Steam ring redesign improves FCC bottoms stability
Installing a newly designed steam ring in a main fractionator stabilised fluctuations in the flash point of FCC bottoms
AJAY KHANDE, AMIT KUNTE, DIPANKAR SAHA and PINTU MAITY
Bharat Petroleum Corporation
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The fluid catalytic cracking (FCC) process is colloquially named ‘the heart of the refinery’ on account of its flexibility and high selectivity towards gasoline. This major secondary conversion process converts low value high-boiling (450°C+), high molecular weight hydrocarbon fractions of petroleum crude oils like gas oils and fuel oils into premium marketable products such as gasoline, LPG, and other products.
A typical FCC process includes cracking heavy hydrocarbon feedstock to lighter hydrocarbon products in a riser reactor by contacting with hot regenerated catalyst followed by separating cracked product from spent catalyst: catalyst with coke, stripping the spent catalyst using steam in a stripper, regenerating the spent catalyst by burning coke deposited on the catalyst, and routing the hot regenerated catalyst to the riser to participate in the cracking reaction again.
The product vapours are separated in a main fractionator and a gas concentration unit into dry gas, LPG, cracked gasoline, light and heavy cycle oils, and bottoms. The gas concentration unit is a collection of compressor, absorbers, and distillation columns that separate the separated product fractions drawn from the main column.
Post fractionation and separation, the dry gas is used as fuel gas for refinery heating processes; LPG and gasoline are directly used as ready products. Light cycle oil (LCO) is used in limited quantity as a blending component in the diesel product pool. The limitation on blending of LCO is attributed to the fact that it is rich in aromatic hydrocarbons that negatively affect the diesel cetane rating. A major portion of heavy cycle oil (HCO) is recycled into the FCC riser. The remaining LCO and HCO is used as fuel oil or cutterstock but this practice is now discouraged by various environmental agencies.
The main fractionator bottom stream consists of hydrocarbons boiling above 360°C and some amount of entrained catalyst. The amount of catalyst entrained can be reduced by improving reactor cyclone design and operation. This bottom fraction is filtered and settled in a slurry settler. The filter is periodically backwashed using HCO and reused as FCC feedstock. The product drawn from the settler top is called clarified oil (CLO) and the settler bottom is called slurry. The slurry is recycled into the riser to avoid catalyst loss and optimised CCR and CLO is blended into fuel oil. If CLO is rich in mono-aromatics, it can be utilised in carbon black manufacture.
Bharat Petroleum’s Mumbai refinery operates a three-vessel fluidised bed catalytic cracker unit (CCU). This unit processes 3500 t/d of feedstock and produces 1400 t/d of gasoline. The unit is well integrated with the refinery which processes a variety of high and medium sulphur crudes and produces fuels, speciality chemicals, and lubricants. The CCU was licensed by Royal Dutch Shell (Shell Global) in 1955. It often processes a mixture of vacuum gas oil and unconverted hydrocracker oil and operates towards maximising gasoline production.
The CCU regularly undergoes scheduled turnarounds during which the research and development and technology divisions work in collaboration on maintenance, modifications, revamps, and troubleshooting. Turnarounds provide an essential opportunity for various maintenance issues to be resolved because they cannot be addressed while the plant is operational. They also allow for an internal inspection of equipment that would otherwise be impossible while the equipment is running or while it contains the product. These processes serve to improve the efficiency of the plant and also enable workers to fix or prevent problems before they cause even more costly outages or accidents.
Turnarounds are necessary for a number of reasons. Most of the aforementioned tasks and benefits can only occur if the plant is temporarily shut down. Even though they render a plant non-operational for a duration of time they are an important investment in the plant’s future. However, turnarounds are also important because they are often mandated by various regulatory bodies. This is required with the goal of preventing accidents and improving safety at the plant. In addition to regulations which might require the plant to have a turnaround periodically, they are necessary to meet the warranty requirements on various pieces of expensive equipment.
The unit underwent major retrofits in 1995, during which it was converted into a riser cracking unit from a reactor bed cracking unit. In earlier FCC designs the hydrocarbon feed was injected into a fluidising bed reactor where the cracking reaction took place in the dense phase of the reactor. The residence time of such FCC designs was much longer than modern units operating today. Due to higher residence time, the selectivity towards gasoline was poor and an excess of gas and coke was formed. With the advent of active shape selective zeolites, the need to reduce the reactor’s residence time was strongly felt. Hence riser technology was developed in which the catalyst travels up a tubular reactor with a high height to diameter ratio at accelerated speeds (see Figure 1). The hydrocarbon feed is injected at the base of such risers and the residence time would be 2-5 seconds. At such contact times, over-cracking of feed into coke and gas was circumvented.
During such a turnaround in 2017, after resuming operations it was noticed during routine quality checks of the product streams that the flash point (ASTM D93) of CLO was lower than specification. Flash point is the lowest possible temperature at which hydrocarbon vapour ignites in the presence of a spark/flame. After a few months of observation, it was observed that the flash point was unsteady and kept fluctuating between 45°C and 55°C (see Figure 2). A lower CLO flash translates to loss of lighter products and is a safety concern in operating slurry settler and filtration systems since, with an increase in pressure, the flash temperature decreases. Flash point is an important parameter for safety considerations, especially during storage, compression, and transportation of volatile petroleum products (LPG, light naphtha, gasoline) in a high temperature environment. The surrounding temperature around a storage tank should always be less than the flash point of the fuel to avoid the possibility of ignition.
The source of the issue was isolated near the CLO draw zone. A steam ring is employed to strip lighter molecules from the hydrocarbon mixture (see Figure 3). A potential reason for the fluctuating flash can be wetness in the steam which will evaporate when it contacts hot oil. This cools the oil and renders the stripping steam less effective because the colder oil has a lower vapour pressure. A steam trap installation can promptly solve the problem. Other reasons can be erosion in the shed deck above the CLO draw which affects mass transfer in stripping; too high a steam flow rate which causes excess vapour traffic that interferes with product withdrawal from the fractionator; or a low draw temperature which may cause the steam to contact with sub-cooled liquid (the steam will reduce its partial pressure but not enough to boil off its volatile part).
After thorough investigations and ruling out the above reasons, the root of the matter was isolated in the steam ring itself. Non-uniformity in steam distribution can lead to inefficient stripping. If a steam distributor sparges steam at varying rates along its circumference, the stripping ends up being inefficient. This causes the flash to fluctuate randomly.
In the subsequent turnaround in 2018, the steam ring in the bottom portion of the main fractionator was replaced with a slightly modified design. The average flash point observed in the year 2018-19 was found to be 59°C, fluctuating between 48°C and 90°C (see Figure 4).
This new steam ring was capable of stripping a steam inlet pressure of 5 kg/cm2 compared to an earlier 3 kg/cm2. There was a considerable increase observed in the flash point of CLO but the fluctuations persisted. Since the replacement affected the end result, the assumption that the steam ring is the source of the problem was further cemented.
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