Cracking of hydrocarbons

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.

What is claimed is:  
     
         1 . A process for thermally cracking a hydrocarbon feedstock, comprising the steps of: 
 (a) supplying a hydrocarbon feedstock to a furnace having at least a first and a second section;    (b) preheating the hydrocarbon feedstock, in the first section of the furnace, to produce a preheated hydrocarbon feedstock;    (c) transferring the preheated hydrocarbon feedstock to the second section of the furnace;    (d) heating the preheated hydrocarbon feedstock until at least some of the preheated hydrocarbon feedstock is cracked within a boundary layer at an inside surface of the second section of the furnace;    (e) mixing the cracked molecules of the boundary layer and the uncracked molecules of the bulk fluid layer;    (f) transferring at least some of the cracked molecules from the boundary layer into the bulk fluid layer and substantially instantly self-quenching and substantially halting further cracking in the cracked molecules and substantially simultaneously transferring at least some of the uncracked molecules from the bulk layer into the boundary layer and cracking at least some of the uncracked molecules as a result of the mixing of the cracked and uncracked molecules; and    (g) removing substantially all the cracked and uncracked molecules from the furnace at an exit temperature of less than about 1250° F.    
     
     
         2 . The process of  claim 1 , including the step of: adding steam to the hydrocarbon feedstock while the hydrocarbon feedstock is being supplied to the furnace.  
     
     
         3 . The process of  claim 1 , wherein the preheated hydrocarbon feedstock is transferred to the second section of the furnace at a mass velocity of about 1 lb/sec/ft 2  to about 5 lb/sec/ft 2  of the preheated hydrocarbon feedstock.  
     
     
         4 . The process of  claim 3 , wherein the preheated hydrocarbon feedstock is transferred to the second section of the furnace at the mass velocity preferably in the range of about 2 lb/sec/ft 2  to about 4 lb/sec/ft 2 .  
     
     
         5 . The process of  claim 1 , wherein the hydrocarbon feedstock is preheated to a temperature in the range of about 500° F. to about 900° F.  
     
     
         6 . The process of  claim 5 , wherein the hydrocarbon feedstock is preheated preferably to a temperature in the range of about 600° F. to about 800° F.  
     
     
         7 . The process of  claim 1 , wherein the preheated hydrocarbon feedstock is heated in the second section to a tube metal temperature in the range of about 2000° F. to about 2300° F.  
     
     
         8 . The process of  claim 7 , wherein the preheated hydrocarbon feedstock is heated in the second section to a tube metal temperature preferably in the range of about 2100° F. to about 2200° F.  
     
     
         9 . The process of  claim 1 , wherein the preheated hydrocarbon feedstock is transferred from the first section to the second section of the furnace by radiant tubes that have an internal diameter in the range of about 0.5 inches to about 2.5 inches.  
     
     
         10 . The process of  claim 9 , wherein the transferring of the preheated hydrocarbon feedstock to the second section of the furnace is by utilizing a flow distributor to evenly distribute flow to the radiant tubes.  
     
     
         11 . The process of  claim 10 , wherein the transferring of the preheated hydrocarbon feedstock to the second section of the furnace is by utilizing a venturi meter to evenly distribute flow to the radiant tubes.  
     
     
         12 . The process of  claim 9 , wherein the preheated hydrocarbon feedstock is transferred from the first section to the second section of the furnace by radiant tubes having an internal diameter preferably in the range of about 0.75 inches to about 1.5 inches.  
     
     
         13 . The process of  claim 9 , wherein the preheated hydrocarbon feedstock is transferred from the first section to the second section of the furnace by radiant tubes having a length in the range of about 4 feet to about 12 feet.  
     
     
         14 . The process of  claim 13 , wherein the preheated hydrocarbon feedstock is transferred from the first section to the second section of the furnace by radiant tubes having a length preferably in the range of about 6 feet to about 10 feet.  
     
     
         15 . The process of  claim 1 , wherein the cracked molecules in the boundary layer and the uncracked molecules in the bulk fluid layer are mixed by utilizing internal fins.  
     
     
         16 . The process of  claim 15 , wherein the cracked molecules in the boundary layer and the uncracked molecules in the bulk fluid layer are mixed by utilizing internal fins that are spiral or circular.  
     
     
         17 . The process of  claim 15 , wherein the cracked molecules in the boundary layer and the uncracked molecules in the bulk fluid layer are mixed by utilizing internal fins that have a height in the range of about 0.05 inches to about 0.25 inches.  
     
     
         18 . The process of  claim 15 , wherein the cracked molecules in the boundary layer and the uncracked molecules in the bulk fluid layer are mixed by utilizing internal fins that have a height preferably in the range of about 0.06 inches to about 0.125 inches.  
     
     
         19 . The process of  claim 15 , wherein the cracked molecules in the boundary layer and the uncracked molecules in the bulk fluid layer are mixed by utilizing internal fins that are mounted within the radiant tubes of  claim 10  at a spacing between each internal fin of about 2 inches to about 10 inches.  
     
     
         20 . The process of  claim 19 , wherein the cracked molecules in the boundary layer and the uncracked molecules in the bulk fluid layer are mixed by utilizing internal fins that are mounted within the radiant tubes at a spacing between each internal fin preferably of about 3 inches to about 6 inches.  
     
     
         21 . The process of  claim 1 , wherein the cracked molecules in the boundary layer and the uncracked molecules in the bulk fluid layer are transferred, cracked, self-quenched, and halted within a residence time in the range of about 0.002 seconds to about 0.005 seconds.  
     
     
         22 . A process for thermally cracking a hydrocarbon feedstock, comprising the steps of: 
 (a) supplying a hydrocarbon feedstock to a furnace having at least a first and a second section;    (b) adding steam to the hydrocarbon feedstock    (c) preheating the hydrocarbon feedstock, the preheating occurring in the first section of the furnace, to produce a preheated hydrocarbon feedstock;    (d) heating the preheated hydrocarbon feedstock until at least some of the preheated hydrocarbon feedstock is cracked within a boundary layer at an inside surface of the second section of the furnace;    (e) heating the preheated hydrocarbon feedstock, until at least some of the preheated hydrocarbon feedstock is cracked to produce a boundary layer, which includes cracked molecules of the preheated hydrocarbon feedstock, and a bulk fluid layer, which includes uncracked molecules of the preheated hydrocarbon feedstock;    (f) mixing the cracked molecules from the boundary layer and the uncracked molecules from the bulk fluid layer;    (g) transferring at least some of the cracked molecules from the boundary layer into the bulk fluid layer and substantially instantly self-quenching and substantially halting further cracking in the cracked molecules and substantially simultaneously transferring at least some of the uncracked molecules from the bulk layer into the boundary layer and cracking at least some of the uncracked molecules as a result of the mixing of the cracked and uncracked molecules;    (h) removing substantially all the cracked and uncracked molecules from the furnace at an exit temperature of less than about 1250° F.; and    (i) cooling and removing heat from the cracked and uncracked molecules from the furnace to recover olefins, diolefins, and aromatics from the hydrocarbon feedstock.    
     
     
         23 . A furnace for thermally cracking a hydrocarbon feedstock, comprising: 
 (a) a first section for receiving and preheating the hydrocarbon feedstock to produce a preheated hydrocarbon feedstock;    (b) a flow distributor adapted to transfer the preheated hydrocarbon feedstock to a second section of the furnace;    (c) the second section for heating the preheated hydrocarbon feedstock until at least some of the preheated hydrocarbon feedstock is cracked within a boundary layer at an inside surface of the second section of the furnace; wherein the second section includes: 
 (d) a mixer for mixing at least some of the cracked molecules in the boundary layer and at least some of the uncracked molecules in the bulk fluid layer;  
 (e) a fluid distributor adapted to transfer at least some of the cracked molecules from the boundary layer into the bulk fluid layer and substantially simultaneously transfer at least some of the uncracked molecules from the bulk fluid layer into the boundary layer; and  
 (f) a conduit for removing substantially all the cracked and uncracked molecules from the furnace at an exit temperature of less than about 1250° F.  
   
     
     
         24 . The furnace of  claim 23 , further including a connector for adding steam to the hydrocarbon feedstock prior to preheating the hydrocarbon feedstock.  
     
     
         25 . The furnace of  claim 23 , wherein the flow distributor transfers the preheated hydrocarbon feedstock at a mass velocity in the range of about 1 lb/sec/ft 2  to about 5 lb/sec/ft 2 .  
     
     
         26 . The furnace of  claim 23 , wherein the flow distributor transfers the preheated hydrocarbon feedstock preferably at the mass velocity in the range of about 2 lb/sec/ft 2  to about 4 lb/sec/ft 2 .  
     
     
         27 . The furnace of  claim 23 , wherein the first section preheats the hydrocarbon feedstock to a temperature in the range of about 500° F. to about 900° F.  
     
     
         28 . The furnace of  claim 27 , wherein the first section preheats the hydrocarbon feedstock to a temperature preferably in the range of about 600° F. to about 800° F.  
     
     
         29 . The furnace of  claim 23 , wherein the second section heats the preheated hydrocarbon feedstock to a metal temperature in the range of about 2000° F. to about 2300° F.  
     
     
         30 . The furnace of  claim 29 , wherein the second section heats the preheated hydrocarbon feedstock to the metal temperature preferably in the range of about 2100° F. to about 2200° F.  
     
     
         31 . The furnace of  claim 23 , wherein the fluid distributor includes radiant tubes that have an internal diameter in the range of about 0.5 inches to about 2.5 inches.  
     
     
         32 . The furnace of  claim 31 , wherein the radiant tubes have an internal diameter preferably in the range of about 0.75 inches to about 1.5 inches.  
     
     
         33 . The furnace of  claim 31 , wherein the flow distributor evenly distributes the preheated hydrocarbon feedstock to the radiant tubes in the second section of the furnace.  
     
     
         34 . The furnace of  claim 31 , wherein the flow distributor evenly distributes the preheated hydrocarbon feedstock to the radiant tubes in the second section of the furnace by utilizing a venturi meter.  
     
     
         35 . The furnace of  claim 31 , wherein the radiant tubes have a length in the range of about 4 feet to about 12 feet.  
     
     
         36 . The furnace of  claim 35 , wherein the radiant tubes have a length preferably in the range of about 6 feet to about 10 feet.  
     
     
         37 . The furnace of  claim 23 , wherein the mixer includes internal fins.  
     
     
         38 . The furnace of  claim 37 , wherein the internal fins are spiral or circular.  
     
     
         39 . The furnace of  claim 38 , wherein the internal fins have a height in the range of about 0.05 inches to about 0.25 inches.  
     
     
         40 . The furnace of  claim 39 , wherein the internal fins have a height preferably in the range of about 0.06 inches to about 0.125 inches.  
     
     
         41 . The furnace of  claim 37 , wherein the internal fins are mounted within the radiant tubes of  claim 31  at a spacing between each internal fin in the range of about 2 inches to about 10 inches.  
     
     
         42 . The furnace of  claim 41 , wherein the internal fins are mounted within the radiant tubes at a spacing between each internal fin preferably in the range of about 3 inches to about 6 inches.  
     
     
         43 . The furnace of  claim 23 , wherein the fluid distributor transfers, cracks, self-quenches, and halts cracking within a residence time in the range of about 0.002 seconds to about 0.005 seconds.