TECHNOLOGY Catalyst separation method reduces Platformer turnaround costs

Stephen R. Blashka, J. Gary Welch
CRI International Inc.
Houston

Kelly Nite
Fina Oil & Chemical Co.
Port Arthur, Tex.

Angelo P. Furfaro
UOP
Des Plaines, Ill.

A catalyst separation technology that segregates catalyst particles by density has proved successful in recovering CCR (continuous catalyst regeneration) Platforming catalyst that had been contaminated with wheel catalyst. The technology, known as density grading, was developed by CRI International Inc. Fina Oil & Chemical Co. used density grading to recover heel-contaminated Platforming catalyst, resulting in a $300,000 savings.

CCR Platforming

UOPs CCR Platforming process converts naphtha to high-octane gasoline components and aromatics for petrochemical use. The reforming reactions take place in a series of Platforming reactors loaded with platinum-containing reforming catalyst. CCR Platforming technology incorporates a moving catalyst bed in a system that permits addition and withdrawal of catalyst from the reactor while the unit is operating. As the catalyst circulates through the reactors, it builds up typical carbon levels of 5%. The spent catalyst from the reactors is transported to the regeneration tower, where the carbon is burned off and the catalyst is reconditioned. The regenerated catalyst is then returned to the top of the reactor stack. Fig. 1 (51287 bytes) shows a schematic of the process. CCR Platforming units typically contain three or four reactors stacked on top of one another to facilitate catalyst flow. The naphtha feed is charged to the lead reactor. Reforming reactions are endothermic, so the reactor effluent must be reheated before it is charged to the next reactor. The effluent from the last reactor is sent to product recovery.

Catalyst contamination

A small fraction of the Platforming catalyst does not circulate through the reactor system. This nonflowing catalyst, known as heel catalyst, is held up in the bottom and along the walls of the reactors (Fig. 2)(31829 bytes). When catalyst is unloaded from Platforming reactors during turnarounds, circulating catalyst commonly becomes contaminated with heel catalyst. In the early 1980s, UOP redesigned the reactors to further reduce the amount of heel catalyst. Although the new reactor design has reduced heel catalyst, it has not eliminated the problem of catalyst contamination. Over time, the heel catalyst will build up carbon levels as high as 50%. When the catalyst is unloaded, heel catalyst is released, contaminating the last fraction of catalyst removed from the reactor. The quantity of heel-contaminated catalyst depends on unit size, unit design, and unloading procedures. Typically, about 10% of the catalyst unloaded from the reactors is contaminated with heel catalyst. The heel-contaminated catalyst should not be reused because only a small fraction of the carbon on the heel catalyst is removed in the regeneration section. If returned to inventory, the heel catalyst would pass through the regenerator to the reconditioning section, where the carbon would react rapidly, causing temperature excursions. If heel-contaminated catalyst is reused, there is a high potential for damage to the unit.

When a refiner plans to reuse the catalyst removed during a turnaround, there is a strong economic incentive to use density grading to recover the low-carbon (circulating) catalyst from the contaminated fraction. If the low-carbon catalyst is not recovered, it must be sent out for platinum recovery and replaced with fresh, makeup catalyst.

Density grading

Density grading technology separates catalyst particles of similar size based on individual particle density. Particles with density differences as small as 10% have been separated effectively. Since its introduction in 1987, density grading has been used in a wide range of catalyst applications. It has been used to separate different catalysts that were stacked in the same reactor and became mixed when the reactor was dumped. In one case, a hydrocracker contained pretreating and cracking catalysts of the same size, stacked in one reactor. During unloading, a portion of these catalysts became mixed. The mixture was not reusable, so density grading was used to separate the catalysts after ex situ regeneration. Density grading also has been used to separate relatively uncontaminated catalyst particles from contaminated ones. In one application, a charge of catalyst was contaminated with vanadium. The vanadium had deposited on the catalyst in the top portion of the reactor. When the catalyst was dumped from the bottom of the reactor, the contaminated catalyst mixed with the relatively clean catalyst from the bottom portion of the reactor. Density grading was used, after ex situ regeneration, to recover the uncontaminated catalyst for reuse. This type of application also can be used for catalyst contaminated with other impurities, such as lead or silica. Separating high-carbon heel catalyst from low-carbon CCR Platforming catalyst is an ideal application of density grading. The two catalysts are uniform in size, and there is a large density difference (40-80%) between them, so a precise separation can be made. There also is a large financial benefit in recovering low-carbon CCR Platforming catalyst.

Commercial application

In April 1995, Fina shut down its CCR Platforming unit for a maintenance turnaround. Because Fina planned to reuse the catalyst, it was analyzed for evidence of heel contamination as it was unloaded from the reactor. Fina did this by periodically measuring the carbon content of the dumped catalyst. Near the end of the unloading procedure, when heel contamination is expected to occur, the dumped catalyst was analyzed more frequently (every container was tested). Normally, the catalyst in the reactor stack has a carbon profile such that the catalyst in the top section contains 0% carbon and that in the bottom contains 3-7% carbon. The catalyst is dumped from the bottom of the last reactor and travels plug-flow through the reactor stack; thus, the carbon level on the dumped catalyst decreases as the reactors are unloaded. Near the end of the unloading, however, the carbon level begins to increase as the catalyst is contaminated with the high-carbon heel catalyst. Fig. 3 (32392 bytes) shows the carbon analyses for the catalyst removed from Finas Platforming reactors. The analyses indicated that 14 of the 60 containers of unloaded catalyst were contaminated with heel catalyst. Note that, depending on the factors mentioned previously, this quantity of heel-contaminated catalyst does not necessarily represent the quantity of contaminated catalyst expected from other CCR Platforming units.

The contaminated catalyst was shipped to CRIs processing plant in Lafayette, La., where the catalyst was density graded on a turnaround basis. The catalyst was scheduled to be reloaded in the reactor 7 days after it was unloaded. The contaminated catalyst was split into a light fraction that was to be reloaded in the reactor and a heavy fraction that was to be sent for platinum recovery. The specifications on the light fraction were tight. The peak carbon level in this fraction had to be less than 8%. The peak carbon level was determined by analyzing the five darkest pellets (the minimum amount needed for the analytical technique) selected from a population of 300-400 pellets. This procedure ensured that there would be no heel-catalyst particles in the light fraction. There was no specification for the heavy fraction because it was being sent for platinum recovery. One of the goals of the separation, however, was to minimize the amount of low-carbon catalyst in the heavy fraction in order to maximize the amount of useable catalyst returned to the reactor. To monitor separation quality, samples of the light and heavy fractions were taken at increments of 1,000 lb catalyst feed. The light samples were analyzed for peak carbon, while the heavy samples were analyzed for average carbon.

Separation results

The results of the catalyst separation were excellent (Table 1)(16393 bytes). The purity and recovery of the light fraction were very good, and the amount of low-carbon catalyst in the heavy portion was minimal. A precise separation was achieved partly because of the large density difference (80%) between the heel and low-carbon catalysts. Fig. 4 (22781 bytes) shows the carbon analyses of the catalyst in each container of light material produced. The first 10 containers held high-quality material with peak carbon levels less than 8% and average carbon levels of 2-3%. This catalyst easily could be reused with satisfactory performance. As the last two containers (3,024 lb) of low-carbon catalyst were recovered, the product became contaminated with a small amount of medium-carbon (10-15%) catalyst. This material is considered marginal quality for reuse. Fina elected to use this material because the average carbon content was low and there was only a small amount of higher-carbon pellets. This catalyst was selectively placed in the areas of the Platforming reactors where heel catalyst accumulates, so that the impact of the higher-carbon pellets would be negligible. Recovery of reusable catalyst was high. By weight, 86.4% good, light catalyst was recovered from the mixture. Because the good catalyst has a lower density, the actual recovery on a fresh catalyst (volumetric) basis was 91.4%. Recoveries of 60-90 vol % are typical in the density grading of heel-contaminated catalyst. Without density grading, all of the contaminated catalyst must be sent for disposal. The purity of the heavy fraction also was high. The average carbon content of the heavy pellets was close to the peak carbon level, indicating a high concentration of heel catalyst (85%). Only about 2% of the low-carbon catalyst was rejected with the heavy fraction. This demonstrates the high level of selectivity achieved with density grading.

Cost savings

The light catalyst fraction was returned to the refinery quickly enough that the turnaround schedule was not affected.

Fina reloaded the light material into the CCR Platforming reactors and, in consultation with UOPs technical service personnel, successfully started up the unit. By using density grading, Fina was able to recover 30,714 lb of catalyst that otherwise would have been sent out for disposal. Considering the savings in new catalyst purchases and platinum recovery costs, and taking into account fees for the density-grading service, Finas net savings were about $300,000.

The Authors