MEA in the solvent increased up to a point, and then the refining yields begin to increase as the concentration of MEA in the solvent increased. The fact that refining yields decreased as the concentration of MEA increased may be imputed to two different factors. First, with an increasing addition of MEA, charged particles were adsorbed by the MEA and aggregation occurred as a result of particle− particle collisions, which led to small clusters. Consequently, due to particle−cluster and cluster−cluster collisions, small clusters formed larger aggregates and sedimentation.15,16 In addition, maybe hydroxyl(−OH) and amido(−NH2) of MEA
Figure 6. Effect of solvent/waste oil mass ratios on yield. Single solvent, i-butanol, n-butanol; t = 20 min; T = 30 °C.
that without the addition of MEA, the solvent/waste oil mass ratios up to 9 for the system are stabilized, which is almost 2 times higher than that by addition of MEA. Note that for butanol to be miscible with both the base oil and flocculation of waste oil impurities, the solvent used must be plentiful enough to ensure the system reaches equilibrium, since the addition of MEA keeps the suspended impurity particles separated from waste oil by neutralizing the electric forces, and MEA may also may adsorb two or more particles by “bridging”.23−25 As a result, the consumption of solvent was reduced which exerts the dominating action to dissolve base oil molecules. According to Figure 6, it can be seen that, just as in the single pure solvent case, once stabilization was reached, yields of recovered oil obtained with single pure alcohol were higher than the composition of solvent and flocculant. A possible explication of this finding might be that, in the case of refining waste oil by a single alcohol, impurities did not completely agglomerate and were partly coextracted together with the base 12767
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concentration of MEA over 2g/kg, the metallic concentration in the recovered oil began to increase. It also should be considered that the MEA exhibits alkalescence, which was introduced to neutralize the acid compounds of the waste oil; in principle, more MEA that is added leads to less acidic compounds of the recovered oil. To confirm the last hypothesis, the quality of the oil refined was assessed through the measurement of the concentrations of oxidation products. Relative concentrations are shown in Table 4. It can be seen Table 4. Effect of the MEA concentration on the acid numbera
Figure 9. Effect of flocculant concentration on yield. Composite, ibutanol/MEA, n-butanol/MEA; solvent/waste oil mass ratios 5g/g; t = 20 min; T = 30 °C.
were formed and by hydrogen bonding were adsorbed onto colloidal particles where individual particles can become attached to two or more particles, thus “bridging” them together to sedimentation.23−25 However, after a concentration up to 2g/kg, refining yields begin to increase as the concentration of MEA increases. This phenomenon may be attributed to the equilibrium which was destabilized by the addition redundant of MEA. In other words, maybe the hydrogen bonds between the polar impurities and the dispersant-detergent additives of the waste oil were broken by superabundant MEA.23−25 Therefore, contaminations did not completely agglomerate and were partly coextracted together with the base oil. According to La Mer et al,26−28 the most perfect impact for “bridging” is when the surface of impurity particles is covered with about 50% flocculating agent, otherwise, destabilization occurs because colloidal particles adsorb the surplus of flocculant and, adversely effect the “bridging”. To test this hypothesis, the concentration of a type of impurities (metallic compound) in the recovered oil was determined. Results from these analyses are shown in Figure 10. Confirming the suggested hypothesis, results indicated that the metallic concentration in the recovered oil began to decrease with a larger concentration of MEA, while with a
(g of MEA)/(kg of solvent)
acid number (mg of KOH)/g
0 0.5 1 1.5 2 2.5 3
1.53 0.56 0.18 0.08 0.04
