An environmentally friendly approach for contaminants removal using supercritical CO2 for remanufacturing industry

Applied Surface Science 292 (2014) 142–148

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

An environmentally friendly approach for contaminants removal using supercritical CO2 for remanufacturing industry Wei-wei Liu a,∗ , Bin Zhang a , Yan-zeng Li a , Yan-ming He a , Hong-chao Zhang a,b a b

School of Mechanical Engineering, Dalian University of Technology, Dalian 116024, China Department of Industrial Engineering, Texas Tech University, Lubbock, TX 79409, USA

a r t i c l e

i n f o

Article history: Received 8 October 2013 Received in revised form 19 November 2013 Accepted 19 November 2013 Available online 3 December 2013 Keywords: Supercritical carbon dioxide Cleaning Remanufacturing Oil and grease

a b s t r a c t The cleaning technology plays an important role in product quality during the remanufacturing processing. Remanufacturing cleaning is among the most demanding steps and is a particularly essential process in remanufacturing. In the meantime, remanufacturing cleaning is often the main source of pollution in the remanufacturing process. During the past decades, supercritical fluids due to their unique properties gained an increasingly attention in many cleaning industries. The supercritical carbon dioxide as a novel cleaning technology for remanufacturing cleaning process is discussed, which can realize cleaning and drying at the same time, promoting a greener solution for remanufacturing industry. In this paper, we reported the experimental results of the effect of some operating parameters. The CO2 at different operating pressures, temperatures and residence time was made to continuously flowing over this. The decontamination rate and amount were monitored and compared. The obtained results show that the optimum parameters were operating temperature and pressure of 60 ◦ C and 20 MPa respectively, to have the highest decontamination rate value at the investigated experimental conditions. In additon, the success of supercritical CO2 cleaning effectively promotes the research for next-generation cleaning methods for remanufacturing industry. © 2013 Elsevier B.V. All rights reserved.

1. Introduction In the past decade, environmental awareness, sustainable development, resource shortage and legislative pressure have caused the concern of the world. In this context, remanufacturing as a specific type of recycling makes the fact that the used durable goods can be repaired to a condition like new realized [1]. By means of remanufacturing, most of the used machinery parts can be repaired to a condition like new with warranty to match, which not only alleviates environmental contamination but reduces energy consumption and professional labor used in production [2]. Therefore, remanufacturing engineering has become the tendency and played significant role in the development of the advanced manufacturing technology. Fig. 1 shows the technological process of remanufacturing. It can be found that the core (the end-of-life or used products that enter the remanufacturing process are known as ‘core’ [3]) passes through a number of remanufacturing processes, such as disassembly, cleaning, inspection and sorting, reconditioning, testing, reassembly and painting and packing. As can be seen from Fig. 1,

∗ Corresponding author. Tel.: +86 041184709302. E-mail addresses: [email protected], [email protected] (W.-w. Liu). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.11.102

cleaning is among the most demanding steps and is a particularly essential process in remanufacturing because the quality of core surface cleanliness directly determines the parts surface analysis and the following process as surface inspection, reconditioning, reassembly and painting processing [4]. In the meantime, remanufacturing cleaning is often the main source of pollution in the remanufacturing process. And that is, the cost, quality and environmental performance of remanufactured products are closely related to the remanufacturing cleaning process. In order to test and recondition each remanufacturing core, it is essential to clean all the cores to prepare the surface for the following processes, or the remanufacturing process will be unsuccessful. Therefore, remanufacturing cleaning is a very significant component of the remanufacturing and manufacturing costs. The cleaning process needed to remove contaminations is the most time consuming and a labor-intensive job [5,6]. Methods of cleaning, which can be categorized into wet and dry cleaning, are either environmental-unfriendly or high equipment and operational cost [5]. Utilizing organic materials or other chemical removal technologies [7], wet cleaning is thought to be problematic from an environmental perspective, since they involve various forms of more or less hazardous solvents and detergents [8]. Main concerns arise from the energy consumption of cleaning process in which high temperatures are often required; the potential risk

W.-w. Liu et al. / Applied Surface Science 292 (2014) 142–148

143

Fig. 1. Common technological process of remanufacturing.

emission of air, soil and water; and the hazardous waste that needs to be treated with special care [9]. The contaminants to be removed have different compositions, structures, densities and thicknesses. Contaminant types of core mainly include oil and grease, carbonaceous deposits, incrustation scale, corrosion, etc. Taking a remanufactured diesel engine core as an example, great amount of contaminants as carbonaceous deposits, oil and grease which would be generated during use, are needed to be eliminated in remanufacturing cleaning process [10,11]. Oil and grease are common contaminants which can be easily found in crankcase, on piston surface and cylinder head, and on cylinder liner and fire ring. In remanufacturing process, cleansing agents play an essential part in many oil and grease removal applications. A variety of chemical substances are used as cleansing agents. They can be grouped into two categories: solvents and aqueous detergents. Although various solvents, such as chlorinated organic solvents, hydrocarbons, fluorocarbons, brominated organic solvents, and alcohols, are used according to the required cleanliness, chlorinated solvents are most widely used in industry, accounting for 31% of the total shipment of cleansing agents in Japan in 2007. And this was followed by aqueous detergents (27%), hydrocarbons (19%), and alcohols (16%) [9]. Now, it is commonly seen that traditional cleaning solvents are still being used, albeit in more restricted and regulated manner. Taking chlorinated organic solvents as an example, they are able to remove oil and grease efficiently and economically. However, they also have environmental disadvantages as the facts that they release volatile organic compounds (VOCs) into the atmosphere, with a negative, toxic impact on the environment and with long period of time for extraction the cost of managing the degraded liquids and chlorinated product residues is always extremely high [12,13]. Using an alternative cleaning agent is one possible measure of eliminating the adverse effects of chlorinated solvents from the perspective of hazard management. A number of alternative cleaning agents and processes have been proposed. Aqueous solutions of detergents and/or emulsifiers can be used in many instances. Drawbacks to this approach include the production of large quantities of contaminated wash solution which, in most cases, must be handled as hazardous waste. And this process entails washing, rinsing, and drying. As a consequence, there is an increasing interest for the use of environmentally friendly solvents rather than traditional solvents, which creates a demand for new type of solvents such as supercritical CO2 (SCCO2 ) fluids in remanufacturing process. The use of supercritical CO2 is an innovative replacement of traditional organic solvents, and the technique has been rapidly growing in parallel with the increased more stringent legislation rules against the use of VOCs since the use of large quantities of organic solvents are often toxic [14]. Supercritical fluid (SF) cleaning using carbon dioxide is a promising alternative for industrial cleaning process where parts are cleaned before surface treatment in order to remove dirt since mild temperatures are used (<343 K) [15], carbon dioxide is inert in the

conditions employed, is a by-product of many processes and thus widely available, and it is considered benign since it is already present in the atmosphere. A supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist [16]. Supercritical fluids have been widely used in extraction [17–19], purification [20–22], and cleaning processes [23–27]. Supercritical CO2 is most often utilized in these applications, since its unique properties such as the relatively low viscosity, high diffusivity and low (near zero) surface tension, mass transfer are improved in SCCO2 compared to organic solvents. And its supercritical conditions are easily attained (Tc = 31 ◦ C, Pc = 7.38 MPa) so that separation of carbon dioxide from the product can be also easily achieved by depressurization. Furthermore, with the properties like low toxicity, not flammable or corrosive, carbon dioxide generates almost no hazardous waste. All of these advantages make carbon dioxide, which is relative inexpensive, approved by the EPA as a non-ozone-depleting chemical alternative [28,29]. Such technology of cleaning with supercritical CO2 has been applied in the field of microelectronics, metallic surface cleaning and medical instrument cleaning, where the effectiveness have been illustrated by several other researchers [30]. In addition, the low surface tension of supercritical CO2 makes it an attractive candidate for cleaning porous materials [31]. Small amounts of polar cosolvents can also be added to enhance the solubility of polar materials [32–34]. It is important to model the SCCO2 cleaning of the oil and grease to obtain access to optimum operation conditions. The experimental data of SCCO2 cleaning has the objective of determining parameters of process design, such as operating temperature, pressure and residence time, to make possible the prediction of viability of SCCO2 cleaning processes on an industrial scale, through the simulation of overall cleaning curves. The objective of this research was to study the removal of oil and grease by using supercritical CO2 for remanufacturing industry. It was suggested to provide remanufacturers directions in selection of the optimal parameters for the SCCO2 cleaning process. Carbon dioxide was used in the temperature range of 35–90 ◦ C, at pressures between 10 MPa and 30 MPa. We investigated the influence of the several operational parameters on the decontamination yield and decontamination rate. The parameters analyzed were: operating temperature, pressure and residence time. 2. Mechanisms of SCCO2 cleaning Carbon dioxide can be used in its supercritical fluid state as a replacement for conventional cleaning solvents, reducing the pollution associated with the operations that would otherwise require significant amounts of volatile organic compounds, ozone depleting substances, or hazardous air pollutants. Nowadays, the use of SCCO2 for parts cleaning is a natural outgrowth of the analytical and production scale extraction processes. This wide range of applications have motivated researchers to develop models that would

144

W.-w. Liu et al. / Applied Surface Science 292 (2014) 142–148

describe the various processes and provide a tool for evaluating the performance SCCO2 cleaning and optimizing operating conditions. There are various models available in the literature for supercritical CO2 applications [15–17,35–37]. In general, modeling SCCO2 cleaning involves two different theoretical approaches, namely, kinetic and thermodynamic models. Thermodynamic models are concerned with estimating equilibrium states, i.e. determining the solubility of solutes in supercritical fluid. Kinetic models are concerned with following the progress of cleaning with time, or in other words, the dynamics of SCCO2 cleaning process. The theoretical basis underlying all kinetic SF models is established in this paper. Most of the existing SF models are based on the mass transfer approach, while other models are basically empirical. In 2011, Oliveira et al. [15] reviewed the most important mathematical models applied to the supercritical fluid extraction of both liquids and solids, and their main assumptions, with particular emphasis on the kinetics. And they provided adequate modeling of supercritical fluid extraction. The theoretical mechanism for SCCO2 cleaning is similar to supercritical fluid extraction, so it is not described in this article. The SCCO2 cleaning efficiency is related with its density, diffusion coefficient, polarity and the solubility of the contaminants. Generally, supercritical CO2 , with no or weak polarity, has high dissolving capacity for the substance with no polarity or small molecular weight, while poor dissolving capacity for the substance with strong polarity or large molecular weight. Such practical experience rule is usually called “like-dissolves-like”. Due to the nonpolar molecule, the dipole moment of CO2 is zero so that it has extremely strong dissolving capacity for organic compounds such as silicone, oil and grease, ester, carbureted hydrogen, fluoride. SCCO2 cleaning systems rely upon the solvent properties of CO2 and other unique properties of a supercritical fluid. The solubility of the contaminant has an effect on the mechanism of SCCO2 cleaning. When the temperature and pressure is above its critical point, the contaminants can be extracted from the solid and dissolved into the mobile phase in response to the increased solvating power of CO2 [38]. Apart from solvating power of CO2 , contaminant volatility, which is directly related to temperature, is another factor that determines the solubility of a contaminant in SCCO2 . Basically, supercritical fluid solvating power is a function of fluid density. In general, higher fluid densities, most readily achieved through increasing pressure, result in higher solvating power. However, temperature is an important variable in contaminant solubility since the volatility of a compound is directly related to temperature. According to equipment setup, increasing the temperature in a cleaning system can reduce solvent density, but the increase in contaminant volatility generally overcomes the decrease in density and enhances contaminant removal. Consequently, by regulating the pressure and temperature the contaminant and CO2 can be solvable with each other and then separated by separator reducing the pressure so that solvating power can be maximized to fulfill the

purpose of cleaning. But because of the complexity of the contaminant components, it is difficult to develop a mathematical model for finding out the component solubility in SCCO2 . Therefore, only by experiments can these rules be found to offer some guide to engineering practice. 3. Experimental procedures 3.1. Materials Common lubricating oil and grease of engine are used in the experiment for this research. For the convenience of statistics and discussion, two different types of contaminant samples were put into use: (1) contaminated surface of absorbent pad with lubricating oil, sample size 150 mm × 150 mm × 2 mm, as shown in Fig. 2; (2) contaminated surface of small steel-based block with lubricating grease, sample size 40 mm × 40 mm × 2 mm, as shown in Fig. 3. The lubricating grease is a mixture of hydrocarbons (paraffinic mineral oils), thickeners (mixture of ethylene-propylene copolymer and ethylene-propylene-ethylidene norbornene terpolymer) and a very small percentage of amine-based antioxidant. The solvent was CO2 of 99% purity. 3.2. Experimental installation Supercritical fluid cleaning was performed in a laboratory apparatus equipped with 1000-mL capacity and 304 stainless-steel vessel designed to operate at pressures up to 50 MPa and temperature range from 30 ◦ C to 200 ◦ C, the schematic diagram of the cleaning apparatus is as shown in Fig. 4. The internal diameter of cleaning vessel is 60 mm and its length is 360 mm, giving a length to diameter ratio of six. The apparatus also consisted of two separators operating in series for the recovery the extract, liquid solvent and CO2 . After coming out of the CO2 cylinder, CO2 could be changed from a gaseous state into liquid by condensation and enter the cleaning vessel through the high pressure pump and the pressure could be increased to the pre-set value. The pressurized CO2 was pre-heated by a heating jacket before entering the cleaning vessel. The temperature control of cleaning vessel was also thermally conditioned by another heating jacket; both with a temperature control of ±2 ◦ C and measured by a thermocouple placed inside the reactor in direct contact to the fluid. Pressure was measured during the process by using a pressure gauge with accuracy of ±0.2 MPa. The control of the pressure and flow rate was achieved by both a heated micrometering valve and the pump. 3.3. Experimental procedure The contaminated samples were placed into the high pressure cleaning vessel and the cleaning vessel was heated. When temperature achieved the set value, CO2 was delivered into the cleaning

Fig. 2. Surface of absorbent pad before and after SCCO2 cleaning: (a) uncontaminated surface of absorbent pad without lubricating oil; (b) contaminated surface of absorbent pad with lubricating oil and (c) surface of absorbent pad after SCCO2 cleaning.

W.-w. Liu et al. / Applied Surface Science 292 (2014) 142–148

145

Fig. 3. Surface of small steel-based block before and after SCCO2 cleaning: (a) uncontaminated surface of small steel-based block without lubricating grease; (b) contaminated surface of small steel-based block with lubricating grease; (c) and (d) surfaces of small steel-based block after SCCO2 cleaning.

vessel until pressure achieved the set value. Then, the contaminants contained on the sample were contacted with the supercritical CO2 at a given low flow rate and for a fixed period of time. CO2 can be recycled and reused in the process. Later, the samples were taken out after decompression. Monitoring was carried out from the initial instant when the gas CO2 was introduced into the cleaning unit in the operational conditions. Operating temperatures generally range from 35 ◦ C to 90 ◦ C. The operating pressures are in the range of 10–30 MPa. In each trial, the samples were placed in the high pressure cleaning vessel and cleaned by pure SCCO2 for a total of about 40 min. The decontamination yield and decontamination rate of SCCO2 cleaning was calculated by weighting the samples. The technological process for SCCO2 cleaning can be described as followings. First of all, the CO2 gas with certain pressure turned into liquid fluid after passing through cooling bath. If there is cosolvent, the liquid CO2 was mixed with the cosolvent intensively in the internal storage. After that, liquid CO2 entered the cleaning vessel, which had been heated to pre-set temperature by heating jacket, to be heated and its pressure will also be increased to the pre-set value by high-pressure pump. The supercritical CO2 was flown through the samples at low rate (<10 L/h) to ensure that the equilibrium concentration was established in the supercritical phase. Contaminants entered the first-stage separator together with supercritical CO2 , whose dissolving capacity can be changed by means of altering the temperature and reducing the pressure, forcing the contaminants separated out from CO2 . Similarly, after the first-stage separation, the residual contaminants entered the second-stage separator together with supercritical CO2 for further separation. Finally, contaminants going through separator were collected at the drain outlet while the supercritical CO2 ,

refined by dehydrator, went back to the cooling bath or the CO2 cylinder for recycling. When the established treatment time was over, the apparatus was depressurized. Waste disposal costs for SCCO2 cleaning are lower than competing cleaning technologies since the waste residue is 100% contaminant. 3.4. Analytical method The effect of SCCO2 cleaning was evaluated by gravimetric analysis method. Because only contaminant dissolves in SCCO2 , the weight loss during the cleaning was due only to the contaminant removal. Gravimetric measurement of contaminant residues was performed using a microbalance (Mettler Toledo, Switzerland) with a precision of ±0.0001 g. The samples were weighted: before charging the residual contaminant, after contaminant charging and drying to calculate the amount of residue contained on each sample and, finally, after the SCCO2 cleaning to evaluate the cleaning performance. The decontamination amount and decontamination rate can be expressed as Eqs. (1) and (2). The weight data were the mean value of three separate measurements. This method has some advantages: need for only a small amount of samples, high sensitivity to small weight changes (using a microbalance), and a short measurement time. An additional advantage is that the microbalance can be tared and calibrated during measurements. GC = (G1 − G0 ) − (G2 − G0 ) = G1 − G2 YC =

GC × 100% G1 − G0

Fig. 4. Schematic diagram of the supercritical CO2 cleaning apparatus.

(1)

(2)

146

W.-w. Liu et al. / Applied Surface Science 292 (2014) 142–148

45

24

40

20

Decontamination rate(%)

Decontamination amount (g)

22

18 16 14 12

35

30

25

15MPa, 60 C 20MPa, 60 C 25MPa, 60 C

50°C, 40min

10

20 8 10

15

20

25

30

0.5

Pressure (MPa) Fig. 5. Effect of pressure on the lubricating oil cleaning at 50 ◦ C for 40 min.

where GC is the decontamination amount; YC is the decontamination rate; G0 is the weight of the sample before soiled; G1 is the weight of the sample after soiled; G2 is the weight of the sample after cleaned. 4. Results and discussion The cleaning effects of lubricating oil and lubricating grease samples by supercritical CO2 are as shown in Fig. 2(c) and Fig. 3(c)–(d). The lubricating oil attached on the samples was almost cleaned by SCCO2 while the lubricating grease was not thoroughly cleaned but the solidification of the scale crust remained makes it easy to be removed by polishing process. In order to optimize SCCO2 (pressure, temperature, and residence time, and so on) conditions, the contaminants removal curves obtained at different operating conditions are investigated in this section. Supercritical CO2 was used to extracted the oily machining waste by Fu and Matthews [38]. It was found there was little relationship between solvent flow rate and oil removal (the oil removal was only increased by 3% when the flow rate of CO2 was increased from 0.2 to 0.6 mL/s). Therefore, in our experiments of SCCO2 cleaning, SCCO2 cleaning efficiency is assumed to be independent of solvent flow rate. Specifically, it shows the effect of the most important operation parameters (pressure, temperature and residence time) on the removal of the lubrication oil and grease on the samples.

1.0

1.5

2.0

2.5

3.0

Time (h) Fig. 6. Decontamination rate curves of lubricating grease obtained at 60 ◦ C in different pressure.

in the decontamination rate at higher pressures is mainly due to the increase in the density of SCCO2 when pressure is increased [13,39]. However, it is not supposed to be good with higher pressure all the time, because from the economic perspective, the costs of investment and operation will be increased with the increase of the pressure. As shown in Figs. 5 and 6, it indicates that at a pressure of around 20 MPa maximum decontamination rate can be obtained with the other variables at their optimum levels. 4.2. Effect of temperature Fig. 7 shows the lubricating oil removal curves carried out at 35 ◦ C, 50 ◦ C, 60 ◦ C, 75 ◦ C and 90 ◦ C at constant pressure (20 MPa) for 40 min. The solubility of lubricating oil in CO2 increased with the pressure due to a subsequent increase of the solvent density; however, the effect of temperature was not as straightforward which can be seen from Fig. 7. In the whole range of testing temperature, the decontamination amount increases firstly and then decreases under the condition of the given pressure and time. There are two

30

25

The effect of pressure on decontamination amount of lubrication oil has been studied at a temperature of 50 ◦ C for 40 min. The corresponding decontamination amount data are given in Fig. 5. The decontamination rates for lubricating grease are shown in Fig. 6 with different pressure levels (15 MPa, 20 Mpa and 25 MPa) and for different residence time (0.5 h, 1.0 h, 1.5 h, 2.0 h, 2.5 h and 3.0 h) at the temperature of 60 ◦ C. It can be shown that the decontamination amount dramatically increased considerably with increasing pressure, especially at higher pressures. It is means that an increase in the cleaning pressure of the supercritical CO2 leads to an increase in the amount of contamination extracted. Therefore, pressure is one of the significant factors affecting SCCO2 cleaning. At given temperature condition, the higher pressure in the cleaning vessel and greater density of the fluid, the stronger solvent power for SCCO2 is and the corresponding cleaning time is shorter. The increase

Decontamination amount (g)

4.1. Effect of pressure 20

15

10

5

20MPa, 40min 0 30

35

40

45

50

55

60

65

70

75

80

85

90

95

Temperature (°C) Fig. 7. Effect of temperature on the lubricating oil cleaning at 20 MPa for 40 min.

W.-w. Liu et al. / Applied Surface Science 292 (2014) 142–148

4.3. Effect of residence time

44 42

Decontamination rate (%)

147

40 38 36 34 32 30

50 C, 20MPa 60 C, 20MPa 70 C, 20MPa

28 26

Residence time, the total time that SCCO2 cleaning is carried out, is also an important parameter that needs to be optimized for maximizing contaminants removal yield [40,41]. As shown in Figs. 6 and 8, the cleaning time is important for a complete SCCO2 cleaning of contaminants, which ensures that a maximum removal yield of contaminants. However, the completeness of the cleaning process depends on the amount of SCCO2 available. Increasing cleaning time generally leads to more contaminate removal yield, but much energy will also be consumed due to the long period of cleaning. Therefore, reducing the cleaning time could reduce costs as well as improve energy efficiency. 5. Conclusions

24 0.5

1.0

1.5

2.0

2.5

3.0

Time (h) Fig. 8. Decontamination rate curves of lubricating grease obtained at 20 MPa at different temperature.

aspects of the influence of temperature on lubricating oil solubility in supercritical CO2 fluid. On the one hand, the increase in temperature decreased the density of the solvent by negatively affecting the extraction, and on the other hand, the temperature increase had improved the solute volatility. The overall effect of temperature on removal efficiency is a result of temperature effects on the density of the SCCO2 fluid, on the solute vapour pressure, and on the adsorption of the contaminants on the sample. If the contaminant is volatile, extraction is more efficient at higher temperatures. If the contaminant’s vapour pressure does not increase with temperature significantly, higher temperatures decrease extraction efficiency as a result of decreased fluid density. However, if the contaminants bind strongly to the samples, elevated temperatures are needed for rapid and complete desorption of the contaminants from the sample. The cases when the temperatures are up to 90 ◦ C have been tested in the laboratory. It can be illustrated from Fig. 7 that for the artificial sample, the maximum removal yield can be obtained at around 60 ◦ C while it will decrease when the temperature is above or below 60 ◦ C. Generally speaking, temperature improves the kinetics of the extraction process and contaminant desorption. However, it also decreases the solvent density and hence its capability for solute removal. As shown in Fig. 7, removal yield increased with decreasing temperature. This can be understood from the decrease in temperature that causes an increase in solubility or solvent density. In addition, at this condition, the change of solvent density is more effective than that of solute vapor pressure. Consequently, the decontamination rate increased with decrease in temperature. As shown in Fig. 8, the final decontamination rate of lubricating grease in supercritical CO2 was slightly higher when the temperature is high. Firstly, this may be because increasing the temperature decreased the viscosity of the lubricating grease, so facilitating drag and dissolution. Secondly, this fact indicated again that the reduction in the extraction rate may be due to the existence of an additives layer or the presence of heavy factions adsorbed to the contact surface, whose elimination (desorption), endothermic in character, was favored by increasing the temperature. Porta et al. [23] found a similar result. For example, in order to evaluate the effect of appropriate temperature, they maintained constant pressure at 150 bar during the process of removing dried inks and adhesives residue from the microscopic cells of engraved rollers while the temperature was increased from 40 ◦ C to 100 ◦ C.

One of the formidable challenges faced by remanufacturing is the environmentally friendly cleaning process since a huge amount of water were consumed by conventional chemical cleaning and high-pressure water jet cleaning while secondary pollution may take place. With an increasing emphasis for all businesses to reduce their environmental footprint, there is considerable interest in new technologies aimed at reducing current waste streams. When used as a replacement for conventional solvents, SCCO2 has significant potential to reduce the environmental impact of certain remanufacturing processes. In particular, parts cleaning operations that cannot be performed with other replacement technologies (such as the use of aqueous cleaning agent) may benefit from this technology. The technology of SCCO2 cleaning is coinciding with sustainable development. SCCO2 cleaning has enormous potential in the remanufacturing cleaning application because CO2 is plentiful, inexpensive, nontoxic, nonflammable, recyclable, and SCCO2 cleaning is typically performed in closed-loop systems which leads to be environmental friendly and reduce wasted disposal costs. In particular, when contaminant must be recovered in a highly concentrated form (either because of its intrinsic value or because it is particularly hazardous), SCCO2 cleaning should be considered. The aim of this work was to investigate SCCO2 as a greener solution for remanufacturing cleaning process. In this work, the research on supercritical CO2 cleaning is based on engine lubricating oil and grease. The effect on the decontamination rate with variation of cleaning pressure, temperature and time by SCCO2 was demonstrated. Experimental results presented that the SCCO2 cleaning technique is an effective tool to remove some of the most common compounds of contaminants attached on the core surface, namely lubricating oil and grease, etc. By studying the effects of temperature, pressure and time on cleaning effects, the optimum technological cleaning parameters were obtained. At the investigated process conditions, the SCCO2 cleaning has the low environmental impact due to the fractional separation of the residue and the complete recovery of carbon dioxide. The process may be also less expensive than the traditional chemical cleaning or aqueous cleaning due to the easy solvent recovery. The research will therefore be useful for planning and conducting remediation projects and assessing their success later. However, a more complete and accurate mathematical model for SCCO2 cleaning and comparison among SCCO2 cleaning, traditional chemical cleaning and aqueous cleaning are needed in remanufacturing industry, and the equipment cost and the downstream processing cost are also thoroughly analyzed. Acknowledgements The research leading to these results has received funding from the National Basic Research Program of China (973 Program) with

148

W.-w. Liu et al. / Applied Surface Science 292 (2014) 142–148

grant no. 2011CB013402 and the fundamental research funds for the central universities of China with grant no. DUT11RC(3)80. References [1] S. Bernard, Remanufacturing, Journal of Environmental Economics and Management 62 (2011) 337–351. [2] W.L. Ijomah, C.A. McMahon, G.P. Hammond, S.T. Newman, Development of design for remanufacturing guidelines to support sustainable manufacturing, Robotics and Computer-Integrated Manufacturing 23 (2007) 712–719. [3] I.C. Nnorom, O. Osibanjo, Overview of prospects in adopting remanufacturing of end-of-life electronic products in the developing countries, International Journal of Innovation, Management and Technology 1 (2010) 328–338. [4] W. Liu, B. Zhang, M. Li, Y. Li, H. Zhang, Study on remanufacturing cleaning technology in mechanical equipment remanufacturing process, in: 20th CIRP International Conference on Life Cycle Engineering, Singapore, 2013, pp. 643–648. [5] Y. Chang, J.H. Bae, H.-C. Yi, Ultrasonic cleaning of used plastic parts for remanufacturing of multifunctional digital copier, International Journal of Precision Engineering and Manufacturing 14 (2013) 951–956. [6] K. Sakai, Ricoh’s approach to product life cycle management and technology development, in: Proceedings of the 14th CIRP Conference on Life Cycle Engineering, Tokyo, Japan, 2007, pp. 5–10. [7] M. Lindahl, N. Svensson, B.H. Svensson, E. Sundin, Industrial cleaning with Qlean water – a case study of printed circuit boards, Journal of Cleaner Production 47 (2013) 19–25. [8] V. Sivakumar, F. Chandrasekaran, G. Swaminathan, P. Rao, Towards cleaner degreasing method in industries: ultrasound-assisted aqueous degreasing process in leather making, Journal of Cleaner Production 17 (2009) 101–104. [9] E. Kikuchi, Y. Kikuchi, M. Hirao, Analysis of risk trade-off relationships between organic solvents and aqueous agents: case study of metal cleaning processes, Journal of Cleaner Production 19 (2011) 414–423. [10] M.S. García-Cascales, M.T. Lamata, Selection of a cleaning system for engine maintenance based on the analytic hierarchy process, Computers and Industrial Engineering 56 (2009) 1442–1451. [11] Y.C. Guan, G.K.L. Ng, H.Y. Zheng, M.H. Hong, X. Hong, Z. Zhang, Laser surface cleaning of carbonaceous deposits on diesel engine piston, Applied Surface Science 270 (2013) 526–530. [12] M.G. Bernardo-Gil, I.M.G. Lopes, M. Casquilho, M.A. Ribeiro, M.M. Esquível, J. Empis, Supercritical carbon dioxide extraction of acorn oil, Journal of Supercritical Fluids 40 (2007) 344–348. [13] G. Vasapollo, L. Longo, L. Rescio, L. Ciurlia, Innovative supercritical CO2 extraction of lycopene from tomato in the presence of vegetable oil as co-solvent, Journal of Supercritical Fluids 29 (2004) 87–96. [14] H. Rajaei, A. Amin, A. Golchehre, F. Esmaeilzadeh, Investigation on the effect of different supercritical fluid extraction process on the activation of the R-134 catalyst, Journal of Supercritical Fluids 67 (2012) 1–6. [15] E.L.G. Oliveira, A.J.D. Silvestre, C.M. Silva, Review of kinetic models for supercritical fluid extraction, Chemical Engineering Research and Design 89 (2011) 1104–1117. [16] M. Al-Jabari, Kinetic models of supercritical fluid extraction, Journal of Separation Science 25 (2002) 477–489. [17] S. Machmudah, A. Sulaswatty, M. Sasaki, M. Goto, T. Hirose, Supercritical CO2 extraction of nutmeg oil: experiments and modeling, Journal of Supercritical Fluids 39 (2006) 30–39. [18] L.T. Danh, N.D.A. Triet, L.T.N. Han, J. Zhao, R. Mammucari, N. Foster, Antioxidant activity yield and chemical composition of lavender essential oil extracted by supercritical CO2 , Journal of Supercritical Fluids 70 (2012) 27–34. [19] I. Lemonis, D. Tsimogiannis, V. Louli, E. Voutsas, V. Oreopoulou, K. Magoulas, Extraction of dittany (Origanum dictamnus) using supercritical CO2 and liquid solvent, Journal of Supercritical Fluids 76 (2013) 48–53. [20] A. Shishikura, H. Takahashi, S. Hirohama, K. Arai, Citric acid purification process using compressed carbon dioxides, Journal of Supercritical Fluids 5 (1992) 303–312.

[21] L. Margenat, A.I. Torres, P. Moyna, H. Heinzen, G. González, I. Jachmanián, Lanoline purification by selective extraction of pesticides using supercritical CO2 , Journal of Supercritical Fluids 45 (2008) 177–180. [22] J. Yoon, B.-S. Han, Y.-C. Kang, K.H. Kim, M.Y. Jung, Y.A. Kwon, Purification of used frying oil by supercritical carbon dioxide extraction, Food Chemistry 71 (2000) 275–279. [23] G.D. Porta, M.C. Volpe, E. Reverchon, Supercritical cleaning of rollers for printing and packaging industry, Journal of Supercritical Fluids 37 (2006) 409–416. [24] D. Aslanidou, C. Tsioptsias, C. Panayiotou, A novel approach for textile cleaning based on supercritical CO2 and pickering emulsions, Journal of Supercritical Fluids 76 (2013) 83–93. [25] C. Ventosa, D. Rébiscoul, V. Perrut, V. Ivanova, O. Renault, G. Passemard, Copper cleaning in supercritical CO2 for the microprocessor interconnects, Microelectronic Engineering 85 (2008) 1629–1638. [26] W.D. Spall, K.E. Laintz, A survey on the use of supercritical carbon dioxide as a cleaning solvent, Supercritical Fluid Cleaning (1998) 162–194. [27] M.J.E. van Roosmalen, G.F. Woerlee, G.J. Witkamp, Dry-cleaning with highpressure carbon dioxide – the influence of process conditions and various co-solvents (alcohols) on cleaning-results, Journal of Supercritical Fluids 27 (2003) 337–344. [28] A. Laitinen, J. Kaunisto, Supercritical fluid extraction of 1-butanol from aqueous solutions, Journal of Supercritical Fluids 15 (1999) 245–252. [29] D.L. Tomasko, X. Han, D. Liu, W. Gao, Supercritical fluid applications in polymer nanocomposites, Current Opinion in Solid State and Materials Science 7 (2003) 407–412. [30] M. Viguera, J.M. Gómez-Salazar, M.I. Barrena, L. Calvo, The removal of lubricating oils from metallic contacts with supercritical CO2 , Journal of Supercritical Fluids 73 (2013) 51–56. [31] J. McHardy, S.P. Sawan, Supercritical Fluid Cleaning: Fundamentals, Technology and Applications, Noyes Publications, Westwood, NJ, 1998. [32] J.-J. Jiang, C.-S. Tan, Biodiesel production from coconut oil in supercritical methanol in the presence of cosolvent, Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 102–107. [33] J. Ke, S. Jin, B. Han, H. Yan, D. Shen, Hydrogen bonding of some organic acid in supercritical CO2 with polar cosolvents, Journal of Supercritical Fluids 11 (1997) 53–60. [34] S.N. Reddy, G. Madras, Modeling of ternary solubilities of solids in supercritical carbon dioxide in the presence of cosolvents or cosolutes, Journal of Supercritical Fluids 63 (2012) 105–114. [35] M.N. Baig, G.A. Leeke, P.J. Hammond, R.C.D. Santos, Modelling the extraction of soil contaminants with supercritical carbon dioxide, Environmental Pollution 159 (2011) 1802–1809. [36] M. Lashkarbolooki, A.Z. Hezave, Y. Rahnama, R. Ozlati, H. Rajaei, F. Esmaeilzadeh, Solubility of cyproheptadine in supercritical carbon dioxide; experimental and modeling approaches, Journal of Supercritical Fluids 84 (2013) 13–19. [37] M. Yazdizadeh, A. Eslamimanesh, F. Esmaeilzadeh, Thermodynamic modeling of solubilities of various solid compounds in supercritical carbon dioxide: effects of equations of state and mixing rules, Journal of Supercritical Fluids 55 (2011) 861–875. [38] H. Fu, M.A. Matthews, Comparison between supercritical carbon dioxide extraction and aqueous surfactant washing of an oily machining waste, Journal of Hazardous Materials 67 (1999) 197–213. [39] C. Yi, J. Shi, S.J. Xue, Y. Jiang, D. Li, Effects of supercritical fluid extraction parameters on lycopene yield and antioxidant activity, Food Chemistry 113 (2009) 1088–1094. [40] E. Reverchon, I.D. Marco, Supercritical fluid extraction and fractionation of natural matter, Journal of Supercritical Fluids 38 (2006) 146–166. [41] U. Topal, M. Sasaki, M. Goto, K. Hayakawa, Extraction of lycopene from tomato skin with supercritical carbon dioxide: effect of operating conditions and solubility analysis, Journal of Agricultural and Food Chemistry 54 (2006) 5604–5610.