Environ. Sci. Technol. 2000, 34, 3469-3473
An Environmentally Friendly Process for the Regeneration of Used Oils D I D I E R G O U R G O U I L L O N , * ,† LUC SCHRIVE,‡ STE ´ PHANE SARRADE,‡ AND GILBERT M.RIOS§ Instituto de Tecnologia Quimica e Biologica, Av. da Republica, Quinta do Marques, Apartado 127, 2780-901 Oeiras, Portugal, Commissariat a` l’Energie Atomique, DCC/DTE/SLC, Laboratoire des Fluides Supercritiques et des Membranes, BP 111, 26702 Pierrelatte Cedex, France, and Laboratoire des Mate´riaux et des Proce´de´s membranaires UMR 5635, Ecole Nationale Supe´rieure de Chimie, 276 rue de La Gale´ra, 34097 Montpellier Cedex, France
We describe in this paper a process of filtration of highly viscous liquids assisted by pressurized CO2, and we present preliminary results obtained during the treatment of motor oils (two mineral oils and one used motor oil). Filtration and concentration runs are commentated in terms of permeate flux enhancement and metal retention. The behavior of these fluids during filtration exhibits significant discrepancies from the one observed with previously investigated model compounds. The permeate flux improvement is about 400% for mineral oils and about 200% for the used motor oil (PCO2 ) 15 MPa, ∆P ) 1 MPa, T ) 353 K, cutoff of the membrane: 300 kDa). The presence of many pollutants can explain the lower improvement in the latter case. The performance of the membrane with regard to the retention of metals is very satisfactory: in the case of zinc and copper, the separation is higher than 99.5%. The 32 h concentration run carried out at 393 K made it possible to regenerate 96% in mass of the used oil and to confine the majority of the metals in a black gum extremely viscous.
Introduction Used oils are a toxic waste that has daily harmful repercussions on the environment and on our quality of life. For example, dumping used oils into sewers, on the ground, or into the trash can have disastrous consequences for freshwater and aquatic life because they contain many toxic substances such as lead, benzene, zinc, cadmium, and arsenic. The world awakening in the field of the ecology, illustrated by the conference of Rio in 1992, encouraged many countries to set up a policy of processing wastes. Consequently, a certain priority is given today to the recycling of used oils in many industrialized countries with two objectives: the respect of the environment and the economy of raw materials. Unfortunately, the traditional acid/clay regeneration process, in addition to the many problems of corrosion that it involves, presents the disadvantage to produce great quantities of acid muds whose elimination is generally costly
because of their high percentage of sulfur (about 15%) as well as metals and metalloids (about a few percents). That is the reason, when technical and/or economical conditions do not allow it, incineration in cement factories or waste treatment centers with energy recovering still remains an acceptable solution. In France, for example, although national and European Community regulations give the priority to the regeneration, the elimination of collected oils is still realized by incineration for 65% of tonnages. The economical aspect of the recycling of used oils is also a crucial point in this end of century where it is estimated that the world production of oil at a low price is subjected to decline in the next decade. The reprocessing of used oils would permit on one hand to protect the environment and on the other hand to reduce the imports of oil products. It is thus necessary to optimize the performance of the existing processes of regeneration or to develop new processes that could be competitive from an environmental and economical point of view. The objective of this study is the implementation of a process for the regeneration of used oils based on separation by ultrafiltration membranes. Due to the high viscosity of these effluents, we propose to use supercritical CO2 (SC CO2) as a viscoreductor agent in order to improve the membrane permeability and thus the energetic consumption of the processing (1, 2). The reasons that justified the use of SC CO2 are the following ones: 1) The dissolution of SC CO2 induces a significant lowering of the viscosity of the considered fluid whose amplitude is function of the parameters pressure and temperature, i.e., function of the dissolved quantity of CO2 (3). 2) The solubility of SC CO2 in oils is sufficiently large in a range of pressure and temperature easily accessible at the industrial scale (P < 25 MPa, T < 393 K) for a notable viscosity reduction, without generating an excessive dilution (4, 5). 3) The use of low temperatures (T < 393 K) permits the reduction of the costs in terms of equipment, security, energetic consumption and allows the treatment of thermolabile compounds. 4) CO2 is easily eliminated after processing by a simple pressure drop and does not require any additional and costly separation stage. Moreover it can be recycled upstream of the process. 5) CO2 does not present any risk of ignition or explosion and thus permits reduction of the costs related to particular safety measures. The use of SC CO2 has already been investigated for the regeneration of used oils by extraction (6). Supercritical Fluid Extraction (SCFE) is an advanced separation technology that has the features of high selectivity and low temperature processing. Nevertheless, in that specific case, the yield that is dependent on the solubility of the oil in the SCF is generally low, and therefore the process may not be suitable for the treatment of large quantities. Work which was undertaken to this end consisted initially in investigating the feasibility of the process during filtration of model compounds (poly(ethyleneglycol)s or PEG) using ceramic ultrafiltration membranes. The results thus obtained enabled us to get a better understanding of the influence of the working conditions and hydrodynamics on the efficiency of the process (1). The second part of this study, whose results are exposed hereafter, is devoted to the implementation of the process in the case of real products: the motor oils.
Experimental Section * Corresponding author phone: 351-21-446-9444; fax: 351-21-4411277; e-mail:
[email protected]. † Instituto de Tecnologia Quimica e Biologica. ‡ Commissariat a ` l’Energie Atomique, DCC/DTE/SLC. § Ecole Nationale Supe ´ rieure de Chimie. 10.1021/es991392g CCC: $19.00 Published on Web 07/08/2000
2000 American Chemical Society
Carbon Dioxide and Oils. The carbon dioxide provided by the Carboxyque company was used as received without any preliminary treatment (purity: 99.7% in mass). Two types of oil were employed for this study: 1) two nonadditiveVOL. 34, NO. 16, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Characteristics of the Oils oil
S3699
S3701
S5010
grade a d15 4 viscosity at 313 K/m2.s-1 viscosity at 373 K/m2.s-1 viscosity indexb ignition point/°C molar mass/g.mol-1 aromatic carbon/% mass paraffinic carbon/% mass naphtenic carbon/% mass metalsc/ppm Ba Ca Mg B Zn P Fe Cr Al Cu Sn Pb V Mo Si Na Nid Ti
400 SSU 0.8900 77.67 × 10-6 9.26 × 10-6 94 248 464 8.74 66.06 25.20
Bright Stock 0.9070 5.091 × 10-4 32.70 × 10-6 96 328 669 6.78 65.44 27.78
350 SSU 0.8895 56.75 × 10-6 9.25 × 10-6 144 210 426 15.76 68.93 15.31 30 1890 269 63 968 806 106 3 5 21 7 176 1 9 21 65 1 1
treated oil
36.58 × 10-6 6.13 × 10-6
2 312 61 27 9 297 44 1 0 0 1 1 0 3 12 14 19 0
final residue
5.46 × 10-3 1.82 × 10-3
141 11000 1398 219 5298 622 23 36 154 31 1290 2 46 68 343 198 7
a Ratio between the mass of a volume of oil at 15 °C and the mass of the same volume of water at 4 °C. b The viscosity index is measure of the constancy of the viscosity of a lubricating oil with changes of temperature. c Analysis by ICP/AES (Inductively Coupled Plasma Atomic Emission Spectrometry). d The high levels of Ni measured in the permeate and in the retentate are caused by abrasion of the alloy previously plated by the Kanigen process (electroless nickel plating process).
containing mineral oils, previously dewaxed by solvent extraction (oils S3699 and S3701) [S3699 is a low viscosity oil (69 mPa.s at 313 K), whereas S3701 is a highly viscous one (462 mPa.s at 313 K). Both of them were provided by the French Petroleum Institute (IFP).] and 2) a used motor oil (S5010), previously desessenciated and dehydrated by “flash” distillation. [The experiments realized with this oil consisted initially in determining the influence of the operating parameters (CO2 pressure and transmembrane pressure) on the permeate flux and on metal retention. During these experiments, the permeate was continuously recycled into the bulk solution in order to get “quasi steady-state conditions”. Thereafter, a “concentration run” was realized in order to appreciate the performance of the process in terms of separation and containment of metals during an uninterrupted operation over a more significant period of time. During this operation the permeate was continuously withdrawn from the installation.] The characteristics of these three oils are listed in Table 1. Pilot-Plant Unit and Membranes. A new bench, called FILEAS (FILtration Experimentale Assiste´e par fluide Supercritique), has been specifically built at the French Atomic Commission (CEA) for pilot-scale experiments. The FILEAS bench is mainly characterized by the following: 1) flow rates inside the module between 0.5 and 3.0 m3.h-1 (pump PMH, TF40MP) that make it possible to test the performances of industrial multichannel membranes; 2) a sophisticated piece of equipment that allows the recording of operating parameters as a function of time such as temperature, pressure (Fisher-Rosemount, 3051CD), liquid flow rate (Fisher-Rosemount, DH 006S and DH100S), gas flow rate (Brooks, 5680 and 5681), viscosity (Sofraser, 6001), and density (FisherRosemount, DH 100S); and 3) the possibility to conduct fully continuous and automated runs during several days. 3470
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The bench is presented in Figure 1. In the loading zone, two pumps allow the introduction of liquid CO2 and oil (Lewa, FK2) at the required pressure; then mixing and preheating are carried out just before entering the autoclave. The mixture thus obtained (oil saturated with supercritical CO2) is circulated through the membrane module. Because of the transmembrane pressure, splitting of the solution into two phases takes place. The light compounds rich phase is called the permeate, while the heavy compounds rich phase is called the retentate. In the separation zone, an isothermal pressure release of each phase takes place. This leads, for both of them, to liquid (concentrate or filtrate) and gas streams at the exits of cyclone separators. The CO2 in the gas state can be recycled at the top of the process. Tubular ceramic membranes with a 300 kDa cutoff and a 10.8 × 10-3 m hydraulic diameter were provided by TAMI INDUSTRIES for this study. They are multichannel elements (“clover” geometry) composed of a thin ZrO2 filtering layer deposited on a macroporous support made of Al2O3/TiO2/ ZrO2. Different lengths were used: 25 cm-long membranes (surface of filtration A: approximately 8 × 10-3 m2) for the filtration runs and one 120 cm-long membrane (surface of filtration A: approximately 45 × 10-2 m2) for the concentration run. Procedure. The entire set of experiments was conducted with an approximate 600 kg.h-1 flow rate inside the membrane (≈ 6 ms-1), under various CO2 pressures (0 MPa < PCO2 < 18 MPa) and transmembrane pressure (0 MPa < ∆P < 1 MPa). The temperature was set to 353 K during the filtration runs and to 393 K during the concentration run, to limit the risks of displacement of the membranes in the stainless steel cartridge. Indeed due to the high viscosity of the oils (mainly for low CO2 pressures) shear stresses are very high. The solubility of CO2 in the oils was continuously measured in order to determine the oil fraction in the permeate fluxes.
FIGURE 1. Schematic of the FILEAS bench.
TABLE 2. Measured Permeate Flux for Oils S3699 and S3701 with a 1 MPa Transmembrane Pressurea oil S3699
oil S3701
PCO2/MPa
J/kg.h-1.m-2
XCO2 mass %
µ/mPa.s
J/kg.h-1.m-2
XCO2 mass %
µ/mPa.s
0 6 9 12 15 18 improvement/%
15.60
0
14.3
56.10 67.72 79.38
11.6 16.0 20.0 22.0
5.0 4.3 3.6 3.2
8.47 26.89 31.12 32.87 38.86 45.49 437
0 7.0 10.5 14.9 14.5 17.6
46.5 16.0 11.5 8.4 7.3 6.8
a
409
The CO2 concentrations as well as the viscosities of the corresponding mixtures oil-CO2 are also indicated (T ) 353 K, u ≈ 6 m.s-1).
This operation was realized by measuring the CO2 volume flow rate QCO2 at the separator output by the means of two flowmeters (Brooks, 5680 and 5681). Assuming that the dissolved quantity of CO2 is the same in the permeate and in the retentate, knowing the mass flux of permeate, J, and the CO2 density at the working temperature (7), FCO2, it is possible to calculate the solubility of CO2, XCO2, with relation 1:
XCO2 )
QCO2 × FCO2 A.J
(1)
All the permeate flux measurements presented hereafter were collected after an approximately 1-h operation corresponding to quasi-stationary conditions (flux decline inferior to 5% per hour).
Results and Discussion Mineral Oils S3699 and S3701. The permeate fluxes obtained with oils S3699 and S3701 are plotted in Figures 2 and 3, respectively, and are listed in Table 2. Solubility values for CO2 are also indicated. As in the case of previous filtration runs with CO2-saturated PEG 400 (1), the addition of CO2 highly improves the permeate fluxes of both oils. However, the behavior of the CO2-saturated oil mixtures during filtration is rather different from the one observed with PEG 400. First of all, the permeate flux enhancement is more significant and reaches a factor of about 5 under a 18 MPa CO2 pressure with the oils, whereas it was inferior to a factor 2 with PEG 400. As shown in Figures 2 and 3, the flux variation versus transmembrane pressure is nonlinear. This phenomenon, that was not observed with PEG 400 for such fluid velocity
FIGURE 2. Influence of transmembrane pressure and CO2 pressure on the permeate flux of oil S3699 (cutoff: 300 kDa, T ) 353 K, u ≈ 6 m.s-1). (approximately 6 m.s-1), can be attributed to fouling, resulting from the accumulation of matter at the membrane surface or in the pores. This fouling is responsible for lowering of membrane performance in terms of mass transfer. The hydrodynamic resistance associated to the membrane, Rm, and calculated from the Darcy equation (eq 2), increases with the transmembrane pressure, ∆P.
J)
∆P µRm
(2)
Finally, we did not observe any optimum in permeate flux under our operating conditions (0 MPa < PCO2 < 18 MPa, T ) 353 K). Therefore, the viscosity reduction achieved by addition of CO2 is large enough to compensate the unfavorable dilution due to the dissolution of CO2, and consequently to increase the permeate flux of pure oil. VOL. 34, NO. 16, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Influence of transmembrane pressure and CO2 pressure on the permeate flux of oil S3701 (cutoff: 300 kDa, T ) 353 K, u ≈ 6 m.s-1).
TABLE 3. Measured Permeate Flux for Oil S5010 with a 1 MPa Transmembrane Pressurea PCO2/MPa
J/kg.h-1.m-2
XCO2 mass %
µ/mPa.s
0 12 15 18 improvement/%
15.60 39.26 50.83 53.72 244
0 13.5 16.3 19.0
11.5 3.1 3.0 2.9
FIGURE 5. Evolution of permeate flux, bulk viscosity, and mass concentration factor during the concentration of oil S5010 (cutoff: 300 kDa, T ) 393 K).
a
The CO2 concentrations as well as the viscosities of the corresponding mixtures oil-CO2 are also indicated (T ) 353 K, u ≈ 6 m.s-1).
FIGURE 4. Influence of transmembrane pressure and CO2 pressure on the permeate flux of oil S5010 (cutoff: 300 kDa, T ) 353 K, u ≈ 6 m.s-1). Used Motor Oil S5010. Filtration Runs. The measured permeate fluxes as a function of CO2 pressure and transmembrane pressure at 353 K are gathered in Table 3 and are plotted in Figure 4. An increase of the CO2 pressure still permits to increase the permeate flux. However, it appears clearly in Figure 4 that there is not a significant improvement of the permeate flux for CO2 pressures higher than 15 MPa. Therefore, it may be considered that 15 MPa is the optimum pressure for the filtration of this oil. Under a 1 MPa transmembrane pressure and for a 15 MPa CO2 pressure, the flux increase reaches 224%. The curves obtained do not present the same evolution as in the case of mineral oils. The presence of additives and
impurities in used oil induces a stabilization of the density flux for transmembrane pressures higher than 0.2 MPa. Consequently, the use of higher transmembrane pressures is not justified since it is synonymous of a higher electric consumption that is not compensated by a significant permeate flux enhancement. Three samples were withdrawn during the run realized under a 15 MPa CO2 pressure: one sample of permeate for a 0.2 MPa transmembrane pressure (P1), an intermediate sample of the bulk solution (B), and one sample of permeate for a transmembrane pressure of 0.8 MPa (P2). Metal analysis and kinematic viscosity measurements for the various fractions are indicated in Table 4. The retention of metals calculated using relation 3 increases with the transmembrane pressure:
retention )
[metal]bulk - [metal]sample [metal]sample
× 100
(3)
During this run, the velocity of the bulk solution inside the membrane module was varied from 3 to 13 m.s-1 without any significant influence on the metal retention or on the permeate flux. All these observations seem to indicate that in the case of the used motor oil the main limiting phenomenon is fouling inside the pores of the membrane, rather than polarization at the membrane-solution interface since there is no influence of the fluid velocity. Moreover, the very small dependence of permeate viscosity on ∆P observed in Table 4 could be interpreted by considering that above 0.2 MPa there is a stripping effect at the pore level, probably due to high shear stresses, that balances expected advantages due to fluidization. This phenomenon was already observed with oil S3701 in Figure 3.
TABLE 4. Metal Concentrations and Cinematic Viscosity of the Samples Taken during the Filtration of Used Oil S5010a metals
oil S5010
P1 concn (ppm)
retention (%)
B concn (ppm)
P2 concn (ppm)
retention (%)
Zn Fe Cu Cr ν/ m2.s-1
968 106 21 1 53 × 10-6
4 12 99.5 90.0
1172 90 30 1 63 × 10-6
2 10 90
a P1: permeate sample for a 0.2 MPa transmembrane pressure; P2: permeate sample for a 0.8 MPa transmembrane pressure; B: intermediate bulk sample (T ) 353 K, u ≈ 6 m.s-1).
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TABLE 5. Cost Estimate for a Regeneration Unit with a 300 Ton/Year Production Capacity paying off (8 years) for a 111 000 USD unit membrane (2-year lifetime) energy consumption (100 kW.h per ton produced, 0.0072 USD/kW.h refrigerating fluids CO2 losses (estimated to be 5% of the 60 tons required) elimination of the residues management, maintenance and assurance (7% per year) labor total
13 875 USD 1100 USD 2150 USD 370 USD 1500 USD 3250 USD 7770 USD 30 000 USD 60 015 USD 200.05 USD/ton
Concentration Run. This run was carried out under a 15 MPa CO2 pressure at 393 K with a low transmembrane pressure (0.2 MPa). The evolution of permeate flux, bulk solution viscosity, and mass concentration factor (i.e. the ratio between the mass of used oil and the mass of recovered residue) during the concentration are plotted in Figure 5. The duration of the operation was 32 h, and 21.3 kg of used oil was treated. The quantity of purified oil (permeate) was 20.5, and 0.880 kg of final residue (concentrate) was recovered in the installation, corresponding to a mass concentration factor of 27. The results concerning the retention of metals and the kinematic viscosities of the different fractions are listed in Table 1. The purified oil exhibits a significant discoloration and a lower kinematic viscosity compared to the initial oil. On the contrary, the final residue
is a black gum that is extremely viscous and contains the majority of the metals. Based on these preliminary experimental results, a raw estimate of the production costs associated with a regeneration unit with a capacity of 300 ton/year leads to a price of approximately 200 USD/ton of purified oil (Table 5).
Literature Cited (1) Gourgouillon, D.; Schrive, L.; Sarrade, S.; Rios, G. M. accepted for publication in Sep. Sci. Technol. 2000. (2) Schrive, L.; Gourgouillon, D.; Nunes da Ponte, M.; Rios, G. M.; Sarrade, S. Ultrafiltration applied to viscous liquids fluidified with supercritical carbon dioxide, Proceedings of the 5th International Conference on Inorganic Membranes, June 2226, 1998, Nagoya, Japan, pp 298-301. (3) Gourgouillon, D.; Avelino, H. M. N. T.; Fareleira, J. M. N. A.; Nunes da Ponte, M. J. Supercrit. Fluids, 1998, 13, 177. Gourgouillon D.; Nunes da Ponte M. Phys. Chem. Chem. Phys. 1999, 1(23), 5369-5376. (4) Mehrota, A. K.; Svreck, W. Y. Can. J. Chem. Eng. 1988, 66, 656; Can. J. Chem. Eng. 1988, 66, 666. (5) Yu, J. M.; Huang, S. H.; Radosz, M. Fluid Phase Equilib. 1989, 53, 429. Huang, S. H.; Radosz, M. Fluid Phase Equilib. 1990, 60, 81. (6) Jian, C.; Yaohua, F.; Yufu, Z. Regeneration of used oils by supercritical fluid extraction, First industrial Chemical Engineering Technology Topical Conference, November 7-12, 1993, St. Louis, MO, USA, pp 262-264. (7) Span R.; Wagner W. J. Phys. Chem. Ref. Data 1996, 25(6), 15091594.
Received for review December 15, 1999. Revised manuscript received April 21, 2000. Accepted May 22, 2000. ES991392G
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