A topic that frequently comes up in consultation is how to posture a hydrotreater or hydrocracker when a short, temporary shutdown is needed (with no catalyst change). These short outages can last from a few hours to a few weeks and sometimes, rarely, a few months. Often, the biggest choice in how to posture a unit is whether to leave the reactor system under a H2 atmosphere or put it under a N2 atmosphere. This month’s blog will discuss the hazards associated with these short “Pit Stop” shutdowns. We will start with the ugly incidents and then discuss the hazards and principles involved.
Examples of Incidents from Improper Reactor Postures
- Diesel Hydrotreater Runaway
A diesel hydrotreater experienced a power outage resulting in loss of recycle flow from the reciprocating compressor and loss of feed to the unit. The reactor was at about 550°F (290°C) at the time of the outage. The reactor was left stagnant under a H2 atmosphere for a few days while waiting for power to be restored. As an older reactor design, the unit only had temperature indicators (TIs) on the reactor inlet and outlet.
After a few days, the operators noticed that the top of the reactor was starting to tilt. They found that the reactor shell had yielded due to extremely hot temperatures. The reactor was cooled with N2, and the coked catalyst was jack hammered out of the reactor. Eventually, the portion of the shell that melted was cut out and the two halves of the reactor were welded back together. It was estimated that the shell temperature must have been well over 2000°F (1100°C) for the shell to yield.
- A stagnant reactor under H2 atmosphere can have reactions occur developing into a reactor runaway, even at a moderate temperature of 500-550°F (260-290°C).
- Unless heat is removed, a runaway reaction can reach temperatures that can melt chrome-moly steel – even in a hydrotreater.
- Never leave a reactor stagnant under a H2
- Recognize that the latest catalyst technology (with much higher activity) creates greater risk of temperature excursions in hydrotreating and hydrocracking reactors.
- VGO Hydrotreater
A refinery experienced a power outage causing a hydrotreating unit to lose feed and makeup H2. The steam turbine on the recycle compressor provided recycle H2 flow for about 5-10 minutes before the steam header pressure sagged. A small amount of makeup H2 flow was used to purge through the reactor.
The pressure on the unit was reduced to about half of normal pressure. Operators then decided to keep some pressure in the unit to make it easier to startup later. They chose to watch the Tis and depressure the reactor further if any temperatures started to climb.
After several hours, the reactor temperatures in the bottom bed started to climb very quickly. Operators attempted to activate the emergency depressuring system, but the system malfunctioned. To depressure the unit, an operator had to open a valve manually in the field. By the time the unit was depressured, reactor temperatures had climbed to over 1500°F (815°C).
During the investigation, it became obvious that reactions had been occurring before the Tis started climbing in a portion of the bed that did not have Tis in it. The small purge on the reactor eventually brought the elevated temperatures close enough to the Tis to reveal what was happening. Procedures for this unit were updated to make sure that the reactor would be fully depressured upon loss of recycle H2 flow in the future.
- Reactions are likely to occur in an idle reactor under H2 that may not be shown by reactor Tis when there is only a small purging flow.
- Reactors should always be fully depressured upon loss of recycle H2 flow – even if N2 is not immediately available.
- Reactor emergency depressuring systems should be tested periodically to prove functionality.
- Hydrocracker Hurricane Shutdown
A VGO hydrocracking unit was being shut down due to an approaching hurricane. The procedure called for the operators to stop feed to the unit, conduct a short hot strip to remove the bulk of the oil, and then cool the reactor to 450°F (230°C). After cooling, the procedure correctly required a switch to a N2 atmosphere.
Unfortunately, operators ran out of time and had to switch the unit over to N2 without cooling it down to evacuate before the approaching storm. The reactor sat at a temperature of about 700°F (370°C) for more than 2 weeks before operators could attempt to restart the unit.
Operators were able to successfully startup the unit following normal restart procedures. Unfortunately, the reactor showed a high pressure drop and extremely poor activity upon restart. The catalyst had to be changed out much sooner than planned and the excessive pressure drop was determined to result from the large amount of coke found in the reactor. It became obvious that a reactor sitting for a long time at elevated temperatures caused substantial coking to occur, even under N2 atmosphere. Procedures were changed to allow a longer time for shutting down this unit for a hurricane.
- A hydroprocessing reactor should be cooled to 450°F (230°C) or below for a pit stop shutdown, even if the reactor will be put under N2.
- Enough reactions can occur at high temperatures in an inert reactor to form substantial coke – in this case, enough coke to cause a substantial pressure drop in the reactor.
- Plan enough time to shut down a unit correctly, even when faced with a surprise like a hurricane.
Hazards from improper idling of a Hydroprocessing Unit
Reactor TIs are not valid without recycle flow
Modern hydroprocessing reactors are often equipped with dozens of temperature indicators (TIs) in the catalyst beds and the reactor shell. This plethora of TIs can mistakenly convince us that we can accurately measure the temperature in all parts of the reactor and catalyst bed. Some refiners have postured reactors in a risky position thinking they can act, “if they see a reactor temperature starting to rise.”
When can we trust TIs to accurately measure temperature? The answer is only when there is adequate flow through the bed. During normal operation with high flow rates of recycle gas, the TIs do a great job measuring temperature of the flow as it passes the location of the TI. In trickle bed reactors, flow calculations show turbulent, well-distributed flow only with high rates of recycle gas and a Reynolds number higher than 75. (Calculating the Reynolds Number of flow through a packed bed and establishing a minimum number for adequate turbulent flow for good distribution may be the topic of a future blog.) The bottom line is that we can only trust temperature measurement when there is recycle gas flowing through the reactor.
Experience and heat transfer calculations have taught us that a catalyst bed is a pretty good insulator when recycle gas is not flowing. In fact, a TI in a reactor with minimal flow is only affected by conditions within 12 – 18 inches of its location! If you draw an 18” sphere around all the TIs in a reactor, you will see that most of the catalyst bed is not adequately covered by temperature measurements under minimal flow conditions. This is especially true since many reactors have TIs located only at the inlet and outlet of each bed.
Reactor Runaway Risk
When we have a pit stop on a unit, it is tempting to leave the reactor under a H2 atmosphere because it will facilitate a faster startup when the unit is ready. However, leaving a reactor under a H2 atmosphere will allow reactions to occur. If there is not adequate flow to remove the heat created by the reactions, then a temperature excursion and potential runaway will occur.
Since reactions can occur under a H2 atmosphere, and we can’t adequately measure reactor temperatures without recycle gas flowing, we are left with two possible safe postures for the reactor during a pit stop shutdown:
- If recycle gas will be flowing at normal rates, then the reactor can be left under a H2 atmosphere at moderate temperatures (~450°F/230°C) and operating pressure.
- If recycle gas will not be flowing at normal rates, then the reactor should be switched to a N2 atmosphere at moderate temperatures (~450°F/230°C).
These two postures are the only safe reactor postures for a short unit pit stop, as illustrated by the previously cited incidents.
Catalyst Coking and Pressure Drop Risks
The examples listed at the beginning of this article illustrate the risk of catalyst coking if catalyst is left at elevated temperatures and low H2 partial pressure. One incident even shows the risk of catalyst left at high temperatures under N2! In a few cases, the coke formation was so substantial that the coke caused a high pressure drop.
As discussed, it is wise to lower catalyst temperatures to 450°F (230°C) or less during a pit stop shutdown. I have never seen catalyst coke or deactivate when it is kept at 450°F (230°C) or less either in H2 recycle gas circulation or under stagnant N2.
Catalyst Desulfurization Risk
A question that must be answered is how long can one keep catalyst under recycle H2 without the catalyst becoming desulfided and possibly reduced? Let’s assume that you decide to idle your hydroprocessing unit for one week by pulling feed and keeping the recycle H2 flowing. You should cool the reactor to 450°F (230°C) or less and take H2S measurements in the recycle gas periodically, but how long can you sustain this operation without risk to the catalyst?
Catalyst vendors may disagree with my answers, but here is my experience:
- For fresh catalyst, defined as in service less than one month, the risk of catalyst desulfiding and reducing is real. The amount of H2S in the recycle must be kept high enough to prevent the catalyst from losing sulfur. Your catalyst vendor can provide guidelines for this scenario.
- For aged catalyst, defined as in service for more than one month, the risk of catalyst desulfiding is minimal if there is any measurable H2S content in the recycle gas and reactor temperatures have been lowered to 450°F (230°C) or less. As catalyst ages, coke formation tends to stabilize the sulfur on the active metals and helps to prevent desulfiding. Many vendors say the H2S must be 1000 ppm, some say 100 ppm, but I have never seen catalyst desulfide if there is at least 1 ppm of H2S in the recycle gas.
Here is my recommended procedure for a pit stop shutdown:
- Lower reactor temperature and pull feed from the unit.
- Raise reactor temperatures 50°F (28°C) above the typical catalyst temperature and use recycle H2 to strip the bulk of the oil from the catalyst
- Once most of the oil has been stripped from the catalyst, cool the reactor at up to 100°F/hour (56°C) to 450°F (230°C).
- Stop amine circulation if the unit has an H2S absorber to keep H2S in the system.
- Stop any H2 bleed from the system to keep H2S in the system.
- Test for CO in the makeup H2 and recycle H2 streams to make sure that no nickel carbonyl will form. Although it may not be planned to change catalyst or open the reactor loop, the CO test only takes 5 minutes to make sure that nickel carbonyl won’t be present if the work scope changes.
- If CO is less than 10 ppm, proceed
- If CO is greater than 10 ppm, purge it out of the system with a hydrogen stream that contains less than 10 ppm CO or switch the reactor system to N2 (if the available N2 contains less than 10 ppm CO).
- If the unit will continue to circulate recycle H2, then keep the reactors at ~450°F (230°C) while circulating.
- Measure the H2S level in the recycle gas to keep it above 1 ppm.
- If the recycle gas falls below 1 ppm, then add some sour gas to the system to raise H2S levels or inject DMDS to create H2
- If it is not possible to keep H2S above 1 ppm, then switch the system to N2.
- If the unit will not continue to circulate recycle gas, then switch the system to N2.
- The most efficient method to switch from H2 to N2 is by pressuring and depressuring the recycle gas at least 3-4 times
Is switching to N2 necessary if the pit stop will not last very long?
This is a question I have heard many times. It is usually presented with a short time as the justification such as, “Do we really have to switch to N2 because the recycle compressor will only be down for 30 minutes?”
I wish I had a nickel for every time I heard the phrase that it will only take 30 minutes – and then I wish I had a penny for every time it was supposed to take 30 minutes and then took hours or even days……
Most fixes take longer than anticipated and are more complicated than hoped for. My philosophy has always been, “know that it is safe.”
Application in your unit(s)
- Does your unit have clear procedures to perform a “Pit Stop” shutdown?
- Do your procedures have criteria in them for how to posture the reactor depending on the situation?
- Are your personnel trained on the hazards of a stagnant reactor under an H2 atmosphere?
Do you need assistance in posturing a hydrotreater or hydrocracker when a short, temporary shutdown is needed? Feel free to drop us a line.