Steam catalytic cracking of naphtha over ZSM-5 zeolite for production of propene and ethene: Micro and macroscopic implications of the presence of steam

One option to produce more ethene and propene can be to crack naphtha type fractions in dedicated smaller FCC units. We present here the results obtained for high temperature steam catalytic cracking (SCC) of a representative naphtha product (n-heptane) with ZSM-5. It has been found that under those conditions the presence of steam produces an irreversible dealumination of the zeolite as well as a reversible deactivation due to the interaction of water with active sites with a negative effect on protolytic cracking. A kinetic decay model that takes into account the two phenomena has been developed. The apparent activation energy is lower in the presence of steam. It appears that whilst the presence of steam is vital when processing heavy feeds to achieve a better feed dispersion and a more effective catalytic cracking in conventional fluid catalytic cracking (FCC) units, in the case of steam catalytic cracking of naphtha (n-heptane) the presence of steam has a negative effect on the final performance of the catalyst. On the other hand, whilst steam does not modify ethene and propene selectivity, significantly decreases H2 and CH4 formation, as well as formation of potential coke precursors.

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► Steam causes an irreversible and a reversible loss of activity in cracking process.
► The apparent activation energy is lower with steam caused by a selective interaction.
► Catalyst deactivation can be reproduced by a model with two deactivation processes.
► High reaction temperatures strongly favour the yields of ethene and propene.
► Steam has an inhibiting effect on protolytic cracking and recracking.