Operations adjustments can better catalyst-cooler operations

The resid-fluid catalytic-cracking unit at LG-Caltex Oil Corp.'s Yosu, South Korea, refinery was designed by Stone & Webster and began operations in December 1995.

The coolers are part of the resid fluid catalytic-cracking (RFCC) unit. This refinery's RFCC unit has two regenerators, and the two catalyst coolers are installed only on the second-stage regenerator.

In the course of troubleshooting its system, LG-Caltex also learned how to better operate and design catalyst coolers. The authors describe recommended leak-detection and leak-repair procedures in this article.

The RFCC unit at LG-Caltex's Yosu refinery is designed for 50,000 b/d of unhydrotreated Light Arab atmospheric tower bottoms (ATB) or 63,000 b/d of unhydrotreated Murban/Oman ATB.

Typical feed to the unit is a mix of unhydrotreated Mideast, African, and other ATB. The Conradson carbon residue (CCR) content is generally 5-7 wt %, the nickel content about 3-17 ppm, and the vanadium content 5-32 wt %. Total metals on the equilibrium catalyst (Ecat) is 10,000-12,000 ppm, and the nickel/vanadium ratio is normally about 50/50.

The company's operating philosophy is to maximize the unit throughput in the gasoline mode. The unit operates against multiple constraints depending upon seasonal demands and the feed quality. Maximum feed rates are 70,000-78,000 b/d.

Catalyst-cooler operation

When there is a leak in a tube, the standard procedure is to isolate the water flow to this tube and abandon the tube. Isolating the tube, however, lowers the heat removal capacity of the cooler.

There is a maximum number of tubes that, when isolated, the catalyst coolers cannot perform enough cooling, and the RFCC unit throughput must be reduced. Before this occurs, however, the operator shuts down the unit to replace the damaged tubes with new ones.

Fig. 1 [166,546 bytes] shows a sketch of the catalyst cooler with the catalyst slide valves, the standpipe, and the air flows.

The unit is equipped with two Stone & Webster Engineering Corp./Beijing Design Institute (BDI) catalyst coolers, designated Cooler A and Cooler B. Each cooler is designed for 63 metric tons/hr of 60 kg/sq cm (850 psig) saturated steam. The two catalyst coolers and their associated systems are replicates of one another.

Saturated steam from the catalyst coolers and saturated steam from the slurry-steam generators are used for reboiling the debutanizer. Excess steam is sent to the CO boilers. In the CO boilers, the steam combines and is superheated with the CO boiler saturated steam. The mixture drives downstream equipment, which includes the main air blower and the wet-gas compressor.

The catalyst coolers are each able to produce up to approximately 75 metric tons/hr of saturated steam. Production is limited by the size of the steam drum pressure-control valve.

The typical water-circulation rate for a catalyst cooler is 1,800 tons/hr for a 27:1 water/steam ratio. The catalyst side of the catalyst cooler is designed for catalyst circulation rates of 0-20 tons/min with typical rates on the order of 5-10 tons/min.

The flow of fluidization air to the catalyst cooler is typically operated at a rate less than 50% of design to maximize the air volume to the combustion zones of the regenerators. Thermocouples tell LG-Caltex that reducing the fluidization air below this minimum rate results in uneven catalyst flow through the catalyst cooler.

Each cooler contains 32 tube clusters. There are four tubes in a cluster. Fig. 1 shows a diagram of one cluster.

To minimize the number of necessary isolation valves, the clusters are arranged so that there is one set of isolation valves for a pair of clusters (Fig. 2) [77,469 bytes]. When a tube leak occurs, this arrangement requires two clusters to be taken out of service at a time.

Since clusters can be blocked in, each set of clusters is provided with a thermal relief valve (TRV). These relief valves leak quite often. Before May 1997, because the TRVs did not have isolation valves, the catalyst cooler had to be taken out of service to repair the relief valves.

During the May 1997 turnaround, block valves were installed on the inlet to each TRV. These new valves allowed the TRV to be removed without shutting down the catalyst coolers.

The LG-Caltex coolers operate over a wide range of catalyst-circulation rates; thus, the design of the standpipe is critical to allow smooth-catalyst circulation and to generate sufficient pressure to return the catalyst to the bottom of the regenerator. Fig. 3 [88,018 bytes] shows one of several cases in which the actual pressure was higher than the design pressure. Higher pressure improves catalyst circulation.

The original pressure-balance design of the catalyst-circulation system used conservative, low-density values. The design assumed a density of 500 kg/cu m (31.2 lb/cu ft) in the catalyst cooler bed and of 350 kg/cu m (21.8 lb/cu ft) in the standpipe. These relatively low design densities result in a rather long standpipe.

Actual densities are more on the order of 600 kg/cu m (37.4 lb/cu ft) for the bed and 500 kg/cu m (31.2 lb/cu ft) for the standpipe. Thus, a shorter standpipe can be used in future installations.

The catalyst-cooler steam drum is near the bottom elevation of the catalyst cooler. This configuration presents certain problems when starting and stopping the water circulation.

When starting circulation, the drum level must be high and the circulation must be done at a low rate to prevent emptying the drum while filling the bundle and associated piping.

When the circulation is stopped, the reverse problem occurs. The drum will fill quickly with liquid, and it is possible to contaminate the steam with water. LG-Caltex has learned that by operating the steam drum at a level less than 45%, the problem can be minimized.

Maintenance history

In late May 1996, the unit was shut down for some planned refinery electrical work. LG-Caltex took advantage of this downtime to inspect the catalyst coolers. The company's inspectors discovered that there was considerable erosion on the clusters. The majority of the erosion was near the catalyst entry, apparently from the incoming catalyst flow (Fig. 4) [15,950 bytes].

Operators originally had three parameters with which they controlled heat removal in the regenerator: catalyst-circulation rate, fluidization-air rate, and catalyst level.

Typically, the catalyst level was maintained such that 50-70% of the cluster length was in the dense catalyst. After inspection of the erosion patterns, LG-Caltex raised the catalyst levels to keep the entire cluster in the dense catalyst. The level currently runs at about the center of the inlet nozzle or approximately 2 m above the top of the cluster.

In October 1996, a final leak occurred in Catalyst Cooler B, and four more clusters were isolated. In November 1996, the unit was shut down for planned modifications to the downstream DeSOx unit and the catalyst coolers were again inspected.

The erosion on the tubes appeared to be caused by the inlet flow of the catalyst, the same conclusion as that found in the May inspection.

The October leak was a result of a rupture in the common steam outlet tube in Cluster 24, which caused external erosion to Cluster 29. The actual reason for the rupture was not determined but it could have been as a result of local overheating.

The catalyst coolers operated until the May 1997 turnaround without any further leaks. Even with eight clusters (25%) isolated, Catalyst Cooler B was removing heat at its design capacity because of the original conservative design of the system. In the May 1997 turnaround, both catalyst coolers were removed and the damaged clusters replaced.

Seven clusters were replaced in Catalyst Cooler A, four of which had been isolated during operation. In Catalyst Cooler B, a total of twelve clusters were replaced, eight of which had been isolated during operation. Except for the four clusters in Cooler B, which were isolated in October 1996, all damage was a result of erosion on the upper 50% of the cluster from the inlet catalyst flow.

There was some speculation that the rupture in October 1996 could be the result of heat from unequal circulating-water flows. The two clusters that shared common isolation valves also shared a common restriction orifice for flow distribution. To resolve this, during the 1997 turnaround, all common restriction orifices were replaced by individual orifices at the inlet to each cluster.

Since the May 1997 turnaround, there have been no leaks, and the unit has not been shut down.

Leak detection

Typical operational symptoms of a catalyst cooler leak include:

• A decrease in catalyst-cooler steam-drum pressure
• Out-of-balance steam production and boiler feed water makeup
• A decrease in the second-stage regenerator dense-bed temperature
• An increase in opacity at CO boiler
• A decrease in catalyst-cooler bed temperatures
• An increase in the second-stage regenerator pressure
• Instability of catalyst-cooler, slide-valve pressure differential.

Recommended steps for determining which cluster is leaking are:

1. Maintain water circulation.
2. Decrease the feed rate to operate with only one catalyst cooler.
3. Close catalyst cooler-inlet slide valve.
4. Maintain lift air and fluidizing air (this will cool the catalyst).
5. Stop water circulation.
6. Drain water and catalyst at the bottom of J-bend.
7. Close all inlet and return valves on clusters.
8. Open and check the vent valve on the return lines of each cluster. Continuous venting from a vent valve after initial depressuring indicates that cluster is leaking.
9. Isolate and blind leaking cluster.
10. Open all inlet and return valves on clusters.
11. Start water circulation.
12. Close the catalyst cooler outlet-slide valve, fill approximately 10% with catalyst, and start catalyst circulation.
13. After confirming good catalyst circulation, bring to normal operation.

The Authors

Louis R. Anderson is technical service manager for Stone & Webster Engineering Corp. In this position, he is responsible for training, commissioning, start-up, and continuing technical service for Stone & Webster's RFCCUs worldwide. Anderson holds a BS in chemical engineering from the University of Texas, Austin.

Hyung Soon Kim is an operations manager for the resid FCCU at LG-Caltex Oil Corp. He is in charge of the reactor, the regenerator, the CO-boiler, Desox, and utilities at the unit. Kim has 13 years of experience in refining, which includes operations planning for 7 years. He was involved in the design, commissioning, and start-up for LG-Caltex's resid FCCU project. Kim holds a BS in chemical engineering from Chunnam National University, Kwangju, South Korea.

Tae Gyung Park is an operations manager for the resid FCCU at LG-Caltex Oil Corp. He has been in charge of LG-Caltex's resid FCC's main fractionator, gas-treating unit, and sulfur-recovery unit for 4 years. Park has 11 years of experience in refining, which includes being in technical service for 6 years. He was involved in the design, commissioning, and start-up for LG-Caltex's resid FCCU project. Park holds a BS in chemical engineering from Hanyang University in Seoul.

Hyun Joo Ryu is an executive director for resid FCCU/petrochemicals at LG-Caltex Oil Corp. He is in charge of LG-Caltex's resid FCC and petrochemicals operation. Ryu has 22 years of refining experience. He has been a technical service engineer, shift manager, and operations planner. Ryu was also involved in the design, commissioning, and start-up for the resid FCCU project. Ryu holds a BS in chemical engineering from Chunbuk National University, Chunju, South Korea.

Suk Jin Jung is an operations engineer for the resid FCCU at LG-Caltex Oil Corp. He is supporting the operations of the main fractionator, gas-treating unit, and sulfur-recovery unit under Park after finishing 1 year of field work. Jung holds a BS in chemical engineering from Pohang University of Science & Technology (Postech), Pohang, South Korea.

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