Patent Publication Number: US-4651531-A

Title: Working fluids for Rankine cycle

Description:
This is a division of application Ser. No. 555,336 filed Nov. 28, 1983, now U.S. Pat. No. 4,530,773. 
    
    
     This invention relates to novel working fluids for a Rankine cycle. 
     Heretofore water has been almost exclusively used as a working fluid in the Rankine cycle in which thermal energy is converted into mechanical energy by repeating a cycle comprising vaporizing a liquid medium with heating, expanding the vapor in an expansion device to produce mechanical energy, and then cooling it to condense and compressing by a pump. Although long applied as a working fluid to steam engines, water has the following drawbacks. It has a limited application due to its high freezing point and great specific volume on vaporization; requires a larger installation and is lower in efficiency when using a low-temperature heating source; and readily freezes and is not workable at low temperatures. 
     While various organic working fluids have been proposed instead of water to eliminate the above drawbacks. Most of them are found to be unsatisfactorily usable due to their inflammability, corrosivity, and so on. Among these fluids trichlorofluoromethane (hereinafter to be referred to as Flon-11) is used in practice. However, Flon-11 has the following drawbacks as the working fluids for the Rankine cycle. It has low efficiency in conversion of thermal energy into mechanical energy, having further reduced efficiency particularly at a higher heating temperature range where it is decomposed to change its thermodynamic properties. When thermally decomposed, Flon-11 produces free chlorine to cause corrosion to the apparatus. Because of the above-mentioned drawbacks, Flon-11 is not suited as a working fluid which is usable for any kind of heat sources. Thus, there is an intensive demand to the development of working fluids usable for the Rankine cycle with an improved conversion efficiency and thermal stability. 
     To meet such demand for satisfactory working fluids, we conducted extensive research and found that a mixture of certain substances having different properties exhibits a superior properties to those of working fluids comprising a single substance which have heretofore been studied and that, particularly, a mixture of dichlorotetrafluoroethane (hereinafter referred to as Flon-114) with a member selected from the group consisting of difluoroethane (hereinafter referred to as Flon-152), and dichlorodifluoromethane (hereinafter referred to as Flon-12) gives markedly excellent properties as a working fluid for the Rankine cycle. 
     It is an object of this invention to provide working fluids for a Rankine cycle which fluids can exceedingly increase the efficiency of conversion of thermal energy to mechanical energy. 
     It is another object of this invention to provide working fluids for a Rankine cycle which fluids have a markedly high stability at a high-temperature range and can be used with safety. 
     These objects and other characteristics of this invention will become apparent from the following description. 
     The foregoing objects can be achieved by using a working fluid which comprises Flon-114 and a member selected from the group consisting of Flon-152 and Flon-12. 
     The working fluids of the invention can be easily prepared by mixing the above specified components in an appropriate ratio. 
     The working fluids of this invention comprising a mixture of Flon-114 with one of Flon-152 and Flon-12 have the following advantages over heretofore known working fluids. 
     The working fluids of the present invention produces a markedly higher efficiency in conversion of thermal energy into mechanical energy than those produced by a heretofore known working fluid such as Flon-11 or Flon-114 alone. 
     Further, while an inflammable or explosive medium have a limited application for use as a working fluid, the working fluids of the invention is neither inflammable nor explosive. Flon-152, which is inflammable by itself, can be made less inflammable when mixed with Flon-114, and becomes substantially noncombustible in the range of content of less than 40% of Flon-152. 
     The mixture of Flon-114 with Flon-12 is usable in a wide range of composition. Generally, the mixture is used at the content of 90-10 weight % of Flon-114 and 10-90 weight % of Flon-12. 
     According to the present invention, particularly significant improvement of the efficiency in power output can be obtained with use of a working fluid comprising about 90 to about 60% by weight of Flon-11 and about 10 to about 40% by weight of Flon-152 or a working fluid comprising about 90 to about 10% by weight of Flon-114 and about 10 to about 90% by weight of Flon-12. 
    
    
     Referring to the accompanying drawings, 
     FIG. 1 is a flow sheet of the Rankine cycle carried out for conversion of thermal energy into mechanical energy. 
     FIG. 2 is a pressure-enthalpy diagram (P-H diagram) of a mixture according to the invention which comprises 70% by weight of Flon-114 and 30% by weight of Flon-152. The points A, B, C, D and E appearing in FIG. 2 correspond to those in FIG. 1 and following FIG. 3 which represent the respective points of state in the Rankine cycle carried out in Examples 1 to 4 below. 
     FIG. 3 shows a temperature-entropy diagram of the Rankine cycle in which a mixture of Flon-114 and Flon-152 is used as a working fluid. The alphabets (A to E) appearing in FIG. 3 correspond respectively to those in FIG. 1 and represent points of state of the working fluid. 
     FIG. 4 is a pressure-enthalpy diagram (P-H diagram) of a mixture according to the invention comprising 80% by weight of Flon-114 and 20% by weight of Flon-12. The points A, B, C, D and E appearing in FIG. 4 correspond to those in FIG. 1 and following FIG. 5 which represent the respective points of state in the Rankine cycle carried out in Examples 5 to 8 below. 
     FIG. 5 shows a temperature-entropy diagram of the Rankine cycle in which a mixture of Flon-114 and Flon-12 is used as a working fluid. The alphabets (A to E) appearing in FIG. 5 correspond to respectively to those in FIG. 1 and represent points of state of the working fluid. 
    
    
     Referring to the Figures, a working fluid is heated and vaporized in the evaporator 4 to produce vapor of high temperature and high pressure. The change of state is shown by D, E and A in FIGS. 3 and 5 where the liquid-phase working fluid is heated to a boiling temperature at which the total fluid vaporizes. The vapor is then superheated (as shown by the change from E to A), and enters the expansion device 1 in which the superheated vapor is adiabatically expanded. At the time, the temperature and pressure are lowered and the state of the fluid is changed from A to B as shown in FIGS. 3 and 5 for the fluid to conduct the work. The vapor-phase working fluid at lowered temperature and pressure is then sent to the condenser 2 where it is cooled by a low-temperature heat source to condense and get into a liquid phase (as indicated by the change from B to C in FIGS. 3 and 5). Thereafter the fluid is returned to the pump 3 to repeat the cycle. In FIGS. 3 and 5 the point a represents the state of a heat source, e.g. warm water, when it enters the evaporator, b represents the state of the heat source when it leaves the evaporator and the arrow on the line from a to b indicates the direction of flow of the heat source. The point d represents the state of cold water at the inlet of condenser, the points e represents the state of the same at the outlet of the condenser and the arrow on the line from d to e indicates the direction of flow of the cold water. 
     Usable as the expansion device for a Rankine cycle system are for example rotating or reciprocating displacement expansion devices and turbine expansion devices. Boilers commonly used to produce steam are usable as the evaporator for the system. Illustrative of usable condensers are those of the type as used in refrigerating apparatus. Employable as the pump are pressure liquid feed pumps for organic solvents generally used in chemical industries. 
     The invention will be described below in more detail with reference to examples and comparison examples wherein the ratio of the constituents will be indicated in % by weight unless otherwise specified. 
     EXAMPLES 1-8 AND COMPARISON EXAMPLE 
     The Rankine cycle illustrated in FIGS. 1 to 3 or 1, 4 and 5 was carried out with the use of the respective working fluids having compositions given in Table 1 below in the same apparatus under the following conditions: 
     (i) temperature of hot water at point a in FIG. 3 or 5: 110° C. or 140° C. 
     (ii) temperature of cold water at point d in FIG. 3 or 5: 25° C. or 32° C. 
     The output characteristics were determined in the electric power generation obtained from conversion of thermal energy to mechanical energy at a charging rate of 1,500 t/h of hot water. The results are given in Table 2. The evaporation temperature was set at a temperature at which the highest output was produced and the condensing temperature was 35° C. 
     
                       TABLE 1                                                     
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Comp.         Example                                                     
Medium  Example   1     2   3   4    5   6   7    8                       
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Flon-114                                                                  
        100       90    80  70  60   80  60  40   20                      
Flon-152          10    20  30  40                                        
Flon-12                              20  40  60   80                      
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                                           TABLE 2                                 
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                 Comp.                                                    
                      Example                                             
Power output characteristics                                              
                 Example                                                  
                      1  2  3  4  5  6  7  8                              
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Temperature of hot water: 110° C.                                  
Adiabatic enthalpy drop (AE)                                              
                 2.1  2.7                                                 
                         3.0                                              
                            3.3                                           
                               3.6                                        
                                  2.5                                     
                                     2.6                                  
                                        2.5                               
                                           2.4                            
(Kcal/kg)                                                                 
Flow rate of working fluid (GF)                                           
                 1500 1300                                                
                         1190                                             
                            1100                                          
                               1030                                       
                                  1420                                    
                                     1390                                 
                                        1400                              
                                           1440                           
(ton/hr)                                                                  
Gross power output (POW)                                                  
                 3380 3710                                                
                         3950                                             
                            4020                                          
                               4060                                       
                                  3940                                    
                                     3910                                 
                                        3880                              
                                           3850                           
(KW)                                                                      
Pump power for working fluid                                              
                  330  380                                                
                          400                                             
                             430                                          
                                450                                       
                                   390                                    
                                      440                                 
                                         500                              
                                            560                           
(PIA) (KW)                                                                
Pump power for cold &amp; warm                                                
                  680  680                                                
                          650                                             
                             630                                          
                                620                                       
                                   690                                    
                                      680                                 
                                         680                              
                                            700                           
water (PIB) (KW)                                                          
Net power output (PWT) (KW)                                               
                 1930 2220                                                
                         2470                                             
                            2550                                          
                               2580                                       
                                  2420                                    
                                     2360                                 
                                        2250                              
                                           2150                           
Temperature of hot water: 140°0 C.                                 
Adiabatic enthalpy drop (AE)                                              
                 3.0  3.7                                                 
                         4.1                                              
                            4.4                                           
                               4.7                                        
                                  3.4                                     
                                     3.5                                  
                                        3.4                               
                                           3.3                            
(Kcal/kg)                                                                 
Flow rate of working fluid (GF)                                           
                 2640 2310                                                
                         2120                                             
                            1980                                          
                               1880                                       
                                  2530                                    
                                     2520                                 
                                        2530                              
                                           2760                           
(ton/hr)                                                                  
Gross power output (POW)                                                  
                 8810 9390                                                
                         9610                                             
                            9760                                          
                               9790                                       
                                  9660                                    
                                     9740                                 
                                        9800                              
                                           10020                          
(KW)                                                                      
Pump power for working fluid                                              
                  770  900                                                
                          990                                             
                            1080                                          
                               1160                                       
                                   930                                    
                                     1080                                 
                                        1250                              
                                           1500                           
(PIA) (KW)                                                                
Pump power for cold &amp; warm                                                
                 1230 1210                                                
                         1160                                             
                            1130                                          
                               1120                                       
                                  1250                                    
                                     1240                                 
                                        1250                              
                                           1310                           
water (PIB) (KW)                                                          
Net power output (PWT) (KW)                                               
                 6150 6620                                                
                         6830                                             
                            6940                                          
                               6890                                       
                                  6830                                    
                                     6750                                 
                                        6620                              
                                           6520                           
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     The results, as shown in Table 2, reveal that the working fluids comprising Flon-114 and one of Flon-152 and Flon-12 permit to exhibit electric power output characteristics which are significantly superior to that obtainable with use of working fluid comprising Flon-114 alone. The results further reveal that the working fluids permit to produce markedly increased electric power output in the range of content of higher than 10% by weight of Flon-152 or 10 to 40% by weight of Flon-12. The results also reveal that the working fluids comprising Flon-114 and Flon-152 or Flon-12 afford improvement of efficiency of the cycle because of its small superheating degree of steam at the outlet of turbine. 
     In a glass tube were sealed a working fluid comprising Flon-114 or a mixture of Flon-114/Flon-152 (weight ratio 70/30) or Flon-114/Flon-12 (80/20) together with iron piece and lubricating oil. The sealed tube was heated at 150° C. for 100 hours and then the resulting working fluids were determined for concentration of halogen and amount of decomposition material by gas chromatography. The results are given in Table 3 below. 
     
                       TABLE 3                                                     
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          Conc. of   Amount of                                            
Working   halogen after                                                   
                     decomposition                                        
fluid     heating (ppm)                                                   
                     material (%)  Remark                                 
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Flon-114 +                                                                
          32         0.1           This                                   
Flon-152                           invention                              
Flon-114 +                                                                
          40         0.2           This                                   
Flon-12                            invention                              
Flon-114  44         0.2           Controle                               
Flon-11   180        0.7           Controle                               
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     The results shown in Table 3 reveal that the working fluid of the invention comprising a mixture of Flon-114 and Flon-152 or Flon-12 forms a reduced amount of halogen ion and reduced amount of decomposition material determined by gas chromatography in contrast with the control working fluid comprising Flon-11 or Flon-114 alone. With the mixed working fluid of Flon-114 and Flon-152 or Flon-12, it is derived that the reduced concentration of halogen formed at a high temperature reduces the corrosion of metal materials of the apparatus due to the working fluid and the reduced amount of decomposed material results in inhibition of change of thermodynamic properties as a working fluid for the Rankine cycle due to the decomposition material and in inhibition of reduction of the cycle efficiency. 
     Consequently, the working fluids of the invention comprising a mixture of Flon-114 and Flon-152 or Flon-12 is markedly superior to the heretofore known working fluids, such as Flon-11 in the energy-conversion efficiency, heat exchanging characteristics, thermal stability, and the like.