Analysis of coking on the outer wall of the lift pipe in sedimentators, centrifuges, and separators
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At the beginning of operation of a certain unit, the feedstock was light, the oil slurry yield was low, and the reaction temperature was maintained at approximately 502–505°C, with a minimum of 498°C. Large fluctuations in system temperature and pressure, or vibrations, can cause the coke deposits on the outer wall of the lift pipe to fall off, thereby blocking the ash hopper or the material leg. In 2006, operations were halted for emergency repairs on the double-acting slide valve; the two devices were not opened for inspection, so it is not possible to determine whether coke particles fell off during that shutdown. The subsequent heating up during startup was rapid; the two vessels were heated at a rate of 25°C per hour. It is also possible that during this heating process, the coke blocks and the equipment itself did not heat up evenly, resulting in the coke blocks falling off. The legs of the Duse settler led to significant loss of catalyst, with a high solid content in the slurry. Even after taking corrective actions, it was not possible to stop this loss of catalyst, which resulted in coking at the lower part of the distillation tower and posed serious threats to the plant’s operations. The reasons for coking can be as follows: The low-speed \"stagnation zone\" on the outer wall of the riser allows particles or droplets of heavier components to accumulate there, creating the conditions for coking; this area is the main site where coking occurs. The main factors contributing to coking are: ① Fine catalyst particles and droplets adhering to the outer wall of the riser tube; these suffer from a low radial centrifugal force, resulting in collisions and adhesion to the outer wall of the riser tube ； ②The high-boiling aromatic components in the reaction gas oil adhere to the surface of the reactor walls, forming \"char nuclei\"; over a certain residence time, dehydrogenation and condensation reactions occur, resulting in the formation of char. ③An increase in the high-boiling-point components in the feed, operational fluctuations, or a lower reaction temperature can all lead to an increased degree of \"wetting\" on the surface of the catalyst particles, allowing the catalyst particles and droplets of heavier components to deposit on the outer wall of the riser and cause coking. The catalyst particles that enter the top of the settler are relatively small and are greatly affected by gas diffusion; these tiny particles tend to adhere to the outer wall of the rise pipe, leading to coking. The initially formed coking occupies the available space in the ring, which further increases the tangential velocity of the airflow in the centrifuge; the pressure gradient within the \"stagnation zone\" also increases, leading to further intensification and thickening of the coking. The internal soft slag hardens, and as this process continues layer by layer, slag deposits form on the outer wall of the rise pipe in the cyclone separator. When the thickness of these slag deposits reaches a certain level, the effective circular space decreases, causing the tangential velocity to increase significantly. This increased scouring force on the slag layer on the outer wall of the rise pipe limits the thickness of the slag deposits, resulting in uneven grooves being formed on the surface of the slag. When the reaction temperature is low, the degree of \"wetting\" on the surface of the catalyst particles increases, causing the catalyst particles and droplets of heavy components to deposit on the outer wall of the lift pipe and form coke.
I’ve learned it.* I still have some more materials here.
Reduce coking in the shell side of the settler; minimize unvaporized oil. Ensure good feed atomization to reduce large particles as much as possible. Use a proper stratified feeding method, as well as an appropriate amount of inert medium. Maintain an appropriate low pressure, and use a reasonable catalyst-to-oil ratio. Design the pre-lift section and vaporization section properly to reduce plug flow. Address the issue of oil slurry recycling. Ensure stable transportation of regenerated catalyst. Reduce coking in the shell side of the settler; lower the vapor pressure of oil and gas in the shell side of the settler. For systems with a rough spin–top spin structure, reduce operating pressure and increase the amount of inert medium used, thereby reducing the amount of oil and gas carried out with the catalyst from the rough spin separator. Use one-pass cracking. Reduce coking in the shell side of the settler; increase the temperature of the settler and reduce heat loss. Allow a large volume of catalyst to pass through the settler. Other methods include reducing the residence time of oil and gas, minimizing the space required in the settler, and allowing a large volume of oil and gas to pass through the settler. Reduce coking in the shell side of the settler by using surfaces that do not allow sticking, relying on catalyst flushing, such as VSS, VQS, etc. Non-adherent surfaces are used to reduce the damage caused by coking; measures to prevent coking are employed to minimize its effects on economic performance. The goal of any enterprise is to maximize profits. In the case of catalytic cracking, this means increasing the yield, that is, reducing the dry gas output – it is estimated that each percentage point improvement results in a profit of 15 yuan. Reducing consumption mainly refers to energy consumption (about 1.5 yuan per 1 kg of EO per ton). Safe operation to reduce coking and economic benefits: The soft connection technology between the primary rotor and the top rotor brings significant advantages to catalytic cracking units. In some units, severe coking problems occur; yet even though Unit 3#FCC experiences coking issues repeatedly, its light product yield is 2 percentage points higher than that of the other unit. Reducing the operating pressure excessively, using maximum steam to lower the partial pressure of oil and gas, and employing the maximum amount of catalyst along with oil and gas in the settler can help reduce coking; however, this reduction should not be too great. Therefore, the goal is to find the most economical way to minimize coking.
There are many factors that contribute to coking on the outer wall of the riser pipe in a settler, but the main cause is the prolonged residence time of oil and gas in the dilute-phase, low-speed zone of the settler. The oil and gas containing catalyst adhere to the dead zones on the pipe or vessel walls, where they coalesce and form coke. This problem has now been resolved in catalytic processes by connecting the inlet of the coarse spinner directly to the outlet of the lift pipe, and connecting the outlet directly to the inlet of the primary cyclone, thereby allowing the oil and gas to pass quickly through the secondary cyclone without going through the settler. In traditional catalytic processes, a rapid separator is installed at the outlet of the lift pipe; the oil and gas then enter the coarse spinner via the settler. As a result, large amounts of oil and gas accumulate in the dilute phase region of the settler, leading to coking. With this direct connection approach, it is important to ensure that the balance pipes are not buried within the bed layer of the stripping section, as fluctuations in the bed layer there can cause material loss. Furthermore, the cause of coke formation is also related to the amount of material stored in the stripping section. The specific reasons are related to the device structure, the properties of the raw materials, and the operation process.