Abstract:
The invention can be used to improve the yield of degraded heat transfer oil ( 1 ) recovered from a solar thermal facility, comprising the successive separation of heavy components ( 4 ) (ortho-, meta- and para-terphenyl) and light components ( 7 ) (phenol and benzene). The plant comprises: an air-cooled cooler ( 2 ) which cools the degraded heat transfer oil ( 1 ) to obtain a cooled oil ( 12 ); a distillation column ( 3 ) which separates the heavy components ( 4 ) and a vapour ( 15 ) of light components ( 7 ) plus oil from the cooled heat transfer oil ( 12 ); and a rectifier ( 6 ) which separates the light components ( 7 ) and the regenerated oil ( 17 ) from the vapour ( 15 ). The method comprises the cooling of the degenerated heat transfer oil ( 1 ) in the air-cooled cooler ( 2 ), the separation of the heavy components ( 4 ) in the distillation column ( 3 ), and the separation of the light components ( 7 ) and the regenerated oil ( 17 ) in the rectifier ( 6 ).

Description:
OBJECT OF THE INVENTION 
       [0001]    The present invention is applicable to the technical field of solar energy capture in parabolic trough collector plants, specifically in the regeneration of the heat transfer oil used in said plants. 
         [0002]    The object of the invention is a plant for regenerating degraded heat transfer oil for solar thermal facilities and a method for carrying out said regeneration. 
       BACKGROUND OF THE INVENTION 
       [0003]    Parabolic trough collector solar plants normally use a heat transfer fluid that can be both thermal oil and water, in direct steam generation collectors. In the case of collectors that use oil as a heat transfer fluid, a thermal oil is heated in the collectors which, after passing through a heat exchanger, allows the production of pressurized steam that is subsequently expanded in a conventional Rankine cycle turbine. 
         [0004]    There are a large variety of compounds which are used as thermal oil for solar plants, each having specific characteristics. Specifically, one of these oils is composed of a eutectic mixture of biphenyl and diphenyl oxide (marketed under the name Dowtherm A or Therminol VP1). The properties of this type of oil make it especially suited for heat transfer within a temperature range of up to 400° C. 
         [0005]    The thermal stability of a fluid is determined by its composition. When the temperature of the oil reaches the working values of the solar thermal plant (temperatures of up to 400° C.), the molecular bonds of the fluid structure break to form two large types of degradation products: light compounds and heavy compounds. 
         [0006]    In the specific case of the aforementioned oil, it undergoes slow decomposition into light components (low boilers), essentially benzene and phenol, and heavy components (high boilers), essentially o-terphenyl, m-terphenyl, p-terphenyl and 2-phenoxybiphenyl. The concentration of these oil degeneration products should not exceed certain limits due to the fact that light components, having a high vapour pressure, increase the pressure in the system and can cause cavitation in the pumps, while the heavy components limit the efficiency of the heat transfer as, among other things, they reduce the specific heat of the heat transfer oil and increase its viscosity. 
         [0007]    In order to avoid accumulation, said heavy components and light components must be periodically removed from the facility, as they modify the properties of the heat transfer oil. 
         [0008]    It has been experimentally verified that simple flash distillation (single-step distillation) followed by partial condensation of the vapour current generated in the distiller is unable to effectively regenerate the oil. The separation of significant amounts of heavy components is necessarily associated with considerable oil losses. Thus, for example, the elimination of 70% of the heavy components entails oil losses of approximately 30%. 
         [0009]    The technical problem being addressed consists of describing a thermal oil regeneration plant capable of minimising oil degradation and oil losses and improving regeneration performance, as the price of said oil is considerably high. 
       DESCRIPTION OF THE INVENTION 
       [0010]    The present invention solves the addressed problem by means of a plant for regenerating the heat transfer oil circulating through the collectors of a solar thermal plant, according to a first object of the invention, and a method for carrying out said regeneration, according to a second object of the invention. 
         [0011]    The regeneration plant comprises:
   a distillation column (stripper) to separate the heavy components;   a boiler;   a rectifier for separating the light components;   an air condenser;   a backflow tank;   a first pump and second pump.   
 
         [0018]    The plant of the invention can additionally comprise an air cooler. In such case, said air cooler is the first element of the plant. The degraded oil reaches the air cooler, at a high temperature, from the solar facility. Preferably, the temperature at the inlet of the air cooler is comprised between 295° C. and 310° C. The temperature at the outlet of the air cooler is preferably comprised between 95° C. and 110° C. Cooled oil is obtained at the outlet of the air cooler. 
         [0019]    Said temperature of between 95° C. and 110° C., preferably 100° C., is sufficiently low to prevent heavy components from being carried away by the vapour that flows out of the top of the distillation column, with respect to the case of not using pre-cooling, which substantially improves regeneration performance. Cooling the degraded oil before introducing it in the distillation column improves the performance of said distillation column. 
         [0020]    In the absence of an air cooler, the degraded oil is introduced directly in the distillation column. In the event of having an air cooler, at the outlet of said air cooler the cooled oil is introduced in the distillation column. The distillation column comprises a column of first perforated plates made of carbon steel, and a boiler. The oil enters the distillation column through the upper part thereof and is heated in said distillation column by the boiler. 
         [0021]    The harmful heavy components (o, m and p-terphenyls) are separated into tails by gravity, together with a small amount of oil, while practically all the oil and light components pass through the successive first plates until they reach the upper part of the distillation column, called the head, where they abandon the distillation column in the form of vapour. 
         [0022]    Preferably, the boiler is a thermosyphon-type heat exchanger that works by means of natural circulation of the hot oil from the solar facility. 
         [0023]    The vapour, substantially free of heavy components, which abandons the distillation column through the head thereof, accesses the rectifier. The rectifier is a distillation column having a plurality of second perforated plates made of carbon steel. The regenerated oil is recovered in the lower part of the rectifier and the light components are recovered in the upper part thereof, which are subsequently condensed in the air cooler and conveyed toward a backflow tank. 
         [0024]    Additionally, the invention includes a first pump and a second pump. The first pump conveys the regenerated oil toward its working fluid circuit in the solar thermal collectors. The second pump conveys the light components from the backflow tank toward the rectifier. The circulation enabled by the second pump is necessary because the pressure in the tank is lower than in the rectifier. 
         [0025]    By means of the invention, regenerated oil with 99.4% purity is obtained, compared to the initial 94.3% purity of the oil prior to regeneration. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0026]    In order to complement the description being made and with the object of helping to better understand the characteristics of the invention, according to a preferred practical embodiment thereof, a set of drawings are included wherein the following is represented in an illustrative and non-limiting manner: 
           [0027]      FIG. 1  shows a schematic view of the plant according to the invention. 
       
    
    
     PREFERRED EMBODIMENT OF THE INVENTION 
       [0028]    Following is a description of a preferred embodiment of the invention, with the aid of the single  FIG. 1 . 
         [0029]    The plant for regenerating degraded heat transfer oil ( 1 ) from a solar thermal facility according to the invention is used to regenerate the oil used in said solar thermal facility, which is a eutectic mixture of biphenyl (26.5%) and diphenyl oxide (73.5%). Table 1 shows the characteristics of this oil, specifically of Dowtherm A. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Properties of Dowtherm A thermal oil 
               
             
          
           
               
                   
                 Vapour 
                   
                 Specific 
                 Thermal 
                   
               
               
                 Temperature 
                 pressure 
                 Viscosity 
                 heat 
                 conductivity 
                 Density 
               
               
                 ° C. 
                 (bar) 
                 (mPa · s) 
                 (kJ/kg/K) 
                 (W/m/K) 
                 (kg/m 3 ) 
               
               
                   
               
             
          
           
               
                 12 
                 0 
                 5.52 
                 1.55 
                 0.14 
                 1065.9 
               
               
                 15 
                 0 
                 5 
                 1.558 
                 0.1395 
                 1063.5 
               
               
                 20 
                 0 
                 4.29 
                 1.573 
                 0.1387 
                 1059.6 
               
               
                 30 
                 0 
                 3.25 
                 1.601 
                 0.1371 
                 1051.7 
               
               
                 40 
                 0 
                 2.56 
                 1.63 
                 0.1355 
                 1043.8 
               
               
                 50 
                 0 
                 2.07 
                 1.658 
                 0.1339 
                 1035.8 
               
               
                 60 
                 0 
                 1.72 
                 1.687 
                 0.1323 
                 1027.8 
               
               
                 70 
                 0 
                 1.46 
                 1.715 
                 0.1307 
                 1019.7 
               
               
                 80 
                 0 
                 1.25 
                 1.744 
                 0.1291 
                 1011.5 
               
               
                 90 
                 0 
                 1.09 
                 1.772 
                 0.1275 
                 1003.2 
               
               
                 100 
                 0.01 
                 0.97 
                 1.8 
                 0.1259 
                 994.9 
               
               
                 110 
                 0.01 
                 0.86 
                 1.828 
                 0.1243 
                 986.5 
               
               
                 120 
                 0.01 
                 0.77 
                 1.856 
                 0.1227 
                 978.1 
               
               
                 130 
                 0.02 
                 0.7 
                 1.884 
                 0.1211 
                 969.5 
               
               
                 140 
                 0.03 
                 0.64 
                 1.912 
                 0.1195 
                 960.9 
               
               
                 150 
                 0.05 
                 0.58 
                 1.94 
                 0.1179 
                 952.2 
               
               
                 160 
                 0.07 
                 0.53 
                 1.968 
                 0.1163 
                 943.4 
               
               
                 170 
                 0.09 
                 0.49 
                 1.996 
                 0.1147 
                 934.5 
               
               
                 180 
                 0.13 
                 0.46 
                 2.023 
                 0.1131 
                 925.5 
               
               
                 190 
                 0.18 
                 0.42 
                 2.051 
                 0.1115 
                 916.4 
               
               
                 200 
                 0.24 
                 0.39 
                 2.079 
                 0.1099 
                 907.1 
               
               
                 210 
                 0.32 
                 0.37 
                 2.107 
                 0.1083 
                 897.8 
               
               
                 220 
                 0.42 
                 0.34 
                 2.134 
                 0.1067 
                 888.3 
               
               
                 230 
                 0.54 
                 0.32 
                 2.162 
                 0.1051 
                 878.7 
               
               
                 240 
                 0.69 
                 0.3 
                 2.19 
                 0.1035 
                 868.9 
               
               
                 250 
                 0.87 
                 0.28 
                 2.218 
                 0.1019 
                 859 
               
               
                 260 
                 1.08 
                 0.27 
                 2.245 
                 0.1003 
                 849 
               
               
                 270 
                 1.33 
                 0.25 
                 2.273 
                 0.0987 
                 838.7 
               
               
                 280 
                 1.63 
                 0.24 
                 2.302 
                 0.0971 
                 828.3 
               
               
                 290 
                 1.98 
                 0.22 
                 2.33 
                 0.0955 
                 817.7 
               
               
                 300 
                 2.38 
                 0.21 
                 2.359 
                 0.0939 
                 806.8 
               
               
                 310 
                 2.84 
                 0.2 
                 2.388 
                 0.0923 
                 795.8 
               
               
                 320 
                 3.37 
                 0.19 
                 2.417 
                 0.0907 
                 784.4 
               
               
                 330 
                 3.96 
                 0.18 
                 2.448 
                 0.0891 
                 772.8 
               
               
                 340 
                 4.64 
                 0.17 
                 2.479 
                 0.0875 
                 760.9 
               
               
                 350 
                 5.39 
                 0.16 
                 2.511 
                 0.0859 
                 748.6 
               
               
                 360 
                 6.24 
                 0.15 
                 2.544 
                 0.0843 
                 735.9 
               
               
                 370 
                 7.18 
                 0.15 
                 2.579 
                 0.0827 
                 722.8 
               
               
                 380 
                 8.22 
                 0.14 
                 2.616 
                 0.0811 
                 709.2 
               
               
                 390 
                 9.37 
                 0.13 
                 2.657 
                 0.0795 
                 695 
               
               
                 400 
                 10.64 
                 0.13 
                 2.701 
                 0.0779 
                 680.2 
               
               
                   
               
             
          
         
       
     
         [0030]    The initial composition by weight of the degraded heat transfer oil ( 1 ) is shown below. The initial conditions of said degraded heat transfer oil ( 1 ) are 302° C. and 33 bar: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Benzene 
                 0.4% 
               
               
                   
                 Phenol 
                 1.4% 
               
               
                   
                 Ortho-Terphenyl 
                 2.9% 
               
               
                   
                 Meta-Terphenyl 
                 0.5% 
               
               
                   
                 Para-Terphenyl 
                 0.5% 
               
               
                   
                 Dowtherm A 
                 94.3% 
               
               
                   
                   
               
             
          
         
       
     
         [0031]    Table 2 below shows the composition of the currents flowing in and out of the plant of the invention. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Composition of the currents flowing in 
               
               
                 and out of the plant of the invention. 
               
             
          
           
               
                   
                 Oil to be 
                 Heavy 
                 Light 
                 Regenerated 
               
               
                   
                 regenerated 
                 components 
                 components 
                 oil 
               
               
                   
                   
               
             
          
           
               
                 Flow (kg/h) 
                 6,500 
                 253.5 
                 117 
                 6129.5 
               
               
                 Benzene 
                 0.4 
                 — 
                 24.1 
                 173 ppm 
               
               
                 Phenol 
                 1.4 
                 — 
                 60.3 
                 0.3 
               
               
                 Ortho- 
                 2.9 
                 68.4 
                 — 
                 0.3 
               
               
                 Terphenyl 
               
               
                 Meta- 
                 0.5 
                 12.1 
                 — 
                 152 ppm 
               
               
                 Terphenyl 
               
               
                 Para- 
                 0.5 
                 12.2 
                 — 
                 115 ppm 
               
               
                 Terphenyl 
               
               
                 Dowtherm A 
                 94.3 
                 7.2 
                 15.6 
                 99.4 
               
               
                 Temperature 
                 302° C. 
                 351 
                 138 
                 275 
               
               
                 (° C.) 
               
               
                 Pressure 
                 33 
                 1.5 
                 1.5 
                 1.5 
               
               
                 (bar) 
               
               
                   
               
             
          
         
       
     
         [0032]    As shown in Table 2, the facility of the invention is appropriate for treating a volume of degraded heat transfer oil ( 1 ) of 6500 kg/h, which allows processing of the total amount of oil used in the solar thermal plant, 1300 tonnes, in a total of 200 hours. The indicated volumes can have variations of up to +−10% without significantly affecting the performance of the plant of the invention. 
         [0033]    The plant comprises the following elements:
   an air cooler ( 2 );   a distillation column ( 3 ) for separating the heavy components ( 4 );   a boiler ( 5 );   a rectifier ( 6 ) for separating the light components ( 7 );   an air condenser ( 8 );   a backflow tank ( 9 );   a first pump ( 10 ) and second pump ( 11 ).   
 
         [0041]    The air cooler ( 2 ) receives the degraded heat transfer oil ( 1 ) from the solar thermal facility and lowers its temperature from an inlet temperature of 302° C. to an outlet temperature of 100° C. The air cooler ( 2 ) incorporates means for controlling outlet temperature (not shown), which comprise a frequency converter or various individually controlled fans. The air cooler ( 2 ) also incorporates inlet and outlet temperature meters and a flow meter. 
         [0042]    The output product of the air cooler ( 2 ) is cooled oil ( 12 ), which is introduced in the distillation column ( 3 ), which comprises a column of first perforated plates ( 13 ) made of carbon steel, and a boiler ( 5 ). The cooled oil ( 12 ) enters the distillation column ( 3 ) through the upper part thereof and is heated in said distillation column ( 3 ) by the boiler ( 5 ), which is a thermosyphon-type heat exchanger, in order to evaporate the products in the lower part of the distillation column ( 3 ) by means of a current of the hot oil coming from the solar thermal facility. 
         [0043]    The boiler ( 5 ) incorporates temperature and flow meters and a flow control valve (not shown). 
         [0044]    The harmful heavy components ( 4 ) (o, m and p-terphenyls) are separated into tails by gravity, together with a small amount of oil, while the rest of the oil and light components penetrate the distillation column ( 3 ) until reaching the head ( 14 ), where they abandon said distillation column ( 3 ) in the form of vapour ( 15 ). 
         [0045]    The distillation column ( 3 ) incorporates a pressure transmitter at the inlet and a pressure and temperature transmitter at the outlet. It also incorporates a plurality of temperature transmitters distributed throughout the interior thereof, one every three first plates ( 13 ), and one temperature transmitter in the lower part thereof, four level switches and an automatic valve at the outlet of the heavy components ( 4 ). 
         [0046]    The distillation column ( 3 ) incorporates an electric resistance (not shown) in the lower part thereof for the purpose of maintaining the oil in a liquid state (250° C.) in the event that the power supply is interrupted. 
         [0047]    The vapour ( 15 ) free of heavy components ( 4 ) abandons the distillation column ( 3 ) through the head ( 14 ) and accesses the rectifier ( 6 ), which is a distillation column having a plurality of second perforated plates ( 16 ) made of carbon steel. The regenerated oil ( 17 ) is recovered in the lower part of the rectifier ( 6 ) and the light components ( 7 ) are collected in the upper part thereof, which are condensed in the air condenser ( 8 ) and conveyed toward a backflow tank ( 9 ). 
         [0048]    The rectifier ( 6 ) incorporates pressure and temperature transmitters at the outlet, temperature transmitters distributed throughout the interior of the rectifier ( 6 ), an additional temperature transmitter in the lower part and level switches, in addition to incorporating an electric resistance in the lower part thereof for the purpose of maintaining the fluid in a liquid state (30° C.) in the event that the power supply is interrupted. 
         [0049]    The air condenser ( 8 ) is equipped with means for controlling outlet temperature, preferably a frequency converter or a plurality of independently controlled fans. 
         [0050]    The accessory elements, such as valves, auxiliary equipment and instruments, have been described in detail, although these may vary in the final construction. 
         [0051]    Additionally, the invention incorporates a first pump ( 10 ), which circulates the regenerated oil ( 17 ) back to the solar thermal facility, and a second pump ( 11 ) that recirculates the light components ( 7 ) from the backflow tank ( 9 ) to the rectifier ( 6 ).