The profitability of an ethylene plant depends very much on upgrading the coproducts. Upgrading the raw C, stream is becoming a great challenge and the question which arises is how to reconcile the butadiene surplus and the isobutene shortage. Significant changes in the world C, markets are starting to generate contradictions. Western European and Far East countries are being forced to destroy by Co-cracking increasing amounts of crude C, streams containing butadiene and isobutene. Simultaneously, demand is very strong for MTBE owing to the growth of unleaded gasoline and the provisions of the Clean Air Act. While Co-cracking has offered a short term solution, it is now being replaced by selective hydrogenation of butadiene which saves the isobutene for MTBE production and produces n-butenes. N-butenes are then converted to isobutene by skeletal isomerization for boosting MTBE production. Another way to upgrade the n-butenes is the metathesis to propylene. This process increases the propylene/ethylene ratio of the steam-cracker while avoiding a decrease of ethylene production These three technologies are presented in this paper.
INTRO DUCTION The profitability of an ethylene plant depends not only on the price ratio of ethylene to feed but also on upgrading of the coproducts. Most long range worldwide forecasts predict that the ethylene and propylene demands will grow more rapidly than the butadiene demand (Table I).
Since butadiene is produced as co-product of ethylene and propylene in naphtha cracking, with only, to 1995, a 2.3% a year increase in butadiene consumption and propylene and ethylene consumption are set to grow by 4.5 and 3.7% a year respectively, it can be predicted further increases in butadiene surpluses, especially in countries where the steamcracker feedstocks are mainly naphtha, chiefly in Europe and Asia. Therefore upgrading the raw C4 stream will become a great challenge and the question which now arises is how to reconcile the butadiene surplus and the isobutene shortage. These significant changes in the world C, markets are start- TABLE I World olefin demand annual average growth, YO per year 1986-1990 1990-1995 1995-2005 Ethylene 5.6 3.7 3.1 Butadiene 4.4 2.3 2.3 Propylene 5.9 4.5 3.9 ing to generate contradictions. Western European and Far East countries are being forced to destroy by Co-cracking increasing amounts of crude C, streams containing butadiene and isobutene.
Simultaneously the demand will be very strong for MTBE owing to the growth of unleaded gasoline and the provision of the Clean-Air Act. This trend has already led to the construction of big units for production of isobutene by isobutane dehydrogenation. On the other hand some ethylene producers, who are r