Evaluation of Polymer Compatibility with Bio-oil Produced from

Nov 19, 2015 - The performance of each polymer was determined by changes in its mass and cross-sectional area. An additional performance metric was th...
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Evaluation of Polymer Compatibility with Bio-oil Produced from Thermochemical Conversion of Biomass Martin R. Haverly,†,‡ Lysle E. Whitmer,‡ and Robert C. Brown*,†,‡ †

Department of Mechanical Engineering, and ‡Bioeconomy Institute, Iowa State University, Ames, Iowa 50011, United States ABSTRACT: Large-scale production of transportation fuels and commodity chemicals through thermochemical conversion of biomass will require a better understanding of the appropriate materials of construction. Processes such as pyrolysis, solvent liquefaction, and hydrothermal processing produce oxygenated bio-oils that can damage materials commonly found in fuel and chemical production facilities. Recent investigations have examined the corrosive effects of these bio-oils on various kinds of steel, but very little effort has been given to determining their effect on polymers used for valve seats and standard gaskets. This project evaluated the performance of several common polymers used in process control equipment after exposure to a simulated thermochemical environment for 48 h. The performance of each polymer was determined by changes in its mass and crosssectional area. An additional performance metric was the change in polymer hardness, as measured on the Shore durometer scale. All of the polymers tested exhibited an increase in cross-sectional area and mass after exposure to bio-oil, with a decrease in hardness. An increasing temperature tended to exacerbate these effects. Bio-oils with a low molecular weight and a high degree of polarity were found to be the most detrimental. From this, it was concluded that the polarity of both the polymer and bio-oil contributed most significantly to the observed effects. Nitrile butadiene rubber and Viton displayed the most significant change in all categories. Polytetrafluororethylene and polyether ether ketone were found to be the most resistant to all types of bio-oil in all conditions.

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INTRODUCTION Concern over the widespread use of petroleum-derived products and the threat of depleting oil reserves has encouraged research into the development of renewable fuels and chemicals from the thermochemical conversion of biomass. Thermochemical processing typically occurs at elevated temperatures and pressures, often on the order of 300−1200 °C and 1−250 bar. These intense reaction conditions promote rapid deconstruction of the complex polymer matrix found in biomass, resulting in a wide range of chemical compounds, which can be used in the production of transportation fuels and chemicals.1 In recent years, fast pyrolysis and direct liquefaction have received significant attention as a result of the ability to produce liquid bio-oil from raw biomass. The high concentration of oxygen in biomass results in biooil with physical and chemical properties distinct from crude petroleum. The oxygen manifests itself in a variety of functional groups, such as acids, alcohols, ketones, aldehydes, and phenols. These functional groups are responsible for the relatively unstable and reactive nature of bio-oil. Furthermore, inorganic components inherent to most biomass have been shown to catalyze reactions that contribute to the reactive nature of biomass liquefaction products.2 The compounds that contribute to bio-oil instability and reactivity also impact the materials of construction commonly found in chemical processing plants. An investigation by Darmstadt et al.3 examined the effect of storing bio-oil obtained from vacuum pyrolysis of softwood bark on corrosion of aluminum, copper, and 316 series stainless steel. The study suggested that bio-oil acidity has the most adverse effect on aluminum and copper, but the high chromium content of 316 series stainless steel prevented any significant corrosion to occur for this material. Similarly, Brady et al.4 recently © 2015 American Chemical Society

published a summary of the investigations by Oak Ridge National Laboratory on the most suitable low-cost alloys for the construction of thermochemical liquefaction processing equipment. This review concluded that the presence of trace amounts of elemental S, Na, and Cl lead to a majority of the corrosion and cracking tendencies of the materials tested. In addition to the adverse effect that bio-oil has exhibited on metals used in the construction of thermochemical processing facilities, polymers used in these environments are subject to undesirable degradation. This has potentially catastrophic effects on both the safety and operational capacity of processing plants, because polymers are often used in critical applications, such as gaskets and valve seats. Although polymer manufacturers and distributers often release compatibility metrics based on ambient temperature testing with pure compounds, compatibility with a complex substance, such as bio-oil, remains in question. Recent work has been performed to address this question for other complex substances, such as biodiesel blends. Haseeb et al.5,6 investigated the impact of palm biodiesel and biodiesel blends on common fuel system elastomers. Their work concluded that the ester functionality inherent to biodiesel caused degradation in polymers that would otherwise remain unaffected by hydrocarbon diesel. This is a clear example of the detrimental effect that oxygenated functionalities can have on ordinary polymers. The present study explored the effect of bio-oil produced from thermochemically processing biomass on various polymers commonly found in processing plants as well as any Received: July 13, 2015 Revised: November 8, 2015 Published: November 19, 2015 7993

DOI: 10.1021/acs.energyfuels.5b01943 Energy Fuels 2015, 29, 7993−7997

Article

Energy & Fuels synergetic effect that may occur at elevated temperatures and pressures. The performance of the polymers tested was determined by the change in polymer mass and cross-sectional area after the samples were submerged in bio-oil for 48 h. An additional performance metric was the change in polymer hardness, as measured on the Shore durometer scale.

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EXPERIMENTAL SECTION

Selected Polymers. The six polymers selected for this study were nitrile butadiene rubber (NBR), ultrahigh-molecular-weight polyethylene (UHMW PE), polyether ether ketone (PEEK), polytetrafluoroethylene (PTFE), Viton, and Kalrez, with the last two being registered trademarks of DuPont Performance Elastomers LLC. These polymers were selected because of their extensive use in industrial applications as both valve seats and fitting gaskets. To create the individual samples, 30 × 30 cm sheets of the materials were purchased from McMaster-Carr. Viton and Kalrez were purchased directly from DuPont in 30 × 30 cm and 15 × 15 cm sheets, respectively. Furthermore, each of the 30 × 30 cm sheets were purchased in a nominal thickness of 6.4 mm, while the 15 × 15 cm sheet of Kalrez was only available in a maximum thickness of 3.2 mm. After the raw materials were procured, 12 equal samples were cut out of the middle of each sheet, with a final individual sample size of 1.9 × 7.6 cm. The sample size was largely dictated by the internal dimensions of the compatibility test cell, but it was desirable to have a significant sample surface area to facilitate the interaction between the individual polymers and bio-oil during each test. Thus, the surface area for each sample was 41 cm2, except for the thinner stock Kalrez, which had a surface area of 35 cm2. Bio-oils. Three bio-oils were investigated in this study. These were various fractions of bio-oil recovered from the fast pyrolysis of red oak at 500 °C. The bio-oil groups were recovered via a novel fractionation process at Iowa State University, in which bio-oil vapors are selectively condensed via careful manipulation of the vapor temperature of the bulk stream.7 In this case, the bio-oils were condensed according to broad groups of compounds defined by the range of dew points as follows: heavy ends (HE, 345−100 °C), intermediate fractions (IF, 100−75 °C), and light ends (LE, 75−18 °C). HE account for waterinsoluble high-molecular-weight compounds that are primarily products of the thermal decomposition of lignin and anhydrosugars derived from the cellulose and hemicellulose. This mixture of highmolecular-weight compounds has a kinematic viscosity of approximately 3500−4400 cSt at 60 °C. Similarly, IF represent the highly polar, low-molecular-weight compounds produced from lignin, such as phenol. The LE are primarily low-molecular-weight compounds produced from cellulose and hemicellulose. The LE contain a large amount of light acids, such as acetic and formic acids, as well as a considerable amount of water produced during the pyrolysis process. Because of the low-molecular-weight compounds in both the IF and LE, these mixtures have a relatively low viscosity of approximately 36− 50 and 1.3 cSt at 40 °C, respectively. Figure 1 depicts the distribution of primary products in each group detailed above. A more comprehensive explanation of the reactor and recovery system can be found in the studies by Pollard et al.7 and Rover et al.8 Compatibility Tests. After the polymer samples were cut to the test size as described above, each was loaded into an individual cell, as shown in Figure 2. The test cells were constructed from a 15 cm long section of 316 stainless-steel tubing with a 2.5 cm outer diameter. The tubing was capped on one end, and the open end was swaged, so that all of the test cells could be connected together, as shown in Figure 3. A total of 50 mL of bio-oil was then added to each test cell, such that the polymer sample was fully submerged. Once all of the test cells were loaded, they were connected together and purged with nitrogen gas to ensure that the head space did not contain any molecular oxygen. The whole test apparatus was then either left at ambient conditions or pressurized and heated, as prescribed by the test plan shown in Table 1. The basic experimental setup matrix was applied to each polymer tested. It should be noted that, as a result of the

Figure 1. Distribution of primary products commonly found in the bio-oil used for this study, shown as a mass fraction of the whole biooil (WB) (adapted with permission from Pollard et al.7) (HE, heavy ends; IF, intermediate fractions; and LE, light ends).

Figure 2. Diagram of the test cell used to expose polymer samples to bio-oil. Each sample was loaded so that it was fully submerged in biooil. relatively low melting point of UHMW PE (∼130 °C), the elevated temperature test condition was not applied to this polymer. The compatibility tests were carried out for 48 h, after which the test apparatus was brought to ambient conditions. Each polymer sample was removed from the test cell and washed clean of any residual bio-oil with deionized water. Once dry, the samples were evaluated for compatibility. 7994

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other samples. Thus, data from the different samples can be compared to each other. Hardness, however, is measured on the nonlinear Shore durometer scale, and therefore, these values cannot be reported as a percent change. They are instead shown as the difference between the final and initial hardness. The percent change in sample mass is shown in Figure 4. The general increase in sample mass suggests that the polymers

Figure 3. Compatibility test apparatus made from stainless-steel tubing. Each test cell shared common headspace to ensure equal topping pressure for each cell. From left to right, the polymers shown are polytetrafluoroethylene (PTFE), polyether ether ketone (PEEK), nitrile butadiene rubber (NBR), ultrahigh-molecular-weight polyethylene (UHMW PE), Viton, and Kalrez.

Figure 4. Sample mass generally increased as a result of exposure to bio-oil (UHMW PE, ultrahigh-molecular-weight polyethylene; PEEK, polyether ether ketone; PTFE, polytetrafluoroethylene; NBR, nitrile butadiene rubber; HE, heavy ends; IF, intermediate fractions; LE, and light ends).

tended to absorb bio-oil. This phenomenon is also corroborated by examination of the sample cross-sectional area, as shown in Figure 5. Here, it can be seen that an increase in the

Table 1. Compatibility Test Conditions Were Set at Two Levels for Each Bio-oil Tested (HE, Heavy Ends; IF, Intermediate Fractions; and LE, Light Ends) test ID

heavy ends

intermediate fractions

light ends

25 °C, 1 bar 150 °C, 42 bar

HE, low HE, high

IF, low IF, high

LE, low LE, high

Analytical Methods. The compatibility of a polymer sample with a bio-oil was evaluated on the basis of its change in physical characteristics. A significant change in any of the physical properties of a polymer, such as hardness or cross-sectional area, would be evidence of poor compatibility in bio-oil applications. Before a sample was loaded into the test apparatus, its initial properties were measured. The sample mass was determined by a Denver Instruments SI-234 analytical balance with a precision of ±0.1 mg. The cross-sectional area was determined with a Mitutoyo Digimatic Micrometer capable of reading to ±1.27 μm precision. Width and thickness measurements were taken at the middle and ends of the sample. The three corresponding width and thickness measurements were multiplied together to find the cross-sectional area, and the three areas were averaged to find the average crosssectional area for each sample. Polymer hardness was measured using either the Shore A or Shore D scale according to a modified ASTM D2240-05 with a Mitutoyo 811 analog durometer (±0.5 of reading). Three measurements were taken for each sample and then averaged to ensure a uniform hardness value representative of the sample as a whole. Post-run sample analysis was identical to the procedure described above.

Figure 5. Sample cross-sectional area generally increased after exposure to bio-oil (UHMW PE, ultrahigh-molecular-weight polyethylene; PEEK, polyether ether ketone; PTFE, polytetrafluoroethylene; NBR, nitrile butadiene rubber; HE, heavy ends; IF, intermediate fractions; and LE, light ends).

cross-sectional area tracks well with an overall increase in mass. This implies that bio-oil is able to penetrate deep into the polymer matrix and increase the overall interaction of the polymer molecules with the bio-oil. Such polymer swelling is well-known, but previous studies have primarily been limited to binary or ternary systems of pure solvents and a polymer.9,10 The complex mixture of chemical compounds found in bio-oil pose a significant challenge in trying to elucidate the specific causes for swelling and other physical effects. There are, however, generally understood correlations between polymer properties and general resistance to chemical attack or degradation. Chemical composition,

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RESULTS AND DISCUSSION Mass and cross-sectional area metrics were compared against the starting values for each polymer. This allowed the results to be normalized to the initial condition and the data to be presented as a percent change. Presentation of the data in this way neutralizes any disparity that exists between the starting samples, such as the initial surface area between Kalrez and all 7995

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highly polar nature of the low-molecular-weight compounds found in IF and LE bio-oil. Given the use of these polymers as mechanical seals, swelling is generally unacceptable and can be seen as a failure mode for a given polymer. For example, if a polymer valve seat were to increase in area, the control characteristic of the valve may change, causing erratic behavior, or the seat may even overflow from its housing, causing it to become dislodged entirely. The mechanical loading inherent to these various polymer applications has been shown to impact the resistance of a polymer to aggressive liquids. Work by Ch’ng et al.18 has shown that torsion and tension loading similar to that found in a valve seat significantly increased the level of swelling for several polymers immersed in biodiesel. Conversely, compressive loading, such as that experienced by a compression gasket, has been shown to actually improve the resistance of a polymer to swelling as a result of the increased resistance to liquid diffusion into the compressed polymer.19,20 In addition to physical changes manifested by swelling, the chemical degradation imposed by bio-oil can also impact the mechanical properties of a polymer. Of particular interest is any significant change in hardness that can compromise the utility of a polymer as a sealing mechanism. An overall decrease in polymer hardness can be attributed to a reduction of secondary bonding within the polymer and a subsequent separation of the polymeric chains.20 Figure 6 demonstrates that this general

polymer polarity, crystallinity, density, and cross-linking are among the properties that determine polymer performance in a solvent. It stands to reason that the chemical composition of a polymer significantly impacts its overall properties. Most polymers consist of a carbon-chain backbone, which can then be substituted with a variety of functional groups. The polymers used in most chemical processing applications are often fluorinated as a result of the wide range of practical functionality that can be achieved through controlled addition of fluorine. Chemical resistance, melting point, thermal stability, dielectric constant, and many other polymer properties are enhanced through this process.11 These findings are corroborated by the results of this study in which PTFE and Kalrez, both of which have a relatively high fluorine content, performed well across all of the tests. It should be noted that Viton is also a fluorinated compound. Although it did not perform as well as several of the other polymers, for reasons subsequently discussed, it generally fared better than NBR, which is a similar polymer but entirely lacking in fluorine. The polarity of a polymer is largely dictated by polarity of its monomers; i.e., a polymer made up of nonpolar monomers is likely to be nonpolar, whereas a polymer made of polar monomers may be either polar or nonpolar depending upon its structure. The overall polarity of a macromolecule is determined by bond angles of the constituent molecules.12 Polymer polarity ultimately affects its compatibility with solvents of opposite polarity. For example, NBR is a polar polymer as a result of the presence of the nitrile group on the 2propenenitrile monomer and, therefore, has superior performance to nonpolar rubber when subjected to nonpolar hydrocarbons.13 Many of the compounds found in bio-oil are polar as a result of oxygenated functionality, such as aldehydes and ketones.14 Of the polymers tested, both NBR and Viton are relatively polar; therefore, their significant swelling, as illustrated in Figures 4 and 5, is not surprising. Crystallinity, cross-linking and density are interconnected in the relative benefits offered to the chemical resistance of a given polymer. For example, the tightly packed structure of crystalline cellulose makes it more recalcitrant to chemical treatment than amorphous cellulose, starch, and hemicellulose.15 This same principle applies to synthetic polymers, where a tightly packed crystalline structure makes it difficult for chemical reagents to penetrate the polymer matrix. In a similar fashion, cross-linking provides more intramolecular strength to a given polymer, which resists solvent molecules from penetrating between the polymer chains. Thus, a polymer with more of a crystalline structure or intramolecular cross-linking will swell to a lesser degree than one with either an amorphous structure or limited cross-linking.12,16 This phenomenon affects the propensity of a solvent to be absorbed by a polymer. As previously discussed, polymer absorption of a given solvent is dictated by a wide range of factors, including the size of solvent molecules.17 Intuitively, small solvent molecules are more prone to absorption into the polymer matrix than large solvent molecules, and by extension, polymers with tightly packed crystalline structures and/or strong cross-linking are more likely to only absorb bio-oil comprised of relatively small molecules. This is corroborated by the experimental data in which the polymers tended to absorb more of the LE and IF bio-oil than the HE fraction, where most of the high-molecular-weight molecules are collected. The trend is especially noticeable for the ambient condition cases and is likely compounded by the

Figure 6. Sample hardness generally decreased after exposure to biooil (UHMW PE, ultrahigh-molecular-weight polyethylene; PEEK, polyether ether ketone; PTFE, polytetrafluoroethylene; NBR, nitrile butadiene rubber; HE, heavy ends; IF, intermediate fractions; and LE, light ends).

decrease in sample hardness occurred for almost all of the polymers tested. Although considerable change in hardness is not preferred, softening can be permissible in some applications, such as a ball valve seat, where the moving parts are mechanically restrained. However, if a seat or gasket were to soften excessively it would be much too prone to damage for it to be useful in any long-term capacity. Therefore, significant change of any kind can also be generally regarded as a failure mode for a given polymer. Further examination of the results suggests that an elevated temperature tends to exacerbate any negative effects that bio-oil had on the polymers. Increasing the temperature is understood to have a strong effect on the intermolecular forces within a polymer. Increasing molecular motion of the individual molecules as well as a general relaxation of cross-link bond forces have been shown to occur at elevated temperatures.12,16 7996

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(7) Pollard, A. S.; Rover, M. R.; Brown, R. C. Characterization of biooil recovered as stage fractions with unique chemical and physical properties. J. Anal. Appl. Pyrolysis 2012, 93, 129−138. (8) Rover, M. R.; Johnston, P. A.; Whitmer, L. E.; Smith, R. G.; Brown, R. C. The effect of pyrolysis temperature on recovery of bio-oil as distinctive stage fractions. J. Anal. Appl. Pyrolysis 2014, 105, 262− 268. (9) Nandi, S.; Winter, H. H. Swelling behavior of partially crosslinked polymers: a ternary system. Macromolecules 2005, 38 (10), 4447−4455. (10) Okeowo, O.; Dorgan, J. R. Multicomponent Swelling of Polymer Networks. Macromolecules 2006, 39 (23), 8193−8202. (11) Ebnesajjad, S.; Morgan, R. Fluoropolymer Additives; William Andrew, Inc.: Norwich, NY, 2011. (12) Ebewele, R. O. Polymer Science and Technology; CRC Press: Boca Raton, FL, 2000. (13) Yasin, T.; Ahmed, S.; Yoshii, F.; Makuuchi, K. Effect of acrylonitrile content on physical properties of electron beam irradiated acrylonitrile−butadiene rubber. React. Funct. Polym. 2003, 57 (2), 113−118. (14) Vanderbosch, R. H.; Prins, W. Fast pyrolysis. In Thermochemical Processinng of Biomass; Brown, R. C., Ed.; Wiley: Hoboken, NJ, 2011; pp 131−133. (15) Brown, R. C.; Brown, T. R. Biorenewable Resources: Engineering New Products from Agriculture; Wiley: Hoboken, NJ, 2013. (16) Nielsen, L. Cross-Linking−Effect on Physical Properties of Polymers. Journal of macromolecular science. J. Macromol. Sci., Polym. Rev. 1969, 3 (1), 69−103. (17) Treloar, L. R. G. Physics of Rubber Elasticity, 3rd ed.; Oxford University Press: Oxford, U.K., 2005. (18) Ch’ng, S. Y.; Andriyana, A.; Verron, E.; Kahbasi, O.; Ahmad, R. Ahmad, Development of a Novel Experimental Device to Investigate Swelling of Elastomers in Biodiesel Undergoing Multiaxial Large Deformation. Exp. Mech. 2013, 53 (8), 1323−1332. (19) Andriyana, A.; Chai, A. B.; Verron, E.; Johan, M. R. Interaction between diffusion of palm biodiesel and large strain in rubber: Effect on stress-softening during cyclic loading. Mech. Res. Commun. 2012, 43, 80−86. (20) Chai, A. B.; Andriyana, A.; Verron, E.; Johan, M. R.; Haseeb, A. S. M. A. Development of a compression test device for investigating interaction between diffusion of biodiesel and large deformation in rubber. Polym. Test. 2011, 30 (8), 867−875.

This behavior is likely compounded by increased molecular interactions that can occur at higher temperatures, thereby increasing the mass diffusion rate of a solvent into the polymer. Furthermore, as the polymers approach their respective melting temperature, the overall molecular integrity of the polymer can be compromised.

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CONCLUSION In general, PEEK was the best performing polymer. It was extremely resilient to all reagents at all conditions. PTFE had a similar performance to PEEK in most of the evaluation categories, except for hardness, in which it tended to soften to about 95% of its starting value. Although this softening is not necessarily desirable, as described previously, PTFE is an inherently softer polymer than PEEK and is favored in applications that require mechanical compression of the polymer. Therefore, this study recommends PTFE for most sealing applications for biomass thermochemical liquefaction.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +1-515-294-7934. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this research was provided by the Gary and Donna Hoover Endowment in Mechanical Engineering at Iowa State University and the U.S. Department of Energy, Contract EE0005974.

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NOMENCLATURE NBR = nitrile butadiene rubber UHMW PE = ultrahigh-molecular-weight polyethylene PEEK = polyether ether ketone PTFE = polytetrafluoroethylene HE = heavy ends IF = intermediate fractions LE = light ends

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REFERENCES

(1) Brown, R. C. Thermochemical Processing of Biomass: Conversion into Fuels, Chemicals and Power; John Wiley & Sons: Hoboken, NJ, 2011; Vol. 12. (2) Diebold, J. P. A Review of the Chemical and Physical Mechanisms of the Storage Stability of Fast Pyrolysis Bio-oils; National Renewable Energy Laboratory (NREL): Golden, CO, 2000; NREL/SR-57027613. (3) Darmstadt, H.; Garcia-Perez, M.; Adnot, A.; Chaala, A.; Kretschmer, D.; Roy, C. Corrosion of Metals by Bio-Oil Obtained by Vacuum Pyrolysis of Softwood Bark Residues. An X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy Study. Energy Fuels 2004, 18 (5), 1291−1301. (4) Brady, M. P.; Keiser, J. R.; Leonard, D. N.; Whitmer, L.; Thomson, J. K. Corrosion Considerations for Thermochemical Biomass Liquefaction Process Systems in Biofuel Production. JOM 2014, 66, 2583−2592. (5) Haseeb, A. S. M. A.; Masjuki, H. H.; Siang, C. T.; Fazal, M. A. Compatibility of elastomers in palm biodiesel. Renewable Energy 2010, 35 (10), 2356−2361. (6) Haseeb, A. S. M. A.; Jun, T. S.; Fazal, M. A.; Masjuki, H. H. Degradation of physical properties of different elastomers upon exposure to palm biodiesel. Energy 2011, 36 (3), 1814−1819. 7997

DOI: 10.1021/acs.energyfuels.5b01943 Energy Fuels 2015, 29, 7993−7997