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Dec-2016

Scale Catcher technology

Feed containing fines, in-organic matter, sludge, rust etc. can cause excessive pressure drop build-up issues in hydroprocessing reactors.

Emir Zahirovic and Peter Bendtsen
Haldor Topsoe
Viewed : 729
Article Summary
This prevents units from delivering maximum value, as refiners are forced to perform premature unit shut-downs for costly catalyst skimming operations, causing loss of production and loss of refinery overall profits.

Installation of scale catchers in such units will prolong run length, as the scale catchers will help collect the fines, scales etc. in the feed and thereby avoid premature plugging of the distributor tray and catalyst bed below.

Haldor Topsoe’s High Efficiency Liquid Phase Scale Catcher (HELPsc™) is a specialised patent pending reactor internals technology, that works in synergy with the graded bed. Installation of the package will significantly prolong the cycle lengths of units suffering from pressure drop problems, due to plugging of the catalytic bed with particles originating from feed or the upstream equipment. Likewise, Haldor Topsoe’s Gas Phase Scale Catcher can improve cycle length for gas phase naphtha units.

Feed containing fines, inorganic matter, sludge, rust etc. can cause excessive pressure drop build-up issues in hydroprocessing reactors. This prevents units from delivering maximum value, as operators are forced to perform premature shutdowns for costly catalyst skimming operations, causing loss of refinery overall profit.

Installation of scale catchers in such units will prolong run length, as the scale catchers will help collect the fines, scales etc. in the feed and thereby avoid premature plugging of the distributor tray and catalyst bed below.

Haldor Topsoe’s High-Efficiency Liquid Phase Scale Catcher (HELPsc™) is a specialised patent-pending reactor internals technology that works in synergy with the graded bed. Installation of the package will significantly prolong the cycle lengths of units suffering from pressure drop problems due to plugging of the catalytic bed with particles. Likewise, Haldor Topsoe’s gas phase scale catcher can improve the cycle length for gas phase naphtha units.

First generation scale catcher

Haldor Topsoe has developed a number of scale catchers for many applications. For trickle bed reactors, our first generation scale catcher has proven successful in over 30 installations. The installation of a Haldor Topsoe scale catcher has shown that cycle length can be improved significantly.

Figure 1 shows Haldor Topsoe’s first generation scale catcher.
The scale catcher is installed above the tangent line in the reactor immediately above the top distribution tray. The design is very simple, easy to maintain, and clean. The scale catcher utilises the unused space in the top of the reactor, providing improved utilisation of the reactor volume.

The scale catcher provides the feed sufficient time to settle, while slowly overflowing onto the distribution tray below. The design ensures that the turbulence is low, and retention time is long enough to allow the heavier particles to settle at the bottom of the scale catcher compartments before the feed travels to the catalyst bed below.

The process gas stream bypasses the scale catcher compartments entirely. This limits the pressure drop across the scale catcher to a negligible level. The collection of several inches of sediment consisting of FeS, carbonaceous material (dry and sludge), and other inorganic material is common, depending on the conditions of the unit. Once the scale catcher is filled up, the entire process stream (liquid and gas) will simply bypass the scale catcher. During turnaround, the scale catcher can be cleaned easily and is completely accessible from the top of the reactor for vacuuming out the particles that have settled. Once cleaned, it is ready for the next cycle.

Figure 2 shows the improvement in cycle length for a North American Refinery’s FCC pretreater after installation of a Haldor Topsoe scale catcher. The plot shows how the unit’s cycle length was prolonged from 380 run days before installation of the scale catcher to 630–680 run days after installation.

Second generation scale catcher – HELPsc
Haldor Topsoe’s second generation scale catcher provides a dual-stage system for improved scale catching efficiency by combining sedimentation and filtration stages. The design of the HELPsc technology reduces pressure drop build-up during the entire life cycle of the unit.

The HELPsc is installed above the distribution tray and, like the others, does not occupy reactor active catalyst space, providing maximal utilisation of the reactor volume, see Figure 3.

It is designed in such a way that the gas flow is not restricted at any time during the cycle length, and the pressure drop over the HELPsc is still negligible.

When the filters are saturated and fully plugged with particles, the liquid bypasses the filters and flow directly to the distribution tray.

Operating principles
The liquid and gas reaching the HELPsc have two paths.
1. The gas will bypass the system and reach the distributor tray below without any restrictions, (Figure 4).
 2. The liquid is directed to the sedimentation section of the HELPsc, which is the first of two solid liquid separation stages of the system, see Figure 5.

The sedimentation chambers provide extended residence time for the inlet liquid, and larger particles will settle down at the bottom of the chamber. Beside the settling of the larger particles, some smaller particles also agglomerate, and some settle at the bottom of the section as well.
Figure 6 shows the cumulative size distribution of particles collected at the sedimentation section in one of Haldor Topsoe’s HELPsc.

The above figure illustrates that 50% of the collected particles are smaller than 200 µm, proving a high separation rate of the larger particles.
After the sedimentation section, the liquid enters the catalyst filter elements (Figure 7A). The filter elements consist of two parallel filter sections, each consisting of two filters in series.

Initially, only the first layer of the filter sections is exposed to the particles in the feed streams. The particles are captured by the first filter, while an almost completely particle free liquid will pass to the second filter and thus pass down to the distributor tray (Figure 7B).

During the cycle, the exposed filters are gradually saturated by the trapped particles, and the liquid level will rise until the filter is fully plugged. Thereafter, the liquid starts to overflow the first filter and expose the second filter to the complete feed stream (Figure 7C).

When the second filter is saturated with particles, the liquid will start to overflow directly to the tray, and the filtration section of the HELPsc will have reached its capacity.
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