Technologies for increasing propylene production via the oil route

Thread Content

The petroleum route is the main source of propylene, and its development has progressed from hydrocarbon thermal cracking to shape-selective catalysis for high-propylene yield. This route initially used steam cracking technology, which involves the thermal cracking of naphtha to produce propylene. With the successful development of Y-type molecular sieves, the FCC technology using Y-type molecular sieve catalysts has seen progress ； Compared to thermal cracking, FCC can use heavy oil as a feedstock, which reduces the reaction temperature and increases the P/E ratio of the products. To further increase the yield of propylene, a shape-selective ZSM-5 molecular sieve was added to the conventional FCC catalyst. However, due to the relatively low reaction temperature in the FCC process for propylene production, although the P/E ratio is high owing to the presence of the catalyst, the propylene yield is not high. Recently, a catalytic thermal cracking technology coupled with steam cracking and FCC has been developed to further increase the propylene yield. With the growing demand for propylene, technologies that use catalysts to further convert low-carbon olefins produced in processes such as steam cracking, FCC, methyl tert-butyl ether, and methanol cracking into propylene have also seen rapid development. Thermodynamic analysis and experimental verification have shown that the use of SAPO-34 molecular sieves with shape-selective properties can effectively improve the selectivity for propylene. Ethylene/butylene disproportionation technology and propane dehydrogenation technology are also technologies developed to further increase propylene production. 1.1 Hydrocarbon Steam Cracking Steam cracking is a thermal cracking reaction of hydrocarbons carried out in a cracking furnace at 800–900°C, using naphtha, light diesel, ethane, etc. as raw materials. The reaction follows a radical mechanism, with ethylene as the main product and propylene as a by-product. In the steam cracking process, the feedstock and operating conditions have a significant impact on the propylene yield, while the P/E yield ratio increases as the relative molecular mass of the feedstock increases ； Reducing the operating severity can also increase the P/E yield ratio. Although using heavier feedstocks or reducing the operating severity can increase the propylene yield, the aforementioned methods will all reduce the ethylene yield to varying degrees. At the current stage, demand for ethylene is also high; it is clearly not practical to increase propylene production by reducing the yield of ethylene. Furthermore, due to limitations such as the design of the steam cracking unit and operating conditions, the potential to improve the P/E yield ratio using the aforementioned methods is very limited, and it cannot meet the rapidly growing demand for propylene in a fundamental manner. Moreover, globally, the newly added steam cracking units (mainly in the Middle East) use ethane as a feedstock and can only produce a small amount of propylene as a by-product. From the above analysis, it can be seen that the main purpose of the steam cracking unit is to meet the market demand for ethylene ； The potential to significantly increase the propylene yield through improvements to the steam cracking unit itself, in order to meet the growing market demand for propylene, is very limited. 1.2 FCC technology for high propylene production: Compared with steam cracking, the FCC technology using Y-type molecular sieves shifts the mechanism of hydrocarbon cracking from a radical mechanism to a carbocation mechanism, thereby effectively increasing the P/E yield ratio. The propylene yield in typical FCC units ranges from 3% to 6%, while FCC technologies that produce more propylene can achieve a maximum propylene yield of 18%. Compared with the traditional FCC process, the FCC process for higher propylene production focuses on improvements to the catalyst and reactor, enabling the selective cracking of low-carbon hydrocarbons in the gasoline fraction into propylene while maintaining the quality of other FCC products, thereby increasing propylene production. 1.2.1 Catalyst improvement: The catalysts used in FCC processes for high-propane production must not only possess good capabilities for converting heavy oils, resistance to heavy metal contamination, hydrothermal stability, and mechanical strength, but also require a high acid strength, moderate acid density, and high selectivity for propylene. There are a wide variety of FCC catalysts that produce propylene on both domestic and international markets, each with its own characteristics. However, they all share one common feature: ZSM-5 molecular sieves or modified ZSM-5 molecular sieves are incorporated into conventional FCC catalysts either through compounding or by adding additives. The ZSM-5 molecular sieve catalyst developed by Mobil is an MFI-type molecular sieve featuring two cross-linked pores formed by decagonal oxygen rings; it has a high silica-alumina ratio and weak hydrogen transfer capacity. Under FCC conditions, it can selectively catalyze the conversion of olefins or olefin precursors in gasoline fractions into C3-C5 olefins, thereby increasing propylene production while keeping the yields of dry gas, coke, and light diesel essentially unchanged. When 5% by mass of ZSM-5 molecular sieve is added to FCC catalysts, the increase in the yield of low-carbon olefins is equivalent to that resulting from an increase in reaction temperature by 150°C. By modifying the ZSM-5 molecular sieve (such as with phosphorus), the stability of the catalyst and its ability to increase propylene production can be further improved. 1.2.2 Process improvement: Depending on the desired product of the FCC process, the corresponding process conditions also vary. To produce more propylene, higher reaction temperatures and a higher catalyst-to-oil ratio, as well as a longer residence time, are required ； To produce gasoline with a low olefin content, lower reaction temperatures, a higher reagent-to-fuel ratio, and longer residence times are required; therefore, it is difficult to simultaneously achieve high production of low-carbon olefins and a reduced olefin content in gasoline within a single riser or downflow bed reactor. In addition to developing new catalysts and additives, seeking effective reactor design solutions is an important approach to addressing this issue. To meet the demand for high-propane yield in FCC processes, a range of new processes have been developed both domestically and internationally, such as the MIP-CGP high-propane yield process, the two-stage riser reactor high-propane yield process, the flexible multi-effect high-propane yield FCC process, the combined downflow bed and riser reactor process, ExxonMobil’s new dual-riser process, and UOP’s PetroFCC process. An analysis of these processes shows that although each process has its own unique characteristics, they all employ the basic principle of \"zoned reaction,\" aiming to carry out different types of reactions under the most suitable conditions in order to achieve multi-objective product formation. For example, in the industrial test of flexible downward-bed heavy oil catalytic cracking conducted by Sinopec Group and Tsinghua University at the Jinan Refinery, the propylene yield was nearly double that obtained in a riser reactor using the same feedstock, while the dry gas yield decreased. The concept of zoned reaction will be a direction for the future development of FCC processes to increase propylene production. 1.3 Technology for Producing Large Quantities of Propylene via Catalytic Thermal Cracking: Catalytic thermal cracking technology refers to a process in which the dual effects of thermal cracking and catalytic cracking are utilized to produce large amounts of propylene at reaction temperatures higher than those used in traditional FCC processes, but lower than those used in steam cracking. A range of catalytic thermal cracking processes and corresponding catalysts have been developed both domestically and internationally, achieving significant progress, with some of these processes already being put into industrial use. China’s Deep Catalytic Cracking (DCC) technology is the pioneering process in catalytic thermal cracking, and it was put into industrial use in 1990; to date, 7 units of this technology have been operational both domestically and internationally. Subsequently, a series of technologies have been developed both domestically and internationally, utilizing riser or downflow bed reactors at harsh reaction temperatures (above 550°C) and combining steam cracking with catalytic cracking to maximize the propylene yield; the catalysts used are all Y-type zeolite catalysts supplemented with ZSM-5 molecular sieves. Some catalytic thermal cracking processes have also been developed on the basis of steam cracking; for example, a naphtha cracking process developed in Japan to increase propylene production, which allows the P/E yield ratio to increase from 0.6 in traditional methods to 0.7. Compared with conventional thermal pyrolysis, this process uses ZSM-5 molecular sieve loaded with 10% lanthanum by mass as a catalyst in a fixed-bed reactor, at an operating temperature of 650°C. Although catalytic thermal cracking technology combines the effects of catalytic cracking and thermal cracking to increase propylene production, it has drawbacks as well; for example, the yield of ethylene is much lower than that obtained through steam cracking ； Due to operation at high temperatures, compared to catalytic cracking, **the yields of gasoline and diesel are reduced, and the gasoline and diesel produced contain excessive amounts of aromatics, making it difficult to meet the** standards for gasoline and diesel ； It increased the yield of by-products such as dry gas and coke, while reducing the overall yield of the target product. Therefore, although the propylene yield can exceed 20% using catalytic thermal cracking technology, the feasibility of a process cannot be determined by a single indicator. As presented at the 2007 U.S. Petroleum Refining Conference, taking into account current economic competitiveness and actual market data, the optimal yield for producing propylene in FCC units is 10%-12% ； Catalytic thermal cracking technology involving an increased reaction temperature was not proposed to maximize the propylene yield. 1.4 Technology for producing propylene via the cracking of low-carbon olefins: As crude oil processing capacity and ethylene production capacity continue to increase, refineries and ethylene plants are generating larger amounts of C4 and C4+ compounds that are rich in olefins ； Some C:olefins are also produced during the methanol/dimethyl ether cracking process. How to utilize this substantial amount of valuable resources to increase their added value has become an urgent task for refining and chemical companies aiming to improve their economic efficiency. Under the pressure to protect the environment, reducing the olefin content in FCC gasoline has also become another urgent task for refineries. In response to the current shortage of propylene and the need to convert C4–8 olefins into high-value products, a range of research efforts have been carried out both domestically and internationally to transform these low-carbon olefins into high-value propylene. Abroad, there are technologies such as the Olefin Conversion Process (MOI) developed by Mobil, the Propylur process developed by Lurgi, and the Superflex process developed by Arco ； In China, technologies for the conversion of low-carbon olefins have been developed by institutions such as the Beijing Research Institute of Chemical Technology under Sinopec, and the Shanghai Research Institute of Petrochemical Technology; while technologies for the conversion of butylene have been developed by the Dalian Institute of Chemical Physics under the Chinese Academy of Sciences and China University of Petroleum (Beijing). The aforementioned process of producing propylene via the cracking of low-carbon olefins typically uses ZSM-5 molecular sieves as catalysts, and is carried out in fixed-bed or fluidized-bed reactors at temperatures of 400–600°C and pressures of 0.1–0.2 MPa ； Hydrocarbon cracking follows a carbocation reaction mechanism; the cracking of butenes and pentenes occurs primarily through dimerization followed by cracking, whereas hexenes and hydrocarbons with higher carbon numbers undergo cracking directly. During the cracking of low-carbon olefins, hydrogen transfer reactions also occur simultaneously, converting the feedstock into stable alkanes, aromatics, or even coke, which reduces the selectivity and yield of propylene. Therefore, finding ways to limit the occurrence of these hydrogen transfer side reactions is an important focus for the development of catalysts and process improvements for the production of propylene from low-carbon olefins. Thermodynamic equilibrium also exists in the mutual conversion of lower hydrocarbons, and the yield of propylene is constrained by this thermodynamic equilibrium. Based on the fact that mesoporous SAPO-34 molecular sieves can significantly improve the propylene selectivity in methanol/dimethyl ether conversion, UOP and the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, explored the process of catalytic cracking of butene to produce propylene using SAPO-34 molecular sieve catalysts. The Department of Chemical Engineering at Tsinghua University conducted a detailed study on the catalytic conversion of butene, pentene, and hexene over SAPO-34 molecular sieve catalysts. Thermodynamic calculations and experimental results show that the use of SAPO-34 molecular sieves with pores as small as 0.43 nm and weak acidity on their outer surfaces enables the suppression of the formation of isobutylene and molecules larger than isobutylene, thereby significantly improving the selectivity for propylene during the catalytic cracking of 1-butene; under conditions of 500°C and 1.4 h-1, the propylene selectivity can reach 65% ； The cracking experiment results of pentene and hexene over the SAPO-34 molecular sieve catalyst also show that the use of this catalyst can significantly improve the selectivity for propylene. The process of producing propylene via the cracking of low-carbon olefins utilizes a wide range of raw materials. It can be combined with other propylene production methods aside from propane dehydrogenation techniques, such as steam cracking, FCC, and processes that produce propylene from methanol/dimethyl ether. Moreover, it has the lowest overall production costs; therefore, this process is highly attractive and is set to experience rapid development in the coming years. 1.5 Ethylene/Butene Disproportionation Technology The ethylene/butene disproportionation technology is a process that involves the disproportionation of ethylene and 2-butene to produce propylene. The catalysts used in this process are all metal catalysts, and there has been a gradual shift from heterogeneous catalysis to homogeneous catalysis. The currently industrialized technology is mainly OCT technology from ABBLummus Company, and IFP Company’s CCR-Meta-4 has also been demonstrated on an industrial scale. Other related process technologies include the ethylene/butylene disproportionation technology developed by BASF, Sasol, UOP, and Lyondell. The Dalian Institute of Chemical Physics, Chinese Academy of Sciences, and the Shanghai Research Institute of Petrochemical Technology, Sinopec Corporation, have also carried out research on the disproportionation of ethylene/butylene to produce propylene, making significant progress in this area. Although the ethylene/butylene disproportionation technology has been industrialized, it has high requirements for raw materials. High-purity ethylene and 2-butene are also scarce resources in China. Therefore, the process of using this disproportionation technology to consume some ethylene in order to increase propylene production is not feasible at present; in China, it can serve as a technical option for increasing propylene production. 1.6 Propane dehydrogenation technology Propane dehydrogenation technology is another method for obtaining propylene. The propane conversion rate is limited by the equilibrium conversion rate, with a typical one-pass conversion rate of 34%-40%. High temperature and low pressure are favorable for increasing the equilibrium conversion of propane, but at high temperatures side reactions increase, the selectivity for propylene decreases, and catalyst deactivation accelerates as well. The propane dehydrogenation technology is fairly mature; currently, there are 10 propane dehydrogenation units in operation worldwide, of which 8 use UOP’s Oleflex technology and 2 use ABBLummus’ Catofin process. The catalysts used are mainly Cr-based and Pt-based catalysts. Our country has also carried out a series of efforts in propane dehydrogenation, but no propane dehydrogenation production facilities have been established yet. Globally, the key to improvements in propane dehydrogenation technology in recent years has been to reduce costs and achieve economies of scale in production. At the same time, the economic viability of propane dehydrogenation technology depends to a large extent on the price difference between raw propane and the resulting propylene. Improvements in propane dehydrogenation technology, coupled with economies of scale in production, will further widen the price gap between propane and propylene, enhancing the economic viability of such facilities and making propane dehydrogenation technology an attractive method for increasing propylene production. China is relatively short of propane resources, especially wet natural gas fields with high propane content; therefore, the conditions for developing propane dehydrogenation processes in China are not yet ripe. However, our country has numerous catalytic cracking units, and most of these units are equipped with gas separation systems. By coupling the dehydrogenation of propane and butane with the catalytic cracking process, it is possible to increase the production of propylene; meanwhile, the cost of the reaction-separation process increases only slightly, while the yield of propylene from the catalytic cracking process doubles ； Moreover, oil field liquefied petroleum gas can be used as a raw material to increase propylene production. In summary, the feedstocks for the petroleum-based route to increasing propylene production have evolved from primarily naphtha to a combination of various feedstocks, such as heavy oil, naphtha, low-carbon olefins, as well as propane and butane ； The development of catalysts for increasing propylene production has also progressed from steam cracking technology that did not use catalysts, to FCC technology that employed Y-type molecular sieve catalysts, and further to selective catalysis technology that uses ZSM-5 or SAPO-34 molecular sieve catalysts. The use of a catalyst effectively reduced the reaction temperature and increased the yield and selectivity of propylene. The development of processes for increasing propylene production has mainly gone through a transition from high-temperature thermal cracking to medium-temperature selective catalytic cracking. The initial naphtha cracking was thermal cracking carried out at high temperatures, following a free radical reaction mechanism, with ethylene as the main product ； Moreover, at the steam cracking temperature (800–900°C), the Gibbs free energy per carbon atom of ethylene is lower than that of propylene; therefore, both from a reaction mechanism perspective and from a thermodynamic standpoint, it is not favorable for improving the selectivity for propylene. The FCC process that was developed later operates at temperatures between 400–600°C, during which the Gibbs free energy per carbon atom for propylene is lower than that of other olefins and alkanes ； Furthermore, due to the presence of the acidic catalyst, the cracking of hydrocarbons follows a carbocation reaction mechanism, with propylene being the main product; thus, both from the perspective of the reaction mechanism and thermodynamics, this operating range is favorable for improving the selectivity of propylene ； Furthermore, by using ZSM-5 or SAPO-34 molecular sieves with shape-selective properties, the aggregation of large molecular hydrocarbons in the system was effectively reduced, thereby further improving the selectivity for propylene. Therefore, the process of producing propylene through hydrocarbon cracking using shape-selective molecular sieve catalysts at lower temperatures (400–600°C) is currently the main approach for increasing propylene production via petroleum-based routes. This post was last edited by brucehan on 2009-1-5 13:24]