Accelerating reactor cool down prior to turnaround
The post-pandemic demand for transportation fuels and petrochemicals requires higher process complexity.
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Much of the complexity involves catalytic conversion technology, particularly hydroprocessing reactor units. With the need to mitigate CAPEX, improvements to existing reactors avoids having to invest in building new units.
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. 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. Many refiners will upgrade existing hydroprocessing units 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)
· 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.
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. There is immense pressure to work as efficiently as possible to get the unit or the entire plant back on stream as soon as possible. However, there are always events that are on the ‘critical path.’ If those events can be shortened, it’s possible to be able to decrease the turnaround duration.
The normal cool down process generally consists of two distinct phases. During the first phase, feed is blocked in and the furnace is shut down. The recycle gas compressor circulates hydrogen-rich gas through the feed/effluent exchanger, through the furnace (though the furnace is not operating), and into the reaction vessel where the gas picks up heat from the hot catalyst bed.
The second phase requires purchasing liquid nitrogen with a vaporizer/feed system and injecting it ‘once through’ into the reactor system. Given the very large mass of the reactor metal and catalyst to be cooled, this expensive operation generally takes several days.
On the high side, if moving the vessel off the critical path allows an entire refinery to be back online 2 days sooner, then a 150,000 bpd per day refinery with a $25 refinery feed marginal value could net $7,500,000 without ever considering the nitrogen and other logistical savings.
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.
For this reason, a patented process has been developed by Aggreko that can typically accelerate catalyst cooling from 200°F to under 100°F within 12-24 hours. The process takes recycle gas compressor discharge and cools the stream against a water/glycol solution in a heat exchanger positioned downstream of the recycle gas compressor. The recirculated solution is then cooled in a mechanical chiller.
Accelerating catalyst cool down will allow reactor manway entry much quicker, possibly moving the vessel off the ‘critical path’, and onto a faster unit startup. Alternatively, if this unit is not on the critical path, this unique cooling arrangement 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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