New Type of Cracking Furnace Radiant Coil Design
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This paper presents the economical benefits for a new radiant coil design which improves the performance of the thermal cracking furnace. The main drivers for this design are: increase of selectivity, capacity, run length, radiant coil lifetime or a combination of these. Cracking furnaces are the heart of the ethylene plant. These furnaces convert hydrocarbon feedstock into ethylene, propylene and other cracked gas products by thermal cracking in the presence of steam. Typical examples of hydrocarbon feeds are ethane, propane, butanes, naphtha's, kerosenes and atmospheric and vacuum gasoils. In modern cracking furnaces, the cracking coils are arranged vertically in one or more passes. Conventionally, ethylene cracking tubes are arranged in the firebox in one lane wherein the lane is heated from both sides by burners. A single lane is called an in-line arrangement whereby all the reactor tubes are arranged in the same vertical plane. Alternatively, the tubes can be placed in a staggered arrangement whereby the tubes are arranged in two vertical parallel planes with the tubes arranged in a triangular pitch towards each other. In a furnace with conventional geometry (in line or staggered geometry), the lanes are heated from both sides by burners located in the bottom and/or sidewall. The inlet sections and outlet sections are heated equally by the burners. It has been found that this leads to less-optimal cracking conditions because of a not so advantageous heat distribution. The cracking process is an endothermic process and requires the input of heat into the feed. For the performance (selectivity) of the cracking process it is desirable to maximize the heat input to the inlet section of the cracking coil. The run length of the furnace will improve if the heat distribution over the hotter outlet tubes improves. The design of the new radiant coil results into an optimization of the heat input into the cracking tubes. Different thermal stress and different thermal creep conditions exist between the inlet sections and the outlet sections as result of the differences in temperature and heatflux between the two sections. Creep is the irreversible expansion which occurs when heating a metal and is the result of thermal stresses inside the metal due to heating. Thermal stress (caused by thermal expansion) is the reversible phenomenon when heating any material. Both phenomena have to be taken care of in the design of the coil and cause the above mentioned restrictions in the cracking coil mechanical layout. This paper presents a new type of cracking coil where the outlet sections are thermally shielded by the inlet sections. This improves the heat distribution over the radiant coil and improves the coil performance. Simultaneously the lay-out improves the mechanical stability of the coil. Johan van der Eijk 14.10.2009