Coking in the large oil and vapor line at the top of the coke tower

Entrainment of coke powder or foam: The entrainment of coke powder in the coke tower is primarily related to the velocity of the oil and gas stream within the tower as well as the safety head height. When determining the diameter of the coke tower, a permissible velocity of less than 0.15 m/s is generally adopted. As the processing capacity of the plant increases, the velocity of the oil and gas stream rises; in some current coking plants, this operating velocity has already exceeded 0.15 m/s, resulting in coke powder and foam not being able to settle and being carried along to the outlet. When determining the height of the coke tower, a certain safety head should be reserved. This safety head is generally the distance from the tangent line at the top of the tower to the top of the foam layer; it usually ranges from 3 to 5 meters. In some foreign designs, this value is 2 to 3 meters. The higher the altitude, the lower the utilization rate of the coke tower; however, it prolongs the residence time of the oil and gas in the tower, which is beneficial for reducing coking in the oil and gas lines and the distillation tower. The design diameter at home is generally around 5 meters. However, in actual operation, due to full or overloaded operation, the safe head space in the coke tower is reduced, which shortens the residence time of oil and gas inside the tower. Coke powder and a large amount of unreacted waxy oil are carried into the main oil and gas line, where cracking and condensation reactions continue to occur. Gradually, more and more coke powder accumulates, resulting in the oil and gas flow area in pipes with diameters of DN350–DN500 being reduced to only φ40–φ100. 2.2 Deposition of coke powder: Under normal circumstances, delayed coking units are equipped with measures to inject quench oil into the oil and gas line at the top of the coke tower. If the temperature and flow rate of the quench oil are chosen appropriately, the temperature of the quenched oil and gas remains around 420°C. If the temperature of the quench oil is too high or its flow rate is too low, the temperature of the oil and gas after quenching will exceed 420°C; this results in residual reactions occurring within the pipelines. If the quench oil is not injected at the appropriate location, the cooling effect will be inadequate, and the coke powder carried by the oil and gas cannot be effectively removed. As a result, the liquid contained in the cooled oil and gas does not get eliminated in time and remains in the outlet pipelines, while large amounts of coke powder accumulate on the pipe walls. Over time, more and more coke powder builds up, eventually leading to blockages in the oil and gas pipelines, an increase in pressure drop, and difficulties in operating the system. 2.3 As world crude oil is extracted, its properties become increasingly poor: its specific gravity and viscosity increase, the levels of impurities and heavy metals rise, and the amounts of water and salts increase. This makes it necessary to carry out pre-treatment before processing. As is well known, the coking unit in an oil refinery functions like a “trash bin” for the city; this is also one of the characteristics of the coking unit: it can process raw materials that no other units are able to handle. As a result, the raw materials extracted from or transported from oil fields become increasingly of poor quality, and the residue fed into the coking unit is even worse. An increase in the salt content in residue leads to foaming within the coke tower; the foam layer rises, carrying large amounts of coke powder into the oil and gas pipelines. Further deposition of this coke powder results in coking in those pipelines. 3. Several measures to reduce coking in large oil and gas pipelines: Only through proper analysis can the right solutions be found. Based on the above analysis, the main causes of coking have been basically understood, and the following measures are proposed to address the coking problem in oil and gas lines. 3.1 Controlling production load Generally, the design determines the specifications and models of equipment strictly based on the scale of the facility, the properties of the raw materials, and the requirements for the products. With the raw materials and processing volume fixed, the diameter and height of the coke tower determined accordingly. The margin of abundance, or operational flexibility, is generally 60% to 110%. If it exceeds the device’s maximum elastic range, a series of problems will arise, one of which is coking in large oil and gas pipelines. Therefore, it is not possible to blindly pursue high processing capacities; instead, the coke tower must be designed based on the properties of the raw materials being processed, so that the gas velocity and height inside the tower remain within safe limits. Alternatively, the processing capacity of the coke tower can be increased by reducing the time required to produce coke. 3.2 Selection of appropriate quench oil: There are three common types of quench oil: diesel, middle distillate oil, and wax oil. The selection of quench oil can be considered from two aspects: one is the quenching effect, that is, the temperature of the oil and gas after quenching. When the temperature of the oil and gas after rapid cooling remains stable, the likelihood of continued reactions within the oil and gas lines is reduced, resulting in less coking in the pipelines. Conversely, the coking becomes more severe. The quenching effect using these three types of oils as quenching oil is diesel > intermediate oil > wax oil. The second aspect is economic efficiency: diesel is used as quench oil, with large amounts of it being injected into the coke tower. The high-quality oil products obtained through separation are also fed back into the coke tower, creating a cycle. The heat contained in the large quantities of diesel separated in the distillation tower cannot be utilized effectively; therefore, this excess low-temperature heat must be removed using air coolers or water coolers. When paraffin oil is used as the quenching oil, it does not vaporize easily; as a result, the oil vapor condenses after quenching, leading to coking in the pipelines. Taking everything into account, it is more appropriate to choose mid-stage oil as the quenching oil. 3.3 Determining the optimal location for injecting quench oil: In early designs, three quench oil injection lines were generally added at the outlet of the main oil-gas line; the diameter of these lines was usually DN25, and they were inserted obliquely into the main oil-gas line. Since the quench oil injection point was located far from the outlet of the oil-gas line, it was not possible to cool the temperature of the oil and gas just coming out of the tower to below 420°C through atomization, which made the section of pipeline prior to quench oil injection prone to coking. Moreover, the coke formed inside the pipeline could not be removed; through years of experience, a new injection location was designed. The injection point of the quench oil was lowered to the base of the oil-gas line just as it exits the coke tower, and the quench oil pipeline was enlarged to DN40; injection through built-in circular spray nozzles was employed, which effectively prevented coking in the oil-gas line.

I firmly support the opinion regarding the sofa. My unit has been in operation for nearly 470 days without any signs of coking. Initially, a patented cooling technique was used, but it led to significant coking; this issue was resolved during subsequent maintenance by reverting to the original design, which involves three-point injection at a 120° distribution. From then on, there was no more coking.

The answer is very comprehensive; it’s worth learning from!* ! !

The method of injecting quench oil is key to controlling coking, and the three-point injection method (with annular distribution pipes inside) at the interior of the coke tower at the outlet of the large oil-gas line is an effective approach.

How is the amount of quench oil to be injected controlled, based solely on the temperature at the top of the coke tower? Is there still some basis for it? Our facility uses paraffin oil and sludge oil as quench oils; of course, since the temperatures of these two types of oil are different, the amount of oil that needs to be injected also varies

During normal production and tower replacement, the temperature at the top of the old tower is strictly controlled to remain below 420 degrees Celsius.

Thank you to the original poster; I’ve learned a lot of new things

Be sure to add quench oil; our factory uses the middle-stage reflux oil from the distillation tower, which gives good results

Some designs have good intentions, but their actual performance is rather disappointing.

Coking in large oil and gas pipelines is mainly caused by an excessively thick coke layer, or by the inability to inject the defoamer or by using too little of it； The main reason is that in the early stage after cutting off the old tower, the amount of quench oil injected during minor blowing is high, and it is at this time that coke powder is most likely to be carried along.