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Ind. Eng. Chem. Res. 2007, 46, 266-272
Improvement of the Waste-Oil Vacuum-Distillation Recycling by Continuous Extraction with Dense Propane Jesusa Rinco´ n,*,† Pablo Can˜ izares,‡ and Marı´a Teresa Garcı´a§ Departamento de Ingenierı´a Quı´mica, UniVersidad de Castilla-La Mancha, Facultad de Ciencias del Medio Ambiente, AVda. Carlos III, s/n. 45071 Toledo, Espan˜ a, Facultad de Ciencias Quı´mica, AVda. Camilo Jose´ Cela n° 10, 13004 C. Real, Espan˜ a, and Escuela UniVersitaria Polite´ cnica de Almade´ n, Plaza Manuel Meca, 1. 13400 Almade´ n (C. Real), Espan˜ a
Dense propane has been used as a solvent for the continuous countercurrent extraction of base oil from waste lubricant oil. The aim of the work has been to identify the best processing conditions to separate base oil suitable for the formulation of new lubricants, avoiding the coextration of waste-oil impurities such us oxidation products and metallic compounds. Experiments have been performed during 6 h in a 1.7 m extraction column operated countercurrently at 30 kg/cm2 and using an oil flow rate of 0.5 g/min. The effects of operation variables such us solvent/used oil ratio, column packing, and column temperature gradient have been investigated. From experimental results obtained, the best operating conditions have been determined: solvent/ used oil ratio ) 5 g/g, column packing ) Raschig rings, and column temperature varying from 80 °C (at the column bottom) to 95 °C (at the column top). Finally, the propane-extracted oil at optimum conditions was vacuum-distilled and found to be comparable to a SN-150 virgin oil. 1. Introduction The lubricating oil after a time of use in vehicles must be replaced because of the degradation of the fresh lubricant components and the contamination from metals, varnish, gums, and other asphaltic compounds coming from overlay on bearing surfaces. This used lubricant oil constitutes a serious pollution problem, since its dumping may contaminate water and earth and, if burnt as a low-grade fuel, harmful metals and other pollutants may be released into the air. However, a proper collection system and recycling process of this oil would help to reduce its environmental impacts and to preserve valuable natural resources. Over the years, many methods have been proposed to remove contaminants from used oil. The old acid clay recycling process used sulfuric acid of high concentration to remove the asphaltenic material, but it generated undesirable and highly polluted acid tar.1 To overcome this problem, modern acid-free procedures using liquid organic solvents2-8 or propane9-14 for the extraction of base oil have been proposed. The base oil obtained in these processes was still contaminated and could not be used in the formulation of new lubricants without being submitted to a severe treatment. Then, processes based on high vacuum distillation followed by a polishing or decolorizing treatment were proposed. However, they present serious problems related to coking and column fouling during distillation, and some form of pretreatment to remove additives and contaminants from the oil becomes necessary. On the other hand, it is well-known that waste-oil impurities are usually kept in stable dispersion by dispersant-detergent additives and electrostatic interactions between heteroatoms and asphaltic compounds, just as in the crude oil.1,15 Then, the first thing to do to separate the undesirable compounds from the base oil fraction of the waste oil is to break such a stable suspension. * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: 34-925-26 88 00. Fax: 34-925-26 88 40. † Facultad de Ciencias del Medio Ambiente. ‡ Facultad de Ciencias Quı´mica. § Escuela Universitaria Polite´cnica de Almade´n.
In a previous work,16 it has been demonstrated that propane has the capacity to destabilize the dispersion and separate base oil from waste-oil impurities. Furthermore, the effect of pressure and temperature on the separation efficiency and yield was investigated in a semibatch apparatus, and the optimum conditions for processing the used lubricant oil were determined (P ) 30 kg/cm2 and T ) 90 °C). At such conditions, pure propane is at the vapor state. However, since the extraction yields attained in that work16 were comparable to those obtained with liquid propane and much larger than those corresponding to a solvent in the vapor state,15,16 it may be suggested that the propane-oil mixture leaving the extractor operated at 30 kg/cm2 and 90 °C was probably at the liquid state. On the other side, the semibatch process mentioned above cannot be used in industry to treat tonnes of used lubricant oil, and a continuous process should be employed. For this reason, and based on results obtained in that preliminary work, in this paper we analyze the continuous production of base oil by propane extraction in a countercurrent column. Accordingly, the major aim of this work is to study the continuous extraction of used lubricant oil with propane in order to identify the best processing conditions to recycle the waste lubricant oil at industrial scale. 2. Experimental Section 2.1. Materials. Liquid propane (purity 95%) was supplied by Praxair S. A. (Madrid, Spain). Used lubricant oil was supplied by Emgrisa, S.A. Since water and light hydrocarbons are undesirable for the formulation of new lubricants, prior to the runs the oil was treated in a rotary evaporator at 60 °C for 30 min. Elimination of water was also necessary because it may modify the solubility parameter of base oil components in propane. After this treatment, the oil concentrations of water and light hydrocarbons were
