May-2021

Increased propylene from heavier residues

A specially developed catalyst enables a refiner to raise propylene selectivity from heavier feeds at maximum feed rate.

KUNIAKI MANABE, Taiyo Oil Company
CAREL POUWELS, PAO SHENG Wang and TOMOCHIKA KOTANI, Albemarle Catalysts

Article Summary

Although the Covid pandemic has diminished global demand for refined products, especially in transportation fuels in Asia and the Middle East, regional demand for light olefins has continued to grow. Propylene, a building block for the petrochemicals sector, is one chemical gaining strong traction in demand. Propylene is typically produced utilising steam cracking and fluid catalytic cracking. The heart of gasoline producing refineries, the FCC unit, has demonstrated versatility in producing widely variable yield distributions, flexibility in handling a wide range of feed sources, and operating reliably with on-stream efficiency in consistently producing on-spec products while maximising profitability. This article describes the successful introduction of Albemarle’s state of the art residue-to-propylene FCC catalyst technology to Taiyo Oil’s operations in Japan.

Taiyo Oil
Taiyo Oil Company, an integrated oil company in Japan’s Ehime prefecture, produces high quality, environmentally friendly petroleum and petrochemical products from its Shikoku and Yamaguchi operations. The company has two crude oil distillation units of 138 000 b/d capacity and one vacuum distillation unit with a capacity of 30 000 b/d. The company has worked actively to upgrade, expand, and rationalise its operations to respond to different demands for petroleum. While Taiyo Oil bears the responsibility of maintaining supply and thus maintaining the lifestyles communities are accustomed to, as a handler of petroleum and petrochemical products, the company is subject to critical appraisal by communities with regard to safety and environmental issues.

Taiyo aims to become a company that achieves sustainable management with the establishment of an ‘integrated operating system’ that oversees activities ranging from the importation of oil to the manufacturing and sales of petroleum and petrochemical products. Both the Shikoku and Yamaguchi Operations have been certified for ISO 9001 (Quality Management System) and ISO 14001 (Environmental Management System) and operate with consideration for quality assurance, quality maintenance, and environmental preservation.

Designed by UOP, the FCC unit at the Shikoku Oil Refining complex operates RxCat technology and thus is one of the most modern of FCC units with the capability to produce high volumes of propylene and promote conversion. Last year, Taiyo celebrated 10 years of commercial operation of its residue-processing FCC unit. Since the FCC unit’s commercial start-up in November 2010, the operation has progressed towards greater efficiency with revamps, more challenging process conditions, and operations against the limitations of the process, thereby pushing the unit to enhanced output.

Operations strategy
Taiyo Oil’s FCC feed is 100% untreated residue, meaning that feed quality fluctuates more than usual in terms of Conradson carbon residue and in comparison to feeds hydrotreated specifically with respect to the content of contaminant metals. In response to the feed’s characteristics, the Taiyo Oil team developed a specialised operations strategy wherein catalyst addition rate is flexibly adjusted based on feed metal content rather than on Ecat added metal content. This helps achieve more stable performance.

The refinery team monitors FCC feed qualities and operational situations to ensure that the unit is always operated to maximise profit and provide efficient operating conditions. This is accomplished by maximising reactor temperature, maximising catalyst circulation, and optimising the RxCat system. Typically, the FCC is constrained by the regenerator temperature and LPG downstream flow rate. In 2019, Taiyo decided to take on the challenge of processing heavier feed to enhance unit profitability within the existing unit’s limitations. This meant that FCC unit delta coke and LPG olefins selectivity needed to be improved to overcome the unit constraints. Even before the introduction of heavier feed, however, the regenerator dense temperature could reach as high as 760°C. As such, the main challenges were to maintain maximum catalyst cooler duty condition and to reduce regenerator temperature without sacrificing operational severity once heavier feed had been introduced.

In short, Taiyo’s strategy is to improve profitability by processing a heavier resid feedstock, which is more difficult to crack. In addition, this feedstock contains more metals like iron, and propylene selectivity is improved.

Typical solutions by operations
Quite a few options are available to handle the challenges of processing heavier feedstocks, given there are no limitations in operations. Operators can choose to process a heavier feed and then allow the regenerator temperature to increase or run the cat cooler at higher duty, provided more air blower capacity is available. However, these actions will potentially lower conversion.

In the case of Taiyo’s FCC unit, the goal to process a heavier feed became much more challenging as the unit is limited by regenerator temperature, cat cooler duty, and air blower capacity. Reduction of throughput is one solution; however, this option is typically not economically desirable.

While a heavier feed is more difficult to crack, it is also associated with an increase in deleterious metals such as nickel and vanadium. In Taiyo’s case, the worst impacts come from iron. While nickel and vanadium enhance dehydrogenation activity and increase hydrogen and coke, iron can have a dramatic impact on conversion and slurry yields. Moreover, their interactions with the catalyst can hurt the morphology of the particles. In addition to loss in the accessibility of catalyst particles, this can lead to fluidisation problems. If a FCC unit is diffusion limited or close to a diffusion limitation, an increase in iron can be detrimental to operational reliability and performance.

Apart from catalyst design solutions, the typical approach to mitigate iron poisoning is by utilising a catalyst management system. As Taiyo has a wealth of experience with strong variations in iron content, the company has developed a sophisticated catalyst management tool2 to diminish the potential for iron poisoning. In analysing the FCC feed as soon as possible, the catalyst addition rate can be quickly adjusted accordingly. This tool enables Taiyo to control catalyst activity as well as unit conversion and slurry yields, resulting in more stable operation.

The third challenge involves maximisation of propylene. Commonly, this can be approached by increasing the reactor outlet temperature and, where possible, through changing feed preheat to maximise catalyst to oil ratio. While a reduction in feed rate can also help increase propylene yield, this is not often applied. At maximum throughput and maximum reactor temperature, the highest propylene volume is typically obtained.