NEW CATALYST IMPROVES SULFUR RECOVERY AT CANADIAN PLANT

Elmo Nasato
Goar, Allison & Associates
Tyler, Tex. R. Steve MacDougall
Hibernia Management &
Development Co. Ltd.
St. John's, Newf. Jan A. Lagas
Comprimo B.V.
Amsterdam

Installation at Mobil Oil Canada Ltd.'s Lone Pine Creek, Alta., gas plant of a second-generation Superclaus catalyst has, combined with the first-generation catalyst, resulted in higher overall sulfur recovery at lower reactor temperatures.

Superclaus reactor inlet temperatures have been reduced from 255 to 2000 C. and as a result have saved on utility costs and reduced tail-gas how and CO2 emissions.

Initial results indicate overall plant sulfur recovery has improved to the 98.7-98.9% range, up from the 98.0-98.3% first-generation catalyst performance level.

The enhanced second-generation catalyst has also proven more operationally flexible than the first-generation catalyst.

IMPROVED CATALYST

The Superclaus process was introduced in 1988 as an enhancement to the Claus tail-gas cleanup process that lowered operational costs and simplified operations (OGJ, Oct. 10, 1988, p. 68).

In 1990, Mobil Canada's Lone Pine Creek plant became the first North American site retrofitted with the Superclaus process (OGJ, Feb. 10, 1992, p. 61).

To achieve higher sulfur recoveries, an enhanced second-generation Superclaus catalyst was developed which improves the selectivity of the oxidation to sulfur.

Mobil installed this catalyst at Lone Pine Creek during a scheduled June 1992 maintenance shutdown. The plant was the second commercial plant in the world to operate with the second-generation catalyst.

The second-generation Superclaus catalyst is a silica (Si) based extrudate with higher surface area than the alpha-alumina (a-Al) based first-generation catalyst. The Si-based catalyst is more active at lower operating temperatures than the (a-Al).

In order to utilize the overlapping performance characteristics of the Si and (a-Al) catalysts, a mixed combination of Si and a-Al is used at Lone Pine Creek.

Sulfur-plant turnaround activities included the installation of an automated Superclaus bypass, mesh-pad installation in condenser No. 3 outlet channel, and replacement of the original a-Al catalyst by a mixed a-Al/Si catalyst combination.

In initial (a-Al/Si Superclaus operation, sulfur recovery has increased from the preturnaround levels of 98.0% to current levels of 98.8%.

Also, the Superclaus reheater duty has been reduced; the outlet temperature has dropped from preturnaround 2550 C. to the current temperature of 2000 C. Subsequently fuel-gas consumption and hence CO2 emissions have been reduced.

SUPERCLAUS PROCESS

The Superclaus process involves modification of the conventional Claus operation-control system and application of a catalyst for selective oxidation of H2S to elemental sulfur. 1

The Superclaus stare uses a specially designed catalyst to promote the nonequilibrium, partial oxidation reaction of H2S in the presence of excess air shown in Reaction 1 in the accompanying box of reactions.

Oxidation of H,S to SO, is minimized in order to maximize sulfur recovery by controlling the inlet concentration of H2S to the Superclaus stage and the inlet temperature of the process stream to the Superclaus reactor.

The complete oxidation reaction is shown in Reaction 2.

Oxidation of H2S to SO2 is a rapid, exothermic reaction which generates about 600 C. temperature rise for every 1 vol % H2S in the process stream introduced to the Superclaus reactor.

Hence, the activity of the Superclaus catalyst indicates both the activation temperature (inlet process temperature to the Superclaus reactor) and the H2S concentration to the Superclaus reactor to hold SO2 formation to a minimum.

Sulfur yield from H2S by the Superclaus reactor is in the 85% range.

CATALYSTS

Performance of the Superclaus catalyst is determined by the kinetic control of the sequence shown in Reaction 3.

The catalyst has been designed to minimize the rate constant ratio k2/k1. A distinction must be made first between the different routes along which sulfur vapor may be oxidized to SO2.

One is the reversed Claus reaction. The others are catalytic oxidation and gas-phase oxidation of sulfur vapor.2

To prevent the reversed Claus reaction, neither the active material nor the catalyst support may promote the Claus equilibrium; that is, the catalyst must be chemically inert in this respect.

The final requirement is that the optimum operating temperature should be between about 200 and 3000 C. to avoid sulfur vapor condensation and to prevent gas-phase oxidation. This also poses limits to both the specific surface area of the catalyst and the choice of the active material.

Fundamental research based on the "kinetic control" concept has produced catalysts that meet all the requirements described.

The first-generation catalyst consists of an a-alumina support coated with a mixture of iron and chromium oxides. This is a low surface area, wide-pore material. Figs. 1 and 2 show behavior of the catalyst in the oxidation of H2S.

The figures show that very high sulfur yields are possible with this catalyst. Neither high water-vapor concentrations nor excess oxygen negatively affect the optimum yield.

The performance of the a-Al catalyst with respect to the activity for H2S conversion and SO2 production is shown in Fig. 3.

The second-generation catalyst which is commercially available now consists of a silica support with a much higher surface area. The higher surface area of the silica per volume of catalyst results in a yield per curve at much lower temperature.

Fig. 4 shows the performance of an equal mixture of both catalysts. The yield curves of both catalyst types are compared in Fig. 5, which illustrates that the maximum yield to sulfur for the mixture of Si/et-Al has shifted with approximately, 500 C. to the left compared to the maximum yield point of a-Al.

Comprimo B.V. and Gastec of Holland developed, in cooperation with the University of Utrecht and the catalyst manufacturer Engelhard, the Superclaus catalyst from 1984 to 1987.

The Superclaus first-generation a-Al catalyst has an iron oxide (Fe2O3) base and chromium oxide (Cr2O3) layer; the second-generation catalyst is, as stated, silica based with only an Fe2O3 layer.

Iron oxide is the active component in both catalysts.

Although physically both catalysts are extrudates, the Si-based catalyst has a surface area of about 90 sq m/g compared to the of a-Al based area of about 10 sq m/g.

Increased surface area of the Si-based catalyst makes the Si-based version more active than the a-Al type; thus, the Si-based has a lower activation temperature than the a-Al catalyst.

A lower activation temperature means that a higher percentage of H,S can be introduced to the Superclaus reactor and still minimize SO2 formation.

Comparison of the catalysts is summarized in Table f.

CATALYST LOADING

So that the operating ranges of the a-Al based and Si-based catalyst could be fully used, the catalyst was loaded in a layered manner.

The original 13.6 cu m of a-Al catalyst was removed along with the 3 and 6-mm diameter ceramic support, 10 x 10 and 4 x 4 mesh support screens, and carbon steel support grating.

Installed in the reactor vessel were new carbon steel support grating, 4 x 4 and 10 x 10 mesh stainless steel support screen, stainless steel screen along the full length of the vessel and along the vessel walls to a level above the calculated catalyst depth, and 3 and 6mm diameter ceramic support material.

A catalyst volume of 13.6 cu m was installed as follows: a 5.4 cu m a-Al layer was topped by a 8.2 cu m mixed layer consisting of homogeneously mixed quantities of 4.1 cu m of Si catalyst and 4.1 cu m of a-Al catalyst.

Fig. 6 shows the order.

Manual loading of the catalysts with the use of wood planks (for walking) to load the premixed top layer minimized potential damage.

Upon completion of the catalyst loading, the preinstalled screen was folded down to the top of the catalyst bed. Next, a 10-x-10 mesh screen was placed on top of the catalyst and tied to the screen along the sides of the vessel.

Consequently, the entire support material and Superclaus catalyst are totally enclosed in 10-x-10 mesh screen.

EQUIPMENT MODIFICATION

Besides the catalyst changeout, other sulfur-recovery unit (SRU) modifications included installation of an automated Superclaus reactor bypass line and installation of a mesh pad in the condenser No. 3 located upstream of the Superclaus reactor.

The Superclaus stage is equipped with a 12 x 16-in., steam-jacketed bypass line containing one automated steam-jacketed valve in the 24-in. main process line and one in the 12-in. bypass line.

The bypass line ties into the 24-in. process line downstream of sulfur condenser No. 3 and re-enters the 24-in. process gas line following sulfur condenser No. 4.

Functionally, the bypass allows for easier and safer Superclaus start-ups and protects the catalyst from fluctuations of high H2S concentrations in the process gas, uncontrollable Superclaus temperature excursions, and shortage of Superclaus oxidation air, all without shutting down the entire SRU.

The bypass line is automatically activated via the programmable logic controller (PLC) by the following process conditions:

Superclaus bed temperatures greater than 3500 C.
H2S concentration greater than 2.0 vol % to the Superclaus reactor.

Superclaus oxidation-air flow rate below the minimum value of 470 cu m/hr for stoichiometric oxidation of H2S.

Carryover of liquid sulfur mist from condenser No. 3 has occurred at Lone Pine Creek. Combination of the elemental sulfur mist with the Superclaus oxidation air forms SO2, which represents lost recovery.

Thus, to minimize the problem, the outlet channel section of condenser No. 3, immediately upstream of the Superclaus stage, was extended and retrofitted with a stainless-steel horizontal demister pad.

PERFORMANCE

The Lone Pine Creek Superclaus stage was recommissioned in June 1992.

Initial results, as recorded by the Lone Pine Creek continuous stack-emission monitor and verified by a third-party stack survey, have indicated a daily recovery of 98.8%, compared with a 98.0% preturnaround recovery.

The higher recovery, is being achieved at a lower Superclaus reactor inlet temperature of 2000 C., compared with the preturnaround 2550 C. temperature. Reduced reactor inlet temperature requires much less fuel gas (11 cu m/hr) compared with preturnaround fuel requirements (65 cu m/hr).

Consequently, fuel-gas consumption and air-blower requirements are reduced, and utility savings are realized. Furthermore, tail-gas volumes and CO2 emissions are also reduced. Table 2 summarizes the comparison of the preturnaround a-Al Superclaus operation with the current Si/a-Al process operating conditions.

Preliminary results indicate a daily recovery of 98.7-98.9% with the Si/a-Al second-generation Superclaus catalyst operation. In addition to being an improvement over the a-Al recovery level, as stated, the new recovery level is also a significant improvement over the pre-Superclaus Claus recovery levels of 96.5-97.0%.

Historical representation of the Claus, a-Al catalyst, and the current Si/a-Al catalyst sulfur recoveries are shown in Fig. 7.

The combined Si/a-Al catalyst operation's lower reactor inlet temperature reduces the fuel gas-fired reheater duty.

REFERENCES

Borsboom, J., Lagas, J.A., Wolfer, W., Bosch, J., and Goar, B.G,. "Superclaus process proves reliability," Hydrocarbon Technology International, London, 1991, pp. 31-36.
Nisselrooy, P. van, and Lagas, J.A., "Superclaus reduces SO, emission by the use of a new selective oxidation catalyst," Sulfur '93 International Conference, Apr. 4-7, 1993, Hamburg.