(140d) Assessing the Influence of Alloy Composition On Coke Formation in Steam Cracking Reactors

Authors: 
Muñoz G., A. E., Ghent University

Reducing coke deposition in the reactor (a.k.a. the radiant coil) is one of the major challenges in steam cracking. It is well known that the reactor material has a huge influence on the amount of coke deposited, and that, hence, selecting the proper reactor material may improve profit margin by millions of dollars each year. Therefore, in collaboration with a major ethylene producing company, a methodology to study the performance of alloys and coatings has been developed: the laboratory scale Jet Stirred Reactor (JSR) set-up. The JSR is used to assess the effect of different reactor materials for different feeds under simulated industrial  conditions, which can include the use of additives and different decoking strategies.

Via thermogravimetric analysis, the amount of coke deposited is measured over time, and initial (catalytic) and asymptotic (pyrolytic) coking rates are calculated. Rates are compared to those of a reference material (centricast 25Cr-35Ni micro alloy). A significant range of behavior is observed (see Fig. 1). The number of coking cycles has an impact on the coking rates. Experimental continuous coking-decoking cycles suggest that some alloys ?age? better than others. SEM and EDX analyses of the samples obtained after coking for three cycles were carried out with two different penetration depths, and have shown that metals are present in the coke layers, and that concentration gradients can be observed. Significant changes on the relative amounts of metals take place when comparing the surfaces of the coked and the blank samples.

 Figure 1: Asymptotic coking rates (normalized with Reference as basis)
for all tested materials over three continuous coking-decoking cycles

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