Organic fluid mixtures as working fluids for the trilateral flash cycle system
The requirements for power generation systems have been reviewed together with the various energy sources available for them. Geothermal energy has been examined in more detail and the principal methods of recovering power from it which are currently employed are discussed. A novel method for improved power recovery from geothermal sources called the Trilateral Flash Cycle (TFC) system is described which has the special requirement of an efficient two-phase expander. Optimum results are obtained from this cycle if a working fluid is used which leaves the expander as dry saturated vapour. A binary mixture of hydrocarbons was therefore sought which by variation of the constituent proportions, would satisfy this requirement for a range of inlet temperatures when the condensing temperature is constant. Methods of estimating mixture properties are reviewed and the chosen thermodynamic model, as well as a computational procedure for evaluation of vapour-liquid equilibria of organic binary mixtures at high pressures, are described. This is based on the Redlich-Kwong- Soave cubic equation of state. By this means a mixture of n-pentane and 2,2-dimethylpropane (neopentane) was found to be the most suitable for the TFC system for expander inlet temperatures between 150-180'C. Temperature-entropy (T-S) diagrams of this organic binary mixture were obtained for several compositions. Bubble and dew pressures of (n-pentane + 2,2-dimethylpropane) have been determined experimentally for five different compositions at six different temperatures, (333.15 K, 353.15 K, 373.15 K, 393.15 K, 413.15 K, and 433.15 K). Vapour pressures of pure n-pentane and pure neo-pentane were also determined at these temperatures. The critical point of neo-pentane was measured to assess the accuracy of the isothermal compression apparatus used. Theoretical predictions were found to be in good agreement with experimental measurements.