Compatibility Assessment of Fuel System Elastomers with Bio-oil and

Jul 12, 2016 - ABSTRACT: Bio-oil derived via fast pyrolysis is being developed as a renewable fuel option for petroleum distillates. The compatibility...
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Compatibility Assessment of Fuel System Elastomers with Bio-oil and Diesel Fuel Michael D. Kass,*,† Christopher J. Janke,† Raynella M. Connatser,† Samuel A. Lewis, Sr.,† James R. Keiser,† and Katherine Gaston‡ †

Fuels, Engines, and Emissions Research Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, United States National Bioenergy Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States

‡

ABSTRACT: Bio-oil derived via fast pyrolysis is being developed as a renewable fuel option for petroleum distillates. The compatibility of neat bio-oil with six elastomer types was evaluated against the elastomer performance in neat diesel fuel, which served as the baseline. The elastomers included two fluorocarbons, six acrylonitrile butadiene rubbers (NBRs), and one type each of fluorosilicone, silicone, styrene butadiene rubber (SBR), polyurethane, and neoprene. Specimens of each material were exposed to the liquid and gaseous phases of the test fuels for 4 weeks at 60 °C, and properties in the wetted and dried states were measured. Exposure to bio-oil produced significant volume expansion in the fluorocarbons, NBRs, and fluorosilicone; however, excessive swelling (over 80%) was only observed for the two fluorocarbons and two NBR grades. The polyurethane specimens were completely degraded by the bio-oil. In contrast, both silicone and SBR exhibited lower swelling levels in bio-oil compared to neat diesel fuel. The implication is that, while polyurethane and fluorocarbon may not be acceptable seal materials for bio-oils, silicone may offer a lower cost alternative.

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INTRODUCTION The United States relies heavily upon petroleum imports to meet its domestic energy needs. A large percentage of imported petroleum crude is refined into distillates, such as diesel and fuel oil, which are subsequently used in transportation, home heating, and power generation. To reduce this dependency upon foreign petroleum and domestic fossil fuels, the United States and other countries are evaluating bio-oil as an alternative to petroleum-derived distillates.1 Bio-oil is attractive as a distillate substitute because its combustion properties are similar to distillate fuels. In fact, there has been limited introduction of bio-oil to power boilers for home-heating applications and power generation. Currently, the majority of bio-oil is produced through fast pyrolysis of biomass2−6 Feedstocks are primarily hardwood tree pulp, the type of which along processing variables have been shown to affect the resulting yield and chemical profile of the fuel product.7 As a result, the composition of these oils varies widely, but they usually contain significant quantities of oxygenates, such as ketones and phenols.5,6 The fast pyrolysis method to produce bio-oil employs rapidly heating biomass feedstock (typically pelletized wood) to temperatures approaching 500 °C in the absence of oxygen.7 Liquid yields can be as high as 75% depending upon the reactor configuration, feedstock type, and processing. This oil typically has a higher viscosity and water content than diesel fuel, as shown in Table 1.2 At this stage, the bio-oil is immiscible with petroleum-based fuels. Additional upgrading, including hydrotreating and deoxygenation, are necessary for bio-oil to be used with conventional transport fuels, such as diesel fuel, kerosene, and gasoline.3 Much of the oxygen exists as ketones, aldehydes, anhydrosugars, furanics, phenolics, and carboxylic acids, such as acetic and formic acids, as well as larger molecular weight and © XXXX American Chemical Society

Table 1. Selected Properties of Fast Pyrolysis Bio-oil and Diesel Fuel property density at 20 °C (g/cm ) viscosity at 20 °C (cS) lower heating value (MJ/kg) ash (wt %) water content (wt %) oxygen content (wt %) 3

bio-oil

diesel

1.2 13 17.5 0.13 20.5 42.5

0.85 2.5 42.9 90%) compared to the other NBRs. The results for the marine grade are somewhat surprising because this NBR type provided the best compatibility and durability properties of the six types with oxygenated fuels.31 While the swell levels for NBR 3 and NBR 4 are unacceptably high, it is important to note that neither of these NBR grades are used in fuel storage and transport infrastructure and, therefore, would not be in contact with bio-oil. The other four NBR grades exhibited moderate swelling (∼20%), which is considered acceptable for many infrastructure sealing applications. The remaining three materials (neoprene, SBR, and silicone) all experienced significant expansion when immersed in the baseline diesel fuel. For neoprene, exposure to the baseline diesel fuel imparted a moderate 20% swell, but higher expansions (over 60%) were noted for SBR and silicone. For these three elastomers, exposure to bio-oil produced swelling around 30%, which is higher than obtained with the neoprene specimens exposed to the baseline diesel fuel. However, the opposite effect occurred with SBR and silicone rubbers. For these two elastomers, the level of swelling was dramatically reduced following exposure to bio-oil. While the observed 30% swelling in silicone and SBR may not be ideal for some dynamic sealing applications, it is

Figure 3. Flowchart showing the exposure protocol and test methods. The specimens were exposed to the test fuels for a period of 4 weeks (after which they were fully saturated), then removed, and measured for volume, mass, and Shore A hardness while in the wetted (or saturated) state. The volume change for each specimen was determined using the protocol in ASTM D471-06, while the hardness measurements were performed according to ASTM D2240.25,26 The hardness was measured at five locations on each specimen, and for the original condition, they were found to match the hardness values provided by the suppliers. This correlation with the specifications of the manufacturers is important because the specimen thicknesses were less than the ASTM D2240 specification of 0.635 cm. Following measurement of the wetted properties, the elastomers were ovenheated at 60 °C (in air) for 20 h. After drying, each specimen was once again measured for volume, mass, and hardness. The changes in these properties from the original (untreated) condition were used to assess compatibility. For the vapor-exposure specimens, only the dried hardness was measured.

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RESULTS All of the elastomer specimens immersed in the baseline diesel fuel were all structurally intact upon removal. The same was true for the NBR, fluorosilicone, neoprene, SBR, and silicone specimens exposed to the bio-oil. However, the polyurethane D

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Figure 4. Percent volume change for wetted elastomers in diesel fuel and bio-oil.

Figure 5. Point change in hardness for wetted elastomers in diesel fuel and bio-oil.

acceptable for use in many static conditions. The implication is that both SBR and silicone may be acceptable for use as infrastructure seals with bio-oil. The accompanying hardness measurements (expressed as the point change from the original value) are shown in Figure 5 for each elastomer type. A negative value means that the hardness was reduced (or softened), while a positive value indicates

hardening or embrittlement. As seen in Figure 5, measurable softening was noted for each elastomer type exposed to bio-oil. In all cases, a decrease in hardness (softening) was associated with volume expansion. The fluorocarbon, NBR, fluorosilicone, and polyurethane specimens exposed to the baseline diesel fuel showed negligible hardness change, consistent with the negligible volume changes associated with these materials. E

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Figure 6. Point change in hardness as a function of the percent volume change for elastomers exposed to liquid bio-oil.

Figure 7. Percent volume change for elastomers immersed in diesel fuel and bio-oil after drying for 20 h at 60 °C.

Interestingly, the extent of softening observed for fluorocarbons, NBRs, and fluorosilicone immersed in bio-oil did not necessarily correspond to the degree of swelling, as shown in Figure 6. This effect is especially true for NBRs, where NBR 1, NBR 2, NBR 5, and NBR 6 exhibited different hardness values, even though the extent of swelling was similar. The softening observed for neoprene corresponded well with the observed

volume swell, but both SBR and silicone rubber showed equivalent softening in both diesel fuel and bio-oil, even though they exhibited much higher swelling in the baseline diesel fuel. The volume and hardness changes for each elastomer type (after drying at 60 °C for 20 h) are shown in Figures 7 and 8, respectively. As seen in Figure 7, most of the materials remained as swollen after drying. The implication is that bio-oil F

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Figure 8. Point change in hardness for elastomers immersed in diesel fuel and bio-oil after drying for 20 h at 60 °C.

Figure 9. Point change in hardness for elastomers exposed to the vapor phases of neat diesel and bio-oil. Specimens were prepared by drying for 20 h at 60 °C.

associated high molecular density and low porosity (i.e., small capillary size) of these elastomer types.32 However, the exceptionally high retention (as evidenced by the high volume expansion) cannot be attributed to capillary forces alone.

was retained in the elastomer structure and very high levels of swell were noted for the fluorocarbons, NBR 3, and NBR 4. Fluid retention has been observed by the authors and other researchers in fluorocarbons and is attributed to capillary forces G

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dried specimens. Silicone rubber was also softened by the biooil vapors, albeit to a lower level than observed for the wetted and dried specimens. Both of these elastomers were unaffected by the bio-oil vapors. The material most affected by bio-oil was polyurethane. In fact, the polyurethane specimen was dramatically softened (by almost 60 points) by the vapors, which is consistent with the severe degradation that polyurethane incurred when exposed to the liquid bio-oil.

Clearly, other intermolecular attraction forces associated with solubility are involved. Interestingly, the volumes associated with several of the NBR grades exposed to neat diesel (especially NBR 1, NBR 2, and NBR 6) actually declined (shrank) from their original values after drying. A negative volume change indicates that dissolution and extraction of one or more components making up the elastomer had taken place. Another interesting finding is that the volumes for SBR and silicone rubber remained swollen (albeit at relatively low levels) after drying and that the dried volumes for both of these materials were significantly higher for the diesel fuel exposure. These results suggest that silicone, which had previously shown high swelling in gasoline and diesel fuel, may be acceptable as seals for bio-oils for some applications. The corresponding point change in hardness results for the elastomers (from their original values) are shown in Figure 8 for the immersed samples after drying. As expected, softening was observed for those specimens having retained fluid (volume expansion) after drying. This effect is especially true for the specimens immersed in bio-oil. The neoprene material was unique in that there was no corresponding decrease in hardness that accompanied the 15% volume expansion that was observed after drying. However, neoprene was appreciably softened in the wetted state, and this softening corresponded to a moderate level of swelling (around 35%). The likely explanation is that the bio-oil extracted the plasticizer component, and the resulting increase in hardness resulting from the plasticizer loss served to offset the softening incurred by the retained fuel. Four of the six NBR grades showed modest embrittlement (greater than 5 point increase in hardness) following exposure to neat diesel fuel. This embrittlement (hardness increase) is attributed to plasticizer extraction by the diesel fuel. It is likely that plasticizer compounds were also extracted by the bio-oil, but this effect may be masked by the high levels of retained fuel, which dramatically lowered the hardness, causing a general hardness decrease in these samples. SBR showed high levels of volume swell (after drying) and correspondingly large reductions in hardness. Silicone also remained swollen in the dried state following exposure to either test fuel. However, the specimens exposed to bio-oil did not show any significant changes in hardness from their original values. The hardness results for the specimens exposed to the test fuel vapors are shown in Figure 9. All of the materials exposed to the vapor-phase region of the exposure chambers were removed intact, including the polyurethane specimens. After removal, these specimens were dried for 20 h at 60 °C and remeasured for hardness using the Shore A method also used on the immersed specimens. The chemical composition of the vapor region can be expected to differ from the base liquid fuel. The fuel components having high volatility can be expected to predominate in the vapor region, whereas they would be expected to exist in lower concentrations in the liquid region. As a result, the dried volume change results can be expected to be different from those shown in Figure 8 for the immersed and dried specimens. The majority of the elastomers were unaffected by the test fuel vapors, but there were several noteworthy exceptions, which include SBR and silicone exposure to diesel fuel and polyurethane exposure to bio-oil. The SBR specimen experienced a 25 point drop in hardness, which is similar to the result obtained from the wetted and

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CONCLUSION In general, common fuel storage and dispensing infrastructure elastomers showed marked difference in swell and hardness when exposed to bio-oil versus a neat diesel fuel. The study examined the effects of neat bio-oil and No. 2 diesel fuel with representative samples of fluorocarbon, NBR, fluorosilicone, polyurethane, neoprene, SBR, and silicone. The two fluorocarbon materials, Viton A401C and Viton B601, showed excessively high levels of volume expansion when exposed to the neat bio-oil. Fluorocarbons are known to be incompatible with ketones, which were prevalent in the bio-oil fuel, as shown earlier in Figure 1. Given this result, any system identified to contain and deliver bio-oil needs to consider replacement of any fluorocarbon seals and hoses. Additionally, polyurethane was severely degraded by the bio-oil, which may preclude its use in these systems as well. Except for the small-engine fuel line hose and marine grades, NBR showed moderate volume expansion. This level of swelling is acceptable in many sealing applications and would not necessarily preclude the use of NBR; however, existing NBR-based hoses may need to be replaced to prevent warpage. The fluorosilicone results mirrored those of the more compatible grades of NBR, and the same recommendation holds for this material. Although SBR showed modest swelling in bio-oil, it exhibited pronounced softening, which reduces its durability. Because SBR is not used as a seal for fuel applications, no significant impact is expected. Silicone showed modest to high swelling in bio-oil, but the extent was lower than that observed for diesel fuel. As such, silicone seals used to store and dispense a diesel fuel are likely to be compatible with bio-oil. In fact, silicone may offer a low-cost replacement for other elastomers, such as fluorocarbons.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 865-946-1241. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the United States DOE’s Bioenergy Technology Office. The authors gratefully acknowledge the support and guidance from Jonathan Male and Alicia Lindauer, DOE, and Tim Theiss, ORNL. The authors are also grateful to Esther Wilcox and Katelin Wheeler, NREL, for their help in providing bio-oil and facilitating shipment to ORNL for this study.

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