Environ. Sci. Technol. 2008, 42, 8534–8540
Comparison of Life Cycle Emissions and Energy Consumption for Environmentally Adapted Metalworking Fluid Systems A N D R E S F . C L A R E N S , * ,† JULIE B. ZIMMERMAN,‡ GREG A. KEOLEIAN,§ KIM F. HAYES,⊥ AND STEVEN J. SKERLOS| Civil and Environmental Engineering, University of Virginia, Charlottesville, Virginia, Environmental Engineering, Yale University, New Haven, Connecticut, and School of Natural Resources and Environment, Civil and Environmental Engineering, and Mechanical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2125
Received March 24, 2008. Revised manuscript received August 25, 2008. Accepted September 03, 2008.
A number of environmentally adapted lubricants have been proposed in response to the environmental and health impacts of metalworking fluids (MWFs). The alternatives typically substitute petroleum with vegetable-based components and/or deliver minimum quantities of lubricant in gas rather than water, with the former strategy being more prevalent than the latter. A comparative life cycle assessment of water- and gasbased systems has shown that delivery of lubricants in air rather than water can reduce solid waste by 60%, water use by 90%, and aquatic toxicity by 80%, while virtually eliminating occupational health concerns. However, air-delivery of lubricants cannot be used for severe machining operations due to limitations of cooling and lubricant delivery. For such operations, lubricants delivered in supercritical carbon dioxide (scCO2) are effective while maintaining the health and environmental advantages of air-based systems. Although delivery conditions were found to significantly influence the environmental burdens of all fluids, energy consumption was relatively constant under expected operating conditions. Global warming potential (GWP) increased when delivering lubricants in gas rather than water though all classes of MWFs have low GWP compared with other factory operations. It is therefore concluded that the possibility of increased GWP when switching to gas-based MWFs is a reasonable tradeoff for definite and large reductions in aquatic toxicity, water use, solid waste, and occupational health risks.
Introduction Metalworking fluids (MWFs) are used as coolants and lubricants in metal cutting and forming to extend the life of tools and achieve faster production rates. They are typically * Corresponding author phone: 434-924-7966; fax: 434-982-2951; e-mail:
[email protected]. † Civil and Environmental Engineering, University of Virginia. ‡ Environmental Engineering, Yale University. § School of Natural Resources and Environment, University of Michigan. | Mechanical Engineering, University of Michigan. ⊥ Civil and Environmental Engineering, University of Michigan. 8534
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a combination of water, lubricant, and additives formulated as an emulsion or dispersion where the water serves as the coolant and the lubricant serves to reduce friction between metal and tool. Each year U.S. manufacturers consume over 9 billion liters of water-based and straight-oil MWFs with the vast majority being associated with water-based MWFs (1). Although water-based MWFs are reused and recirculated over a period of weeks or months, MWFs ultimately deteriorate and must be replaced (2). These water-based MWFs and their corresponding maintenance and disposal/treatment systems can be expensive. Research suggests that the life cycle costs of MWF systems can range from 4-16% of metals manufacturing costs (3). Water-based MWFs have been widely cited for their negative environmental health impacts (Figure 1). A search of approximately 150 articles on the topic of metalworking fluids reported that ecological toxicity, water use, and fugitive emissions are key issues. Ecological toxicity concerns associated with MWFs exist due to their chemical content (e.g., chlorinated fatty acids and chelating agents), their ability to carry metals through waste treatment plants, and other treatment issues such as biochemical oxygen demand and oil content. The United States Environmental Protection Agency has recognized the problem and established the Metals Products and Machinery Rule in 2003 to improve disposal practices at metals manufacturing plants (4). Worker health and safety concerns related to MWFs (not listed in Figure 1), are also widely reported and associated with the misting of water-based MWFs, which leads to pulmonary exposures of chemical additives, surfactants, and bioaerosols among manufacturing workers. This connection has been revealed over the past two decades through extensive worker health studies (5). An authoritative cataloging of these health concerns was conducted by the Occupational Safety and Health Administration in 1998, leading to the recommendation to significantly reduce allowable workplace mist concentrations (6). The environmental and health concerns associated with MWFs have led to the development of environmentally adapted lubricants (EALs). EALs are defined by the European Union as lubricants that have high biodegradability and low toxicity with performance equal to or better than conventional alternatives (7). To date, the EAL concept has essentially been synonymous with the substitution of petroleum-based lubricants with vegetable-based lubricants in conventional water-based formulations (8). Recently, manufacturers have successfully applied Minimum Quantity Lubrication (MQL) strategies involving low-volume sprays of petroleum-based oil delivered in compressed air. In practice, MQL processes deliver oils at a rate of 0.1-2 mL/min with line air applied between 2 and 5 atm at 60-140 L/min. In principle, MQL systems deliver oil exactly at the rate the oil is consumed by the process, while in reality the excess is typically carried out with the metal part and is hardly noticeable in a practical sense (9). External MQL sprays that are not delivered through the tool can generate mists that require control strategies similar to those of water-based MWFs, although these mists do not contain chemical additives or microbial byproducts and are more easily controlled due to their low volume and focused application (10). The MQL approach eliminates large volumes of aqueous waste that, over a period of a month, can exceed a hundred thousand gallons from a single large factory. MQL sprays also can be applied in a manner that eliminates mist, maintenance, and disposal operations associated with waterbased MWFs thereby reducing environmental burdens (11). 10.1021/es800791z CCC: $40.75
2008 American Chemical Society
Published on Web 10/22/2008
FIGURE 1. Concern with the environmental health impacts of MWFs as reported in the academic literature (information on how the figure was developed is provided in the Supporting Information). Even though MQL can solve the most serious environmental and health issues associated with today’s water-based MWFs, it cannot be universally applied since it does not provide sufficient cooling for many operations (12). As a result, the diffusion of MQL technology into practice has been slow. To overcome some of the limitations of air-based systems while maintaining the advantages of MQL, research has demonstrated the feasibility of using minimum quantity sprays of lubricants dissolved in supercritical carbon dioxide (scCO2; 13). Above its critical temperature and pressure (Tc ) 31.1 °C and Pc ) 7.37 MPa), CO2 effectively dissolves lubricants while the rapid expansion out of a nozzle forms chilled microparticles of lubricant. This spray extends the benefits of MQL to more severe manufacturing operations. In addition, CO2-based sprays can deliver oil in much smaller quantities than air-based systems because the oil is soluble. Oils can be effectively delivered at a rate as low as 0.01-0.1 mL/min within a CO2 spray of 0.1-30 g/min at 10.1 MPa or above. Despite these low oil flows, CO2-based MWFs can perform better than conventional water-based MWFs in many applications. Although all EALs promise to reduce the impacts of conventional MWFs, the relative environmental emissions of these systems have yet to be reported. The purpose of this paper is to provide a quantitative assessment of environmental burdens and a semiquantitative assessment of health impacts associated with EALs. From a green design perspective, such predictive modeling is a powerful tool that can guide decision-making. The impacts of EAL technologies were evaluated relative to a conventional benchmark MWF and four systems were selected for comparison (1) benchmark microemulsion of petroleum oil-in-water, (2) microemulsion of rapeseed oil-in-water, (3) dispersion of mineral oil-incompressed air, (4) solution of rapeseed oil-in-supercritical CO2. The benchmark microemulsion of mineral oil-in-water has been extensively discussed in the literature (14, 15) and includes water, oil, and an emulsifier system comprised of a mixture of nonionic and anionic surfactants (16). In practice all water-based MWFs include other proprietary additives that are not required in MQL systems such as corrosion inhibitors, defoaming agents, and biocides. These additives were not included for consistency of the analysis.
Development of LCA Model Figure 2 provides a representation of the scope of this analysis. The service of a MWF provided to one machine tool for one year was selected as the functional unit. It was assumed that the number of parts produced per unit time did not vary
depending on the MWF type. The machining time associated with one year of production was based on the schedule (102 hr/week for 42 weeks/year) used at a major automotive powertrain facility in Detroit, MI. Many manufacturing facilities are moving away from centralized distribution systems because of high maintenance costs and the difficulty in modifying these systems as production lines change. Auxiliary processes such as filtration and continuous circulation that are required to maintain a large volume of water-based MWF in a centralized system were not included here since MQL systems are not centralized, and only large factories use centralized water-based systems. The reference country for the analysis is Germany since data on bio-based oil and surfactant production were complete and readily available in the literature. To be consistent, water production and electricity generation were also based on German data (17). Emissions associated with the transportation of MWFs from production facilities to use facilities were excluded because they were expected to be negligible. Emissions factors were obtained from the literature or from SimaPro software and the model was constructed in Excel. Impacts from the transportation of lubricant carriers were found to be small, two orders of magnitude lower than transportation from other life cycle stages, because these commodities are typically produced and consumed without long-distance transportation.
MWF Production Each MWF is composed of lubricant oil (vegetable or petroleum based) and a lubricant carrier (water or gas). Emulsifiers are used to stabilize the oil-in-water-based systems and are not required in gas-based systems. Either petroleum-based oil or bio-based oil can be carried in water or gas. Presently, petroleum oil dominates air-based MQL systems while scCO2 systems have been applied using biobased oil. Materials production is discussed in the following paragraphs. Petroleum-Based Lubricants and Surfactants. The benchmark MWF contains mineral oil and surfactants based on petroleum. Producing these components requires the distillation and processing of crude oil which creates several byproducts so the emissions of this process were allocated by mass assuming 1.9 L of lubricating oil are produced per barrel (159 L) of crude oil (18). Sodium petroleum sulfonate is an anionic surfactant produced by reacting linear alkyl benzene with sulfur (19). Diisopropanolamine is a nonionic surfactant made by reacting propylene oxide and ammonia (20). Vegetable-Based Lubricant and Surfactants. The processes required to convert rapeseed and corn crops into lubricant oil and surfactants include the production of diesel for farm equipment, the production of fertilizers and pesticides, and the distillation of crude oil for process chemicals used to purify the bio-based components. Rapeseeds (i.e., canola) are produced through seedbed preparation, sowing, fertilizing, pesticide application, growth, and harvesting (18). The seeds are converted to oil through drying, storing, crushing, and refining. Alcohol sulfate is an anionic surfactant made from coconut oil (21) and polyethoxylated glycerol ester is a nonionic surfactant made by reacting glycerol with straight-chain alcohol ethoxylates (22). Lubricant Carriers. Water production by municipal distribution systems was accounted for using German data sources (17). The CO2 used in industrial processes is typically recovered as a saleable waste product from the steam reforming of hydrocarbons as required to produce hydrogen (23). Most of this hydrogen is used to produce ammonia so VOL. 42, NO. 22, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Schematics of the MWF systems considered in this research. Dotted lines indicate system boundary between components or processes included/excluded in the life cycle model. environmental burdens are calculated assuming the CO2 was produced as a byproduct of ammonia synthesis using a price allocation of impacts (24). The price allocation was selected here since CO2 can be produced using other significantly different processes and CO2 use in MWF is unlikely to influence CO2 markets. A cost allocation for CO2 on a mass basis is presented and allocations for other materials discussed in the Supporting Information.
(assumed here as 5% by mass and further discussed in the Supporting Information) (27). Due to the large variability that can be observed among similar manufacturing facilities, the energy results are reported as a function of the consumption rate of the fluid. For water-based MWF systems, this is a bulk liquid replacement rate (usually L/week) and for MQL this is a flowrate (g oil/min or L gas/min) and a lubricant flowrate (mL/hr) (28).
Use Phase
End-of-Life
MWFs are pumped to manufacturing processes from a reservoir attached to the tool. The pumping energy is a significant factor in the analysis and depends on the MWF application rate which can vary greatly in practice (25). For this reason, emissions and energy consumption for this work are reported explicitly as a function of flow rates typical of industrial practice. The production of the MWF delivery equipment (e.g., pumps, compressors, tanks) was not considered since the impacts are relatively small compared to the use phase and the equipment has a working life of several decades (26). The methods used to estimate the power consumption from the pumps and compressors in use are discussed in the Supporting Information. During use, MWF deteriorates and is also lost to evaporation, leaks, spills, mist, and residue on chips and workpieces
For MQL systems, the MWF is used once then the gas diffuses into the surroundings and the remaining oil is carried out on the manufactured part. Therefore no disposal treatment operations were considered for the MQL systems. For waterbased systems, the MWF is collected and recycled until it fails (or is replaced on a schedule) after a period of weeks or months. Two options were considered in this analysis: disposal directly to a municipal wastewater treatment plant and disposal by evaporation followed by incineration (2). Advanced recycling technology can significantly extend the life of water-based MWFs and this was also considered (27).
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Results The environmental emissions associated with the use of MWFs were characterized according to seven categories:
FIGURE 3. Life cycle burdens for the four MWF systems based on expected application conditions. White lines for aqueous systems represent reductions possible if implementing microfiltration-based recycling. water use, toxic emissions to water, solid waste, land use, nonrenewable energy consumption, global warming potential (GWP), and acidification. Each category is described in detail in the Supporting Information. The results are presented in Figure 3. It is observed that for water-based MWFs, a switch from petroleum to rapeseed oil lubricants can produce modest reductions in GWP and acidification but these reductions are associated with increases in energy, solid waste, and land use. The reduction in GWP for rapeseed oil is associated with sequestered CO2 from growing the oil seed, but this comes at a price of increased nonrenewable energy required to farm and process the rapeseed oil and to produce the surfactants necessary to emulsify the vegetable oils. The general result that switching from petroleum-based to bio-based feedstocks creates these tradeoffs has been previously reported (e.g., in biodiesel studies (29)). The results further suggest that despite strong interest motivated by environmental concerns to switch from aqueous mineral oil to aqueous vegetable oil MWFs, the primary concerns listed in Figure 1 are left largely unchanged. On the other hand, Figure 3 illustrates that switching from waterbased to gas-based MWFs results in a significant reduction of environmental emissions within all the major categories of Figure 1. Specifically, the switch from water- to gas-based MWFs results in major reductions in water use, toxic emissions to water, and solid waste. Gas-based MWFs do release more GWP per year than conventional mineral oil in water MWFs but this increase is small in an absolute sense and it is likely to be smaller in large manufacturing facilities where the circulation pumps and coarse filtration systems create significant energy loads. The GWP increase for the gas-based MWFs (about 800 kg CO2-eq/yr) is driven by compressors in the air-based system and CO2 emissions at the end of life for CO2-based systems. The 800 kg is about 1/70th of the CO2 emissions demanded from a typical machine tool that creates the need for MWF (30), assuming an average machine tool demand of 30 kW with an emission factor of 136 g CO2/MJ (17). The conclusion that gas-based MWFs have lower environmental emissions than water-based MWFs for most impact categories is robust to different end-of-life options that can be considered for water-based fluids. As shown in Figure 3, the emissions from water-based MWFs can be reduced significantly by microfiltration recycling. These gains, however, are smaller than the gains made by moving from emulsified MWFs to MQL systems. Incineration does recover some energy from the waste MWF but it is not an improvement because it does not reduce water use or aquatic toxicity and it significantly increases GWP and acidification. These results are further discussed in the Supporting Information. The results shown in Figure 3 are based on typical MWF applications but can change significantly depending on the
preferences of factory operators and machinists. Operating flowrates, pressures, and replacement intervals (for waterbased MWFs) all have a significant impact, for instance, on nonrenewable energy consumption as shown in Figure 4. The general conclusion is that it is possible to run any of the systems in a way that it is much better or much worse than the alternatives, although it should be noted that for severe machining operations very low flow rates do not provide adequate cooling and lubrication so fluids are often delivered in excess. For the petroleum oil-in-air system, increases in the air compression and oil flow rates can drive up the life cycle energy requirements of the fluid quickly. This is in contrast to the scCO2 system, which generally consumes much less oil, but for which energy per unit of CO2 flowrate is higher due to the energy embedded in the ammonia synthesis process which produces CO2 as a byproduct. The energy requirements of the air and scCO2 compressors are significantly higher than the energy requirements of the emulsified MWF pumps but the mass flows of air or scCO2 are much lower than the flow of water. Figure 4 also does not consider the energy required to continuously circulate and treat waterbased MWFs in centralized systems nor does it include microfiltration for the aqueous systems (A and B). Therefore, the energy requirements for delivering the gases are on the same order of magnitude as that for delivering water. The results suggest that the energy loads for all four MWFs are comparable but that for MQL and scCO2-based MWFs, modest reductions in flow rates can produce large energy efficiency gains. Such gains seem likely given that both technologies are relatively new.
Discussion Ultimately a practitioner must decide if switching to a gasbased MWF carrier is an appropriate approach to reduce solid waste, toxic emissions to water, land use, and acidification. These tradeoffs are regional in scale (e.g., does an improvement in local water quality merit an increase in GWP?) and also consider some factors not included in Figure 1. This section considers how some qualitative issues such as worker health and safety, manufacturing performance, and ease-of-maintenance would reinforce the conclusion that gas-based MWFs are a more sustainable solution to meeting cooling and lubrication needs in manufacturing than water-based MWFs. Worker Health and Safety. The move to EALs has been driven largely by a desire to create safer workplaces (31). Chronic inhalation of water-based MWF mists has been shown to be responsible for serious health risks (32). As a result, the National Institute for Occupational Safety and Health has set limits for workplace exposure to MWF mists on a mass per volume basis. However, this standard only VOL. 42, NO. 22, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Nonrenewable energy requirements for the production and use phase for 4 MWF systems: (A) petroleum-in-water; (B) rapeseed oil-in-water; (C) petroleum oil-in-air; and (D) rapeseed oil-in-CO2 as a function of operating parameters: flow rate; replacement frequency; oil flow. Nominal operating conditions based on typical industry practice are indicated by the circular mark in each panel.
TABLE 1. Respiratory Toxicity of Common Metalworking Components (Adapted from Ref 33) component
class
RD50Pa
post-exposure recovery
required in gas-based MWF
sodium petroleum sulfonate boramide triazine petroleum oil
surfactant corrosion inhibitor biocide lubricant
103 129 137 3188
poor yes poor; death within 24-72 h moderate
no no no yes (or vegetable alternative)
a
RD50P measures the mist concentration resulting in pulmonary irritation (lower values correspond to more irritation).
addresses mist quantity not its content. Water-based MWF mists can harbor bacteria, as well as their byproducts, and contain surfactants, biocides, and defoamers. None of these materials are present in gas-based MWF mists which contain only oil. This is notable since surfactants and biocides have been found to impair lung function at concentrations at least ten times lower than oil (Table 1) (33). More importantly, it has been shown that factory mist can be nearly eliminated in gas-based MWF systems, especially if these MWFs are delivered through the tool (12). The mists produced from gas-based MWF systems are easy to control if needed by using existing ventilation and filtration systems already in place in many manufacturing settings (34). On the basis of these data, it can be argued that MWF systems delivered in gas are a better solution than those delivered in water from the worker health standpoint. Worker safety is less clear because the concerns associated with the MWF alternatives are markedly different. Water-based fluids can cause dermatitis and other skin irritations. They also tend to accumulate oily sludge on and around the factory over time. Spills can also be a rather regular workplace hazard which would be eliminated if using gas-based MWFs. On the other hand, a less likely but more serious safety issue is that air or CO2 lines or tanks could rupture, causing serious acute hazards for workers if contingencies for these are not adequately addressed. Ease of Maintenance. Recirculated water-based MWFs are known to become contaminated by hydraulic oils, grease, dirt, and other debris (2). The contamination negatively 8538
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impacts their performance and leads to functional deterioration and breakdown. In addition, water-based MWF ingredients such as biocides and defoamers (not considered in Figure 3) lose their efficacy over time and must be replaced typically at a rate faster than the bulk MWF. Another maintenance problem is that unintended fugitive emissions can occur through uncontrolled or unexpected pathways. These losses are particularly important when the MWF is being stored in large reservoirs. There is little basis to calculate the frequency of these events though the magnitude of such emissions is lessened when individual MWF systems are associated with a single machine tool because tank sizes are smaller and the transmission losses are negligible. Manufacturing Performance. In this analysis it was assumed that all MWFs have comparable machining performance and lead to identical tool replacement rates (35). In practice this is highly unlikely. In fact one motivation for developing scCO2 MWFs is that oil-in-air MWFs are not generally applicable to severe machining operations using advanced materials such as titanium. In some cases, tool life increases ranging from 100-1000% have been observed when using scCO2-based MWFs relative to air- or water-based MWFs (36). In this case, the environmental emissions associated with the life cycle of tools are reduced by a factor ranging from 2 to 10 times. However, it should be noted that the usual response to this type of gain in tool life performance would be for the manufacturer to increase the machining rate so that parts can be produced faster.
There are several other performance criteria to consider including dimensional accuracy and workpiece surface finish. These are both strongly dependent on the quality of lubrication and cooling functions of MWF, and if specific quality tolerances are not achieved, the parts must be remade, leading to additional environmental emissions. This is especially significant to consider in the comparison of MQL systems described in this paper. The cooling limitations of air-based MWFs can lead to dimensional accuracy outside desirable tolerance limits. In contrast, scCO2-based fluids extend the applicability of MQL systems by producing cooling on par with or better than traditional water-based fluids. In summary, based on the calculations presented in Figure 3 and the discussion in this section it can be concluded that a switch from water- to gas-based MWF systems is a move toward more sustainable manufacturing. In the field of green engineering, technology solutions rarely present themselves without trade-offs. Switching from water- to gas-based MWFs will result in significant reductions in aqueous/solid waste, occupational health risks, and maintenance with relatively small increases in GWP and nonrenewable energy for single machine-tool systems as modeled here. This cost is even less when considering centralized water-based MWF systems or the additives required to stabilize these fluids. While waterbased MWF systems have undergone intense optimization over the past six decades, gas-based MWF systems have only recently been studied in depth and applied in manufacturing operations. Given their relative positions on the experience-efficiency curve, we expect that significant performance improvements in gas-based MWF systems are still possible relative to conventional waterbased systems.
Acknowledgments We thank Ashwini Miryala and Bradley Osinski for their assistance collecting the data for this study. This work was funded by EPA STAR fellowship program and NSF grant BES 0607213 and does not necessarily reflect the view or opinions of these agencies.
Supporting Information Available More information regarding the scope and boundary of the life cycle model, the functional unit, life cycle stages, modeling parameters, and background inventory data. This information is available free of charge via the Internet at http://pubs.acs.org.
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ES800791Z
