Advances in a catalyst cooler technology
In this article reliability and flexibility of operation is assessed, and commercial experience of the technology in over 20 FCC units is discussed
Ting Y Chan and Dalip S Soni, ABB Lummus Global Inc
Zhang Fuyi, Luoyang Petrochemical Engineering Corporation/Sinopec
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In catalytic cracking of heavy oil, direct catalyst cooling is often employed to maintain regenerator temperature within desirable limits. High regenerator temperature (greater than approximately 1350°F) is detrimental to catalyst activity maintenance, yields and equipment life.
A new generation of catalyst cooler design has been developed and commercialised by China’s Luoyang Petrochemical Engineering Corporation (LPEC), a company organised within Sinopec that has been engaged for a number of years in the research, development and commercial application of catalyst coolers for fluid catalytic cracking of heavy oil. Its catalyst coolers have been in continuous operation for more than 10 years without failures.
The newest cooler technology has been commercially proven, with nine units in commercial operation and seven under construction. The main features which differentiate the technology are:
Pneumatic dense-phase control. The catalyst circulation rate is controlled by fluidisation and control air with a dense-phase catalyst return instead of by an expensive slide valve followed by a dilute-phase catalyst return.
Finned tubes. A special proprietary finned tube design was developed and commercialised to enhance the heat transfer. The heat flux is 50 to 80 per cent higher than that of the conventional bare tube. This allows the use of fewer tubes.
Large diameter tubes. To increase system reliability and reduce investment, LPEC developed the use of relatively large diameter steam generation tubes (from 273mm to 325mm in diameter). Steam generation tubes can be individually isolated, while in operation, for leak protection.
Natural circulation. Hot water is circulated by natural circulation, eliminating the expensive hot water high pressure circulation pump.
Integrated steam drum. A unique design in which the steam separation drum is integrated with the cooler in a single compact unit has been developed and commercialised. This lowers investment and reduces installation space requirement.
The design objectives guiding the development of the new catalyst cooler system were flexibility, simplicity, reliability and efficiency.
Flexibility. Different feedstocks (light or heavy), conversions, product slates, and changes in equilibrium catalyst activity result in different heat removal requirements. Thus, a modern catalyst cooler must have a design flexibility that allows for a wide range of adjustment in heat removal duty.
The new cooler can operate from zero to 100 per cent of design duty, and is designed so that it can be shut down while the FCCU is in operation. Full of settled catalysts, it can be restarted while the FCC is in operation, even after an extended (six months) outage.
Simplicity. A modern catalyst cooler must be simple in design, requiring fewer supporting systems, not only to save capital investment but to lower maintenance costs and increase system reliability. The LPEC catalyst cooler system is comprised of the catalyst flow control system and hot water circulation system. The design is simple and does not require an expensive supplementary air blower system, a specially designed high pressure water circulation pump, or a slide valve catalyst flow control system.
Reliability. The time between turn-around for an FCC unit is usually three years or longer, so the catalyst cooler must also be reliable. In addition to appropriate boiler feed water (BFW) quality and correct operation, sound engineering design and high-quality fabrication, including refractory lining, (proper construction, correct material selection and quality controlled installation, etc) play a critical role in ensuring reliability. The new cooler’s individual steam generation tubes further improve reliability because tubes can be isolated in the case of a leak.
Efficiency. High efficiency heat transfer contributes to both profitability and reliability. LPEC developed and commercialised a finned tube design that greatly increases the heat transfer of the fluidised bed and reduces the number of tubes required for a given duty.
Based upon these design objectives, the company developed and commercialised a new generation of FCC catalyst cooler designs.
Hot regenerated catalyst is withdrawn from the regenerator dense bed to the catalyst cooler. Upon entering the cooler, the dense-phase catalyst flows downward, exchanging heat with hot circulating water by partially vaporising the water in the evaporation tubes. The cooled catalyst is returned to the regenerator by a dense-phase return line which can be designed as a central internal return line (Figure 1) or as an external return line (Figure 2) (on following page).
The heat duty is adjusted by three operating parameters:
1.Varying the fluidisation air flow rate in the shell side to adjust the density of the down-flowing dense-phase catalyst.
2.Varying the control air flow rate to adjust catalyst circulation rate. When the control air is fully shut off, the flow rate of the catalyst in the internal or external catalyst return becomes zero and the catalyst cooler is in complete, backmixed operation. In this mode of operation, the aeration air is used to adjust heat removal, but the heat flux is greatly reduced.
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