Sep-2024

Robust and reliable measurement solution for delayed coke drums

The delayed coking process is one of the crucial processes in the petroleum refining industry. It is used to upgrade and convert low-value residue oil from other processes into higher-value liquid and gas products.

Berthold

Article Summary

The by-product left in the coke drum after the process is called petroleum coke. Since delayed coking is a continuous batch process, one coke drum is alternately fed with process material while the second drum is being cooled and emptied. Therefore, pairs of coke drums can be normally found in refineries (Figure 1).

The process feed, usually residue oil from the preceding vacuum distillation tower, is fed to the bottom of a fractionation column, where it is mixed with heavier cracked products of the previously, thermally cracked feed. From this point, it is pumped along with hot steam to a furnace to heat it up to the thermal cracking temperature. The feed is then introduced from the bottom into the coke drum, where the thermal cracking takes place. During the cracking process, heavy hydrocarbon molecules break down into lighter hydrocarbons and solid petroleum coke. The produced vapours leave the coke drum via overhead lines and are returned to the fractionator. As they try to escape the coke and the viscous liquid, a layer of foam tends to accumulate on the surface of the coke. During this stage, the worst-case scenario is a “foam-over” event, where the foam reaches the top of the coke drum and eventually reaches the fractionator via the overhead lines. Such an event would result in costly production losses, with several weeks of downtime required for intensive cleaning and maintenance work. The targeted use of anti-foam chemicals minimises these events, as the operator keeps the foam layer to a minimum. However, excessive usage of these chemicals can poison the downstream catalyst and reducing its useful life. Therefore, to maintain optimal operational efficiency and prevent foam-over events, precise dosage of the anti-foam chemical is essential.

When the coke bed level reaches the targeted height, the filling process is switched to the standby drum, which has been cooled, emptied and prepped for a new cycle. Once the switch is made, steam is applied to steam quench the filled drum. This steam quenching is used to reclaim the remaining cracked hydrocarbons. The vapour flow continues towards the fractionation tower until most hydrocarbons have been stripped from the now offline drum. Subsequently, the vapour is directed to the blowdown system, enabling the drum to be cooled down and depressurised back to atmospheric levels. This causes the foam, which was created while filling the drum, to collapse. However, this collapse is not happening instantaneously. During steam quenching, pressure can decrease in the coke drum while vapour velocities increase. In this process, more hydrocarbons that have undergone cracking can emerge from the solution. This will likely cause the foam level to rise rapidly before collapsing, sometimes to the point of overfilling the drum. After the steam quenching step, the hot drum is cooled down, by filling it with water. Once the drum temperature and pressure have been reduced, the drum is opened at the top and bottom allowing the coke to be removed by water-cutting. During this cleaning process, remnants of coke may remain on the wall of the drum.

Regarding this process, reliable and accurate level measurement is crucial for obtaining a safe and efficient coking cycle. However, it is also a very challenging measurement since the entire process passes through stages with different conditions, such as changes in temperature, gas pressure, product density, composition, and surface geometry. Many different types of technologies have been tried and tested as level measurement solutions, including differential pressure, radar, and radiometric devices. However, due to the extreme process conditions inside the drums, differential pressure and radar units tend to either fail or prove extremely unreliable. Thus, radiometric level measurements are usually the only reliable solution to determine the level in the coke drum.

A frequently used radiometric solution is neutron backscatter measurement (NBS), where both the source and detector are located outside the coke drum within one housing. However, this technique has some issues. Since the source and detector are installed in one housing, special precautions are necessary for any access and maintenance of the detector unit itself. Moreover, the system needs to be mounted directly to the coke drum to avoid losing too much signal strength. This can cause issues with electronics due to higher temperatures. The measured value is greatly affected by water inside and outside the coke drum, such as rain or water-soaked insulation, which can cause faulty readings.

NBS employs a distinct measurement principle, focusing on the detection of hydrogen to determine hydrogen density. Typically, a single system covers a relatively short measuring range of approximately 700 mm. For example, fast-rising foam may go undetected by NBS due to its lower hydrogen content compared to the residual. Consequently, the presence of less hydrogen in the foam results in a decreased level output, even though the level is increasing in the drum. Thus, NBS gauges act as level switches during normal operations, but during conditions when foam can increase or decrease rapidly, they act more as hydrogen density gauges. Therefore, one of the main disadvantages of using various NBS gauges is that foam ups and their height, e.g. during steam quenching, cannot be reliably detected. However, this capability is crucial for operators to prevent undue shutdowns due to foam-overs. Overall, NBS is a measurement solution that demands highly skilled process technicians to ensure proper calibration. These experts must be proficient in interpreting level readings and adept at handling the specific operational conditions.

Continuous radiometric level solutions with state-of-the-art gamma scintillation technology are not affected by the discussed issues, making them the optimum solution for this measuring task as shown below.

Basics of radiometric level measurement using gamma radiation
A radiometric level measurement using gamma scintillation technology is a fully non-contact method, meaning it is non-invasive and non-intrusive, as the equipment is installed outside the process vessel. This type of measurement is based on a simple yet sophisticated concept – the principle of attenuation of gamma radiation.

A typical radiometric setup includes:
A Radioactive Source: This source emits gamma radiation and is housed within a shield for safety.
And on the other hand:
A Scintillation Detector: This device detects the gamma radiation and is positioned opposite the radioactive source.