Patent Publication Number: US-11035259-B2

Title: Method and system for stack heat recovery

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional patent application Ser. No. 62/648,068, filed on Mar. 26, 2018; the entirety of which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention is generally directed to the recovery of heat from chimneys or stacks. More particularly, the present invention is directed to an improved process for heat recovery during manufacturing processes of a variety of products. 
     BACKGROUND OF THE INVENTION 
     Many industries employ heating material to drive moisture from a product during the manufacture of the product. This moisture is carried away with hot air and is typically discharged up a chimney or stack and is emitted into the atmosphere. In many processes, however, this moisture laden exhaust is not suitable for direct emission and must be treated to remove volatile organic compounds (“VOCs”) and other pollutants. 
     Thermal oxidizers are one method for reducing these pollutants. They easily destroy 95 percent or more of the VOCs delivered to them, require little labor and can be monitored for compliance simply and cheaply. A thermal oxidizer raises the temperature of the exhaust to a high enough level that the pollutants are burned or oxidized and reduced to elemental compounds. 
     However, there is still vapors that have been processed by the thermal oxidizers with energy that is wasted up the exhaust. Therefore, what is needed is a new method and system that recovers heat from an exhaust stream during a manufacturing process to improve overall efficiency, allowing for energy produced by the system that can be added back into the process with no additional external energy needed to be input. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
         FIG. 1  is a process flow diagram of the present invention using a condensing heat exchanger. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1  a process flow diagram of the heat stack recovery system  10  in accordance with the invention that utilizes a condensing heat exchanger  12  is depicted. The present invention recovers energy from an exhaust stream during manufacturing processes such as thermal oxidizer discharge and regenerative thermal oxidizer discharge and inputs it back into the manufacturing process from which it extracted or used in other processes within the plant. Exhaust streams during manufacturing processes consist primarily of nitrogen, carbon dioxide, a small amount of oxygen and trace amounts of ash. Because the exhaust stream  14  is at a temperature of typically less than 500 degrees F. and is moisture laden, a condensing heat exchanger  12  is employed. This heat exchanger  12  has relatively cold recirculation water on one side of the tubes or plates while the hot exhaust or vapor stream passes the other side of the tubes or plates. The energy is transferred from the hot exhaust  14  to the cold recirculation water; thus cooling the exhaust vapors. Once the vapor temperature has dropped to the dew point, condensation takes place and is collected into the condensate tank  16 . The cooled exhaust is then sent to its normal discharge stack  18 . 
     The condensate is typically ultra-pure due to the distillation of the water from the dryers as well as passing through thermal oxidation that removes contaminants. However, it is also low pH due to the acidic nature of the combustion products. Because of the water purity, it can be easily pH-adjusted with a base to bring the pH above neutral, i.e. 7.0 pH, to prevent any corrosion of system components. The condensate can also be sprayed back into the hot exhaust stream to drop the temperature to dew point more quickly  20  and/or be used as make up water to the flash tank  22 . 
     The condensing heat exchanger recovers heat energy via exhaust temperature and condensing of the water in the exhaust stream. This heat energy is absorbed by the water stream inside the heat exchanger. The water stream including the heat energy is called recirculation water. Recirculation water is pumped from and back to a low pressure flash vessel. The flash vessel receives the heated recirculation water and flash cools it. The cooled recirculation water is then returned to the heat exchanger to recover more heat. The steam flashed in this vessel is routed to a suitable part of the process to offset steam produced by other sources. 
     As discussed above, the recirculation water is pumped from the flash tank to the condensing heat exchanger where it recovers heat, picks up energy and its temperature rises. It then returns to the flash tank where it is at a higher temperature than can be maintained in the pressure of the tank. A portion of the recirculation water flashes off as low pressure steam  24  (evaporates); giving off energy necessary to lower the remaining recirculation water temperature to an equilibrium point that matches the pressure of the flash tank. This cooled recirculation water is then pumped back to the condensing heat exchanger to extract more heat. 
     The low pressure steam that is produced by the flash tank is used in other parts of the plant or in the process in accordance with the invention. As this steam exits the system, water has been removed from the recirculation water and must be made up to maintain a constant volume. The pi-adjusted condensate from the condensate tank is used for this purpose. If there is additional external make-up water  26  is required, existing plant boiler make-up water, external to the heat recovery system is used. Those of skill in the art will appreciate that external make-up water is expensive. Because the system in accordance with the invention uses condensate water to maintain a constant volume of recirculation water, it decreases the amount of external make-up water that is required over conventional systems by 20% to 75% as hereinafter described. 
     The condensed water is collected in a receiver tank. As previously disclosed the condensed water is typically ultra-pure to very clean (i.e. boiler quality water) due to the flashing process of the dryer—distillation—and the thermal oxidation process that destroys any contaminants that would be driven off by the drying and carried with the exhaust gases. Because of the purity of the water, it is typically at a low pH due to the carbon dioxide that is present in both the dryer and thermal oxidizer. Water at a low pH is typically corrosive to the equipment it comes into contact with. Therefore, the pH may be adjusted to neutral in the receiver tank and can then be filtered if ash is present. A portion of the water and/or condensate stream is sprayed via a fine-spray nozzle in the exhaust gas stream prior to the condensing heat exchanger to lower the vapor temperature to help fully saturate the stream to increase the effectiveness of the exchanger. By fully saturate we means 100% relative humidity. If the vapor stream is at 100% relative humidity, the heat exchanger must remove the sensible heat to get to 100% relative humidity. Once at this point, latent heat—phase change can take place. Latent heat can transfer much more energy per unit area than can sensible heat. Any condensate not used to cool the vapors can be directed to the flash tank make up as described below. 
     Condensate recovery can vary from 20% to 75% of the steam produced. This variability is due to the water-absorption properties of exhaust vapors from differing combustion processes. For example, the combustion processes employed in conventional systems can be operated in ways that can produce constituent levels in the exhaust vapors—particularly with regard to oxygen (O 2 ) and nitrogen (N 2 ). As the O 2  is decreased, the N 2  increases. This changes the point at which water will condense from the exhaust vapors. In atmospheric air (oxygen at 21%, nitrogen at 78%, and carbon dioxide &lt;1%), water will begin to condense at 210-212 F. After combustion, (oxygen at 2% and nitrogen at 86%, and carbon dioxide at 11%), water will not condense readily until the temperature falls below 175 F. 
     The combustion processes differ in the amount of excess oxygen allowed thus dictating the final nitrogen levels. The condensate produced is reduced as the condensing temperature drops. These are all physical characteristics of variable gas compositions. Heat sink temperatures (flash tank), circulation water rates, etc. can and will vary at each installation location of the heat recovery system in accordance with the invention thus varying the amount of recovery of the condensate, which in turn drives the reduction in the amount of external make-up water required. 
     Example I 
     A combustion process used in the stack heat recovery system uses 15% oxygen, 80% nitrogen, 212 degrees F. at 100% relative humidity (6800 gr/lb of water) into the stack heat recovery system in accordance with the invention. This results in an output of 15% oxygen, 80% nitrogen, 175 degrees F. at 100% relative humidity (2400 gr/lb of water). The yield of condensate is (6800 gr/lb-2400 gr/lb) divided by 6800 gr/lb equals 65% water condensate recovery. The condensate water is treated with a base to increase the pH to approximately 7.0. The condensate is then routed to the flash vessel to increase or adjust the recirculation water by the volume of steam evaporated thereby using 65% less external make-up water to adjust the volume of the recirculation water. 
     Example II 
     A combustion process used in the stack heat recovery system uses 2% oxygen, 86% nitrogen, 212 degrees F. at 100% relative humidity of 6800 gr/lb of water. This results in an output of 2% oxygen, 86% nitrogen, 175 F at 100% relative humidity or 5400 gr/lb of water. This yields (6800 gr/lb-5400 gr/lb)+6800 gr/lb equals 21% water condensate recovery. The condensate water is treated with a base to increase the pH- to approximately 7.0. The condensate is then routed to the flash vessel to increase or adjust the recirculation water by the volume of steam evaporated thereby using and 21% less external make-up water is required. 
     Although the present invention has been described with reference to various aspects of the invention, those of ordinary skill in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.