Process Optimization Of Two-stage Riser Catalytic Cracking Of Inferior Feedstock

Fluid catalytic cracking(FCC) process plays a pivotal role in converting heavy oil into light products in China. As crude oil is getting heavier and more inferior, the feedstocks of FCC units become more inferior and complicated. In the conventional process, feedstocks with different compositions and cracking performance are simply blended, and then react in one reactor under the same operating conditions. However, it is difficult for this process to optimize the operating conditions, even leading to competing reactions among different feeds and deterioration of the product distribution. Therefore, the key question for cracking inferior feedstocks efficiently is how to control appropriate reaction conditions and reaction depth for feedstocks with different reaction performance. Meanwhile, FCC units provide approximately 75% gasoline pool of vehicle fuels in China, but the olefin content of FCC gasoline is usually as high as 40–60 vol %. Facing the increasingly strict environmental regulations on gasoline, how to upgrade FCC gasoline efficiently is another technological problem of FCC process.To solve the problem that the inferior feedstock is difficult to convert, this work first took coker gas oil(CGO) as a research object, using three processing schemes to enhance the catalytic cracking of CGO. Then, the effects of processing parameters on the reactions of CGO cracking, such as thermal cracking and hydrogen transfer reactions, on the sulfur and nitrogen distributions, on the conversion chemistry of sulfur- and nitrogen-containing species, on the conversion of saturates, aromatics, resins, and asphaltenes(SARA), and on the variation of acid amounts of coked catalysts were analyzed. Furthermore, the compositional and structural identification of nitrogen compounds in cracked heavy oils was carried out by electrospray ionization(ESI) Fourier transform ion cyclotron resonance mass spectrometry(FT-ICR MS). The results show that the optimal operating conditions for CGO cracking were high reaction temperature and large catalyst-to-oil ratio with a short residence time. This operation could change the reaction pathways of nitrogen-containing species, restrain the adsorption and coking of nitrogen compounds on the catalyst surface, thus more acid sites were available for the cracking of nitrogen-free hydrocarbons, while the nitrogen compounds were enriched in the cracked heavy oil. The synergistic process by selectively recycling a certain amount of light FCC naphatha(LCN) from the upper position of the riser reactor could provide appropriate conditions for CGO cracking and gasoline reformation, realize the coupling of the two process and control the reactions of nitrogen compounds. It could increase the feed conversion and the yield of desired product, as well as upgrade gasoline at a higher efficiency.To minimize the naphtha loss during the upgrading process and increase the olefin conversion, several parameters, such as the properties of LCNs, the residence time, and the reactor structure, were investigated in a pilot-scale riser FCC apparatus. The results indicate that a certain amount of fractions with the higher boiling range could increase the reforming efficiency. Moreover, a relatively short residence time was beneficial for efficiently upgrading LCNs. Additionally, the influence of the reactor structure should be brought to our attention. When a novel structural-changed reactor with multinozzle feed system was used, the significantly increased olefin conversion and decreased naphtha loss could be achieved. The calculation of hydrogen balance indicated that due to the decrease of dry gas and coke yields, more hydrogen in the feed could be distributed into the desired products. After that, the EMMS-based multi-scale 3D-CFD method combined with the species transport equations was used to simulate the gas-solid flow in novel reactors. The results show that the novel swirling gasoline nozzle designed by this work could increase the catalyst density of the mixing zone, enhance the uniform mixing of feeding gasoline with the oil gas and catalyst in the reactor, relieve the formation of secondary flow caused by the high-speed jet of gasoline nozzles, reduce the back-mixing degree of oil gas in the mixing zone, and shorten the mean residence time of gasoline in the reactor.Finally, on the basis of two-stage riser FCC process, the high efficient conversion of inferior feedstocks and upgrading of gasoline were achieved by the optimization of the feeding mode and the reaction conditions for different feedstocks, as well as the creative design of reactor.In order to solve the problems that blending coker gas oil(CGO) into conventional FCC feedstocks will lead to the significant decrease of conversion, the decline of product selectivity, and reprocessing gasoline in the FCC units always causes yield loss which cannot be ignored, a two-stage synergistic(TSS) process was proposed, which cracking CGO and conventional feedstocks in different reaction zones and combining the process of CGO cracking and gasoline upgrading in the second stage riser. The simulated experiments of the process show that the higher conversion and more desired products could be achieved, at the same time, the olefin content of gasoline decreased by 13.5 wt %.To solve the problems that when processing inferior feedstocks, the feed conversion would decreased, the contradiction between maximal diesel yield and increasing feed conversion, and the low olefin conversion of gasoline when upgraded by the conventional riser reactor, the two-stage riser-multiple functions(TSR-MF) FCC process was modified by the optimization of reaction conditions and the reactor structure. The results show that the synergistic process of cracking HCO and upgrading LCG in the second-stage riser could significantly enhance the conversion of HCO while reducing the olefin content of gasoline at less expense of gasoline yield. Furthermore, the novel structure riser reactor and gasoline feed nozzle could increase the catalyst density of the gasoline reaction zone, enhancing the contact of oil gas and catalyst. Thus, the significantly reduced olefin content of gasoline and increased feed conversion could be achieved. Due to the significant increase of HCO conversion, the fresh feedstock could be cracked under mild conditions for producing more diesel without negative effects on the feed conversion. Compared with the TSR FCC process, in the TSR-MF FCC process, the increased feed conversion, diesel and light oil yields could be achieved, as well as the olefin content of gasoline decreased by 17 wt %.