Faster Reactor Cooldowns

Faster Reactor Cooldowns

One of the most frustrating situations to endure is the slow cooldown of a reactor (or series of reactors) when shutting down a hydrocracker, hydrotreater, or renewable feed unit. It often seems that the entire refinery is watching the reactor cool, especially as reactor temperatures drop below 200⁰F. This blog article will cover ways to speed up the cooling process while maintaining equipment integrity and process safety. Investing a little time and effort into the cooldown process can pay big dividends!

Without procedures and processes to accelerate the cooldown, it can take 80-100 hours to fully cool down a reactor. The chart below shows a typical reactor cooldown curve indicating both the hottest catalyst temperature and the hottest skin temperature (typically the skin temperature on the bottom head of the reactor). Your reactor(s) may have a different cooldown time, but I’ll bet the curve looks similar to the chart below.

Considering that maintenance costs and lost profit opportunities can range from tens of thousands to hundreds of thousands per day, the opportunity to improve reactor cooldown can easily be worth at least a few million dollars for many units!

Pull the cracked stocks early

I don’t fully understand the reasons, but 40 years of experience have shown that pulling out cracked stocks 8-12 hours before pulling all the feed helps to reduce the light hydrocarbons on the catalyst.

Maximum cooldown rate for equipment safety and reliability

A well-established maximum cooldown rate for high-pressure equipment is 100⁰F per hour. It is imperative that this rate not be exceeded to avoid high stresses in the thick metal of high-pressure equipment. During the initial cooldown, it is easy to cool down at 100⁰/hour using the reactor furnace to adjust the cooling rate. The cooldown rate of 100⁰/hour applies directly to the reactor skin temperatures, but this rate is also a good idea for reactor catalyst temperatures. Having the same cooldown rate keeps things simple and helps keep portions of the reactor skin not covered by thermocouples within the cooling rate limits.

Smooth and steady

The cooling rate should also be smooth and steady. I came across one operator who cooled the reactor 100⁰F in 15 minutes and then took a 45-minute break! This “unsteady” cooling rate is extremely hard on equipment health.

Hot stripping— how long and how hot?

Here are rough guidelines to minimize the time required for hot stripping:

  • A rough rule of thumb is that the hot strip time can be cut in half for every temperature increase of 50⁰F, so hotter is much better.
  • The hot strip should be maintained for a minimum of 6-8 hours or at least two hours past the time that liquid stops appearing in the cold separator (whichever is longer).
  • Hot stripping should be done at normal operating pressure while maximizing the flow of recycle gas.
  • Reactors that have experienced maldistribution or high-pressure drop will need additional hot strip time because the flow channels through the reactor. When in doubt, go longer and hotter to save time later.

 

See my next article for a detailed discussion of hot stripping and the pros and cons of alternate procedures using chemical injections.

Feed/effluent exchanger bypass

A bypass around the feed/effluent exchanger(s) (shown below) decouples the heat transfer from the product to the feed and drastically speeds up reactor cooling. Any unit that does not have a feed/effluent bypass should add one soon. The bypass is also an important safety feature to help cool a reactor in an emergency. Use the feed/effluent bypass to speed up cooling when the cooling rate begins to fall below 100⁰F/hour.

Nickel carbonyl testing

A previous blog article discussed the hazards of nickel carbonyl formation and the procedures to prevent formation above 1 ppb. As a reminder, the reactor should not be cooled down below 400⁰F until the carbon monoxide (CO) content of the makeup and recycle gas is less than 10 ppm measured with the apparatus shown below.

Remember that renewable units tend to produce a substantial amount of CO during normal operation, so an efficient method to remove the CO must be in place for a quick reactor cooldown.

Keep the pressure on

Maximizing recycle gas flow helps to cool the reactor faster, but you get more mass flow from the recycle gas when you keep the pressure high as long as possible.

Brittle fracture prevention

Another reminder: as reactor skin temperatures approach the minimum pressurization temperature (MPT), the reactor system pressure must be lowered to prevent brittle fracture. The typical pressure limit when any metal temperature is below MPT is typically 25-30% of the maximum allowable working pressure. Some units have developed MPT “curves” rather than an MPT “point.” The curves allow the pressure to stay higher while gradually reducing pressure with the change in reactor skin temperatures. Becht metallurgists can help you develop MPT curves for your reactor if desired.

Whoa there—slow that compressor!

As the reactor catalyst temperatures approach the compressor outlet temperature, slowing down the compressor will lower the temperature of the gas going to the reactor and help the reactor cool down further.

Cool nitrogen vapor injection

Many refineries bring in nitrogen pumper trucks and inject cool nitrogen vapor into the recycle stream going to the reactor. Injecting nitrogen vapor can be done without special facilities, but the temperature of the vapor going to the reactor should be maintained above 40⁰F to minimize metal stresses in the equipment and to prevent water from freezing.

Nitrogen injection is usually started as the reactor temperatures approach ~300⁰F to accelerate cooling down to final target temperatures. The nitrogen injection isn’t cheap, but it does help cooling and can have a good payout on the cost with time saved.

Cold nitrogen liquid injection

A few refineries inject liquid nitrogen into the recycle stream for added cooling. While liquid nitrogen injection will cool faster than cool nitrogen vapor injection, the injection point requires a specially designed quill and spool to prevent the cold liquid from cracking the steel. Metallurgists and mechanical design experts should be consulted to obtain a properly designed system for liquid nitrogen injection.

Never inject liquid nitrogen without piping specifically designed for this purpose.

Temporary cooling exchanger

A recent innovation that offers significant help in cooling a reactor is to add a temporary cooling water exchanger to the outlet of the compressor as shown in the diagram below. Rental companies offer exchangers that can be installed prior to a reactor shutdown. Some refineries have installed piping stubs and valves to facilitate the exchanger installation.

Water flooding the reactor

An innovation that a few companies use extensively is water flooding the reactor. Water flooding has many advantages: it cools down the reactor(s) quickly and does not require inert entry into the reactor. Reactors that dump water and catalyst together will dump catalyst faster and more completely. A few disadvantages are the need to separate water from the catalyst/water slurry dumped out of the reactor and to dispose of the water.

For the water flooding process, the reactor is cooled until the hottest metal temperature is below 200⁰F. As you know, you never want water to boil with the stainless-steel lining inside the reactor. Once the appropriate temperature is reached, the reactor is blinded and the top head removed. Water is then injected into the reactor at an appropriate, calculated rate so as not to quench the reactor shell too quickly.

Once the reactor is filled and cooled down, the catalyst/water slurry is dumped with the reactor under an air environment. The wet catalyst prevents pyrophoric reactions. The air environment is much safer than an inert environment and it facilitates entry into the reactor for inspecting internals and catalyst loading without an inert environment.

Many years ago, a diesel hydrotreater reactor was shut down without water flooding and then, four years later, it was shut down again using a water flooding procedure. A comparison of the two reactor cooldown curves (below) shows that water flooding cuts about one-third of the time off the total cooldown—reducing the total cooldown time from about 90 hours to less than 60 hours.

Water flooding the reactor(s) must be done after an appropriate management of change process, and it must be done using the right procedures. For example, the reactor foundation must be checked to make sure it can tolerate the additional weight of the water. The correct processes must be put in place to separate water from the catalyst and to dispose of the water. I highly recommend you seek the advice of a licensor and catalyst handling company with extensive experience in water flooding reactors.

Application in your unit(s)

  1. How long has it been since you reviewed your shutdown procedures to improve the speed and efficiency of the shutdown and cooldown processes? Many refineries do not put much emphasis on this area.
  2. What is the “size of the prize” for reducing shutdown time on your hydrocracking and hydrotreating units. The cost to implement many of the ideas listed in this blog is small, but the payout can be huge.
  3. Are your reactor cooldowns smooth and steady to help protect your equipment?
  4. Do you have the correct written procedures to prevent nickel carbonyl formation and brittle fracture?
  5. Have you optimized the hot strip time for your reactor shutdowns? (More to follow on this topic in the future!)

 

Until my next article, stay safe! You can always reach out to discuss optimizing your own reactor cooldown procedures or other hydroprocessing challenges. I also invite you to attend my upcoming Hydrotreating and Hydrocracking Process Technology training class from April 2-4, 2025, taking place in Houston, Texas, and virtually.

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About The Author

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Jeff Johns has over 35 years’ experience in the petroleum refining industry. He was honored as a Chevron Hydroprocessing Fellow (Chevron’s highest technical recognition) for contributions to Chevron and to the industry. Jeff has expert knowledge of hydrocracker and hydrotreater design/operation, optimization, and troubleshooting, and has substantial experience in other key refinery processes. Jeff managed hydrocracking and hydrotreating technology in Chevron’s refineries worldwide where he developed and implemented best practices and projects to improve safety, reliability, and profitability. One of his special interests as a technology mentor was developing and delivering training. For 20 years, Jeff led an ad hoc Industry Committee of hydroprocessing experts dedicated to sharing safety and reliability information among North American Refiners. He was a member of the AFPM Q&A Panel in 2004 and directed multiple technology seminars as a member of the AFPM Q&A screening committee. Jeff served on the Board of Directors for Advanced Refining Technologies (ART). Jeff holds a B.S. degree in Chemical Engineering from the University of Utah. He holds six patents in hydroprocessing technology.

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