Menno de Wind, J. J. L. (Hans) Heinerman, Seck Leong Lee, Frans L. Plantenga
Akzo Chemicals Division
Amsterdam
Chris C. Johnson, Debbie C. Woodward
Akzo Chemicals Inc.
Houston
As industry acceptance of ex situ presulfided catalysts increases, so does its range of applications in the refinery. Along with fluid catalytic cracking pretreaters, hydrocrackers, and mild hydrocrackers, the list now includes tail gas treaters.
In addition, recent research involving the use of water-soluble sulfur compounds for off site catalyst presulfiding has shown promise.
BACKGROUND
Hydrotreating catalysts consist of a high-surface-area alumina carrier loaded with highly dispersed metal oxides.
The most common metal oxides are NiO or CoO in combination with MoO3.
The metal oxides are not active as such, but need to be transformed into the active metal sulfides. However, metal-sulfides such as MoS2, NiS, and CoS are unstable in an oxygen-containing atmosphere, as applied during catalyst production.
Traditionally, the catalysts are therefore sulfided in the reactor ("in situ" presulfiding) with the aid of a sulfiding agent and hydrogen.'
In situ presulfiding is typically done with easily dissociating compounds such as dimethyldisulfide (DMDS), dimethylsulfide (DMS), or carbondisulfide (CS,) as spiking agents in a light gas oil (LGO) feed.
Eurecat S.A., together with Akzo Chemicals, developed the off site presulfiding method. Akzo designates such presulfided catalysts "EasyActive." With off site presulfiding, the refiner can:
- Avoid the use of spiking agents
- Minimize the time required for start-up
- Eliminate possible mistakes during start-up.
This presulfiding method was first marketed in 1986. It has since become a widely accepted technology. The amount of Akzo catalysts sold in the EasyActive form, as a percentage of the total sales, has increased from 2% to 42% since 1986.
Pilot plant tests and commercial experience have shown that these ex situ activated catalysts give at least the same activity as catalysts activated by the conventional in situ presulfiding technique. 3
The homogeneous sulfidation of all the catalyst in the reactor has in many cases led to even better performance of units employing ex situ sulfided catalyst.
PRESULFIDED CATALYST
In the off site presulfiding process applied at Eurecat, an organic sulfide dissolved in an organic solvent is brought onto the catalyst. After heat treatment, the sulfide is fixed on the catalyst surface and the solvent is removed.
The sulfur compound is chemically bound to the catalyst, although the Mo, Ni, or Co oxides are not yet completely converted into the corresponding sulfides. The latter step has to take place in the reactor under H2 atmosphere.
Accordingly, the presulfided catalyst contains:
- An amount of sulfur corresponding to the amount of metal oxides
- Some hydrocarbon residue from the organic moiety of the sulfur compound
- A trace of the solvent.
These materials fill part of the pores of the catalyst, resulting in a higher density and loss on ignition (LOI) than the original oxidic material.
The catalysts are stable in air, and there is no risk of releasing the sulfur compound.
Upon H2 activation the active metal sulfides are formed according to the following reactions:
[SEE FORMULA]
The sulfidation step is exothermic, with a heat release of approximately 50 kcal/kg catalyst.
The hydrocarbon residue is simply released from the catalyst.
START-UP PROCEDURE
There are two different procedures for activating the presulfided catalyst, once loaded in the reactor, based on:
- Using hydrogen (gas only)
- Using hydrogen and feedstock.
The general start-up procedure is as follows:
- Purge the unit with N2 (02
- Do not dry separately, but maintain the temperature above 100 C. once the procedure has been started.
- Switch to H2 and increase the pressure to 30 bar (if the unit allows). Set the gas flows at normal rates and full recycle.
- a) Raise the reactor inlet temperature to 150 C. at 10-20 C./hr.
b) Bring the feed at normal rates at 120-140 C. and recycle (if possible).
c) At 130-150 C., an exotherm may be observed. Hold the reactor inlet temperature until the exotherm has stabilized. Drain water.
- Increase the temperature from 150 C. to 320 C. at 10-20 C./hr. At 250-260 C., the HDS will start (second exotherm). Maintain at 320 C. for 2 hr.
- Bring the unit to normal operating conditions and stop recycle of feed.
Start-up takes place at 30 bar and, if possible, with feed and gas recycle. If the liquid feed cannot be recycled, a "gas only" activation is recommended.
The major part of the activation takes place at about 130-150 C. This usually results in a distinct exotherm. It is at this temperature that most of the H2O is released.
Typical activation parameters with hydrogen and feed (Case 1) are given in Tables 1 and 2.
EXOTHERM
Fig. 1 shows the reactor inlet and outlet temperatures for a typical hydrogen/feedstock activation. It is obvious that an exothermic reaction takes place near 130-150 C. The outlet temperature rises faster than the inlet temperature.
The second exotherm, observed at about 260 C., indicates the start of the hydrodesulfurization (HDS) reaction.
In general, the magnitude of the exotherms depend on two factors:
- The rates of heat release and heat removal
- The heat capacity of the catalyst and reactor.
The first factor is controlled mainly by the H2 partial pressure (ppH2). At higher ppH2, the rate of heat release increases. Also, the temperature at which the activation starts is lower at a higher ppH2.
During gas-only activation, it is therefore advisable to apply pressures below 70 bar.
The second factor is controlled by the gas and feed rates. The heat capacity of the catalyst and the reactor walls is, of course, a constant factor, although the latter depends on the thickness of the reactor walls.
The presence or absence of the liquid feed has a substantial effect on the observed temperature change (AT). In liquid activations, the AT almost never exceeds 20-30 C. (Fig. 1). Hence, there is no real limitation on the pressure of such a start-up.
In gas-only activations, exotherms greater than 100 C. have been observed. This temperature rise can be minimized by applying the maximum gas rate.
Fig. 2 shows the observed maximum AT for a number of commercial gas-only cases, as a function of the treatgas rate per unit volume of catalyst. Although the data scatter, the expected quenching effect of the treatgas is indeed observed.
In all cases of Fig. 2, full recycle was applied at around 30 bar total pressure. The catalyst volume varied from 20 cu m to 400 cu m, and the time required to start up, from 7 hr to 17 hr.
SULFUR RELEASE
Table 2 shows the H2S in the recycle gas during activation for Case 1. The observed decrease in H,S with increasing temperature is typical of a presulfided catalyst activation.
In the initial phase, some of the sulfur is released in the form of H2S. At a later stage when higher temperatures are reached, the H2S is taken up by the catalyst again.
During gas-only start-ups, sulfur is released only as H2S-no sulfides have been detected. During liquid activation, part of the sulfur may be released as sulfides dissolved in the feed.
An example of this behavior is shown in Fig. 3, displaying the sulfur content of the gas oil at the reactor outlet during a liquid phase start-up. In this case the gas oil was recycled.
Sulfur loss during activation can be minimized by recycling the gas and/or fees. (If a liquid recycle cannot be applied, a gas-only activation is preferred.)
Furthermore, it is important to minimize the time that reactor temperatures are between 150 and 200 C. At higher temperatures the catalyst will rapidly pickup the sulfur compounds.
The heat generated by the start-up usually helps to achieve this target easily. In commercial activations, the sulfur release has never been known to lead to improper activation or low activity (i.e., real sulfur loss), including the cases of gas-only or once-through procedures.
In pilot plant activations, the risk of sulfur loss is higher when the catalyst remains at low temperatures too long and no recycle of feed and treatgas is applied. Because of the isothermal operation (no IT), the catalyst may not be heated rapidly enough by the sulfidation reactions.
With the EasyActive technology, it is impossible that sulfur release will lead to blockage of downstream equipment by any sort of sulfur compound. the sulfur will only be released as H2S or, when liquid phase activation is applied, as dissolved organic sulfides.
In addition to the exotherm and the prevention of sulfur loss, another important practical implication is the formation of water according to the sulfidation reactions mentioned previously.
This of course also occurs during conventional in situ presulfiding.
With EasyActive activation, however, the water is formed in a relatively short span of time. Therefore, extra attention should be paid to draining water from the separator system. Conditions in the separator system should be such that good separation of water occurs.
COMMERCIAL EXPERIENCE
Presulfiding has become an industrial standard method for activation of hydroprocessing catalysts. Eurecat has sulfided more than 17,500 tons of catalyst.
During its introduction phase, Easy active catalyst was mainly used for smaller units treating naphtha, kerosine, and gas oils. More recently, large reactors holding 200-300 tons of catalyst have been started up this way. These types of reactors are mainly in use in fluid catalytic cracking (FCC) pretreatment, hydrocracking, and mild hydrocracking operation.
Data from the successful activation of a hydrocracker pretreater with highly active NiMo catalyst (Case 2) are shown in Table 3. This unit, holding over 200 cu m of pretreat catalyst, was activated in 1 day with gas-only, on full recycle.
The normalized weighted average bed temperature (WABT) required to attain the desired product nitrogen content was 5 C. lower than that used in the previous cycle with a conventional catalyst.
Case 3, shown in Table 4, is a nearly 300 cu m FCC pretreater holding Ketjenfine 840. This unit was also started up with gas only on full recycle.
Within 30 hr this unit was producing on-spec product. Because the gas rate was rather low, the maximum AT was around 80 C.
Previous papers have indicated the benefits and economic incentives to using exsitu presulfided catalysts instead of applying in situ sulfidation."
These advantages are:
- No use and handling of spiking agents that are toxic, flammable, and have an odor
- Less chance of environmental violations
- No additional (injection) equipment needed
- Homogeneous sulfidation of all catalyst in the reactor
- Less time required for start-up
- Less off-spec product
- Easy activation-less manpower and attention needed, producing less chance of mistakes.
SPECIALTY USE
Until recently, EasyActive catalysts had never been used in tail gas units. But in the spring of 1991, a Beavon unit was successfully started up using presulfided catalyst KF-124LD-1/8. ("Nonsulfided" KF-124LD has found widespread use in tail gas treating units.)
Normally, a tail gas unit is sulfided using the offgas from the Claus unit. This gas is typically high in SO2, however, and low in H2S, resulting in a lengthy sulfiding procedure. Further, significant SO, emissions occur during the sulfiding of these units.
This particular refiner had always exceeded the U.S. Environmental Protection Agency (EPA) regulated maximum of 250 ppm (wt) SO2 over a 6-hr rolling average during its typical unit sulfiding/start-up. This usually resulted in fines imposed by EPA. The option was to reduce refinery processing rates-a very expensive alternative.
Several concerns with using EasyActive surfaced when reviewing the refiner's start-up procedure in advance of the turnaround. The main concern was that the reducing gas generator (RGG) is purged with air for a few minutes before bringing in hydrogen to assist in lighting-off this heater (Fig. 4).
Once hydrogen is brought in, the atmosphere is a reducing one, eliminating any possibility of catalyst oxidation. However, the duration and temperature of the resulting air contact with the presulfided catalyst were both determined to be low enough not to present problems.
Other concerns were the low pressure of the unit (0.7 bar) and the low hydrogen flow.
Fig. 5 depicts the reactor temperatures during activation. Although there was indication of maldistribution based on the sporadic temperature profile, the start-up proceeded smoothly. There were no regulatory violations, and start-up time was reduced by 6-8 hr.
Economic and environmental incentives are indeed an important driving force to use presulfided catalyst. However, in the past few years it has become very clear that one of the most important reasons to use this technology is the ease of activation.
NEW DEVELOPMENTS
Eurecat has spent much effort optimizing its presulfiding plant. This resulted in building a new plant at the company's La Voulte, France, site. The new plant came on stream in the fall of 1989.
Akzo Chemical's research and development department has focused on one alternative route, i.e., ex situ presulfiding with water-soluble agents.
WATER-SOLUBLE AGENTS
Research at Akzo laboratories indicated that water-soluble sulfur compounds can also be used for off site presulfiding.
In several cases, improved activities were observed. Moreover, the water-soluble agents could have environmental advantages during production and may, lead at least to a simpler sulfidation plant.
Akzo Chemicals has filed a patent application on this type of presulfidation.
Table 5 gives examples of water-soluble sulfides that have been used to prepare ex situ presulfided Ketjenfine 165 catalysts. After aqueous impregnation with sufficient sulfur for stoichiometric sulfidation of the metals, the catalyst samples were dried at 100 C. and evaluated in a vacuum gas oil (VGO) HDS test.
Activation of the catalyst was carried out in test reactors. First the reactor was flushed with nitrogen to remove air. Next, hydrogen gas was passed over the catalyst at a pressure of 50 bar while the temperature was increased to 150 C.
Subsequently, a light gas oil containing 1.2 wt of sulfur was introduced, and the temperature was raised to 368 C. Thereafter, a vacuum gas oil was used for the HDS measurements. The test conditions are presented in Table 6.
Several sulfur compounds appeared to give higher activities than the standard presulfiding process (Table 7). To investigate the reproductibility of the test results, three subsequent tests were carried out with Ketjenfine 165 EasyActive and with diethanoldisulfide (DEDS) impregnated Ketjenfine 165.
The results, presented in Table 8, clearly indicate that ex situ presulfidation with DEDS gives improved HDS activities. Analogous improvements have been obtained for Ketjenfine 742.
REFERENCES
- Eijsbouts, S., Heinerman, J.J.L., and Elzerman, "MoS2 Structures in High Activity Hydrotreating Catalysts," Akzo Catalysts Symposium. June 1991, Scheveningen, The Netherlands.
- Hallie, H., OGJ, Dec. 10, 1982, pp. 69-73.
- Plantenga, F.L., Akzo Catalysts symposium 1988, paper H-3.
- Asim, M.Y., Akzo Catalysts Technical Seminar, October 1989.
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