UPGRADING THE C4 CUT FROM STEAM CRACKING - VIA THE HYDROGENATION OF BUTADIENE, THE ISOMERIZATION OF BUTENES AND METATHESIS INTO MTBE OR PROPYLENE

The evolution of the outlets for C-4 cuts from steam cracking shows qu ite contradictory results. On one hand, European and Asian petrochemis ts are more constrained to recycle this type of effluent, which contai ns butadiene and isobutene, to the steam cracker. Likewise, the demand for isobutene for MTBE production is such that it has to be produced by the dehydrogenation of isobutane. This situation is effectively cau sed by the surplus of butadiene, a by-product of ethylene, and for whi ch the demand is not increasing as fast as the demand for ethylene. To improve cracking performances during the recycling of the C-4 cut, bu tadiene is more and more selectively hydrogenated. Under these conditi ons, rather than cracking it, the new processes could make it possible to better upgrade it. Indeed, after selective hydrogenation, most of the cut can be transformed into MTBE by implementing the new n-butene isomerization process. Another upgrading scheme consits in transformin g isobutene into MTBE and n-butenes into propylene by metathesis with ethylene. These new route may find competition in the transformation o f butenes into alkylates, for automotive gasoline, with or without MTB E production. After a brief review of the different markets for the pr oducts involved, a technico-economic study serves to classify the diff erent upgrading routes of the C4 cut from steam cracking, by comparing both the minimum profitable selling prices for a discounting rate of 12% and the discounted cash-flow rate of return for fixed selling pric es. Calculations are performed within a context in which the C4 cut is devalorized in relation to naphtha with a price of $152/t. When butad iene no longer finds any outlets, and when its price is at a very low level such as $270/t, its extraction is hardly profitable, and propyle ne production by methatesis proves to be more advantageous. However, i f the steam cracker is integrated in a refinery, butenes may also be a dvantageously transformed into alkylates or MTBE, more especially to m eet a demand for octane or oxygenated products for fuels. These differ ent routes for upgrading MTBE are more advantageous than MTBE producti on from butanes, requiring heavy investments for the separation of but anes, the isomerization of n-butane and the dehydrogenation of isobuta ne. Likewise, the transformation of butenes into propylene is more pro fitable than obtaining propylene by the dehydrogenation of propane. A sensitivity study, for the price of butadiene and also for the price o f other products, enables isoprofitability curves to be plotted, which delimite price zones favorable to one or the other of the products co nsidered, taken two by two.