Published July 2003
The technology for producing olefins by thermal steam cracking appears to have reached a stage of maturity in which improvements in yield and product selectivity are becoming increasingly difficult. For the conventional steam cracking process, ethylene yields are improved by raising the cracking temperature and reducing residence time, i.e., increasing the cracking severity. These severe conditions, however, are constrained by the metallurgy of the cracking tubes and rapid coking tendency in the cracking coils. At present, the maximum skin temperature for cracking tubes made of Cr-Ni alloys is about 1100°C (2010°F).
In this report, we evaluate recent developments in cracking furnace design, including the use of ceramic materials for ultra-high temperature cracking. Silicon carbide ceramics can withstand temperatures up to 1400°C (2550°F), have high conductivity and low surface catalytic activity towards coke formation. We have developed a conceptual design and preliminary economics for a steam cracking process using ceramic furnaces, based on technology patented by IFP (Institut Français du Pétrole). Although the process is still in development stage, our analysis shows that the technology offers competitive economics when compared to conventional cracking in metal tubes.
An alternative route to improving the performance of steam cracking is to carry out the reaction in the presence of a catalyst. Catalytic cracking, particularly of liquid feeds such as naphtha and gasoil, can achieve higher olefin yields at moderate reaction conditions coupled with reduced energy consumption. This report evaluates the main technology developments in catalytic steam cracking using metal oxide and zeolite-based catalysts. A promising technology, although still in laboratory research stage, is Japan's AIST process using a lanthanum-modified ZSM-5 catalyst. When cracking light naphtha at 650°C (1200°F), the process achieves an overall ethylene-plus-propylene yield of about 70 wt% and provides significant potential savings in both investment and production costs.
