You are currently viewing: Articles



Apr-2015

Tube alloy degradation in a steam cracking furnace

Understanding catalytic coke growth mechanisms to better predict and optimise furnace service times with tube alloy degradation

GLEN A HAY and GHONCHEH RASOULI, Virtual Materials Group, Inc.
TOSHIHARU MORISHITA and JINICHIRO USAMI, Mitsubishi Chemical Corporation
Viewed : 4118
Article Summary
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 VMGSim1 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 HK40.3 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.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

In order to bring the effect of tube coatings or feed inhibitors into the simulation model, the overall coke growth rate is calculated using Equation 2:

rCoke = rAsym * rAsymInhib* (1 + rCat * lThick * rCatInhib) (2)

where rAsymInhib is a reduction in asymptotic coke formation due to using inhibitors or coatings; and rCatInhib is a reduction in catalytic coke formation due to using inhibitors or coatings, which is a function of tube coating or feed inhibitor effects on the catalysed coke growth mechanism.
Although this study focused mostly on the changing content of tube alloy, the roughness of the tubes is also suspected of playing a role in the reduced service time seen in older tube coils. In addition, the metals deposit as oxide films and their effect is not as simple as direct weight percentages, as described here, and carburised layers are created where carbon can more easily intrude. These are two details of the service time performance over tube aging that could potentially be further refined if more performance data and analysis were available.
Current Rating :  4

Add your rating:



Your rate: 1 2 3 4 5