Abstract:
A device and method for processing decoke effluent to remove particulate matter and pollutant gases is provided, with particular concern for meeting ever more stringent environmental standards.

Description:
FIELD OF THE INVENTION 
     The invention pertains to a device and method for processing decoking effluent that efficiently minimizes nitrogen oxides, carbon monoxide, and particulate matter. 
     BACKGROUND OF THE INVENTION 
     Ethylene is the building block of the petrochemical industry. Cracking furnaces are the heart of an ethylene plant, producing hydrogen, methane, ethylene, propylene, butadiene, and other cracked gases. 
     A fundamental issue with thermal cracking of hydrocarbons is coke formation and deposition inside the radiant coils and also inside the inner tubes of the transfer line exchangers along the run. Coke formation results in the need to periodically decoke a furnace. Decoke is required when radiant coil inlet venturi pressure ratio (“VPR”) reaches the end of run limit (“EOR”) for uniform flow distribution through the coils or maximum tube metal temperature (“MTMT”) of the radiant coils at EOR is reached in any one of the radiant coils. 
     Decoke air together with dilution steam or medium pressure steam is often used during the decoking process to remove coke deposited in the radiant coils. During furnace decoke, two types of decoking process are traditionally applied; either decoke effluent from the last transfer line exchanger is directly routed to the bottom of the firebox for combustion of coke particles with combustion air, or decoke effluent from the last transfer line exchanger is directly routed to the decoke cyclone for separating coke particles from decoke air and steam. After coke removal, decoke effluent goes to atmosphere via a vent stack. Coke is collected at the bottom of the decoke cyclone. 
     However, emission requirements have become extremely stringent to comply with Environmental Protection Agency (“EPA”) and National Ambient Air Quality Standard (“NAAQS”) requirements. An ethylene plant cannot be built if the emission requirements are not satisfied. 
     The size of emitted particles is directly linked to their potential for causing health problems. Particles less than 10 microns (“PM 10 ”) pose the greatest problem for health as they can get deep into the lungs and reach the bloodstream. 
     Ethylene cracking furnaces also produce flue gas containing pollutants such as nitrogen oxides (NO and NO 2 ), carbon monoxide (CO) and particulate matter. The increasingly stringent environmental regulations in the United States and elsewhere require new control methods to minimize these atmospheric pollutants, including particulate matter with a diameter of 2.5 μm or less (“PM 2.5 ”). For example, recent requirements for projects along the United States gulf coast have required not more than 0.01 Lb/MM Btu (HHV basis) or 10 PPMV (dry basis) of NO x , 0.0025 Lb/MMBtu (HHV) of total PM 10 +PM 2.5 , and 0.012 Lb/MMBtu (HHV) of carbon monoxide. Such controls are needed during normal cracking mode and also during steam plus air decoke mode. 
     Based on current ultra-low NO x  burner technology, it is impossible to achieve the Lowest Achievable Emission Rate standard of 0.01 Lb/MMbtu (HHV) of NO x  in an ethylene cracking furnace stack. To achieve this standard, a selective catalytic reduction (“SCR”) unit integrated with an ammonia injection grid (“AIG”) must be installed in the convection section of the furnace. 
     Another furnace emission is the decoke effluent. Conventional decoking passes the effluent through a decoke separator then vents it to atmosphere. This approach probably cannot meet the standards for CO removal and also has limited ability to remove PM 2.5 . 
     An alternative process involving routing decoke effluent to the firebox also cannot achieve the requirements for complete particle removal. This is particularly true during furnace decoke. Further, using such a process in cracking furnaces integrated with an SCR unit in the convection section may foul the SCR catalyst. 
     Accordingly, it is desirable to provide a decoke unit for ethylene cracking furnaces that meet new and anticipated standards for the removal of nitrogen oxides, carbon monoxide, and particulate matter. It is further desirable that such a decoke unit be compatible with existing technology such as SCR units. 
     SUMMARY OF THE INVENTION 
     The invention comprises equipment for and a method of routing decoke effluent from the outlet of a furnace decoke motor operated valve (“MOV”) to a specially designed decoke system. The decoke system comprises a decoke cyclone unit integrated with a lock hopper and valves. The decoke cyclone acts to remove most coke particles from the decoke effluent into the lock hopper. The decoke effluent is preferably oriented to make a tangential entry into the decoke cyclone. Optionally, one or more additional cyclones may be used, in parallel or in series, to further eliminate coke particles from the decoke effluent. 
     A lock hopper is connected to the bottom of each decoke cyclone by one or more valves, such as knife gate valves or other type of valve suitable for solids handling at high temperatures that can provide an adequate seal to isolate the lock hopper from the decoke cyclones. This valve (or valves) between the decoke cyclone and lock hopper provide solids handling control to discharge the stream containing coke particles. Optionally, the valve or valves may have water wash or steam wash capabilities to periodically clean the valves to prevent coke particles from plugging them or restricting operations. 
     To provide fully automatic operations during decoking, the valves are preferably equipped with actuators or motor driven. During decoke mode, valves between the decoke cyclone and lock hopper must be open so that coke is collected in the lock hopper. During normal cracking mode, valves between the decoke cyclone and lock hopper are normally closed but can be opened occasionally for water washing purposes. During coke unloading during normal cracking mode, the valves between the decoke cyclone and lock hopper must be closed to avoid back flow from the firebox to decoke cyclone and then to the lock hopper, to avoid damage or injury in case the furnace firebox reaches its high pressure trip set point during operation. An interlock is preferably provided so that the valve located at the outlet of lock hopper can never be open if the valves between the decoke cyclone and lock hopper are not closed. 
     Secondary decoke effluent from the top gas outlet of the decoke cyclone (or the last cyclone, if multiple cyclones are used) is routed to a piping system at the bottom of the furnace firebox for full particle combustion, and to remove both CO and PM 2.5  successfully. This piping system is specifically designed to avoid interference with furnace burner operation and efficiency, and may be engineered to accommodate the specific size and flow needs of a particular plant design without departing from the spirit of the invention. 
     As discussed below, a computational fluid dynamics (“CFD”) modeling demonstrates both CO and PM 2.5  destruction using the design of the present invention. 
     In this design, no external injection or treatment is required for the decoke effluent prior to entering the decoke cyclone. Additionally, there is no water/steam injection to the decoke effluent line before entering the decoke cyclone. Normal operating temperatures and pressures range from 300 to 750° F. and 1 to 5 psig, respectively. 
     EXAMPLE 
     A conceptual study of coke combustion and CO destruction was done by CFD. The objectives of the study were to observe CO content changes in the firebox by calculating CO amounts at the firebox inlet and outlets, and to observe the fate of coke particles within the firebox, calculating the combustion and coke travel residence time within the firebox and any entrainment and coke escape into the convection section. Coke particles were approximated as carbon particles with carbon&#39;s physical properties and combustion characteristics. 
     The model used a typical particle size distribution. All radiant pyrolysis tubes were assumed to be uniform in temperature at around 1500° F. The reaction:
 
CO+½O 2 ⇄CO 2  
 
was treated as reversible, with an activation energy of 21,700 Btu/Lb·mol, and a pre-exponential factor of 2.0E9 for the forward rate coefficient.
 
     Result: 
     With a CO mole fraction at the decoke nozzle inlets of 0.00963, the CFD model indicates that the mole fraction of CO at the outlet will be 2.166·10 −8 , or effectively zero. Accordingly, CFD indicates that the present invention will combust 99.999% of CO. Additionally, coke particles are fully combusted by the time they reach approximately fifty percent of the firebox height. 
     Thus, the present invention provides a means of meeting newer emission standards during decoke operations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic side view of a cyclone and lock hopper combination of the present invention. 
         FIG. 2A  is a schematic bottom view of a furnace box of the present invention. 
         FIG. 2B  is a schematic side view of the manifold, risers, flow restrictors, and nozzles of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , cyclone  10  receives raw de-coke effluent via inlet pipe  14  to cyclone inlet  16 , which is preferably a tangential entry into cyclone  10 . Heavier coke particles settle in cyclone  10  into cyclone lower section  18 , where they are transferred via first lock valve  24  and second lock valve  26  into lock hopper  12 . When it is desirable to remove the accumulated coke from lock hopper  12 , lock hopper valve  28  may be opened and the accumulated coke removed. 
     Those of skill in the art will understand that one or more cyclones  10 , coupled with one or more lock hoppers  12 , may be used in any combination, only with regard to engineering need. The lighter effluent from such combinations may be used in this invention without departing from the spirit thereof. 
     Lighter effluent, including unwanted gases (CO &amp; NO x ) exit cyclone  10  through cyclone flue  20 , and into treated effluent header  22  to a furnace box ( 210  of  FIGS. 2A and 2B ). 
     Referring now to  FIGS. 2A and 2B , furnace box  210  receives treated effluent through inlet port  214 . Furnace box  210  comprises furnace fuel gas inlet  212 , providing fuel gas to multiple burners  216 . Treated effluent passes through inlet port  214  into manifold  218 , and into staggered risers  220 . Treated effluent passing through staggered risers  220  may pass through flow restrictors  222  (present on one or more of risers  220  for flow balance). Staggered risers  220  pass treated effluent to nozzles  224  for ejection into the furnace box  210 , without disruption of the furnace burners&#39; function. Innocuous final combustion gas exits as furnace flue gas into the convection section. 
     Those of skill in the art will recognize that the design of manifold  218 , risers  220 , flow restrictors  222 , and nozzles  224  may be altered to optimize conditions for a particular installation without departing from the spirit of the invention.