Higher molecular weight organic molecules generally have higher critical points than lower molecular weight ones. High critical point allows higher temperature sources to be used without superheat and therefore at greater efficiency. However, continuing to increase molecular weight causes the slope of the saturated vapor line to become more positive (Figure 2). A positive slope implies that the fluid will be expelled from the turbine as a superheated vapor, increasing the exit temperature of the turbine and reducing cycle efficiency. Some of this excess heat can be recaptured using a regenerative heat exchanger. This increases efficiency but at the expense of additional capital equipment and a reduction in the maximum achievable efficiency.
The effects of increasing molecular weight on the slope of the saturated vapor line can also be partially mitigated by changing the molecular structure to limit the number of available molecular orientations. Ring structures do not allow the twisting that occurs in straight chain or branched alkanes, they therefore have fewer degrees of freedom and more vertical saturated vapor lines. As an example of this behavior, both heptane and toluene shown in Figure 2 have seven carbons, but the ring structure of toluene shifts the critical point up while simultaneously making the saturated vapor line closer to vertical. Effectively its saturated vapor line behavior is more like that of a lower molecular weight fluid but it retains the high critical point of a high molecular weight fluid. This makes toluene a potentially more efficient working fluid than pentane when higher temperature sources are available.
Based solely on their T-s behavior it would appear that molecules such as R-245fa and pentane for lower temperature sources and toluene for higher temperature sources would be nearly ideal working fluids, and in fact these are common fluids used in ORC equipment. All have nearly vertical saturated vapor lines and the critical point can be as high as 600°F (316°C). However, thermal and chemical stability is another issue that needs to be considered, and it is generally in conflict with choices that would be made solely based on the thermodynamic properties of the fluids themselves. This is an area with significant uncertainty for ORC technology. All organic materials undergo degradation and cracking processes as temperature increases. Degradation is accelerated by the presence of air in the system. For this reason, the maximum working fluid temperatures for organic Rankine cycle systems must be limited to avoid temperatures which would cause decomposition. In addition, care must be taken to ensure that the system is free from atmospheric contamination.
In theory, smaller molecular weight organic molecules should be more resistant to thermal cracking. However, very little decomposition data is available in the academic literature in the temperature range of interest for waste heat recovery. Research measurements were conducted in the early 1900’s at substantially higher temperatures with the intent of determining the mechanism of decomposition and measuring decomposition rates (Marek and McCluer 1931; Paul and Marek 1934; Morgan et al. 1935; Frey and Hepp 1933; Pease 1928). More recent measurements were taken in the temperature range of interest but were not performed over a range of temperatures (Andersen and Bruno 2005). The results of these studies have been re-plotted in Figure 3. Reaction rate constants as a function of temperature were determined using the Arrhenius equation.
The early temperature dependent studies cited here have concluded that the decomposition mechanism is a simple first-order decomposition reaction for all molecules studied. While there may be other, faster, processes that dominate decomposition at lower temperatures first order decomposition should provide a lower bound of the decomposition rate. Extrapolating the results into a lower temperature regime allows an estimate of decomposition rate at the temperatures of interest for ORC’s.
There are several trends of interest from Figure 3. First, smaller molecules decompose more slowly than larger ones. This is expected assuming entropy is the driving force for first order thermal decomposition. Branched molecules decompose more rapidly than straight chains, and ring structures, particularly benzene, are more stable than linear chains. This is also expected because of the added stability introduced by carbon’s resonant structures. Finally, when similar molecules were tested at lower temperatures, their stability was significantly worse than predicted by high temperature data extrapolation. This may indicate that some mechanism other than thermally activated first order decomposition becomes dominant at lower temperatures. One possibility is that the surface of the reaction vessel catalyzes the decomposition at low temperatures.
上一页 [1] [2] [3] [4] [5] [6] [7] 下一页
工业余热回收利用英文文献翻译(5000字) 第3页下载如图片无法显示或论文不完整，请联系qq752018766
