Bayernoil mild hydrocracker CATnap application
On 21 January, Bayernoil, Neustadt, successfully shut down the mild hydrocracker (MHC) using the CATnap catalyst passivation process applied by Cat Tech (Europe) Ltd
Frank Wentzlau, Bayernoil Raffineriegesellschaft mbH
Gary Welch and Karl Thew, Cat Tech International Ltd
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The CATnap technology was chosen as a means to reduce catalyst reactivity, eliminate dust problems, reduce the shutdown time window and minimise potential volatile hydrocarbon (LEL) issues. This was the maiden catalyst replacement for this unit. With no history, there were many uncertainties with how the unit would respond during shutdown and unloading.
The application was a total success, accomplishing all of the objectives. The unit was shut down one day quicker than it would have taken with the “conventional” procedure. There were no issues with reactive catalyst, and toxic and pyrophoric dust was completely eliminated. The unit was LEL free after a normal pressure/depressure procedure. The catalyst was not unloaded under air, which will be an option to accelerate the procedure and minimise nitrogen cost further for future applications.
Bayernoil was the third refiner in Europe to apply the technology. CATnap is a process to chemically treat hydrotreating and hydrocracking catalyst so that they can be safely handled in air. This provides the potential to eliminate inert entry for catalyst removal from reactors, avoiding a dangerous operation and minimising the cost of nitrogen for unloading.
Other benefits include reduced downtime through shutting down the reactor without hot hydrogen stripping and better efficiencies in unloading. Dust particles adhere to the catalysts, eliminating dust problems that can produce very toxic and pyrophoric material. Furthermore, CATnap is an effective way to mitigate LEL problems in units with a history of high LEL.
The CATnap process involves applying an organic chemical during oil recirculation while cooling the unit. The chemical coats the catalyst surface with an organic film that retards oxygen penetration to the reactive metal sulphides in the catalyst pores. The carrier oil must have the proper balance between viscosity, pour point and flashpoint. The viscosity should be high enough to help the chemical provide a chemical barrier, but the pour point must be low enough to allow the catalyst to be free flowing at the catalyst removal temperatures. This was a bit of a challenge at Neustadt, where the ambient temperature was -10°C and below in January. The IBP and flash point need to be sufficiently high to ensure no light hydrocarbons are left in the reactor, which might be difficult to purge out and produce high LEL in the reactor.
For the MHC, a middle distillate was chosen as the carrier oil. It had an IBP >200°C, pour point of -3°C, flash point of 85°C and viscosity of 3.4 cSt at 40°C. At 08:00 on 21 January, the feed rate was reduced to 170 m3/hr and the temperatures began reducing at ~25°C/hr. After five hours, the reactor inlet temperatures were around 300°C and the carrier oil was brought to the unit to flush out the normal feed oil. Samples were taken at the feed drum and fractionator bottoms and analysed for viscosity and flash point to follow the flushing operation. After about seven hours of flushing, the unit was put on internal oil recycle.
The CATnap passivation chemical was injected over two hours. This was followed by five hours of oil circulation while continuing to cool the reactors. The inlet temperatures could only be reduced to ~130°C, which was a bit higher than the target of 120°C. This was deemed to be acceptable and the bulk oil was pushed out with maximum hydrogen flow.
Reactor cooling and gas freeing
This completes the application of the CATnap process. The de-oiling, depressuring and gas freeing are according to the normal procedures. It took about 24 hours to depressure the unit and get the reactor wall temperatures down to ~40°C. This was pretty much according to schedule. The anticipated timeline and the licensors recommended timeline/procedure are show in the figure below. By eliminating the diesel flush, heating and hot hydrogen strip, the CATnap procedure reduced the shutdown timeline by one day.
We went through ~7-8 pressure/depressure cycles with nitrogen to gas free the reactors. The pressure was increased to ~3 bars and depressured to ~0.6 bar. The hydrocarbon content was measured at the outlet of the second reactor. It was found to be 600 ppm. This would be equivalent to ~10% LEL. That is borderline acceptable, but gas samples at reactor outlets are always suspect. It was decided to open the reactors. The initial reading on the first reactor was 60 ppm hydrocarbon and gave 0% LEL on an explosivity meter. This was most encouraging. It is possible that additional time could have been saved by reducing the number of pressure/depressure cycles.
Bayernoil elected to unload the catalyst under nitrogen rather than air. Since the catalyst was primarily dumped, the cost of nitrogen and the safety issues were not as severe as in vacuuming operations. We did not want to apply soda ash to the reactors and tray sections for passivation and cleaning. Our preference was to maintain the unit under nitrogen for unloading, cleaning and reloading.
The reactor was entered for tray removal. Many of the tray sections had double and triple nuts on the hold-down bolts, making their removal much more time consuming than planned. Catalyst was supposed to be unloaded by dumping through dump nozzles on each bed. There were some problems with free-flowing catalyst on bed 3 of the first reactor. This is probably a result of the catalyst being dense loaded and several unit upsets during the run cycle. The catalyst was very compacted. As a rule, CATnap will mitigate this problem. Hot hydrogen stripping will dry out these compacted pockets and cause them to cake worse. CATnap does not dry out the catalyst and provides some lubricity for free flowing.
Our actual CATnap application went very much according to plan. The timeline was achieved and the unit was LEL free. Catalyst was stable to air and there were no dust issues. The compacted catalyst unloading and tray work are not impacted by the CATnap technology and were subject to normal catalyst handling delays and problems; however, inert entry could have been eliminated and thereby reduced any timeline effects. Bayernoil will have a second CATnap application for our CHD unit in March 2011. This unit offers new challenges in that it does not have a feed drum for oil recycle and it has a documented history of serious LEL problems.
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