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

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