Discussion on issues in the delayed coking process?

The adjustable recycle ratio technology of the Luoyang Plant is the mainstream process in China, and it is recommended to use this process.

For our new project, the old process is still being used: the crude oil is taken from the tank and fed into the crude oil buffer tank, where it exchanges heat with diesel and wax oil. It then enters the convection section of the heating furnace, proceeds to the distillation tower, and finally the combined oil goes into the radiant chamber. It is under construction. Please share any constructive suggestions you may have, thank you!

The second is the process developed by the Beijing Institute based on the 1 million tons per year coking capacity of Shanghai’s plants. Its advantage is that the final temperature for heat exchange with residue oil is relatively high; however, the temperature at the bottom of the distillation tower is high and coking tends to occur easily. This is also the main process used by the Beijing Institute in designing coking plants today； The third is the process developed by Luoyang Institute; this process adds a circulating oil extraction facility to the conventional one. The circulation ratio is adjusted by mixing circulating oil with vacuum residue, and the heat from the reaction gases is removed through the return flow of circulating oil. By eliminating the process in which the reaction gas oil is directly heated in exchange with vacuum residue inside the tower, it is not only possible to adjust the recycle ratio flexibly, but it is also possible to **reduce the tendency to coking in the lower part of the distillation column at low or ultra-low recycle ratios. At the same time, since the depressurized residue fed into the heater does not come into direct contact with the reaction gas and oil containing coke powder, the coke powder content in the feed pump of the heater can be significantly reduced. This helps to slow down wear on the feed pump of the heater and extend its service life. The disadvantages are: the process is too complex and the equipment investment is high. There is considerable debate over the second and third approaches, and it is difficult for the two design institutes to convince each other.

Our unit uses the third type, one designed by Luoyang Institute. However, I think we also need to consider the properties of the crude oil to be processed in your unit, and determine whether it is necessary to first send it through a distillation tower to vaporize the light components

The second type is used very effectively in Huizhou’s oil refining operations; it is convenient to adjust the circulation ratio. Coking at the bottom of the distillation tower occurs rarely, as the circulation pump plays a key role in this regard. Moreover, there is no need for the residue oil to exchange heat with the oil gas, which further reduces the likelihood of coking! !

The set we just installed also uses the third approach, which can effectively adjust the circulation ratio. . . . . .

This allows for the efficient use of energy, reduces the energy consumption of the device, and helps to save costs; especially for private enterprises, the third solution is the best choice

We are using the second process, and so far it has worked fairly well. I would like to ask everyone: what is the typical temperature control at the top of your distillation column?

Disadvantage of the first approach: Coking is likely to occur at the bottom of the distillation tower. The temperature at the bottom of the distillation tower is usually around 380°C, and this temperature rises further as the recycle ratio increases, which facilitates coking at that location； 2 It is difficult to utilize waste heat at low temperatures: the final temperature after heat exchange is too high, which leads to an excessively high temperature at the bottom of the distillation tower. This limits heat integration between units as well as the optimization of the heat exchange networks within each unit, resulting in high fuel consumption per unit of production. 3. The circulation ratio adjustment is not intuitive: The circulation ratio is controlled by a three-way valve to regulate the flow rates of the convective oil entering the upper and lower sections of the distillation tower, making it even more difficult to achieve an ultra-low circulation ratio. Disadvantages of the second approach: 1. It is inconvenient to adjust the circulation ratio: the circulation ratio is controlled by a three-way valve to regulate the flow rates of the convective oil entering the upper and lower sections of the distillation tower ； 2 It is difficult to achieve an ultra-low recycle ratio: The temperature of the oil and gas at the top of the coke tower is around 420°C. To keep the temperature in the evaporation section below 395°C, it is necessary to exchange heat between part of the convective oil and the high-temperature oil and gas, so that the heavier components in the oil and gas can condense at the bottom of the distillation tower; as a result, it is hard to reduce the recycle ratio below 15%. The third option is the best: in this catalytic cracking process with adjustable recycle ratio, the residue from the cracking unit no longer enters the distillation tower. The gas oils from the coke drum have their temperature reduced by the recycle oil from the bottom of the distillation tower. After absorbing heat from outside, this recycle oil is used, in one stream, as reflux at the upper part of the distillation tower to control the temperature in its evaporation section; in another stream, it serves as reflux at the lower part of the distillation tower to control the temperature at its bottom. A third stream of this recycle oil is used to adjust the recycle ratio of the entire plant. This approach not only eliminates the constraint imposed by the temperature at the bottom of the distillation tower on the final temperature after conversion, but also allows for flexible adjustment of the recycle ratio, enabling operation at ultra-low recycle ratios.

The delayed coking process used by Changling Petrochemical is the third type of process. I’ve seen that the liquid level at the bottom of their fractionation towers is always full; it’s common to see them having to send the oil from the bottom of the towers elsewhere. Apparently, this oil can’t be sold at a good price. The company where I work uses the first type of process, and it works well for us. The clean filter at the bottom of the distillation tower has not been seen in half a year, and the lowest temperature at the bottom of the tower can reach 400 degrees. However, under normal circumstances, we require the temperature not to exceed 380 degrees; when it rises above 380–385 degrees, the flow rate for the bottom circulation should be increased by 20–30 cubic meters. The liquid level should not be set too high, around 70% is sufficient. I run a private business; it’s already good enough if I can get oil, let alone any high-quality oil or crude oil. The unit processes oil with a water content of around 6% and mechanical impurities of about 3% (what we call ‘water and oil’ is actually the oil left at the bottom of the tank after cleaning). Processing such oil is truly challenging; high water content in the feed leads to significant vibration in the outlet pipes, large fluctuations in flow measurement instruments, and considerable swings in the vapor line of the coke tower, resulting in constant ‘squeaking’ noises. . ”The sound of friction caused by the swinging of the vapor line can be heard from 100 meters away; the entire derrick vibrates as a result. Sigh. . . Later, I had no choice but to ask the operator to keep the mixer for the raw materials running continuously, for fear that if the raw materials settled for too long, the water would settle at the bottom of the tank. If the raw material pump then tried to draw water in, it would be a disaster – an accident would definitely occur. Raise the temperature of the feed material in the buffer tank to 140–170 degrees, which improves the dehydration effect. Water and some light components enter the distillation tower through the equilibrium line at the top of the buffer tank; fluctuations may occur in the distillation tower, so it is essential to maintain proper control over the top temperature and system pressure. The top temperature is usually set at 105 degrees; for processing this type of oil and water, I have asked my supervisor to raise it to around 120 degrees. . . The device is much more stable!