Valorisation de la coupe C4 de vapocraquage via l'hydrogénation du butadiène, l'isomérisation des butènes et la métathèse, en MTBE ou en propylène Upgrading the C4 Cut from Steam Cracking via the Hydrogenation of Butadiene, the Isomerization of Butenes and Metathesis Into Mtbe Or Propylene

Abstract

Le surplus de butadiène au niveau mondial contraint les pétrochimistes à recycler la coupe C4 au vapocraqueur. De plus en plus, le butadiène est hydrogéné avant de recraquer toute la coupe. Une fois hydrogénée, cette coupe peut être beaucoup mieux valorisée, soit en MTBE avec isomérisation des n-butènes (procédé ISO-4), soit en propylène et MTBE en utilisant le procédé META-4. L'étude technico-économique montre que cette dernière voie offre la meilleure rentabilité (TRI = 21,5 %). Si le vapocraqueur est intégré à une raffinerie, les butènes peuvent également être transformés en alkylats ou en MTBE, pour répondre à une demande en octane ou en oxygénés pour les carburants. Ces diverses voies de valorisation sont plus intéressantes que la production de MTBE à partir des butanes via la déshydrogénation de l'isobutane ou que la production du propylène par déshydrogénation du propane. Une étude de sensibilité aux différents prix des produits envisagés permet d'établir des courbes d'isorentabilités, délimitant des zones de prix favorables à l'un ou l'autre des produits, pris deux à deux. The evolution of the outlets for C4 cuts from steam cracking shows quite contradictory results. On one hand, European and Asian petrochemists are more constrained to recycle this type of effluent, which contains 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 caused by the surplus of butadiene, a by-product of ethylene, and for which the demand is not increasing as fast as the demand for ethylene. To improve cracking performances during the recycling of the C4 Cut, butadiene is more and more selectively hydrogenated. Under these conditions, 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 transforming isobutene into MTBE and n-butenes into propylene by metathesis with ethylene. These new route may find competition in the transformation of butenes into alkylates, for automotive gasoline, with or without MTBE production. After a brief review of the different markets for the products involved, a technico-economic study serves to classify the different 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 prices. Calculations are performed within a context in which the C4 cut is devalorized in relation to naphtha with a price of $152/t. When butadiene 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 propylene production by methatesis proves to be more advantageous. However, if the steam cracker is integrated in a refinery, butenes may also be advantageously transformed into alkylates or MTBE, more especially to meet a demand for octane or oxygenated products for fuels. These different routes for upgrading MTBE are more advantageous than MTBE production from butanes, requiring heavy investments for the separation of butanes, the isomerization of n-butane and the dehydrogenation of isobutane. Likewise, the transformation of butenes into propylene is more profitable than obtaining propylene by the dehydrogenation of proprane. A sensitivity study, for the price of butadiene and also for the price of other products, enables isoprofitability curves to be plotted, which delimite price zones favorable to one or the other of the products considered, taken two by two.