Delayed coking fractionator bottom circulation

The main function of the bottom circulation is to prevent coking at the bottom of the tower; it serves only to stir the liquid at that location, thereby preventing the oil there from coking due to lack of flow over time. There is also a return flow above the liquid level. I’m not sure where yours is located – in our case, it’s used at the feed point for the raw material. It has not been used from the start of operation until shutdown; perhaps it is intended to be used to counteract excessive gas volume in the carbon tower, so as to prevent the pressure of the raw material oil from becoming too high.

It is generally not necessary to extend below the liquid surface. . . If there is a path that leads down below the liquid level, I think that is intended to reduce the fluctuation in the liquid level readings caused by frequent water hammer effects on the float at the bottom of the tower as it receives backflow liquid. . . :loveliness:

Upper and lower feedings serve different purposes: the upper feeding is used to control the liquid level at the bottom of the distillation tower, while the lower feeding is primarily used to control the temperature at the bottom of the distillation tower

Two feed streams can be set up, primarily for mixing purposes to prevent coking

This usually has two channels; I think it is mainly used for controlling temperature, and it can also serve as a means of adjustment!

Typical coking units have feed at both the top and bottom; the top feed is used to control the liquid level at the bottom of the distillation tower, while the bottom feed is primarily used to control the temperature at the bottom of the distillation tower in order to prevent coking

It should be divided into two circuits during design, in order to regulate temperature; the two circulation circuits are switched depending on the temperature level, thereby ensuring thermal balance.

I think it’s to stir at the bottom of the tower, preventing the accumulation of coke powder and facilitating long-term operation of the unit

The upper feed is used to control the liquid level at the bottom of the distillation tower, while the lower feed is primarily intended to remove heat from that area, preventing overheating and coking, as well as serving a certain stirring function

Our design involves three pathways entering the tower: one above the liquid level, in contact with the oil and gas, used to control the circulation ratio； One path leads inside the tower to a circulation loop below the liquid level, where it does not come into contact with oil and gas. Its main functions are to stir the mixture; when the circulation ratio is very low (0.1–0.15, meaning the volume of fluid circulating upward is minimal), it helps maintain a constant flow rate at the outlet of the circulation pump and ensures stable flow there (in some systems, the upper and lower circulation processes are controlled separately). The third path goes directly to the bottom of the tank, with an annular distributor at its outlet – there are no control valves here. Its purpose is to stir the sedimented coke powder at the bottom and prevent coking. This valve should be opened appropriately during startup, but no adjustments are needed during normal operation ；

The amount of bottom circulation in a process using traditional techniques differs from that in a process with a flexibly adjustable circulation ratio

A process with a flexibly adjustable circulation ratio is adopted (i.e., the Luoyang Institute process)

Most delayed coking units in our country use the conventional circulating oil process shown in Figure 7. Since the conditions for contact between the hot oil-gas and the residue feed inside the fractionation tower are fixed, the circulating oil volume corresponding to different ratios of feed from the top and bottom can only range between 0.35 and 0.45; therefore, it is difficult to further reduce the circulation ratio. However, as analyzed from Figure 9 on the principle of cyclic oil formation, cyclic oil is formed when the oil and gas feedstock mixes with the feedstock supplied from above and below, and part of the oil and gas feedstock condenses to create cyclic oil. The circulation ratio can be changed as long as the way in which the oil and gas feed meets the raw material feed is altered. At the end of 2008, a refining company carried out modifications to reduce the circulation ratio in its two coking units. An extension was added below the liquid level at the bottom of the distillation column after the residue oil entered this column, thereby reducing the contact time and area between the residue oil and the hot oil gases, and thus achieving the goal of lowering the circulation ratio. The specific modification method is shown on the right side of Figure-9 below (the red line represents the additional residue feed extension line). Figure 9: Analysis of the principle behind the generation of recycle oil in the distillation column of a conventional cyclic oil process, as well as modifications to reduce the recycle ratio. After reducing the recycle ratio, the processing capacity of the two delayed coking units increased from 3,200 tons per day and 1,100 tons per day to 3,500 tons per day and 1,400 tons per day respectively. Due to the reduced heat exchange between the hot oil gas and the residue, there is less temperature drop of the oil gas and less condensation; as a result, the rising oil gas carries more heat than before the modification, causing the temperature in the evaporation section above the feed to rise while the temperature at the bottom of the distillation tower decreases. After operating for half a year, the unit suffered coking at the end of 2008. During inspections of the bottom of the distillation column, it was found that neither the bottom of the column nor the bottom filter had experienced coking, representing a significant improvement over the previous condition. Tables -7 and -8 below show the changes in some operating parameters of the distillation column and the product yields. Table 7: Changes in operating conditions of the fractionation tower before and after renovation
Item | Before Renovation | After Renovation
--- | --- | ---
Wax oil collection tank temperature (°C) | 314 | 323
Evaporation section temperature (°C) | 377 | 368
Oil and gas inlet temperature (°C) | 409 | 402
Bottom of tower temperature (°C) | 365 | 355
Convection outlet temperature (°C) | 333 | 335
Radiant feed flow rate (t/h) | 185 | 156
Daily raw material processing capacity (t/d) | 3101 | 3370
Recovery ratio | 1.35 | 1.15

Go ahead and make it visible to more people, so they can think about it.

Our bottom circulation pump has two destinations: one is to return to the fractionation tower along with the crude oil, and the other is to the inlet of the radial feed pump

What enters the tower directly is below the liquid level, serving to stir the mixture