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Apr-2012

Optimised reactor internals for a hydroprocessing unit

Optimised distributor and quench box design can improve catalyst performance and unit reliability

TAI-SHENG CHOU
H2Advance
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Article Summary
Catalytic reactor improvement projects satisfy clean fuels requirements with reduced capital costs. In response to the refining industry’s need to meet clean fuels specifications with minimal capital investment, there has been acceleration in the development of catalysts with higher catalytic activity. It is also known that utilisation of state-of-the-art reactor internals can maximise catalyst utilisation while improving unit reliability.

The function of reactor internals is to ensure a full utilisation of the achievable catalyst activity. Therefore, designing reactor internals for hydrotreating, hydrocracking or hydroisomerisation dewaxing processes is as important as catalyst activity in improving unit performance and reliability. Indeed, optimum design of reactor internals for hydrocracker or dewaxing reactors alleviates concerns of hot spot formation (or temperature excursion/runaway), resulting in improved unit yield and reliability.

The performance of reactor internals depends on:
• Process design for achieving uniformity of flow distribution and temperature spread
• Mechanical layout and design for overall internals integrity
• Supervised field installation to ensure tray seal and acceptable tray levelness.

Consequently, applications of reactor internals require many years of accumulated experience to ensure successful reactor project execution.
Lessons learned from past project executions and post-startup monitoring can serve as guiding principles for future reactor projects.
This article evaluates optimised distributor and quench box design for improving both catalyst utilisation and unit reliability. It illustrates a compact quench zone dsesign with a reduced number of manways, with the purpose of reducing capital costs while shortening unit turnaround time. It also discusses other important issues related to reactor internals. Lastly, this article highlights hydrocracker upgrading using good thermometry and optimised quench internals to illustrate the consequent unit reliability improvements.

Distributor tray optimisation
An optimised reactor distributor should provide uniform fluid flow distribution over the cross-sectional area of the reactor. In addition, an optimised distributor should encompass the following characteristics:
• Reduced sensitivity to tray out-of-levelness
• Good flow coverage in the reactor wall region
• Good spray angle for tray elements
• Low pressure drop
• Ease of maintenance
• Maximum catalyst loading
• Minimal tray leakage.

Reduced sensitivity to tray out-of-levelness
A tray installation involves tray beams and tray panel pieces, where some degree of out-of-levelness for the installed tray pieces is inevitable. Often the specification requires tray out-of-levelness to be less than 10mm high to low (or ± 5mm). In view of this narrow tolerance, the distributor element design should provide a reduced sensitivity to tray out-of-levelness.

One solution to minimise sensitivity to tray out-of-levelness is utilisation of bubble-cap or vapour-assisted lift trays. Chimney trays with proper orifice designs (including orifice diameter and its elevation on the chimney) will also provide comparable low sensitivity to tray out-of-levelness. This is due to the nature of orifice flow, where the volumetric liquid flow rate through the orifice is proportional to the square root of the liquid head above the centre of the orifice. Consequently, a small difference (less than 10mm) in the liquid height among chimney pipes will result in minimal variation in the liquid rates between the two chimney downcomers.

Good flow coverage in the reactor wall region
It is important to provide good distributor element coverage near the reactor wall region. The wall flow region (within 6in or 15cm of the wall) comprises 19% of the total cross-sectional area for a 10ft ID reactor. Some older generation bubble-cap distributor elements occupy a large footprint, making it rather difficult to fit the bubble-cap elements near the reactor wall. Since this region represents a high percentage of the total catalyst volume, it is vitally important to provide sufficient coverage for distributor elements here.

A chimney downcomer distributor with a reasonable spacing between downcomers can be neatly fitted in the reactor wall region. Chimney elements occupy a small footprint, which facilitates sufficient element coverage near the reactor wall. A chimney downcomer or distributor with good element coverage in the reactor wall region improves catalyst utilisation for the loaded catalysts.

Good spray angle for tray element
The liquid spray angle created by the shearing force of the vapour on the liquid sheet is critical for providing sufficient liquid coverage for the catalyst located below the distributor. If the flow distribution element cannot provide sufficient coverage for the top layer of inert/ catalyst, provisions have to be made for liquid flow dispersion below the distributor. An increase in the distribution element density will improve the liquid coverage over the cross-sectional area of the reactor. However, for tray elements designed with a minimal spray angle, proper liquid flow distribution in the active catalyst bed may require a deep layer of large-sized inert packing for facilitating liquid redistribution. This is highly undesirable in view of wasted reactor volume.

Liquid spray angle created by the shearing force of the vapour on the liquid sheet at the outlet of the elements can significantly improve the uniformity of the liquid distribution. However, the efficiency of the fluid distribution depends on the spray angle achievable. In general, chimney downcomers should provide an optimised liquid spray pattern at the exit of the downcomer. A downcomer tray with 5in spacing between downcomers can attain nearly 100% liquid coverage, with about 7in (18cm) clearance between the bottom of the downcomers and the top of the inert packing.
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