Abstract:
A hydrocarbon cracking process and apparatus for recovering ethylene from hydrocarbon raw material streams. More particularly, the hydrocarbon cracking process and apparatus of this invention utilizes a furnace with a convection section and a radiant section in which the hydrocarbon feedstream is cracked by heating it to a particular temperature and then is self-quenched.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims the benefit of provisional application with the U.S. Serial No. 60/378,307, filed on May 7, 2002, which hereby is incorporated by reference in its entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates to a process and apparatus for cracking hydrocarbon feedstocks. More specifically, the invention is directed to a process and furnace in which a hydrocarbon is cracked by heating it to a particular temperature and then self-quenching itself within the furnace.  
           [0004]    2. Description of the Related Art  
           [0005]    Thermal cracking of hydrocarbon feeds, which is also known as hydrocarbon pyrolysis, to produce olefins, diolefins and aromatics is a common petrochemical process. This process is frequently referred to as steam cracking since the hydrocarbon feeds are usually mixed with steam when they are heated to an incipient cracking temperature and cracked. Hydrocarbon feedstocks typically include, but are not limited to, ethane, propane, naphtha, or gas oil. The cracking takes place in a cracking furnace that typically comprises a radiant section, a convection section, and heat recovery equipment. In the convection or preheat section, a mixture of the hydrocarbon feed and steam are heated to an incipient cracking temperature, normally in the range of 1100° F. to 1300° F. The radiant, or cracking, section is where the cracking occurs. The heat recovery equipment recovers heat from the cracked hydrocarbons and the furnace flue gas.  
           [0006]    The steam cracking process described above has been in commercial use for over sixty years and most of the current cracking furnaces are similar, even though differences exist depending upon the furnace designer. The hydrocarbons being cracked in the radiant section of the furnace pass through high alloy tubes that receive heat from the burning of natural gas and/or the less desirable light gasses produced in the furnace tubes. The tubes typically range from one to three inches in diameter and are forty to eighty feet in length. Some furnaces have a plurality of smaller tubes at the inlet of the radiant section joined to a smaller number of larger tubes, up to eight inches in diameter, at the outlet of the radiant section of the furnace. The tubes in the radiant section of the furnace exit the radiant section and then pass into exchangers, which cool and quench the furnace effluent to stop the cracking reactions taking place within the tubes.  
           [0007]    Much has been learned about the desirable characteristics of the crucial radiant section of these furnaces that effect the yield of desirable olefins and diolefins produced from any hydrocarbon feed. Namely, it has been learned that the yield of desirable olefins and diolefins is increased when the partial pressure of hydrocarbons in the radiant section of the furnaces is decreased. It has also been learned that yield of desirable olefins and diolefins is increased when the effective residence time, during which time the cracking occurs, is reduced.  
           [0008]    As a result of this knowledge, recently installed cracking furnaces have residence times in the radiant sections of from 0.1 to 0.3 seconds. The furnace effluent is then quenched as soon as possible after leaving the radiant section of the furnace. The typical residence time between exiting the furnace and being quenched is between 0.01 to 0.04 seconds, which has a detrimental effect on the yield of the desirable olefins and diolefins.  
           [0009]    In one representative example of a known hydrocarbon cracking furnace, there is a convection section and a radiant section. The radiant section is cubed shaped and contains only vertical radiant tubes that are between 1″ and 4″ in diameter. The radiant tubes can have internal fins, but they are not required. The cracking residence time for this furnace is 0.07 seconds to 0.2 seconds. However, weaknesses exist in this design. Even though the residence times are short, there is still a potential for coking within this residence time range. Additionally, this furnace relies on external cooling to quench the cracking reaction once the effluent stream has left the furnace. The requirement for an external heat exchanger increases energy costs and capital costs.  
           [0010]    To prevent coking within cracking furnaces, others have attempted to chemically halt coking by the addition of decoking fluids or by making process changes to their hydrocarbon cracking process. In one proposed process, a decoking fluid is injected into the hydrocarbon feedstock to prevent coking. In addition, the cracking temperature is lowered to 1550° F. to 1850° F. and uses a residence time of 0.01 to 0.10 seconds. While the coke formation may be decreased, this process requires the addition of expensive chemicals to prevent decoking and has decreased efficiencies since lower cracking temperatures are used. Additionally, in order to cool and stop the cracking process, additional heat exchangers are required.  
           [0011]    Most furnaces operate at low discharge pressures ranges of four to fifteen psig. However, there is a considerable pressure drop in the radiant section due to coking that is detrimental to the yields of desirable products. When the partial pressure of hydrocarbons increases, then the yield of desirable products decreases.  
           [0012]    A need exists for a hydrocarbon cracking process and furnace that will increase the yield of desirable products, decrease coke formation within the furnace, decrease the residence times within the cracking process, and provide potential cost saving benefits such as lower capital costs, lower energy consumption, and lower pressure ratings for equipment.  
         SUMMARY OF THE INVENTION  
         [0013]    Typically in hydrocarbon cracking processes, the stream to be cracked is heated such that the bulk temperature of the stream is substantially higher than the incipient cracking temperature of the hydrocarbon being cracked. Cracking occurs in this bulk stream. In this invention the bulk stream to be cracked is heated to below the incipient cracking temperature but is passed through a very hot tube such that the boundary layer between the bulk fluid and the hot tube is heated to a sufficiently high temperature to cause cracking to take place only in the boundary layer. This results in the reaction being self quenching as the molecules pass back and forth between the hot boundary layer and the cool bulk fluid, which is at a temperature below incipient cracking temperature. Mixing is needed to enhance the transfer of the molecules between the bulk fluid and the boundary layer. Mixing is ideally performed by utilizing internals fins within the furnace tubes in which the cracking process occurs. Since cracking only occurs in the boundary layer the cracking residence time is substantially lower than with current technology and therefore the yields of the most desirable products are greatly enhanced.  
           [0014]    In addition to the new hydrocarbon cracking process, a new furnace has been developed to optimize the new hydrocarbon cracking process. The new furnace is designed to operate at the higher tube metal temperatures, with lower residence times, required for this process. The new furnace has at least two sections, but can have more. The first section, or convection section, is for preheating the hydrocarbon feedstock fed to the furnace. The second section, or radiant section, is where the cracking process occurs. Within the radiant section is a plurality of furnace, or radiant, tubes. As opposed to most furnaces, the radiant tubes within the present furnace are much shorter and typically have smaller diameters than previous models. The radiant tubes also have internal fins internally mounted within to assist in the transferring of molecules between the bulk fluid layer and the boundary layer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0015]    So that the manner in which the features, advantages and objects of the invention, as well as others which will become apparent, may be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which form a part of this specification. It is to be noted, however, that the drawings illustrate only a preferred embodiment of the invention and is therefore not to be considered limiting of the invention&#39;s scope as it may admit to other equally effective embodiments.  
         [0016]    [0016]FIG. 1 is a simplified process flow diagram of the improved hydrocarbon cracking process in accordance to the present invention;  
         [0017]    [0017]FIG. 2 is a partial cross-sectional view of a furnace for hydrocarbon cracking hydrocarbon feedstocks in accordance with the process in FIG. 1;  
         [0018]    [0018]FIG. 3 is a cross-sectional view of a furnace for hydrocarbon cracking hydrocarbon feedstocks in accordance with the process in FIG. 1, taken along the line  3 - 3  of FIG. 2; and  
         [0019]    [0019]FIG. 4 is a partial cross-sectional view of a radiant tube with internal fins in accordance with the apparatus of FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    This invention is directed to a process and apparatus  10  for cracking a hydrocarbon feedstock  12  to recover olefins, diolefins, and aromatics from a feedstock  12 . The apparatus  10  is typically called a furnace. It shall be noted that any type of furnace vessel  50  could be utilized, including, but not limited to, a heater, boiler, kiln, kettle, or cracker.  
         [0021]    The hydrocarbon cracking process begins by supplying a hydrocarbon feedstock  12  to the furnace  50 . The furnace  50  has at least two sections. The first section  14  is typically called a convection section or a preheat section. Many times, steam  16  is added to the hydrocarbon feedstock  12  prior to entering the furnace  50  or prior to preheating the feedstock  12 , either of which would be considered as adding steam  16  to the hydrocarbon feedstock  12  while it is being supplied to the furnace  50 . All embodiments of the present invention are believed to work, whether or not steam  16  has been added to the hydrocarbon feedstock  12 . In the convection section  14  of the furnace  50 , the hydrocarbon feedstock  12  is preheated to a temperature in the range of about 500° F. to about 900° F., or more preferably in the range of about 600° F. to about 800° F. The manner in which the feedstock  12  is preheated will be apparent and is also known to one skilled in the art.  
         [0022]    Once the hydrocarbon feedstock  12  has been preheated, the feedstock  12  is transferred to the second section  18  of the furnace  50 . The second section  18  is typically called a radiant section, a cracking section, or a fired section of the furnace  50 . The second section, or radiant section,  18  typically contains a plurality of radiant tubes  20  through which the preheated hydrocarbon feedstock  12  travels, as shown in FIGS. 2 and 3. The second section  18  also contains a plurality of burners  22  that supply the heat that is necessary for cracking the preheated hydrocarbon feedstock  12  contained within the radiant tubes  20 . Natural gas or the lighter undesired cracked gasses are typically used as fuel gas, but any suitable alternate can be used.  
         [0023]    The radiant tubes  20  are typically constructed of a high alloy material to enable the tubes  20  to withstand the severe conditions within the furnace  50 . However, any material suitable for this type of process is a suitable substitution, provided it has the requisite strength and durability to function within a furnace and is compatible with the chemicals in the method. The tubes  20  have a length in the range of about 4 feet to 12 feet or more preferably from about 5 feet to about 8 feet. The tube  20  diameters vary between about 0.5 inches to about 2.5 inches, or more preferably 0.75 inches to about 1.5 inches.  
         [0024]    In a preferred embodiment of the invention, when the preheated hydrocarbon feedstock  12  is transferred to the radiant tubes  20  in the radiant section  18  of the furnace  50 , a flow distributor  24  is installed at the inlet of each radiant tube  20 , but external to the radiant section  18 . The function of the flow distributor  24  is to evenly distribute the preheated hydrocarbon feedstock  12  flow to each of the radiant tubes  20 . A venturi  25  is a suitable flow distributor  24  for this purpose. However, other devices or techniques that adequately perform the foregoing described function could, if desired, be used in the present method and apparatus, provided that they are constructed with materials that are compatible and the chemicals used in the present method and the severe conditions of the method.  
         [0025]    The preheated hydrocarbon feedstock  12  is transferred to the tubes  20  in the radiant section  18  at a mass velocity of about 1 lb/sec/ft 2  to about 5 lb/sec/ft 2 , or more preferably between about 2 lb/sec/ft 2  to about 4 lb/sec/ft 2 . If steam  16  has been added to the hydrocarbon feedstock  18 , the mass velocity remains the same.  
         [0026]    The burners  22  within the radiant section  18  are those typically known in the art. The only requirement is that the burners  22  need to be able to supply enough heat to reach a tube metal temperature for the radiant tubes  20  in the range of about 2000° F. to about 2300° F., or more preferably between about 2100° F. and 2200° F.  
         [0027]    Once the preheated hydrocarbon feedstock  12  is in the radiant section  18  of the furnace  50 , the feedstock  12  is heated until the radiant tubes  20  have a tube metal temperature in the range of about 2000° F. to about 2300° F. The preferred range is between about 2100° F. and 2200° F. When the desired tube metal temperature is reached, at least some of the preheated hydrocarbon feedstock  12  is cracked, which produces a layer of cracked molecules within the tubes  20  called a boundary layer  24  (not shown). At least some of the remaining uncracked, or partially or lesser cracked, preheated hydrocarbon feedstock  12  remains in what is called a bulk fluid layer  26  (not shown), which is also within the same tubes  20  as the boundary layer  24 . The bulk fluid layer  26  contains uncracked molecules. Within the radiant tubes  20 , the cracked molecules in the boundary layer  24  and the uncracked molecules in the bulk fluid layer  26  are mixed together. To assist in the mixing, the use of internal fins  28  within the radiant tubes  20  is the preferred method of mixing the molecules in the two layers, as shown in FIG. 4.  
         [0028]    In this situation, cracking of the preheated hydrocarbon feedstock  12  will only occur in the boundary layer  24  between the inner surface of the hot radiant tubes  20  and the bulk fluid  26  passing through the tube  20 , which will be below the incipient cracking temperature of the feedstock  12 . Some portion of feedstock  12  entering the radiant tube  20  will be in the boundary layer  24 , which will be at temperatures well above incipient cracking temperature and thereby cause these molecules to crack.  
         [0029]    Each tube  20  contains internal fins  28  that are about 0.05 inches to about 0.25 inches in height, or more preferably in the range of about 0.0625 to about 0.125 inches high. The fins  28  preferably have a spiral or circular configuration. However, it is believed that other fin configurations will work and should be included within the scope of this invention. The fins  28  are mounted internally within the tubes  20 , as demonstrated in FIG. 4. The mounting space between each fin  28  tip, or pitch, is in the range of about 2 inches to about 10 inches, or more preferably about 3 inches to about 6 inches.  
         [0030]    As a result of the mixing of the cracked and uncracked molecules, at least some of the cracked molecules in the boundary layer  24  are transferred into the bulk fluid layer  26 , which substantially instantly self-quenches and halts further cracking in the cracked molecules. Substantially instantly is defined in the range of about 0.002 seconds to about 0.005 seconds. Since such short time periods are too difficult to accurately measure, for simplification, this time is considered to be substantially instantly. Substantially simultaneously, uncracked molecules in the bulk layer  26  are transferred into the boundary layer  24  and become cracked molecules. Substantially simultaneously is defined as being as quick as practically possible.  
         [0031]    Once the cracking process occurs in the radiant tubes  20 , all of the streams from radiant tubes  20  are combined and exit the furnace  50  at an exit temperature of less than about 1250° F. Upon exiting the furnace  50 , the effluent stream  30  can then be cooled and heat recovered from the stream by conventional heat exchanger and recovery systems whereby olefins, diolefins, and aromatics are recovered. With proper control of temperatures of the tubes  20  and the cracked hydrocarbons  30  exiting the radiant section  18 , it is believed that the proper conversion of the hydrocarbon feedstock  12  will be obtained.  
         [0032]    Even though this new process cracks the hydrocarbon feedstock  12  at a temperature in the range of 2000° F. to 2300° F., coke formation is kept at a minimum. Increased temperatures usually cause more degradation to coke. However, the chance for coking is significantly reduced in the present invention since the residence times are so short when compared to other comparable hydrocarbon cracking furnaces. The residence times for the present invention range from 0.002 to 0.005 seconds, as opposed to the 0.01 to 0.04 second range of prior hydrocarbon cracking processes.  
         [0033]    There are several advantages of the improved hydrocarbon cracking apparatus over the current designs. The first advantage is that the effective residence time for cracking, which is the time the hydrocarbon feedstock or partially cracked molecules spend above the incipient cracking temperature, will be between 0.002 and 0.005 seconds. Only the molecules in the boundary layer  24  will be above incipient cracking temperature. The lower residence time will result in enhanced yields of desirable products from the cracking furnace  50  when compared to conventional cracking furnaces. The lower residence time also decreases coke formation, as previously discussed.  
         [0034]    Another advantage of the current invention is that when the furnace effluent stream  30  exits the radiant section  18  with the bulk fluid below the hydrocarbon feedstock incipient cracking temperature, it is believed that mixing will cause an instantaneous quenching and cessation of cracking of the preheated hydrocarbon feedstock. Heat can then be recovered from the cracked effluent in much simpler and lower cost processes. Currently, additional heat exchangers are required to quench and stop the cracking process. This change is a considerable capital cost savings since no heat exchanger equipment is needed to quench the cracked hydrocarbons and stop the cracking process. Additionally, lower pressure rated equipment can be used since the pressure of the hydrocarbons will be lower than in current processes, which also reduces capital costs.  
         [0035]    In addition to the shorter residence times and lower discharge pressures from the furnace, the pressure drop through the radiant tubes and the heat recovery equipment will also be much lower than in other current cracking apparatus. The lower pressure drop increases the yield of desirable products compared to currently accepted cracking technology.  
         [0036]    A further advantage of the new hydrocarbon cracking process is that the net energy required for this process is believed to be substantially lower than available alternate designs. Since the cracking is self-quenching, the lower net energy requirement is due to not having to cool the cracked hydrocarbon stream to stop the cracking process, which reduces the need for additional heat exchangers to quench the stream. Energy will be saved by removing the need for a heat exchanger to quench the reaction.  
         [0037]    It is to be understood that the invention is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as obvious modifications and equivalents will be apparent to one skilled in the art. Accordingly, the invention is therefore to be limited only by the scope of the appended claims.