M. Genç, Tüpraş, Istanbul, Turkey; and B. Ö. KANTAR, M. F. ÖNEN and O. TUNCA, Tüpraş, Kocaeli, Turkey
Tüpras utilizes proprietary fluid catalytic cracking unitsa (FCCUs) at its Izmit and Izmir refineries. The Izmit unit was designed in 1968 and started up in 1972 with a side-by-side style reactor/regenerator. The riser disengager consisted of rough cut cyclones, and the regenerator was a bubbling-bed type. The original FCCU was designed for a capacity of 1,750 m3/d of vacuum gasoil (VGO) feed with heavy cycle oil (HCO) and naphtha recycle. The unit was originally designed to operate in a partial combustion mode.
The unit can process primarily VGO mixed with 15 vol%–50 vol% hydrocracker bottoms, heavy coker gasoil (HCGO), lube oil extracts, naphtha, vacuum residue and atmospheric residue. The total fresh feed capacity is 2,250 m3/d. The slurry settler bottoms stream is recycled to the riser via a separate feed nozzle, which was provided under a revamp project in March 2010. Other changes during the 2010 revamp included the bottom entry feed injection nozzle, a new spent catalyst distributor, a new air grid and new regenerator cyclones. The bubbling-bed regenerator in the unit operates at full combustion mode.
Tüpras wanted to optimize and maximize the production of high-value products, particularly propylene. It was planned to replace the internal parts of the reactor for maintenance reasons, and this presented an opportunity to apply updated process technology to the existing FCCU—when replacing equipment, it is usually cost-effective to replace them with improved designs. The existing equipment of the reactor regenerator section, fractionation section and the gas absorber section were utilized effectively to meet the desired product objectives. An engineering study of the revamp was conducted to determine the potential of the existing unit, the extent of improvement/modifications to the existing equipment that could be recommended, and the minimum level of investment required to achieve the objectives.
The revamp of the unit included the reactor, a regenerator design change, flue gas energy recovery and particulate removal from the flue gas. The reactor termination device was upgraded to a vortex disengager stripper (VDS) and all feed nozzles were elevated, while the regenerator remained the same bubbling-bed type. The main objective was to increase gasoline yield, achieve higher gasoline and dry gas selectivity, maximize propylene yield, obtain higher energy recovery from the flue gas and comply with best available technique (BAT) emissions. This article focuses on the expected and obtained results from the revamp of the unit.
RESULTS
Design changes and the effects on the yields. Prior to 2020, the reactor had a bottom entry feed nozzle, which was changed with elevated feed nozzles that have more efficient feed and catalyst contact, and decrease delta coke and dry gas yield. The feed injection point was moved from the turbulent bottom zone to a more uniform catalyst zone in the riser. Simply changing the nozzle types can lead to a 1 wt%–2 wt% increase in gasoline yields. However, multiple design changes were carried out in the reactor section, so the contribution of each change could not be measured individually. Droplet size, pressure drop, stages of atomization, spray pattern and the minimization of the utilities used for uniform spray are all significant for the design of the feed nozzles.
Hydrocarbon containment was also one of the design change objectives for the reactor. Yield selectivity can be increased obviously by higher containment levels in the primary separation device. After the hydrocarbon vapors are released to the reactor shell from the second-stage cyclones, thermal cracking occurs, and higher gas production and lower gasoline selectivity are observed.
Tüpras had a traditional Tee design as the riser termination device. This type of design offers no containment and can lead to post-riser overcracking. The new reactor has a chamber with single-stage cyclones connected to it. Hydrocarbon vapors exiting the stripper section and diplegs are retained in the chamber. Hydrocarbon containment is > 99% for this type of termination device. The increase in conversion caused by the switch from a Tee-type design to a vortex separation system can be almost 2 vol%. Either the microactivity test (MAT) results or the riser outlet temperature must be increased for vortex separation systems to boost the conversion. Even though thermal cracking promotes delta coke and dry gas yields, it also has an effect on gasoline yields. Therefore, the catalyst activity or riser outlet temperature must be more than the unit’s old operating parameters to compensate for the loss that may be observed in gasoline yields due to decreased thermal cracking.
A stripper design change is another parameter than can increase yield selectivity, and decrease delta coke and dry gas yield. Additive coke and cat-to-oil coke should be decreased by the internal change of the stripper section. Cat-to-oil coke contains the hydrocarbon moved from the reactor to the regenerator by catalyst circulation. Additive coke is formed by concarbon, nitrogen and aromatics in the feed. Stripper residence time, the surface area of the internals, hot stripping and low pressure are all important in efficient stripping. Additive coke and cat-to-oil coke compromise 8 wt% and 9.7 wt% of the total coke in Tüpras’ FCCUs, respectively. Due to the increase of the internal surface area, the amount of steam used for the stripping can be decreased, as well. There can be a change in hydrogen per coke amount of up to 1.5 wt%, and a total conversion benefit of up to 2 vol% can be achieved with an internal change from trays to packings.
While achieving high yields in the reactor section is a key objective, another main goal must also be maintaining the light ends to fuel gas in the gas concentration section. Getting the highest absorbtion and recovery in the first and second absorber is also important to achieve a recovery of ~98%. This is only possible by maintaining a minimum tray efficiency of 85%, enough overhead and intercooling capacities, as well as enough flow for the lean and sponge oils that are used for the absorber sections. Otherwise, liquefied petroleum gas (LPG) or propylene that is produced by ZSM-5 will be lost to fuel gas.
Yield and flue gas emissions effects. The change in gasoline yield was particularly notable—the increase in gasoline yield was almost 6 wt%, while the increase in light cycle oil (LCO) yield was around 2 wt%. There was a decrease in clarified oil (CLO) and coke yields of 1 wt% and 2 wt%, respectively. The difference between LPG yields before and after the revamp was almost 5 wt%. The amount of C2- slightly increased to 0.17%, which is low.
The other important parameter was the amount of 1,3-butadiene in the LPG product and absorber gas recovery: it decreased to 75% of its previous value due to the reduction in thermal cracking. Absorber gas recovery was almost 98% after the modification in the gas concentration section. The decrease in 1,3-butadiene content enabled the increase of the riser outlet temperatures, achieving higher conversions while maintaining the expected LPG quality. Getting high absorber gas recovery also affected propylene recovery and C3+ recovery from the absorber gas and prevented losing valuable products to fuel gas.
A research octane number (RON) increase of 0.4 resulted in an increase in the naphtha hydrotreating unitb performance and less delta octane loss in the gasoline product (FIG. 1). Riser outlet temperatures can be changed between 515°C and 550°C while keeping the highest temperature in the regenerator as 750°C, the thermal degradation value for the catalysts.
A new hybrid shell-and-tube cooler was added to the flue gas section to achieve 9.34 MMkcal/hr of heat removal and limit the flue gas temperature to 290°C to prevent any sulfur oxides (SOx) dewpoint corrosion. The flue gas cooler is a hybrid shell-and-tube design, and the flue gas was filtered to decrease the particulates down to < 10 mg/Nm3 at BAT levels. BAT limits are given in TABLE 1 for new and existing units. SOx, nitrogen oxides (NOx) and carbon monoxide (CO) emissions can be decreased by up to 90% by the use of a blend of different additives, especially in full-burn operation.
Takeaways. Shifting the yields to gasoline and propylene are now the main objectives for refinery FCCUs. Drastic changes cannot occur in the yield profile and selectivity by catalyst changes alone. Different additives and catalyst uses in combination with design changes can play an important role in shifting the yields from the bottoms to lighter ends. FCCU reactors and regenerators are the main equipment that enable refiners to proces heavier feed and achieve the highest margins. It was observed in the revamp project that high-yield profiles for gasoline and propylene were achieved by a combination of flexible design, and catalyst and additive selection. Without upgrading technology, it is not possible to maintain the highest expected yields with only catalyst or additive changes, although some improvement can be achieved. HP
NOTES
a UOP FCCUs
b Axens’ Prime G
Miray Genç is a Senior Process Specialist responsible for troubleshooting, process, operations, catalyst, additive changes, revamp and design projects of FCC/DCU/VSB/Prime G+/Merox units at the Tüpras refineries. Previously, she worked as a Process Superintendent for hydrocracker, SMR, SRU, VSB, DCU and treatment units, and also as a chemical engineer at the Turkish Council of Research and Development/Middle East Technical University (METU) collaboration. She holds an MS degree in polymer science and technology, and a BS degree in chemical engineering from METU.
Begüm Öztürk Kantar is a Process Superintendent responsible for FCC/DCU/Prime G+/Merox units at the Tüpras Izmit refinery. Her previous experience includes positions as a Process Chief Engineer responsible for FCC/Prime G+ units and a Process Engineer for CDUs and VDUs. She holds a BS degree in chemical and biological engineering from Koç University and continues an MS degree in software engineering in Bogaziçi University.
Mehmet Fatih Önen is an Operations Supervisor responsible for daily operations, including maintenance of units, optimizing production, troubleshooting, monitoring of FCC/LPG Merox/Caustic neutralization/CDU/VDU units at the Tüpras Izmit refinery. He holds MS and BS degrees in chemical engineering from Istanbul Technical University (ITU).
Ozan Tunca is a Process Engineer responsible for monitoring, troubleshooting and optimizing Prime G+ and FCCUs at the Tüpras Izmit refinery. Previously, he has worked as a Process Safety Engineer in the oil and gas and petrochemical industries. Tunca earned a BS degree in chemical engineering from Yildiz Technical University.