Patent Publication Number: US-2011072819-A1

Title: Heat recovery system based on the use of a stabilized organic rankine fluid, and related processes and devices

Description:
BACKGROUND 
     This invention generally relates to systems and processes for recovering and utilizing waste heat. More specifically, the invention relates to organic rankine cycle (ORC) systems which benefit from improved working fluids, and to methods for recovering waste heat from various sources, such as power plants. 
     Large amounts of waste heat are generated by many different types of industrial and commercial operations. Examples of the heat sources are combustion engines, combustion turbines; nuclear power plants, coal-burning plants; and coal gasification plants. One method of generating electricity from these types of waste heat is to apply a rankine cycle. 
     Fundamentally, the rankine cycle is often water- or steam-based, and usually includes a turbine-generator, an evaporator/boiler, a condenser, and a liquid pump. While water/steam-based rankine cycles are useful for recovering heat at relatively high temperatures, organic rankine cycles (ORC&#39;s) are very efficient at recovering heat from some of the lower-temperature operations mentioned above, e.g., at temperatures of about 100-300° C. Moreover, organic rankine cycles are currently being developed to recover heat from higher-temperature heat sources, e.g., up to about 450° C. 
     A number of working fluids have been used or considered for use in organic rankine cycles Examples include thiophene, various hydrofluorocarbons, pentanes, butanes, and silicone oils. In terms of physical and thermodynamic properties (e.g., vapor pressure, vaporization enthalpy, and normal boiling point characteristics), thiophene is an especially attractive candidate for advanced organic rankine cycles. 
     While thiophene has many desirable properties for this application, there is a key drawback as well. The compound is very susceptible to oxidation at temperatures of about 300° C. or more. Thus, in the presence of even small amounts of oxygen, the compound can oxidize rapidly, yielding byproducts such as acidic off-gasses. The byproducts may cause various problems, such as corrosion within the heat recovery system, e.g., within piping and heat-exchange systems. Most heat recovery systems require working fluids which are stable for several years or more. Therefore, the propensity for thiophene oxidation makes it less attractive for this purpose—especially because of the fact that oxygen can inadvertently enter the heat recovery system in a number of ways, e.g., through various seals in the apparatus. 
     In view of these considerations, it can be seen that new working fluids for heat recovery systems would be welcome in the art. Moreover, since thiophene may be an ideal working fluid for some systems based on the organic rankine cycle, modified fluids based on thiophene would be of special interest. The new working fluids should exhibit oxidative stability at elevated temperatures, while also possessing the thermodynamic properties needed for efficient heat capture. They should also be compatible with the heat-recovery equipment, and relatively economical to incorporate into a commercial system. 
     BRIEF DESCRIPTION OF THE INVENTION 
     One embodiment of this invention is directed to a heat recovery system. The system includes a thermally-stable, organic working fluid which comprises a mixture of thiophene or a derivative thereof, and at least one hydrocarbon having a boiling point in the range of about 25° C. to about 125° C. The hydrocarbon is present at a level of about 1% to about 25% by weight, based on the weight of the mixture. 
     Another embodiment is directed to a waste-heat recovery system, comprising at least one organic working cycle. The organic working cycle includes a working fluid which comprises a mixture of thiophene or a derivative thereof, and at least one hydrocarbon having a boiling point in the range of about 25° C. to about 125° C. 
     An additional embodiment relates to a method for recovering waste-heat from a power plant. The method comprises the step of directing the waste-heat to a heat-recovery system as described herein, so as to function as at least a portion of the heat source in the system. 
     Still another embodiment is directed to a photometric sensor system for the detection of oxidative activity in an industrial process which is carried out at elevated temperatures, and which utilizes at least one fluid. The fluid comprises a mixture of a thiophene-based compound and at least one hydrocarbon, and changes color upon oxidation. The sensor system further comprises at least one detector in optical contact with the fluid. The detector is capable of detecting a color change in the fluid, as further described below. 
     Another embodiment relates to a method for detecting oxidative activity in an industrial process which is carried out at elevated temperatures, and which utilizes at least one fluid, as described herein. The method comprises the step of measuring color changes in the fluid with a color-change detector. The oxygen-sensitive fluid possesses a specific color at an initial time setting, and then undergoes a measurable color change over time upon exposure to oxygen. The color change can be correlated to oxidation of the hydrocarbon, which is indicative of oxidative activity in the industrial process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of an exemplary heat recovery system based on some of the embodiments of the present invention. 
         FIG. 2  is a schematic of an exemplary photometric sensor system according to some embodiments of this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The compositional ranges disclosed herein are inclusive and combinable (e.g., ranges of “up to about 25 wt %”, or, more specifically, “about 5 wt % to about 20 wt %”, are inclusive of the endpoints and all intermediate values of the ranges). Weight levels are provided on the basis of the weight of the entire composition, unless otherwise specified; and ratios are also provided on a weight basis. Moreover, the term “combination” is inclusive of blends, mixtures, alloys, reaction products, and the like. Furthermore, the terms “first,” “second,” and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. The modifier “about” used in connection with a quantity is inclusive of the stated value, and has the meaning dictated by context, (e.g., includes the degree of error associated with measurement of the particular quantity). The suffix “(s)” as used herein is intended to include both the singular and the plural of the term that it modifies, thereby including one or more of that term (e.g., “the compound” may include one or more compounds, unless otherwise specified). Reference throughout the specification to “one embodiment”, “another embodiment”, “an embodiment”, and so forth, means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the embodiment is included in at least one embodiment described herein, and may or may not be present in other embodiments. In addition, it is to be understood that the described inventive features may be combined in any suitable manner in the various embodiments. 
     As mentioned above, the organic working fluid for the present invention comprises thiophene, or a derivative thereof. Thiophene has the empirical formula C 4 H 4 S, and is a heterocyclic, aromatic compound, having a five-membered ring. As used herein, a “derivative” of thiophene can be any closely-related compound in structure, but is usually a compound in which one of the hydrogen atoms is substituted with a methyl group or with a halogen, i.e., fluorine, chlorine, bromine, or iodine. (While the term “thiophene” is used primarily herein, it should be understood that the derivatives are implied as well). 
     The working fluid further includes at least one hydrocarbon compound. A number of hydrocarbons can be used in combination with thiophene. Usually, the hydrocarbon has a boiling point in the range of about 25° C. to about 125° C. The hydrocarbon compounds can be aromatic, aliphatic, or cycloaliphatic. As further described below, one theory (and as such, non-binding) regarding the benefit of the hydrocarbon is that the hydrocarbon constituent appears to function as a “scavenger” for oxygen within the heat recovery system. Thus, the hydrocarbon is preferentially oxidized, thereby impeding or preventing the oxidation of the thiophene constituent for an extended period of time. 
     Examples of the aromatic compounds are toluene and various xylenes (e.g., ortho-, meta-, or para-xylene). Combinations of any of these materials may be used as well. (Benzene might also be used, but is typically not preferred because of environmental and health concerns). 
     Non-limiting examples of suitable aliphatic compounds include iso-pentane; n-pentane; 2,3-dimethylbutane; 2,2-dimethylbutane; 2-methylpentane; 3-methylpentane; n-hexane; 2,2-dimethylpentane; 2,4-dimethylpentane; 2,2,3-trimethylbutane; 3,3-dimethylpentane; 2,3-dimethylpentane; 2-methylhexane; 3-methylhexane; 3-ethylpentane; n-heptane; 2,2,4-trimethylpentane; 2,2-dimethylhexane; 2,5-dimethylhexane; 2,4-dimethylhexane; 2,2,3-trimethylpentane; 3,3-dimethylhexane; 2,3,4-trimethylpentane; 2,3,3-trimethylpentane; 2,3-dimethylhexane; 2-methylheptane; 4-methylheptane; 3,4-dimethylhexane; 3-methyl-3-ethylpentane; 3-ethylhexane; 3-methylheptane; 2,2,4,4-tetraethylpentane; 2,2,5-trimethylhexane; n-octane; 2,2,4-trimethylhexane; and various combinations thereof. In some preferred embodiments, the aliphatic compound is selected from the group consisting of iso-pentane, n-pentane, 2-methylpentane; 3-methylpentane; n-hexane; and various combinations thereof. 
     Non-limiting examples of suitable cycloaliphatic compounds include cyclopentane; methylcyclopentane; cyclohexane; 1,1-dimethylcyclopentane; trans-1,2 dimethylcyclopentane; cis-1,2 dimethylcyclopentane; methylcyclohexane; ethylcyclopentane; 1,1,3-trimethylcyclopropane; cis-trans-cis-1,2,4-trimethylcyclopropane; 1,1,2-trimethylcyclopropane; cis-cis-trans-1,2,4-trimethylcyclopropane; cycloheptane; trans-1,4-dimethylcyclohexane; 1,1-dimethylcyclohexane; cis-1,3-dimethylcyclohexane; 1-methyl-1-ethylcyclopropane; trans-1,2-dimethylcyclohexane; cis-1,4-dimethylcyclohexane; trans-1,3-dimethylcyclohexane; and combinations thereof. In some preferred embodiments, the cycloaliphatic compound is selected from the group consisting of cyclopentane; methylcyclopentane; cyclohexane, and various combinations thereof; with cyclopentane itself being most preferred for some applications. 
     The relative amounts of thiophene and the hydrocarbon compound(s) in the mixture can vary significantly. Various factors are used to determine how much hydrocarbon is appropriate. They include the specific hydrocarbon employed, and its own chemical and physical properties (e.g., normal boiling point); the type of heat recovery system in use; the general temperature range at which the working fluid will be vaporized and carried through the system; and estimates of potential air or oxygen leakage into the heat recovery system. In general, enough thiophene should be present to obtain the thermodynamic benefits of such a compound, while enough hydrocarbon should be present to, in effect, thermally-stabilize the thiophene. Usually, the hydrocarbon (total amount) is present at a level in the range of about 1% to about 25% by weight, based on the weight of the mixture. In some specific embodiments, the level of hydrocarbon is about 5% by weight to about 20% by weight, and most often, about 5% by weight to about 10% by weight. 
     As mentioned previously, another embodiment of this invention relates to a heat recovery system, utilizing the thermally-stable organic working fluid discussed previously. A large variety of heat recovery systems can be employed in this embodiment, and substantially all of them operate on the principle of the organic rankine cycle. Non-limiting examples of various configurations are found in U.S. Pat. No. 7,225,621 (Zimron et al); U.S. Pat. No. 7,174,716 (Brasz et al); and U.S. Patent Publication 2009/0000299 (Ast et al), all of which are incorporated herein by reference. 
       FIG. 1  depicts a simplified, exemplary heat recovery system  10 , which utilizes waste heat (or any other type of process heat) from at least one source  12 . Non-limiting examples of the heat source include: combustion engines, combustion turbines; nuclear power plants, coal-burning plants; coal gasification plants; petroleum coke gasification systems; steam plants; geothermal systems, biomass combustion systems, municipal waste combustion systems; municipal waste gasification systems; space heating assemblies; general cooling systems; and various combinations thereof. (In other words, there could be multiple waste heat sources  12 ). The waste-heat temperature will vary, depending on the source, and in some embodiments, is in the range of about 200° C. to about 600° C. 
     The waste heat from source  12  is directed to an evaporator (e.g., a boiler)  14 , in which the organic working fluid (not specifically shown) is vaporized. Those skilled in the art understand that the working fluid being heated may be contained in one or more heat exchanger systems, and these systems then direct the heated fluid to the evaporator. Moreover, a number of pumps may be employed in the system. 
     The vaporized working fluid is then directed to a turbine-generator system  16 . The particular type of turbine-generator system is not critical to this invention. Many variations of each individual component (i.e., the turbine  18  and the generator  20 ) are possible, and their arrangement can vary as well. The vaporized working fluid expands in the turbine-generator  16 , producing electrical power, via the generator. The electrical power output can be directed to any number of sites, e.g., to feed a common base load, such as a power utility grid. 
     An organic fluid condenser  22  is in direct- or indirect communication with the turbine-generator system  16 . The condenser  22  condenses the vaporized, expanded working fluid after it exits system  16 , so that the fluid is again returned to its liquid state. A pump  24  or other suitable means is then used to direct the condensed fluid back to evaporator  14 , to begin the cycle again. Heat from the condensation step can be transferred to cooling water; can be directed to another heat-recovery unit or boiler; or can be used for any other conventional purpose. 
     Again, many variations are possible, in terms of each unit of the heat recovery system. Several of the variations are described in the patent publication of Ast et al, described previously. Some embodiments described therein rely on two rankine cycles, which can efficiently utilize waste heat from at least two different sources. Each cycle can include its own working fluid, which may comprise the composition described herein. The configuration in the Ast reference also includes a cascaded heat exchange unit, through which the working fluids can circulate. In any of the various heat recovery systems which can be designed, the use of a working fluid which is very stable at relatively high temperatures, for extended periods of time, represents a distinct system advantage. 
     As mentioned previously, another embodiment of this invention is directed to a photometric sensor, i.e., a “sensor system”. The sensor is based on a discovery of the present inventors, regarding changes in color which were observed, during the course of oxidative activity within the working fluid composition. The sensor comprises at least a portion of the working fluid, i.e., the mixture of a thiophene-based compound and at least one hydrocarbon, as described previously. The sensor further comprises a detector means. The detector means can be in optical contact (e.g., through a sight glass) with the working fluid mixture, and may be used to monitor changes in fluid color, which indicates oxidative activity. The detector means may be electronic (e.g. a color-sensitive photodetector), or it may be based on visual observation by an operator. 
     The thiophene/hydrocarbon mixture possesses a specific color at an initial time. The mixture then undergoes a measurable color change over time, which can be correlated to oxidation of the hydrocarbon compound. As explained previously, oxidation of the hydrocarbon represents a signal that oxidative activity is occurring in the industrial process, e.g., in a heat recovery system in which the oxygen-sensitive mixture is the working fluid. Thus, in one embodiment (usually a higher temperature environment), it is possible to employ the sensor to determine how much oxygen or oxygen-containing gas has entered the system in which the sensor is operating. Such a determination can be very useful for many different types of systems and processes, one of which is the ORC-based system described herein. 
     It should be understood that photometric sensors may include a number of features and related components. Non-limiting examples include color filter arrays; electrical circuitry; recording equipment; image sensors; shade guides; photocells, photoarray detectors, light-emitting diodes (LEDs), and light meters. One or more of these features and components can be incorporated into the present sensor system, by those skilled in the art. 
       FIG. 2  is a non-limiting, simplified illustration of the sensor system  30 . The system includes working fluid  32 , which may function, for example, as part of a heat recovery system  34 , as described herein, e.g., in  FIG. 1 . The working fluid can be monitored in-line, i.e., during its passage through the heat recovery system; or it can be a liquid sample which is periodically diverted or taken from the system. 
     Detector  36  can constitute any means for determining color and change-in-color, e.g., a color-sensitive photodetector, which is known in the art. Moreover, one or more conventional processor/controller devices  38  can be used to coordinate and process data obtained from the detector. As mentioned previously, the detector may alternatively (or additionally) be based on observation by an operator. 
     Examples 
     The example presented below is intended to be merely illustrative, and should not be construed to be any sort of limitation on the scope of the claimed invention. 
     Various samples of working fluids were tested for thermal stability. In one set of tests, a 10 cc stainless steel bomb was initially placed in a 300° C. oven for a week under an air atmosphere, to pre-oxidize the surface. The bomb was then cooled to ambient temperature, and a weighed witness coupon (made of stainless steel) was placed inside the bomb. 
     The bomb was then evacuated, and subsequently charged with the fluid to be tested (and, optionally, air), by injection through a septum, using a gas-tight syringe. The bomb was then sealed. The amount of air charged was calculated, based on the desired, initial oxygen concentration. The bomb was then weighed and placed in the oven at 300° C. for 60 hours. After this exposure time, the bomb was removed from the oven, cooled, and weighed, to determine whether a leak had occurred. The headspace of the bomb was sampled, by withdrawing a sample to a second, evacuated 10 cc bomb, followed by chromatographic analysis (GC-MS). The first 10 cc bomb was then opened for inspection and weighing of the coupon, and for analysis of the liquid sample. Headspace acidity was measured by moistened EM Merck pH strips that were suspended inside the first 10 cc bomb for 15 seconds, after unsealing, but before removing the liquid. Table 1 provides a summary of the content of each sample that was tested; and related properties and characteristics. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Sample 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                 C 
                 D 
               
               
                   
                   
                   
                 Thiophene + 
                 Thiophene + 
               
               
                   
                 A 
                 B 
                 10% Cyclo- 
                 10% Iso- 
               
               
                   
                 Thiophene 
                 Thiophene 
                 pentane 
                 pentane 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Fluid Charge 
                 0.546 
                 0.572 
                 0.537 
                 0.529 
               
               
                 (g) 
               
               
                 Air charge (cc) 
                 — 
                 0.75 
                 0.75 
                 0.75 
               
               
                 Witness 
               
               
                 Coupon: 
               
               
                 After Heating 
                 0.1585 
                 0.1823 
                 0.1876 
                 0.1867 
               
               
                 (g) 
               
               
                 Initial (g) 
                 0.1585 
                 0.1824 
                 0.1876 
                 0.1866 
               
               
                 Delta (g) 
                 0.0000 
                 −0.0001 
                 0.0000 
                 +0.0001 
               
               
                 Appearance 
                 Bright 
                 Discolored 
                 Discolored 
                 Discolored 
               
               
                 Liquid 
                 Clear 
                 Light yellow 
                 Dark yellow 
                 Yellow 
               
               
                 Appearance 
               
               
                 pH of Vapor 
                 5 
                 2-3 
                 4-5 
                 4-5 
               
               
                 Headspace 
               
               
                 GC/MS* 
               
               
                 COS** 
                 0.7 
                 28.1 
                 6.7 
                 7.9 
               
               
                 SO 2   
                 0.8 
                 7.1 
                 3.2 
                 0.7 
               
               
                 CS 2 *** 
                 0.2 
                 2.3 
                 2.1 
                 0.7 
               
               
                   
               
               
                 *Gas chromatography/mass spectrometry; area of parent ion peak as percentage of fragment, with m/e = 60 for thiophene 
               
               
                 **Carbonyl sulfide 
               
               
                 ***Carbon disulfide 
               
            
           
         
       
     
     Compositions A and B were comparative samples, and did not include the hydrocarbon constituent. As indicated for samples B, C, and D, the stainless steel witness coupon became discolored in the presence of air, which was expected. In the case of sample B, the light yellow appearance of the liquid was an indication that the thiophene component, without any hydrocarbon being present, was undergoing some degree of degradation. The increased vapor acidity (shown by a lower pH as compared to sample A) was attributable to the presence of acidic byproducts like COS, SO 2 , and CS 2 , which was another indication of thiophene degradation. 
     Samples C and D were within the scope of the present invention. In the case of sample C, the dark yellow liquid color was a favorable result, indicating that the cyclopentane component was being sacrificially oxidized, i.e., in preference to any degradation or decomposition of the thiophene. Similar results were seen with sample D, which employed isopentane as the hydrocarbon constituent. Moreover, the higher pH values for samples C and D, as compared to sample B, indicated that the amount of acidic byproducts being formed was considerably decreased. The relatively low levels of COS, SO 2 , and CS 2  provide further support for this conclusion. 
     It is clear that the thiophene portion of the compositions corresponding to samples C and D are thermally- and chemically more stable at elevated temperatures in the presence of oxygen, as compared to the thiophene in sample B. Thus, the advantages of using organic working fluids based on these thiophene-hydrocarbon compositions is also apparent. Moreover, these working fluids are relatively economical to prepare and use, and, in general, are fully compatible with heat recovery systems currently in use. 
     The present invention has been described in terms of some specific embodiments. They are intended for illustration only, and should not be construed as being limiting in any way. Thus, it should be understood that modifications can be made thereto, which are within the scope of the invention and the appended claims. Furthermore, all of the patents, patent applications, articles, and texts which are mentioned above are incorporated herein by reference.