NEW PRESULFIDING TECHNIQUE PROVES SUCCESSFUL IN COMMERCIAL TRIALS

J. Gary Welch
CRI International Inc.
Houston

Paul Poyner
Conoco Inc.
Ponca City, Okla

Robert F. Skelly
Criterion Catalyst Co. LP
Houston

An improved ex situ catalyst-presulfiding procedure has proven successful in several commercial trials, including one at Conoco Inc.'s Ponca City, Okla., refinery.

The new technique overcomes some of the difficulties experienced with earlier presulfiding technologies.

Hydrotreating catalyst presulfided using CRI International Inc.'s new procedure was loaded into Conoco's gas oil desulfurizer in mid-1993. The large exotherm experienced with earlier-generation presulfided catalysts was eliminated with the new procedure.

HISTORY

In 1988, CRI International introduced a presulfiding technology that incorporated elemental sulfur into the pores of hydrotreating catalyst (see Terminology).1 (Other presulfided catalysts are produced using an organic polysulfide.2) Fig. 1 illustrates how start-up time is reduced when presulfided catalysts are used.

The use of elemental sulfur dramatically reduced the cost of ex situ presulfiding, making it much more competitive with in situ sulfiding. But, although these presulfided catalysts were convenient, there were a few drawbacks to the technology.

After loading the presulfided catalyst, the unit is pressurized and feedstock introduced. The feedstock is put on full recycle, otherwise part of the sulfur may be washed out of the system, resulting in insufficient sulfur for complete sulfiding.2 In extreme cases, it is possible to remove enough sulfur to cause plugging in downstream equipment.

Once the system is on recycle, the unit temperature is increased until activation occurs, which is indicated by three events:

Rapid temperature rise
Rapid increase of H2S levels in the recycle gas
Release of copious amounts of water formed from the sulfiding reactions.

When the activation temperature is reached, the temperature is held steady until the conversion of the catalyst's metal oxides to metal sulfides is completed. The reactions initiate at the top of the catalyst bed and travel downward through the bed, producing a heat front that passes through the catalyst bed in a matter of minutes.

This procedure is good for achieving rapid start-ups, but the reactions are highly exothermic. In extreme cases, the exotherms can be great enough to reduce the metal oxides to base metal. If this happens, the catalyst will not achieve its optimum activity level.

The rapid release of water also can cause complications. The refiner must pay close attention to the liquid level in the high-pressure separator and be prepared to drain the water as it is formed to avoid overflow. About 1 gal of water is produced from every 200 lb of catalyst activated.

In the case of conventional, in situ presulfiding, a "hold" step is performed while the sulfiding agent is injected. This typically requires 8-12 hr if the unit is operating on oil flow, but it can take 2-3 days if there is only gas flow.

Once the low-temperature sulfiding is completed, the unit temperature is increased for the high-temperature sulfiding step. The net result is that in situ sulfiding can take 20-30 hr, compared to 8-12 hr required for start-up on ex situ presulfided catalyst.

Fig. 2 illustrates a fairly typical activation for presulfided catalyst in a unit under oil flow. The data for the figure were taken from a gas oil desulfurization unit at Conoco's Ponca City refinery.

The reactor was loaded with about 160,000 lb of Criterion 424 catalyst presulfurized by the elemental sulfur process. Oil and gas flow were established, and the reactor temperature was increased.

Fig. 2 also shows that, when the inlet temperature reached about 325 F., the reaction began to take off. It propagated through the catalyst beds in only 20-30 min. As the reaction went through the catalyst bed, the bed temperatures increased rapidly. The temperature increase at the bottom thermocouples was about 100-150 F., even with oil flow through the unit.

Once the exotherm has passed through a reactor, the unit temperature typically is increased to about 600 F., held for several hours, then adjusted to run temperature.

If it is not possible to start up the unit with liquid feedstock flow, the catalyst can be activated under flowing hydrogen. This will intensify the exotherms, however, because the liquid feedstock acts as a heat sink for the exothermic reactions.

Fig. 3 shows how severe exotherms can be with a polysulfide-presulfurized catalyst in a hydrocracker pretreater. In this example, the bed outlet temperature increases from 350 to 675 F. in about 10 min, producing a bed _T of about 300 F. This is not good for the catalyst and has proven to produce erratic catalyst activity.

Similar exotherms have been observed with catalyst presulfurized using elemental sulfur. Thus, CRI does not recommend gas-phase activation of catalyst presulfurized with elemental sulfur.

One of the major advantages of ex situ presulfiding is that it produces an even distribution of the sulfiding agent throughout the reactor, and even throughout the individual catalyst pellets. Thus, if the unit has poor flow distribution during start-up, the activity will not be affected adversely.

In fact, industry has had excellent results with the activity of ex situ presulfided catalyst. In the few cases where good activity was not obtained, the problems were caused by washing of sulfur from the catalyst or by excessive exotherms.

NEW PROCESS

Recognizing the weaknesses in the early ex situ presulfiding processes, CRI embarked on a development program in the late 1980s. Two deficiencies had to be overcome:

Excess sulfur removal under oil flow
Large exotherms caused by rapid activation.

After spending 3 yr and $1 + million, CRI's development program produced a breakthrough method for presulfiding catalyst.

The new technology utilizes a novel organic matrix to form a complex with the sulfur and metal oxides on the catalyst. Catalysts presulfided in this manner contain a range of metal oxy-sulfides which are converted to sulfides over a wide temperature range, thus eliminating sharp exotherms.

The sulfur is protected by the organic matrix so that it cannot be washed off the catalyst until the conversion reactions take place. Catalysts presulfided by this process, called actiCAT, are easy to activate and yield consistently high activity.

REDUCED EXOTHERM

One of the keys to the successful development of the process is the simulation of commercial operations in the laboratory.

The pilot-plant facilities in CRI's Houston research and development center helped optimize procedures and assess catalyst activity, but were not well suited to investigating the exothermic nature of the activation process. To resolve this weakness, CRI utilized differential scanning calorimetry (DSC), which measures precisely the heat released from chemical reactions.

As shown in Fig. 4, a sample is placed in a sealed chamber equipped with a temperature probe. The chamber and sample are then heated. When reactions take place in the sample material, heat is released.

The temperature probe in the sample senses the temperature increase. This increase is then compared to that of an inert reference material and the instrument determines the differential heat release (_H). To fully simulate a hydrotreater reactor, the sample chamber can be pressurized with hydrogen gas.

This sophisticated approach measures and simulates actual reactor conditions accurately and produces valuable information. DSC thermograms for actiCAT-processed catalyst, and for the earlier generation of catalysts sulfurized using elemental sulfur and polysulfides, are shown in Fig. 5. The heights of the DSC traces indicate the rate of heat release, and therefore the expected temperature rise, of a commercial reactor.

One of the most important characteristics of the new process is a slow rate of reaction over a broad temperature range. The catalysts start activating at about 250 F. and continue up to a temperature of about 600 F. The bulk of the reaction occurs between 400 and 550 F. This dramatically reduces the temperature spikes.

By contrast, the material presulfided with polysulfide reacts rapidly over a temperature range of about 350-400 F. And the reaction of the elemental sulfur product is equally sharp at about 500 F.

The reduced exotherms predicted by DSC for actiCAT-presulfided catalysts have been corroborated by commercial experience. Fig. 6 presents the temperature profiles for the Conoco gas oil desulfurizer shown in Fig. 2. This unit was started up in May 1993 with actiCAT-presulfided Criterion 424 catalyst.

Comparing the two figures, it is obvious that the new product (Fig. 6) eliminates the exotherm experienced with the first-generation product (Fig. 2).

The unit was loaded with the new catalyst and pressured up. Feed was started, and then heated at a rate of about 75 F./hr. Because there is no discernible heat front, it was not necessary to hold at an activation temperature.

A more severe test of controlling and minimizing exotherms is a gas-phase activation (i.e., without any oil circulation). Without oil as a heat sink, there is a potential for very large exotherms.

Fig. 7 shows the maximum bed temperature compared to the inlet temperature for start-up of a commercial naphtha desulfurizer loaded with 100,000 lb of Criterion 424 catalyst presulfurized by the new process. At about 375 F., the catalyst started activating and the _T became positive. It took about 3 hr for the reaction front to pass through the catalyst bed.

By activating over an extended time (still very short by in situ presulfiding standards) and broad temperature range, it was possible for the hydrogen gas flow to remove the heat from the system so that no thermocouple was ever more than 50 F. greater than the inlet temperature.

The unit was on stream making specification product in 7 hr.

SULFUR RETENTION

Another major improvement with the new process is the product's ability to retain sulfur during oil circulation. Table 1 shows the results of flushing catalysts with diesel oil in a pilot-plant simulated start-up.

The catalysts were heated to 250 F. over 7 hr while the amount of sulfur removed was measured. The product presulfided with elemental sulfur retained less than 20% of its sulfur, while the product presulfided with polysulfide retained about half of its sulfur. The actiCAT product, however, kept about 90% of its sulfur, a critical feature when starting up a unit.

High levels of sulfur released into the oil stream can deposit at cold spots downstream of the reactor (for example, heat exchangers). This deposition can cause restricted flow and even plugging, in extreme cases.

More importantly, presulfided catalysts typically are prepared with only enough sulfur to stoichiometrically convert the metal oxides to metal sulfides. (There is little or no excess sulfur on the catalyst.)

If some of the sulfur is washed out of the system, there is a significant risk of not having enough sulfur to sulfide the catalyst completely, which can lead to inferior activity. Sulfur release is a major concern if mechanical problems develop during start-up, causing oil to circulate over the catalyst for an extended time.

Another advantage of the new process is that the bulk of the sulfur is not released until activation temperatures are reached. Until that time, the sulfur is bound to the catalyst as metal oxy-sulfides, and is protected by a polymeric matrix.

THE FUTURE

The new process has proven very reliable and has solved many of the problems with the earlier technologies. CRI is now expanding the technology to include other catalyst systems.

In particular, hydrocracking catalysts are of prime interest. Hydrocracking catalysts are normally activated with H2S in flowing hydrogen. Oil circulation is used rarely because of concerns with uncontrolled hydrocracking reactions. As a consequence, several days normally are required for presulfiding. Because hydrocrackers have high upgrading value, this lost time is a great expense.

The first commercial application of actiCAT-processed catalysts in a hydrocracker took place in 1993. Activation without oil flow occurred in a highly controlled fashion, with exotherms never exceeding 25 F. through any catalyst bed.

This is a major improvement over prior experience. The unit previously had used catalyst presulfided with polysulfides and experienced temperature excursions of 200 F. or more.

CRI currently is applying actiCAT technology to a variety of noble-metal and nonnoble-metal hydrocracking catalysts. In response to increased demand for actiCAT-presulfided catalysts, manufacturing facilities at CRI's Lafayette, La., plant were expanded in 1994. And new facilities were installed in the Luxembourg plant in 1994 to make the new process readily available to European and Middle Eastern refiners.

REFERENCES

Seamans, J.D., Welch, J.G., and Gasser, N.A., U.S. Patent 4,943,547 (1990).
de Wind, M., Heinerman, J.J.L., Lee, S.L., Plantenga, F.L., Johnson, C.C., and Woodward, D.C., "Air quality and economics spur use of presulfided catalysts," OGJ, Feb. 24, 1992, p. 49.