Greenluma Steam Cracker Furnace

In order to optimise service run length for a steam cracking furnace it is essential to understand the conditions surrounding and including the tube coil for that run. These running conditions, such as temperatures, pressures and steam dilution, allow coke growth trends to be predicted and minimised to ensure the most favourable plant operational economics. In order to understand and optimise trending of multiple service runs throughout the life span of the tube coils, different considerations must be taken into account. This article reviews a simulated case study using the software package VMGSim 1 to explain the mechanisms causing reduced run times over the lifespan of a tube coil at a Mitsubishi Chemical plant site in Kashima, Japan. Coke formation mechanisms Coke formation in an ethylene cracker reduces the tube cross section, the heat flux to the reacting gas mixture and yield; it increases pressure drop and consequently reduces service time.

Coke growth can happen through pyrolytic and catalytic mechanisms. Both mechanisms play an important part in the formation of coke within tube coils in a cracking furnace. At the early stages, coke formation mainly occurs through the catalytic mechanism. This type of coke growth is driven by the tube alloy itself when metal sites such as iron or nickel are contacted by process material and filamentous coke is produced. Detailed kinetic models of this can be developed by including surface reactions, segregation processes, and the diffusion of carbon through specific metal particles such as nickel. 2 Chromium content in the tubes can be used to inhibit the catalysing effects of tube metals and is often found in higher service temperature tube coil materials such as Inconel or HK40.

Cracking furnaces. Fuel gas/oil. To other furnaces. Dilution steam. Decoking effluent. Cracked gas from other furnaces. More pass inlets.

3 One must be careful regarding less obvious effects of trace components in the tube alloys such as silicon and aluminum and the interactions between iron, nickel and chromium content that make any direct correlations cumulatively incorrect (see Table 1). Over time, pyrolytic coke growth soon becomes the dominant mechanism within the remaining service time of the furnace tube coils. This mechanism is directly related to the concentration of components within the process material and the running pressure and temperature.

The simulation model developed and used for the analysis within VMGSim applied a molecular structure-type model for prediction of the coke growth rate profiles throughout the tube coil using the PIONA oil characterisation environment. 5,6 Coke formation from each type of molecular group is predicted and general kinetic rates could be derived from open literature using this generalised structure. 7,8,9 The classifications and groupings for kinetic rates in many of the papers available showed types of molecular structures from olefin to more dehydrogenated and ringed components were already recognised as different influences towards overall pyrolytic coke growth rates. A combined equation to determine coke growth is shown (see Equation 1) where the first term consists solely of pyrolytic coke formation and provides an asymptotic growth rate: rCoke = rAsym * (1 + rCat * lThick) (1) where rAsym is asymptotic coke growth due to pyrolytic coke formation, which is a function of the local temperature, pressure, and composition; rCat is catalytic rate of coke formation, which is a function of the tube alloy material; and lThick is thickness effect related to coke thickness, that is a function of the local coke thickness. Alloy degradation effects on service run times As coke builds on the inside of the tube coils, the added roughness, reduced internal diameter and heat flux resistance cause the inlet coil pressure and furnace box temperatures to increase to keep outlet product specifications constant.

Once a maximum tube coil temperature or pressure drop is reached, the inner tube coil must be cleaned. In this process of decoking, the tube coil metallurgy is affected and the metal content of the surface of the tube coil changes. Regular operation of the cracking furnace also alters the composition of the tube surface as iron, nickel, chromium and other elements can be found in coke formed within the coil during service time. 7 Possibilities of tube coating and feed inhibitors Tube coatings come in the form of aluminum, magnesium, zinc, and other metals and their associated oxides. These coatings are specifically good at reducing catalytic coke growth since they hide the iron and nickel sites that would typically catalyse the coke formation surface reactions.

10 Inhibitors used are commonly sulphur, phosphorous, aluminum, or silicon based and also focus on reducing the catalytic coke growth by passivating the metal surface. 11,12 Alloy degradation effects on service run times As coke builds on the inside of the tube coils, the added roughness, reduced internal diameter and heat flux resistance cause the inlet coil pressure and furnace box temperatures to increase to keep outlet product specifications constant. Once a maximum tube coil temperature or pressure drop is reached, the inner tube coil must be cleaned. In this process of decoking, the tube coil metallurgy is affected and the metal content of the surface of the tube coil changes. Regular operation of the cracking furnace also alters the composition of the tube surface as iron, nickel, chromium and other elements can be found in coke formed within the coil during service time. Bananarama venus zippy the pinhead. 7 Possibilities of tube coating and feed inhibitors Tube coatings come in the form of aluminum, magnesium, zinc, and other metals and their associated oxides. These coatings are specifically good at reducing catalytic coke growth since they hide the iron and nickel sites that would typically catalyse the coke formation surface reactions.10 Inhibitors used are commonly sulphur, phosphorous, aluminum, or silicon based and also focus on reducing the catalytic coke growth by passivating the metal surface.