Fundamental Studies On The Technology Of Fluid Catalytic Cracking With Two-Stage Risers For Maximizing Propylene

Propylene is one of the major raw materials used in organic synthesis, whose global consumption significantly is upgrading, recently. Steam cracking with naphtha, which is the conventional process for producing propylene, is falling into a limited development in our country because of the shortness of naphtha with high quality. As heavy oil is relatively abundance and cheap, the FCC process using heavy oil as feedstock for maximizing propylene is gaining more and more attention now.On the basis of the TSRFCC technology and basic studies on reaction rules and catalyst of converting heavy oil, C4 mixture gas and FCC light gasoline to propylene, a new technology, named TMP, for maximizing propylene and producing light oil with good quality is proposed, in which heavy oil, C4 mixture gas and FCC light gasoline are all catalytically cracked over the catalyst specially designed.The catalytic cracking is a decarbonization process of heavy oil with low hydrogen content to light petroleum products with high hydrogen content. In this process, hydrogen is redistributed by the rule that the product is lighter and it gains more hydrogen. Taking the hydrogen balance into account, the yield of dry gas that the hydrogen content is more than 20% should be debased in order to provide enough hydrogen for propylene and light oil.The catalyst designed for selectively converting heavy oil to propylene should not contain more Y or USY zeolite but more ZSM-5 zeolite, otherwise the selectivity to propylene may decline. Then the catalyst with high ZSM-5 content and appropriate ability of converting heavy oil was developed and applied in commercial FCC unit. In order to yield propylene from heavy oil selectively, the temperature is higher than that of the conventional FCC process; the catalyst to oil ratio should be as large as possible and the resident time should not be too long.Large amount of butene and FCC gasoline with high olefin content, which are both perfect feeds for propylene, are also obtained while heavy oil is converted to propylene selectively over ZSM-5 catalyst. Most of olefin in FCC gasoline are C₅⁼ and C₆⁼, which belong to the light gasoline fraction below 80℃. So, in the TMP technology, butene and light FCC gasoline are recycled.The yield of propylene can reach 18.25% when C4 mixture gas is cracked over the ZSM-5 catalyst under 600℃and 0.32 s, which is higher than that over the Y catalyst. The yield of dry gas increases markedly as the resident time is prolonged. It is obviously that the ideal conditions for converting butene to propylene are high temperature, short contact time and catalyst with high ZSM-5 content. In addition, butene is converted to propylene via the dimerization-cracking mechanism and there are no distinct differences about yield and selectivity of propylene among of those isomers of butene. It is shown that, as butene, high temperature, short contact time and ZSM-5 catalyst are also benefit to light FCC gasoline selectively converting to propylene according to the experimental research.On the basis of basic studies as mentioned above, a new technology (TMP) with characteristics that is combined feeding, low temperature, large catalyst to oil ratio and suitable resident time is proposed for realizing maximizing propylene, producing light oil with high quality and restraining the production of dry gas. The ratio of catalyst to oil can be boosted without enhancing the outlet temperature of riser when combined feeding of C4 mixture gas/light gasoline with heavy oil is carried out. With large ratio of catalyst to oil, requirements of catalytic converting light feedstock and heavy oil to propylene can be both met. The yield of dry gas may decline without enhancing the outlet temperature. In addition, it can be achieved that decreasing the yield of dry gas and improving the selectivity to propylene for converting light feedstock with the shortened resident time because the reaction of light feedstock is inhibited by injecting heavy oil. Experimental results showed that the yield of propylene was more than doubled while the yield of dry gas was almost changeless when recycle oil was combined feeding with C4 mixture gas or light FCC gasoline.The experiment simulating the TMP technology with the diesel oil separation from oil vapor of the first stage riser was carried out on the experimental unit using the FS-AR feedstock and the LTB-2 catalyst, under the milder reaction conditions. The results showed that yields of LPG and propylene were 41.7% and 21.22%, respectively, and yields of gasoline and diesel oil were 14.78% and 18.98%, respectively. If combined feeding of C4 mixture gas/light gasoline was used in the second stage riser, the yield of propylene may reach more than 25%. In October 2006, the TMP technology was tested in Daqing refinery. It was shown that the yield of propylene was 15.81%, the yield of total desired products were 83.41% and the yield of dry gas + coke + loss were only 13.32% when the real conditions did not meet the designed value, which showed the advantages of the TMP technology in maximizing FCC propylene...