In Figure 3, k is the reaction rate constant, the dashed vertical line near 1000°F is the low temperature bound of the early temperature dependent studies, and the horizontal dashed lines represent the threshold for the rate constant where 1% of the fluid decays in the time period indicated. These can be used as a guide to estimate the maximum allowable working fluid temperature for a given fluid decomposition tolerance. However, the lack of data in the temperature range of interest for technologically important working fluids is a concern that needs to be addressed by the research community.
An ORC working fluid is circulated at temperatures between ambient and the maximum temperature of the system. So if, for example, one percent of the fluid decomposing is tolerable and the maximum temperature of the system is such that one percent is expected to decompose per year at that temperature, satisfactory operation for ten or more years may be possible because the fluid is not exposed to the maximum temperature for a large fraction of the cycle. To determine more accurate measures, these temperature dependent rate constants could be combined with modeling that takes into account the temperature profile of the fluid with time. This also highlights the importance of avoiding hot spots in a heat exchanger that is near the maximum allowable temperature.
In addition to the data presented in Figure 3, there is some variation in reports regarding acceptable working temperature for some fluids. For example, Andersen and Bruno (2005) concluded that toluene has an unacceptable decomposition rate at 600°F, but Marciniak et al. (1981) reported that it can be used to 750°F (399°C), though they do so without reference or supporting data. Cole found toluene to be a stable working fluid to 677°F (358°C) and was expected to be stable at least to 750°F (399°C) provided that oxygen was excluded from the system (Cole et al. 1987). Researchers in that study suggested that years of operation should be possible between fluid changes. Baton (2000) reported that in one facility operating with toluene, working fluid decomposition products of toluene were found after several thousand hours of operation at 700°F (371°C) hot side temperature. But another facility operating at 750°F (399°C) hot side temperature had not shown any signs of decomposition. Differences between laboratory measurements of decomposition and field observations might be explained by differences in measurement methodology. Specifically, the field workers are looking for visibly obvious signs of decomposition such as black chunks or residue, while laboratory workers are using instruments to measure concentrations of decomposition products quantitatively. If this is the case, it may indicate that some of the decomposition products are largely benign.
Figure 4 highlights the conflicting needs of high critical point and high resistance to thermal decomposition for the nalkanes. For example, if 1%/year degradation is the maximum allowable for a heat source at 500°F (260°C) a fluid would have to be chosen which is working above its critical point. The data displayed for decomposition is meant to illustrate the tradeoff that exists between stability and critical point, the data used is based on the early high 本文来自辣~文\论|文/网，
毕业论文 www.751com.cn 加7位QQ324'9114找源文 which shows decomposition of pentane faster than 1%/30 days at 600°F (316°C) while extrapolating higher temperature data into this range suggests it would be closer to 1%/year.
Fluorinated refrigerants such as R-245fa (1,1,1,3,3-pentafluoropropane) are also actively used as ORC fluids. Angelino and Invernizzi (2003) reported that this compound is stable for at least 50 hours at 572°F (300°C) but at 626°F(330°C) decomposition is rapid (Angelino and Invernizzi 2003). Their results are in contrast with a representative of the manufacturer of R-245fa, who suggests that working fluid temperatures much above 300°F (149°C) should be avoided due to observations of fluorine formation attributed to decomposition (Zyhowski 2008). Further, manufacturers of ORC equipment using R-245fa generally limit the maximum working fluid temperature to 300°F. While the C-F bond strength is high, the contributions of entropy driving the decomposition is also larger for the refrigerants than it is for alkanes of the same chain length due to the larger number of atom types. This is what drives the auto ignition temperature of the HCFC’s to be lower than alkanes of the same chain length (The Engineering Toolbox 2008; BOC Gases 2008). Therefore, a lower decomposition temperature should be expected for these compounds than their non-halogenated relatives. An explanation for the difference in opinion regarding operating temperature may simply be the length of time over which the experiments were conducted and the sensitivity limits available to measure decomposition. Because the decomposition process is a thermally activated one, moving from 572 to 626°F (300 to 330°C) could increase the decomposition rate from 1%/day to 1%/ hour (which was the stated maximum sensitivity for Angelino and Invernizzi 2003). A change this dramatic is consistent with the activation energies for alkane thermal decomposition. The greater bond strength of the C-F bond compared to the C-H or C-C bond would likely make the activation energy larger and the change in decomposition rate even more abrupt.
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