PETROLEUM-DERIVED ADDITIVE REDUCES COKE ON HYDROTREATING CATALYST

Upgrading heavy oils is becoming increasingly important as the world crude slate gets heavier and demand for light products increases. But most upgrading processes must contend with problems related to coke formation during hydrotreating.

Commonly encountered problems include catalyst decay, increased pressure drop in reactors, furnaces, heat exchangers, and other equipment, and reduced product stability.

Three researchers have found that materials having high radical-scavenging ability can reduce coke formation when applied to hydrotreating heavy oils. And these materials can be produced from heavy petroleum fractions.

Hidehiro Higashi and Hiroki Arao of the Catalysts Research Institute, a branch of Catalysts & Chemicals Industrial Co., Kitakyushu, Japan, and Junichi Kubo of Nippon Oil Co.'s central technical research laboratory in Yokohama, Japan, released their findings in an unpublished report.

COKE FORMATION

It is widely known that hydrogen-donating hydroaromatics effectively reduce coke formation in coal liquefaction and heavy oil upgrading. 12 But the amount of such solvents necessary to satisfactorily reduce coke formation increases costs and causes technical difficulties. These effects can be attributed to the insufficient radical-scavenging abilities of hydrogen-donor solvents.

Coke is formed through free-radical reactions, however, and superior radical scavengers can inhibit this process.

Materials with high radical-scavenging properties (such as antioxidants) generally are not durable enough to heat and cannot be used in severe conditions. Specially treated heavy, highly aromatic petroleum products, on the other hand have no functional groups containing heteroatoms (O, N, S, P, etc.) and are very resistant to heat.

RESEARCH

In their other studies, the researchers found that materials with good radical-scavenging effects can be produced from petroleum products. 3 The team produced additives by hydrogenating highly aromatic, heavy petroleum fractions at the following conditions:

• Temperature, 330-360 C. (626-680 F.)

• Hydrogen pressure, 100-150 kg/sq cm ( 1,420-2,130 psi)

• Liquid hourly space velocity (LHSV), 0.2-0.5 hr-1

• Catalyst, commercial CoMo/Al2O3 desulfurization catalyst.

Table 1 shows the properties of the material produced under these conditions. The material was found to have a radical-scavenging ability about 70 times that of tetralin, a common hydrogen-donating hydroaromatic.

PILOT PLANT

In a pilot test, the material was applied to hydrotreating Arabian Heavy atmospheric resid. The process was carried out in a pilot plant equipped with a downflow fixed-bed reactor (ID, 19.4 mm; length, 3,200 mm; catalyst volume, 400 ml). Feedstock properties are shown in Table 2.

Operating conditions were:

• Reactor temperature, 383- C. (721 F.)

• Reactor pressure, 150 kg/sq cm (2,130 psi)

• LHSV, 0.2 hr -1

• Hydrogen/feed molar ratio, 700

• Catalyst, MoNiV/Al2,O3.

The additive was added to the process at 3 wt % of the feedstock. The test results are shown in Table 3.

The amount of dry sludge produced when using the additive is only 6% of that produced without the additive. The additive also improved the results of the ASTM spot test, which represents product stability. The spot test is thought to detect coke precursors, thus further substantiating the additive's inhibiting effects.

REFERENCES

1. Carlson, C.S., Langer, A.W., and Hill, R.M., Ind. Eng,. Chem., Vol. 50, 1958, p. 1067

2. Fisher, I.P., Southrada, F., and Woods, H.J., OGJ, Nov. 22, 1982, p. 111.

3. Kubo, J., Ind. Eng. Chem. Res., Vol. 31, 1992, p. 2587.

   Copyright 1993 Oil & Gas Journal. All Rights Reserved.