with the rapid development of China's economy, the demand for p-xylene as one of the most important basic organic chemical raw materials has shown a strong growth trend in the past five years. Due to the rapid development of downstream products (mainly PTA industry), the PX market demand will rise rapidly in the next few years. It is estimated that the demand will increase by an average of 24.9% and the annual consumption growth rate will reach 22.4%. It is estimated that in 21, the consumption of PX in PTA plant in China will reach 54-61Mt, and the construction of plant capacity lags far behind the growth of demand, and the gap between demand and output of PX in China will further widen.
The typical production method of p-xylene is to separate p-xylene from the thermodynamically balanced mixed xylene (C8A) generated by naphtha catalytic reforming by multistage cryogenic crystallization separation or molecular sieve simulated moving bed adsorption separation (adsorption separation for short). For the treatment of ortho-and meta-xylene and ethylbenzene, mixed xylene isomerization (isomerization for short) technology is often adopted to isomerize them into para-xylene. Toluene disproportionation and transalkylation technology are effective ways to make full use of the cheap toluene and c9 aromatics/C1 aromatics (C9A/C1A) in industry to convert them into mixed xylene and benzene. For aromatics complex, more than 5% of mixed xylene is produced by this technology, which is the main means to increase the production of p-xylene in industry. Selective disproportionation of toluene is a new way to produce p-xylene. In recent years, with the continuous improvement of catalyst performance, the process has made great progress. With the continuous improvement of ethylene production capacity, the total amount of toluene will show an upward trend, thus making the process have a good market prospect.
this paper summarizes the progress of these two technical routes for increasing p-xylene production in recent years, and puts forward the technical development trend in this field.
1 toluene disproportionation and transalkylation process technology
1.1 Typical production process flow
The traditional toluene disproportionation process flow is the Tatoray process with hydrogen fixed bed jointly developed by UOP Company of the United States and TORAY Company of Japan in the late 196s. Shanghai Research Institute of Petrochemical Technology (SRIPT) has been developing this technology for more than 3 years, and the developed S-TDT process was industrialized in 1997. Compared with Tatoray process, S-TDT process allows C1 heavy aromatics to be contained in the raw materials, and HAT toluene disproportionation catalyst with international leading level is used, so the energy consumption and material consumption of the plant are low, which makes the process have excellent technical and economic indicators.
The brief process flow of S-TDT toluene disproportionation process is as follows: C9A raw material containing toluene and C1 heavy aromatic hydrocarbon is mixed with circulating hydrogen, and then heated to the required reaction temperature by a heating furnace after heat exchange at the inlet and outlet of the reactor, and then enters a fixed-bed adiabatic reactor, where benzene and mixed xylene are generated under the action of a catalyst. The reaction effluent passes through the heat exchanger at the inlet and outlet of the reactor, then is cooled and enters the high-pressure separation tank, and the separated aromatic liquid enters the downstream fractionation unit. Part of the separated gas is discharged, and most of the gas enters the circulating hydrogen compressor after being mixed with supplementary hydrogen, and is used as circulating hydrogen after being pressurized.
1.2 research and development progress of toluene disproportionation and transalkylation technology
1.2.1 TA toluene disproportionation catalyst and Tatoray technology
Tatoray toluene disproportionation and transalkylation technology was jointly developed by UOP Company of the United States and TORAY Company of Japan. Since the technology was industrialized in 1969, it has been stable in operation, long in operation period and advanced in technical and economic indicators. At present, there are more than 5 devices in the world. TA-4 catalyst was used in this process in 199s, and TA-5 catalyst has been applied in industry since 1997. At present, TA-4 and TA-5 catalysts are mainly used in foreign Tatoray process.
UOP company has recently developed a new generation of TA-2 catalyst for metal hydrodealkylation. Due to the function of metal hydrocracking, the heavy aromatic hydrocarbon treatment capacity of the catalyst is improved, and the mixed feed with toluene content of 3% can be processed, and the raw material is allowed to contain alkane content of 1%. Compared with the original TA-4 and TA-5 catalysts, the long-term stability of TA-2 catalyst has also been improved.
1.2.2 HAT series toluene disproportionation catalyst and S-TDT technology
In order to meet the needs of capacity expansion and transformation of aromatic hydrocarbon combined plant without changing the reactor and compressor, SRIPT researched and developed HAT series toluene disproportionation and transalkylation catalysts, among which HAT-95, HAT-96, HAT-97 catalyst has been successfully applied to toluene disproportionation plants with a domestic scale of 1.3-12.3 Mt/a since 1996, and S-
TDT toluene disproportionation technology and catalyst with HAT catalyst as the core technology have been exported to Iran. Table 1 lists the main performance indexes of the industrialized HAT catalyst. As can be seen from Table 1, from the catalyst HAT-95 to the catalyst HAT-97, the treatment capacity of the catalyst is greatly increased, while the hydrogen-hydrocarbon ratio is getting lower and lower. The existing device can achieve the purpose of capacity expansion only by replacing the catalyst without replacing the compressor. At the same time, because the allowable C1A content in the reaction feed is getting higher and higher, the disproportionation device can process more and more heavy aromatics, which effectively improves the output of benzene and mixed xylene and improves the economic benefit of the device.
compared with similar foreign industrial catalysts, the aromatic hydrocarbon treatment capacity of hat catalyst has been greatly increased, and the industrial operation results show that its comprehensive performance has reached the international advanced level. The HAT-99 catalyst which has been researched and developed takes C1A as the third reaction raw material, allowing the mass fraction of C1A in C9A raw material to reach 25%-3%. The successful research and development of HAT-99 catalyst will effectively improve the utilization rate of heavy aromatics, thus greatly increasing the production of mixed xylene and p-xylene.
In recent years, toluene disproportionation unit is required to be able to process high-content C9A raw materials, so as to produce more C8A and meet the demand of p-xylene capacity expansion. SRIPT studied the disproportionation and transalkylation of toluene and C9A catalyzed by macroporous β zeolite. The experimental results of MXT-1 catalyst showed that the mass fraction of C9A in the reaction feed was as high as 5%, the total molar conversion was over 46% and the molar ratio of C8A aromatic hydrocarbon to benzene was over 3.7 at high space velocity and low hydrogen-hydrocarbon ratio. Compared with HAT mordenite catalyst, MXT-1 catalyst has higher mixed xylene yield, and the industrial side-line test in disproportionation production plant has been completed.
1.2.3 MTDP-3 toluene disproportionation and transalkylation technology
MTDP-3 toluene disproportionation and transalkylation technology is a technology developed by Mobil company that can process a certain amount of C9A. Because ZSM-5 molecular sieve is used in this technology, the mass fraction of C9A in the reaction feed should not be higher than 25%. It is the competitive advantage of this technology to allow the operation under the condition of low molar ratio of hydrogen to hydrocarbon (less than or equal to 3).
On the basis of MTDP-3 technology, in order to improve the capacity of processing C9A and part of C1A raw materials, Mobil Company and Taiwan Province China Petroleum Company (CPC) jointly developed the TransPlus process, which was first industrialized in Linyuan Petrochemical Plant in Taiwan Province, China in 1997. This technology uses a catalyst with good light-weight function of heavy aromatics, so that it can process raw materials containing a certain amount of C1A and C9A. It is said that the maximum allowable mass fraction of C1A in C9 raw materials can reach more than 25%, and the mass fraction of C9A in reaction mixed raw materials can reach more than 4%, but there is no industrial data reported so far. Typical operating conditions are: reaction temperature is 385-5℃, reaction pressure is 2.1-2.8MPa, mass space velocity of aromatic hydrocarbon is 2.5-3.6h-1, molar ratio of hydrogen to hydrocarbon is not more than 3, and the total conversion rate is 45%-5%.
1.2.4 Other process technologies
Xylene-Plus of p>Arco-IFP Company was industrialized in 1968, using rare earth Y zeolite, with low activity and selectivity of 28%-3% and 92.5% respectively; Because of the use of moving bed reactor, the catalyst needs to be continuously regenerated, which consumes a lot of energy. Toluene and C9A can be used as raw materials. C allowed in raw materials? The content of 9A is low, so far there are only four sets of industrialized devices in the world.
The T2BX process of p>Cosden Company was industrialized in 1985, with a high operating pressure (4.1MPa) and a conversion rate of 44%. Mordenite was used as the catalyst, and toluene and C9A aromatics could be used as the reaction raw materials. There is no new report in recent years.
2 Technology of preparing high concentration p-xylene by shape-selective disproportionation of toluene
2.1 Overview
Shape-selective catalysis can effectively inhibit side reactions, greatly improve the selectivity of target products, simplify the separation process, and greatly reduce energy consumption and investment, thus effectively improving the economic benefits of the plant. But the shape-selective disproportionation reaction of toluene can only be used for pure toluene raw materials.
In order to obtain high para-selectivity in shape-selective disproportionation of toluene, appropriate pore size of molecular sieve and external surface passivation are very important. Passivation of the outer surface of molecular sieve crystal aims to make paraxylene rapidly diffuse out of the pore channels of molecular sieve, so that isomerization reaction will no longer occur on the outer surface of molecular sieve, and mixed xylene with thermodynamic equilibrium can be generated.
Up to now, the patent reports on the selective disproportionation of toluene with ZSM-5 molecular sieve mostly come from Mobil company, and a few of them involve ZSM-11 molecular sieve with similar pore structure to ZSM-5 molecular sieve.
2.2 Technologies developed abroad
2.2.1 MSTDP and PXMAX toluene shape-selective disproportionation technology
The first industrialized toluene shape-selective disproportionation technology was the MSTDP process with in-situ modification technology introduced by Mobil Company in 1988. The MSTDP plant was successfully operated in EniChem Refinery in Gela City, Italy. The technical indexes of its industrialization are: toluene conversion rate is 25%-3%, para-selectivity is 85%-9%, and the molar ratio of benzene to xylene in the reaction product is 1.44.
in p>1996, the company introduced ex-situ modified PX-MAX technology, with a selectivity of over 9% for p-xylene and a toluene conversion rate of about 3%. Compared with MSTDP technology, the molar ratio of benzene to xylene in the reaction product by PXMAX technology is reduced, so more p-xylene can be obtained.
2.2.2 PX-PLUS toluene shape-selective disproportionation technology
In 1997, UOP introduced the PX-PLUS process, which was said to be superior to the MSTDP process. The main indexes are: toluene conversion rate is 3%, para-selectivity is 9%, the molar ratio of benzene to xylene in the reaction product is 1.37, and the yield of p-xylene is about 41% (calculated by converted toluene). In 1998, the first device was industrialized.
UOP company thinks that this technology has a good complementary effect when combined with the aromatic hydrocarbon combined plant which uses molecular sieve to adsorb and separate crude p-xylene. After simple crystallization and separation, the mixed xylene with high concentration of p-xylene produced by PX-PLUS technology can obtain high-purity p-xylene products. The mass fraction of p-xylene in the residual liquid is still above 4%, which is much higher than that in the common mixed xylene, and can directly enter the adsorption separation unit.
2.3 domestically developed technology
domestic research in this field started in the early 199s, and RIPP completed the industrial side-line test of 1L catalyst in 1999. The main results are as follows: the conversion of toluene is over 3% and the para-selectivity is over 9%, but the molar ratio of benzene to xylene is about 1.6.
in 1997, Sript studied the catalyst for selective disproportionation of toluene with high yield of p-xylene, and achieved good results. The results of laboratory research show that the conversion and para-selectivity of toluene are 3% and 9% respectively, and the molar ratio of benzene to xylene in the reaction product reaches 1.4. At present, the expansion test of the catalyst has been completed, and the industrial side-line test is being prepared.
3 Dealkylation process technology of heavy aromatics
With the increase of refining capacity, the scale and quantity of aromatics production facilities such as continuous reforming also increase, which accelerates the development of dealkylation process of heavy aromatics. Mixed xylene is produced by hydrodealkylation of C9A and above aromatics, which can effectively reduce the plant scale and make full use of all heavy aromatics resources. The reported technologies in this field abroad include Toray TAC9 technology of UOP company, ATA technology of ZEOLYST company and GT-TransAlk technology of GTC company.
3.1 Toray TAC9 technology for producing mixed xylene from heavy aromatics
Toray TAC9 process is a technology for selectively converting C9-C1 aromatics to produce mixed xylene. Since C1A is also completely used to produce mixed xylene, this technology can obtain additional mixed xylene products from heavy aromatics. Like Tatoray technology, TorayTAC9 process also uses the fixed-bed reaction technology in the presence of hydrogen to prevent coking. The main hydrogen consumption comes from dealkylation of chiral aromatic hydrocarbons and cracking of non-aromatic hydrocarbons. In order to ensure a high yield of mixed xylene, benzene and toluene produced by the reaction are separated by the deheptane tower and returned to the reactor feed.
The mixed xylene yield of this technology is affected by three aspects: the total ratio of methyl to phenyl, the distribution of isomers of C9A and C1A, and the value of C9/C1A in the feed. For pure C9A feed, the yield of mixed xylene is about 75%, and the yield of light fraction is about 21%. With the increase of C1A content in feed, the yield of mixed xylene decreased.
This technology was first applied in industry in 1996, and the catalyst had good stability. The first operation period was more than two years. By 1998, this technology had been used in two plants, and the plant scale reached 85 kt/a..
3.2 dealkylation and transalkylation technology of ZEOLYST/sk heavy aromatic hydrocarbon
this technology is developed by zeolyst company and Korea.