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Consider new processes for clean gasoline and olefins production

09.01.2011  |  Dharia, D.,  Stone & Webster Inc., A Shaw Goup Co., Houston, TexasLong, J.,  SINOPEC Research Institute of Petroleum Processing, Beijing, Peoples Republic of ChinaXu, Y. ,  Sinopec Research Institute of Petroleum Processing, Beijing, ChinaZhang, J.,  Sinopec Research Institute of Petroleum Processing, Beijing, ChinaBatachari, A.,  Shaw Energy & Chemicals Group, Houston, TexasYuan, E.,  Shaw Energy & Chemicals Group, Houston, TexasGim, S.,  Shaw Energy & Chemicals Group, Houston, TexasXu, S.,  Shaw Energy & Chemicals Group, Houston, Texas

Advanced technologies promote propylene yield while reducing olefins in gasoline

Keywords: [gasoline] [diesel] [fluid catalytic cracking] [propylene] [sulfur] [olefins] [China] [naphtha] [regulations]

Refiners must meet increasingly stringent specifications for cleaner gasoline, as shown in the gasoline standards defined by the Worldwide Fuel Charter (WWFC), a global agreement between the major motor manufacturers in the US, Europe and Japan.1 Fig. 1 shows the trend of WWFC standards toward progressive reductions of sulfur, olefins, benzene and aromatics. The high level WWFC categories of gasoline have been or soon will be, adopted by the gasoline standards of the US, Europe and many other countries.

  Fig. 1. Progressive reductions of sulfur,
  olefins, benzene and aromatics in gasoline.



For the gasoline components with similar molecule weights, isoparaffins, iso-olefins and aromatics provide the highest octane numbers. The gasoline specifications actually require more isoparaffins to compensate for the octane number losses due to reductions of olefins and aromatics mandated by newer regulations.

The fluid catalytic cracking (FCC) naphtha contributes high percentages of olefins and sulfur in gasoline pools. Hydrotreating is a common method to remove sulfur in FCC gasoline. However, the olefins are also saturated by hydrotreating, which causes significant octane loss. A desired approach is to improve the FCC to process and produce more isoparaffins with less olefins, sulfur and benzene in the FCC naphtha.

In addition to gasoline, the refinery FCC process also produces one-third of the global propylene supply. It is desirable for refiners to produce high-quality gasoline, while an FCC unit is shifted to higher propylene production. This article will discuss several new FCC technologies—maximizing isoparaffins, and clean gasoline and propylene, that can meet the increasingly stringent environmental regulations for cleaner gasoline and produce more propylene. These processes can produce much better quality gasoline than a conventional FCC unit, while almost doubling the propylene yield.

Two-zone riser.

Major chemical reactions in an FCC riser include cracking, hydrogen transfer, isomerization and alkylation of hydrocarbon molecules. The rates of various reactions change with the conversion depth of the feedstock while the hydrocarbons pass through the riser.

The cracking reactions are endothermic and will take place in the initial step, or the lower section of riser, where the feed and hot catalysts are in intimate contact. The other converting reactions (hydrogen transfer, alkylation and isomerization, etc.) will occur in the later step, or the middle or upper section of the riser. In the sequential reactions, olefins are produced from cracking reactions and then are consumed by subsequent secondary reactions, thus converting some olefins to paraffins or aromatics as shown in Fig. 2.

  Fig. 2. The two step sequential reactions to
  produce and consume olefins in an FCC riser.



However, those converting reactions in the second step are exothermic and are favored or accelerated under a temperature lower than the cracking temperature. The conventional FCC riser, which is essentially a straight pipe, does not have applicable means to provide the two distinct temperature zones, i.e., the high temperature zone for cracking reactions and the low temperature zone for converting reactions.

Based on the two-step reaction mechanism, an innovative two-zone riser was developed in late 1990s to optimally accommodate the desired FCC reactions to reduce olefins in FCC gasoline.2 As shown in Fig. 3, the two zones are connected in series with a larger diameter Zone 2 on top of a smaller diameter Zone 1. Zone 1 is designed, similar to a conventional FCC riser, to operate at high temperature and short residence time, while Zone 2 operates at lower temperature and longer residence time. The riser temperature profile is controlled by a quench stream or recycled catalyst injected between the two zones, with the two zones operated at distinct environments that favor the endothermic and exothermic reactions sequentially.

  Fig. 3. Configuration of the two-zone riser.



The novel two-zone riser reactor was investigated and studied by extensive cold-modeling tests and CFD simulations to optimize its configurations, dimensions and fluidization conditions. Optimal design of the new riser reactor is a combination of a dilute pneumatic transport section in Zone 1 and a fast fluidized bed in Zone 2 to best suit for the FCC reaction mechanisms.

The Zone 1 operates at a dilute pneumatic transport regime and is just like a conventional FCC riser, with high temperature, large catalyst-to-oil ratio and short residence time. The high temperature and large catalyst-to-oil ratio increase the cracking reactions to convert heavy oils into intermediate components including olefins, while the short residence time prevents excessive secondary cracking reactions. The higher conversion in Zone 1 can also improve the gasoline octane number by decreasing the n-paraffins and cyclo-paraffins content that depress octane numbers.

Zone 2 is a fast fluidized bed, different from that in conventional FCC risers. The temperature of Zone 2 is lower, while the residence time is longer than in Zone 1. These operating conditions favor reactions to convert olefins and other intermediates into isoparaffins and aromatics. Commercial applications have shown that the two-zone risers can maintain all the benefits of modern FCC technology with high conversions, and at the same time reduce nonselective cracking reactions to decrease dry gas and coke make.

The chemical reaction mechanisms on the reduction of olefins, sulfur and benzene in the two-zone riser are:3

• Reduction of olefins in the gasoline is mainly achieved because of the double-molecule reactions involving olefin molecules in Zone 2, form isoparaffins and aromatics.

• Sulfur compounds (mainly mercaptans and thiophenes) in FCC gasoline are formed from cracking or regrouping of the sulfur compounds in the feedstock. Zone 2 enhances hydrogen transfer reactions to favorably convert mercaptans and thiophenes into either gas sulfur (H2S) or coke sulfur, thus lowering sulfur content in the gasoline. The H2S would be less likely to react with olefins to form gasoline sulfur since the concentration of gasoline olefins are much lower in Zone 2 than in conventional riser.

• Alkylation reaction between benzene and olefins in Zone 2 effectively reduces both benzene and olefins in gasoline, while more alkyl benzenes are produced.

Isoparaffin technology.

The first commercial application of the new isoparaffins technology was in 2002 for a revamp of an FCC unit at Sinopec’s Gaoqiao Petrochemical Branch Co. (Gaoqiao) in Shanghai, China. Since then, to meet market demand and maximize clean gasoline production, 17 isoparaffin units have been installed in China (14 revamps of FCC and three grassroots application), with the unit capacities ranging from 0.44 to 2.8 million tpy. Industrial applications have shown that isoparaffin units have significantly improved the product yields and qualities, with overall operation costs similar to a conventional FCC.4

Similar to a conventional FCC unit (FCCU), an isoparaffin unit consists of a riser reactor, a catalyst regenerator and gas plant to separate the reactor effluent into liquefied petroleum gas (LPG), gasoline and other products. The unique features include:
• A two-zone riser (Fig. 3), with the two zones operated at different conditions to favorably promote chemical reactions that improve the quality and yield of gasoline
• A quench stream or recycled catalyst is injected into the second zone, to lower the temperature and increase the catalyst-to-oil ratio in Zone 2
• Optional proprietary catalyst enhances the converting reactions while cracking heavy feeds.

This process can provide several advantages over conventional FCC:
• Cleaner gasoline
• Reduces content of olefins (20%–50%), sulfur (20%–40%) and benzene (up to 33%)
• Improved octane numbers
• Higher yield of gasoline
• Increase isobutane (feedstock for alkylation), up to 40% in LPG
• Higher total liquid yield and less dry gas and slurry.

Operation and performance data from Sinopec’s Gaoqiao FCCU before and after the isoparaffin revamp are listed in Table 1. The results show that isoparaffin technology has reduced the olefin content by 20 vol%, benzene content by 30 vol% and sulfur by 44 vol% in FCC gasoline, with a slight improvement in octane numbers. In addition, this process increased gasoline yield by 5.1 % over FCC, while dry gas and coke were significantly reduced. The isoparaffins in the gasoline were 34 vol% higher.

 



Clean gasoline and olefin process.

The two-zone riser configuration has added superior controllability to the FCC reactions, and it can dramatically increase propylene yield while improving the quality of gasoline. Compared with the isoparaffin process, the clean gasoline and olefin reactor is operated at higher temperature and longer residence time in Zone 1 of the riser, for deeper catalytic cracking reactions to increase the propylene yields. The recycled catalyst increases the catalyst-to-oil ratio to an even higher level in Zone 2, and some gasoline olefins can be selectively cracked further into propylene. At the same time, more gasoline olefins are converted to isoparaffins and aromatics in Zone 2, in the way similar to the isoparaffins process. It is the combination of catalyst formulation and specific operating conditions in the two-zone riser that produces both the higher yield of propylene and better quality gasoline than a conventional high-severity FCC.

The clean gasoline and olefin catalyst has an unique matrix and pore structures and acidity distributions to maintain proper activities, even after being coked in Zone 1 of the riser.3,5 To maximize the propylene yield, an active component with the MFI structure was mixed into the catalyst during its preparation. A metal element was also added to enhance cracking gasoline olefins to propylene. The new catalyst design can retain the active sites after passing Zone 1, and continue to promote hydrogen transfer and cracking reactions in Zone 2.

Since 2004, to meet the market demand for production of both clean gasoline and propylene, 18 clean gasoline and olefin units have been installed in China (15 revamps of FCC and three grassroots units), with capacities ranging from 0.5 to 3 million tpy.

The unique design features of clean gasoline and olefin are similar to those listed in the isoparaffin process with the exception that a proprietary catalyst and different operating conditions are required. Applications of the clean gasoline and olefin technology have shown several advantages over conventional FCC:
• Cleaner gasoline
• Reduction of olefins (20%–50%), sulfur (20%–40%) and benzene (up to 33%)
• Improved octane number
• Higher yield of propylene (up to 9 w% of feedstock) and LPG
• More isobutane (up to 40%) in LPG
• Less dry gas and slurry.

Table 2 lists performance test data of the clean gasoline and olefin unit at Sinopec’s Jiujiang Petrochemical Branch Co. Table 2 shows that the clean gasoline and olefin technology had reduced olefins and sulfur in gasoline by 67 vol% and 23 vol%, respectively, while propylene yield (based on the feedstock) increased by 2.7 wt%, compared to the residue FCC unit before revamp. The research octane number (RON) of gasoline was significantly improved.

 



Economic benefits.

At present, there are a total of 35 clean gasoline and olefin and isoparaffin units operating in China, with many more units in the design or construction phase. The total throughput of the units has reached more than 50 million tpy, accounting for about 50% of the total FCC capacity of China. Performance data from the operating gasoline and olefin and isoparaffin units showed that even with inferior feedstock qualities, the yields of dry gas, slurry and coke are reduced on average by more than 1.5 wt%, while cleaner gasoline and/or more propylene are produced.

On average, the olefins and sulfur content in gasoline were reduced 14.3 vol% and 28 vol%, respectively, while the RON and MON increased 0.4 and 1.2 points. Commercial data has also shown that gasoline and olefin and isoparaffin units consume less energy than conventional FCC units. This benefit is primarily attributed to the lower feed pre-heat temperature and less reaction heat required in the two-zone risers.

Fig. 4 shows one of the historical back-testing results of gasoline and olefin and isoparaffin unit profitability over conventional FCC. It is clear that both gasoline and olefin and isoparaffin units would always be more profitable than conventional FCC, although the profit margins appear to fluctuate with prices of feedstock and products. The profit margins of gasoline and olefin unit would be more sensitive to market demand, especially with regard to propylene prices.

  Fig. 4. Historical back-testing of profitability
  over conventional FCC. (Based on US Gulf
  Coast price and 1.4 million tpy unit.)




 

Continuing development.

Dry gas and coke are the FCC byproducts with low added value. The yields of these byproducts can be significantly decreased by using the flexibility of the two-zone riser operations. The same level of catalytic cracking conversion can be reached either by a higher temperature with a lower catalyst-to-oil ratio, or by a lower temperature with a higher catalyst-to-oil ratio. Experiments have shown that the temperatures and catalyst-to-oil ratios in the two zones of the riser can be readily optimized and controlled to minimize dry gas and coke at the same conversion level as a conventional FCC.

Aromatics in the gasoline from the new process are in a range of 10 vol%–25 vol%, which is well below the allowable aromatics limits of 40% or 35% in the standards (Fig. 1). An effective way to improve the octane number is to shift some aromatics from light diesel to gasoline. Efforts are ongoing in researching new ways to convert light diesel into premium gasoline components. Light diesel contains a high percentage of mono-aromatic rings with long alkyl branches. These long alkyl branches can be catalytically broken from the aromatic rings in the two-zone riser to produce more premium gasoline components that are rich in isoparaffins and mono-aromatic rings with short alkyl branches. The gasoline can have a high octane number (RON > 100) and antiknock index, while benzene content is less than 0.5 wt%.

The olefins content in the gasoline from the new processes, is predominately iso-olefins, which is much lower than that in conventional FCC gasoline. For this reason, the octane loss from olefins saturation in the downstream desulfurization hydrotreatment will be much less than that seen for conventional FCC gasoline. Studies and experiments have shown that the octane loss of gasoline in hydrotreating is less than 0.2 RON, in comparison with the loss of 1.0 RON for conventional FCC gasoline in the same level of desulfurization severity.

Looking ahead.

The innovative two-zone riser reactor has been developed and applied for two new FCC technologies. Gasoline quality produced from the new gasoline and olefin and isoparaffin technologies has been dramatically improved by decreasing olefins, sulfur and benzene with an increase in gasoline octane number when compared to a conventional FCC. HP

LITERATURE CITED

1 Worldwide Fuel Charter, Fourth Ed., September 2006.
2 Long, J., Y. H. Xu, J. S. Zhang and M. Y. He, “Research and Development of Maximizing Iso-Paraffins (MIP),” Technology, Engineering Science, Vol. 1, No. 2, December 2003.
3 Gong, J. H., Y. H. Xu, C. G. Xie, J. Long, et al., “Development of MIP Technology and Its Proprietary Catalysts,” China Petroleum Processing and Petrochemical Technology, No. 2, June 2009.
4 Cheng, C. L. and Y. H. Xu, “The MIP Technology and Its Commercial Application,” China Petroleum Processing and Petrochemical Technology, No. 2, June 2009.
5 Qiu, Z. H., J. Long, H. P. Tian and W. Peng, “Development and Application of the CGP-2 Catalyst in the MIP-CGP Process,” China Petroleum Processing and Petrochemical Technology, No. 4, December 2007.

The authors 

 
  Dr. Jun Long is the president of SINOPEC Research Institute of Petroleum Processing (RIPP) in Beijing, China. He leads the company’s research and development activities for oil refining technologies and also serves as the supervising professor of RIPP’s doctoral students. Dr. Long has received more than 10 awards at the provincial and ministerial level and holds more than 140 patents. He has a PhD in applied chemistry from the China University of Petroleum.  

 
  Dr. Youhao Xu is the deputy director of the FCC Process Research Department of SINOPEC Research Institute of Petroleum Processing (RIPP) in Beijing, China. He has authored 60 papers and holds more than 150 patents in the FCC process. Dr. Xu holds an MS degree in chemical engineering from East China University of Science and Technology and a PhD in applied chemistry from RIPP.

 
  Prof. Jiushun Zhang is the deputy chief engineer for SINOPEC Research Institute of Petroleum Processing (RIPP) in Beijing, China. He holds 78 patents and has published 54 papers. He earned a BS degree in petrochemical engineering from Fushun Petroleum Institute and has 30 years of FCC experience.

 
  Dilip Dharia is program manager, High Olefins FCC, at Shaw in Houston with responsibility for all aspects of the High Olefin FCC program including technology development, business development and licensing of technologies worldwide. He also assists in business development and licensing of FCC Technology. A Shaw employee for more than 30 years, Mr. Dharia has worked on projects in the areas of refining, petrochemical and gas processing.  

 
 

Andy Batachari is vice president of Asia Sales for Shaw in Singapore. He has more than 30 years of experience with business development, sales, project management and engineering design. Mr. Batachari has broad-based skills in international oil refining, gas processing and the petrochemical EPC business. He has extensive international knowledge, having lived and worked in Japan, Korea, Taiwan, Thailand, Algeria, England, Germany, India, Malaysia, Singapore and China. Mr. Batachari first worked at Shaw as a process engineer and progressed through the ranks as a project manager, project director, and regional business development director. He holds an MS degree in mechanical engineering from Brooklyn Polytechnic. 


 
 

Ed Yuan is a senior technology specialist at Shaw in Houston. He has 37 years of experience in process design and technology development in petroleum refining. The majority of his experience has been related to FCC technology. He has a PhD in chemical engineering from Iowa State University.  


 
 

Steve Gim is manager, Financial and Technology Valuation, for Shaw’s refining team in Houston. With 20 years of experience in the energy and process industries, he helps refiners meet their techno-economic challenges in today’s environment. He previously held various management positions in Enron, Reliant Energy and SUEZ North America. Mr. Gim obtained a BS degree in chemical engineering from the University of Texas and an MBA from Rice University, He is also a Chartered Financial Analyst (CFA). 


 
 

Simon (Xiaomin) Xu is a separations specialist with Shaw in Houston. Previously, he spent 10 years with Koch Engineering in Texas and nine years with China University of Petroleum in Beijing, primarily involved in R&D, design and troubleshooting of distillation and process units. He earned both a bachelor’s and Ph.D. degrees in oil refining and chemical engineering from China University of Petroleum. 





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