Present Development Status and Future Trend of Aromatics Technologies
Year:2019 ISSUE:2&3
COLUMN:ORGANICS
Click:217    DateTime:Jan.22,2019


By Hou Yuxuan, Song Qianqian, Sinopec Research Institute of Petroleum Processing


Aromatics are most basic raw materials in organic chemical industry. The output and the scale of benzene (B), toluene (T) and xylene (X) are only next to ethylene and propylene. They are extensively used in the production of synthetic materials such as fibers, resins and rubbers, as well as organic chemical intermediates and products. The global output in 2017 was 47.19 million tons for benzene, 26.54 million tons for toluene and 56.55 million tons for xylene. The rapid development of the polyester sector in China has boosted the demand of upstream raw material PX in recent years. PX has become a variety of aromatics with the biggest consumption. The consumption of PX was 22.87 million tons in 2017, the output was only 9.30 million tons and the import dependence was over 50%. 

   Aromatics mainly come from petroleum and coal tar. BTX coming from petroleum accounts for over 95% of the total BTX output. Catalytic reforming units and naphtha steam cracking units are major sources of aromatics. New technologies for expanding raw material sources such as the production of aromatics from methanol and the production of aromatics from biomass like cellulose are also in the process of active research and development. 

Combined processes for the production of aromatics

   The traditional commercial production of aromatics is mainly completed in large-scale combined units composed of typical processes such as catalytic reforming/steam cracking – aromatics extraction – disproportionation/trans-alkylation – isomerization – X rectification – adsorption stripping.

   1. Catalytic reforming and steam cracking 

   The catalytic reforming technology is the major process for the production of aromatics. Thirty-eight percent of benzene and 87% of xylene in the world come from catalytic reforming units. Typical processes mainly include the Aromizing continuous reforming process of Axens, the CCR Platforming process of UOP and the ultra-low pressure continuous reforming process and the reflux moving-bed continuous reforming process of Sinopec. The continuous reforming process technology is already well-developed. The research and development of the technology is mainly focused on the upgrading and updating of catalysts. 

   At the time of producing ethylene, propylene and butadiene, naphtha steam cracking units also co-produce great quantities of cracked gasoline with a BTX content of around 60%. The recovery of BTX from cracked gasoline usually uses a two-stage hydrogenation process route. BTX are stripped from C6-C8 cuts of hydrogenated cracked gasoline through aromatics extraction. Through further hydrogenation the reformed gasoline process of UOP can achieve the production of high-purity benzene through solvent-free extraction. The APU process of SK uses precious metal/molecular sieve catalysts. The technology used in Sinopec for increasing the output of BTX from heavy cracked gasoline uses C9+ cuts of cracked gasoline as feedstock and de-alkylation is achieved through using a fixed-bed process and a precious metal supported adhesive-free zeolite bi-functional catalyst. The technology has quite high C9+ conversion rate and stability. 

   With impacts from the rapid development of new energy vehicles, the demand growth of oil products in China is gradually slowed down. The demand growth of low-cost chemical raw materials such as BTX is however making a constant increase. Technologies for increasing the output of BTX chemical raw materials in oil refining will therefore gain more extensive applications in future.

2. Increase of PX output through aromatics conversion

   The toluene disproportionation/trans-alkylation technology uses toluene/benzene and C9+ aromatics as feedstock and converts them into xylene through trans-alkylation. 

   In foreign countries quite a few toluene disproportionation/trans-alkylation processes have achieved commercialization. Major ones include the Tatoray process of UOP, the TransPlus process and the MTDP-3 process developed by ExxonMobil. The Tatoray process is the commercialized technology used most often today. Its latest catalyst TA-20HP uses metal additives and the processing ability of heavy aromatics is therefore increased. The technology achieved commercialization in 2006. The TransPlus process can process 100% C9 aromatics feedstock. Its process flow is basically identical to the Tatoray process. The MTDP-3 process uses fixed-bed or radial-flow reactor. Catalyst used is ZSM-5 renewable catalyst. It can process pure toluene feedstock or up to 25% C9 mixed aromatics feedstock.

   The S-TDT process of Sinopec uses HAT series catalysts. Commercial test was made in 2013 to recently developed HAT-100 catalyst. The catalyst can help increase the output of xylene to the maximum. The benzene/C9+aromatics trans-alkylation technology that achieved commercial application in 2008 uses BAT-100 catalyst and benzene/C9+ aromatics feedstock to produce toluene and C8 aromatics. The technology can achieve the readjustment of resources and product structures between benzene, toluene, xylem and C9+ aromatics in combined units for the production of aromatics. 

   Trans-alkylation is an important component of combined units for the production of aromatics. It can play the role of increasing the output of xylene, readjusting the product structure of aromatics and improving the production flexibility of facilities. 

   At the time of increasing the output of PX, the xylene isomerzation technology can convert ethyl benzene (EB) into xylene through isomerization or generate benzene through de-ethylation. The Isomar process of UOP is a typical well-developed xylene isomerization process in foreign countries today. In the process xylene isomers are produced from C8 mixed aromatics. UOP has also developed EB de-alkylation catalyst I-500 to increase the xylene selectivity. The LPI process liquid-phase xylene isomerization technology of ExxonMobil uses low-temperature operation and the cost is therefore greatly reduced.

   Sinopec has developed EB conversion catalyst PAI-01 and de-ethylation aromatics isomerization catalyst LHC-01 and the production cost is therefore reduced. SKI series isomerization catalysts of Sinopec have always occupied a considerable market share in China. Sinopec proposed EB conversion catalyst RIC-200 in 2010 and the complete process technology was used in a 600 kt/a PX unit in 2013. 

   Isomerization processes are already well-developed. The research and development is focused on the development and modification of new molecular sieves, the increase of product yield and the improvement of catalyst stability so as to reduce the loss of xylene. 

   Toluene shape-selective disproportionation processes use pure toluene as feedstock to produce mixed xylene with a high concentration of PX. They have features of simple stripping operation and low energy consumption.

   The PxMax toluene disproportionation process of ExxonMobil achieved commercialization in 1997. The process uses EM-2300 catalyst and the PX selectivity is >96%. The PX single-pass conversion rate of SD-01 catalyst used by Sinopec is >90%. The catalyst first achieved commercial application in 2005. The complete toluene disproportionation process technology achieved commercial application in 2006. 

   As toluene shape-selective disproportionation processes generate quite a lot of benzene and feedstock can only be pure toluene, it is still unable to replace traditional disproportionation/trans-alkylation processes but can be used as a supplementary means for output increase. 

   3. Stripping of aromatics 

   Technologies for the stripping of aromatics include rectification, extraction, extractive distillation, adsorption and crystallization. Adsorbents and processes of the PX adsorption stripping technology have been monopolized by UOP and Axens (formerly IFP) for long years. The Hybrid Eluxyl process developed by IFP combines adsorption technology with crystallization technology on the basis of Eluxyl adsorption stripping and a higher PX recovery rate is therefore reached. The latest ADS-47 absorbent used in the Parex process of UOP can reduce pressure drop and use the capacity of facilities to the maximum. BP has developed a method for stripping PX and EB from C8 aromatics through pressure swing adsorption (PSA). Para-selective non-acidic hollow molecular sieve with MFI structure and adsorbent with a vapor-phase PX/EB adsorption volume > 0.01g/g are used. 

   Sinopec has developed second-generation RAX-3000 adsorbent on the basis of first-generation RAX series PX adsorbents and the adsorption volume is increased by over 8%. The complete process technology first achieved commercial application in 2013 in a combined unit for the production of aromatics with a PX capacity of 600 kt/a. 


Other technologies for the production of aromatics

   1. Aromatization of light hydrocarbons 

   In addition to catalytic reforming and steam cracking as traditional technologies, light hydrocarbons used as feedstock such as LPG, light olefins and reformate raffinate can also be converted into BTX through aromatization. There are two process routes on the basis of catalysts used. One process route uses modified ZSM-5 molecular sieve catalyst. Feedstock is mainly C2-C5 light hydrocarbons. The process route has advantages of simple process flow, no need of strict feedstock refining and low construction cost. The yield of aromatics can reach over 60%. The other process route uses Pt/KL alkali molecular sieve catalyst. It mainly processes C6-C7 paraffinic hydrocarbons. The yield of BTX is higher than the traditional reforming process, but requirements on feedstock refining are stringent. 

   In 2006 Shandong Qiwangda Group used the nano-catalytic light hydrocarbon aromatization technology developed by Dalian University of Technology and constructed the first light hydrocarbon aromatization unit in China. The LHTA-M moving-bed process and the LHTA-F fixed-bed process developed by Sinopec achieved commercial application respectively in 2011 and 2013. The technology developed by PetroChina for the production of high octane gasoline components through mixed C4 hydro-aromatization (LAG) uses the fixed-bed C4 aromatization process to convert C4 hydrocarbons into aromatics-enriched production raw materials. 

   2. Light-weighting of heavy aromatics 

   The light-weighting of heavy aromatics can also be used as an approach for increasing the output of aromatics. It is mainly achieved through direct de-alkylation, ring-opening cracking or trans-alkylation. The TransPlus process mentioned above can process certain amounts of C10 heavy aromatics. The latest-generation TransPlus 5 technology can process up to 100% C9+ feedstock and increase the output of BTX. The HAT-plus heavy aromatics light-weighting technology developed by Sinopec uses C10+ aromatics as feedstock and can achieve the light-weighting of heavy aromatics under mild conditions. 

   3. LCO technology 

   The total content of heavy aromatics in catalytic cracking light cycle oil (LCO) is as high as 80%. The LCO-X process of UOP uses LCO as feedstock to produce high-quality xylene and at the same time co-produce naphtha and super-clean diesel. 

   4. Methylation of toluene/benzene and methanol 

   The GT-TolAlk methylation technology jointly developed by GTC of the United States and IPCL of India uses fixed-bed reactor and special high-silicon zeolite catalyst. The mass fraction of PX in products can reach 85%. The EMTAM process of ExxonMobil uses fluidized-bed catalyst to convert methanol and toluene/benzene into PX and fine readjustments can be made to the ratio of methyl groups and aromatic rings. The 200 kt/a toluene methanol methylation unit completed commercial test in Yangzi Petrochemical Co., Ltd. in 2012 and passed acceptance at the end of 2017. CAS Dalian Institute of Chemical Physics and Shaanxi Coal Chemical Technology Engineering Center have jointly developed the technology for producing PX from methanol and toluene and coproducing low-carbon olefins (TMTA). The technology uses fluidized-bed special catalyst and has achieved commercial enlargement. In 2013 Shaanxi Coal Chemical Technology Engineering Center, CNOOC Huizhou Refining & Chemical Co., Ltd. and Sinopec Luoyang Engineering Co., Ltd. signed a cooperation agreement on the technical development of a 200 kt/a TMTA commercial demonstrative project. The project is making headway today. 

   5. Production of aromatics through methanol isomerization (MTA) 

   The MTA technology involves a series of complicated process steps such as hydrogen transfer, oligomerization, cyclization, dehydrogenation, alkylation and de-alkylation. Although some relevant patents have been released, there are yet no reports on its commercialization. Major catalyst for the production of toluene through isomerization is ZSM-5 with the variety developed by ExxonMobil as the lead. Tsinghua University has developed the fluidized-bed FMTA technology. The 30 kt/a methanol feedstock test unit constructed by China Huadian Corporation with the FMTA technology made successful wet commissioning in 2013 and methanol could be fully converted. CAS Shanxi Institute of Coal Chemistry has developed the MTA technology using a two-stage fixed-bed process. Catalyst used in the technology is MoHZSM-5 (ion exchange) molecular sieve and 100 t/a pilot test was completed in 2017. 

   6. Conversion of cellulose into BTX 

   University of Massachusetts of the United States has developed a production process for converting cellulose biomass into aromatic compounds. Anellotech of the United States has made the technology commercialized. VTT (Technical Research Center) of Finland has developed a process for converting ligno-cellulose biomass into BTX. Gasified ligno-cellulose used as syngas is taken to a Fischer-Tropsch synthesizer and an aromatization reactor to generate BTX.

   The development of expanding technologies for the production of aromatics will effectively solve the problem of raw material supply shortage, make an organic combination of coal chemistry, natural gas chemistry and petroleum chemistry and promote their balanced development. 

   Through developments for several decades, quite a few special aromatics technologies in China with independent intellectual property right have been formed in catalytic reforming, aromatics extraction, toluene disproportionation/de-alkylation, xylene isomerization and adsorption stripping. The demand of basic chemical raw materials will remain to be brisk in future. Technologies for the production of aromatics will still be core technologies with development priority. Accelerating the development of complete technologies for the production of aromatics with diversification of raw materials, flexible readjustment of product structures, environmental protection and independent intellectual property right will play a positive stimulating role to the development of the aromatics sector in China.