Jul-2024

Radiometric principles in monitoring refining and petrochemical operations (TiA)

Accurate measurements, such as monitoring catalyst powder levels in the fourth-stage separator of an FCC, require maximum precision.

Marco Tescari
VEGA

Article Summary

Collaboration with licensor companies and leading EPC companies to upgrade refinery FCC units has led to the introduction of the fourth-stage cyclone in recent years, which is responsible for separating catalyst particles from cracked hydrocarbons after they exit the riser reactor. Efficient separation is crucial for maximising catalyst recovery, as any loss of catalyst can impact the economics of the FCC process.

The fourth-stage cyclone helps in removing fine catalyst particles from the product stream. This is essential for producing high-quality products with the desired specifications. Reduced catalyst carryover helps prevent contamination of the final products, ensuring they meet the required purity standards.

Proper separation in the fourth-stage cyclone ensures that only cracked hydrocarbons are sent to the main fractionation unit.  At the same time, catalyst particles are circulated back to the riser reactor for further catalytic cracking. This promotes the efficient and continuous operation of the FCC unit, contributing to a consistent production rate. The fourth-stage cyclone plays a role in minimising the carryover of catalyst fines into downstream units, such as the main fractionation column. Minimising catalyst losses is essential for the economic viability of the FCC process, as catalyst make-up and regeneration are significant operational costs.

Efficient separation in the fourth-stage cyclone helps prevent the accumulation of coke on the catalyst particles. Coke formation can lead to catalyst deactivation over time, so maintaining catalyst activity is essential for achieving optimal conversion rates and product yields in the FCC process.

In summary, the fourth-stage cyclone separator is a critical component in the FCC unit, contributing to catalyst recovery, product quality, temperature control, and overall process efficiency. Its proper functioning ensures the economic and reliable operation of the fluid catalytic cracking process in petroleum refineries.

Powder monitoring
Against the backdrop of reliable FCC operations, the lower part of the fourth-stage cyclone, dust level monitoring is of vital importance for correct control of the emitted powder reduction. After exiting the cyclone, the catalyst powders are conveyed to a collection tank and loaded on trucks to landfill sites.

Optimising the fourth-stage cyclone yield is essential, where the temperature is lower in the fourth-stage cyclone separator compared to other cyclones due to heat loss from the gas and dust. Regardless, the process temperature is higher than 700°C.

Without going into the construction details, the cyclone, whether side-by-side or stacked, or if specially designed by the licensor, typically has an overall height around of 10m and a diameter of about 1.5m.

Powder monitoring usually takes place in the last metres of the cylindrical section of the cyclone that ends with a conical part. Relying on radiometry represents the only reliable and safe solution for monitoring the level. Installation of the instrumentation is, in fact, external to the cyclone. A nuclear source emits energy towards a detector mounted on the opposite side of the cyclone. Mounting on parallel structures to the cyclone, but detached, is common practice. The number of sources depends on the required measurement range. For about 3m of total measuring range, two sources with low-energy activity are sufficient.

The measurement system is based on a physical law in which the energy emitted by the source undergoes an exponential decay law relating the attenuating factor to the density and thickness of the matter crossed. If the cyclone was empty, a high value of received energy would be read on the receptor.

As the dust settles, the energy value recorded on the receptor, with the empty cyclone, decreases until it is almost zero. This occurs when the dust level has reached the maximum required elevation. The precision, accuracy, and resolution that radiometric technology can offer is very high. With these measurements, it is often required that the degree of reliability that the system must achieve to reduce the risk of accidents during its use complies with the safety standards defined by the IEC 61508 standard in the safety integrity level (SIL 2) level. At the very least, a low-level alarm is dedicated to a nuclear switch detector.

VEGA nuclear detectors, all equipped with Plics electronics, could meet this requirement. SIL 2 is always ensured in single-channel architecture.

In addition, beyond compliance with SIL standards, PLICS electronics offers:
• Storage of up to 250 events
• Storage of up to 250 parameter changes
• Storage of up to 100,000 measured values
• Diagnostic functions according to NE 107
• Storage based on a clock that reliably records the time.

Safety
Management of cyclone safety is improved with extensive diagnostic functions. Event storage based on a clock that reliably records the time shows how VEGA meets the highest safety and reliability requirements while also facilitating the work of the operators in the refinery. In addition to the standards (such as SIL), having user-friendly instruments can help plant security. 

Since VEGA electronics are incorporated into the nuclear receptor, it is not only possible to have the display on the receiver head, but also to connect the receiver directly to the distributed control system (DCS). This simplicity, which also minimises commissioning time, responds to the requirement for better system safety and radio protection.

ALARA
An industrial system of measurement based on radiometry is engineered following the ALARA principle (as low as reasonably achievable). The activity of the source is minimised as much as possible considering the required measuring role and the regulations of radio protection in the country in which the plant is located. However, in addition to the high standards and promoted practices established by international regulations in radio protection, it is also possible to consider optional solutions that could help maximise safety in radio protection.

Wireless communication, through the Bluetooth standard, available in the electronic solution Vega Plics, represents a further possibility to manage the detector located almost 20m from the source. The option to choose the type of detector provides a balance in maximising simplicity with concrete technical requirements. For several years, the scintillation detector has been the reference model for industrial measurements, which can be provided in a mechanically rigid or flexible version.

Detector sensitivity
The rigid receptor with polyvinyl toluene (PVT) scintillator is highly sensitive. However, the flexible detector made in styrene, working according to the principle of optical fibres, allows great advantages, although physically slightly less sensitive. Other advantages include:
• Being able to follow the tank profile
• Low weight, which facilitates transport and installation
• The diameter is approximately half of a rigid receptor.

The advantage of having a diameter of almost half of the rigid detector is to centre it in a narrow energy bundle, making it intercept the energy at its maximum power. This way, the opening can be minimised on the horizontal plane of the energy beam coming from the source.

For example, in the case of a fourth-stage cyclone with a diameter of only 1.5m, the energy beam on the detector (on the horizontal plane) is less than 15cm in width. The Fibertrac 31 instrument, with a diameter of only 4.5cm, can be placed in the very centre of this beam. With optical fibre technology, the energy arrives at a point propagated along the entire length of the fibre.

The distribution of energy is thus very uniform, managing to optimise the primary task of a receptor, which is to block the radiation and manage to transform maximum incident radiation in the electrical signal. This is useful for  generating an output that represents the level reading. The theoretically lower sensitivity of the scintillator in styrene fibres compared to that in PVT is thus compensated by the concrete benefits that these fibres offer.

In the fourth-stage cyclone (see Figure 1), as in other classic refinery applications, redundancy of measurements is often required. Installing two Fibertrac 31s (see Figure 2), one beside the other, requires only 9in of space. Receptors can be huddled together without any interference in reading, each sending a signal to a different system. Therefore, on the horizontal plane, the area where the useful energy is needed can be optimally minimised. These receptors can be easily installed side-by-side in the perfect centre of the energy beam. These technical precautions are why Vega has succeeded in meeting the maximum demand for radio protection, precision, and measurement accuracy while also containing transport costs and installation.

This short case study originally appeared in PTQ's Technology In Action Feature - Q3 2024 Issue

For more information: m.tescari@vega.com