Taking hydroprocessing reactor cooling off the critical path

Rapid cool-down of a typical hydroprocessing unit’s recycle gas compressor offgas can be used to cool spent reactor catalyst while avoiding the need for liquid nitrogen cooling during pre-turnaround phase.

Rene Gonzalez
Editor, PTQ

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Article Summary

The increasing demand for petrochemical feedstocks from refineries requires higher process complexity. Much of the complexity involves catalytic conversion technology, particularly hydroprocessing reactor units. However, with the need to mitigate CAPEX, improvements to existing reactors avoids having to invest in building new hydrotreaters and hydrocrackers.

Instead, new multi-functional catalyst formulations combined with upgraded reactor internals and downstream fractionation can significantly increase product yield. However, safely removing spent catalyst loads can become cost-prohibitive considering the amount of nitrogen and time required to cool down the reactor prior to manway entry.

In a margins-compressed market, even a 24-hr reduction in the pre-turnaround phase can save refiners millions of dollars. Quickly isolating hydroprocessing units from the process loop (for the purpose of recharging with the newest catalyst formulations) requires rapid cool down of the hot reactor beds to below 100°F for targeted upgrades.

Unclear pricing policies combined with relatively high production costs are why a lot of refiners  upgrade existing hydroprocessing units (instead of buying new reactors), driven by the following factors:
- Take reactor cool-down off the critical path and ensure the projected turnaround completion date is met (or even reduced)

- Supply glut in gasoline and gasoil, along with long-term LSFO objectives, compel HDT, HC and HDS unit revamps

- Increasing range and complexity of HDT, HC and HDS catalysts predicate more frequent catalyst change-outs, incentivizing operations managers to look for reactor and catalyst bed cooling solutions that can reduce the pre-turnaround phase by one or two days

- Barring no complications with spent catalyst cool-down, a common objective among refiners involves installing latest-generation reactor internals and catalysts, enabling the hydrotreating and hydrocracking of heavier and more difficult feeds such as deasphalted oil, vacuum gas oil (VGO), while increasing conversion capability

- Rapid and efficient availability of a wide variety of peripheral services during the pre-turnaround cooling stage, such as run-down cooling of certain intermediate and final product streams and control of refinery flare calorific value throughout all phases of process unit turnarounds.

Reducing turnaround duration can dramatically increase refinery profitability and mitigate margin compression.  Historically, cooling reactor catalyst has often been on the critical path when maintenance was being performed on those units.   An alternative approach to catalyst cooling eliminates nitrogen injection and shortens the duration of phase 2 cooling segment (from about 200°F to below 95°F).

Once the plant shuts down, a considerable amount of money is being spent without any revenue being generated. During a turnaround, refiners must cool the catalyst in hydroprocessing units and catalytic reformers from the unit’s normally high operating temperatures to near ambient temperatures in two separate steps. The first step uses the equipment in the unit to cool the catalyst to approximately 200°F. The second step has often used liquid nitrogen but faces numerous disadvantages.

Accelerating catalyst cool down will allow entry to the reactor vessel much quicker, possibly permitting the plant to move this vessel off the ‘critical path’, so the plant can restart the units faster. Alternatively, if this unit is not on the critical path, unique cooling arrangements can still allow the plant to implement a capital project or perform other work on the unit that might not otherwise have been possible.

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