Optimized process for the preparation of olefins by direct conversion of multiple hydrocarbons

A process for direct conversion of heavy hydrocarbon with a distillation range higher than 350° C. with steam to produce lower olefins mainly being ethylene and light aromatics, which comprises:

• (a) mixing and bringing into contact the heavy hydrocarbon feed with steam and a solid granular catalyst in the reaction zone of a piston flow reactor having a single lift pipe to proceed catalytic pyrolysis, thereby forming an oil gas, wherein the catalyst comprising AL2O3 and/or SiO2 and an active component selected from the group consisting of oxides of alkali metals, alkali earth metals, transition metals and mixtures thereof, and aluminum silicate modified with oxides of alkali or alkali earth metals;
• and wherein conditions in the reaction zone are:
• temperature 600-900° C, pressure 0.13-40 MPa (absolute), total steam/hydrocarbon ratio 0.1-1.0 wt/wt, total catalyst/oil ratio 5-100 wt/wt and the catalyst contact time 0.02-5 s;
• and
• (b) separating quickly the oil gas from the catalyst after reactions and quenching the oil gas, wherein the catalyst is recycled for reuse after regeneration;
• and the oil gas enters a fractionation and separation system to proceed the separation, wherein a product gas mainly containing ethylene, propylene, bytene, butadiene, the by-product ethane, and a liquid product rich in aromatics is obtained;
• the highly pure by-product ethane from the sepatation system and/or the gases containing ethane from other sources return to the pyrolysis reactor of the piston flow from the upstream inlet of the reactor and come into contact with the catalyst of higher temperatures and high activities, fast pyrolysis takes place at temperatures higher than 780° C. to produce ethylene and meanwhile, the catalyst cools down and deactivates, steam is introduced at the same time when ethane feed is introduced from the upstream inlet of the reactor;
• heavy hydrocarbon feed is introduced at position certain distances from downstream of the inlet for ethane, the hydrocarbon introduced later plays a role of quenching those introduced earlier, the reaction temperature at this moment is 680-800° C;
• the feeding positions of the ethane and the heavy hydrocarbon are determined as such that the residence time of ethane, which is difficult to pyrolyze, is long, while that of heavy hydrocarbon, which are easy to pyrolyze, is short, the differences in the residence times of ethane and heavy hydrocarbons in the reactor are 0.001-3 s.