Abstract:
A tube-and-shell heat exchanger system is disclosed which provides for partial heating of the cooler stream as it flows through a first compartment in the shell and conducting the partially heated stream to the outlet end of a second compartment in the shell to maintain the outlet end of the tubes at a higher temperature. The higher temperature at the outlet ends of the tubes avoids rapid fouling of tubes near the outflow end. There are provided slide bushings for tubes passing between the compartments in the shell. The slide bushings make possible heating of greater volumes of the cooler stream and maintaining the outlet end of tubes at higher temperature, while extracting more heat from the hot stream. The slide bushings provided may also be used to replace conventional expansion joints. The system is particularly useful in carbon black plants, where the hot smoke stream containing combustion gases and carbon black is used to preheat the air stream used for burning fuel in the reactor of the plant.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to high-temperature heat exchangers for gas streams. More specifically, improved apparatus and method for recovering heat from the furnace effluents stream in a carbon black plant to preheat the combustion air stream are provided. 
     2. Description of Related Art 
     In the typical carbon black production process, fuel and air are combusted in a furnace to provide the necessary temperature and energy for the carbon black production step. Oil feedstock is injected directly into the combustion gases, still inside the furnace, where the feedstock is dehydrogenated in a pyrolytic reaction to form carbon black and other gaseous products. The final stream, after all the reactions are complete, is referred to as “smoke.” In order to completely stop all the reactions and to cool the furnace effluent, the smoke is quenched by direct contact with water. After the water quench, the smoke stream is still very hot and can be used to heat other process streams, such as the combustion air stream. 
     Preheating the combustion air stream significantly increases efficiency of the carbon black production process by reducing the amount of fuel required while also increasing the capacity of a carbon black production unit. Various processes and apparatuses for preheating the combustion air stream in a carbon black production process are known to the industry. Most carbon black production processes use a vertical shell-and-tube heat exchanger to preheat the combustion air stream by indirect contact with the smoke stream exiting the water quench. The smoke stream typically flows upwards through the tubes while the air stream is forced downward through the shell. Combustion air exiting the current industry standard air preheater may be heated to temperatures up to about 800° C. 
     Several problems must be considered when designing a preheater for carbon black production. First is the tendency of the smoke stream to deposit carbon particles inside the tubes, thus fouling those surfaces and reducing heat exchange efficiency. If the smoke stream is cooled too much, the fouling becomes particularly pronounced. Thus, most preheating processes and apparatuses are designed to keep the smoke stream hot in order to reduce fouling as much as possible. 
     Several remedies exist in current practice to maintain the smoke streams at high temperatures and prevent fouling. First, a sheath may be constructed around the top portion of the tubes, thus creating a stagnant air gap between the sheath and the tube surface. The air gap reduces heat exchange in the area of the sheath and thus reduces cooling of the smoke stream. Unfortunately, the amount of heat transferred to the combustion air is also reduced, resulting in a less efficient preheater. Furthermore, the sheathing complicates the heat exchanger manufacturing process and adds to exchanger cost. 
     A second remedy is to decrease flow of combustion air through the preheater. U.S. Pat. No. 4,737,531 discloses a method by which a control valve causes a fraction of combustion air to bypass the preheater. A lower flow of combustion air through the preheater transfers less heat away from the smoke stream, keeping the smoke at a temperature higher than the temperature at which high rates of fouling occur. This method requires a complicated and costly control system and preheats only a portion of the combustion air. 
     A third design uses a double tubesheet (two parallel, closely spaced, tubesheets) in a heat exchanger and two stages of air compression (“Improvements to High Temperature Airheater,” presented at Carbon Black World 96, Nice, France, Mar. 4-6, 1996). Hot gas from a reactor, carrying carbon black smoke, is passed through the tubes of a shell-and-tube heat exchanger. The double tubesheet in the shell around the inflow end of the tubes creates two separate heat exchange compartments on the shell side. Compressed air is fed to the air pre-heater from a first compression stage. About 20% of the air stream from the first compression stage is diverted to a second compression stage and forced between the double tubesheets, which form a small compartment on the shell side of the exchanger. Air flows radially inward across the tubes between the double tubesheets and is then directed up a center tube in the shell to the top of the heat exchanger. The preheated air then encounters a baffle system at the top of the heat exchanger where it mixes with compressed unheated air from the first stage of compression. The combined stream then flows through the shell side countercurrent to flow of the smoke stream in the tubes. 
     The slip stream that is further compressed and sent to the top of the tubes serves to increase temperature of the top of the tubes, which reduces fouling in the top section of the tubes, but shielding of the top section of the tubes is still normally required to prevent fouling. Shielding of the tubes, which decreases heating of the incoming air, causes loss of efficiency of the pre-heating process, as discussed above. 
     The double tubesheet at the end where the hot smoke stream enters the pre-heater addresses another problem of combustion air preheaters—mechanical failure of the tubes and the tubesheet caused by high temperature of the smoke stream. Cool air transfers heat away from the tubes and tubesheets in the entry zone and reduces thermal stress on the heat exchanger. Without the double tubesheet, lower temperature of the incoming stream and insulation in the tubes to decrease heat transfer rate are necessary, both of which cause loss of efficiency. 
     One additional drawback of the third design is a limitation of the volume available between the double tubesheets, and thus a limitation of the temperature that can be attained in the air stream that is to be directed into the shell at the outflow end of the tubes. The heat exchanger tubes are welded to a sleeve which is welded to the middle tubesheet and lower tubesheet, and the tubesheets are welded to the shell in this design. This results in the elimination of air leakage from across the center tubesheet, but it causes other problems. The tubes expand during operation due to their increased temperature. This expansion places stress on the sleeves and tubesheets, and the stress may cause failure, especially at the point where the sleeves are welded to the tubesheet. The greater the distance between the tubesheets, the more stress is created. Limitations in the amount of stress that the tubesheets can tolerate restrict the distance between the tubesheets in the prior art design. Thus, the maximum volume between the double tubesheets and the maximum flow through that compartment of the shell is restricted. 
     While the third design makes improvements in the operation of air preheaters for use in carbon black plants, increased complexity in design of the heat exchanger and increased cost of a second compression step are necessary. Shielding of the top of the tubes may still decrease efficiency. 
     All of the above mentioned high temperature air preheater designs utilize expansion joints which are welded to the upper tubesheet. Commercially available expansion joints have a significantly larger diameter than the tubes, and thus allow for little tubesheet material between tubes in the upper tubesheet. Also, the expansion joints put extra stress on the bottom and top tubesheets. The combination of little top tubesheet material and stress often causes tubesheet failure. Further, the commercially available expansion joints themselves are often prone to fail. 
     What is needed is a heat exchanger system that has reduced complexity and cost while retaining and improving efficiency of the heat recovery process and increasing service life of the system. 
     SUMMARY OF THE INVENTION 
     A slide bushing inserted in the middle tubesheet of a tube-and-shell heat exchanger allows for tube expansion during heat exchange with very hot gas passing through the tubes and restricts flow between the tubes and tubesheet. A slide bushing mechanism may also be used in place of an expansion joint to seal between tube and the terminal tubesheet, in which case the slide bushing contains rings and is designed to allow very small leakage across the tubesheet. 
     Apparatus and method are provided for pre-heating an air stream by heat exchange with the hot smoke stream in a carbon black producing unit. Three tubesheets separate the shell side of the heat exchanger into two compartments. Slide bushings allow thermal expansion of the tubes through the middle tubesheet. The entire combustion air stream is compressed and passed through the compartment of the shell between the first and second tubesheets at the inflow end of the tubes where it is heated, then passed through an insulated conduit outside the shell and back into the shell at the outflow end of the tubes. The air stream is heated sufficiently in the first compartment to minimize fouling of the tubes at the outflow end. Leakage through the slide bushings from the first compartment to the second compartment may be allowed to provide cooling to the tube and sleeves. The air stream then passes through the shell countercurrent to flow in the tubes and to an exit manifold near the middle tubesheet. 
     The slide bushing is welded or otherwise joined to the middle tube sheet and is designed to closely fit around the tubes. The slide bushing may be designed for metal to metal contact at operating temperature and may include a slit to reduce frictional force between the bushing and the tube. Alternatively, the slide bushing may have ring grooves which hold one to several rings in place. The rings may be ceramic. Ceramic paper, well known in industry, may be placed between the slide bushings and the ceramic rings to decrease leakage around the rings. 
     Further features and advantages of the invention will be understood from the following detailed description of preferred embodiments with reference to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow diagram of a carbon black production unit using the present invention. 
     FIG. 2 is a drawing of the inside of the shell of the heat exchanger of the present invention. 
     FIG. 3 depicts a first embodiment of a slide bushing of the present invention. 
     FIG. 4 depicts a second embodiment of a slide bushing having ring grooves. 
     FIG. 5 depicts an embodiment of a seal ring for use in a slide bushing. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, air preheater system  10  is shown. Air is compressed in compressor  12  and sent to compartment  14  where it is heated by gases from reactor  20  which have been directed through base  15  and into the tubes of a tube-and-shell heat exchanger. Base  15  is shown at the bottom of a vertical heat exchanger, but may alternatively be placed at the top of a vertical apparatus or at either end of a horizontal apparatus. For ease of description, the use of the terms “top”, “bottom” and “middle” will be used herein. In any case, base  15  is located at the end of the heat exchanger where a hot process stream enters the tubes. The volume of air compressed by compressor  12  is preferably equal to the combustion requirements in reactor  20 . Outlet pressure at compressor  12  depends on flow resistance in air preheater  10 , but will usually be in the range from about 6 psig (41 gage kilopascals) to about 12 psig (82 gage kilopascals). 
     After compressed air is heated in compartment  14  to an elevated temperature, often in the range of 150 to 200° C., it is passed through an insulated conduit outside the heat exchanger to top inlet manifold  16 , where it enters the shell side of the heat exchanger and flows countercurrent to gas flowing inside the tubes and exits at air exit manifold  18 . From air preheater  10  the air is piped to reactor  20 . The stream containing the carbon black from reactor  20  exits the tubes in the air preheater into bonnet  22  and goes to a bag house or other equipment for separating and pelletizing the carbon black. 
     FIG. 2 shows the arrangement inside the shell of air preheater  10  with base  15  and bonnet  22  removed. Tubes  24  contain the hot gas carrying carbon black. First or bottom tubesheet  26  and second or middle tubesheet  28  form first compartment  14  in the shell side of the heat exchanger. The air stream within this compartment is directed across the entire tube bundle. Second tubesheet  28  and third or top tubesheet  30  form second compartment  17 . Baffles  32  and baffle supports  34  improve efficiency of heat transfer from hot gas in tubes  24  to combustion air passing countercurrent in main compartment  17  before exiting through lower manifold  18 . 
     Tubes  24  are rigidly attached and sealed to tubesheet  26 , using techniques well known in industry. Expansion joints well known in the industry (not shown) or terminal slide bushings  25  of the present invention may be used to provide a seal between the outflow end of tubes  24  and tubesheet  30 . In the apparatus and method of this invention, tubes  24  preferably pass through middle tubesheet  28  within slide bushings  40 . Slide bushings  40  are designed to control leakage of air through second tubesheet  28  and allow for thermal expansion of the tubes. The slide bushings prevent high thermal stresses in the tubes and tubesheets that are present when tubes are fixed to the second tubesheet, as in prior art designs. Velocity stack inserts  42  are also depicted in FIG.  2 . As known in the art, they serve to protect tubes  24  from the hot turbulent gases near the inflow end of the tubes and can be replaced when necessary. 
     FIG. 3 shows a detailed view of one embodiment of slide bushing  40 . Slide bushing  40  is designed for a metal to metal contact between tube  24  and slide bushing  40  at operating temperatures. Slit  41  reduces frictional force between slide bushing  40  and tube  24  after thermal expansion in the diameter of tube  24 . Some air leakage is allowed from compartment  14  to compartment  17 . The difference between the outer diameter of the slide bushing  40  and the inner diameter of the tube  24  can be from about 0.005 inch (0.013 cm) to about 0.02 inch (0.051 cm) at room temperature, but is selected to prevent leakage of not more than about 15 percent of the air rate entering compartment  14 . A selected amount of leakage may be desirable to provide cooling for the tubesheet and tube. 
     FIG. 4 depicts a second embodiment of slide bushing  40 . In this design, at least one bushing ring groove  46  is cut into slide bushing  40 . FIG. 5 depicts seal ring  44  which fits into this groove  46 . The split at the seal ring  44  may be at any angle but in a preferred embodiment is perpendicular to a line tangent to the ring. The seal ring system of FIG.  4  and FIG. 5 can be used to control air leakage rate to less than 1 percent of inflow rate into the shell side of the heat exchanger, if desired. The number of rings  44  fit in a bushing ring groove  46  may vary from one to ten. In a preferred embodiment two rings  44  are utilized for slide bushing  40 , and the rings are aligned so that the splits in the rings are opposed 180°. Rings are selected to seal between the outside diameter of tubes and tube rings  44 . Ceramic paper  48 , well known in the industry, may be placed between the bushing ring groove  46  and seal rings  44  to further reduce leakage. Suitable ceramic rings are available from Kyocera of Elk Grove Village, Ill. 
     At low rates of air leakage through slide bushing  40 , the air may be heated to about 500 ° C. The leaking air then mixes with the hot air in main compartment  17  of FIG.  2 . Since the leaking air is such a small volume stream, and since it is heated while leaking, the air preheater loses very little heat exchange efficiency due to the leakage. Further, some leakage of the air is desired to cool the tubes as they pass through the middle tubesheet. 
     The air which does not pass through second tubesheet  28  depicted in FIG. 2 exits compartment  14  at a temperature preferably of about 175° C. or more. The preheated air passes through insulated conduit  43  and enters compartment  17  through upper inlet manifold  16 . Mechanical shielding of the top of the tubes to prevent fouling inside the tubes is normally not needed because the air temperature entering manifold  16  is higher than in prior art apparatus. 
     Air exiting the exchanger at exit manifold  18  is at a temperature higher than the air exiting preheaters in the prior art that are operating at the same rate of production and with the same feed streams, because of the improved efficiency of heat transfer in the heat exchanger. The higher temperature preheated air is used to burn a fuel (normally natural gas) and the higher temperature combustion stream is more effective in pyrolyzing carbon black oil which is fed to the reactor. The result is lower fuel requirement and lower costs of production and increased unit capacity. This mixture of gaseous products and carbon black is quenched with water in water quench section  11 , shown in FIG.  1 . The quench is used to stop the reactions. 
     The smoke stream exits the reactor  20 , depicted in FIG. 1, at temperatures as high as approximately 1650° C. The smoke my exit the water quench section  11  at approximately 1000 ° C. The quenched smoke stream is then directed to the air preheater. Since the preheater operates at higher inlet temperatures than inlet temperatures of prior art preheaters, less water is required in the quench process, allowing more heat to be recovered in the preheater. The slide bushing design of this invention will make possible higher inlet temperatures at the preheater, greater efficiency of heat transfer from the hot inlet gas and longer lifetime of the heat exchanger equipment. 
     The exchanger may utilize a commercially available expansion joint at the top of each tube. Referring again to FIG. 2, expansion joints (not shown) may be welded between the tubes and third tubesheet  30  to allow for thermal expansion in length of the tubes between tubesheet  28  and tubesheet  30 , as is well known in the art. Alternatively, according to the present invention, the exchanger will utilize terminal slide bushings  25  fixed in third tubesheet  30 . Terminal slide bushings  25  preferably have the design depicted in FIG. 4, so as to allow only very small leakage rates. Bushing ring groove  46  of the terminal slide bushing may be designed to hold one to ten seal rings  44 , but in a preferred embodiment, two to five seal rings are utilized. The rings are preferably positioned so that splits in neighboring rings are angularly displaced so as to maximize resistance to flow though the splits of successive rings. Groove  46  is sized to minimize the gap between rings  44  and the groove  46  at operating conditions so as to maximize resistance to flow along the surfaces between rings. Ceramic paper  48 , well known in the art may be utilized between the rings  44  and ring groove  46  to decrease leakage between the rings and the groove. Terminal slide bushings  25  having the features shown in FIG. 4 allow very low rates of leakage of gas through tubesheet  30  of FIG.  2 . 
     A variety of materials can be used in the apparatus of this invention. In a preferred embodiment, the half of tubes nearest base  15  and velocity stack inserts  42  are made of 310 stainless steel that has undergone an aluminum co-diffusion process on the interior surfaces. The process is available from Alon Surface Technologies of Tarentum, Pa. The aluminum co-diffusion process offers excellent resistance to oxidation and sulfidation at the higher temperatures. The tubesheets, slide bushings, lower shell, baffle plates, and upper tubes may be made from 304H stainless steel. Carbon steel  516 - 70  is preferably used for a portion of the upper shell and lifting lugs. Commercially available expansion joints, if utilized, are preferably made of INCONEL alloy. Other choices of materials may be suitable in other embodiments, depending on operating temperatures. 
     The slide bushings allow thermal expansion of tubes in the first compartment, which makes possible extension of this compartment over a greater length so that all of the combustion air stream can be directed through this compartment. Further, the additional cooling near the inflow end of the tubes reduces the need for insulation between the velocity stack and the tube in the double tubesheet area and further decreases cost and enhances exchanger efficiency. The distance between the two tubesheets near the inflow end of the tubes is selected to optimize temperature of the air stream exiting this first compartment. The distance is preferably selected to prevent rapid fouling of tubes near the outlet end of the tubes. The distance between the first and second tubesheets may vary from about one-quarter to one-and-a-half times the inner diameter of the exchanger shell. For example, in one carbon black plant, the shell inside diameter is about 49 inches (124 cm) and the distances between the two tubesheets is designed to be about 22 inches (56 cm), or 0.45 times the diameter. The specific distance ratio of tubesheets to shell inner diameter should also be chosen to provide adequate cooling of the first and second tubesheets. 
     Directing the entire combustion air stream through the first compartment offers many advantages. First, since the combustion air stream follows only one path, only one compression step is needed and no control system is needed, thus eliminating costly parts of previous designs. Since the entire combustion air stream is heated in the first compartment, the temperature of the combustion air stream entering the shell around the outlet end of the tubes is substantially hotter than in previous designs. The hotter combustion air stream reduces fouling in the tubes without using tube shielding. Elimination of the tube shielding makes the entire heat exchanger more efficient and less costly to build than a comparable heat exchanger with shielding. Another advantage is that a higher air flow through the non-insulated first compartment more easily transfers heat away from the tubes and the velocity stack inserts. Cooler velocity stack inserts will last longer, and this will reduce heat exchanger maintenance costs. 
     Use of terminal slide bushings in the top tubesheet eliminates many of the problems faced by currently available expansion joints. The terminal slide bushings put less stress on the tubesheets and also have smaller diameter than the current expansion joints, allowing more tubesheet material between tubes. Additional tubesheet material and reduced stress reduces the likelihood of tubesheet failure and increases exchanger service life. Also, the terminal slide bushings themselves are less likely to fail than currently available expansion joints. The inventive terminal bushings further add to the service life of the exchanger. 
     While the pre-heater of this invention has been discussed especially with respect to its application in the carbon black industry, it should be understood that the apparatus and methods of this invention can be applied to any tube-and-shell heat exchanger where excess cooling of tubes near the outflow end is to be avoided or where excessive thermal stresses may occur within the tubes or tubesheets of the heat exchanger. Either the process stream through the tubes or through the shell of the heat exchanger may be gaseous or liquid or a combination thereof, but they will normally be gaseous. The deposit causing fouling can be suspended solids or solids precipitated upon cooling. 
     It should also be understood that some of the characteristics achieved by the “bushing” described herein can be achieved by selected procedures in forming a tubesheet. Such a tubesheet would be equivalent to a tubesheet adapted to receive the bushings and the bushings sealingly attached therein. For example, holes in a tubesheet can be drilled to diameters having a selected diameter greater than the tube diameters, a groove can be cut in the hole and a ring or a plurality of rings can be placed in the groove. Alternatively, a plurality of grooves can be cut in each hole of the tubesheet. 
     Although the present invention has been described in connection with preferred embodiments, the invention is not limited thereto. The embodiments and features disclosed herein are provided by way of example only. It will be easily understood by those of ordinary skill in the art that variations and modifications can be easily made within the scope of this invention as defined by the following claims.