Mixed-distillate hydrotreating reduces costs

David J. Podratz
Murphy Oil USA Inc.
Meraux, La.

Karl Kleemeier
Murphy Oil USA Inc.
El Dorado, Ark.

William Jay Turner, Brian M. Moyse
Haldor Topsoe Inc.
Houston

• Feeds and products of MDH unit (Fig.1) [60,994 bytes]

In its Meraux, La., refinery, Murphy Oil USA Inc. worked with Haldor Topsoe Inc. to increase the cycle lengths of Murphy Oil's mixed-distillate hydrotreating (MDH) unit by more than 42%.

Reducing the pressure-drop buildup and catalyst-deactivation rate allowed increased cycle lengths.

This particular Murphy Oil refinery processes low-sulfur diesel and fluid-catalytic cracking (FCC) feed in its MDH unit. Typically, middle distillates and FCC feed are hydrotreated in separate, dedicated units, but Murphy Oil performs these two functions in one unit.

The Clean Air Act Amendment (CAAA) of 1990 required U.S. refiners to manufacture low-sulfur, 500 ppm(wt) diesel for on-road use. Also, many refiners pretreat FCC-unit feed to improve yields. Murphy Oil has combined these two functions into one unit to reduce capital costs.

The operation of Murphy Oil's MDH unit shows that it is possible to successfully coprocess middle and heavy distillates in the same reactor.

In 1997, for the unit's Cycle 5 run, Murphy Oil and Haldor Topsoe worked together to eliminate pressure-drop problems that shortened Cycles 1-4 of the MDH unit.

During the MDH unit's Cycle 6 run, Murphy Oil realized a substantial increase in cycle-length. Based on the success of Cycles 5 and 6 coprocessing operations, Murphy Oil used Haldor Topsoe catalyst for Cycles 7 and 8.

Cycle 7 ended in mid-January with a turnaround, and Cycle 8 started in March.

To further increase cycle length, the company added a third reactor to the unit for Cycle 8. Again, Murphy Oil chose Haldor Topsoe catalysts for this cycle.

Coprocessing conditions

Certain relevant operating parameters and unit objectives must first be defined to coprocess combined middle and heavy distillate streams: reactor pressure and temperature, recycle-gas rates, and fractionation.

To realize improved FCC feed quality in terms of aromatic saturation, denitrogenation, and demetallation, the operating conditions for an FCC-pretreating unit should prevail in a coprocessing operation. That is, hydrogen partial pressure should be in 1,000+ psig range, and reactor-inlet temperatures should be capable of reaching at least 800° F.

Recycle-gas rates are of prime importance in a coprocessing operation. The gas rate and hydrogen purity must be determined to ensure adequate hydrogen availability throughout the reactor. If adequate hydrogen is not available down toward the bottom of the reactor, the probability of coke formation on the catalyst increases. If this occurs, the catalyst activity will rapidly decrease to the point where the catalyst must be replaced.

Finally, adequate reactor-product fractionation must be available for a coprocessing operation. Without product fractionation, the concept of coprocessing is virtually impossible.

Catalysts for coprocessing

Proper catalyst selection for use in distillate coprocessing ensures the success of the operation. The selection depends on the desired unit objectives.

Murphy Oil's objectives are to produce 0.05 wt % sulfur diesel and improve FCC feed quality. The degree to which these objectives are met or exceeded requires careful study and close cooperation between the catalyst vendor and the refiner.

In general, the catalyst system consists of both a NiMo catalyst and a CoMo catalyst. The spatial arrangement of catalyst types depends upon the sulfur species present, the contamination level, and the unit objectives.

Cycle length is also an important catalyst consideration.

NiMo catalyst. In FCC pretreating, various catalyst functions are required. These functions not only include hydrogenation and demetallation activity, but also Conradson carbon conversion.

To accomplish all these functions, multiple layers of specific NiMo catalysts must be used in the catalyst loading. Typically, a catalyst loading consists of a demetallation-specific catalyst layer, followed by a layer of Concarbon conversion catalyst (with a lesser degree of demetallation activity), and, finally, the main-bed catalyst.

The demetallation and Concarbon conversion catalysts act to protect the metals contaminant-sensitive, main-bed catalyst, which provides the bulk of the hydrogenation activity.

CoMo catalyst. In a coprocessing operation, the NiMo catalysts will provide a measure of desulfurization activity and aromatic saturation on the diesel fraction. However, for low-sulfur diesel production, a CoMo catalyst is preferred.

By the time the combined, partially treated feed passes out of the NiMo catalyst beds, all that is required of the CoMo catalyst is desulfurization activity. Therefore, normally, only one layer of CoMo catalyst is required.

Cycle length. Another important consideration is cycle length.

Although a long cycle length is desired, it must be balanced with requirements for product-sulfur specifications. For example, although long cycle lengths can be achieved with low temperatures, at low temperatures, the product-sulfur specifications would not be met.

When determining the catalyst loading, the amount of NiMo catalyst compared to CoMo catalyst should be taken into account. Ideally, it is desirable to have the catalyst ratio such that overall activity decline for both types of catalyst reaches end-of-run (EOR) at the same time. This is not always possible as a result of factors such as vessel sizing and reactor internals.

New MDH unit

The Murphy Oil refinery in Meraux, La., coprocesses both middle and heavy distillates.

The Meraux refinery is a 100,000 b/d facility that typically processes crude oil with about a 30° API gravity and 1% sulfur.

The MDH unit, which coprocesses the middle and heavy distillates, is a UOP LLC design that started up in 1993.

The unit consists of two reactors in series, with a combined reactor-catalyst volume of 5,700 cu ft. This unit operates without a recycle-gas scrubber. Recycle gas is purged, and hydrogen makeup rates are adjusted to control H2S buildup.

To date, the unit has operated with seven completed catalyst cycles and is currently in Cycle 8. Cycles 5 and 6 will be discussed in this article.

The MDH-unit flow sheet indicates the various feed and product streams (Fig. 1).

Table 1 [45,809 bytes] outlines both the feed components and combined feed properties charged to the unit for Cycles 5 and 6.

The relevant operating conditions are described in Table 2. [18,572 bytes]

In order of priority, the operating goals of the MDH unit are to:

1. Produce 500 ppm(wt) sulfur diesel fuel
2. Maximize Conradson carbon removal
3. Maximize hydrogen uptake (as evidenced by API gravity increase)
4. Maintain a gas-oil sulfur content below 0.4 wt %
5. Increase cycle length.

Murphy Oil met the first four objectives in Cycles 1-4 of the MDH unit. It did not, however, meet the desired cycle-length objective until Cycle 6. The refinery implemented catalyst changes in Cycles 5 and 6 to increase cycle length.

Cycle 5

Prior to Cycle 5, the catalyst system was limited by pressure drop rather than activity. To help extend the cycle length and control the pressure drop, Murphy Oil chose Haldor Topsoe Inc.'s catalyst-grading system recommendation over a competitive topping system.

Murphy Oil installed the following graded catalyst system in the top of the first reactor:

• TK-10 inert-topping material
• TK-711 3/16 in. Raschig ring catalyst
• TK-711 1/8 in. Raschig ring catalyst
• TK-831 1/8 in. Trilobe catalyst.

The balance of the first reactor was loaded with another vendor's high-activity NiMo catalyst. The second reactor was loaded with the same vendor's high-activity CoMo catalyst. The ratio of NiMo to CoMo catalyst is in the range of 50/50.

Haldor Topsoe's TK-10 is a high-void fraction (55%), inert hold-down material shaped into a 7-hole, 5/8-in. diameter tablet. The TK-711 Raschig ring catalyst was designed with a large pore radius (80 ?) to remove metals from the heaviest fraction of the feed.

Combining TK-10 with Raschig ring catalyst has not only increased the storage capacity for coke fines and other entrained material, but has also enhanced flow distribution.

The TK-831 catalyst, in addition to metals removal, was designed for high-activity Concarbon and asphaltene reduction. TK-831, as a result of its higher metals content, exhibits a greater HDS activity than TK-711.

No apparent pressure-drop increase from start-of-run (SOR) conditions was observed after 5 months of Cycle 5 operation. At about 6 months into the run, however, reactor temperatures started to increase at an unexpected rate.

As the reactor weighted average bed temperature (WABT) approached 800° F. (EOR), color problems began to develop in the diesel product. At this point (7 months into the cycle), the reactor was brought down for a catalyst changeout.

Fig. 2 [64,956 bytes] presents a comparison of pressure-drop data for Cycles 4 and 5.

The first four cycles were limited by pressure drop rather than catalyst activity. The use of Haldor Topsoe's grading system prevented the premature buildup of pressure drop and allowed all of the catalyst activity to be used in Cycle 5.

Cycle 6

With the pressure-drop limitations essentially eliminated in Cycle 5, attention now centered on further increasing cycle length.

Early on in Cycle 5, Haldor Topsoe performed a pilot plant test for Murphy Oil. Using feed obtained from the MDH Unit, the tests were carried out with the vendor's main bed NiMo TK-525 and CoMo TK-554 catalysts. The ratio of catalyst types was about 50/50.

TK-525 was designed specifically for FCC-feed pretreatment, in which good HDS, high HDN, and high aromatic saturation activity are required.

TK-525 has also shown excellent activity maintenance and stability in commercial operation.

TK-554 was developed for deep HDS activity on distillate fractions, gas oils, and blended feedstocks. The primary application of TK-554 is in <0.05 wt % sulfur-diesel production.

The pilot-plant tests showed that the product properties would be as good, or slightly better, than those observed from the incumbent catalyst system. It was also anticipated that the cycle length would be increased over the incumbent catalyst system as a result of the stability of the catalysts and the use of the grading.

Based on the success of the pilot-plant data and the elimination of the pressure-drop problem, Murphy Oil decided to try a full loading of Haldor Topsoe catalysts.

As such, the following catalysts were installed in the reactors:

• TK-10 inert-topping material
• TK-711 3/16 in. Raschig ring catalyst
• TK-711 1/8 in. Raschig ring catalyst
• TK-831 1/16 in. Trilobe catalyst
• TK-751 1/16 in. Trilobe catalyst
• TK-525 1/20 in. Trilobe catalyst
• TK-554 1/20 in. Trilobe catalyst.

Fig. 3 [63,870 bytes] shows a comparison of WABT vs. cycle length for Cycles 5 and 6.

After 10 months of operation (a 42% increase in cycle length over Cycle 5), the unit was brought down for catalyst change to match a planned refinery-wide turnaround schedule.

Before the unit was brought down, reactor temperatures were increased to fully use the catalyst activity. Based on reactor temperatures and product properties, there was still good catalyst activity, however, left at the time of the changeout.

The Authors